BCAS

The "Photography" In Astrophotography

By John H. (2000)

click here for pdf file (Acrobat Reader required) ~ coming soon //

An Introduction

Taking pictures of astronomical subjects - astrophotography, can be involve a lifetime of doing stellar photography or one or two shots of an interesting moonrise or comet in the sky with a telephoto lens. It can involve equipment expensive enough to require a second mortgage on your home or a simple 35 mm camera steadied on a tripod or a fence post. It can grow and change with your own interests in astronomy and provide a visual record of your hobby in the same way that your snapshot album records the changes in your family life. And because every sunset is different, there is no end of interesting things to photography in the sky. It combines two interesting hobbies - photography and astronomy in a fashion that is sure to provide enjoyment of both.

All promoters of astrophotography give the same advice to novices. Start simple and work your way to the more difficult as your experience, inclination and budget allow. Astrophotography, like any other hobby, can consume a lot of time as well as money, but it does not need to be particularly expensive or time-consuming in the early stages. Most photographers already have the basic equipment and the only cost is for film and processing.

Providing you with a starting point for your astrophotographic efforts is the purpose of this workshop and outline. Owning a telescope is not a requirement but if you do, you will learn how to turn it into a powerful photographic tool with some simple techniques and a minimum of additional attachments.

The Main Steps in Astrophotography

The main steps in the progression from a novice at star photography to an expert amateur astrophotographer includes developing expertise in a series of stages with gradually more elaborate equipment and techniques. But please realize that you do not need to complete all these steps to enjoy the results of taking star photos. Elaborate equipment is NOT required to produce some beautiful, even artistic photos. And you can proceed at your own pace.

Here is the usual sequence of stages: (as suggested by Dickensen and Dyer and endorsed by Hlynialuk)

Step 1: Constellations, aurora, planet groups, comets, meteors with a camera on a fixed tripod.

Step 2: Moon through telescope eyepiece or in a long telephoto lens (400 mm and up).

Step 3: Tracked exposures with a barn door type camera mounting.

Step 4: Tracked exposures with a camera mounted piggyback on a polar aligned telescope.

Step 5: Eyepiece projection shots of the moon and planets.

Step 6: Guided deep-sky exposures through the eyepiece of a telescope.

The stages above are suggested as a logical sequence to follow to eventually become an expert in the field but is not meant to suggest that fine astrophotos require the full course. Many excellent photographs have been obtained by simple techniques by amateurs at various levels of expertise. The satisfaction of getting beautiful shots of the heavens occurs all along the path towards the "top" but does not require one to master all levels before very pleasing results start to occur.

Targets for Astrophotography

There is a natural progression of targets to photograph which starts with wide field subjects that can be recorded on basic equipment and leads eventually to high magnification photography through the telescope. To some extent this follows the natural progression of many who have become interested in astronomy. They first wonder at the star-spangled sky on a dark night, then do binocular scanning of the heavens and eventually purchase a telescope for those deep-sky views of planets, nebula and gas clouds. The list below follows the same progression:

the constellations and the changing planets’ wanderings against the starry background;

northern lights and lunar and solar halos;

the moon and sun in and out of eclipse;

meteor trails and comets;

clusters of stars and other objects that require long telephoto lenses or telescopes;

deep-sky objects which require special equipment and skill to successfully capture on film.

Basic Astrophotography Equipment

A common misconception among beginners is that a telescope is required to take star photos. This is like suggesting that scuba diving equipment is required to enjoy a day at the beach. The basic list below is all that is needed to produce some stunning astrophotos.

The Basic List

• 35 mm cameras with normal 50 mm lenses and manual shutter settings (B setting a must)
• tripod (the heavier the better)
• cable release with lock
• wide angle and telephoto lenses in the range from 24 mm to 400 mm or so

Intermediate

• a barndoor tracker (described elsewhere);
• a telescope from 2 to 8 inches aperture of the refractor, reflector or hybrid type;
• a rigid mounting system for the telescope preferably with slow motion controls on both axes;
• adapters to mount the camera piggyback onto the telescope;

Advanced Astrophotography

• a telescope of 4 inches aperture or larger;
• a clock drive motor and gear system on the mounting to allow automatic tracking of the stars ;
• suitable adapters to mount the camera onto the eyepiece holder;
• a guiding eyepiece (a high magnification eyepiece with illuminated crosshairs);
• a frequency controller to make corrections in the drive rate of the clock motor;
• a set of filters for enhancement of features on planets for example.

Basic Equipment: Cameras, lenses, film and telescopes

Cameras: Most 35 mm cameras work well as long as they are capable of being set to hold the shutter open (B setting). Electronic cameras are at a disadvantage here since they require battery power to do so and drain the batteries fairly quickly especially in the winter. There may be many older manual cameras out there that can be useful as astrocameras. Adapters are still available for many screw-type mounts as well as a variety of telephoto lenses. Get rid of the shoulder strap since it will get tangled in other equipment.

Lenses: Any lens from wide-angle to long telephoto is suitable in astrophotography but modern fast lenses are hard pressed to keep the images of stars as nice round points at the edges of the field of view. There is no more difficult task to test the quality of the optics. Test out prospective purchases beforehand if possible. In any case it is routine procedure to stop down one f stop for focal lengths of up to 100 mm or when using 2X or 3X multipliers. Experiment for the best results.

Film: Slide vs Print - the age old debate

With respect to the films used, astrophotographers usually employ one format or the another and can give you good reasons why the other is worthless. When film choices were limited, a lot of those arguments made good sense. But now, with the variety of films of both kinds available, the choice is less clear cut. My own preference is for slide film because of the fact that it is easier to show large audiences a slide than a print. On the other hand, prints can be rapidly processed and come in smaller roll sizes that give you the opportunity to correct exposures quicker.

My main objection to prints is that whenever a negative is converted to a positive, you may be at the mercy of your processor - whatever colour he or she feels that day will be the colour you get. In addition, damage to the negatives is more likely since they are generally handled more. On the other hand, your processor might be someone who is willing to experiment to give you the best results. Those individuals willing to meet the special needs of astrophotographers are much appreciated. Consider yourself fortunate if your processor is also an astrophotographer.

The main advantage of slide film, once you have a quality processor, is that you get to view the photons that arrived at your film virtually in their original form. Furthermore when projected on a screen in a dark room you have an enlargement of the scene that has the three dimensional aspect of the original - something just not possible with a print. In any case, the best slides can be easily converted to enlargements at your local photo shop or if you have a slide duplicator you can do it yourself. Slide copying is actually used by serious astrophotographers to increase the contrast and colour saturation of slides and can be quite effective at bringing out faint details in the originals.

Best Colour Print Films for Astrophotography

An article by Alan Dyer in the March/April (1997) issue of SKY NEWS lists the most recent test results of film suitable for astrophotography. Out of two dozen films tested, 8 came out on top . These are listed in order of ISO speed rating. Slide films are also discussed below. There are always new films appearing on the market, however. Don’t be afraid to experiment or have the advice of the experts in film reviews that appear regularly in Astronomy or Sky & Telescope.

*some of these professional films are available locally but usually in 36 exposure rolls and sometimes in minimum 5 roll packs. Inquire for availability.

Colour Slide Films: Kodak

The list of films available in slide format is shorter but there is a good representation from slow speed films for bright objects like eclipses and the moon to exceptionally high speeds for deep sky work.

Kodak has Kodachrome 25, Kodachrome 64 and Kodachrome 200 which are suitable for sunsets, day-sky phenomena, the sun and slide duplicating. Lunar eclipse pictures can employ Kodachrome 64 for exceptionally fine resolution when the moon is still relatively bright. You’ll probably need a faster film for the total phase or for earthshine. Higher speed film from Kodak includes Ektachrome 200 and 400 which have been my favourites for many years. The newest film from Kodak leapfrogs the ISO ratings to the 3200 range - Ektachrome P1600.

Ektachrome P1600 is a remarkable film with excellent colour balance in long exposures. Slide films in the past have been notorious for their shift in colour during typical star photos.The different colour dyes lost sensitivity at different rates and produced colours than were never actually there. Now with the introduction of P1600, we have a fast film with excellent grain structure which can be pushed to 3200 (at your request) with a small loss of colour balance and increase in grain. Kodak recommends processing at 1600 for best results. And those results are impressive.

My personal experience with this film has provided surprise after surprise. I have never in the past been able to overexpose photos of constellations but clearly this is possible with P1600. More than 1 minute of exposure at 135 mm produces an overexposed photo. What normally took 6 to 7 minutes to record at 270 mm now takes about 2-1/2 to3 minutes and those long telephoto shots of nebula and clusters which showed only a few bright stars now look like fireworks!

This film is excellent for comet photography. Even at 2000 mm, a 60 second shot through my 8-inch telescope gave lots of details in the inner coma of Comet Hale-Bopp as well as some streamers of both gas and dust tail.

This film can be used with 100 mm telephoto shots of 20 second duration to give beautiful comet pictures with no guiding necessary. P1600 is my personal recommendation for a choice of film for both comets and deep-sky but be careful not to overexpose it at shorter focal lengths.

One minor disappointment was with a wide angle 28 mm lens. I expected more stars to show up on the 2 minute exposures I took but these probably should have been exposed longer. More experimentation required here.

Colour Slide Films: Fuji

Fuji also provides a variety of slide films that are suitable including Fujichrome 400 and RD400. Fuji film in the past (Fuji 100 and 100D, 400 and 400 D) has been known to shift colours for longer exposures and produced redder colours than normal or other unnatural shifts. Fuji’s attempts to fix this lead to a new film which was worse than the original in terms of the way it rendered red objects. Further attempts to fix this "fixed film" have been erratic. They seem to have come close with their 400 ISO films but I find them still rather blue in long exposures. If you use Fuji print films there are no problems, but I have not yet seen a Fuji slide film that matches Kodak’s or 3M’s (see below).

Other films that have been tested for astrophotography are Scotch Chrome 400 at the lower speed end and Agfachrome 1000 as well as Scotch Chrome 1000 at the higher ISO speed. The penalty for high speed however with these is a grainy reproduction. These films for that reason are not as popular as Fujichrome 1600D, Ektachrome P1600 and Scotch Chrome 800/3200* (known as Scotch Chrome 400 in the US). These are all designed to be push processed (base speed is 400 ISO) to 800 or usually 1600. At that speed, the films are only slightly grainier than at 800 but the increased speed is worth it. Pushing to 3200 gives a noticeable increase in grain and is only used for special circumstances when speed is important and correct colour balance is not.

Scotch Chrome 800/3200 is manufactured in Italy by 3M and is often sold as "house brand" film in many stores. If the box says "Made in Italy" there’s a good chance the film is made by 3M. The base rating of this film is ISO 400 but should be pushed to 800 with no increase in grain size or colour imbalance. At 1600 the grain size is only slightly smaller and the colour is still excellent! At 3200 there is noticeable grain size and colour shifting.

Black and White Films

Selecting colour films requires a careful match to the subject matter. No one colour film is a good all-purpose film for the wide range of brightness and exposure times. However, In B&W photography, there is clearly one outstanding film - Kodak Tech Pan 2415. It is a high contrast, medium speed, very fine-grained emulsion for scientific applications. It has extremely fine resolution, extended red sensitivity (a good thing for B&W film to record nebula and hydrogen gas clouds) and an ISO rating of 125. It needs to be gas-hypered to increase its sensitivity for deep sky work however. This can usually be done by the supplier (Lumicon) or in a home gas hypering tank.

Another excellent film is Kodak’s T-Max in 100, 400, and 3200 ISO ratings. T-Max 100 and 400 can be pushed one stop to double their speeds while 3200 can be doubled to 6400 or even 12,500. This in itself is remarkable but tests show that it can even be pushed to 25000 or even 50000 for special purposes!!

Some testing of this aspect appears to be called for. Furthermore, B&W home processing requires very little equipment and it is thus possible to see the results of your sky-shooting efforts almost instantly.

Getting Your Film Processed

Many photographers still process their own films in a home darkroom and there is no reason why you should not continue to process your astro negatives or even slides in that way. It has also become much simpler to do colour processing in the home as well with the one or two solution systems from Kodak and Unicolor.

Films used for astrophotography like Tmax and 2415 (B&W films) are simple to process in a standard developer like D-76 and can be pushed to higher speeds by leaving them in the developer longer. This is also true for slide films which use the E-6 chemistry. More solutions are involved in doing slides but once you are done, of course, there are no print chemicals to mess with.

If you send your films away for processing, insist on a quality lab like Colortron or BMG (both in Toronto) for those photos of once in a lifetime events. Use the regular processing outlets for the non-critical stuff.

Be aware of the fact that star negatives and slides are not normal photos handled by the staff of many labs. Help them to locate the frame boundaries by recording a bright light onto the first frame so that the technicians have a reference point. Don’t take the chance of having some or all of your negs cut in half.

Develop a good relationship with your local processor. Many will be very happy to provide you with the specialized services that you require in processing your pictures. Even if a lab charges a bit more for the quality service, that service is important in helping you obtain astrophotographs of those important astronomical events.

Computers and Astrophotography

The most exciting development in the last few years has been the advent of digital imaging with electronic cameras (CCD’s are described in the Advanced Techniques section) or by the scanning of existing photographs to produce a computer image. Manipulation of the images using software like Adobe Photoshop or PhotoFlash allows some incredible things to be done with your star photos. Faint features in the tail of comets can be enhanced, contrast improved, scratches and other marks can be removed and the image improved dramatically. The age of the electronic darkroom has arrived. And then you can send copies of your favourite star pictures to your friends over the Internet.

If you are not so much interested in the observational aspect of astronomy (and prefer doing your astronomy from an armchair) and have access to a computer, then computer astrophotography may be to your liking. Using the Internet, it is now possible for a modest registration fee, to access a telescope at a remote site in Arizona or the UK and take pictures with their instruments! The images can be downloaded to your computer via the phone line and you can dispense with the messy hands-on stuff completely! Many web sites also exist which offer live video coverage of major events like eclipses and comet appearances. Most of them are run by amateur astrophotographers with very simple equipment and you only need to be "on the web" to take advantage of them. The BCAS may soon have one of these video camera web sites.

It sometimes makes me think about why I put up with mosquitoes, dew, - 20 C temperatures, sticky shutters, dead batteries in the middle of an exposure, having to dodge beer bottles from passing trucks, and chasing away dogs that mistook my telescope stand for a tree. I wonder...

Choosing the Best Telescope for Astrophotography

To some extent, the perfect telescope for astrophotography is a contradiction in terms. It needs to be light enough for portability but massive enough that it does not wobble when bumped. It must be capable of photographing wide fields but still provide high power. Your final choice may be to get two telescopes - a small high quality refractor for the wide field shots and a "light bucket" for the long exposures of deep sky objects.

In fact this is what many amateurs have come close to doing by purchasing a Schmidt-Cassegrain. All S-C’s nowadays come with accessory lenses that can be mounted ahead of the eyepiece or camera to change it from a long f/ratio (f/10) - ideal for planet and some deep-sky work toa rich field (f/6.3) which can be used to image the complete disk of the full moon with black space all around. The addition of a few medium to long telephoto lenses that can be used piggyback gives a fairly complete range of focal lengths from 50 mm to 2000 mm and more.

Additionally, S-C’s have mountings which have been designed to be both sturdy and portable (solid tripods that fold into reasonably compact packages) which provide a steady mounting for the delicate work with long exposures under clock drive.

Any telescope is suitable for photography as long as it allows cameras to be attached to it either piggy-back style or onto the eyepiece holder. The necessity for the camera to be rock steady for long exposures however puts an added requirement on the telescope that may not be the case for one to be used for only visual work and the occasional short exposure of the moon through the eyepiece. This is where the mounting becomes a concern.

The Mounting: More Critical than the Telescope

For serious astrophotography, the mounting is more important than the telescope.

Your mounting should be capable of supporting an ever increasing mass of cameras, lenses, etc. as your astrophotographic interests change. In the past this has been accomplished by making mountings massive, but very rigid, yet light, tripods are becoming the rule nowadays for both refractors and S-C telescopes. Larger reflectors have more variety in mountings, but good design here can also reduce mass. There are many opportunities here for home building - I have seen many very expertly constructed telescopes that have cost half the price of a commercial instrument which have all the features. In fact, many commercial instruments have at one time been amateur designs that eventually entered the market as mass-produced telescopes. But if your pocketbook is tight and if you are at all handy with tools, a home-built telescope may be a good, usable and inexpensive system.

Characteristics of Good Mountings and Those to Avoid:

Mountings have some very simple basic design features. First of all, there are always two axes of movement required (more than this only makes the telescope more complicated to use). The telescope must rotate about a polar axis and about a declination axis. The polar axis is always the one that is aligned parallel to the earth’s axis (pointing to the star Polaris) and the declination axis allows for movement perpendicular to the polar motions. The mounting should have a clock drive mechanism on the polar axis with a way to make fine adjustments as well as a slow motion adjustment on the declination axis. Nothing more is needed to accurately track stars for long exposure pictures. As a rule the more robust the mechanics of an instrument, the better, but make sure you check for smooth motions of the controls as well as solidity. There should be no detectable snags or tension changes as you rotate the controls. Make sure the manufacturer has used special low temperature lubricants in the bearing to avoid undue stiffness in the winter.

Many department store telescopes have impressive looking mountings with fancy knobs and gears which are for the most part unusable. Avoid these, especially if the mounting vibrates like jello when you bump it lightly. (Even good mountings have some vibration, but motions must die down quickly for the telescope to be capable of providing steady images.)

One final bit of advice. The "best buy" in telescopic optics is a pair of binoculars and a tripod adapter to provide a secure mounting for them. Binoculars (10x50 for eg.) are ideal instruments for locating objects in the night sky. Their light weight, portability, and reasonably high magnification are the recommended optics for comet watching and eclipse viewing (500 mm telephotos!). They provide nice views of the larger deep-sky objects and can even be used on terrestrial objects. (Imagine that.) Their only disadvantage is that it is not easy to photography through binoculars.

The Six Steps To Successful Astrophotography

STEP 1: Fixed Camera Exposures: (requires tripod, locking cable release, regular or wide angle lenses, medium to fast film, record book to keep track of exposure times, f setting, etc)

The main concern in tripod photography is the fact that with the rotation of the earth, objects in the sky appear to rotate about the pole star (Polaris). If you face north and look 45 degrees up (halfway to the point overhead, the Big Dipper guides you to the North Star. The stars there appear to rotate in counterclockwise circles as you face north. Facing east will reveal stars rising up from the horizon and passing in an arc overhead. A west view shows stars setting while a south view shows the top part of the circle of star motion around the south pole. All these effects are due to the fact that we are on the surface of a globe which is spinning on its axis. Imagine stars to be on the inside of another, larger hollow globe which is rotating around an axis running through the Earth’s poles. The ancients believed that these spheres (and more) were in fact real and explained the motion of moon, sun, stars, and planets.

A time exposure of the sky with camera pointed in these directions will show star trails as circles or arcs if the exposure exceeds about 30 seconds with a 50 mm lens. There is some variation in this depending on where the camera is pointed and certainly with greater magnification (longer telephoto lenses) the trailing occurs with shorter exposures.

It is up to you to decide whether you want star trails or not and to match the exposure times to the focal lengths and film speeds. Both types of picture have appealing aspects. The "star tunnel" effect showing star trails around Polaris is striking but to see the clouds of the Milky Way the stars look better as points. Experiment with both.

See below for details (from the Backyard Astronomers Guide) on what to photography and the exposure times required to record various subjects.

Table 1: Astrophotography Subjects, Film, Exposure Times

If you wish to avoid star trails on your photographs Table 2 gives the limits of focal length vs exposure time. Note that star trailing is worse the closer you are to the celestial equator. This is an imaginary line that is an extension of the Earth’s equator in the sky and which is used as a reference for stellar mapping. The counterpart to latitude is right ascension and longitude is referred to as declination. The equivalent of the stellar Greenwich meridian is a line going through a point in the constellation Ares - the First Point of Ares.

Matching focal length with ISO and exposure times is critical for keeping the images as points. Make sure you keep careful records of exposure times, f settings, the time, equipment used, special techniques, etc. These are absolutely essential! You will never remember the details later on even the simplest exposures.

Here is a sample of the headings from a page in a typical astrophotographers notebook:

• Date
• Time (EST)
• Film
• Exp.#
• Exp length
• Focal
• length
• F ratio
• Mount
• Subject
• Remarks

The Remarks column can include comments after you have processed the films and seen the results. This is very helpful when setting up for a similar picture on a subsequent occasion. This kind of record could be combined with a more conventional anecdotal journal which is great reading on those cloudy nights around the fire.

Table 2: Maximum Exposures to Avoid Star Trails:

(both tables from BYAG-Dickenson/Dyer)

STEP 2: Moon Shots Through Telescope/Telephoto (requires telescope with or without clock drive and adapters to connect camera to eyepiece, cable release, slow to medium films preferred for better resolution)

This type of photography is the first type recommended for novices with telescopes since it does not require accurate guiding due to the short exposures. Once the telescope has been centered on the moon, and the clock drive engaged, the drift is not sufficient to blur pictures that are only fractions of a second long. Clock drives are not required for exposure of the moon which are usually less than ½ s for all phases but the thin crescents with earthshine. For capturing the earthshine effect, exposures up to 60 seconds may be required with the slower films (200 ISO) and accurate tracking is necessary (see STEP 5). For faster films (1000 - 3200) try 2 to 10 seconds respectively. These exposures are short but tracking is preferred.

Table 3 below indicates the size of the moon image for a given focal length of lens. A reasonable size for slide projection requires a 500 mm telephoto or longer. Enlargements can be made from 200 mm shots but unless the film is very fine-grained and consequently slow, photos from less than 200 mm pictures don’t stand much enlarging.

Table 3: Maximum time before drifting

All objects in the sky appear to rise in the east and set in the west due to the rotation of the earth. Most telescopes with clock drives are set up to track with the stars (the sidereal rate). The moon however travels in its orbit in a direction that carries it from west to east in our sky at a slow rate (about 4% slower than the stars). At this lunar rate, the moon will drift very slowly backwards so that a sidereal rate is not quite correct. This effect shows up most in high magnification shots (over 1000 mm ) and limits the exposure to under 10 seconds - hardly enough time to capture earthshine or the total phase of a lunar eclipse. Using a high speed film helps but with a few exceptions, high speed equals more grain and fuzzier images.

A solution other than switching to a higher ISO film involves using the lunar tracking rate that is built into many drive correctors or setting the drive corrector at a slightly slower rate and zeroing in on a feature on the moon.Now we are bridging the gap to STEP 5. Practice getting good photos of crescents and earthshine (less than 10 seconds) before progressing to the next stage.

Table 4 below provides suggested exposures for various phases of the moon. But so much depends on sky conditions like transparency, haze, proximity to city lights, etc. that these are only a guide. Keep careful records and bracket the exposures by 2 shutter speeds above and below.

Table 4: Lunar Photography Exposure Data:

(nominally f/16)* Intermediate values of ISO in the table can be interpolated.

*Use faster shutter speed by one setting for f/11, 2 times faster for f/8 etc. But always bracket exposures one or preferably two speeds either side of the recommended value.

** Lunar rate drive required for these exposure times. NOTE: Lunar eclipse photography is treated in more detail in the section "Special Techniques Lunar and Solar Eclipse Photography" but in general expose as you would for Earthshine above during the total phase of the eclipse.

STEP 3: Barndoor Trackers (requires barndoor tracker, a fairly heavy tripod, cable release, regular, wide angle and low power telephoto lenses, medium to fast films.)

A barndoor tracker is a simple platform that allows a camera to track the stars for up to 10 minutes. Longer times will expose the main flaw - the inability to accurately polar align the device. Sighting Polaris through a piece of pipe or similar sighting device gives only a crude alignment that eventually produces star trails on your film. Some photographers have used small telescopes mounted on the trackers to locate Polaris more accurately and this technique does increase the exposure times available. Others have attached clock drive motors through gearing to turn the screw - this also makes it easier since the human element is eliminated from the tracking.

Simple barndoor trackers can track fairly accurately for 5 to 10 minutes or so before the tracking method fails to match the true sky motion. Most trackers use a ¼-20 screw thread that is turned by hand - ¼ turn for every 15 seconds (remember that 30 seconds will allow stars to trail on a 50 mm frame).

However, it is important to align the trackers as carefully as possible to the pole star and to maintain a steady tracking motion. Barndoor tracking requires concentration for only a short time and can be a simple and effective way to increase exposures for those spectacular wide angle shots. This type of tracking doesn’t get nearly as tedious as tracking through the eyepiece (STEP 4 and 7). It is even possible to get some decent telephoto shots with no trails if you are prepared to track for 5 minutes or more. In combination with a high speed film, this is the simplest way to get spectacular comet photos.

The advantage of barndoor trackers is that an evening’s construction project gives very good to excellent results for wide to medium views!

The disadvantages are the tedious turning of the screw for longer exposures and the bain of all photographers - light pollution. Some photographers have prepared tapes of their favourite music with superimposed time marks indicating when to turn the screw to make the longer exposures more pleasant. A neat idea! As for sky glow, with the super fast films (ISO 1000 and up), 1 or 2 minute exposures may be the limit before background light starts to degrade the contrast of the images on your film, especially for short lenses like 50 to 100 mm. Travel to a truly dark site may be the only solution.

STEP 4: Piggyback Exposures on a Tracking Telescope (requires telescope with accurate clock drive, frequency controller, camera mount, cable release, medium to long telephoto lenses, high speed film)

The requirements of this type of astrophotography are fairly rigorous. You need

1. A telescope with accurate clock drive;
2. A guiding eyepiece of 10 to 15 mm focal length with an illuminated crosshair or reticle;
3. A clock drive frequency corrector or fine adjustment hand controls on both axes of the telescope;
4. A rigid camera mount on the telescope for a camera with heavy telephoto lenses;
5. A counterweight set to balance off the heavier equipment;
6. Other paraphernalia like power supplies, dew zappers etc. if you are at a dark sky remote site.

The length of exposures in this system depends both on your choice of lenses and your personal tolerance for discomfort.

Generally the longer the telephoto lens, the less time is available before a stationary camera produces trailed images. See Table 2 above. Note that with a 200 mm or longer telephoto, star images are trailed after about 3 seconds of exposure. Very few films can record much with this short an exposure. The barndoor tracker will allow for exposures with regular and low power telephotos but do not work well with focal lengths over 200 mm or so.

Piggyback photos require a correctly aligned telescope (an art in itself), the selection of a guide star in the field of view of a guiding eyepiece, the careful monitoring of the drift of the star (clock drives never track accurately) and corrections to the tracking rate of the telescope that depend on the power of the telephoto lens used (higher f.l. require finer tracking corrections).

The field of view of telephoto lenses depends on the focal length according to table 5 and determines the tracking tolerance at the guiding eyepiece.

Since guiding eyepiece magnifications and telescope parameters vary greatly, it is necessary to determine the specific tolerance for your system and lenses as follows:

1. Locate a star in your guiding eyepiece fairly close to the celestial equator.

2. Center it in your crosshairs and turn off the drive for:

12 seconds for a 50 mm lens

6 seconds for a 100 mm telephoto

3 seconds for a 200 mm and so on...

3. The amount the guide star moves from the center of the crosshair is the distance the star can move without producing a noticeable trail on the film.

Obviously for lenses greater than 800 mm the tolerances are tight! Most modern telescopes come with manual slow motion controls or electronic ones than will allow you to track guide stars accurately. Telescopes without these gadgets will limit what you can do with them as you become more experienced and your astrophotography interests change. Plan ahead in your purchases.

Knowing the tracking tolerance allows you to determine how much drift can be tolerated but does not limit the length of the exposure. This is determined by the speed of the film and the amount of background light present at your site. The general rule of thumb is that a 400 ISO film will start to pick up the faint glow from the air (sky fog) after about 15 minutes exposure. For the higher ISO films available today, sky fogging becomes the limiting factor much sooner. Recent tests with P1600 Ektachrome film indicated that with a 135 mm telephoto only a minute of exposure starts to record sky fog at a fairly dark site. At 200 mm sky fogging becomes noticeable after 6 minutes exposure. The full potential of these high speed films may only be reached at the very high focal lengths (1000 to 2000 mm)!

Field of View vs Focal Length

Table 5 below lists fields of view available from various focal lengths of lenses. Use as a comparison the following objects:

• moon and sun @ 0.5 degrees across,
• Big Dipper bowl @ 10 degrees wide, (yes, 20 full moons would fit along the width of the bowl!)
• entire Big Dipper @ about 35 degrees wide.

(note: a 50 mm lens will just fit the Big Dipper along the long dimension.)

Table 5: Field of View vs Focal Length

STEP 5 : Eyepiece Projection Through a Telescope (requires telescope with accurate clock drive and a range of eyepieces, camera mount, cable release, adapters to attach camera to eyepiece holder - not just a T-adapter , medium speed films)

Photography using eyepiece projection differs only in the exposures times with respect to the subject selected for capture on film. The normal choices are the moon, sun, and planets. The special requirement for solar photography is a filter to reduce the light to safe levels both for viewing with the eye and photographically. The other two subjects, the moon and planets do not require a reduction in light intensity although coloured filters can be used to enhance certain features of their surfaces.

The main difference between lunar closeups and planetary photos is that the latter require extreme focal lengths to record an adequate size on the film. Moon images of good resolution can be enlarged from the negatives to a fair degree, but planets are such small objects that a reasonable size (2 to 4 mm) is required on film to overcome the problem of tiny grainy images that do not enlarge well.

Method 1: Negative Projection:

A focal length of 10 000 mm or so is required to zoom in on a small section of the moon. This is not a normal telephoto lens system and requires special techniques whereby a lens-less camera is mounted on the eyepiece of the telescope. The difference is that a telescope lens is also used in the light path to increase the magnification. One approach is to use a 2 x Barlow (It needs to be good quality) or a camera teleconverter (2X for ex.) . Barlows are mounted in the eyepiece of the telescope and the camera is positioned behind it using an adapter that clamps the two units together. Alternately, the teleconverter is mounted on the camera and the combination is attached with an adapter to the eyepiece of the telescope. The result is a negative projection system that gives about 10 000 mm and an enlarged image on film. With this system, you can effectively get down into individual craters on the moon!

Method 2: Positive Projection:

To get the necessary magnification for decent planet image size, even longer focal lengths are required. This is achieved by positive eyepiece projection. An extension tube is mounted in the eyepiece holder of the telescope, a normal lens is placed into it, and a camera is mounted behind the eyepiece lens. Focal lengths of up to 40 000 mm are possible with focal ratios of f/45 to f/200 but f/100 is most often used. Exposure times up to 10 seconds are normal for this type of photography so an accurate tracker is essential. The high magnifications produce trailing very quickly.

Films For Eyepiece Projection:

Films recommended are the medium speed films - high speed films are too grainy. Try Ektachrome 200 or 400, Fujichrome 400, or Fujicolor Super HG400. Black and white photos should be taken on Kodak Tech Pan 2415 which can be used unhypered.

Exposure Times Depend on the Planet:

Exposure times vary from planet to planet. Venus is the brightest but has little surface detail. Less than 1 second exposures are usually fine. Jupiter and Mars are favourites with lots of surface structure but they are less bright. Try 1 to 4 seconds for both Mars and Jupiter. The rings of Saturn require 2 to 10 second exposures.

STEP 6 : Deep Sky Photography At Prime Focus (requires telescope with accurate clock drive and a range of eyepieces including a guiding eyepiece with illuminated crosshairs, drive frequency controller, camera mount, cable release, off-axis guider or separate guiding telescope, medium to high speed films)

This is by far the most demanding form of astrophotography and unfortunately the starting point for some aspiring photographers. Many find their enthusiasm coming to an abrupt end when the desired results do not appear on film.

Prime focus photography concentrates on relatively small objects, less than 1 degree in size requiring focal lengths from 750 mm to about 2500 mm - the focal length of your telescope. Cameras are mounted directly at the prime focus of the telescope with adapters that are special to your camera-telescope system. Sometimes a focal reducer lens is used to increase the field of view for especially large objects like M31 - a galaxy that is about 6 times the size of the full moon. Older versions of focal reducers produced a vignetted field. This refers to a brighter circle in the centre with a gradually darker ring around the outside, like looking through a blackened tube. Newer models of these telecompressors have corrected most of this.

The most commonly photographed objects at prime focus are the Orion Nebula and the Lagoon or Ring Nebula visible during the winter and summer respectively. Exposures of 5 to 10 minutes record these objects since they are so bright - both visible to the naked eye.

All deep sky exposures require accurate guiding since the magnification shows up even minor star trailing. There are two methods of do this - using a separate guide scope or an off-axis guider.

Guide Scope

This method requires a separate telescope mounted rigidly on the main one you are using for photography. Both have to be aligned to point generally in the same direction (within 3 degrees). The guide scope must have a fairly high power eyepiece (200 to 400 x) with a cross hair of some sort (perhaps illuminated) to keep track of the motion of the guide star. A drive frequency controller is required which speeds up or slows down the clock drive to correct for the always present errors in the gears, the electric voltage supplied to the drive, and refraction in the atmosphere. Using a separate guide scope has one advantage since they can be somewhat more easily adjusted to give a bright guide star in the field of view. You may not be able to guide on the object directly since it is usually a fuzzy object to begin with.

Off-Axis Guiders

These devices mount between the camera and the telescope and sample a small amount of light directly from the object being photographed. This insures that there is no trailing of the image since you are actually monitoring the object by its own light. A high power guiding eyepiece is used to monitor the objects motion with fine corrections of the clock drive again required. Many off-axis guiders allow some movement within the guiding field to locate a suitable guide star, but bright ones are not always available. Guiding on a faint star makes the whole process especially difficult.

In addition, guiding tolerances are very tight and the whole process can be very grueling when your eyes are watering, the mosquitoes are biting or your skin freezes to the metal eyepiece when you touch it. If there is any place in astrophotography where an automatic guiding system would be welcome, this is it. In fact, such systems do exist and have made deep sky photography much easier.

Special techniques: Lunar and Solar Eclipse Photography

Lunar Eclipse Photography

Successful lunar eclipse photography involves taking a lot of pictures and keeping only the best ones!

Here are a few guidelines from Alan MacRobert adapted from Sky & Telescope mag (Sept 96):

Table 6: Lunar Eclipse Photography Exposure Guide

Exposure Suggestions (seconds)

The minimum focal length for getting a good-looking moon is about 300 mm. A focal length as long as 500 mm will still give a field wide enough to include stars or planets in the background. With 500 mm and a 2-second exposure you could get away with a camera on a fixed tripod, but background objects might not show well even on fast film. Longer exposures or focal lengths will require a tracking mount to prevent blurring due to the turning of the Earth. A simple, do-it-yourself tracking camera mount is described on page 32 of the May 1996 S&T.

A barndoor tracker can give very good results for exposures up to 5 minutes or so but the moon does not move at exactly the same rate as the stars. Exposures of the moon which are much longer than a few minutes will produce minimal blurring at only the shorter focal lengths. But of course the lunar image on the film is then quite small (1.8 mm for a 200 mm fl). Longer telephoto lenses aggravate the problem since they produce a larger moon image and magnify its motion proportionately. For telephoto shots over 1000 mm, it is necessary to track on the moon accurately and requires a telescope mount properly aligned to the pole and tracking at the lunar rate.

Taking long exposure photos of the eclipsed moon requires the expertise and equipment of Step 4 described earlier. There may be a way out of this bind however with the higher speed films available. Do some experimenting here.

See Table 3 for more details of exposure times f/ratios etc. but a rough guide is that for 2000 mm the moon image is about 18 mm. Two seconds of exposure will be the maximum before the moon blurs even if it is on a telescope which is being driven to track the stars. For half the focal length, you get half the image size and twice the maximum exposure. To get those gorgeous full frame shots of the coppery moon during mid eclipse requires 30 seconds or so piggyback on a telescope tracking at the lunar rate with a 1000 mm lens or better. And some measure of luck.

Simpler tripod shots of lunar eclipses can be very satisfying however, and there are several different techniques that give interesting results.

Eclipse Streak:

Lock the shutter open, let the moon move across the frame for the duration of the eclipse. The result is a trail of light the width of the moon which changes from white to red and back to white again. The field of view of a 50 mm lens is large enough to accommodate the full motion along the diagonal (a periods of 2 to 3 hours) but you can try shorter spans centered around mid eclipse if you want to use a longer lens. Stop down the lens to f/8 or 11 to reduce the fogging of the film due to sky glow and city lights. Using a slow film under 100 ISO helps. A double exposure of an extra 2 to 4 seconds at f/2.8 can be made to add a foreground landscape. Evergreens are always nice.

Multiple Exposure:

Instead of a continuous exposure, you might also try many short exposures taken at regular intervals. You get a picture with many moons each recording a different aspect of the eclipse. You need a camera that allows multiple exposures to be made without advancing the film between frames. (Sometimes the rewind release button can be used to fool your camera.) Don’t take exposures more frequently than 2 minutes apart since the moon’s motion will not carry it far enough to prevent overlapping images. Use table 5 to determine the field of view available with the lens you use. Make sure you have one shot at the very middle of the eclipse. This could be a bit longer than the partial phases.

Use Fig 1. to help you determine the positioning of the images on the frame. This type of shot requires lots of planning but is well worth the effort.

Piggyback Photos:

1. Short telephoto:

Using the techniques described in Step 2 and Step 4, it is possible to get some excellent results. Larger images of the moon are possible as well as pictures of the eclipsed moon in front of background stars. With 50 mm to 100 mm lenses, piggyback shots give nice starry backgrounds and do not require especially careful guiding. Use the normal sidereal rate rather than the slower lunar rate.

• Long telephoto or telescope:

For eye-popping photos of the eclipsed moon in all its colour shoot through the eyepiece of the telescope. The exposure times are basically those for the phases provided in Table 4 but be prepared to experiment and above all bracket your exposures. During the total phase longer exposures are necessary to pick up the reddish glow. Try 1 to 20 seconds depending on the speed of the film and the tracking tolerance given in Table 3. During these longer exposures, the uncovered portion of the top of the moon will shine through and overexpose the film - unavoidable if you wish to record the deep colour of mid eclipse.

Note: Even veteran photographers bracket exposures by at least 2 shutter speeds over and under the best guess. You might wish to change to a faster film for the total phase and use a slower film for the partial phases. A second camera loaded for the total phase is another alternative.

The Shadow Tunnel Effect:

One last technique that is the most difficult. The idea is to produce a stunning photograph which shows multiple moon images as it moves through the shadow of the earth. The photograph is a visual impression of the moon appearing to move through a circle of darkness - the shadow of the earth as it is projected in space. The setup requires multiple exposures made at precisely calculated times to show the arc of the dark umbral shadow cutting the disk of the moon both before and after the middle of the eclipse. A minimum of 500 mm is required to give a large enough image and the telescope must track a star in the vicinity of the moon. Exposures need to be adjusted correctly to photograph partial phases and totality and there can be only a few seconds leeway to get a symmetrical series of moon images. A lot has to go right to get a successful photo.

Solar Eclipse Photography:

Photographing the sun is to a great degree similar to lunar photography. It is placed in a separate category because of the necessity for proper filtration of the extremely bright solar image when taking pictures directly through the telescope or through any focal length of lens.

It is not safe to view the sun with the naked eye or camera because only a few seconds of bright light can permanently damage the retina or destroy camera components. It is safe however to view a totally eclipsed sun, because for those short minutes, the moon blocks out all of the solar radiations. It is also safe to photograph the sun at this time but the focal length will determine the size of the image on the film. You can use table 5 to determine the size of the image since the moon and sun have very close to the same apparent size in the sky. A focal length of 500 mm and up is recommended.

During the partial phases of the eclipse, a solar filter is mandatory. This is usually a disk of glass or mylar plastic coated with a partially transparent coating of aluminum or other metal. Arc welding filters are suitable but give the sun an unnaturally green colour. Neutral density filters (density factor of 5 required) can be used on the camera but should not be used to view the sun with the eye.

The only way to be certain of proper exposure through any of these filters is to test a range of exposures on the full sun and pick out the ones that give the best solar image. That exposure can be used for all but the very last stages of the partial phase before totality.

During totality, the brightness of the sun allows you to use fairly slow ISO films that give a better resolution due to the finer grain structure in these films. The exposure times can vary from a few seconds to a few hundredths of a second, there is no one correct value. What occurs is that the long exposures record the outer corona - a pearly region of wisps that extends out from the sun to 3 or 4 solar diameters. The short exposures record the bright red prominences along the sun’s edge.

Advanced techniques: Cold Cameras, Gas Hypering and CCD’s

Amateurs have employed various advanced techniques in their attempts to capture more of those elusive photons on film in shorter and shorter times. One of the most recent ones (now in disfavour) involved cooling the film down to the temperature of dry ice (-80C). The additional detail recorded on many different films was felt to be worth the effort of home building a specialized cold camera that allowed the film to be cooled without actually immersing it in dry ice. Moisture had to be scrupulously excluded since water freezes at 0C and a way to advance super cold film had to be devised, etc. Not a project for the faint of heart.

A less difficult approach to increasing film sensitivity involved the fact that both moisture and oxygen gas tends to attach itself onto the surface of the emulsion and in some way prevents the film from doing its job collecting photons. It was found that by replacing these with another gas, the film could reach its true sensitivity. Amateurs started gas-hypering film by baking it in a mixture of nitrogen and hydrogen for a day or so and noted incredible increases in film response. The film that is the most popular in this form is Kodak 2415 B&W film. It is especially sensitive to the red end of the spectrum and records very well the light given off by gas clouds in space. Several companies provide hypered film of different types so you do not need to do it yourself. (Hydrogen is explosive remember.)

A more recent and serious challenger to the use of film in astronomical cameras is a version of the same electronic chip used in video cameras. The CCD (charge coupled device) has revolutionized not only normal photography but also astronomical picture taking. These CCD cameras can record individual photons and produce pictures in much shorter times. A normal photo of a faint galaxy that may have taken an hour or more with conventional film can be recorded in a few minutes on an electronic chip. The image is usually input into a computer where it can be enhanced, colour-coded and manipulated to produce the most detailed image. Many amateurs of the old school find the results so incredible that they wonder out loud about whether the pictures are of real objects or imaginary ones created in the binary innards of the microprocessor. The debate continues but the details appear to be real.

Astrophotography and Murphy’s Law

All photographers at one time or another have left the lens cap on during an exposure or forgot to load film into the camera. Recently I tried to record Comet Hale-Bopp at an f setting of 22 with the self timer instead of the B setting. These flubs are a part of your endeavors to photograph the stars and should be accepted as inevitable. They make great conversation during the cloudy nights indoors and eventually even become funny stories.

Being organized is the only way to minimize the boo-boos that plague any activity where a lot has to go right for a successful photo. Here are some hints: (You might call them anti-Murphy’s Laws)

1. Keep detailed notes of your attempts exposure by exposure. Do not trust to memory. It will fail.
2. Double check your aperture and exposure settings. Most star photos are taken at full aperture or closed down one stop. Make sure the lens cap is off and your shutter speed is set to B (for time exposures) or to the correct shutter speed.
3. Always have a spare set of camera batteries with you preferably in an inside pocket to keep them warm.
4. Don’t cold soak your camera by leaving it outside at night (especially in the winter) before trying to use it. Shutters stick and batteries drain faster in a cold environment. (This does not apply to film - cooling film to - 20 C improves its light response considerably but it does get stiff and picks up static charges that may appear on your negatives like fuzzy lightning bolts.) If you use two cameras outdoors in the winter, keep one inside your jacket while you use the other and alternate. Wrap your camera in a plastic bag before you come in from a cold night outside. Moisture can get into the camera and onto the film ruining it or degrading your equipment.
5. Give yourself plenty of time to set up equipment. It is much harder to do it in the dark when your fingers are cold or when you are in a rush to record a fleeting event.
6. Summer time is not the time to put away your warm winter coat. Dew and heat radiating from your body makes summer evening viewing damp and cool. Always wear a touque or other warm hat to keep the body heat it. Most heat is lost through your head.
7. Doing photography in the Canadian winter is an experience in arctic survival. Dress appropriately.
8. Repellent is a must for bug season photography.
9. Focusing is critical. Focus on a bright star to get the tightest image possible. It may be necessary to get a special ground glass viewing screen if focus problems persist.
10. Do not wind the film too quickly. Sprocket holes can tear if the film is cold and dry winter air promotes static electricity discharges across the film. They look like faint lightning bolts on your images.
11. Purchase a small rechargeable flashlight (small enough to hold in your mouth to free your hands in a pinch). Put some red cellophane over the end or paint the lens red to preserve your night vision. The views through the viewfinder are always faint and you may not be able to see much if your eyes have not adapted to the dark. This take about 15 to 20 minutes.
12. Dew is a problem on lenses and other optical surfaces so either purchase a dew control device (basically low wattage heater wire) or use a small hair blower to zap the moisture. On 120 AC these work fine, but battery powered dryers drain even a car battery fairly quickly.
13. Overexpose the first frame on a roll by shooting a bright light source. This provides a starting point for the processor so that they know where the frame edges are. You will not be happy when you receive a roll of pictures from the processor all neatly cut in half.
14. And finally the GOLDEN RULE OF ASTROPHOTOGRAPHY: If your pictures all turn out to be star trails, tell your audience that was what you were trying for.

References:

All of the photographic tables and data lists were obtained from THE BACKYARD ASTRONOMER’S GUIDE by Terence Dickinson and Alan Dyer published by Camden House (1991) for about $40. The extensive knowledge of these and other authors was used in producing this set of articles. I have learned much from these sources and gratefully acknowledge their extensive experience. Errors and omissions are of course mine and I humbly accept responsibility for these.

Additional material (recommended colour films) was obtained from Sky News magazine produced by the Museum of Science and Technology in Ottawa.

Sky and Telescope magazine also has online information on the Internet. http://www.skypub.com

Adventures In Astrophotography With A Small Telescope

http://www.eclipsechaser.com/eclink/astrotec/aphotsmt.htm

Articles Related to Astrophotography

http://www.aa6g.org/Astronomy/articles.html

Astrophotography By Jerry Lodriguss

http://www.astropix.com/

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