I have seen several questions on astrophotography for amateurs, and I was thinking that I could share my experience here. Of course, it will not replace books that one can find on the subject. I strongly recommend "Astrophotography for the Amateur" (Michael Covington) as the book to own if you want to start in this field.
Everything described here is from my own experience, and covers only
the basics of astrophotography. I am still far away from Fletcher
and Hallas, but I enjoy what I do. I tried to explain my own
evolution, from simple picture of the sky to prime focus one. It is
not the only way, but it helps to practice the basics before going
to long focal deepsky astrophotography.
1) GET STARTED: PHOTO ON TRIPOD
Before even starting with photography with a telescope, I recommend
to practice photography using a simple tripod. There are many things
to do with a simple tripod:
Moon and Planets at dawn or dusk: choose a nice foreground landscape (trees, mountains...). Evening just after the New Moon are very nice for that. Full Moon set above cities are very nice too. I usualy use a 100-400ASA slide film.
Constellations: Using a fast film (I like the Kodak Ektar 1000) and short exposures (30"-45" for a 50mm lens), one can record a lot of details on constellations. I got some nice results on the Milky way, especialy around Sagittarius. Those pictures are also a perfect test for the photo shop: is the sky coming black from their lab?
Star trails: Long exposure will record star movement over the sky. Combined with a nice foreground, you can get some pretty nice results. I like to use a flash light or a Flash to show some trees or my telescope on the pictures. And if get lucky, you may record a shooting star! I usualy use a 400ASA film for those pictures, except when I want to do a very long rotation around the north pole for which I use a 100ASA film.
Other: There are several other pictures to take with a tripod: bright comet, aurora, eclipses (lunar or solar)...
To do astrophotography on a tripod, you need a very minimal material:
A camera: If possible, use a one that doesn't run off battery for long exposure. Some exposures like star trails need very long exposures, and a camera that runs off battery will die during the night, usualy in the middle of an exposure! There were several discussions on what is the best camera for astrophotography. If you want to invest at this point on a camera, I strongly recommend an Olympus OM1. It is easily available on the second market for $150-$200 with a simple lens, and it makes nice outdoor pictures too.
A lens: At this point, a short focal is recommended (35mm-50mm). More open is the lens, better it is (f-ratio around 1.5).
A cable-release: It allows to keep the camera open during the exposure. One can find it in any photo shop. Choose one that has an automatic lock system, it is easier than the one with a screw.
A tripod: choose a light one, that you can take with you during your observing trip, hiking...
I am still doing those kind of pictures. I really enjoy the search
of the perfect foreground, the perfect composition. And nothing can
be more enjoyable than taking the crescent moon in the morning after
a long observing (or photographing!) night...
2) TRACKING?
Tracking is essential to do long exposure on the sky and having
round stars. With tracking, you can use slower film with less grain,
or use longer focal to show more details on the sky.
A very simple way to track is to use three pieces of wood and a screw! It is both cheap and fun to build, and you can have very nice results.
If you have a simple telescope, you can fix your camera on top of it (piggy backed). I built a camera stand for my old 4.5" using a piece of wood and some rope (I also replaced those flexible cable with electronic buttons that created much less vibration)! You can buy a piggy back, which is very expensive for what it is but it works fine. You can also buy a rotative head for fixing your camera from a local photo shop or from Orion for example; it will help orienting the camera to get the field of view you want.
A lot of people with an equatorial telescope are wondering how to start in deep sky photography. I really think that the best is to do piggy back. You will need an equatorial mount for long exposures, as Alt-Az mount will have field rotation.
With short focal lengths (50-150mm), you won't need a very good polar alignment. If you want to do long exposure (1hr for example) or use a long focal lens (200mm-500mm), you will certainly need a good polar alignement.
I personaly use the drift method to align my telescope (a SCT 8"). It takes approximatly 30' for a good alignement for a 500mm focal lens. First, level your tripod and orientate it to Polaris (not necessary, it just helps). If you have a Polar alignement circle, it can also help to use them.
Look at a star near the Equator, South. Track the star in RA only, and look if the star goes up or down in your eyepiece (supposing your are looking straight). Rotate the base of your telescope to adjust it until the star is not moving anymore.
Then, move to a star at East (West works also). Do the same, but adjust the latitude this time (the angle between your telescope axe and the horizon plan). By switching several times from South to East (West), you should be able to adjust your polar alignement quite quickly. Of course, the first time you will spend a lot of time; take notes of what you are doing, and it will be much quicker after.
To practice polar alignement is very important if you want to move to prime focus deepsky photography. That's why I always recommend to start with piggy back first: it is much easier to do!
Piggy Back photography offers a wide range of new projects:
Constellation: you will get much more details than exposures on tripod, and you can use finer grain films. And why not your own photographic atlas? Great as finding chart later!
Milky way: why not a mosaic of the milky way? In two hours, I did the milky way in August using a 50mm lens and Ektar 1000 film, 3 minutes each pictures. Great results!
Comet or asteroids: bright one are easily accessible with a 50mm-150mm focale lens. 10-20 minutes with Ektar 1000 and a 135mm (F/2.4) gives nice results.
Movement of planets over the sky: show how a planet retrograds over a constellation.
Shooting stars: Add a friend at several miles from you, and you
can calculate the position of the meteors; Add a rotational screen
front of your lens and you can calculate the speed!
3) PRIME FOCUS
Prime focus is certainly a goal for a lot of people: be able to
record faint objects, show spiral arms in galaxies, record the
central star of a planetary nebula... But there are certainly a lot
of traps not to fall in!
First of all, material is very important for deep sky astrophotography:
Telescope: Large aperture is a plus, especialy if you have a fix observatory. If you need a portable solution like me, SCT have great advantages. I personaly choosed Celestron Ultima 8, which is very good for astrophotography, at least a good price/performance ratio. If you have a location where you like to observe, it is nice to build a fixed tripod. Then, you will just have to come and install your telescope, the polar alignement will be almost already done. Of course, your telescope should have a drive corrector to track on RA and allow correction. A DEC motor is also useful, even if one can still take pictures with the Ultima 8 without DEC motor; it just makes life easier.
PEC: Periodic Error Correction is a great invention. You spend some time (usualy ten minutes) to setup the PEC, but guiding is much easier after.
Reducer/Corrector: A short f-ratio is nice for deepsky astrophotography, it allows shorter exposures. For SCT, there are reducer that also correct the field of view. It allows also to have a full image on the film. I heard that one can stack reducer/corrector but I never tried. Even if it is optional, I strongly recommend this accessory. Celestron has a very nice one for SCTs.
Off-Axis Guider: Guiding is critical for nice round star on long exposures. Lumicon has an excellent one, I use the Orion Ultra Guider which is nice too.
Finding a guiding star is not always easy. Usualy, I use a low power eyepiece to find the star, then switch to a higher reticuled eyepiece. A 9mm is nice for a SCT 8" f/10 or f/6.3; a 12.5mm eyepiece is not enough power. If I can't find a guiding star, I just change target!
Front Cover: I usualy use a plain carton. I cover the telescope with it, then open the camera and wait for the vibration beeing stabilized, then I remove the cover. Most of the time, the guiding star is still centered. Very useful... And while you have the carton, save a piece to make a dew protector!
Deep Sky filter: If you live in light polluted area, a Deep Sky filter can help you to limit the sky fog effect. It is an optional accessories; but after several pictures taken, it will become necessary.
We already talked about polar alignement. Of course, alignement for prime focus is much more important than for piggy back; I usualy spend two hours to do it, and I can do up to 60' exposures without field rotation. During summer Star Parties, I like to walk around while the night is coming talking with other people on what they are planning. Two hours can pass very quickly...
Focusing is also very important for prime focus astrophotography. Do not expect to do proper focus just by looking through the camera, it is not precise. I always use a razor edge to cut a star beam (well, also my finger sometimes, so becareful).
Principle of this 'foucault' method is easy. Point the telescope to a star, and do an approximative focusing through the camera. Then, open the camera body (without a film!), and open the shutter in Bexposure. You should see the star pattern, as a donut shape through a SCT telescope. Take a razor blade, and tape it on the camera, exactly where the film should be.
By moving the telescope (I usualy tape the razor blade in order to be able to cut the beam just by moving in DEC), the razor edge will cut the star beam. If the razor is out of focus, the star pattern will disapear slowly from one side or the other depending if your are inside or outside of focus. At the focus point, the star pattern disapears suddendly, and it is hard to tell from which side.
This is an easy method, and it takes around 5'-10' to do it. It is very precise, and pictures will gain in sharpness.
Choosing a film for deepsky astrophotography is not easy. There are hundreds of articles on this subject, and it is changing all the time as new film are coming out.
Basicaly, Hyper film are the best; I used a few times TP2415 Hyper and Ektar 1000 Hyper, and I got the best results. Lumicon sell Hyper films, it is a good alternative if you don't have your own Hyper system.
Otherwise, Ektar 1000 is a nice film for bright objects. I heard about Kodachrome 1600P, but never had a chance to try it. TMAX 3200 is a very good Black & White film to practice the technic, including the photo lab technic. I would use those film to try and improve the technic, and then buy Hyper film for better results.
Now, the most difficult may be to find the object and to center it! Long hours practicing visual observation will certainly help. I am a Star Hopping fan, I enjoy following star patterns in my finderscope... and having the object right in the middle! Several computer program can help to create perfect charts for the search; I use Guide CD-ROM or Megastar. The SKY, a little bit expensive, looks nice too. Of course, digital circles can also help.
I spent several hours to calibrate my telescope. I draw pictures to show where the camera field of view was in the finder depending on the camera orientation. And I calibrated my finder with Guide charts too. Now, I am able to imagine the object position inside the finderscope (especialy when there is a group of galaxies and you want all of them in the field).
Also, I know where is the guiding eyepiece field. Sometimes, I use a low power eyepiece and try to locate the object within the guiding field (through the prism in the off-axis guider). It works pretty well for 12-13 magnitude galaxies. then, I just move the telescope to center the object, and try to find a guiding star.
There are hundreds of projects to do with prime focus
astrophotography, beyond the simple art of photographying what you
see. Let go your imagination!
4) STORING
Astrophotography overall is fun, and it is not so hard to do. Buy a
good book, try easy shots first, then move to piggy back and prime
focus. You will get nice results very quickly and you will have to
store them.
First, always note what you are taking (subject, film, lens, fratio, camera, exposure, date/time). I used to take notes on paper, but I always lost them! Now, I have one record-book for all my visual observations as well as my astrophotos; it works great.
Then, protect your films; I use binders with film protector sheets, sorted by Date/Time.
I started to move to Photo CD (Kodak) format; it is way too expensive, but it helps to keep all my best pictures, and to process them using a computer. The only problem: you can't record them after processing, except if you have GB of disc!
Last but not least, it is always nice to have an album with your best pictures. I use to sort them my Date/Time, it is easier and less maintenance. But a sort by subject would be better.
And if you take nice pictures, scan them and post them on the net!
Olivier THIZY
thizy@ns.net