Astrophotography Using Film
General Comments
Astrophotography using normal film is also available at Kitt Peak. Just as with CCD imaging when the camera is attached we cannot look through the telescope. Common exposure times run about 15 minutes to more than 30 minutes. If you have specific equipment for a particular camera of guider you are welcome to bring it along with you. We can do piggyback photography which is mounting the camera with a lens on top of the telescope. This will allow you to use the various lenses you might bring.
We can also attach the camera to the telescope at prime focus. A common misconception in terms of astrophotography is that once the camera is attached, the telescope will track on the sky automatically without any assistance. Unfortunately at prime focus this is NEVER the case. Even the most technologically advanced telescopes in the world require computer control to guide accurately so we need to guide. We do this by attaching a CCD camera to do the guiding for us. By putting a guide star in the frame of the CCD camera we can have it guide the telescope mount during our exposure.
Using film has some advantages over CCD imaging. First, with 35mm camera it is possible to capture a larger field of view. Second: although slow, film allows for creating color images in a single exposure unlike the CCD camera which requires three exposures in three different filters and then assembling them in software. Finally, film technically has a small grain (pixel) size than the CCD camera.
Please visit here to see some examples of film pictures.
Why is piggyback photography trivial?
Piggyback photography is easy because the tracking errors are generally much smaller with short focal length lenses than the images of the stars on the film. However when you piggyback with longer focal length lenses we need to guide with a CCD camera, the autoguider in the CCD camera (at the back of the telescope) corrects for tracking errors and gives very good results. The autoguider will be making corrections that are very small compared to your film because the plate scale is different. Piggyback photography is a good choice for taking pictures of targets too large to fit in the telescope: the Andromeda Galaxy, the North American Nebula, the Milky Way star clouds, etc. So, all you have to do is set the camera up and walk away until the exposure is complete.
Which film should I use?
I would reccomend using relatively fast film. I have have my best results with Fuji 400 ASA. Faster films tend to be a bit grainy- although they do reduce exposure times.
What are the typical exposure times?
Exposure times depend on the speed of the film. However, 15 minute exposures with 400 ASA in the piggyback mode record very faint details (much faster optical system). With the same speed film, longer exposures of up to an hour will give you pictures like you see in books! (Exceptions are the moon and planets).
Why is focussing at Prime Focus so difficult?
Unfortunately, everyone’s camera is slightly different and there is no universal method of focussing for different cameras. The two methods I use are:
Focus using the diffraction pattern of a star caused by the grooves in screens of some camera viewfinders. I find this method works very well; however, if you do not have a screen with some graduations in it this will not work.
Focus by taking readings from the digital readout of the JMI focuser. I have had some luck in running the focus in and out while taking the average of the results and calling it focussed. However, this method relies entirely on how consistent I am.
You may judge my abilities to focus from the examples posted in the astrophotography section.
Exposure times for planets and the moon are very short since these targets are very bright.
My camera is new-fangled and technologically advanced!
The best camera for film astrophotography is an older camera body with a manual shutter. If you want to use a new battery operated shutter camera, remember to bring lots of batteries. Most battery operated cameras hold their shutters open by battery power as well. With astrophotography, you will run those batteries dry quickly. Even worse is when the battery goes in the middle of your exposure! Better are the SLR cameras with bulb settings in which the shutter is held open by mechanical means.
