Film

Astrophotography with Film
by Dennis Allen

In this age of digital cameras, you may wonder why I bother with film. Well, a couple of reasons. One, design. I built my telescope to be equally useful for visual or photographic work. Two, cost. While film is more expensive than computer memory, a suitable digital camera is still more expensive than a used Nikon F or Pentax K-1000 camera.

I built a 24" truss-tube dobsonian with a truss-tube kit from AstroSystems (for complete construction details, click here). To do astrophotography, I purchased a third generation dual-axis equatorial platform. This platform gives me an hour of tracking, making photographic work possible and visual use quite enjoyable. To help locate faint objects, I installed a Lumicon NGC Sky Vector. As you will read later, the Sky Vector computer is also useful in photography.

To do photography work, I went to the local camera shop and purchased a used Nikon F camera. For astrophotography, you need a camera that can lock the shutter open for the duration of the exposure. You also need a shutter release cable with a lock mechanism.

I also purchased a Newtonian easy guider from Lumicon. This off-axis guider mounts the camera onto the 2" focuser, allowing prime focus photography. Now the off-axis guider comes with an adapter to a t-ring, so I had to purchase a t-ring to Nikon F bayonet ring. The camera shop, however, figured out how to remove the t-ring adapters altogether. So now the off-axis adapter is directly connected to the bayonet ring, which is a nice feature. In photography, the camera needs to be closer to the main mirror than the eyepiece. The closer, the better.

Setting up for a shot takes a few steps. First, rewind your platform. Insert your camera/off-axis guider into the focuser. Find a bright star and focus it in the camera view finder. To help in focusing, I replaced the standard Nikon screen with a wide-field "B" screen.

Now the off-axis guider has a pick-off mirror and a place to insert your illuminated reticle (lighted cross-hair) eyepiece. A double-lined cross-hair box is preferred. You have to slide this eyepiece in and out until the stars focus (don't touch the focuser). Find the object you want and center it. In the cross-hairs find the nearest star. Use the platform's controller to move this star to one of the corner's of the cross-hair box. Now you're ready to take a shot.

In astrophotography, there's a lot of trial and error. Brighter objects like M8 will need less time than dimmer objects like M20. Try 5, 10, 15 or even 30 minute shots (use an egg timer). As for film, try 800 speed Fuji or 800 speed GoldMax film. Both films are good, but tend to saturate after 15 minutes (I find little difference between 15 minute shots and 30 minute ones). For dimmer objects, take longer shots or try 1600 speed and even 3200 speed film. Again, try different films, different speed films, and different exposure times.

Suppose you have an object too dim to see in the camera, then what? Well, what you need to do is find and center the object using an eyepiece. Then memorize the star pattern in the eyepiece and/or finder scope. Replace the eyepiece with your camera, focus the camera on a bright star, and return to the object as best you can.

A telescope computer, like the Lumicon Sky Vector, can help. Before you start your run, turn on your computer and align it by shooting two stars (for an equatorial platform, the mode is "ET"). Now find your object and align the computer on it. After you replace the eyepiece with your camera, the computer should be able to take you back to the exact spot.

Club 15" telescope

Our club has a 15" dobsonian sitting on a second generation equatorial platform. Very similar to my telescope setup. In fact, I used the 15" scope before going to my 24" scope. The 15" scope is smaller and has a longer focal ratio, so exposure times are doubled from that of my 24" scope. Unlike my telescope, however, the 15" scope has a 60mm (1200mm focal length) refractor mounted as a guide scope. Using this guide scope, you don't need an off-axis guider. You just need an illuminated reticle for the guide scope and a camera with a 2" adapter.

Conclusion

As I said, there's a lot of trial and error. To increase my productivity, I mounted a camera adapter to the top of my telescope. There I can use my other camera to take piggy-back shots while I'm doing prime focus shots. For milky way mosaics, 10 minute shots with 800 speed film and a 50mm lens work fine. Again, try different speed films, different exposure times, and also try different lens sizes.

Tip: Always keep checking your film. Last weekend I took a set of shots with Fuji 800 speed and 1600 speed film. Fuji must have changed the emulsion, because neither film picked up red nebulas very well. They did do a nice job on galaxies, however. Better than before.

Anyway, guiding by eyeball is harder than digital photography, but you can still get good images. I haven't done too badly, so click here to see the results.

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This web page was last updated 04/03/08