Photographic Methods

... still under construction ...

Astrophotography is hard work. There are many things that can go wrong, and they do. Also, the pictures never seem to turn out as well as the ones in the books and magazines. But when you do get a good shot ...

My first astro photos were prime focus shots of the moon. I then moved to eyepiece projection of the moon and planets, followed by piggy back astrophotography. More recently, I have begun deep sky photography. I am still learning the techniques and am not an expert by any means.

Piggy Back Photography

This refers to attaching a 35mm SRL camera to a tracking system so that stars are not trailed on long exposures. The camera can be piggy-backed on the optical tube of an equatorially mounted telescope or attached directly on an equatorial mount. Some photographers build their own barn-door tracking system.

Piggy back photography is one of the most forgiving types of astrophotography and can give beautiful pictures of star fields, constellations, and the Milky Way. You should master this technique before attempting prime focus photography of deep sky objects.

The mount needs to be roughly aligned with the north pole and track the stars as the earth rotates. Most any motorized equatorial mount should track accurately enough for modest exposures with a short focal length lens. Popular lenses range 50mm to 150mm, and typical exposures are from 10 to 30 minutes. See my piggy back photographs for some specific exposure information.

The camera focus should be set at infinity. Close the aperture down by one f/stop from the full open position to minimize distortions from lens aberrations. For best results, use a dark sky site. Good piggy back shots are hard to beat.

Lunar Photography

This is the easiest type of prime focus astrophotography. The camera is attached directly to the telescope using a T-adapter. (Alternatively, an eyepiece projection adapter can be used without the eyepiece.) A telescope with a focal length of 1500 mm to 2000 mm is ideal for fitting the whole moon into a frame of a 35mm camera.

Focus the telescope on the moon while looking through the camera. Choose your exposure depending on the f/ratio of your scope, the speed of the film, and the phase of the moon. The exposure probably will be much less than 1 second. Bracket your exposure times. You may also want to try bracketing your focus. Lock the mirror in the up position before the exposure if your camera has that capability. If it doesn't, watch out for possibly blurred images due to mirror slap. But don't confuse this with poor focus.

The telescope should be equatorially mounted and aligned with the north pole. Alignment and tracking are not critical since exposures are short.

Eyepiece Projection

For planetary pictures or closeups of the moon's terminator, eyepiece projection is necessary to increase the magnification beyond the intrinsic focal length of the telescope. This is done by inserting an eyepiece into an eyepiece projection adapter. The adapter attaches to the telescope, and then the camera is attached. The effective focal length, F.L., (and also the f/ratio) of the system is increased by a magnification factor according to the following equation. An analogous relation exists for the f/ratio (substitute f/ratio for FL and fl)

magnification = distance / eyepiece
F.L. = f.l. * magnification

f.l. is the focal length of the telescope optical tube. Distance is measured in mm from the eyepiece to the plane of the film, and eyepiece is the focal length of the eyepiece (also in mm).

You should use a good quality plossl or orthoscopic eyepiece. Choose the eyepiece size to give an f/ratio of f/90 to f/120 for planetary imaging. Focus on the planet as carefully as possible. Try the following exposures for 200 ASA color film at f/100: 5" for Saturn, 2" for Jupiter, 8" for Mars, <1" for Venus. Bracket the exposures (and maybe the focus) to find what works for you.

For photographing the lunar terminator, choose the eyepiece that gives the magnification you want. Try X seconds on 200 ASA assuming f/100.

The telescope should be equatorially mounted and aligned with the north pole. Aim for steady nights with the planets as high overhead as possible.

Prime Focus Photography

Long exposures of deep sky objects require good polar alignment and careful guiding. I use an ST-4 autoguider off-axis with the Taurus astrophotography system. Initial photographs were taken on Konica 3200, a very fast but grainy film. Next I tried Fuji G 800 with good results. From my backyard, I can expose Fuji 800 for about 30-40 minutes at f/6.3 (I live in a suburban area of town, total population about 40,000). From a truly dark site, exposures of 60-75 minutes at f/6.3 are better.

More recently I have been using Kodak Pro 400 film. I also tried the Multispeed PMJ but it did not give as good of results as the Pro 400.

Developing

Most of my photographs have been with off-the-shelf color print films. I have them developed and printed at a local 1-hour photo lab. Sometimes I have the prints redone with a slightly different exposure, but the lab now usually does a pretty good job.

Images were scanned from photographic prints using an inexpensive color scanner. The contrast and brightness was adjusted with Photoshop. Very few of the images received additional enhancements. The images were converted into either gif or jpeg files for viewing on the web. Jpeg files tend to maintain the best image quality. I tried to keep the image size small enough so that download time would not be excessive.

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