Eyepiece Projection

Recently we purchased T-adapter and T-ring along with the variable adapter for eyepiece projection imaging. The real intent was to start out taking a series of lunar photos at Newtonian focus using an 35 mm SLR. The 1200 mm focal length of our Newtonian should put an image of the (full) moon approximately 10–11 mm across at the film plane; plenty big enough to make some nice enlargements and yet small enough that we can take pictures even though the telescope is on a Dobsonian mount.

Alas, we discovered what many others have found—the camera will not come to focus due to insufficient in-travel on the focuser. The cure is to obtain an low-profile focuser. Time to spend another USD$80 or so.

However, since we already had the tools for eyepiece projection, I decided to try it by taking a couple of rolls of film of Jupiter. I was a little worried about vibrations due to the mirror slap of the SLR but figured it wouldn’t hurt. So, I used 3 rolls of Fuji Superia Xtra ISO 400 film and took multiple frames at each exposure. My effective focal ration was near f/41, so I used the table from the back of Michael Covington’s Book and shot frames from 1/4 second all the way out to 1/125 second. For each shot, I refocused using the logic that I couldn’t get it wrong all the time. The ground-glass focusing screen in my Pentax Spotmatic made it tough to get the focus, but I could generally tell I must be close because Jupiter’s moons would become pinpoints and I could make out a little banding on Jupiter.

The first two rolls had identical exposures and I had one of them pushed one stop. The last roll was the one which extended to 1/125 second, and I had it pushed two stops. Every exposure was repeated 4 or 5 times. Again, the idea was that I hoped to have at least one good frame in the lot.

Now the bad news: I have exactly two frames that have anything remotely resembling Jupiter. The remainer are indistiguishable from a badly out-of-focus picture of a distant light bulb. Clearly, the mirror slap is a bigger issue than I realized, at least at f/41. The two “good” frames (and I use that term quite loosely) were taken at 1/125 second. Oh, and did I mention that I took the film to a professional shop where I had it all developed, printed, and a set of contact sheets done for USD$80? Ouch!

All hope is not yet lost for our original project, taking moon pictures. At f/8, the effects of mirror slap should be less noticable. Plus, except for the smaller crescent phases, there should be enough light that, with moderately fast film, the exposure should be very short hopefully “freezing” the slap. That’s what I think saved the two frames which came out showing at little of Jupiter’s banding.

The moral: for planetary shots, you probably want a camera with mirror lock up.