[APML] Re: Need Help Focusing Telephoto Lens

["Kevin Wigell" <kwemail@twcny.rr.com>]

Thanks to all who responded to my questions.

Aberration: I assume that this would show up more toward the edges than the
center. When I'm out of focus with this lens, I'm out of focus all the way
from center to edge. And I have successfully taken some sharp photos with
this lens. Just not consistently. So I don't think it's aberration.

Flexure: Am I correct that flexure would show up as elongated stars, rather
than donuts? My stars are showing up as tiny circles with dark centers. I
don't think this is a symptom of flexure, am I wrong about that?

The equipment I'm using is a 10" LX-200 (not the best for AP I know),
Olympus OM-1, Vivitar 300mm telephoto lens. I have an Intenscreen in the
OM-1. For guiding, I'm using the STV. Initially, I was guiding through the
LX-200 at prime focus with the camera piggybacked. I couldn't get it to
work. I tried locking the mirror down, but the LX-200 and the camera would
still not track the same. The images were always streaked. So I got the
e-finder for the STV. Now I'm using the e-finder to guide (mounted on one
side of the OTA) and the camera is mounted on the other side of the OTA. So,
both the e-finder and the camera are rigidly mounted to the OTA. At this
point, in this mode, I'm not using the LX-200 optical system for either
guiding or imaging, but rather it's just a mount for the camera and e-finder
guidescope. In fact, the cover could be on the OTA while I'm using it in
this mode. This seems to work pretty well for guiding, as I said before, my
donuts are perfectly round. Just out-of-focus donuts.

I don't have a magnifier for the OM-1 viewfinder, but that certainly seems
like a good suggestion for getting good focus. I'll have to look into that.
I hadn't considered making a Hartmann mask for the telephoto lens either,
but for about $0.25 in cardboard or construction paper, I could certainly
give that a try.

My latest attempts were 60 min exposures of M31. I haven't yet finished
scanning the negatives, just enough to tell they were out of focus. I
usually don't lock up the mirror when taking piggybacked photos with this
lens. The STV reported good tracking, with average guide errors around 2
arc-seconds. I don't think that small an error would show up with a 300mm
lens. I had the proper parameters entered in the STV for the e-finder (1"
diameter objective, 4" focal length).

I think temperature shifts may be a big contributor to my problems. I was
focusing on distant terrestrial objects at about 50F, and photographing at
about 25F. That could certainly throw off my focus, I suppose.

I'll also check out the links that some of you suggested.

So, with a KE focuser for the OM-1, I would need two OM-1 camera bodies, one
for focusing and one holding the film? Do I understand that correctly? (The
costs of this hobby never cease to amaze me.)

Thanks again for all the suggestions. By the way, here's a link to my
website with some of the very few of my images that have turned out. Be
forewarned that *none* of these images are anywhere close to the quality
I've seen on this list. http://home.twcny.rr.com/kevinwigell/index.html
Comments are welcome, and yes I know, I have a long way to go.

Kevin Wigell



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