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Hi Yi-Zen,
Welcome to the world of astrophotography, where if
things can go wrong, they will go wrong. There are quite a few things
that could be causing your guide errors with the STV and the trailed stars in
your photographs. You may have to provide some additional information
before the APML folks can help you find the answers.
First the guiding: The STV should be able to
guide with a 100 mm lens fl (E-Finder) to about 1 arc sec in the mount motion is
smooth, the STV mounting is rigid, and the ground or pier isn't being shaken by
people walking around. You mentioned using a rented GM-8. I don't
have experience with that mount, but mounting a scope, ball head camera and STV
on a dovetail bar sounds like a recipe for flexure. I have also heard that
owners of GM-8's have to work on their mounts, sometimes replacing the drive
components to get low and/or smooth periodic error. Getting 2 arc sec
errors with the STV is probably what you should expect with that mount,
unmodified.
Second the star trails: The length of
exposure generally determines the amount of error or star trail length.
elongated stars can come from:
1. inaccurate polar alignment causes field rotation
which rotates around the "guide star".
2. Using a guide star some distance from the field
of view of the camera/scope will cause more of #1. The solution id to
guide on a star near the center of your field of view, since you have a separate
guide setup for the STV.
3. Longer exposures allow flexure between the
guider and camera. The mount rotates at 15° per hour, enough to make the
gravity direction change and flex your setup. The solution is more
rigidity, which means more weight, which means larger mount, an expensive
solution. Alternatively, move the STV as close to the camera or scope as
the dovetail will allow.
4. The flexure of a ball head mount depends on the
quality of the ball head, and the overhanging weight of the camera/lens. I
once tried using my Nikon F on a ball head that used a nylon bolt through the
plate to mount the ball head. I got lots of flexure from the nylon bolt
stretching. Going to a steel bolt solved most of that. Most of this
flexure is due to the RA rotation at 15° per hour, so shorter exposures may give
better results.
I know you will get lots of help here on the
APML.
clear Skies,
Don Westergren
Hello everyone,
I'm a newbie to deepsky astrophotography, and would like to get some help. During the last new moon period I got together the following setup: 1) Losmandy GM-8 mount with Gemini GOTO system (rented) with dual scope platform 2) On one side of the dual scope platform I mounted a dovetail bar with an STV + camera lens (180mm or 105mm) on one end and a ball head with a Nikon F4 on the other end. 3) The other side of the dual scope platform carried a Borg 100 ED. The STV head was mounted on the dovetail plate directly with a single screw tightened as tight as possible - same for the ball head on the dovetail plate. First problem: I tried to autoguide with the STV and saw very large errors, on the order of 2-4 arc seconds, sometimes even larger. Is this normal? I did notice that seeing wasn't too good, which surprised me somewhat, as I was observing from an elevation of 4-5K feet or higher (SE AZ). What are the possible causes of large errors? I'd like to try to investigate this more carefully but would appreciate inputs on what to look out for. Second problem: almost all my shots (except one) came back with trailed stars. The longer exposures seem to have the longer trails. (I shot mostly with 55mm and 180mm lenses, and also through the Borg at 640mm.) Most of them were trailed in the NE-SW direction. Only one was trailed in the E-W direction. The latter shot was of the Rho Ophi region and the rest were of Cygnus, Mel 111, M101 and Lagoon regions. I am wondering if this trailing is related to the first problem. What are the other possible reasons for this? Flexure? Bad guiding? Severe backlash? I thought that perhaps the ball head was flexing, but there is trailing for the shots through the Borg as well. I noticed that the camera lens can be pulled away from the CLA-5 adaptor a little (i.e. the lens does not sit on the flange tightly) - could this be a source of flexure? Thanks for any inputs! Yi-Zen
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