Thanks for your swift response, Gene. I might as well play my hand and tell
you why I am asking these questions. I am pursuing an idea that may be crazy
or may be viable. Perhaps some of you guys can help me determine if its one
or the other.

The desired results are:
1) To prolong the time an object can be tracked exactly (or exposure time if
you willl) for a poor/good/excellent PA. This applies to people without an
autoguider or without useable guide stars. It might be useful even with an
autoguider.
2) Being able to track asteroids, comets and flying saucers (if you have
their orbital elements :-)
3) Being able to track satelites. This is really a special case of 2 but it
poses its own problems and has a low priority right now.

>From Gene's response I conclude that the PA error is the prime culprit in
tracking errors, when imaging for a limited time period at a given position
of the sky. As a consequence determining and eliminating this error should
fulfill objective 1.

To determine the error: Connect a computer to the telescope center a known
star. Let the scope track the star for some time, then recenter the star,
read the new position and elapsed time. Now compute the polar axis for the
mount. This is the tricky part. I don't have the necessary math figured out
yet - I'm not even sure it can be done based on one observation. If more
than one observation is needed, mirror shift and flexure will come into play
when moving to another part of the sky to repeat the observation - not good.

To correct the error: Once the axis of the mount has been determined it
should be no big deal having a computer issue small corrections as time goes
by. The algorithm used should utilize a combination of position readouts and
guide speed adjustments rather than goto's I think.

Being able to determine and correct the PA error, achieving objective 2 is
simply yet another set of corrections based on the orbital elements.

The same goes for objective 3, but here the speed of the motors impose some
restrictions. I don't want to waste energy on this problem right now. Maybe
later.

During summer the sky does not get darker than nautical twilight here in
Denmark and there is not much observing to do. So I'm ready to invest some
time in this, in fact writing a small test program is no problem once the
math is done. But first I'd like to hear your opinions. Can anything useful
come from it ? Anyone ?

greetings
Henrik

-----Original Message-----
Date: 15. juni 1998 23:37
Subject: Re: [M]: Unguided images


>
>Well, without good PA the others don't matter! Ignoring PA,
>I would rank the factors as:
>
>Mirror shift - 60%
>Flexure - 20%
>PEC - 20%
>Refraction - NA - You shouldn't be shooting that low!
>
>> What is the expected max. exposure time for a average to good
>> mount without guiding?
>
>At what focal length? Piggy back with a 50mm may not show
>*ANY* drift. At 1,000mm you might be able to go 10
>minutes without material drift. At 2,000mm with good PA you
>should be able to go 2-5 minutes, 10 minutes if you have a
>*VERY* good mount (AP1200, Tak NJP). Another factor is what
>you call a good exposure. Some people are perfectly happy
>with oval shaped stars. I am not.
>
>Take care,
>
>Gene Horr

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