Old 13-10-2015, 08:57 PM
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mikejenningz is offline
Join Date: Oct 2015
Location: Leeds, England
Posts: 2
Smile Guiding - Dec axis problems

Hi Guys,
Since I started with astrophotography I have been fascinated with autoguiding and how it can very much increase exposure time to really make good images of DSOs possible. I have now got the necessary kit and have been practicing, but I have a niggling problem that Iíd love to solve.

My setup is a pier mounted Celestron 8Ē SCT connected to a DSLR as my imaging camera. Driven by a Celestron advanced GT mount, I also have a ST-80 refractor as my guide scope. For guiding my camera I use a QHY5 connected with a ST-4 cable to the mount. I use the latest PHD2.5 software on my laptop.

Iíve used drift alignment for polar alignment, but I heard some advice that leaving polar alignment a little off could help with guiding a little. The idea is that the DEC axis would generally only have to move in one direction slightly chasing the guide star and eliminating the problem of backlash where the Dec axis has to change direction to track the guide star.
I have been running this system for a while and it works ok, it has allowed me to do some guided astrophotography but periodically whilst guiding the Dec axis goes off the scale. There does seem to be a pattern with it. As the Dec axis climbs to the limit where a correction is required a moderate correction pulse seem to send it completely off the scale.

Please see some of my screen captures showing the problem, and also the result of the PHD2 guiding assistant. I can get 10 mins good guiding sometime but then tracking in Dec seems to send a pulse and sends the tracking off the scale. I have tried changing the pulse sizes for calibration, but the problem is re-occurs with difference numbers of calibration steps.

I guess I need to find a way of isolating the problem, I know issues causing it could be either backlash, flexure, or even periodic error. I wonder if the polar alignment error is a little too far off, but the PHD2 guiding assistant didnít say that it needed correcting. I need to find a way of testing for each possibility.

In terms of any flexure in my system, it seems to be minimal, PHD calibration seems to go ok. I have generally tended to take guide exposures of 0.5 sec, and I have now heard that longer exposures are better and eliminate the effects of seeing. Anyway, thanks for reading , hope it can get this problem resolved.

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Old 13-10-2015, 09:14 PM
Somnium's Avatar
Somnium (Aidan)

Somnium is offline
Join Date: Oct 2014
Location: Sydney
Posts: 1,669
Hi Mike, not sure what the issue is but if you cant solve it and given that you have a permanent set up, have you thought of really dialling in your polar alignment and only dithering the RA? you should be able to get long exposures that way without the issues in your dec axis
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autoguiding, guiding, phd2

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