guiding shifts over 1hr

[2stroke]

Heya guys ripped off 1hrs worth of 3mins subs the other night and noticed my stars move about 10mm off to one side (ossp v2 color aka qhy8l) from first shot to last. My guide scope is an st80 has piniched optics as i havent had time to colliminate or fiddle with the optics, its a home rebuilt job lol. Guide cam is a meade dsi pro, mount heq5, scope ed100 f9. There is some fluxture but its not to bad soild plastic housing holding guide scope, the mount was a 1 run alignmaster job only. I'am using phd with a gpusb as i haven't had much luck with pulse, can someone give me some input on whats going wrong please, the stars were nice and round though 3min subs only.

[Daniel-K]

Id ditch align master and go with ascom. I PA with ascom and Im doing 120 second subs unguided. Ive tried align master found to be Rubbish to be honest. If its got pinched optics then why not try a finder guider?

[2stroke]

I'am getting close to 2mins un-guided using alignmaster when i do 3 runs lol so its pretty good my end for polar alignment, best piece of software i've used so far, it comes down to the thread pitch not being fine enough that i can't get it any better with this cheap Vixen knock-off Chinese mount haha. I will fix the optics up, that not a problem though have being looking at a finder guider due to having less flexture then using a st80, though i herd guide stars can be trouble when you only have low mag ones to choose from.

[ollypenrice]

What you are describing is almost certainly polar misalignment. It is a perfect description of what happens if you are misaligned. Could your tripod be settling during the run? Or the mount head be moving?

Olly

[peroni]

What you are describing is almost certainly polar misalignment. It is a perfect description of what happens if you are misaligned. Could your tripod be settling during the run? Or the mount head be moving?

Olly

I get something similar at present as I have my mount on grass while the patio is finished :-(

Sent from my phone :-D

[opticalpath]

I agree with others that this could be PA error.  However, if you're getting drift whilst auto-guiding, it could also be caused by differential flexure,  i.e. the guide scope is keeping the guide star beautifully centred (as far as the guide camera is concerned), but meanwhile the imaging scope is gradually pointing in a slightly different direction!  This is quite a common problem unless guide scope and camera are very solidly mounted.  It results in a gradual drift of star images, often in a diagonal direction, perhaps too small to notice in individual exposures but apparent over a long run.  Auto-guiding will go some way to correcting a small PA error.  If guiding is working correctly, PA error will result in field rotation and summed images (without rotation alignment) will show arcs.  So first check your PA, then check for flexure.

To asses your PA error, take a series of images UN-guided so that the auto-guider doesn't confuse the picture or disguise the problem. Adjust PA if you see obvious drift.  If you start PHD guiding but check the 'disable guide outputs' box, PHD's guide log will record exactly what corrections would otherwise have been applied.  In other words, it will show you exactly how the mount tracked natively, without guider correction.  The log has all the data you need to see drift as well as periodic error etc.

Once PA is confirmed to be reasonably good, check for differential flexure: Start auto-guiding (with guide outputs switched back on) and let the mount run guided for a couple of hours.  Take an image at the beginning of the run and another at the end of the run and compare them.  If there is still significant drift, look at the PHD guiding log.  That will show you what the guide scope thought the x- and y- errors (and corrections) were over the period.  If the errors and corrections in the log remained quite small over an hour or two (i.e the auto-guider did its job and kept the guide star centred), but the imaging camera is showing a gradual drift of stars in your images, that means you have differential flexure or some other source of movement between imaging and guiding systems.  Then the fun starts!

Adrian