1ST foray with PHD - guide star constantly drifting to 1 side!

[dc509]

After a few efforts, finally got PHD fully hooked up and taking images through my DMK31. Calibration seemed to go OK (it did not complain, but is there another way to tell if it's gone OK?), but once I started guiding, the guide star would drift out of it's little box within a couple of minutes. I tinkered blindly with the options in the graph box to no avail. My graph, compared to the usual ones I've seen here, is a dog's dinner. I know that my polar alignment is not up to much - does that explain what's going on here? I thought PHD would help with that, but the guide star constantly drifting to one side bothers me. Any help gratefully received!

d

[samtheeagle]

I'm certainly no guiding expert, but by the looks of that graph it could well be your polar alignment. It's the DEC axis that done a bunk (the red one), and I think that's indicative of poor polar alignment. In theory, if you're perfectly polar aligned the DEC should never need to be moved at all. But a massive shift of that extent makes me think that you're quite a long way off!

I'm sure that those more learned in the ways of guiding will be along soon to elaborate

[dc509]

Thanks - thought it was down to my laziness, and looks like I was right! Was hoping auto-guiding would compensate for not doing drift alignment, but maybe not...

d

[samtheeagle]

Well I've never drift aligned, and I don't get anything like that kind of DEC drift. That kind of movement suggests that you're a looooong way out, or maybe guiding at a very long focal length so the effect is exaggerated greatly.

All I've ever done is level the scope, rotate the RA axis to the correct date and time, and then use the polar scope to get polaris in the reticule piper. I'm probably not going to get 10 minute subs like that I grant you, but it's always been good enough for me thus far.

[martin_h]

You dont have to drift align to get it close.....but try to get it closer on the polar alignment with the polar scope.

Guiding will iron out some errors but if you are way out it wont stand a chance.

[swag72]

What scope are you guiding with? Once we know that, it may be useful as well to see the settings that you have in the 'brain icon'. It may be that some of those can be tinkered with to get you a better graph.

[dc509]

It s SW ST80. I tried a bunch of settings , mostly the RA aggressiveness (everything from 25 to 100), and I tinkered with many others to no avail. I've attached the brain screen as it was when I gave up (same as the graph above). Thanks for all the help so far, by the way.

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[samtheeagle]

I can only really comment from my own limited experience with PHD here, but the default settings are a very good place to start. I've made very few adjustments for my specific setup, and those were just to try and optimise things. Everything worked perfectly well straight out of the box with the default values. I'd be tempted to put everything back to defaults (hover over each value in the "brain" dialogue to see the default setting in a tool-tip).

Perhaps before even trying to autoguide you can line up your kit with a bright star or planet, get it centred in a medium to high powered EP, and then see how the tracking goes for a 5 minute spell. If the polar alignment is way out you'll find the target will have wandered across the FOV.

[dc509]

Samthe eagle, I think you are right. Better polar alignment, less shortcuts - I need to be more accurate in my aligning.

I will report back when the clouds clear...

d

[mark7331]

I have to agree with Sam here. The defaults are pretty good and getting a decent polar alignment makes all the difference.

If I'm correct in thinking the ST80 has 400mm focal length then one setting I would change is the "Calibration Step". The PHD manual says that for that focal length the step should be about 1000ms.

Mark

[adamsp123]

I would suggest you download the latest version and start, as other have said, with the default settings.

[dc509]

Thanks - I've had another go based on the advice and the results are significantly better! Still get drift after 10 mins or so, but have managed to take 5 min subs (5x5m, lum only of M101 below). Am well chuffed by the fact that the stars are nearly round and not "lang streaks o'p***" as my Aberdonian mother would have described them.

d

[Starflyer]

I'm glad you managed to get it working, that's a nice M101

The setting that stands out to me is Max Dec Duration. If you're reasonably polar aligned then 60ms is a good setting, if your PA is out then 60ms corrections will never catch up with the drift in DEC, try 500ms if you have problems again.

As mentioned, the max calibration step is a little on the low side. Watch the number of steps it's taking during calibration and alter this setting to get 10 - 15 steps. More steps won't hurt but it doesn't improve the guiding accuracy and wastes time when you could be imaging. A higher setting here will give you less calibration steps.

I'm not sure what RA Hysteresis does but when guiding with a similar setup mine performs best with a setting of 10 or 11 here.

Balance is as important as polar alignment when guiding, perform your initial balance with everything attached that you'll be using when you image.

I never drift align, maybe I should but imaging time is precious when you don't have a fixed setup. However I do take the time to get polaris in the correct position through the polarscope - the free software polarfinder is a great help here.

Cheers,

Ian

[mark7331]

That looks like a great result D. Nice round stars. Glad it's working better for you.

Ian, brilliant advice - I didn't really know how many calibration steps I should be seeing so thank you.

FYI:

I'm not sure what RA Hysteresis does but when guiding with a similar setup mine performs best with a setting of 10 or 11 here.

The RA Hysteresis setting takes into account the previous adjustments before making another one. A low setting means more adjustments, higher settings mean less.

Cheers

Mark

[dc509]

Thanks again for all the advice. I wasted a lot of time last night, redoing the alignment over and over again before realising how important balance is. Using a dual bar with 2 very different tubes has made it quite difficult to balance and last night my dec was all over the place. took advantage of a lazy sunday to sort it out properly and am hoping that tonight will give me a chance to test it out (should be clear)!

PS I've been using adamsp123's alignment procedure (using phd) and it's very simple... http://stargazerslounge.com/imaging-discussion/120593-phd-drift-polar-alignment.html

d

[Starflyer]

Here's my hint for balancing a dual bar setup;

Do it during daylight if possible so you're not rushing the job, and again do it with everything loaded that you'll be using that night. When it's balanced to your liking stick a few pieces of insulating tape to the dual bar, dovetails and guidescope to indicate where the balance points are... next outing you'll be there or almost there in minutes

Thanks for the info Mark... I'll have to read the manual one day

[dc509]

I am very pleased to report that thanks to the advice here, tinkering with the guiding and phd settings has paid off and I am collecting 10 min subs with, to my partially trained eyes, very decent guiding. The phd graph bubbles pleasingly around the origin for RA and dec, and it's been doing that for 2 hours now.

Thanks again for the invaluable help - I'll post the results as soon as I have hacked them to death in DSS/PS. At that point, the questions will start flowing again!!

d