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PHD Problem with Guiding

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#1 orlyandico

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Posted 16 February 2011 - 09:22 AM

What does this mean?

Posted Image

Look at the DEC error. It drifts off scale, then eventually corrects. Later on, it stayed pretty much in the middle.

My polar alignment isn't perfect, but it isn't terribly shoddy either (drift alignment 30 seconds via CCD method).

#2 adamsp123

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Posted 16 February 2011 - 10:10 AM

cable snag? wind?

#3 Peter in Reno

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Posted 16 February 2011 - 10:20 AM

What mount do you have?

Excessive Dec backlash? Maybe needs mesh adjustment at Dec axis?

Looks like you have a sensitive guide camera. I can easily tell you are guiding M42. What kind of guide camera?

Peter

#4 orlyandico

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Posted 16 February 2011 - 11:34 AM

it's an old Meade DSI Pro I.. Vixen GP with DD-1 stepper drives and GPUSB-AH.

i think it was cable snag.. because after i left the laptop (which I literally had in my lap..) to wash dishes, the DEC flattened out. need to work on my brain settings though..

#5 dale c

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Posted 16 February 2011 - 12:02 PM

In addition to the DEC track, the RA track looks like there is a problem also. I notice you have the min motion setting set to 0.03 I don't know how PHD handles such a small number but I would think with a qhy5 camera and about a 600mm guide scope a 0.3 min motion setting is more realistic.
Dale

:foreheadslap: I see I adamsp123 guide scope info but the image scale looks about the same. :tonofbricks: Dale

#6 orlyandico

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Posted 16 February 2011 - 12:18 PM

dale, yup someone in my local club told me to up that number.

the RA was not a problem, over 40 minutes (4+ worm periods) it behaved, average PE was 20 arcseconds. Except for a huge spike every worm period!

Posted Image

i copied the 0.03 settings from some forum blindly.. obviously not such a good idea..!

#7 dale c

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Posted 16 February 2011 - 12:32 PM

Looks like you may have some binding or rough spots in your RA worm bearings. I had some surface rust in mine not too long ago (thanks to the pacific northwest humidity) which was giving me a problem in RA. Sounds like you are getting things under control.
Dale

#8 Falcon-

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Posted 16 February 2011 - 12:54 PM

The DEC problem you see there looks like taking up backlash to me. I know with my CG-5 when I first start guiding it DEC often runs away like that until backlash is taken up, then after that the graph flattens out (just as you described).

Yours seems to have taken quite a long time to flatten out, but that may be a difference in your DEC motor's guide rate.

#9 adamsp123

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Posted 16 February 2011 - 01:50 PM

That would seem to be one hell of large backlash to account for that graph especially considering the 2000ms DEC setting I stick will cable drag or similar.
I also agree set the min motion up, I use a 50mm finderscope and set that to about 0.5, I don't want to chase the seeing or keep sending large amounts of correction data which I believe increases the chance of unecessary oscillations in the guiding

#10 jmasin

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Posted 16 February 2011 - 03:04 PM

I would agree that's some crazy backlash if that's it... but I don't know the mount.

Definitely the 0.03 px min move # is too small. Regardless of what you say about the RA not being a problem, you PHD graph says otherwise and I think you can approve. That oscillation in RA is due to too much adjustment and bouncing back and forth, likely due to the aggressive min move and 100% setting. IMHO you are making PHD do too much.

I guide at 5"/px and image at 1"/px, so I've set min move to 0.15, which is just under a 1px move for the main imaging scope scale. I set RA aggressiveness to 0.95 (or 0.9 if seeing is bad) and do 3 to 5 min exposures. This has served me fairly well and never resulted in runaway adjustments. YMMV.

One other thing I'd add, I don't know if this makes a difference, but I always try to select a guide star without any nebulosity around it if possible. This can be hard with the M42 region. I do this so the light is starlight only, and not some misshapen source that may cause an inaccurate calculation of the centroid. Again, I have no idea if this matters, but it's been my practice and served me well.






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