Guide star is lost after dithering.

[Terry Shaub]

I've been getting great guiding for months. I restarted everything; Calibration, darks, etc. Slew to the target, plate solve, start PHD2 and the graph shows very good guidance. 

After the image is done, dithering times out, and PHD2 loses the guide star. I reset the guide star, and again see good guiding. And the same thing happens. I have recalibrated, and applied the suggestions. 

iOptron CEM60ec

I still can't get guiding. https://openphdguiding.org/logs/dl/PHD2_logs_xH3A.zip

[bw_msgboard]

Hi Terry.  These dithers are truly grotesque - you're issuing dither commands that represent very nearly 1 arc-minute of movement!  Have you done the math, are you completely sure you have to do such huge dithers?  With your current mount guide speed while imaging at Dec = 60, which is what you were doing, it's going to take over 20 seconds just to move the mount in RA.  And during that time, your scope will be wandering off by itself for reasons of polar alignment error if nothing else. It's no wonder you're losing the guide star.  Have you checked your polar alignment lately?  If you are going to continue with such huge dithers, you should disable the option in PHD2 for "Fast recenter after calibration or dither". 

 

 

Even then, you're going to see really long delays before the dither is completed and settled.  I suggest you look at your dithering parameters again and dial this down - those are specified in your imaging app, not in PHD2.

 

Good luck,

Bruce

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Terry Shaub
Sent: Tuesday, October 12, 2021 1:48 PM
To: Open PHD Guiding
Subject: [open-phd-guiding] Guide star is lost after dithering.

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[TERRY SHAUB]

Hello Bruce,

Ouch!

I was "perplexed" with your response...

"These dithers are truly grotesque - you're issuing dither commands that represent very nearly 1 arc-minute of movement!... " 

""If you are going to continue with such huge dithers......"

 

I can appreciate that you have contextual knowledge that many users just don't have. I had no idea what you were talking about until that last part... "those are specified in your imaging app, not in PHD2."

"It's no wonder you're losing the guide star."

Your allegation was that I was intentionally doing something wrong... not doing the math...and this..."“It's no wonder you're losing the guide star.”

If you had started with,  "Your problem is in the dithering input in you imaging app." I would have have said thanks and went to work..

I still consider myself a newbie after the last 20 months. Not all of us can connect the dots until we have the a higher level of contextual knowledge. 

I appreciate your knowledge, but not your rudeness.

I'll probably never ask for help here again. 

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/8A887F5370A142B29D857D2F06967465%40HomeDesktop.

[mj.w...@gmail.com]

Bruce and the other developers always welcome your thanks for the hours of FREE time they put into helping struggling astrophotographers :->

Did you read the "How to ask for PHD2 Help" link on the home page,  as you didn't send a log that included a Calibration ?

Have you read any of the PHD2 Help and How To guides available via the Help menu ?

Then you would have Calibrated at Dec = 0 instead of Dec = 24

And read what to do when the Calibration resulted in an Error Message concerning differing guide rates  ?   Last Cal Issue = Rates, Timestamp = 10/9/2021 12:02:23 AM

Norm rates RA = 4.4"/s @ dec 0,     Dec = 7.9"/s;     ortho.err. = 2.0 deg

Dithering is typically 12 pixels of the imaging camera.

Your approximately 12 pixel Dithers of the GUIDE CAMERA would result in 12 x image scale 4.98 arcsec/pixel = 60 arcsecs of the imaging camera.

Or a Dither of 60 pixels of the imaging camera,  assuming you are imaging at about 1arcsec/pixel.

Did you read how to use Dither in the PHD2 Instructions ?

Dial down wherever you set Dither, to give Dithers of about 3 pixels max in the guiding.

 "The Gods help those who help themselves"  first.

Michael

Wiltshire UK