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sub success rate seems low to me - please comment

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#1 rcol

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Posted 24 January 2021 - 02:05 PM

I have an older CGEM mount, recently DIY hypertuned, WHICH SEEMED TO BE TRACKING AND POINTING WELL.

 

But, recently, I had the opportunity to run 72 subs of 10 seconds length through a 1600mm scope. I am at 28#. Examining the subs 29% were bad!!
An example of a really bad frame is enclosed. Nothing unusual was going on except a guy on a gas-powered segway cruised by at 11pm.

 

With such short exposures, i would expect nearly 100% good frames. How do I diagnose the problem?

 

tnx for any comments

 

attached file is at 100%

 

 

Attached Thumbnails

  • worst-case.jpg


#2 bobzeq25

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Posted 24 January 2021 - 02:22 PM

The "double stars" look frequently indicates cable drag.  As the mount moves, the cable tightens, and then jumps.

 

28 pounds is not the only factor here.  1600mm magnifies errors.

 

I presume you're not autoguiding.  Or you'd have quantified information.  I strongly suggest you start.  It will both work a lot better (a CGEM would probably have a periodic error of 10-20 arc seconds, even mediocre guiding will reduce that to 1.5), and give you much better clues as to what the problem is.

 

Personally I wouldn't want even 1% of my subs to look like that.  You have a serious problem, somewhere.

 

The only way to measure tracking errors is with an autoguiding program, or looking at your images.  "seems to be tracking well" means nothing.  The errors involved are _tiny_.

 

This ain't a Segway driving by.  <smile>


Edited by bobzeq25, 24 January 2021 - 02:26 PM.


#3 alphatripleplus

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Posted 24 January 2021 - 02:23 PM

How did you check your polar alignment? If it is really far off, you could get trailing even in 10sec.  Or you might have large jumpy periodic error (I'm assuming you weren't guiding if you were doing 10sec subs) -  this could mean some of your subs are bad ( in parts of the worm period where PE manifests itself), while some are good. If it is PE, the motion will be in the direction of RA. If it is polar alignment error, the trailing could be in any direction depending on how you are misaligned in PA. Or as bobzeq25 says, it could be drag of a cable.



#4 rcol

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Posted 24 January 2021 - 02:40 PM

tnx for comments!!

 

First, the pic enclosed is a WORST CASE at 100% scale. most were not thyat bad.

 

Second, yes we were guiding but i suspect some parameters were way off and need to repeat with no guiding

 

Third, we were using celestron vibration isolators under the tripod, and I suspect those as bein part of the problem.

 

Fourth, our PA was very good .. about 15 arc-seconds error total

 

Fifth, we are wireless to the ASI air with careful cable management from there



#5 KTAZ

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Posted 24 January 2021 - 02:49 PM

Post the guide log and folks will be able to review it.

#6 rcol

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Posted 24 January 2021 - 02:58 PM

how canm I tell, when l looking at imges, whether movement is in RA or dec?



#7 bobzeq25

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Posted 24 January 2021 - 03:21 PM

how canm I tell, when l looking at imges, whether movement is in RA or dec?

That's complicated.  The graph in PhD2 is very simple.


Edited by bobzeq25, 24 January 2021 - 03:22 PM.


#8 alphatripleplus

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Posted 24 January 2021 - 03:38 PM

how canm I tell, when l looking at imges, whether movement is in RA or dec?

 

Here's one way: If you have a composite stacked image of the good subs, platesolve it using astronomy.net and the orientation of your field will be included in the results.


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