Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Why do I only get oval stars in my images at best?

  • Please log in to reply
14 replies to this topic

#1 hidalgo

hidalgo

    Sputnik

  • *****
  • topic starter
  • Posts: 33
  • Joined: 11 Jan 2008
  • Loc: So. Calif.

Posted 05 July 2020 - 11:32 AM

I've spent the last 3 nights trying to fix this problem with my images. My stars are oval at best and occasionally peanut shaped and ALWAYS elongated in the same direction no matter the target. I should be able to get round stars without having to BUY more stuff. I have a Celestron RASA 8" on a CGEM II mount and a Lacerta MGEN II autoguider and ZWO ASI294MC Pro camera and have been using Nebulosity to capture. My polar alignment is solid. I checked every screw I can tighten on the mount. I use 2 alignments stars and 4 calibration stars. I've experimented with many of the tolerance, aggressiveness and auto guide rate settings in the Lacerta. PEC seemed ineffectual or worse. To get reasonable photos I've been restricted to one minute subs to avoid the mis-shapened stars What am I missing? The photos are all cropped test shots of 2 minutes each except one wide shot. The subjects were around Altair and Graffias. Thanks for helping!oval stars 2.jpg oval stars 3.jpg oval stars 5 whole frame.jpg oval stars 6.jpg oval stars 7.jpg



#2 Hesiod

Hesiod

    Skylab

  • *****
  • Posts: 4,133
  • Joined: 13 Jan 2013

Posted 05 July 2020 - 12:07 PM

It is always along the N/S axis?


  • hidalgo likes this

#3 sg6

sg6

    Cosmos

  • *****
  • Posts: 8,368
  • Joined: 14 Feb 2010
  • Loc: Norfolk, UK.

Posted 05 July 2020 - 12:13 PM

Unfamiliar with scope and mount but I would have said it was more to do with tracking then optics. The mount is well within capacity.

 

Is the mount GPS? Assume it is as most that size are now. Just wondering if the mount data is right.

 

When you say Polar Alignment is spot on - What says it is spot on?

I just never trust software, useful but software can be made to say anything.

Have you a polar scope to confirm.


  • hidalgo likes this

#4 rk2k2

rk2k2

    Mariner 2

  • -----
  • Posts: 226
  • Joined: 10 Jun 2018
  • Loc: Cool, CA

Posted 05 July 2020 - 12:23 PM

Just going by memory from a couple years ago as I had something similar.  I happened to notice when I was focusing on a bright star with my camera in live view at highest magnification, the star would slowly drift slightly and then 'pop' back and repeat.  Not suggesting it's what your problem is but looking at a star in this way might help isolate the cause- mine turned out to be excessive play in my RA


Edited by rk2k2, 05 July 2020 - 12:37 PM.

  • hidalgo likes this

#5 nemo129

nemo129

    Gemini

  • *****
  • Posts: 3,400
  • Joined: 03 Jan 2010
  • Loc: WMass

Posted 05 July 2020 - 12:38 PM

To eliminate possible guider issues, you could try a couple of short unguided subs and examine the star shapes.


  • hidalgo likes this

#6 hidalgo

hidalgo

    Sputnik

  • *****
  • topic starter
  • Posts: 33
  • Joined: 11 Jan 2008
  • Loc: So. Calif.

Posted 05 July 2020 - 01:02 PM

Yes, always n/s axis.

 

I don't have GPS for this mount. I use the Celestron All Star alignment AND a polar scope. I have checked with the scope an hour or so later and polaris is where it's supposed to be in the polar scope.

 

An idea occurred to my sleep deprived brain. Perhaps it could be the filter and filter drawer (Starizona). I will try a different filter and reorient the camera as well.



#7 Alex McConahay

Alex McConahay

    Cosmos

  • *****
  • Posts: 9,245
  • Joined: 11 Aug 2008
  • Loc: Moreno Valley, CA

Posted 05 July 2020 - 01:13 PM

That is something besides just a drift.....Note that the spots are doubles. That is, the scope is in one position, shifts a few pixels, and stays in the second position. It is not as if it just one simple smear. 

 

If it happens in just an occasional frame, I would suspect cable or mirror FLOP. (Or the doggy kicked the tripod as it went by.) 

 

If it happens in all frames (and it is in fact north-south), I suspect it is some kind of backlash/slop in the guiding. 

 

How do you know it is north-south? I ask because this looks more like an RA tracking problem. 

 

>>>>>I use 2 alignments stars and 4 calibration stars.

 

If you are talking about celestial alignment, this does not matter. That just tells the scope where it is pointing and a few other parameters to correct the pointing model. It does not really affect tracking all that much. 

 

And what are "Calibration stars?" Where do they come into play?

 

Alex



#8 Cotts

Cotts

    Just Wondering

  • *****
  • Moderators
  • Posts: 10,432
  • Joined: 10 Oct 2005
  • Loc: Madoc, Ontario

Posted 05 July 2020 - 01:15 PM

To eliminate possible guider issues, you could try a couple of short unguided subs and examine the star shapes.

Additional test::  Point scope at Polaris and take an exposure with no tracking at all....  Examine star images....this will eliminate (or confirm) collimation of the RASA as a problem....

 

I assume you are using a separate scope for the guiding.  There is a good possiblity of flexure between the guide and main scopes... OR a cable dragging or adding its weight to the guider......Your elongations could be from just that....check the routing of all wires, the seating of the guide camera in the guide scope, the mounting of the guide scope within its rings, the mounting of the guide scope rings to the main telescope the seating of the ZWO in the RASA...your auto guider could be blissfully ticking along with great numbers on the display while some aspect of your main scope "wiggles" relative to the guider... 

 

A picture of your set-up would help folks diagnose and point out potential weak points....

 

Dave


  • Andrew Brown likes this

#9 Hesiod

Hesiod

    Skylab

  • *****
  • Posts: 4,133
  • Joined: 13 Jan 2013

Posted 05 July 2020 - 02:05 PM

N/S drift makes me to think to issues with PA, but the irregular shape of the drifted stars is awkward.

Were this happening in just few subs I would think to mechanical issues but in such case would be quite awkward; I do not know if it is the case also with the CGEM II but some mounts try to compensate the polar error by tracking also in DEC.

That may cause both the guide and the mount to try to displace the telescope, turning your stars into irregular trails



#10 hidalgo

hidalgo

    Sputnik

  • *****
  • topic starter
  • Posts: 33
  • Joined: 11 Jan 2008
  • Loc: So. Calif.

Posted 05 July 2020 - 03:48 PM

That is something besides just a drift.....Note that the spots are doubles. That is, the scope is in one position, shifts a few pixels, and stays in the second position. It is not as if it just one simple smear. 

 

If it happens in just an occasional frame, I would suspect cable or mirror FLOP. (Or the doggy kicked the tripod as it went by.) 

 

If it happens in all frames (and it is in fact north-south), I suspect it is some kind of backlash/slop in the guiding. 

 

How do you know it is north-south? I ask because this looks more like an RA tracking problem. 

 

>>>>>I use 2 alignments stars and 4 calibration stars.

 

If you are talking about celestial alignment, this does not matter. That just tells the scope where it is pointing and a few other parameters to correct the pointing model. It does not really affect tracking all that much. 

 

And what are "Calibration stars?" Where do they come into play?

 

Alex

Backlash could be an issue (I haven't dealt with that YET). On this mount the 2 alignment and 4 calibration stars are part of the celestial alignment procedure (not for tracking but I do it to help me practice and diagnose this problem). Perhaps it isn't north/south but "up down" in the photos. It's always the same orientation though ( \ ) no matter where the target is in the sky.



#11 hidalgo

hidalgo

    Sputnik

  • *****
  • topic starter
  • Posts: 33
  • Joined: 11 Jan 2008
  • Loc: So. Calif.

Posted 05 July 2020 - 03:54 PM

N/S drift makes me to think to issues with PA, but the irregular shape of the drifted stars is awkward.

Were this happening in just few subs I would think to mechanical issues but in such case would be quite awkward; I do not know if it is the case also with the CGEM II but some mounts try to compensate the polar error by tracking also in DEC.

That may cause both the guide and the mount to try to displace the telescope, turning your stars into irregular trails

I should try some shots without the DEC motor. I am pretty confident the PA is in good shape.



#12 Der_Pit

Der_Pit

    Surveyor 1

  • *****
  • Posts: 1,521
  • Joined: 07 Jul 2018
  • Loc: La Palma

Posted 05 July 2020 - 03:56 PM

If it's N/S, i.e., declination, the first suspect is DEC backlash.  I don't know the possibilities of the Lacerta, can you disable N-S guiding separately?  And/or measure BL?That would also allow to use drift scans to check polar align (what PHD2 PA does). 

You could try to do a less-than-perfect polar alignment, which will create a drift in DEC, then the guide only needs corrections in one direction



#13 hidalgo

hidalgo

    Sputnik

  • *****
  • topic starter
  • Posts: 33
  • Joined: 11 Jan 2008
  • Loc: So. Calif.

Posted 05 July 2020 - 04:10 PM

I've made a list of new tests to do this evening based on all your input. THANK YOU!



#14 Alex McConahay

Alex McConahay

    Cosmos

  • *****
  • Posts: 9,245
  • Joined: 11 Aug 2008
  • Loc: Moreno Valley, CA

Posted 05 July 2020 - 04:45 PM

While you are doing it, plate solve to determine angle of rotation....

 

ANd then, just for the heck of it, take a thirty second exposure. Let it sit for five seconds doing nothing, and then for fifteen while pushing the "North" button on your handpaddle (or virtual handpaddle). Then let it run another ten seconds or so. What kind of streak do you get? The dim dot will be the five seconds, the dimmer line the direction North while heading to the bright dot at the north.

 

Does the direction of that line match the direction of the drift in your regular pics?

 

Alex



#15 17.5Dob

17.5Dob

    Fly Me to the Moon

  • ****-
  • Posts: 6,386
  • Joined: 21 Mar 2013
  • Loc: Colorado,USA

Posted 05 July 2020 - 09:05 PM

Can you confirm your Autoguider is even working ??? Even with a really bad PA, a properly functioning guider should be able to correct for it...I was about to brainfart and say check your PHD log...




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics