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

Help identifying the source of oblong stars

  • Please log in to reply
2 replies to this topic

#1 bnickeson

bnickeson

    Ranger 4

  • -----
  • topic starter
  • Posts: 318
  • Joined: 24 Oct 2016

Posted 11 April 2021 - 08:11 PM

Things finally went well enough last night for at least one light frame to get an image where I can start to debug the stars I'm getting on (relatively) new equipment.  The astrobin link below contains a full resolution image where you can see the various star shapes throughout my image field.  While they're not awful, I don't consider them up to my standard and I know they can be better.  So I'm looking for some help in identifying potential issues.

 

My gear is a Celestron 8" Edge HD with 0.7 focal reducer within a Moonlight focuser.  I then have an OAG, a few spacers, and a QHY168C imaging camera. Guiding last night was quite good (best I've ever gotten actually) with RMS being below 0.5" and individual wayward deviations being no more than about 1".  My imaging scale is 0.69"/px so only a couple pixels would be affected by wayward movements of the mount.  Transparency was OK before the dew set in, and seeing was average to good.  This image was taken with the OTA pointing about 60 degrees up so I would think any flexure would be minimal, especially with the Moonlight focuser being built like a tank.  Collimation was done a couple months ago by looking through an eyepiece at a defocused bright star and verified with the imaging camera as well.  It was about as good as I can possible get - no eccentricity was noticeable.

 

As you can see, stars in the middle of the image have a slight comet shape to them with the tail off the to left.  The original bayered image is easier to see this in.  Stars in the upper left are fairly good, in the upper right they're bloated to the lower right side.  Stars in the lower left again have a comet-like shape to them with the tail being to the upper right, and stars in the lower right corner are fairly egg shaped tangential to the center of the image.  I'm mostly perturbed by the stars near the center since I expect at least some image degradation around the edges.  FWHM per PixInsight Subframe Selector is a little over 3 and eccentricity is about 0.58.  Are there any ideas on sources for these various oblong stars?  

 

get.jpg?insecure


  • limeyx likes this

#2 jerahian

jerahian

    Apollo

  • *****
  • Moderators
  • Posts: 1,399
  • Joined: 02 Aug 2018
  • Loc: Maine

Posted 11 April 2021 - 09:11 PM

I know you said your collimation was good, but this still looks like a collimation issue to me.  Two things I did to ensure my collimation was actually accurate:

 

1) I adjusted the corrector plate, with the secondary mirror removed, to ensure the opening is centered and aligned along the central axis of the tube (you would see perfectly concentric circles of reflections between the opening in the objective and the primary mirror if centered properly).  I found this video mildly helpful to illustrate what I mean, even though it's trying to do this for a Hyperstar/Edge setup.  BTW, my secondary mirror was quite a bit off axis when I got around to checking it.

 

2) I started using Metaguide for collimation instead of the defocused star method.  The defocused star method is a good process to get you almost there, and then I use Metaguide to really dial it in.  Here is a video of the author of Metaguide, freestar8n (Frank), presenting his full software solution on TAIC channel.  This is the section of that video where he talks about and demonstrates how it's used for collimation.

 

I'm fairly certain this is your issue and it's where I would begin.

 

Good luck,

Ara


  • dswtan and bnickeson like this

#3 bnickeson

bnickeson

    Ranger 4

  • -----
  • topic starter
  • Posts: 318
  • Joined: 24 Oct 2016

Posted 12 April 2021 - 08:22 PM

Thanks for the tip on Metaguide.  I'll download it and use it on my next trip out.  Looks way easier than guesstimating using the defocused star method.




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