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

Star Shape/Color Issue

  • Please log in to reply
8 replies to this topic

#1 dontsayflavor

dontsayflavor

    Lift Off

  • -----
  • topic starter
  • Posts: 4
  • Joined: 30 Apr 2020
  • Loc: The Biggest Little City, USA

Posted 29 June 2020 - 11:21 PM

Hi All,
 
So I've been lurking on here for a while now, but finally got all my astro gear up and running and decided it was time to jump in head first.  I've managed to take some decent first photos that have exceeded my expectations, but have noticed some odd star shapes/colors that have stumped me.

 

Odd Stars

 

I feel like I'm looking into a sea of frog eggs waiting to hatch into space tadpoles...

 

Because it will be asked, I'm shooting with an SV80ST w/ Field Flattener and unmodded Nikon D5100, ISO 800, 45sec x 80 subs unguided on a CEM60 with iPolar.  Image is a screenshot of a portion of the sky near M57, but shows up on all my images so far.  Also, while this is the stacked and edited version with lights, darks, and flats, the same star shape/color shows on single subs as well.  What am I looking at here?  Stars are not egg shaped, comet shaped, etc, so I'm assuming it's not collimation or coma.  Could it be poor seeing or tracking?  The Frogpocalypse?

 

Thanks in advance for the help!

 



#2 Tapio

Tapio

    Mercury-Atlas

  • -----
  • Posts: 2,835
  • Joined: 24 Sep 2006
  • Loc: Tampere, Finland

Posted 29 June 2020 - 11:38 PM

Show us full frame - or all corners.

#3 james7ca

james7ca

    Cosmos

  • *****
  • Posts: 8,518
  • Joined: 21 May 2011
  • Loc: San Diego, CA

Posted 29 June 2020 - 11:40 PM

Looks like you are out of focus, or that combined with some other optical aberrations. If that sample is toward the corner of your field then the field flattener may not be working very well. But, it's somewhat unlikely that you will get small and perfectly circular stars right into the corners of an APS-C format with any given field flattener (some will be better than others -- but it shouldn't be as bad as you've shown).



#4 DSOs4Me

DSOs4Me

    Messenger

  • *****
  • Posts: 457
  • Joined: 20 Jan 2017
  • Loc: New Jersey

Posted 29 June 2020 - 11:43 PM

If that shot isn't zoomed in then I would say you are out of focus



#5 dontsayflavor

dontsayflavor

    Lift Off

  • -----
  • topic starter
  • Posts: 4
  • Joined: 30 Apr 2020
  • Loc: The Biggest Little City, USA

Posted 30 June 2020 - 11:10 AM

Show us full frame - or all corners.

 

 

Looks like you are out of focus, or that combined with some other optical aberrations. If that sample is toward the corner of your field then the field flattener may not be working very well. But, it's somewhat unlikely that you will get small and perfectly circular stars right into the corners of an APS-C format with any given field flattener (some will be better than others -- but it shouldn't be as bad as you've shown).

 

 

If that shot isn't zoomed in then I would say you are out of focus

 


 

Thanks for the input.  Here's the full frame shot:

 

M57 Stacked Lpc Cbg St Fin
 
For the focusing I used a 3d printed Bahtinov mask and thought I got it pretty darn close, but I'll have to double check that next imaging session.  I do see some of the out-of-roundness in the corners, but it's not nearly as noticeable to me as the red discoloration at the center of almost all of the stars in the image.  If it is not remedied by better focusing, it sounds like I may want to try shooting with a different camera, or are you saying that somehow the field flattener may be less than ideal? It's also a SV.


#6 Tapio

Tapio

    Mercury-Atlas

  • -----
  • Posts: 2,835
  • Joined: 24 Sep 2006
  • Loc: Tampere, Finland

Posted 30 June 2020 - 12:24 PM

Okay, the center looks pretty good and edges a bit soft.

It might be the best you cet with that camera (some reducer/flatteners have hard time flattening APS-C sensor cameras).

But try if you can get better focus next time.



#7 james7ca

james7ca

    Cosmos

  • *****
  • Posts: 8,518
  • Joined: 21 May 2011
  • Loc: San Diego, CA

Posted 30 June 2020 - 06:30 PM

It's difficult to tell with the small scale of the full frame example you provided. I'd just post a small 1:1 crop of the center and the two worst edges (or all four edges).



#8 dontsayflavor

dontsayflavor

    Lift Off

  • -----
  • topic starter
  • Posts: 4
  • Joined: 30 Apr 2020
  • Loc: The Biggest Little City, USA

Posted 13 July 2020 - 10:14 PM

Okay, the center looks pretty good and edges a bit soft.

It might be the best you cet with that camera (some reducer/flatteners have hard time flattening APS-C sensor cameras).

But try if you can get better focus next time.

 

 

It's difficult to tell with the small scale of the full frame example you provided. I'd just post a small 1:1 crop of the center and the two worst edges (or all four edges).

Thanks again for the advice.  I managed to get out again last night and try some new things out, both a different camera and more manual focusing fun.  It looks like much of the problem was attributed to poor focusing, but still getting some misshapen stars on the edges (will post soon, trying to figure out how to put together an image that shows center and corners without pixinsight or other astro program).  That said, am I correct in assuming that manually focusing with a dslr with no computer connection is more of an experiment in trial and error?  I've found that even with the Bahtinov mask, focusing on the small camera screen can be pretty challenging.  Any recommendations here? 

 

Also, you mention sensor size/ FF and it's effect on star shape, at least in regards to the crop sensor cameras.  In the not to distant future I hope to get an ASI183MM, as it seems a good fit for the scope and has room to grow.  I don't know if the solution is solved by throwing more money at it, but I also don't want to disappoint myself if I end up with the same or similar issues with the dedicated cam...



#9 dontsayflavor

dontsayflavor

    Lift Off

  • -----
  • topic starter
  • Posts: 4
  • Joined: 30 Apr 2020
  • Loc: The Biggest Little City, USA

Posted Yesterday, 12:41 PM

It's difficult to tell with the small scale of the full frame example you provided. I'd just post a small 1:1 crop of the center and the two worst edges (or all four edges).

 

Here's a different image, different APS-C camera, in focus, but with same star shapes.  To me, stars look round on left and change to more egg shape the farther right you look in the picture.
 

StarTest1



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