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

Focus, Wind, Tracking, or Something Else?

astrophotography imaging refractor
  • Please log in to reply
4 replies to this topic

#1 tknana

tknana

    Lift Off

  • -----
  • topic starter
  • Posts: 12
  • Joined: 01 Sep 2019

Posted 19 November 2019 - 05:51 PM

Hello,

 

Tried to shoot the Rosette Nebula a few nights back and was having focus or wind issues...not sure. See attached.

 

To me, it looks as if I'm having focus issues. However, I tried the same exact frames at 30s and 60s and seemed to be getting better, sharper images (HFR's as low as 1.02 at 30s, not 3.05 like you see below). Sadly, didn't save those subs for comparison to show you.

 

So, I'm wondering if the length of the exposures was the issue and wind was affecting it? Wouldn't wind issues show streaks or star trails, not blobby stars?

FYI, wind was light, but gusts of maybe 7-8mph from time to time.

 

Questions:

 

- Could wind cause the issues you see in that sub?

 

- If it was focus (not wind), why were the shorter subs better than the longer? Can the focuser creep out of focus over the course of a 3m sub? And if so, why did subsequent 30s subs seem to look okay?

 

Thanks for any help or thoughts,

Temu

Attached Thumbnails

  • Rosette Issue - 1.jpg


#2 kathyastro

kathyastro

    Viking 1

  • *****
  • Posts: 973
  • Joined: 23 Dec 2016
  • Loc: Nova Scotia

Posted 19 November 2019 - 06:03 PM

On first guess, I would say focus.  If you are sure that's not it, then wind could do it, though it would take an astonishingly uniform pattern of wind-induced mount vibration to keep the stars that circular and even.  A focus issue is more likely.

 

Focus can and will creep with changes in temperature.  With my 8" Newt, a 0.5 degree Celsius drop in temperature is enough to give blobby stars.  Focus can also change due to gravity acting on the camera load, if the focuser has any play or slip at all.



#3 John Miele

John Miele

    Soyuz

  • *****
  • Posts: 3511
  • Joined: 29 May 2005
  • Loc: North Alabama

Posted 19 November 2019 - 07:12 PM

I'm going to say virtually certain you are out of focus. If you look carefully at the fainter stars you can just begin to see a dark spot in the center. Just like when you defocus on purpose and see rings...Maybe a little drift thrown in as well...


  • tknana likes this

#4 tknana

tknana

    Lift Off

  • -----
  • topic starter
  • Posts: 12
  • Joined: 01 Sep 2019

Posted 19 November 2019 - 09:16 PM

On first guess, I would say focus.  If you are sure that's not it, then wind could do it, though it would take an astonishingly uniform pattern of wind-induced mount vibration to keep the stars that circular and even.  A focus issue is more likely.

 

Focus can and will creep with changes in temperature.  With my 8" Newt, a 0.5 degree Celsius drop in temperature is enough to give blobby stars.  Focus can also change due to gravity acting on the camera load, if the focuser has any play or slip at all.

Thanks for the feedback... I assume that it must be focus and I'll check my focuser to see if there's any play or slip in it. That said, I've been able to do 3m, 5m, and 10m subs before without an issue. Wonder what changed...

 

Additionally, I don't understand why the 30s and 60s subs were in focus while the 3m subs weren't? IIRC, I went back after the first few 3m subs and tried 30s again and it seemed to be fine. If focus was slipping during the course of the 3m exposure, surely the follow-up 30s subs would have been blobby too? Does focus somehow change with length of exposure?

Guess I'll just have to trouble-shoot it the next time I can get out. 



#5 Der_Pit

Der_Pit

    Viking 1

  • *****
  • Posts: 976
  • Joined: 07 Jul 2018
  • Loc: La Palma

Posted 20 November 2019 - 07:16 AM

Yes, clearly defocus, with those uniform disks instead of star peaks.

So either look for slip in your focuser, or check the temperature.  Could have been you started imaging before the OTA was temperature stabilized, or even a weather change with cold air rushing in...




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





Also tagged with one or more of these keywords: astrophotography, imaging, refractor



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics