#1  
Old 26-10-2019, 04:30 PM
Ant0nio's Avatar
Ant0nio (Tony)
Quid Clarius Astris

Ant0nio is offline
 
Join Date: Apr 2018
Location: Gympie,Qld. Australia
Posts: 228
Any ideas on this artifact?

Hi all, I shot a sequence of 12 frames of M78 last night & this gradient showed up after pre processing. It's in every frame. I've deliberately over stretched the image to make it obvious.
The night before I shot the same sequence, same setup, same target & no artifacts were present. Details are:

12 x 600sec frames, bias & darks subtracted
Scope - William Optics FLT98
Focal reducer/flattener - William optics AFR IV
Camera - SBIG 8300C
Filter - Baader IR UV

Any ideas what has caused it?
Thanks in advance,
Tony
Attached Thumbnails
Click for full-size image (M78 _600x12 FLT98 FRFF@F5 Baader IRUV Filter SBIG 8300C.jpg)
199.6 KB92 views
Reply With Quote
  #2  
Old 26-10-2019, 06:53 PM
Merlin66's Avatar
Merlin66 (Ken)
Spectroscopy Wizard

Merlin66 is offline
 
Join Date: Oct 2005
Location: St Leonards, Vic
Posts: 7,076
Top left of frame?
There's a spectrum and a secondary reversed spectrum usually caused by extraneous light and a glass edge.....
Reply With Quote
  #3  
Old 26-10-2019, 09:56 PM
Ant0nio's Avatar
Ant0nio (Tony)
Quid Clarius Astris

Ant0nio is offline
 
Join Date: Apr 2018
Location: Gympie,Qld. Australia
Posts: 228
Quote:
Originally Posted by Merlin66 View Post
Top left of frame?
There's a spectrum and a secondary reversed spectrum usually caused by extraneous light and a glass edge.....
Hi Ken, thank you for your expert analysis, I spent hours trying to find some clues to this on the internet to no avail. Your description fits though,
I thought it might have been a finger print on the filter so I took it off to clean it & noticed the glass rattled a bit. The retaining ring had loosened,
so maybe my local light pollution (neighbors floodlights) caught an edge of the filter glass.Sounds feasible.
I think this issue is resolved thanks to your wisdom.
Many thanks,
Tony
Reply With Quote
  #4  
Old 27-10-2019, 02:36 PM
ericwbenson (Eric)
Registered User

ericwbenson is offline
 
Join Date: Sep 2009
Location: Adelaide, Australia
Posts: 200
Hi Tony,

What you have there is diffraction of an off-axis star by the anodized barrel of a tube inside of your OTA/focuser/imaging train. Starlight impinging at grazing angle on the somewhat smooth wall of a tube (with microgrooves from machining) produces that effect. You can possibly see it with your naked eye by observing the daytime sky (or a bright screen in a dark room) without the camera/filter wheel in place. (don't look anywhere near the sun obviously).

I had to solve this problem with my telescope. See the bottom of this thread here:
http://www.iceinspace.com.au/forum/s...d.php?t=105990


Good luck,
EB
Reply With Quote
  #5  
Old 27-10-2019, 08:34 PM
Ant0nio's Avatar
Ant0nio (Tony)
Quid Clarius Astris

Ant0nio is offline
 
Join Date: Apr 2018
Location: Gympie,Qld. Australia
Posts: 228
Quote:
Originally Posted by ericwbenson View Post
Hi Tony,

What you have there is diffraction of an off-axis star by the anodized barrel of a tube inside of your OTA/focuser/imaging train. Starlight impinging at grazing angle on the somewhat smooth wall of a tube (with microgrooves from machining) produces that effect. You can possibly see it with your naked eye by observing the daytime sky (or a bright screen in a dark room) without the camera/filter wheel in place. (don't look anywhere near the sun obviously).

I had to solve this problem with my telescope. See the bottom of this thread here:
http://www.iceinspace.com.au/forum/s...d.php?t=105990



Good luck,
EB
Thank you Eric, that is rather conclusive. The artifacts in the two images are virtually identical. I'm using a set of the cheap Chinese spacers for back focus & the inside surfaces are really quite glossy,
I'll have a go at coating them with flat black & failing that replace them with some of good quality.
Many thanks Eric,
Tony
Reply With Quote
  #6  
Old 27-10-2019, 08:46 PM
Ant0nio's Avatar
Ant0nio (Tony)
Quid Clarius Astris

Ant0nio is offline
 
Join Date: Apr 2018
Location: Gympie,Qld. Australia
Posts: 228
My sincere thanks to both Ken & Eric for taking the time to help me resolve this. Truly a testimony to the value of the collective wisdom available via the IIS forums.
Even though the insight offered by Erics' experience seems to have identified the cause, Kens' contribution has undoubtedly helped me avoid another lurking issue.
Thanks again to both of you & clear skies.
Best,
Tony
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 03:36 PM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Advertisement
Celestron Australia
Advertisement
Bintel
Advertisement
Lunatico Astronomical
Advertisement
Meade Australia
Advertisement
NexDome Observatories
Advertisement
SkyWatcher Australia
Advertisement
OzScopes Authorised Dealer
Advertisement
Astronomy and Electronics Centre
Advertisement