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

Bias/dark frames - color tint - light leak through multiple blankets?

  • Please log in to reply
6 replies to this topic

#1 maxsid

maxsid

    Ranger 4

  • *****
  • topic starter
  • Posts: 389
  • Joined: 11 Sep 2018
  • Loc: Sunnyvale, CA

Posted 23 January 2021 - 10:12 PM

Hello,

I started to create a bias/dark library today.

I used to collect my bias/dark frames only if I needed a new set and was doing this outside after sunset.

 

This time I decided to collect the whole library in one run for different gains/offsets/exposures indoors.

What I notices is that indoors all bias/dark fames had a significant blue tint.

 

This was pretty surprising as the scope was covered with a cover cap and several blankets.

 

The tint was there even for very short exposure bias frames.

 

Does this tint even matter? (I think it does).

Can the day-light really penetrate cover/blankets? (looks like it really can).

 

Master bias shot in the dark (scope cover cap and additional tarp on too)

bias-outside.jpg

 

Master bias shot indoors during daytime (scope cover cap and additional tarp and two thin blankets on)

bias-inside.jpg



#2 bobzeq25

bobzeq25

    ISS

  • *****
  • Posts: 23,451
  • Joined: 27 Oct 2014

Posted 23 January 2021 - 11:55 PM

Basic mistake.  Those histograms are "stretched" data.  The program you're using automatically stretches the data as far as it needs to so you can see something.  The appearance of a light link may be only due to that.  Probably is.

 

What you want to look at is the linear histogram.  For bias, it will be just about a vertical line, separated from the lefthand edge only by the camera offset, often 300-600 ADU.  Not a gaussian curve unless you expand the x-axis.

 

Suitable programs are IRIS and PixInsight.  There are others.

 

I stick the lens cap on and throw my regular telescope over the setup.  Works fine with an astro specific camera, with a DSLR you'll want to get a viewfinder cover, light can leak in that way.

 

https://www.amazon.c.../dp/B06XSJGJB5/


Edited by bobzeq25, 23 January 2021 - 11:58 PM.


#3 DJL

DJL

    Messenger

  • -----
  • Posts: 448
  • Joined: 15 Aug 2020

Posted 24 January 2021 - 11:00 AM

Did you ever do film and paper photo processing? Would blankets have been good enough to make a darkroom? No, you needed a completely dark room with no light leaks around doors and windows. You could use a red light for the print, but for film you needed total darkness.

 

I used a dark filter in a filter wheel plus the telescope end cap and took darks in the daylight. The enclosed is a screenshot from Mac Observatory's finder FITS preview, which is autostretched, and hence has a lot of color noise but it does show the light leak. I used these darks and ended up with weird burned out parts in the resulting stacks.

So I still use my filter wheel for darks, but I do so at night, automated at the end of the light run, still at night, or switch over to darks when clouds appear. With a cooled camera I can save darks for different bias and exposure length.

 

Attached Thumbnails

  • Dark in th Daylight.jpg


#4 maxsid

maxsid

    Ranger 4

  • *****
  • topic starter
  • Posts: 389
  • Joined: 11 Sep 2018
  • Loc: Sunnyvale, CA

Posted 24 January 2021 - 08:28 PM

Yes I did and quite a lot. Photo-plates too. Before CCD cameras it was a big part of my professional activity.

 

Did you ever do film and paper photo processing? 


Edited by maxsid, 24 January 2021 - 08:54 PM.


#5 maxsid

maxsid

    Ranger 4

  • *****
  • topic starter
  • Posts: 389
  • Joined: 11 Sep 2018
  • Loc: Sunnyvale, CA

Posted 24 January 2021 - 08:44 PM

Oh, no. You got me wrong.

I understand histograms and stretching well.

 

What surprised me was that the light really goes through the end cap, blankets and tarp in a dimly lighted room.

What is even more surprising - just a cap and the cap plus multiple additional layers of light protection produce almost identical results.

 

I learned my lesson and made my conclusions.

 

My questions were not about what's going on or what to do but:

Is this possible? Yes.

Does it matter? I think - yes. It has to  be done in the dark.

 

Thank you!

 

 

Basic mistake.  Those histograms are "stretched" data.  


Edited by maxsid, 24 January 2021 - 09:17 PM.


#6 ChrisWhite

ChrisWhite

    Fly Me to the Moon

  • *****
  • Posts: 6,357
  • Joined: 28 Feb 2015
  • Loc: Northeast USA

Posted 24 January 2021 - 08:54 PM

Looks like you figured it out, but I was going to suggest you could have light entering through the optical viewfinder as well....



#7 maxsid

maxsid

    Ranger 4

  • *****
  • topic starter
  • Posts: 389
  • Joined: 11 Sep 2018
  • Loc: Sunnyvale, CA

Posted 24 January 2021 - 09:16 PM

Oh.. guys. Sorry. I got right results but interpreted them wrong.

It was the latest software update which screwed things up - reverted my white-balance in ZWO camera driver back to default.

Mine was WB_R=50 WB_B=50, default is WB_R=52 WB_B=95 which produces twice as much blue.




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