#1  
Old 11-04-2011, 09:15 PM
Ross G
Registered User

Ross G is offline
 
Join Date: Apr 2007
Location: Cherrybrook, NSW
Posts: 4,996
3 Humps on my histogram??

Hello,

I have just started using a QHY8L ccd camera, having upgraded from a Canon 1000d.

Consequently, I have also started taking flat and bias frames.

The camera's first real test was 3 nights of imaging at Coonabarabran in March. It worked well and I have been processing and experimenting with the images as I learn.
I have noticed that applying flats and bias frames gives my images an unsatisfactory colour shift as compared to not using flats and bias frames. For example, my shots of the Horse Head Nebula have a magenta cast whereas without the flats and bias' the reds are red and look more natural.
When I looked at the histogram of the flat frames, they have 3 distinct humps.
Is this normal?
I used a light box to produce the flats.

Thank you.


Ross.
Reply With Quote
  #2  
Old 15-04-2011, 05:18 PM
Cloudyagain's Avatar
Cloudyagain (Neale)
Registered User

Cloudyagain is offline
 
Join Date: Sep 2009
Location: Tower Hill
Posts: 178
I have a OSC CCD camera and a light box which is illuminated by LED's. I have 3 humps on the histogram of my flat fields. This is presumably related to the emission spectrum of the LED's. White light LED's are basically blue LED's with a phosphor coating that is "excited" by the blue light and then emits further light in the red and green bands. I dont know why your images come out with colour shifts though. If the flats are applied to the light images before Bayer interpolation then the 3 humps shouldn't matter. What software are you using? Images Plus does a good job for me with my OSC camera (SBIG).

Neale.
Reply With Quote
  #3  
Old 15-04-2011, 07:10 PM
Ross G
Registered User

Ross G is offline
 
Join Date: Apr 2007
Location: Cherrybrook, NSW
Posts: 4,996
Hi Neale,

Thanks for your reply. It's good to know the 3 "humps" are normal.

I captured with the Ezycap software that came with the QHY8L camera. I then used DeepSkyStacker to debayer, register and stack. DeepSkyStacker was also used to apply the flat and bias frames.
This was my first serious outing with the camera and the first time I have used flats and bias frames.
The colours were different with and without flats and bias frames.

When I get more time I think I will try Nebulosity to see if the results are better.

Thanks.

Ross.
Reply With Quote
  #4  
Old 18-04-2011, 05:18 PM
Hagar (Doug)
Registered User

Hagar is offline
 
Join Date: Oct 2006
Location: Australia
Posts: 4,646
Hi Ross, You will probably find each of the humps corresponds to one of the 3 base colours. Using the levels control in Photoshop and realigning each colour may well correct you colour problems and stop your humping problem.

I hope this helps.
Reply With Quote
  #5  
Old 18-04-2011, 05:21 PM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Hinchinbrook
Posts: 18,233
Just a thought. If your light is too strong you'll get internal reflections and concentric rings/discs in your flats. Might be part of the problem.
Reply With Quote
  #6  
Old 18-04-2011, 06:24 PM
Ross G
Registered User

Ross G is offline
 
Join Date: Apr 2007
Location: Cherrybrook, NSW
Posts: 4,996
Hi Doug,

This is my first serious attempt with flats and bias frames.
Previously using a DSLR I did not see a need but now with the QHY8L it seems essential.

Will I need to do this with each of the flats?

Thanks for your advice.


Ross.
Reply With Quote
  #7  
Old 18-04-2011, 06:28 PM
Ross G
Registered User

Ross G is offline
 
Join Date: Apr 2007
Location: Cherrybrook, NSW
Posts: 4,996
Hi Marc,

I did see ring reflections in my light frames of the Horse Head, probably due to the brightness of Alnitak.

What would be the lowest exposure that I could use for the flat frames?

Thanks and hopefully see you on Wednesday.

Ross
Reply With Quote
  #8  
Old 18-04-2011, 06:34 PM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Hinchinbrook
Posts: 18,233
Quote:
Originally Posted by Ross G View Post
Hi Marc,

I did see ring reflections in my light frames of the Horse Head, probably due to the brightness of Alnitak.

What would be the lowest exposure that I could use for the flat frames?

Thanks and hopefully see you on Wednesday.

Ross
If you dim your lightbox then you can lengthen your exposures. Dim you lightbox until you don't get any reflections and shoot longer flats. I sometime shoot 8s flats with a Ha filter.
Reply With Quote
  #9  
Old 18-04-2011, 06:54 PM
tlgerdes's Avatar
tlgerdes (Trevor)
Love the moonless nights!

tlgerdes is offline
 
Join Date: Mar 2009
Location: Sydney
Posts: 2,280
Hi Ross,

Can you paste a screen shot of your histogram?

What did you use to create your flats?

Also bring them along on to Wednesdays meeting if you are coming.
Reply With Quote
  #10  
Old 21-04-2011, 07:55 AM
Hagar (Doug)
Registered User

Hagar is offline
 
Join Date: Oct 2006
Location: Australia
Posts: 4,646
If we are talking Flats then the problem isn't really a problem it may well be a hot spot in the flat or just a dust mote which is causing this to happen.
The one thing which must be taken is darks or bias frames. Without these the flats will be near useless.
If we are talking the light frames after flats have been used to calibrate then I suggest you just continue to combine then straighten out the colours after calibration and combination.

Using RAW CCD images is a little different to using a DSLR which does a lot of processing of each frame before you even see it. Remeber the CCD image is RAW, Very RAW and needs a little more massaging but gives much improved overall results.

Flats can be a bugger to get just right. Keep at it.
Reply With Quote
  #11  
Old 22-04-2011, 05:27 AM
Ross G
Registered User

Ross G is offline
 
Join Date: Apr 2007
Location: Cherrybrook, NSW
Posts: 4,996
Thanks Doug,

I'll keep on experimenting. From the results I have seen so far I think it will be worth the effort.

Ross.
Reply With Quote
  #12  
Old 05-05-2011, 04:39 AM
mostschaedel's Avatar
mostschaedel (Gerald)
mostschaedel

mostschaedel is offline
 
Join Date: Mar 2007
Location: Ebreichsdorf Austria
Posts: 60
Hi Ross!
had exactly the same challenge when moving from DSLR to QHY8.
Also ImagePlus and all other Calibration SW did not work well on that.

Found a way to make out of the 3 bumps on the RAW image only one bump on the Raw image.
Simply, in Astroart or PixInsight move the raw flat 0.5 pixel up and
0.5 right (keeping the image size).
This averages the 3 bumps to one bump.
Histogram Action can only help on Colour images not
on bw raw images. And you want to calibrate the bw raw images
of your colour camera.

Hope this helps.
Gerald
Reply With Quote
  #13  
Old 10-05-2011, 07:47 PM
Ross G
Registered User

Ross G is offline
 
Join Date: Apr 2007
Location: Cherrybrook, NSW
Posts: 4,996
Hi Gerald,

My apologies, I only saw your reply today.

Thank you for the advice. I will definetely give it a try.


Thank you.


Ross.
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 03:07 AM.

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