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Old 11-04-2011, 09:15 PM
Ross G
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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.
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  #2  
Old 15-04-2011, 05:18 PM
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Cloudyagain (Neale)
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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.
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Old 15-04-2011, 07:10 PM
Ross G
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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.
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Old 18-04-2011, 05:18 PM
Hagar (Doug)
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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.
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Old 18-04-2011, 05:21 PM
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multiweb (Marc)
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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.
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Old 18-04-2011, 06:24 PM
Ross G
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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.
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Old 18-04-2011, 06:28 PM
Ross G
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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
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Old 18-04-2011, 06:34 PM
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multiweb (Marc)
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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.
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Old 18-04-2011, 06:54 PM
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tlgerdes (Trevor)
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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.
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Old 21-04-2011, 07:55 AM
Hagar (Doug)
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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.
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Old 22-04-2011, 05:27 AM
Ross G
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Thanks Doug,

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

Ross.
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  #12  
Old 05-05-2011, 04:39 AM
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mostschaedel (Gerald)
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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
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  #13  
Old 10-05-2011, 07:47 PM
Ross G
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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.
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