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First ever CCD image - Now how to check it?

swag72

By swag72
in Imaging - Discussion

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swag72

swag72

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I'm a little stuck here and not sure what to do now or how to do it.

I've taken my first CCD image and have done a screen grab below. What I don't understand at the moment is how to check it. For example, with my DSLR I looked at it in the Canon software and could see whether it was well exposed etc. Here, I don't know what I am looking for.

Is there some magical figure I am looking for somwhere to tell me it's OK?

Also, can someone suggest a nice FITS viewer that is easy to use and will give me all the information I need? Perhaps Maxim will, but I don't really know where to look or what I'm looking for.

I'm plugging away at these, so hope someone can help before I waste my whole evening!!!!

post-18339-13387768117_thumb.jpg

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Merlin66

Merlin66

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Sara,

I don't use MAXIM, but there must be an interactive help menu which will show you how to display the .fits header info and to display histograms...

There's no ideal exposure with the CCD. It's just a matter of building up as much signal data as you can - more signal...less noise.

Did you take some flats and darks?

When you've collected a series of lights, there will be a process set up within Maxim to allow you semi-automatically stack them and apply both the darks and flats for corrections.

HTH

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swag72

swag72

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Cheers Ken - No flats and darks taken at this time - Figured I wouldn't get too hung up on it all as I am using the scope with the problem focuser! So, I am looking for .fits header info - Anything specific on there numbers wise? I did see .fits header info on Maxim, but a few numbers made no sense to me.

Regarding histogram - If I find it - Will it be the same as the DSLR in principal?

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NickK

NickK

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Usually applications have a maximum value field which shows the maximum value of a pixel in the image. For a 16bit CCD image, the maximum value a pixel can have is 655535 (the minimum being 0).

A histogram would provide you with spread distribution however I usually go for the look rather than histogram graphs!

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Merlin66

Merlin66

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The .fits header, depending on how you have set up the preferences, only gives the basic stuff like when, duration, location, scope etc.....

The histogram will look "similar" to the DSLR....if your camera is a 16bit camera the maximum "well depth" is around 65K ADU, but most sensors loose linearity above 70%, so a max. peak around 45K ADU would be a good start.

You'll probably find the background light pollution starts to creep up long before this figure is reached....

5 min subs would be a good start (if you can)

HTH

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swag72

swag72

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What I am stuggling with a little is the following.

With the CCD image, I have a whole host of screen stretches - So, depending on how I do the screen stretch, depends on what I see on screen, also seems to affect the histogram. Is this changing the actual image? How do I know how I should do the screen stretch? If I can change the histogram with this stretch, how do I know what to look at? Does that make sense? I'd rather look at some figurs to be honest.

Had a look at an image from last night and I've got the following figures.

Min 215

Max 65535

Mean 2645

So obviously I've got some stars that are fully saturated, but as my min isn't down to zero, that must be good? Should I just look at this for starters and sdjust my exposures so that my max is about 45000?

You say about a 5 min exposure - This was only 2 mins in the red channel as when I tried 5 initially I had a totally white screen.

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Merlin66

Merlin66

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In AA5 the screen stretch is just that....the view on the screen. It doesn't change the raw data. You need to apply curves (or similar) to actually change the image.

The max may be only showing you a hot pixel!

Look at where the bulk of the histogram is sitting.....

(Again in AA5 I can move the cursor around the image and see what the intensity values are - you can check out the image "detail" - I also have a "profile" function which allows you to take a slice through the image at any orientation to see the peaks and background...there must be something similar in MAXIM....time to read the manual....)

(For comparison - for faint spectra we sometimes need a 10 stack of 5min exposures to find much above the noise....)

HTH

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dph1nm

dph1nm

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How would you measure that?
I knew someone would ask that, because I don't know the (practical) answer! What you need is a software package which will let you measure the rms (root-mean-square) of a selected area of background pixels on your image (which gives you sky noise combined with read noise) , and on a bias image (which gives you just the read noise). You can then do a bit of maths to work out how much the read noise is affecting things:

e.g. suppose your bias rms was 10, and your image rms was 40. Then the rms from the sky alone would be sqrt(40 ^2-10^2) = 38.7. i.e. the read noise is adding about 3% - hardly worth bothering about - you are correctly exposed.

But now suppose your bias rms was 10, and your image rms measured only 15 - then the same calculation gives a sky noise of 11.2. i.e. the read-noise is increasing this by ~34% - not so good - definitely worth exposing for longer.

NigelM

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