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Help with a flatfield


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Hi all, I'm relatively new to advanced astrophotograpy and I'm a little unsure about a flat field image that i am getting with my telescope. It just doesn't look like most flat fields I see other people getting. Any idea whats going on or what I am doing wrong? Cheers

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10 minutes ago, Marstiphal said:

Hi all, I'm relatively new to advanced astrophotograpy and I'm a little unsure about a flat field image that i am getting with my telescope. It just doesn't look like most flat fields I see other people getting. Any idea whats going on or what I am doing wrong? Cheers

Got a picture ?

Dave

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There's a few things you can include to help us help you. How long is the exposure? What are you using for a light source? Have you got a histogram we can look at? perhaps tell us the steps you took in producing flats. Can see some dust bunnies but I agree with you, doesn't look quite right. At a guess, I'd say the flats might be too short. the peak of the graph should be approx. 1/3 from the black end of the graph

 

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What software are you using to capture with, some ( Maxim for instance ) only displays black and white points whereas Artemis capture displays a full range of grey scale.

It could be OK if it matches the light frames, have you got something to measure the pixel values, you need to aim for an average of about a third of the value between 0 and 65000.

Dave

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Thank you for your replies. The image was taken using Maxim DL6 through a Williams optics 132 and using an Aurora Flatfield panel as the light source (simply placed on the end of the telescope - not too sure if that is a good solution or too close etc.). It was an exposure of 0.02s as in testing that gave me a roughly half well depth ADU for my camera (Atik 4000 - 24,400 ADU so flat fields @ 12,200 ADU). The flat fields average at ~ 12,200 ADU, would this be correct?

11 hours ago, Davey-T said:

Measured your JPEG so only 0 to 256, it's average value is around 55, needs to be around 85 so it's under exposed.

Dave

This was just a dirty screen grab - not sure if that makes a difference or not? I'm also not too sure what the averages are you are stating or how they relate to my image (sorry probably obvious)

 

11 hours ago, Scott said:

There's a few things you can include to help us help you. How long is the exposure? What are you using for a light source? Have you got a histogram we can look at? perhaps tell us the steps you took in producing flats. Can see some dust bunnies but I agree with you, doesn't look quite right. At a guess, I'd say the flats might be too short. the peak of the graph should be approx. 1/3 from the black end of the graph

 

would this be what you are looking for?

lnsferG.jpg

 

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5 hours ago, Davey-T said:

They need to be somewhere between 20000 and 30000.

Dave

Ah maybe I have my maths back to front then. for an Atik 4000 with a Well depth of 40,000 electrons and a gain Factor of 0.61e-/ADU I would have a pixel saturation level of

 

40,000/0.61

= 65,573 ADU full saturation

 

meaning 30000 ADU for flats? I think I may have been multiplying the numbers rather than dividing them giving me a lower number than I should have - I'll go back and give that a go and see if i get better results. Many thanks for all your help.

 

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16 hours ago, Davey-T said:

Use the info thingy in Maxim to measure your flats.

Dave

Yeah it says roughly 12000 and like you said, it should be more like 30000. Next stop, back to the observatory to test longer exposure flats! Thanks again for the help

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