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Measuring ADU values for flats


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hello there 

I've been searching the interweb with no success, i need to find a program that will measure the adu of my flats which isn't too confusing.

i'm currently exposing to half way on the histogram on the lcd display of the dslr(canon 1300d)

I've read that this doesn't work correctly and i should be basing how far to expose using ADU values, i have no idea how to find these.

I use backyard eos for capturing lights and darks but use the dlsr functions for flats.

any help will save me pulling my hair out, thanks haha

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Hi Dave - This link talks about taking flats with a DSLR and also mentions adjusting the EV setting as part of the process.  I use PixInsight which has a statistics process, but unless you were thinking about getting PixInsight, you might want to try the EV adjustment mentioned in the article and see what the correction looks like in DeepSkyStacker.

Taking proper Flat frames with your DSLR (myastroscience.com)

Thanks,
Rob

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9 hours ago, rbeard47 said:

Hi Dave - This link talks about taking flats with a DSLR and also mentions adjusting the EV setting as part of the process.  I use PixInsight which has a statistics process, but unless you were thinking about getting PixInsight, you might want to try the EV adjustment mentioned in the article and see what the correction looks like in DeepSkyStacker.

Taking proper Flat frames with your DSLR (myastroscience.com)

Thanks,
Rob

Thanks for the link, shame it mentions pixinsight as damn thing requires me not to use a Hotmail account haah gonna try use apt flat aids. New software but from the sounds of it, it's better than backyard eos. 

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Try IRIS (it's free), you just hover over any pixel and it gives you its ADU value and it will also give you the mean, median, max and min for the whole image, but it's so old (latest version is 2010'ish) that apart from me, nobody seems to use it these days. It seems to be the precursor to SIRIL (IRIS spelled backwards + L for Linux?)

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Why not just use SiriL? That's being actively developed. And is free.

ASTAP is another very complete, free package with which you can analyze your images. I told it to analyze a recent set of my flats, screen shot of the result below. "Background" is the figure of merit.

And of course if you were using KStars/Ekos you could set a target ADU value and the flats-capturing module would cycle through exposures looking for one that yielded the value  you sought :-).

 

Screen Shot 2021-01-02 at 1.08.54 PM.png

Edited by rickwayne
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Whatever software you use you need to be in the right region of ADU values. CCD and CMOS have 4 main regions. With increasing signal the they are, read noise limited, shot noise limited , PRNU limited and non-linear (saturated).

 For flats you want to be in the PRNU region where the error is proportional to the signal level. So you want as high an adu you can get while the response is still linear I.e. no saturation. 

Regards Andrew 

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On 02/01/2021 at 19:00, rickwayne said:

Why not just use SiriL? That's being actively developed. And is free.

ASTAP is another very complete, free package with which you can analyze your images. I told it to analyze a recent set of my flats, screen shot of the result below. "Background" is the figure of merit.

And of course if you were using KStars/Ekos you could set a target ADU value and the flats-capturing module would cycle through exposures looking for one that yielded the value  you sought :-).

 

Screen Shot 2021-01-02 at 1.08.54 PM.png

i checked out that ASTAP software, analysed some of my recent flats and the 'background' level is only around 2500, im assuming thats the ADU?

is this really low or am i missing something here, can see all the dust spots really well though.

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On 01/01/2021 at 18:41, Dave2292 said:

histogram on the lcd display

Hi

I wonder if you may have read too much into it. 

Point your optics at an evenly lit wall/lightbox/sky and partly depress the shutter button with the 1300 set on Av. Adjust the intensity of the light until you get around a 3s exposure indicated. Now fire 12 or so frames. That's it.

Cheers

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Yes, 25000 is not a magic number or anything. The idea is to have both ends of the histogram (except for hot and cold pixels) in the linear range of the sensor. There are "ends" because the intensity does vary across the frame due to vignetting, dust, or whatever (otherwise you wouldn't need flats!). The precise values don't matter at all. And of course the acid test is whether they work.  If you stretch the flats, you can see where the image-train problems are. If your images don't display those (or artifacts from their removal) after processing , then your flats  are just fine.

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Here's one of mine, for example. Straight out of the camera, the actual values only vary from a little over 25000 to a little over 24000 over the whole range, and it looks perfectly even.
661246013_ScreenShot2021-01-09at7_18_13PM.thumb.png.7c0ecc2e2791d78dd95e0e8ea93ee161.png
Stretched, you can see where there's a whacking great splotch of random snot, and the vignetting is  pretty obvious, especially  on the right side. 1805757911_ScreenShot2021-01-09at7_06_30PM.thumb.png.fdb89d1baec0e72b78b29cf42c73b369.png

None of the vignetting shows up in the final. 

Edited by rickwayne
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  • 3 weeks later...

hello guys , sorry for not replying in a while but ive started doing my flats using the AV flat mode on backyard eos and using an art light box instead of my laptop, seem to be getting better results doing this instead of using the histogram on the back of the dslr. thanks everyone though

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On 01/01/2021 at 17:41, Dave2292 said:

hello there 

I've been searching the interweb with no success, i need to find a program that will measure the adu of my flats which isn't too confusing.

i'm currently exposing to half way on the histogram on the lcd display of the dslr(canon 1300d)

I've read that this doesn't work correctly and i should be basing how far to expose using ADU values, i have no idea how to find these.

I use backyard eos for capturing lights and darks but use the dlsr functions for flats.

any help will save me pulling my hair out, thanks haha

I would try Flats wizard in NINA. 

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