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Questions & Answers/Tips & Tricks => Image Processing & Related Software => Topic started by: johnadams on 19:50:58, 08 November, 2017

Title: Flat analysis
Post by: johnadams on 19:50:58, 08 November, 2017
Hi

Can anyone tell me if this flat looks normal.

Thanks

John
Title: Re: Flat analysis
Post by: Einari on 20:18:13, 08 November, 2017
Not quite as you have something blocking (bottom) in light path.
Flat should be symmetrical.
Can you tell about your imaging setup ?
Title: Re: Flat analysis
Post by: johnadams on 20:25:06, 08 November, 2017
Hi

Using a Veloce Rh200 with QSI 683 wsg . All flats from LRGB filters look like this. Average ADU is 30,000 . This is  a master flat from 30 frames stacked with Pixinsight.
Title: Re: Flat analysis
Post by: Eccoskies on 20:57:13, 08 November, 2017
Are you using an OAG John?

Mark
Title: Re: Flat analysis
Post by: johnadams on 21:03:06, 08 November, 2017
Hi

Yes I am but I am sure its not the prism casting a shadow.
Title: Re: Flat analysis
Post by: Eccoskies on 21:14:43, 08 November, 2017
John,

I have a similar shadow in my flats which is the prism. I do have it as far away from the light path as I can without affecting performance. My problem is the focal reducer causes the shadow from the prism.
Have you got any FR too?

Mark
Title: Re: Flat analysis
Post by: Einari on 05:02:11, 09 November, 2017
Veloce Rh200 being f/3 don't need reducer but there is a rear correcting element near focuser. I don't know if that's a problem with oag.
Title: Re: Flat analysis
Post by: Nomis Elfactem on 10:50:27, 09 November, 2017
Have to say it looks like the OAG prism blocking to me.... if you look at the top corners you can clearly see that the chip is bigger than the light path and the vignetting is quite severe.... out of interest, how long was the exposure and what did you use to illuminate the flat-field ?

S.
Title: Re: Flat analysis
Post by: chris.bailey on 11:06:05, 09 November, 2017
I have to say that with that focal length/sensor combo I would expect the OAG prism shadow to be more of a square intrusion rather than for the full length of one side.

Stretched flats are always dangerous as they tend to magnify what can be very small differences in ADU levels. What does the flat contour plot look like if you run this through the Flat Contour Plot script in PI?

Chris
Title: Re: Flat analysis
Post by: Roger B on 17:49:45, 09 November, 2017
Do your light frames have this ?

Roger
Title: Re: Flat analysis
Post by: johnadams on 20:58:09, 09 November, 2017
Hi

The exposure of the flat is around 8 seconds. Heres a image of the Contour plot but no igea what to look for.
Title: Re: Flat analysis
Post by: chris.bailey on 21:20:22, 09 November, 2017
That looks OK (fish) to me. The light falloff in the corners is very normal and I suspect the darkness along part of the bottom is then the shadow from the OAG or just the normal readout  from the CCD.
Do the calibrated lights work ok?

Chris

ps I find 30,000 ADU for the 8300 chip to be a little on the hot side and limit mine to under 20,000.
Title: Re: Flat analysis
Post by: johnadams on 08:25:28, 10 November, 2017
Hi

This is a 5 min sub after calibration . To me the master flat hasn't remove the shadows along the bottom.
Title: Re: Flat analysis
Post by: chris.bailey on 10:13:34, 10 November, 2017
John

It hasn't done a BAD job of it though! I would be happy with that!

I did a bit of analysis with by Atik 383 a while back after having similar problems with flats not performing 100% and came to the conclusion that the 8300 chip looses a little bit of linearity past about 22,000 ADU. After that I dialled back my flats to have a peak of 20m000 ADU and they worked a lot better. Might be worth a try.

Chris
Title: Re: Flat analysis
Post by: johnadams on 11:02:28, 10 November, 2017
Hi Chris

Thanks for your help. Whats your opinion on this 8 x 2 mins RGB.

Thanks
John
Title: Re: Flat analysis
Post by: chris.bailey on 12:43:04, 10 November, 2017
John

To my eye and based on the .jpg that is about as good as it gets. Bit of residual gradient but thats not a calibration issue. Running DBE on it sort that out nicely.

To be quite honest I only use flats now if I have an issue. I try and clean the CCD optical window carefully and avoid dust motes. I then do everything else with DBE. That is with Ha though were I struggle to get flats to work well enough, a known issue with narrowbandpass.

Chris
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