Author Topic: Flat Challenge - Must be missing something :(  (Read 1903 times)

Online gvasseur

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Flat Challenge - Must be missing something :(
« on: 2017 June 21 16:17:28 »
Good day everyone,

After many nights and a good dozen of targets in the past 6 months spent trying and adjusting my flat techniques, I just can't make it work properly. On top of that, I think I'm adding issues on my own by probably having other flaws in my process. I manage to get some OK shots, but I feel that my flats don't improve anything. I feel I'm not making progress anymore because of that.



And here it is before flats. Adding my flats barely corrects anything.






I would be so grateful to receive some help to identify what I do wrong to that I can start tackling other functions to make my pictures even more acceptable? Does anyone have a process that could work with the hardware I have?

I here is a link to a small set of flat/dark/light of M101: https://drive.google.com/open?id=0B4D_Gnn2piwfb00xd1JBVW5ndkE.

Being an amateur, I just shoot RGB with a Nikon D750 on a 9-1/4 Schmidt Cassegrain and a Hyperstar. Last night it was with 240s exposures.

Many thanks in advance,

Gauthier

Offline Dimitris Platis

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Re: Flat Challenge - Must be missing something :(
« Reply #1 on: 2017 June 22 01:03:49 »
Dear Gauthier,
what exactly do u think needs correcting after adding your flats?

Online gvasseur

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Re: Flat Challenge - Must be missing something :(
« Reply #2 on: 2017 June 22 07:08:41 »
Good morning Dimitris,

Thank you for your prompt response. I should have attached a processed picture before crop.

Here it is the version after processing: the dark strip on top is still there although I thought it was well captured in my flats.



With M101, the target was small enough to enable me to crop things out. But on wider nebulae, it is a real problem.

Shouldn't I expect a better result after application of the flats?

I would really value your expertise,

Gauthier

Offline Alejandro Tombolini

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Re: Flat Challenge - Must be missing something :(
« Reply #3 on: 2017 June 25 15:47:21 »
Hi Gauthier,
It seems that there is a strong light pollution gradient and also clouds that avoid a good correction with the flats. I have also had this problem from the site where I take pictures.

I understand that you have many more image, I suggest you make a good selection of them (in the set you have shared I deleted two of them because de clouds or movement) and try to remove the residual gradient in the integrated image with DBE. I was able to partially remove it.

Saludos, Alejandro

Online gvasseur

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Re: Flat Challenge - Must be missing something :(
« Reply #4 on: 2017 June 26 13:52:45 »
Good evening Alejandro,

I guess you are spot on. That night had clouds and due to the layer on top of the nearby city, there was much light pollution than usual. I didn't know that it could be a cause.

I will proceed as you described.

I'm really grateful for your help,

Cheers,

Gauthier


Offline blueridgeDSIA

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Re: Flat Challenge - Must be missing something :(
« Reply #5 on: 2017 June 28 10:58:43 »
How many flats did you take?  The borders are still underexposed which I would say requires many more flats to properly integrate.

Online gvasseur

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Re: Flat Challenge - Must be missing something :(
« Reply #6 on: 2017 June 30 22:44:25 »
Hi BlueRidgeDSIA,

I took 31 flats for this session. Should have I take more?

Offline blueridgeDSIA

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Re: Flat Challenge - Must be missing something :(
« Reply #7 on: 2017 November 26 12:39:13 »
Hi BlueRidgeDSIA,

I took 31 flats for this session. Should have I take more?

Yes, I would take alot more flats, think 100-120.  Also be sure to use linear fits and dial in the rejection parameters. 

Offline gcardona

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Re: Flat Challenge - Must be missing something :(
« Reply #8 on: 2017 November 26 13:22:36 »
One thing that is very important for flats to work properly is to have both your flats and your lights calibrated with bias and darks of the correct exposure. If one or the other is improperly calibrated, the flats won't work properly because when the calibration routine divides the lights by the flats, the floor value (pedestal, dc-offset) may be different and the division does not work properly.