Flat fields for DSLR

[Isas_Astroatelier]

Hello, while I found a very insightful discussion here on flat fields for among others with a QSI/KAF 8300 I was wondering if anyone has some values for flats with a DSLR. I tried 10s/5s/3s/2s/1s against a flat filed panel but it's too much. The aperture priority setting doesn't give good results since I'm using an LP filter. Will try 1/2/1/4 etc. Anyone with a tip? Thanks a lot and CS! Best, Isa

PS: Below a picture of Orion Nebula taken with a Takahashi FSQ-85EDX with Extender 1.5x, LP filter, Nikon D800. As you can see, despite a master flat I still have this huge dark ring on the picture - masterdark and super bias are efficient so far. The core of the nebula is overexposed so will go easy on it next time, reducing exposure time to 180s (from 300s).

 

 

 

 

[Adreneline]

Hi Isa,

Not sure I can help but it seems a little strange that AV mode does not produce a usable flat for you. I have found it works very well with my Canon cameras.

Can you provide a little more detail as to how you are taking your flats? Are you using a light panel/box?

It would also be interesting to know what software you use. I use BackYardEoS with my Canon to collect the images and calibration frames.

Very nice image of Orion; I have had to go right down to 30s to reveal the core details.

Adrian

[carastro]

I have had 2 modified DSLRs, the first one was easy, just set to AV and 100 iso and it would take the correct flat.  

Not so my 2nd DSLR so I had to devise a new method.  The only way I could make it work was to examine the histogram, this needs to be between 1/3 and under halfway across from the left hand side.  You will have to reduce the length of your exposure until you get a histogram that looks right.  If you are using the day time sky to take flats then this will vary every time depending on how bright the sky is.

If you are using a flat panel, you will have to dim the light sufficiently until you get the correct histogram.  I say dim, because the length of exposure will be much less than you think it will be and even 1sec is too long.

Here is a screen grab of flats I did many years ago showing the histogram just under halfway across you will see that the shutter speed is 1/100 of a sec and I also reduced the iso to 100.   (N.B. this was when I was using AV mode), but even doing Man mode so you can adjust the speed, I think you will have similar length of exposure. 

I hope this helps. 

Edited October 26, 2021 by carastro

[Isas_Astroatelier]

Thank you Carole and Adrian for your useful comments! AV mode was with 1/60s (at ISO 1600 - I thought I'd need the same ISO as for the subs but will try with ISO 100 ...) but this wasn't sufficient so I went a bit overboard yesterday morning trying between 1 and 10 seconds which was obviously too much 😄 

"Not so my 2nd DSLR so I had to devise a new method.  The only way I could make it work was to examine the histogram, this needs to be between 1/3 and under halfway across from the left hand side.  You will have to reduce the length of your exposure until you get a histogram that looks right.  If you are using the day time sky to take flats then this will vary every time depending on how bright the sky is." 

Will try again to reach a histogram as indicated by Carole. Below you'll see the setup from yesterday. 

Thanks again & CS 🌟

[carastro]

Ah, I see you are using a flats panel, so once you have managed to find the optimum length of flat, you can make a note of it and keep it at that.

Carole