Please comment on my flats

[Peter Reader]

Hi all,

Tried to take some flats for the first time in order to correct vignetting I am seeing in my final images. I used an LED tablet on minimum brightness and NINA's flats wizard... which failed for me by saying the images were too dim regardless of exposure length and panel brightness so I arbitrarily took two exposure lengths manually which after stretching seemed to show uneven field illumination so I guessed these might be useable as flats...?

Please would you mind critiquing on the attached flat frames?

I couldn't figure out how to assess the histograms - I know I'm ideally after 50% brightness (?). In case it matters: my light pollution filter was in the imaging train when I took flats.

Cheers

0.2 sec flat.fits 0.02 sec flat.fits

[Peter Reader]

This morning I quickly stretched my stacked light frames of the Monkey Head Nebula from last night without any flats. I then applied the same processing layers to Deep Sky Stacker outputs that contained the 0.02 sec and 0.2 sec flat in my previous post. 

From what I can tell, the 0.02 sec flats are closest to being useful but if anythin over-compensate for the vignetting such that I now have "reverse vignetting" with the edges of my image brighter than the center. The 0.2 sec flats introduced some bizarre colours. I would deduce that I need even shorter exposure time for my flats or of course a dimmer light source.

Any advice greatly appreciated. Deep Sky Stacker summary of the stacking procedure attached also.

[Stuart1971]

Take longer flats, try Around 2-5 seconds with your CMOS camera…👍🏼

[Stuart1971]

Oh and BTW, you should be using Dark flats too…and don’t bother with bias, seems to work well….

Edited January 30, 2022 by Stuart1971

[Adam1234]

Exactly what Stuart said.

Dark flats make a huge difference in the effectiveness of flat frames. Make sure the same exact same settings (gain, temperature, exposure) are used for the dark flats. 

If you struggle to get dim your light source enough to get longer flats, I increase the number of layers between the source and lens (i use sheets of white paper) to control the exposure time.

Hope that helps

[Stuart1971]

Flat frames can work well  other own, sometimes, but they are best calibrated with Dark Flats, which also contain the bias signal too hence why there is not need for separate bias frames….so use Darks, Flats at around 2-5 seconds, and Dark Flats at the exact same duration as the flats…and all at the same Temp,  gain and offset settings, and you should be good to go…👍🏼

[barbulo]

According to his signature, I guess the OP is using a DSLR (1000D), so I wouldn't recommend any dark or dark-flats calibration frames since he cannot match the exact temperature of the light frames. 

18 hours ago, Peter Reader said:

I used an LED tablet on minimum brightness

I would put some white clothes between the LED panel and the scope. Add layers to dim the light as needed. And be sure the fabric is stretched. This is a good guide.

18 hours ago, Peter Reader said:

In case it matters: my light pollution filter was in the imaging train when I took flats.

That's ok: the optical path must be the same as lights'. 

8 hours ago, Peter Reader said:

Why are you calibrating with flats only 19 light frames and not all the 27? You can use the same flats with all the lights taken with the same ISO (whilst the optical path remains unchanged)