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The solving of flats issues


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Bottletopburly

For ages I have had flats issues sort of coincidental I changed dslr to Nikon D5300, but never thought it was the camera as it’s highly regarded for Astro , I had been mainly stacking in DSS , then recently switched to ASTAP for stacking which seems very good and a lot simpler to set up and use , so as I dither I only take bias and flats and load bias as darks what I didn’t realise I still needed to load the bias in bias tab because that’s needed to calibrate the flats, bingo flats now working as they should once data stretched cloudy outer ring gone, though I think my flats panel still to bright that I use for my Newtonian and that’s with dimming with paper and using a t-shirt with a 1/125 exposure giving me just under half way on the back of camera histogram but for this camera I should be aiming for 7/8 of boc histogram but that will get sorted  now I re read the CN thread on this camera  ,  I picked up on CN thread that there is a ring issue with certain Nikon’s caused by lossy compression which later model have an option to turn off , luckily uk user Mark smedley has written a program to fix this , so I now convert all Nef files to Adobe dng raw then run marks program and it sorts the lossy compression, it’s took a while and a combination of things but it does seem I’m heading on the right path just some old data to trawl through to see if I can salvage and data I thought duff .

CN thread here https://www.cloudynights.com/topic/746131-nikon-coloured-concentric-rings/

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interesting using the bias frames as darks, i also use a high dither but have been using darks which is a pain to do with a dlsr as i tend not to do them on the same night so sometimes temperature is slightly different, i`ve also been experimenting with my flats, i use a tablet with white screen turned down to minimum brightness, then a white t shirt and two pieces of paper, then i go for a longer sub length for the flats typically over 4 seconds which still keeps the histogram between a third and half, i found that a longer flats exposure works much better as it illuminates the whole chip perfectly.

 

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Bottletopburly
5 hours ago, red dwalf said:

interesting using the bias frames as darks, i also use a high dither but have been using darks which is a pain to do with a dlsr as i tend not to do them on the same night so sometimes temperature is slightly different, i`ve also been experimenting with my flats, i use a tablet with white screen turned down to minimum brightness, then a white t shirt and two pieces of paper, then i go for a longer sub length for the flats typically over 4 seconds which still keeps the histogram between a third and half, i found that a longer flats exposure works much better as it illuminates the whole chip perfectly.

 

Using bias as darks was suggested  Olly  penrice and seems to be the way when using dithering with a dslr as you can’t temperature match the darks , for my refractor I use a small tablet for darks and the 200pds I have an led light panel but even dimming with frosted acrylic   layers of baking paper  and a two folded t - shirt I too bright on that  I can’t get 1sec flats without clipping for me I can get on the Newtonian 1/20 sec flat with a 7/8 th boc histogram as suggested for the D5300  I need a dimmable  light source I saw a chap on YouTube using this from IKEA though you need a controller too https://www.ikea.com/gb/en/p/floalt-led-light-panel-dimmable-white-spectrum-90436314/ may have a look at that once lock down ends 

tried a tracing led panel but edge lit and light not uniform , could you post a stretched flat are you using a Newtonian? .

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i use one of those white screen apps on my tablet, this allows you to dim the light so it isn`t so bright, even then i use a white t shirt and two sheets of white paper, this allows me to do longer flat subs and i get a better illuminated flat.

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