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I am new to using flats etc so probably something I've done wrong, but my light frames look they are being massively over-corrected for vignetting by my flats when I calibrate them (see pic attached of a calibrated light frame!). I've put an example of a light, flat and dark frame in this folder if anyone can help have a look and tell me if I've done something wrong? https://drive.google.com/drive/folders/1GffIM_bEM6-_ybE1NugnPy95oM_Wx_mC?usp=sharing
I used an iPad with white light to do the flat frames by placing it directly over the lens - they looked good to me, if a bit underexposed. I created DNG files with the exposure brought up slightly to see if that made a difference but it didn't. The original raw file, and one DNG of the flat are in the folder above.
I'm using a Nikon D750 astro-modified and these were with sigma 28mm @ f/1.8 with ISO 1600 and 60 secs exposure.
Any advice much appreciated!!
You say you put the iPad directly over the lens... but do you then add a hood or dew shield for imaging? You must have the image train exactly as you image. I've found that dew shields and extension tubes can add stray light to the image if they are not flocked well enough. This can cause vignetting. The flat will unfortunately also contain this stray light but it must to properly calibrate the lights.
@astrogee didn’t use a lens hood or anything when taking lights, and then balanced the lens and camera on the iPad to take the flats - to me at least, the flat also looks about right for the vignetting in the light frame - but then it massively brightens the edges and darkens the middle for some reason?
Hi Callum, @cwhite939
You indicate : I've put an example of a light, flat and dark frame in this folder
In this case, the overcorrection is not unexpected, you also need to add either bias or darkflats to have a correct calibration workflow. Did you use bias or darkflats as well?
Mabula
