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I shot this over 6 nights. We had an amazing go of clear nights in a row. Now it's all rain until next week. But while it was clear, I managed two nights per filter on my newest scope. A total of ~ 760 3 min images. I think the total imaging time was 37.8 hours. Shot it through my newest scope the RedCat 51mm. It did well, but could have used a belt focuser. I managed to get one together from Moonlite, but only just after imaging this. The scope was able to hold focus a good bit of the night, but the quality curve looked like a swan dive towards the end as the scope lost focus at the end of the night each night. I got out my old trusty AVX to manage the aiming parts of the setup. Guided by an old Orion SSAG I had lying around and a recently acquired ZWO Mini guide scope since it matches the RedCat so well. But I think I'm going to need to swap out the guide cam for something with smaller pixels. Overall though, it worked. Main camera was ZWO ASI 1600 MMC. I used 5nm Astrodon filters.
Hi All, I am posting what I suspect is a newbie mistake question but hoping that someone can assist with the issue of flats.
Although I have been fumbling around the sky, taking snaps at leisure, recently I became serious. I have read up about the different calibration files (flats, darks, bias) and they seemed to make sense; different ways to capture the image defects and extract those from the image of the sky. After a few weeks (months) of further fumbling I went back to the very first target to receive my attention, M42 Orion Nebula.
In short, I took 20x 30s exposures in LRGB and ran these along with 20x LRGB each of darks, bias and flats. To obtain the flats I used a diffuse sheet of perspex (lightbox material) and an LED video lamp that has 180 white LEDs, turned to its lowest setting. Attached below is the stacked Luminance flat and the light image. In the lights I am getting very strong marks from dust and I had thought that the flats would subtract this but looking at the flats the marks are completely different shapes and do nothing to remove them from the lights.
The attached has been further stretched to show the issue. Now, I am obviously doing something wrong but I have no idea what, any pointers from the vast pool of knowledge will be much appreciated.
So after Carole helped me to realise that I was taking my flats wrong, I've reprocessed this image and finally the funny rings have vanished.
TL;DR - I was underexposing my flats, which resulted in failure of calibration to remove vignetting, as well as it introducing weird red concentric oval rings.
Last night was supposed to be clear, so I set up in the garden. Cue the Irish weather. I only managed to get PA and focus (you know, the most fun bits) and then the clouds rolled in.
Anyway, so instead of imaging, I took a bunch of calibration frames. The setup was slightly different to the lights -
1) temp 4 C instead of -1.5 C (no big deal)
2) camera at different angle to telescope (can't correct dust motes)
3) new camera angle adjuster in path (different vignetting and internal reflections)
so I didn't hold out much hope that it would correct the problem. But it did! I used the new bias and darks to correct the new flats, and then the old bias and old darks and new flats to correct the lights. I'm so pleased I've finally figured out what was going on as it's ruined many images for me over the past year+.
Thanks again Carole,
I have recently converted from an old Canon DSLR to a ZWO ASI1600MC-Pro. The individual subs look great with the new camera, but I am having seemingly insoluble problems processing, so I need the assembled wisdom here! The first image shows a single sub on the left (auto-stretched), and the result of stacking some 25 subs on the right, combined with flats. The flats have corrected quite nicely for the dust doughnuts, but given me inverted vignetting. (I should say that I have tried processing with darks and bias as well, but the basic problem of the reversed vignetting remains. Processing was done in Deep Sky Stacker, although I get the same results using PI and the BPP script).
I initially thought this was down to using the wrong ADU values for the flats, so I've tried various from 5,000 up to 30,000 in 5,000 increments, but nothing improves - the higher the ADU the worse the effect seems to get. I've also tried different techniques for taking the flats - grey skies (easy at the moment) with t-shirt layers in front, or a tablet screen on white. Flats are seemingly ok - here is an individual sub on the left and master flat on the right. OK, they are rather brownish, but apart from that they look ok to my (DSLR-attuned) eye. Different ADU values seem to fail to correct for the dust doughnuts, as well as making the vignetting worse.
I have one further thought / problem to add. The over-corrected centre of that processed image (top, right-hand side) shows some odd details. Here's a zoomed version. There are fairly broad swirly lines showing up in there. Actually the lines are across the whole image, but most visible in the centre where the vignetting shows it up.
My only thoughts are that this could be condensation or similar? Might this account for the flats not correcting properly? As I said I'm using the ASI1600MC-Pro, and have it set to cool to -20C for all lights, darks and flats. I've not encountered a problem like this with flats failing to work so miserably, so I'm after any and all knowledge at this point.
To a fair extent I can take out the vignetting with ABE/DBE, but that then leaves me the dust doughnuts. I really don't want to have to 'edit' my pictures to that extent, and feel that the flats should take care of the vignetting.
Thanks for looking.