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I have found an issue when creating "flats" for my Astrophotography calibration files. Equipment Camera: Sony A7III Telescope: Redcat 51. Telephoto lens : Samyang 135mm Aperture Priority or Manual , the highest Shutter Speed on my camera: 1/8000 ISO (any ISO), but usually 6400, 3200 or 1600. Issue: when either of my my Sony A7IIIs (I have 2) are attached to either the Redcat Telescope or Samyang telephoto lens, and I create Flats by pointing at a very bright light source, each of my Flat files, without exception, have a shadow edging at the top of the image (below. This doesn't happen if I attach my 70mm-200mm Sony lens (by way of a test.) No edging is apparent on this lens or any smaller focal length lenses I use. Only on the Redcat and 135mm (prime) Samyang, as below [b]#[/b]  Any advice would be greatly appreciated. Thank you. Richard |
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Are you using the shortest shutter speed to produce flat frames? You should seek exposure times that are much longer than that in general for flats. Use a diffuse covering and make them between 0.5s and 2s each. I could see there being chip-read effects at that quick of a shutter speed, although I don't yet see how it only occurs on some lenses. Either way it may improve their effectiveness! |
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Hi Awesome Astro, Thanks for your reply. My understanding is that instructions for creating flats is to specifically set your camera to aperture mode and the camera will choose the FASTEST speed possible, which, on my camera, is 1/8000s. BTW, I looked more closely at the images from the 70-200m lens and there is a slight hint of shadowing, but very faint. |
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Hi Awesome Astro, Adam Block pointed out in his youtube channels that very short exposure times can lead to issues with flats on CMOS sensors. So try to for longer exposure times and check whether the effect goes away. To me it actually looks more like a vignetting effect, maybe an issue with the shutter curtain at this high speed? Can you try with electronic shutter? Clear skies Wolfgang |
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I create Flats by pointing at a very bright light source This is probably the issue. You really want quite a muted light source and also probably need to cover the scope with something (the white T-shirt method) that will make sure the light source is even. What is your "bright light source"? |
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| To give you a bit more of a target, try to get your flats in the 1+ second range. The issue with many light sources and I suppose mechanical shutter is these can cause artifacts with very short exposure times. From what I see in the flat provided there is no gradient other than the bit at the top edge. I'd expect it to model the curvature of your optics, so brighter in the middle with falloff at the edges generally. Depends on many things though. So, dim the light source and continue with aperture mode, but target 1 or 2 second exposures if you can. |
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FWIW: this is what I would expect a typical flat to look like; kind of slightly dark grey with some vignetting due to the optics/filters/scope and the odd dust-bunny. The one you posted looks far too bright and doesn't show any of the normal gradients I would expect. |
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Yeah follow what people have said above, you definitely shouldn't have an exposure time that short with flat frames, and you want diffuse light hitting the sensor, so cover it with a plain white t-shirt or something. Although it definitely does look like a shutter artifact at this speed, I was confused why it wouldn't appear with all of your lens combinations, but as I type this, perhaps higher focal length lenses don't include its contributions on the sensor because of the different focal length? Not fully sure, but I would start here nonetheless. |
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Thanks for all the replies, from Awesome Astro, Kuechlew, Andy Wray, and Jewzaam. It seems I have had it all wrong from the beginning, and have taken everyone's advice onboard accordingly. Andy, my bright source is a light-box, and obviously too bright for this purpose. (I use the light-box because I shoot mostly from a balcony, which doesn't necessarily have the sun to point to when I need it.) I wrapped a T-shirt around the lens hood 3 times (3 layers), pulled them as taught as I could, get the shutter speed down to 1/320 at ISO 6400. I've attached one of the frames below,, which shows no shadowing (downsized below). Note that it's an Astro-modified camera, hence the pink shade. This seems close enough/good enough for the purpose, or should I add more layers?  |
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| I use copy paper between my drawing panel and scope to get my time into the 2 to 3 second range. It takes 4 or 5 sheets to dim it down enough, this plus the tee shirt. |
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This seems close enough/good enough for the purpose, or should I add more layers? I think it's fine now. Your camera appears to have automatically selected the correct exposure (i.e. the brightness levels on a histogram view are between 40% and 50%) and you have none of those nasty artefacts you had before. If I stretch it I also notice that it has captured your vignetting nicely. |
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Use the same ISO as you shoot lights with. Or a lower ISO. Calibration frames with lower bit depth will not be as good. I note this given you say you're using ISO 6400. That being said, you can go with a lower ISO which means higher full well. And therefore longer time before a pixel is saturated. As noted by Tim above, sheets of papers a good way to dim a light source. I have a tracing tablet with 3 sheets of paper laminated. The lamination is great so I can wipe off any bugs or other things that might have added a permanent spot on the paper. This combined with the panel being dim or bright lets me get to the exposures I want for flats. If you can't dim your light source, add more paper to get that shutter speed in the 1+ second range. |
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| +1 on lower ISO and longer exposure. I've had flats introduce noise and undesired artifacts when they're under 0.5" each. |
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Thanks everyone for the advice. I'll look at adopting the multi-sheets of paper method, probably the laminated way as per Jewzaam's suggestion. Incidentally I usually try to shoot at a much lower ISO, it's just that some of my targets have only a very narrow capture window, 35 minutes or less, before they disappear behind my upstairs neighbour's balcony, so I compensate it by upping the ISO. Images are obviously not as good, but better than nothing. Of course, on the rare occasion that I can I get out into the field, the situation is entirely different. Finally, out of interest, here are 2 images of the target (Rho Ophiuchi/Antares): the first stacked then stretched without Darks and Flats by PixInsight only, complete with light-pollution, dust spots and vignetting. The second, the final result, still without darks or flats, worked with a combination of other software (PI, LR, Affinity Photos and AstroLabs) to give it some degree of recognition.   |
