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Many critics of my photos have suggested that my flats aren't good. So I'm wondering what you all are doing to create your flats. My setup: ZWO294MC Pro Celestron 9.25" SCT Hyperstar Lens Skywatcher Pro Mount EQ6 ASIAIR Pro German light panel Dew shield I place a dew shield on the scope and point the scope straight up. I place either a couple of light filters (or white sketch pad paper) on top of the dew shield. (Even at the LOWEST levels of brightness of the panel I blow out my ADUs without the filters. The histograms are off the chart. ) I place the light panel on top of the filters with a light weight of some kind to keep it down. I play around with the panel light settings until my histograms are even and in the 29,000 range. I take about 20 3 second frames at the gain that I used for my lights (121) Any suggestions are appreciated!! |
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Not sure what "my flats aren't good" means, hence could you share your master flat and/or typical flat frames so that one could look at it/them? CS Manuel |
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| What is your gain and offset? |
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| Try lowering the adu to around 24000. |
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Thanks Guys! Yes, I'll share my master flat shortly. (I'm a newbie on this. Can i share an xisf file via the forum?) Gain and offset are at the recommended levels for a ZWO294MC. I'll try 24,000 as well. |
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| I also wonder if the 1.25 filter size is cutting off some of the light path? I use 2" filters. |
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After I discovered Russell Croman’s Gradient XTerminator I stopped using Flats! Stargazer66207 |
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One easy thing that helps is to take bias frames and darkflats in the same session. I spent quite a while analyzing my overcompensating flats just to find out in the end that my bias/darkflats that I was using from an older session hat different camera offset. Might not be be the cause in your case but it is a good practice I learned the hard way..... Michael |
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I believe you must use 2" filters because of the steepness of the light cone at f/2. Also, as @Elmiko mentioned, the 24,000 adu is about the right target for flats. The dark-flats need to be done as well at the same exposure time as the flats. The gain/offset for all four elements, light, dark, flats and dark flats all must match. |
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| “Recommended” is too vague - gain depends on target, LP, etc. Your gain drives full well which then what you’ll use 50% on. Specifics matter. |
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Flats are indeed really important if you want a nice even background to your shots. Have you ever tried sky flats, from parked position (away from sun), ideally in the blue and just after sunset, or just before sunrise (there's a few 10's minutes ideal where stars won't show) ? To me the histogram position has proven to be irrelevant, as long as you don't clip anything at either end. Much easier to do, no gear needed and much less prone to internal reflection or panel defects. I do 100% of my flats this way, usually automated in the morning while I sleep. |
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My tips for flats Regarding the histogram, with a color camera you need to be careful. I find red has the lowest histogram with my panel, green the highest. I set the adu to keep the bottom of the red peak to be at least 15% if possible, higher is better. Red will otherwise be very noisy and unreliable. But be careful, keep the top of the green histogram below 90% at the most, you do not want to saturate green. Flats should use flat darks, same exposure, gain, temperature, etc as the flats. Don't forget to use good darks with your lights, otherwise flats will not work right. Don't bother with any biases just adds noise if you have good darks. If you don't know that your light panel is perfectly flat, I would at least once take flats at 4 positions of the light panel, 0,90,180,270 degrees. Then compare an image using the different flat sets. If the flattness of your image looks different, then you should in the future take you flats at different positions, and average them to even out those differences. I would take 50 flats to reduce noise. If you dither in your lights that is enough. If you don't dither you may need to take even more flats. Rick |
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Thanks, all your comments are super helpful and I will try each of them. I don't know how yet to respond to each comment separately. (Newbie!) Here's more information: Attached are the xisf (or jpeg if xisf doesn't load) photos for my MASTER 3 sec flat darks and my MASTER 3 sec flat. Also, attached is a typical screen shot of the histogram that I see when I shoot my flats. The gain I use with my camera is 121. The offset is the default value for my 294 which is 30. Most of the time I'm shooting in the Bortle 7 light polluted skies of suburban Detroit. (I'm not sure how to do sky flats with the Hyperstar attached to front of scope? Please advise. Thx) Hope this helps diagnose what I see going on. Of course, now that I've written this I see how to respond separately. masterDark_BIN-1_4144x2822_EXPOSURE-3.00s.jpg masterFlat_BIN-1_4144x2822_FILTER-NoFilter_CFA.jpg  |
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| @fvialet How do you go about automating your sky flats? Place the t-shirt on after your run and then program ASIAIR to shoot flats at a certain time in the am? Do you take the Hyperstar off beforehand? Just seems like the lack of flatness in the t-shirt would cause a serious gradient. Thx! |
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| @Michael Ring yeah, I've been using a dark flat master for a while. I'll try making those for each session for a while. |
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| @Rick Veregin I'm not dithering (at least not on purpose) but when I look at my subframes via Blink in PI the stars do move around very slightly. I don't guide and I don't set anything up to dither. But maybe ASIAIR is dithering for me automatically. |
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| @Rick Veregin I'm not dithering (at least not on purpose) but when I look at my subframes via Blink in PI the stars do move around very slightly. I don't guide and I don't set anything up to dither. But maybe ASIAIR is dithering for me automatically. |
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| @Stargazer66207 when are you using Gradient Xteminator in your process? Right after you get an integrated image and before SPCC? Do you run DBE at all then? |
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Richard Milford: The point is just that your stars are moving to different pixels from frame to frame. And it doesn't need to be in every frame. I also do not guide and take relatively short exposures (1 minute), which means I do sample many pixels with my stars over a session, effectively dithering my image. The only things to be wary of is if the shift is consistently in one direction, this can lead to walking noise. Also, dither must sample new pixels, if it is just moving back and forth between a couple of pixels, that is not so effective. But I generally find with polar alignment effects, wind, tracking, that non guided images rarely show walking noise, especially if images are taken over more than one night, and pre/post meridian, or just you have lots of subs. By the way, DeepSkyStacker is free software for stacking, works really well. Even if you don't want to use it for your final stack, if you stack in DSS it gives you the x,y shift of each frame relative to a reference frame. You can see exactly how much you are shifting each frame, so you can track that you are indeed sampling new pixels. I also use it to make sure I am not drifting too much between subs, which would degrade the quality of my subs. The reason this dithering is important is that if your stars are on exactly the same pixel in every frame, then your master dark and master flat have all the pixels matched too with your image. This means that noise in the pixels of darks and flats add linearly in your image, they are not randomly averaged out, as they are always the same for any particular pixel in your light. When the stars shift, they match to different pixels in your masters, and thus, they are uncorrelated, and will randomly average out with enough images. So lets say you have 20 darks and 20 flats And you create a final image say from 100 frames, and every light frame is shifted to a new set of (x,y) pixels, which is dithering 100 times. The noise from your 20 darks and flats will be equivalent to 20 x 100 =2000 darks and flats. So your noise from those darks and flats will be very low compared to the noise from your 100 lights. Dithering is extremely powerful, as it effectively multiplies the number of flats or darks you took. Taking only 20 darks and 20 flats with 100 images and no dithering, will add a lot of noise to your images, So keep the number of dithers (the number of shifts) * number of flats (or darks) >> number of subs if you can. This should ensure little noise from your flats and darks creeps into your lights. Rick |
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| @Michael Feigenbaum and @Elmiko may have a point with the filter size, perhaps you should try to do flats without filter to find out if your filters are vignetting on Hyperstar. Here's a Flat from my Hyperstar 9.25, I use a bigger APS-C camera and it looks to me that you should see less vignetting with your smaller sensor, this is of course not an apples to apples comparison because the applied stretch can make vignetting appear less or more depending on how agressive it is.  Michael |
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Regarding where in my processing sequence I use GradientXTerminator: I am using PhotoShop (not PixINsight). My first step after converting the image to .tiff format (so I can process it in PS) I first go to "levels" and adjust the black point right up to the left edge of the peak in the histogram. Then, I go to "curves", and do a stretch to lighten up the background sufficiently to that the gradients at the corners show up. THEN I run Gradient XTerminator, after which I go back to "levels" and re-adjust the black point. Stargazer 66207 |
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| @Rick Veregin Thanks Rick, this is extremely helpful! |
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Flats are indeed really important if you want a nice even background to your shots. Have you ever tried sky flats, from parked position (away from sun), ideally in the blue and just after sunset, or just before sunrise (there's a few 10's minutes ideal where stars won't show) ? Ditto for sky flats. They are way better than any I took before using an expensive flat panel and they are a lot cheaper. CS, Wanda |
