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With all the cloudy weather in AZ this week I was only able to get 2 hours of data, but wanted to practice post processing on our first nebula shot. Can’t wait to add to this over the next few weeks. Question. Should I move to 2 or 3 min subs or stay with 1 minute exposures? Edit. Adding the following at the very good suggestion of another user. 120x60s @gain 100 Workflow was WBP high quality preset, with drizzle 2x. DBE, SPCC, BXT, SXT, stretched starless and used curves to adjust a bit further with a small amount of saturation boost, stretched the stars with no further correction, and then finished with NXT and a 50% crop. ZWO AM5 WO Redcat 71 ZWO ASI 2600 MCFMA230 Guide Scope ZWO ASI 120 MM mini ZWO ASIAir Plus No filter  |
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Hi Kevin, Firstly, that's a beautiful shot -- good color, guiding, etc. Much will depend on sky conditions and equipment. You'll want sufficient exposure times so that signal outweighs noise, always. I don't know your sky conditions, but the image would indicate they're not too light-polluted. Also, it looks like you're using a 60 or 80mm scope. With a wide-angle telescope or lens, a light pollution filter goes a long way to heightening detail and keeping background glow at a minimum. This will affect exposure times. A good choice is the L-enhance filters from Optolong. As far as length of exposure, in addition to S/N ratios favoring Signal, your guiding setup will factor in, also. The wider the field, the less accurate guiding needs to be, especially if exposures are 2-3 minutes (obviously with good polar alignment). If you're using a camera with a large chip, data storage is important, also. With the new breed of CMOS cameras, a 20-30mb chip is going to produce many gigs of data, so 120-180 one-minute exposures is going to require a good amount of RAM processing power, not to mention terrabytes of storage capacity. To sum up, 2-3-minute exposures are certainly good, given the above parameters are in line. Be sure, when posting a query for opinions, that you include as much technical info as possible. Obviously, this helps when answering. Keep up the good work, mate! - - Steve |
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Hello, sometimes shorter exposure times (for example 60") can also be important in order not to overexpose bright regions when developing the data (HDR). Best regards! |
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Steve Solon and Terry Chatterton: |
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I like the color and framing. Just a couple of suggestions. 1. What is causing the trailing stars? Only some of the brighter stars show this effect. Was there a bad sub or two that should not have been included? 2. I would go with 2 or 3 minutes, but 60 seconds is probably fine. If you want less noise, just take more images to combine. It is rarely advantageous to take exposures over 5 minutes, and I find 2-3 minutes gives me good results even with a focal length of over 1400mm. 3. The biggest problem I see is that the center of the image is blown out... overstretched. How did you process the image? I would bet that using EZstetch in Pixinsight would give a better result. Or better yet, use the generalized hyperbolic stretch.  |
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1. What is causing the trailing stars? Only some of the brighter stars show this effect. Was there a bad sub or two that should not have been included? No idea. I thought it might be focus or lack there of, so I order a kit for my Redcat so I can install and EAF. 2. I would go with 2 or 3 minutes, but 60 seconds is probably fine. If you want less noise, just take more images to combine. It is rarely advantageous to take exposures over 5 minutes, and I find 2-3 minutes gives me good results even with a focal length of over 1400mm. cool. I’ll stick to 60s secs. I have plenty of disk space to use. 3. The biggest problem I see is that the center of the image is blown out... overstretched. How did you process the image? I would bet that using EZstetch in Pixinsight would give a better result. Or better yet, use the generalized hyperbolic stretch. I did it by eye and trial and error. I will try the beginner option you suggested. Thanks! |
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Kevin... One disadvantage of short exposures is that you can end up with a lot of subframes to review. If you use Pixinsight, be sure to use Blink and the subframe selector first to make sure you only use subframes in the stack that don't have issues. It is an important first step in processing. Good luck. We spend our winters in Tucson, and I sometimes wish I could be there for the "nebula" season too. George |
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Kevin Knight: *** stars are good. Nebula outer regions good. Inside there is something about the focus or sampling thats not right. Could be need tonrun HDR but its soft for that scope. I would try increasing exposure time. Blow out the stars. Concentrate on the core bright nebula. Process for the core. Then add stars in later using 2min subs. That should work. The problem may be your image is doing everything its supposed to for the redcat wide field but we are drawn into the centre wanting more *** |
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Thanks for the all tips and advice. Last night I was able to add some more expose time bringing the total to 299 subs after removing about 20 or so that were clearly bad on the edge IMO. I installed and used EZ soft stretch, and HDR at default settings on the starless image, and bumped up the drizzle to 3x. Everything else is as I originally did: Workflow was WBP high quality preset, with drizzle 3x. DBE, SPCC, BXT, SXT, stretched starless and used curves to adjust a bit further with a small amount of saturation boost, stretched the stars with no further correction, and then finished with NXT and a 50% crop. ZWO AM5 WO Redcat 71 ZWO ASI 2600 MCFMA230 Guide Scope ZWO ASI 120 MM mini ZWO ASIAir Plus No filter  |
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My only suggestion is to darken it up a bit using the histogram transformation or increase the contrast using curves. It still has those little star appendages on some of the brighter stars, but overall a very nice image. George  |
