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| LOL! Or a CDK? |
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I wondered about the aperture impact and almost replied earlier suggesting it, but now I'm not sure that's the case. I dug up some of my own images of this space. Both of these were shot with the same camera and filter (ASI2600MM Pro with Antila 3nm Pro Ha). These are otherwise unprocessed stacks from PixInsight. All I did was a quick star alignment with your shots above so they were scaled/rotated the same way. The left one is 20x300s shot through a Raptor61 (61mm @ f/4.5). The right one is a 30x300s stack shot through an ES AR152 with a 0.65x reducer (152mm @ f/4.2). That's a more than 100% increase in aperture and I'm not seeing that much difference in the stars. I also checked a more recent shot I made of the Fish Head Nebula using my WO FLT132 since it has a piece of this same area in the corner. The stars were about the same. Not entirely trusting my eyes, I analyzed both of these with the FWHMEccentricity script (because I know it counts stars). It found 857 stars in the left image and 1114 in the right. Part of that could be explained by the somewhat cleaner image on the right (with 50% more integration). Still, it's not a staggering difference. Based upon this I would wonder about other possible causes for the lower star count in Gwori's image. I know that for me, a habitual narrowband imager with Bortle 8 skies, star count is something I use in SubframeSelector in addition to, sometimes in lieu of, SNR. Subs with substantially lower star counts indicate something has gone wrong, whether that's thin clouds, bad transparency, or just too much background glow (e.g. shot too low in the sky or too close to dawn -- as noted the Moon isn't a huge problem as long as you're 20+ degrees away from it). Also comparing all five of these images, Gowri's still looks a little softer, in a way that isn't fully explained by aperture (when comparing it to my 61mm aperture image). So theories I'd suggest would include: 1) thin clouds, 2) severely impaired transparency (high humidity, dust, wildfire smoke, smog, etc.), or possibly 3) just slightly out of focus.  |
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| Great analysis Greg! I think you figured it out as we've ruled out most everything else. -john |
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Since this is my very first narrowband image, I am not ruling any of the above out! Focus I am working on with a Moonlite Focuser, thin clouds and conditions, I can watch out for to validate that too. Thanks for the analysis!! |
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I have to keep and eye on my ZWO electronic focuser especially with the Oiii. I had to increase the Oiii focusing exposure to 10 seconds to get the best results. This has been a good learning experience for me also. I'm looking forward to your next project! -john |
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This is a little off topic, but I've noted that the ASIAir/EAF focusing routine is usually less successful moving out vs in (e.g. going from Ha to Oii which in many optical systems means moving out a bit for focus). If it misses and shifts to try again, it'll then go too far out. This gets way worse with a reflector because it'll misinterpret part of a donut shape as a star centroid. While I sleep through most of my imaging, sometimes I set myself an alarm for the point in the routine where I'm switching to Oiii so I can wake up and check it. The nice thing about ASIAir is that I can do that via my phone from bed.  |
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You had to destroy the dream of a simple explanation Greg 😊😊😊. No no ... very good points! It will be interesting to see how Gowri's next projects will turn out, that might prove or disprove some of the ideas here. John, I have my focusing exposures always at 10s 😊, since quite a while actually. Does not really add a lot of "wasted" time and is certainly better. My Plate Solve is at 20 seconds! But all with 2x2, works perfectly and is much faster than 1x1. I also use a crop of the image for the focusing (I use SGP, maybe this is as well an option for your focusing program). I take 20% away from the images (edges), makes a huge difference if you ask me too. |
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| More good tips Uwe! |
