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Dear fellow astrophotographers Along my journey to better images, where progress goes hand in hand with increased expectations and discoveries of new imperfections, I am wondering what is wrong with my stars and in particular whether the grey aura around medium sized stars is a consequence of i) bad seeing (ok, no action required), bad processing (then I am willing to learn) or dirty mirrors (then I will clean everything but only after confirmation that this is the issue). On these images, medium stars have an aura that I would like to get ride of. https://www.astrobin.com/g3nvbk/B/ https://www.astrobin.com/ps27ua/D/ Here is a concrete example from pic #1 above :  Interestingly, I did not identify the same issue on some older images of mine, even after reprocessing with the workflow used of the above images. Here is an example: https://www.astrobin.com/full/95geuq/D/ My processing goes always the following general lines : extract Ha and Oiii from DNB data to prepare a starless using either pixelmath or a foraxx script, after crop, DBE, PMCC, BlurX, noiseX, GHS. The stars are prepared using RGB data, with crop, DBE, PMCC, blurX, noiseX, gentle GHS, starnet+, stretch and intergration with the following expression : Pixelmath expression: RGB/K: ~((~starless)*(~stars)). Up to now, I considered that my mirrors are note in terrible conditions. I have some dirt on the main mirror and also some dew droplets that fall from the OTA onto the primary because the scope parked in home position after hitting the ascom limits instead of parking, as it should do, on the side at ALT=0 AZ=270 for drying in the morning (in that position no droplet can fall on a mirror). That episode left some very discreet and faint staints on the main, nothing more. But my recent images make me wonder. Any hint most welcome ! Clear skies, Patrice |
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| Dirt and then huge amount of it, would only scatter light more-less uniformily around the image plane but this would be hard to detect and even harder to show clearly in any image under standar processing condition for deep sky objects. So, no, isn't you mirror which is dirty, is the way you process things and possibly collimation is a bit off. Obviously they look fuzzy (the stars I mean) but as far as it goes you must have clearly a measure of your average seeing, be it FWHM or whatever else (PI will tell you if you run SubframeSelector) . If the seeing has always been poor then there is nothing you can do about it (i.e., greater than 3"). |
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andrea tasselli: Thanks @andrea tasselli for this very clear answer. Based on available online resources, my seing varies between 0.9'' and 2.5'' at maximum. I will have a look at the subs using SubFrameSelector and inquire new processing method. Damned : there is so much to learn in this hobby ! |
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Does this look familiar?.... https://www.astrobin.com/full/coq4s4/0/ I think it's faint IFN. FWIW, though I have a SCT, not a Newtonian, when my seeing is poor (and 'good' for me starts at about 3"), I don't see that sort of effect, just bloated stars. Cheers, Scott |
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Here's a wider fov of the area showing IFN: https://www.astrobin.com/kownph/?q=ifn&camera= The owl and surfboard are just below and to the left of the blue star furthest to the right. Cheers, Scott |
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Have you tried blinking your subs? I'd be willing to bet you had a few high, thin clouds travel through your frame throughout the night. Other thought is it could be your mirrors slightly dewing up. Before I scrapped my newtonian, I was dealing with secondary fogging issues often, particularly when near zenith. Since you said you have a water droplet issue, I'd also lean heavily towards this being a potential issue as well. Definitely not because your mirror is dirty however. |
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Many thanks for all these inputs. A fiew comments : @Scott Badger : I will try to improve the processing to let IFN appear. That could make sense that brighter stars bring IFN out. @Brian Puhl : Thanks. Looking at the Subframselector stats, nothing suggests the development of dew over the sessions. Besides, I never had dew because I have a fan forcing the air into the OTA and a heater glued to the secondary. I had once dew forming on the inner wall of the tube but that was in very rare circumstances (fros accumating inside the tube because I could not use my shield during that freezing February 2023 night; droplet fall wenn drying). @andrea tasselli : I see that my FWHM is rather poor :  Could that account for the issue here? Best, Patrice |
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Could that account for the issue here? Most likely it has a part maybe even a significant one but processing also is/might be an issue and I would check collimation too. |
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We have different scopes, but even when seeing/fwhm is 5" and above, I don't see that sort of effect. Given that other images of the same area show either IFN, or the same effect around bright stars as your image, that's where my money is.... IFN is cool when you have enough signal to bring it out i.e the IFN signal is sufficiently above the background noise level, but I've had a couple images where it sits right at the border. Not strong enough to work with, in adamblockian, I hadn't 'earned the right', but still strong enough to make for a blotchy background and a PITA to smooth out. Had the issue with the Rose galaxy (https://www.astrobin.com/full/nz9nhm/B/) and you can still see remnants around the two brightest stars next to the galaxy(s). Cheers, Scott |
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andrea tasselli:Could that account for the issue here? @andrea tasselli Based on you advice, I am inquiring collimation and various aspects of my optical train. I ran a bunch of test and got a lot of example of the following pattern using the FWHM script in pixinsight.  I have difficulties to understand this. I would expect good collimation to translate either into uniform patterns or in concentric patterns and wrong collimation to exhibit some clearly asmytrical pattern. Here the FWHM map seems to be inverted : it is as if I was focusing using a corner while the backfocus is impect. Does that sound correct ? Collimation is however ok I believe. Many thanks for this help. Clear skies ! |
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| I think ASTAP gives cleaner interpretation of your field flatness but otherwise I just note that saddle-like results for fwhm distribution are nothing unusual, even in supposedly "perfect" optical systems and a spread of 10% would be considered as good as it gets. Eccentricity is harder to pin-down without the results of the aberration inspector and that on a single frame of short duration to minimize the impact of tracking. But otherwise I don't think that whatever is your coma corrector you have a lot to improve. Processing is, however, another story. |
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andrea tasselli: Thanks @andrea tasselli. I prefer the scenario where I have to learn how to properly process star than the one where I have to inquire again the optics, because I already invested much energy in that. I will then try to gather some litterature on star processing (as opposed to starless processing). Clear skies |
