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An image speaks for 1000 words: I'm getting those ugly halos around all bright stars. Those are 30" subs taken with a Sony a6100 and the Rokinon 135mm f/2 UMC at f/2. No filters, nothing else but the lens hood. Other than that, I'm very happy with the lens performance, even the star shapes are very controlled up to the corners of the APS-C sensor:  This is a 100% crop from the top left corner, I'm more than OK with that. My question is: how could I test where I'm getting those halos from? Any input would be much appreciated, thanks!! |
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| Is the camera modified? |
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Is the camera modified? Nopes! All stock: camera, lens and hood. No filters, no mods, nothing else. |
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| Big fast lens + very bright stars = big halos, that's the expected outcome. Even with the best coatings of the world. Luckily for you those halos can be removed in post-processing. |
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These halos are larger than mine. Maybe this is just a consequence of dew formation in a lens element. Have a look at your individual subs, it should be easy to see if there are any variation in halos size. EDIT... But maybe I'm wrong since the halo edges are too sharp... |
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| I might say something wrong but I would try to change to f4 and see if any change |
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I might say something wrong but I would try to change to f4 and see if any change *That will improve star shape, but shouldn't really effect the bloating. That's coming from internal reflections which will not really change if you change the aperture. |
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Franco, based on the sharpness of the halo its size, my first reaction is that these are reflections between the sensor surface and some protective window in the camera. Such reflections are not uncommon, and unfortunately very little one can do about it. you may check it the following way: if these are reflections of the aforementioned type, the size of the halo is related to the distance between the protective window (or whatever the second reflector is) and the sensor by the following formula [units in brackes]: distance [mm] = diameter of halo [px]*pixelsize[micron]/1000*f-ratio/2 I expect you getting distance on the scale of sub-mm or very few mm -> something near the sensor. Matthias |
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Franco, based on the sharpness of the halo its size, my first reaction is that these are reflections between the sensor surface and some protective window in the camera. Such reflections are not uncommon, and unfortunately very little one can do about it. you may check it the following way: if these are reflections of the aforementioned type, the size of the halo is related to the distance between the protective window (or whatever the second reflector is) and the sensor by the following formula [units in brackes]: The formula you quote gives you the amount of defocus the halos have but does not tell you *where* the defocus occurred. Most often the end result is few mm away from the sensor if applied blindly, where nothing exists whereas the actual defocusing is most likely due to a reflection within the image forming lens (or group thereof) since it very much looks like an image of the exit pupil of the lens itself. |
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| Hi Andrea, correct w.r.t location, note that you need two reflections, and, assuming my suspicion is right based on the radius of the halo, since it is only a mm or so you need two reflections within that distance. Near the sensor you actually have such a situation (sensor + various filters/protective windows on top) and reports are common which also work out in detail if you follow the math (e.g. recently one here on AB on a ASI6200,,). Could also happen in the optical system - classical case is the halo owing to inner reflections in a filter, though for optical elements other than filters I would expect the distance to be larger (may be wrong - speculating, one would need the actual numbers to get further insights). Matthias |
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Franco, I get similar halos ... the first picture below is quite old, with a Rokinon 135mm and a Canon T6i plus the optolomg l-enh filter. Possible dew ? Further down, there's one with my Risingcam Sony IMX571 camera and SVbony dual band Ha/O3 filter, which shows a lot less star bloat. Note that Alnitak always causes some major bloat close to the Horsehead.   |
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Die Launische Diva: That's a nice theory, at least one that I can try to test: First sub:  One hour later:  They look pretty much the same.... (The 2nd one is brighter as the moon was starting to rise above the horizon) (*) On a side note, I'm blown away with the amount of data captured with 30" subs:  The humidity was pretty high that night. The lens didn't get any visible dew, but it instantly fogged once I brought it indoors. Maybe it was dew... I just got a Svbony dew heater strip and might be able to test it next week. I'll be trying to get either the Witch Head or Eta Carinae, I'll let you know the results. Thanks all for the input!! |
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Just found this picture, taken with a very similar camera and the same lens... and showing the same halos, same size: https://www.astrobin.com/yatei5/0/  cc @AstroSully |
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Franco Grimoldi: I would stop down to f/2.8 to reduce this, as f/2 makes the stars look bloated...other than that I'd look into getting the camera astro-modified to allow full spectrum exposures. |
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I would stop down to f/2.8 to reduce this, as f/2 makes the stars look bloated...other than that I'd look into getting the camera astro-modified to allow full spectrum exposures. I'm very constrained on my imaging opportunities. For reference, I rented a cabin and drove 14 hours to get that shot. Here at home I can't make long exposures, so I'm happy with getting double the light at f/2 and "less than stellar" stars... which still look pretty good. My samples are all at 100% magnification. On the other hand, I'm also extremely pleased with the "stock" response of my camera, I like the color balance and the fact that it's able to capture faint nebulosity. Most importantly: it's my only camera and I use it for "real world" applications, so it won't get modified. I'll eventually graduate to an astro camera, but won't happen any time soon, our new president is destroying our currency without mercy (100% inflation within the first weeks at the office). |
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| Im not a big fan of the roki hype and this image is proving yet again its unreliability. The halos, they could probably not even be the lenses fault, but the stars on your image feel extremely soft to me! Not how they should be on a 135mm lens imo. |
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Stefan Pfleger: This is a 100% crop from the center of the frame (different subject, of course):  (My stock Sony camera has 3.91um pixels) This is the lower right corner, also at 100%:  I see very round stars all across the field, all while having an unparalleled light gathering power. Considering that this was shot wide open at f/2 and I haven't applied any de-convolution, blur exterminator or sharpening, I'm amazed at the results. The only apparent drawback of shooting wide open has been a moderate vignetting, which gets easily corrected with GraXpert. |
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Franco Grimoldi:Stefan Pfleger: fair enough, i find it hard to judge sometimes from a processed image, especially when you‘re unsure about the crop. In that case i think something is reflecting in ur system, could be anything. Good luck |
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| I've used 3 different Samyang 135's (and a Samyang 85mm) and none had any distinct halos like that even on really bright stars like Deneb. There's usually only a faint glow that varies depending on the lens optical quality. Here's a Deneb example with the worst Samyang 135 that I've had - https://www.astrobin.com/n2sf0b/. Stars are misshapen with that one but no halos even with a dual band filter. Perhaps you could try it with a different camera? |
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Dmitrijus Tiazlovas: I wish I had another Sony camera to test... |
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Franco Grimoldi:Dmitrijus Tiazlovas: It isn't the camera. |
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As I stated above, dew could be a major cause and it's therefore best to repeat with a dew heater (eg SVbony) in place.   |
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Herbert W Hamber: Nice setup!! I got 3D printed pretty similar (if not the same) mounting rings for the lens. I'm also planning to get a laser finder, I waste too much time finding targets... On top of that, I'm eager to try my brand new SVbony dew heater. By looking at your pictures, I'm a little confused: you've attached it to the focuser ring and/or the camera body... How could I tell the best placement for mine? I thought it was supposed to be attached to the lens hood, close to the front lens element... |
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| It's the first picture where the heater strap is. Not that I think your issue has anything to do with dew. |
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| Yes the dew heater is usually placed on the dew shield (front metal cylinder) otherwise I think it interferes with the focus. Luckily out here in Calif it's usually pretty dry so I can do without it. Buona fortuna e facci sapere ... |
