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Can anyone give any advise as to what I may be doing wrong to get this flare/spike happening in my bright stars? I'm not sure if this normal or if this is due to seeing or atmospheric conditions or if I am stretching the image too much. I would just like to figure out how to get nice round tight stars in my final image, if it is possible to clean these up in Pixinsight. This is basically my first ever image using a dedicated astro camera (mono) I am still very new to this. This is a HaLRGB photo that I have edited in Pixinsight. If anyone needs any other info that I have not included, please let me know Thanks in advance!  |
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| What was used to take the frames? Reflector or refractor? |
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andrea tasselli: A refractor was used to take the images. I have a William Optics GT71 with the flattener/reducer. |
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| Then those are the effects of the clips used to hold the lens in place. I'd assume you can mask the lens down to remove them but I won't advise it. |
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andrea tasselli: Is this what pinched optics are? As soon as you asked what type of scope I had, a lightbulb went off in my head and I remembered seeing something about pinched optics before. I will definitely be contacting William Optics to see what they say. Thanks for your help. |
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| Pinched optics is different - results usually from cold temperatures causing the retaining ring to contract and put pressure on the lens. Tell tale sign is triangular stars. This is simply the blocking effect of the clips causing the aperture of the lens to be non circular. As Andrea said, it may be possible to put a mask that covers these clips and also covers the spaces in between. This will result in a circular aperture but will reduce the amount of light the lens gathers. |
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Pinched optics is different - results usually from cold temperatures causing the retaining ring to contract and put pressure on the lens. Tell tale sign is triangular stars. This is simply the blocking effect of the clips causing the aperture of the lens to be non circular. As Andrea said, it may be possible to put a mask that covers these clips and also covers the spaces in between. This will result in a circular aperture but will reduce the amount of light the lens gathers. Thank you very much for your help |
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I agree with the above comments. I have had pinched optics in a TMB130SS and it will usually create a diamond shaped star. It may only happen in extream cold conditions. I have had simular spicks/flares fron refractors. And they were caused by the spacers between the lens elements. Sometimes there are three and as yours four. Lynn K. |
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Lynn K: Thank you very much! |
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| This is pinched optics caused by four lens cell screws that are to tight. Look at this video on youtube: https://youtu.be/1gtrVODCepE. Your telescope will probably have three rows of screws because it is a triplet. |
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I have a similar situation with my William optics GT81IV with 6AIII reducer flattener. This attached photo was taken back when it was freezing outside, but I don't notice it anymore with the warmer weather which makes me believe it is pinched optics. I did unscrew the dew shield and saw 3 sets of 4 screws but I haven't tried to adjust anything. The middle set of screws appears to have some sort of lacquer on them while the front and rear sets of screws have nothing. |
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Torstein Thomassen: Thank you for the reply. I am really hoping that it is just something easy like that. I have contacted William Optics, the scope is only a few months old. So lets see what they say. |
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This attached photo was taken back when it was freezing outside, but I don't notice it anymore with the warmer weather which makes me believe it is pinched optics I have used mine in weather as cold as 28 degrees and then just last week when it was in the 50's so the cold weather is not really the problem I don't think. I have also read that some people have received their William Optics scopes from the factory in this condition and yes they had to adjust those screws. I will wait for a reply from William Optics before I do anything but that may be my first move is to adjust the screws and see what happens. Fingers crossed |
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Yup I agree with everyone above, I don’t believe its pinched optics. Its the tab’s in the lens. Dale |
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Dale Penkala: Good to know, thanks a lot Dale |
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Hi, I am replying to this post, over a year later, just to inform others who may be using this as a resource. I don't think the star flaring is from pinching or tabs in the objective cell. I have the same phenomenon on my GT-71, complained to William Optics who took the lens cell back, "fixed it", sent it back, with zero change. So I launched a series of tests, and now, after 7 telescopes, have concluded this is "normal" (and what WO told me in the very beginning). It seems all telescopes have flaring of brighter stars to some degree. I even tested my homemade Newtonian, where I glued the primary and secondary to avoid diffraction from mirror clips, and it too had flaring (though there was still some glass in there, due to the corrector, which cannot be easily removed). This is a photo-summary of the 7 telescopes tested so far, as well as the GT-71 stopped down in the front to 60mm: https://www.flickr.com/photos/125134422@N06/albums/72177720308616569 My conclusion is that star flaring is not that unusual, and is not a "defect". Yes, some scopes have more flaring than others, but none are entirely free from it. I don't think it is in the camera, as each scope has its own characteristic "structure" to the flaring. It is interesting how the telescope with Fluorite has a much finer flaring structure than the others. Fluorite has very low dispersion (Abbe number=94.99). I also find it interesting that my much older William Optics 71 Star II Petzval has a very different flare structure than the new RedCat 71 Petzval. Both have identical specifications and use 4 elements, in the same or similar configuration. I expected those two to be the most alike, but that is not what the test results show. I am not done with the testing, but an early conclusion is that the flaring is either from within the glass, or the surface finish, or even the coatings (the latter two might explain the Newtonian flaring). But I do not think it is from physical objects within the OTA or pinching (testing was done at "comfortable" temperatures, ~ 10 Celsius) Steve |
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Us reflector folks learn very early that anything blocking your optical path affects the shapes of your stars. One of the obvious ones you'll see in 90% of reflector images is the mirror clips, which produce a star that looks very similar to yours, only we have 3 clips, and you clearly have 4. Imagine shining a flashlight directly through a paper towel tube and projecting it on a wall. You will see the outline of a tube. Now use your finger or something to block that light... you'll see that 'obstruction' as we call it. The stars are like your flashlight, bright ones will easily show any obstructions down your optical tube. Reflectors will show the clips, refractors will show the clips (if they're not masked) and even camera lenses will end up showing interesting patterns due to the iris mechanism. What you see is exactly that. It's not pinched optics. Pinched optics will misshape stars. Yours are still round. There is nothing wrong with your scope. Can it be fixed? Sure, (maybe?) but it will probably be much more effort than its worth. In the end you'll only end up reducing the amount of light that comes into your scope. You'd be better off just living with it and upgrading your scope down the line. edit: Jeez I just realized this was an old post lol. |
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It was an old post, but my post is from hours ago. My reflector has zero clips. None. If you re-read my post, I glued the primary (and secondary) mirrors. I just added a simulation, using SpheroFront, of a simple secondary support with a 4-vane spider and it shows flaring. So it seems clips are not needed to cause that: https://www.flickr.com/photos/125134422@N06/52962215887/in/album-72177720308616569/ But this is not about the reflector. (These flares are minimal, and possibly from the glass in the coma corrector, or as SpheroFront indicates, just the nature of the diffraction across the circular secondary obstruction combined with four thin vanes) It's about the flaring in the William Optics GT-71 refractor, and other refractors. And a forward, circular aperture stop did not change anything, (Images 7 and 15 in the Flickr album). So it is not something protruding into the light path. So the flaring is from something else. I forgot to mention, I might try a precision pinhole lens with the same camera. It will be difficult as the f/# is in the 100's, so very long integration times will be needed on a very bright star. But if the flares remain. we can no longer blame the optics. |
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I keep on-hand a set of filter size step-down rings that thread into one another so can be stacked as desired and then mounted to the front of camera lenses or smaller diameter refractors. I got these originally to avoid strong star diffraction flares from using camera lenses' internal diaphragms to stop down the f-ratios. The ring set is handy as a testing tool, however, to reduce aperture by one or a few millimeters to test if internal clips are shadowing the light path, or to use anytime one prefers to image at a higher, albeit slower, effective f-ratio to obtain better optical performance from the optic. |
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Jim Lindelien: I haven't had success with this on fast lenses. It just creates crippling vignetting. |
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In the context of the topical discussion here, my point was that if there is any suspicion that some internal small edge clips holding a lens are shading the optical path and introducing diffraction effects, one can rule that theory in or out pretty directly by restricting the objective's diameter with a known circular aperture a millimeter (or more as may be needed, by simple stacking the rings) to block light from reaching such clips. --- To diverge briefly off topic into your experience with vignetting for just a moment, I am not an optical designer, nor do I get very theoretical about all this. I'm an hobbyist and experimentalist: I try lens, f ratio, camera (sensor size) and filter combinations and just remember what has worked better or worse or not at all. Front mounted rings on my Rokinon 135mm f/2 ED UMC to operate at effectively f/2.4 to f/2.8 produce a better quality image than no rings at f/2 or by using the internal diaphragm at those ratios, whether I am using full frame sensors or my older, smaller ASI1600MM. Flats compensate any vignetting in these combinations, but I can't speak for f/8 for example, as I never performed that experiment. The image quality and shorter exposure times have proven a good tradeoff. I have not seen vignetting I'd call crippling, i.e., beyond the ability to compensate with flats, in the combinations I've tried. I also know that I can use a Baader UFC-series filter drawer mount adapter on the front of that lens and insert my 2x2" filters which happen to produce better halo behavior than any other filters I own: smaller filters that could fit between the lens and camera in the conventional position. Sure, it's a square aperture but creates a more pleasing result on star appearance, to me anyway, than the diaphragm petals do, and the haloes are better. When a quality very dark sky night comes along, I stick to setups I already know work, and don't waste any time experimenting on a fine night. I try not to assume any generalized answer or rule or very strongly held opinion, but indeed have formed a couple opinions that are pretty well cast in stone at this point, not worth further time to experiment with. I'll never again try a zoom lens for astrophotography! I've also not found much success with any DX lens (for the 1600MM; of course I would not expect DX to work full frame), so I no longer try those (n.b., in both cases for reasons other than vignetting). Now others on AB will likely share contrasting opinions...but we are off topic. |
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| Interesting that you got it to work with the 135. I agree, maybe an interesting topic for another thread. |
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I think I have reached the end of my testing regarding star-flaring. It evolved into mitigation methods, using various aperture masks. My thinking was that if I could not remove the asymmetric flare patterns, I could superimpose, or perhaps even substitute a more symmetric and pleasing flare pattern, hopefully without losing any significant light gathering area. That, by itself, had interesting results. A full photo summary is here: https://www.flickr.com/photos/125134422@N06/albums/72177720308616569 It is also interesting how much more prominent the diffraction spikes are for the Newtonian. I had expected the diffraction spikes from the four segment aperture mask to be similar. Note that the four segments, placed on the outside edge of the aperture, obscure 2.2% of the total area. If you only consider the 0.5mm thick secondary vanes in the 114mm Newtonian, (not the secondary itself) they obscure approximately 1.1% of the area. Yet those spikes are much more visible. But perhaps the open aperture on the GT-71 is not actually 71mm, making the segments a smaller percentage than calculated, which would explain it. edited: This is most likely due to the fact that the diffracting "edge' is so much shorter on the mask than the full-aperture length vane.. Steve |
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I wanted to do one more test, with a 6-segment mask, and got lucky with an unpredicted 30 minutes of clear sky last night. This time I made the segments smaller, and only protruding into the aperture by 1.0mm, (versus 2.0mm) which produced much finer and fainter diffraction spikes. What is interesting is that for the 4 and 6-segments masks, there is a "shadow", like crepuscular rays, at each spike, but not noticeable in the 20 segment: And for the 2-segment mask, the shadow is only on two of the spikes. https://www.flickr.com/photos/125134422@N06/52974829068/in/album-72177720308616569/ I wonder what causes the shadow? Obviously the segment itself, but why that shape? Maybe this is a clue to the "pac-man" look from the very original issue? Bumps or surface roughness of the OTA baffles or lens cell? (I can't see anything unusual when looking down the scope or shining a light from the back to a projection screen. but then again, it is nearly impossible to see the segments too). The full photo summary album is here: https://www.flickr.com/photos/125134422@N06/albums/72177720308616569 Steve |
