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Hello AB community. I have been trying to figure out what is going on with my 135mm setup. I'm not sure if this is internal reflections, light leakage, or something else. The imaging train consists of : A ZWO 2600MC pro - ZWO filter drawer to lens adaptor - Rokinon 135mm lens. I have attempted to find any place light could be leaking in from with nothing that I could see. The lens adapter to lens does have a slight amount of play in it. I have also removed the camera and places the rest of the train on a light panel to see if I can find any reflections, and there does not seem to be any. I'm running a dew strap at the point where to hood meets the lens if this matter for leakage issues. I am running at F2.8 no filters in Bottle 4.5 ~. Calibrating with flats and flat darks. Current light exposure time is 180 sec. I have tried to search the internet for anything and haven't found too much to help. I can't really find any examples of this at all. This has not happened in every single situation, but most since I started using this in about October. Find below some sample image... please bear with the "ugly" images as they are meant to show the problem and have had minimal steps done. Number 1,2,3 have all been taken with about a 25% moon but many degrees away with about 50 mins each on each image, and the last image is a short 20 mins, but no moon was present.     |
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Hello, In order to rule out light leakage you could put the lens cap on and do a long exposure light frame while pointing some lights at the image train from different angles. But make sure no light is entering from the front as the lens cap does not seal the lens perfectly. If you happen to notice that light is infact leaking try to narrow down from where it is coming. Does this occur on the individual light frames as well? Try stacking without flat frames and see if this issue also comes up. If not this might be a problem with the flat frames. This has actually happened to me once using the Samyang and by simply retaking the flat frames the issue was resolved. (I never figured out why the first batch of flat frames was bad). Good luck! |
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Hmm, not sure I can help Chris. If it was light leaking into the optics late in the train (like between lens and camera, I would think these to look like asymetric flairs. Correct me if I am wrong, but the issue I see is a faint ring of light that is quite large. But not quite a ring... Looks like it has an octagonal component to it? Maybe 9? Because the Rokinon has 9 blades to its f-stop aperature, maybe this is playing a part. Is the lens stopped down in these shots. It doesn't look like there is any real vignetting from the aperature, but maybe there is some light reflecting off of the back of the blades. With no stars, it is hard to know these answers. One way I could imagine light reflecting off of the f-stop blades could be whatever type of filter you are useing. If it is a dichroic interference filter LP type filter, those filters reject unwanted light by basically reflecting the light back out of the lens. This light, which for an LP or UV/IR cut filter would then be the rejected wavelengths. But if this is reflected, you would think that it would just be cut by the filter in any case. Maybe some other internal reflections? Is the lens a used lens? Do you know if someone has worked on the lens. I have seen videos of people modding the lenses and getting into the area where the blades are. It can be tricky putting things back together again properly. Hopefully someone will know better and I apologize for the large number of speculations. Good luck. Edit: And of course I reread your post and find my questions answered by you in the first place. But I see you did have it stopped down a bit. I would try repeating the same artifact and then when you see it, open the aperature wide and see if it is the blades being the culprit. |
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Hi Alan, Thanks, and I also seem to see an octagonal shape as well. The blades were where I kind of thought of right away. Lens was new back in October. I can trying changing the F stop up and down to see if anything changes or not. Thanks for the input on this, and I'm hoping to get it figured out. It is pretty much impossible to see it when a single sub is finished even when stretched. The stars have to be removed to find it, and obviously the more images that sack the more it becomes obvious. Chris. |
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| If it is a faint internal reflection involving the blades when stopped down, then you would thing this would calibrate out with flats. Key would be sure to set you f-stops the same when taking flats. Only other thing is that maybe the way you set up your flats are somehow not able to deal with the blades in an equitable way. |
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| The F-stop remained the same for flats, and I re-took some sky-flats even to try something different than the flat master flats. So I'm still scratching my head on it if it is flats not doing their job or something else. |
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Does this occur on the individual light frames as well? Try stacking without flat frames and see if this issue also comes up. If not this might be a problem with the flat frames. This has actually happened to me once using the Samyang and by simply retaking the flat frames the issue was resolved. (I never figured out why the first batch of flat frames was bad). It appears to be in individual frames after a very heavy stretch. I have stacked without flats and that didn't seem to change the outcome. I have also re-taken new flats from both a flatmaster and sky flats and that also did not remove it. Chris. |
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It appears to be in individual frames after a very heavy stretch. I have stacked without flats and that didn't seem to change the outcome. I have also re-taken new flats from both a flatmaster and sky flats and that also did not remove it. What does a heavy stretch of your flats look like? Or the master flat? In theory it should not be there since it fails to remove the ring, but if it was inherent to your setup, then it really ought to be seen in the flats. If you can't see any sign of it in the flats, then that might be a tool to help solve the mystery. Dumb question, but I assume you did not move focus before taking your flats? Also, try taking an image during daytime of some rather bland object to see if is seen then. Say the side of a bland cloud. It kind of a flat, but. During a daylight image session, wile you have live frames coming in to you computer monitor the image as you rack the focuser in and out. Try at different f-stops and see if you can reproduce the effect. Shoot a distant bland wall. If you can see anything, then try to see if you can block them to find where they are. |
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| That is an image of the exit pupil of the lens, which is quite close to the diaphragm. My take is something is not properly blackened either within the lens itself or between the lens and the sensor. Try opening up to f/2 and then back down to f/8 and see how this light area changes. Hopefully this will point out at where the issue is. |
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Alan Brunelle: I can't see it in the flats from what I can tell. The focus point remained the same as where the last autofocus point would have been during the lights. |
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andrea tasselli: Thanks for the info. This is going to the next thing I try when a night allows it. |
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Chris Parfett:Alan Brunelle: Ok. I think what @andrea tasselli said is spot on. But the question remains as to what is illuminating that. It may be poor blackening or light leakage, but you would think that you would see it in the sort of flats you have done. Stop down all the way and look in the front and back of your lens. It should be obvious if the blades are not blackened. Also in a dark room, blast a bright LED light at the outside of the lens near the f-stops ring and all the joints down there and between the camera. Shield your eyes from the external light. Leave only the camera off and look into the back to see if light is getting through. If you cannot immediately resolve this and cannot return the lens, then I would not use the built-in f-stop. Get an aperture stop down ring and use that. In any case, you should quickly learn what your typical f-stop setting is and that should not change. Buying an aperture stop-down ring at that f-stop and using it should be the preferable way to image anyway. Unless you like the multipoint diffraction spikes on your stars. I'm afraid I have not helped you and not sure I have anything more I can add. |
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Thanks Alan, I will try a few things listed here and see if I can find anything. |
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| @Alan Brunelle @andrea tasselli After some further investigation the ZWO lens adapter does have an exposed aluminum flange that does face the camera sensor. Could this possible be the problem that is maybe causing an internal refection?   |
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| I do the shiny aluminum. Also the diaphragm blades look typical. If it turns out to be the culprit a bit of flat black paint out to do it. Still check for light leaks. |
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| I did run a fairly bright flashlight at the joints and couldn't see anything that made it through. I'm going to paint that up and re-test. The pictures are not the best to show the full aluminum ring edge, but it does have a similar shape to the artifact in the image. |
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I had a similar issue not with my 135mm, but with my widefield refractor. In the end I end up figuring out it was the moon. In your case you don't have aparently that problem with moon, but I bet you have like straigh light in your area. Like lamp posts, neighbour.. something that is reflecting light in your lens. Did you try a dark site? Or at least, a place without direct artifical light? Clear Skies, Cesar |
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HI Cesar, Thanks for the reply. The closest artificial light is about 500 yards +/- away. When I was shooting the M81 area just recently with no moon, the lens was pointing directly away from the closest light. I'm in a rural area with no ultra close neighbours and zero street lights. The ability for me at this time of the year to travel to a dark location isn't ideal until the spring. |
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Chris Parfett: Hello Chris.. well the other thing that I can think off is internal reflection. Some manufacturers like Altair (My filter drawer is from Altair), they recommended to put something around the filter drawer like a cloth or something in order to avoid direct light coming into the imaging train and somehow reflecting in the glass elements (to be honest I’ve never ever done it), that could be one of the things in order to take that out of the table, but from what I saw in your images, you have the blades from aperture engraved in the image, which makes me believe that the reflection is happening before light enters in the aperture blades, and not the other way around. But anyway you should take the possibilities in order to narrowed the problem.. I think you’re using heating bands also, right? Im saying that because, dew can be also the culprit. |
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Observatório Astrográfico do Boqueirão: I am using a dew heating strap, and I check the lens after every run when it comes back to home for any sign of dew before I cap it. I also use the anti dew feature of the camera as well. I attempted to run a bright flashlight around the joints of the drawer and couldn't detect any light getting in, but wrapping it could be another step to figure things out. |
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Well Cris, putting things that way I can't help in any other way. I had the same issue with a refractor of mine, but the problem in my case, the one who was causing me donut artifacts on the files, was moonlight. I was too close to the moon when I did those shots. It was when the Tsuchin-something comet, crossed the Leo Triplet. I tried two sets of Flat Frames, and the problem persists. I didn't know back then what was seems to be the problem, but eventualy I figure it out it was almost direct light comming to the front lens. What I can suggest at the moment is to open the aperture at his fullest. F/2 to be specific. If you want to step down at F/2.8, just use a filter reducer on the front of the lens. List one of these: https://www.amazon.es/dp/B01FS6JDRQ/?coliid=I2666S6DS12HJZ&colid=FFGC41KBZX3C&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it This method also have the benefit of put your stars completly round without any aperture blades difraction spikes. Hope you'll solve that issue soon. Regards, Cesar |
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Hi there! Let me start by introducing myself. My name is OSC Neon Palette, and I use a Nikon D5600 as an amateur astrophotographer from Panama. I'm sorry to everyone for my writing; I'm not native to English  .This reply to @Chris Parfett. I recently encountered internal reflections with my Sigma 70-300mm f/4.0-5.6 APO DG Macro Lens for Nikon, however the circumstances were different. I've been using this lens for more than two years without experiencing any issues, thus this issue drove me insane (i.e., an unprocessed photo between 2023 and 2024). The biggest distinction is that, in 2024, I relocated from the rural Bortle 4 area to a Bortle 7 city.  Initially, I thought it might be my filter (Baader Neodymium Moon & Skyglow 2") or my adapter. However, I obtained the same result with and without the filter. I even tried using another Optolong L-Quad Quadband filter and borrowed an SVBony Duo-Band OSC from a colleague, but the result remained unchanged. I also considered the possibility of flat frames; I stacked images with flat frames (using panel, sky, and manual) and without flat frames, but unfortunately, the result was the same.After reading various posts and responses from other colleagues, I realized that lenses for DSLRs struggle to capture optimal photos at F8 or lower in Bortle 7 conditions. I conducted various tests with many exposure (i.e., 30, 60, 120, 180, 240, 300) in different scenarios with both full moon and new moon phases. Another issue I encountered was that, currently, I can only capture photos from my balcony starting at 40 degrees elevation. To test this theory, I took a comparison shot at F11 versus F8 (both at ISO 800, 40 degrees elevation, 300mm focal length, without filters, and without calibration), and this resolved the issue. I experimented with different focal lengths (70, 100, 135, 200, 250, 300), and F11 consistently produced optimal results. On another hand, I had around 2 hours of data with this issue, but yesterday Pixinsight released a new version with gradient correction, as neither DBE or GrapXert could resolve the aggressive gradient and donuts. After conducting various tests with my data, this issue was resolved using this new feature.  I used the following settings: everything in default, except High Threshold set to 0.40, Scale set to 2.20, Smoothness set to 0.12 with automatic convergence, and Structure Protection disabled for this issue and data.  I hope this response can help others who are experiencing or have experienced the same issue with their DSLRs. Everyone can take this response to perform different tests or improve the English wording . It doesn't bother me at all.Clear Skies, |
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Nice post OSC. In addition to your solution, which may not address the cause, your photos are very clear. You can actually see the blade edges of the f-stop diaphragm. There can be only one cause of that and that means there is a source of light that is illuminating the backside of the f-stop blades. Or the frontside image of illuminated f-stop is being reflected off the backside of one of the lens elements. For you, the added light pollution is adding to the stress of the system. For Chris, it appears the effect is strong enough from the natural skylight. This is another strong reason to not use the f-stop diaphragm of this lens for astro work. Another issue I found with this lens is the stock dew/light shield sucks. I never trusted it and quickly made an extended shield. See my  10 Degrees in Monoceros - Some Familiar Objects - A First Light in OSC rig in this discussion. At my Bortle 4 site, it made an instant improvement in the background of my subs the night I installed it. To give you an idea of what light is getting into the lens with the stock focuser, take a straight stick, align it from the lens edge to the far outer edge of the shield. Everything in the sky within a circle that scribes is light getting to at least the edge of the objective. Much of that light is peripheral light and oblique and that could be the source of the light generating these problems |
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OSC Neon Palette: Hi OSC, thanks for the detailed write up! Nice results from the new PI process on your data. I have also used it with a few different settings and it is not able to remove the artifact. I will continue to use the new process with different setting, but initially it doesn't look like this will be able to be removed. Chris. |
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Alan Brunelle: Hey Alan, what did you make your extended shield from? Edit: I see it in your description. I missed that. Did you leave the OEM shield on and attach it to that? |
