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| I am trying to figure out why I have such horrible vignetting in my images, vignetting that is so strong that I am finding it nearly impossible to remove in post. In a recent conversation with fellow imagers there was a suggestion to remove the f6.3 focal reducer. My equipment is as follows: 5 inch SCT (1250mm) f6.3 focal reducer, 105mm of spacing between the reducer the sensor of an ASI294MC, 1.25 inch UV.IR cut filter. The suggestion to remove the reducer was based on the idea that use of a reducer with a 4/3rds sensor is not a compatible mix of components...is this correct? |
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| Did you try to remove the focal reducer? And if yes, did it solve the issue? |
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I am using the same camera as you with a 1050mm Klevtsov-Cassegrain and a 0.5x reducer. I think that's quite similar to yours. Vignetting is very visible indeed but is also corrected 100% with flats. This is the price of using a reducer, yes it is reduced if you remove it but then you are working with a slower telescope at a much larger focal length so you have completely different field of view, tolerance to tracking errors, required exposure times and so on. How do you place the reducer? For me it is camera+T to 1.25" adapter + reducer. That gives a 45mm distance between the sensor and the reducer which is not ideal as the recommended distance is 55mm. This has the effect of smaller reduction (effective focal length becomes 623mm instead of 525) and some aberrations at the edges. I have decided life is too short to worry about that missing 1cm. But vignetting is the one thing I do not care about at all since flats take very good care of it. To answer your second question I don't think 4/3 sensors have any particular problem with reducers. If anything, the smaller sensor is less prone to any kind of optical issue because it is at the center of the image circle. If you plug a DSLR instead you will probably be horrified at the extent of vignetting and other aberrations outside the central 1/3rd of the image or so  Cheers, Dimitris |
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| Hello DKamen and thank you for the reply. I have the same understanding regarding what to expect when using a reducer. I like the faster imaging and wider field of view that comes with using the reducer but struggle with the consequences. My limited knowledge of Astro gear led me to seek out other opinions and experiences. I believe the real problem I am facing must be with the Flat files or how they are being subtracted from the image? Post processing is where I have no skills, the individual subs look good but when stacked and stretched the vignetting is so bad that it looks like the telescope is imaging through a tunnel. |
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Flats are not subtracted, but divided. Calibrated image = raw / master flat. (the formula is more complicated because you also have dark and bias but let's ignore this for now). All postprocessing software can use flats correctly, including the free software Siril. You probably need to create a good master flat. What I do: At the end of the imaging session, I take the telescope with the camera and focusing as they are (undisturbed) and put a tablet displaying a flat gray image in front of it, at a 90 degree angle. I take 10-15 shots making sure peak signal level is about 20,000 out of 65,000. The shots need not be of the same gain as your lights but it is simpler if they are. Their duration will typically be much smaller than the lights, for me it is 0.15 seconds at 120 gain. Then these frames are stacked to produce the master flat. If your flats are too dark or too blown out or were taken with the optical path altered (say you defocused a lot, defocusing just a little is not really much of a pb) or include a gradient because your light source (tablet) was not placed perpendicular to the scope obviously correction will not work as expected. Best way to be sure is to produce the master flat (simple stacking) and inspect it. It should show roughly the same vignetting as the stacked lights you want to correct. |
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| I am becoming more and more convinced that the core of this problem revolves around my flats! Also, I did use a light tablet but the illumination I chose was white, I shall try grey...it may be that my cal frames even though they have an adu value of 25k they may be over exposed? This would explain why the dust bunnies and vignetting are not as prominent in the flats as they are in the subs. |
