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I can use another set of eyes on this issue I'm running into. I'm testing out a new scope and flattener/reducer. I've tried multiple back focus adjustments and have adjusted the adjustable flattener/reducer. No matter the image or position in the sky I keep getting this color issue. I'm using an ASI294MC Pro. Has anyone seen this before? I'm not sure, but I think this is a type of aberration. I'm focusing using a bahtinov mask at the moment as I haven't set up the focuser yet. The mask is saying that I'm at focus, but I can focus sharper and get the FWHM lower and stars sharper. Then the mask is showing it's not in focus and giving this awful color issue. Any insight as to what this is? Thanks for the help!  |
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| It looks like the colors are not overlapping correctly, probably an optical issue. Just my 2 cents. |
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| That's what I'm leaning towards as well. I did a little search on the web and found that certain aberrations will cause the light to not "line up" properly causing purple or blue colors to appear. Purple from the blue and blue due to the red not lining up. I just want to exhaust any other issues before I bring the scope back and find out it's me and not the scope... |
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| The flattener/reducer doesn't work very well (not many do) with that scope and you have residual astigmatism (with some tilt) and loads of lateral color aberration, which is quite typical of refractors. I would avoid if I were you... |
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| Thanks to the both of you for taking the time to give some feedback. Much appreciated! |
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I’m wondering if that scope doesn’t work well with small pixels. I tried 2 flt91’s with my 294mm pro and returned both of them. Went with another brand unfortunately. The scope build seemed very nice but I couldn’t get useable images. You (like I did) have aberrations in the centre of the field. I spent many nights on both scopes trying to mess with tilt and backfocus but in the end WO asked for the lens cells back. I’d return it if you can. (I also got so fed up with adjusting that damn flattener that I went with a petzval design for my replacement scope lol) |
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Thanks Sean! Yeah, I spent a handful of nights trying to ensure it wasn't user error. All in all I spent 10-11 hours adjusting it. I'm not opposed to putting in time for performance, I just never found it. I've never had such issues with a 4/3 sensor. I thought it would be an easy way to test the scope and make sure everything was performing properly prior to dressing it for photography. The thing that I struggled with the most was the focus issue. I've never had a scope show that it's focused with a mask yet I'm able to focus it considerably more with numbers on NINA that are showing 2.40 with the mask and then I can get it down to 1.98 without. Yet when I put the mask back on it shows that it is considerably out of focus. Is this something that is typical with a refractor, or any other scope for that matter? Maybe I've just never noticed it before and that's normal. I'm not sure. I reached out here as I can't seem to find answers on the web pertaining to this issue I'm running into. Thanks to everyone who is trying to help shed some light on this issue of mine. I appreciate it! |
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This looks like a combination of tilt and backfocus issues to me. Not to cast doubt that your telescope is totally hosed, but the star colours look like a processing issue more than an optical one, at least in the middle. It could also look that way because of how low it might of been shot on the horizon. Do you have an unprocessed sub-frame to look at? The problem with glass configurations (aka refractors) is that a lot of issues can masquerade as each other because the lenses are designed to work at a very specific backfocus, and any deviations can introduce a whole host of chromatic aberrations depending on how bad it is. Here is my FLT 91/Reducer combo with a 2600MC, so the chip is bigger and the pixels are smaller, both of which put a larger strain on the optics. It looked a bit like yours before I got the backfocus and tilt hammered out.  |
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Nice satellites lol. Yeah I’ve seen some nice shots from the flt91. That’s why i tried a second one. I’ve also seen processed pics posted that I wouldn’t settle for. For that kind of money the scope should be pinpoint across a m4/3 sensor. Tilt doesn’t ususally affect stars right in the enter of the frame does it? My camera works fine with no tilt adapters on my new telescope, and i had no problem adjusting the flattener and the reducer on my ed102. Both flt’s I had had aberrations across the whole field that changed with backfocus, and rotation of the camera angle adjuster. Like I said, WO looked at test frames and asked for both lens cells back. I read about issues with the new internal focus gt81’s and decided to go with a different brand. i really wanted the flt91, but a test night with the askar65 sold me on it. It just worked. if you look at the op’s pics you’ll see stars that are stretched circularly in one corner, and in the opposite corner they point inward. I had the same problem. I spent hours and hours with the adjustable flattener turning it 1/16 of a turn, focusing and taking a pic. I even tried a different camera and had the exact same aberrations. I tried without the flattener and had the same aberrations. |
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Sean Mc: The Op's picture suggests both tilt and backfocus issues. If it was just backfocus, then the star shapes would be the same in all the corners. However, they are not and that is where the tilt comes in. |
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| Except that the problem exists in the center of the frame. That means that either the lens cell is decentered or the tube isn’t straight. A tilt adapter won’t help with that. I had the same problem and tried to get rid of the tilt but it’s not possible. I’ve read of other people having the same problem with this scope. Sometimes a replacement fixes the issue, sometimes (as in my case) it doesn’t. |
