I was hoping I can get some guidance with my FLT91. I was imaging with a full frame 6200MC Pro and I get triangular shaped stars at the corners. They point to the center of the image. The combination scope and dedicated reducer should be able to handle a full frame camera. I’m not sure if I am doing something wrong. I do think I have some backfocus issues but I figure I should get elongated stars not triangular. My last image of the Sadr region was corrected by BTX but I really don’t want to rely on it if I can fix the image train. https://www.astrobin.com/hwnzoz/-Harry
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Don't you hate that!
Could be tilt or collimation, which can be annoying to correct. But I suspect it can be fixed by slightly changing your back focus. I had similar artifacts in one corner that were corrected completely by moving the camera a bit closer to the scope body. That is made easy with the FL6AIII by just screwing it in or out a bit and checking. This was with the 2600, which is the APSC version of your camera.
The specs don't take into an account the broad range of variability in everyone's setup. So it's not unusual nor wrong to try shortening or lengthening the flattener a tad and see what happens.
I did just that one night. My scope is the GT71. Disconnected the camera and turned the flattener/camera assembly one turn, reconnected the camera, and took a shot. Doesn't take long to see results, better or worse, and then fine tune.
Try this before the other steps. Refractors and cameras are both closely calibrated at the factory and don't shift easily. It's not a great idea to change those settings until you know you don't have a choice.
Why one corner? Could in fact be a bit of tilt. But it also could be simply that the camera sensor isn't perfectly centered in the optical train and you are seeing the beginning of quality drop-off that occurs in all corners, but visible in the one your camera is closer to. Especially with the 6200, which is trying to deliver the scope's entire FOV.
Good luck! I'd enjoy hearing how the scope works out. It looks like a nice instrument.
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Thank you. I thought it might be tilt along with backfocus.
the scope is in New Mexico. I’ll be going there in 2 weeks and I’ll play with it.
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I have a Pleiades 68, and it is very sensitive to back focus. The Askar back focus adjuster is nice if you have space for it. It allows you to very precisely adjust the BF without having to take the image train apart to add or remove spacers. It works great on my scope.
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Wait a minute. I may have overlooked something obvious in your description.
Just to confirm, are you using a 6200 camera with a flattener-reducer? Normally, a reducer (such as the 6AIII) is designed to shrink the scope's full frame image field down to APSC size. So, of course, one using a full frame camera might see quality drop-off in the corners. WO also offers a 1-to-1 flattener for using the scope's native FOV with a full frame camera.
I apologize if I misunderstand your specifics!
BTW, there's nothing wrong with using a reducer with a full frame camera to maximize FOV, but it would be expected that you would want to crop in a little on the final image to rid it of corner distortions. And/or rely on BlurX to fix.
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Here is a JPEG of the unprocessed, but stacked image. it is pretty evenly distributed that the stars are triangle shaped. If there was tilt i would expect problems on one corner more than the other. There clearly is a back focus issue but I was not clear on the triangle shape.  |
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I had the same problem. Cross shaped stars no matter what i did. Two different flt91’s. Wo wanted the lens cells back. I just returned the scopes and went with something different unfortunately. I hope you can figure it out!
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They are not "triangular", it is astigmatic coma with astigmatic field curvature at the extreme corners with, potentially, poor tracking (witness the elongated stars at the center of the frame). All it points at issues with either position of the corrector/reducer and/or the capacity of said corrector/reducer to do its job. And obviously there is tilt, as the rotation of the astigmatic lines imply, but to a much lower degree that the astigmatic field curvature.
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Thank you, Thank You, THANK YOU!
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