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| In my recent sessions with my WO-FLT91 with the Flat6AIII flattener, I have noticed that the stars in all four corners are elongated, indicative of back focus issues. I checked this in the aberration inspector process in PixInsight and I am consistently seeing he same result in all four corners in all images. The back focus started at 60mm which is the required distance when using the Williams Optics FLT 91 with the FLAT6AIII, I have a filter in the image train so I added a 0.5mm spacer, but got elongated stars in the four corners. I have since added another 1.0 mm in spacers and still the same problem. Next session will see me adding a total of 2mm in additional spacers. Is anyone else using the Williams Optics FLT 91 with the FLAT6AIII and having difficultly with achieving optimal back focus? Any suggestions would be welcome. |
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| I know this is contrary to the typical solution, but what do the stars look like if you remove some spacers? |
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Maybe there's something I don't understand about backfocus, but isn't the Flat6AIII adjustable exactly so you don't have to deal with spacers? I have the FLT120 + FLAT68III and the lens of the flattener is on the telescope side. I can just loosen the ring on the flattener and unscrew it a bit to move the camera back few millimeters. |
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Miguel T.: Miguel, of course you are right I can do that. However I do not have a backyard set up, I have to travel to a viewing site, then assemble and disassemble my rig every session, so to minimize the possibility of something moving during transport and assembly I have locked the adjustment at flattener adjustment at 0 and use spacers to make sure there is no variability in the back focus. As a matter of interest how much have you loosened the adjustment to achieve good back focus? |
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Spencer Collins: Yup tried that, lots of eggs at each corner. |
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Ivan M:Miguel T.: I started at the recommended 10.7mm but I also have a ASI6200MM which of course had tilt issues, so with the help of the ASG photon cage that adds about 3mm extra distance I followed Hocus Focus advices in NINA for both the tilt and back-focus. I haven't checked the distance I'm at now but it's no longer 10.7mm. |
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I have a travelling setup as well but I bought a large enough case to keep the scope set up and ready for imaging. It does make it easier for multi night sessions... anyway. I set my 6AIII at (approx.) 10.5 mm and I seem to be getting decent enough results. Cheers! |
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| What camera are you using? The Flat 6AIII isn't really made to illuminate a full frame camera so I had the same issue with it on my FLT91. It was impossible to get round stars at the corners on a full frame sensor. I actually had to purchase the Flat 68III which has a much larger exit circle to get round stars in the corners. |
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John Rush: Hello Josh and Ivan, Actually I have the Flat68III and having elongated stars at the corners with the 2600. I've been troubled shooting this for months now and still no success. William Optics are making life harder because they are not generous at all in providing required information about their products, on the contrary they provide confusing information!! In addition when you email them they answer after very long time and they do not provide any clear information or instructions. Coming from the Redcat51 and then the FLT91 I believe this will be my last William Optics scope as there are better options out there where you don't have to spend much time troubleshooting instead of imaging.... Enough rant  In my experience with the system there are many causes of elongated stars at the corners, they come from: - Back focus distance, - Extension rings having flexure, - Focus draw tube flexure (you can confirm it if you hold your focus tube up gently and you will find it moves like 1mm or so). - Primary lens needs collimation. - Camera tilt. It's painful and is taking lot of time, but you have to eliminate all these reasons and each setup my have different combination of them!! I noticed that I had focuser tube flexure and after dissecting the imaging train and focus tube multiple times I noticed that when you install the EAF you have to remove the focuser lock thumb screw; as per the instructions of guess who >>>>>> William Optics user manual (See screenshot attached) this lead to the draw tube being un-stable. So I played around to have a similar sized screw which stabilized the focus tube again.. This improved the image but two corners are having noticeable star elongations. I'm left with Camera tilt and Lens collimation. I'm currently working on camera tilt using a special adapter. but If it comes to sense collimation I'm definitely getting a new scope because this will be too critical to adjust and I did not pay William Optics to trouble shoot their equipment Sorry for this long post and hope it helps. Also I will be happy if anybody can help with this as it will benefit us all! Clear Skies  |
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John Rush: For me I have no issues getting round stars on the Flat68III and even less so now that I pair it with the 2600MC. It's only a problem on a full frame with the Flat6AIII on my scope. |
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| Check out post 13 in this thread on cloudy nights https://www.cloudynights.com/topic/790549-need-help-setting-up-wo-flt-91-w6aiii-flattener/. There you will find a file that you can 3d print. You can replace the original locking ring with this 15.4mm spacer tube. This solved my backfocus/tilt issues on the FLT91/FLAT6AIII. |
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Torstein Thomassen: Awesome solution, once I establish the right value. Thanks for this. |
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John Rush: I am using the ASI2600MC Pro and by the time you consider the filter wheel as well, there may well be a tilt issue. Lost of great ideas to help figure this out, thank you. Clear skies |
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| I had the exact issue using the flt91 and same flattener, with a Zwo filter drawer. I got it straightened out. I have a Zwo asi 294 camera 17.5mm, filter drawer 21mm, spacer 16.5 mm, and a .66mm shim for the 1/3 the filter thickness = 55.66mm total backspace to the flattener. In addition, Glen Martins flattener instructions specifies 5.4 mm gap in the flattener. And that fixed my problem. Let us know how you do. Good luck. |
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I think things are getting a bit over complicated here. As someone who both lives in the same area as you and used the exact same camera and telescope configuration as you, I promise you that if you use the adjustable flattener, it won't slip or move if you travel with it. I have traveled south of Longview and as far east as around Brooks and it held together no problem. If you set the adjustable flattner to around 5mm, and then follow the ZWO guide to get the 2600MC to a distance of 55mm from the flattener you should be fine. If it is off by a little bit then you can adjust that 5mm on the flattener a little bit in or out.  |
