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I just purchased a William-Optics Fluorostar 91 telescope and Flat 68 iii field flattener, and have a question about connecting a ZWO ASI 533 MC Pro camera. The W-O web site for the Flat 68 iii shows the back focus for the FLT91 as 75 mm, but also indicates the adjustment distance for a DSLR or mirrorless camera as 5 mm. Because of the large discrepancy between these two numbers, I’m a little confused about what to adjust the Flat 68 iii to, as well as what spacers to use between the Flat 68iii and the ASI 533 camera. Is the distance from the rear glass plane of the Flat 68 iii to the camera sensor supposed to be 75 mm as suggested by the web page? Or do I set the Flat 68 iii adjustment to 5 mm and then use spacers to achieve the usual distance of 55 mm between the flat and camera sensor? That is, since the ASI 533 sensor is located 6.5 mm back from the camera connection point, it seems that I have two choices: 1. Set the Flat 68 iii to 5 mm, and then add 48.5 mm of spacers to achieve a total of 55 mm, or 2. Set the Flat 68 iii to 5 mm, and then add 68.5 mm of spacer to achieve a total of 75 mm? Which if these two is correct, or if I’ve completely misunderstood, what should the total distance be? |
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Do you have the extra adapter? With the extra adapter, I needed 5.6mm including 0.6mm for my filter. Since the adapter is too fat to clear my zwo OAG, i instead used 5.6mm in spacers and set my flattener at 0mm, but that's the equivalent. I believe the adapter basically extends it by 10 or 11mm, so you should try to find the length difference. My opticap train is scope, flattener, 5.6mm spacers, zwo oag, zwo filter drawer asi2600mc. For your 533, you'd have that extra 11mm ring after the filter drawer. |
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Thanks for the info, Paul. My setup is 46.5 mm worth of spacers: M24-M48 adapter @ 16.5 mm Filter drawer @ 20 mm Spacer @ 10 mm The 533 sensor is 6.5 mm back from the front, so a total of 55 mm. I shot some images night before last, and there were some slightly oblong stars pointing inwards in the upper left and lower right corners, but the stars looked OK in the upper right and lower left corners (see the attached screenshot from PI showing the eccentricity). The inward pointing stars suggest that I need to move the camera away a bit, but I was a bit puzzled why the oblong stars were only in two of the corners. Since I don't have any additional spacers, I've bumped the flattener up to 7 mm and will try that and see how it works. Naturally, the next several nights are forecast to be cloudy... - Ken  |
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I think I finally figured out my problem. I originally ordered the W-O M80 to M48 adapter with the scope, but when the flattener arrived it already had an adapter attached so I returned the M80 to M48 adapter I had ordered. Big mistake! The "extra" adapter is apparently longer than the one that comes with the flattener, and is needed on the FLT-91 to get the correct back focus distance. Now that I know that, I'll reorder the adapter, which should make everything work well and give me nicely rounded stars. Ah well, RTFM! - Ken |
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Hi Ken. I dont know if this is of use.   William Optics sent me this. Also you have removed the visual tube? Mark |
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Thanks, Mike. I found the same drawing on CloudyNights, and that made it pretty obvious that I really did need the other adapter. After I received and installed it, things worked perfectly. Eccentricity is usually 0.4 or below, and fairly evenly distributed throughout the field. And yes, I did remove the visual tube; it was the adapter that threw me (I know, RTFM, right?). Now how to figure out how to get clear nights... - Ken |
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| Hi Ken, I just got the Flt 91 with flatter+adapter. I adj the flatter to recommended 5mm then from back of adapter 55mm to sensor and resulted in sharp stars corner to corner. |
