Quote:
Originally Posted by OzEclipse
Hi Chris,
If the image you posted is an uncropped APS C, then the defocussed star diameter is about 15.7mm depending upon the specifics of your APS C sensor and blurring at the edges. If the FSQ 85 is the model with the 450mm focal length, then without any reducer this is a simple geometric similar triangles calculation:-
Extension required is approximately
15.5mm x 450mm = ~82mm
........85mm
I would retake the image with the Estatto at mid-travel rather than at one end measure the out of focus image and repeat the calculation.
Cheers
Joe
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Awesome explanation and diagram!
Quote:
Originally Posted by OzEclipse
Chris,
The 1.01x flattener, unlike a 0.8x focal reducer, will only very marginally change the position of the focal plane. However it will correct, flatten and importantly, act as extra extension that might save you needing to buy another 2 grams of $200 empty tube.
Joe
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I think the 65mm is mandatory. As you noted, he needs circa 85mm. He isn't going to get that from the flattener alone. The flattener comes after the focuser anyway. He needs the extension before the focuser and the OTA to get the focuser to work as it doesn't have a lot of travel.