22554RE: [William Optics] RE: FLT 110, P-Flat IV, SBIG 8300c
- Feb 26, 2014
I am using the same set up with a ST8300m and Starlight Express filter wheel but have the FLT123 scope. I agree Ivor’s approach. I needed to do some minor adjustments by taking a series of star images and fine tuning until I got good stars at the periphery of the frame.
The distance is critical
Yes you definitely need to add a spacer. WO have made the whole setup very confusing which is a shame as it is a great product; it took me quite a while to get my head around it.
Your goal is to get the chip 73.5mm away from FF lens and you have to kind out calculate the spacer back to front as the scale is 66 -,86mm.
By setting the FF to 66 (you went the wrong way) you have made a delta change of -7.5mm, the ST8300C has a back focus of 17.5mm
therefore you need to a spacer close to [73.5 - (7.5 +17.5)] = 48.5mm. This is not way I'd recommend as you want the FF to be in the middle of the range to allow for minor adjustments either way.
I found it easier to approach the problem from a different viewpoint , to keep the FF at 73.5 I need the chip to be 55mm away from the FF collar so you need a spacer of (55-17.5) = 37.5mm. As with mine I couldn't get a perfect spacer so I then made a minor adjust on the FF to compensate.
Hope that helps.
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