WO Zenithstar 61 spacing issue

[micovn]

Hi all,
I would like to replace the camera rotator on a William Optics Flat 61A field flattener (https://www.firstlightoptics.com/reducersflatteners/william-optics-adjustable-flattener-for-zs61.html). I think the StellaLyra with the 54M thread might work (https://www.firstlightoptics.com/rotators/stellalyra_caa_m54.html), but I am concerned that the spacing won't be right. 
The original rotator is roughly 18mm thick (measured with a ruler) whereas the StellaLyra appears to be 35.2mm - 5mm (male thread) = 30.2mm.

The Flat 61A flattener is normally operated at 12.9mm, let's call it 13mm. To account for the extra 12.2mm thickness (30.2mm -18mm) of the new camera rotator I should operate the Flat 61A at about 1mm. Does this make sense? Has anyone else tried to replace the camera rotator on the WO Flat 61A ?

Thanks in advance,

Michele

[symmetal]

The rotator is located before the field flattener lens elements, so the FF spacing requirement is unchanged. If you had the FF adjustable spacing set to 12.9mm before changing the rotator it will still be 12.9mm after changing the rotator no matter what thickness the rotator is. The only change necessary is that as the StellaLyra is 12.2mm longer, you will need to rack the focuser in by 12.2mm to regain focus. 🙂

I have a Flat 61A in my spares box without a rotator on the front, (not sure when I got it or why), and it has an M54 x 0.75 male thread on the front, so the StellaLyra rotator should fit  fine, assuming you can rack the focuser in by 12.2mm from its current focused position to accommodate it.

Alan

[micovn]

3 hours ago, symmetal said:

The only change necessary is that as the StellaLyra is 12.2mm longer, you will need to rack the focuser in by 12.2mm to regain focus. 🙂

Thanks Alan, this makes so much sense!

Of course, in order to have a flat field what needs to stay constant is the sensor-to-flattener distance, everything else doesn't matter, as long as I can reach focus!

Never been good at optics!

Thank again,

Michele