Thanks for the further clarification.
As I'm using an integrated Paracorr, SIPS, I need to find out if the set position of the Paracorr, although correct for my Televue eyepieces, is also optimal for the Antares and if not what additional or reduced spacing is required.
Most NVDs can't be used in prime focus mode with the SIPS placed optimally. Mine (an OMNI-M) has the photocathode unusually close to the telescope end, and still needs 7.5mm more in-travel than a 21mm Ethos, and that's on a 'plain' P2 where part of the power button can be put closer to the focuser that the tunable top's shoulder. On a SIPS scope the power button interferes with the FT focuser, so that costs you 5mm more.
Detuning a Paracorr 12.5mm doesn't do wonders for the spherical aberration and the coma correction, although at 7.5mm off optimal you're still OK (you're not using very high powers which tends to hide the spherical aberration somewhat, and the 40° AFOV effectively means you don't see the worst of coma either).
A reducer only makes things worse, since the effective focal plane of the reducer+NVD is pushed even further out.
You can use the NVD, but you usually just ditch the Paracorr or SIPS and live with the coma of your scope and the extra aberrations caused by the reducer. Which is (given the small AFOV of the NVD) doable for a f/4.75, but less so for a f/3.72.
If you want something better corrected, you're better off removing the SIPS and using a TS Maxfield 0.95x coma corrector (which has a removable stop and can be inserted into the FT focuser). Or -- for more reduction -- the Starizona or TS coma correctors/reducers for 0.73x-0.75x, if you can ge them without a lip.
Once you start to barlow an NVD those woes with a P2 or SIPS disappear if you use a screw-on barlow°, since the barlow will move the effective focal plane closer to the telescope. And then the SIPS or the P2 is a boon since it's easy to ge the coma corrector on the right spot, whereas with screw-on coma correctors getting the spacing right is hard.
°e.g. a Baader VIP element in a M34-T2 adapter, or a Zeiss barlow element plopped into a Baader PushFit adapter with a M42->M48 ring on the Zeiss barlow's M42 threads
Edited by sixela, 05 August 2022 - 05:36 AM.