Effect of Paracorr on Off Axis aperture mask and vice versa

[Moonshane]

Hi all

I am building a 16" f4 scope and will be using a Televue Paracorr with it. I also plan to make a 7" off axis aperture mask and assuming the 'naked' focal length of 1600mm this would effectively create an unobstructed 7" f9 instrument.

Does this mean that for use in this format I don't need to use the Paracorr or is the inherent coma in such a fast 'naked' mirror still there even when used at 7" f9 with the mask?

Furthermore, assuming that the scope does still require the Paracorr, and that this will effectively create a focal length of 1840mm, will this equate to an unobstructed 7" f10.3?

I'd be interested to hear views on this but will hopefully be able to try it out for myself sometime soon!

Cheers

Shane

[FraserClarke]

Short answer; yes, you'll still need it because you are using the off-axis part of the pupil which dominate the coma aberations. I've made quick ZEMAX (optical design software) plot of the PSF for the full aperture and with an 7" off-axis aperture mask. The large circles represent a 10-arcsecond diameter. The small circle is the airy disc (0.34 arcsec). I made plots on-axis, 0.1, 0.25 and 0.5 degrees off-axis. The exact shape of the off-axis PSF depends on where you look in the field, but you get the idea. Bascially, you just get a chunk cut out from the full aperture PSF.

Caveat: Why are you using an aperture mask? probably to try and improve contrast on bright targets like planets? In which case, you don't really need to worry about coma; because all your targets of interest will be small so not affected by coma...

[Moonshane]

superb, thanks TeaDwarf. you were quite right in your assumption about the reason for the aperture mask. I am hoping to compare the performance of this 16" f4 with that of my 6" f11 (i.e. same focal length) on planets. I am hopeful that the performance will be slightly better with the '7" f9' than with the 6" f11, especially as the former has a slightly better figure (1/8 PV vs 1/6 PV). I may also make a 6" aperture mask and use without the PC to make a direct comparison. will be an interesting test!

thanks again

Shane

[Peter Drew]

I've always been a fan of off axis aperture masks on a large enough primary mirror to make it worthwhile, attracted by the promise of refractor like images and no CA. This works up to a point but professional optician colleagues, in the past, have pointed out that this provides an image derived from a new aperture that is not a figure of revolution so would not quite compete with, say, a top end refractor of similar aperture. It would certainly be close and better as the native focal length increased.

[Moonshane]

Thanks Peter

I think your thoughts mirror (pun intended!) my own exactly. I'll never buy a top end refractor, let alone one that's a 7" aperture. so close but 'not quite competing' is good enough for me!

Cheers

Shane