3 replies to this topic

[JWST]

- recently received the Nikon MC II and two EiC-16 converters.

 

- Trick 1: the Nikon MC II can be mounted directly in the EiC-16 without an adaptor. The EiC-16 grabs a narrow unthreaded band on the neck of the MC II, permitting the lower barrel to move without friction against the EiC-16 or against an improperly designed adaptor. Converts the MC II from 7-21 to 4.375-13.125. Bullseye for my preference with 1624 mm / RC8:  371x - 123x

 

- Trick 2: Nikon says the EiC-16 is designed to reduce astigmatism in the SW series. How is that supposed to happen? Well, the Teflon sealing ring on the EiC-16 allows the SW eyepiece to conveniently rotate in place before locking. If there is a small amount of astigmatism built into the EiC-16, this could cancel the astigmatism in the SW eyepiece, just as the TV Dioptrx does. Seems to work, but it was at the end of the observing session. Need to try it again.

 

BTW, the MC II is completely parfocal, unlike the Baader 8-24 MkIV and the SV135 (it is a little non-parfocal - proven with Bahtinov pattern while changing magnification). The SV135 is great. MC II Bahtinov-pattern tests at 371x revealed that the MC II is sharper yet and has less coma.

[CrazyPanda]

- Trick 2: Nikon says the EiC-16 is designed to reduce astigmatism in the SW series. How is that supposed to happen? Well, the Teflon sealing ring on the EiC-16 allows the SW eyepiece to conveniently rotate in place before locking. If there is a small amount of astigmatism built into the EiC-16, this could cancel the astigmatism in the SW eyepiece, just as the TV Dioptrx does. Seems to work, but it was at the end of the observing session. Need to try it again.

 

Astigmatism in an eyepiece should be radially symmetrical around the field, unlike visual astigmatism that requires an asymmetric corrector like the Dioptrix.

 

I would be VERY concerned if a barlow or eyepiece exhibited asymmetric astigmatism such that rotating one against the other would reduce or cancel it out.

 

Any claim that the EiC minimizes astigmatism in the SW line is likely simply due to the fact it's feeding the eyepiece a 1.6x more gentle light cone. It's also possible it's optically matched the the SW line and is doing more astigmatism correction than a longer effective focal ratio suggests, but it would be symmetrical and should not depend on rotational alignment.

Edited by CrazyPanda, 21 June 2025 - 11:13 PM.

[JWST]

Thanks CrazyPanda for this comment. Under better seeing conditions and fresher eyes, I can't say I noticed a big difference in position of the eyepiece vs position of the EiC-16, but the mount, tripod and draw tube at 300x-500x were not visually stable as I rotated the eyepiece while being sure the EiC-16 itself was not rotating. So the target star was moving around.

 

I have a DIOPTRX 0.5 and know how it behaves on my Ethos 6 mm. Any rotational effect of the EiC-16 was smaller. But there was a point in the rotation where the target star seemed to be a bit tighter.

 

Could you explain how radial astigmatism appears compared to coma and spherical distortion? I thought astigmatism was in essence bi-axial and therefore immune from compensation by adjusting the radial symmetric distortions like coma and spherical aberration.

 

With the luxury of two EiC-16s (a second one was necessary to achieve the minimum purchase amount from Kyoei-Osaka), I used one on the MC II at its 7 mm limit and one on the SW 7. Both pulled in the faintest stars in NGC 869 (Double Cluster) the same. But the MC II seemed slightly sharper on individual stars. Smaller FOV and smaller field stop?

 

If one eyepiece were perhaps ever-so-slightly sharper than the other at 371x, are we counting angels on the head of a pin?

Edited by JWST, 25 June 2025 - 02:25 PM.

[25585]

The EiC also flattens the field somewhat. Its also good for binoviewers, instead of a GPC.