Nirvana 16mm - extent of sharp central field

[omega3]

I received my Nirvana 16mm on Mon and managed to try it out last night. It was a little sharper when viewing the moon than my Hyperion zoom at the same FL. That's good.

I managed to use the Nirvana to look at the stars from Albireo to Lyra before the clouds arrived. The field was so wide that I had to peer to the sides to see the edge of the field! But what I noticed was that the stars were only pinpoint in the central 60 degrees, I had to refocus to get those further out to be points of light. Is this due to the fast F4.7 dob that I was using? Would it be the same with the WO UWANs and the Naglers?

Alan

[david o]

Alan,

I'm sure it was due to the fast 'scope.

The WO UWANs should perform similarly to the Nirvanas (clones of the WOs), maybe a little better.

By all accounts the Naglers will be pin sharp to the edge, even in a 'scope as fast as f/4. It's what Tele Vue's rep is built on, after all.

HTH, but bear in mind that I don't own, nor have tried, any of these EPs

[John]

I've used a Nirvana 28mm, and owned a UWAN 16mm and now Naglers and an Ethos. The TV eyepieces are indeed sharp to the edge in my F/4.8 10" newtonian although you can see the coma that all newtonians have towards the edges of the FoV and I'm wondeing if it's coma that you are seeing. The Nirvana's and UWAN's I've used have been 95% as good as Naglers, even in fast scopes.

It takes good quality wide field eyepieces to show coma - with less expensive ones it's masked by the astigmasim that the eyepieces themselves introduce.

Maybe a coma corrector is called for ?.

[michael.h.f.wilkinson]

It could indeed in part be field curvature and coma of the objective. Can you describe how the stars are deformed at the edges?

[omega3]

I had another go last night. I noticed that if I move the position of my eyes, stars became sharp further out up to the last 10-15% of the field. I think the stars that were not sharp were a scatter of light. Next time I get out, I will take more notice of the unsharp stars and report back.

Alan

[John]

Here is a useful web site that nicely explains and illustrates coma and astigmatism (and other aberrations):

Got Aberrations?

[omega3]

Thank you for the link. I took more notice of the distorted stars at the edge of the field and they are elongated with the long axis parallel to the outer edge of the field. So that looks like astigmatism but where would that be from?

Alan

[Doc]

I would say that the WO range are one of the better brands that try to suppress the coma inherant in faster scopes. I use the UWAN range and I would say I see coma in last 10% in my scope.

[michael.h.f.wilkinson]

Thank you for the link. I took more notice of the distorted stars at the edge of the field and they are elongated with the long axis parallel to the outer edge of the field. So that looks like astigmatism but where would that be from?

Alan

Astigmatism is usually from the EP, as I understand it.

[John]

If you are getting noticable astigmatism (rather than coma) with your Nirvana 16mm in any more than the outer 5% or so of the FoV, even at F/4.7, there could be an issue with the eyepiece. Nirvana's and UWAN's aren't perfect but they are pretty good, even in fast scopes.

[Doc]

What I see is like little comet tails around the edge of the FOV. They are not bad but if I refocus I can get these stars sharp but then the centre goes comet like.

As I say it's not bad with the UWAN's but cheaper eyepieces it's not nice at all.

I think this is coma not astigmatism, hopefully someone will know.

[coliea]

Thanks John, that's an excellent resource.

Col

Here is a useful web site that nicely explains and illustrates coma and astigmatism (and other aberrations):

Got Aberrations?

[John]

Thanks John, that's an excellent resource.

Col

No probs Col

Only problem with it is it's a bit like reading a medical dictionary - you start thinking "oh, I think I get some of that ..... and a bit of that one as well.... "

Optical paranoia sets in !.

[Chris H]

If its astigmatism could it not be from your eyes?

[great_bear]

I think this is coma not astigmatism, hopefully someone will know.

It's not coma. You can't "focus away" coma. Also eyepieces don't correct for coma (only the Paracor corrector lens does that, and it's not an eyepiece. Some claimed the University Optics Klee Barlow did too, but I think that's unlikely and probably a misunderstanding)

I suspect that what you're seeing is field curvature revealing some eyepiece astigmatism. The WO UWAN 16mm does the same in our Sky-Watcher Explorer 130P. The field curvature is really strong.

[great_bear]

If its astigmatism could it not be from your eyes?

In conjunction with field curvature? Quite possibly, yes.

The givaway would be in the direction of the comet tails. If they are all in the same direction and extend equally on both sides of every star, then it's your eyes. If they radiate from the center of the view, then it's the eyepiece.