Correction of hyperions at F5?

[Ags]

I find my 24 mm hyperion pretty useless in my F5 ST80. It is fuzzy from about halfway out to the field stop, to my eyes at least. My question is: do shorter focal length hyperions perform a bit better? Do they zoom in on the good part of the field of the 24mm, or do they show the same errors?

[CGolder]

hello Ags, that should not be the case as far as focus as you are receiving the same light path so it all to do with how the eyepiece handles it,

ill be honest with you and say that the longer focal lengths are worse by design but even at F10 the hyperions still shows some fuzzy out of focus stars at the edges

still good value but far from perfect

[Zaphod360]

The 8mm performs very well with my f5 sw200p, I think that the 24mm is regarded as the worst performing ep (relatively) of the set. On slower scopes apparently it's not as obvious, on my scope it's not quite as bad, but I'd be interested to find out if this can be correct as the field of view is lovely.

[Ags]

I have yet to try my 17mm hyperion in the ST80, so I will post the results of that soon.

[Carbon Brush]

My longer FL Hyperions are a relatively recent purchase and have only been used in F8 upwards scopes, without problem. However, I have just put my SW200 on the pier so next clear night...Whenever that may be I will carry out a proper check.

[Ags]

I've noticed with many designs - hyperions, TS Planetaries - that the longer focal lengths are said to be the least well-corrected. Even with the Ethos EPs the 110 degree fields are only offered for the shortest focal lengths.

If I buy an F5 newt next year as planned, then perhaps I should sell both my 17mm and 24mm hyperions to fund a 17mm speer waler...

On another thread it was suggested that the field curvature of the ST80 was killing the view in the hyperion. If that is the case, then the shorter focal lengths will have less trouble as the will see less curvature. But it also makes it difficult to extrapolate from an F5 refractor to an F5 newt.

[ronin]

Would have said that the problem will occur on the longer focal length eyepieces.

I would expect that the image created by an 400mm FL f/5 scope will be curved. Therefore the eyepiece expecting a flat image field will find that the edges of the image will no longer be at the focal plane of the eyepiece so will not be sharp.

A shorter focal length eyepiece will only see a smaller part of the image created by the objective and so this smaller bit will be flatter across the field of the eyepiece. This should therefore give a sharper image across what is seen in the eyepiece.

TS do a flat field eyepiece that is designed to take into account the curvature created by scopes like f/5's. Maybe one of these would be better and less expensive.

Another aspect is that I would expect you are getting some spherical aberations as well.

[andrew63]

it's not so much the eyepiece - but given it's not a great performer edge wise. It's more of the field curvature produced by the lens of the refractor - which gives far more than a mirror based telescope.

andrew

[rwilkey]

I have always been unhappy about the performance of the 24mm Hyperion, but the 8mm is a good performer and sharp to the edges in my scopes, mainly the 200P.

[Ags]

THanks all for your answers.