SCT field curvature

I'm noticing appreciable field curvature when using either of two good 40mm
(2" dia.) eyepieces with my 8-inch f/10 SCT. I can't get the center and the
periphery in focus at once. (My eyes are fairly presbyopic; I can't
compensate by just focusing my eyes.)

Are there eyepieces that compensate for the field curvature?

I know an alternative is to use the f/6.3 flattener and a 25- or 32-mm
eyepiece.

Thanks.

On Mon, 15 Dec 2003 20:47:30 -0500, "Michael A. Covington"
wrote:

I'm noticing appreciable field curvature when using either of two good 40mm
(2" dia.) eyepieces with my 8-inch f/10 SCT. I can't get the center and the
periphery in focus at once. (My eyes are fairly presbyopic; I can't
compensate by just focusing my eyes.)

Are there eyepieces that compensate for the field curvature?

My guess is that you're seeing the defects in the EPs, not the SCT,
Michael. Try to borrow a 35mm Pan - you'll be amazed.

Wayne Hoffman
33° 49" 17' N 117° 56" 41' W
"Don't Look Down"

http://home.pacbell.net/w6wlr/

"WayneH" wrote in message
...
On Mon, 15 Dec 2003 20:47:30 -0500, "Michael A. Covington"
wrote:

I'm noticing appreciable field curvature when using either of two good
40mm
(2" dia.) eyepieces with my 8-inch f/10 SCT. I can't get the center and
the
periphery in focus at once. (My eyes are fairly presbyopic; I can't
compensate by just focusing my eyes.)

Are there eyepieces that compensate for the field curvature?

My guess is that you're seeing the defects in the EPs, not the SCT,
Michael. Try to borrow a 35mm Pan - you'll be amazed.

Hmmm... I didn't think a Pentax XL was a bad eyepiece. Is Panoptic that
much different?

On Mon, 15 Dec 2003 22:52:19 -0500, "Michael A. Covington"
wrote:

Hmmm... I didn't think a Pentax XL was a bad eyepiece. Is Panoptic that
much different?

It is IMHO. The Pans are famous for their flatness of field. Not to
say they are without fault, 'cause you'll see a bit of pincushion, but
no other wide field EP keeps focus across the full field as well.

You should try before you buy (YMMV), but be sure to let us know what
you find.

Wayne Hoffman
33° 49" 17' N 117° 56" 41' W
"Don't Look Down"

http://home.pacbell.net/w6wlr/

My guess is that you're seeing the defects in the EPs, not the SCT,
Michael. Try to borrow a 35mm Pan - you'll be amazed.

Hi:

Perhaps. BUT, yes, the SCT's field _is_ strongly curved. The most cost
effective cure is a Meade or Celestron f/6.3 reducer/corrector. You won't be
able to use a 40mm eyepiece with one, but you won't need to.

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

"Rod Mollise" wrote in message
...
My guess is that you're seeing the defects in the EPs, not the SCT,
Michael. Try to borrow a 35mm Pan - you'll be amazed.

Perhaps. BUT, yes, the SCT's field _is_ strongly curved. The most cost
effective cure is a Meade or Celestron f/6.3 reducer/corrector. You won't
be
able to use a 40mm eyepiece with one, but you won't need to.

Right... I have one of those which I use a lot on my old C5. It's slightly
inconvenient to insert and remove it, compared to just changing eyepieces,
which is why I don't use it more.

Over the years I've vacillated about whether 2-inch eyepieces are a Good
Thing, or whether I should just use the reducer ahead of a conventional
diagonal and eyepiece. Right now I have a Meade 2-inch diagonal, on the
ground that it probably has some advantages w.r.t. optical quality (flat
mirror) and absence of vignetting even if I use only 1 1/4" eyepieces with
it.

I think I've secured an opportunity to do the Panoptic vs. Pentax comparison
in my telescope. If it happens, I'll let everyone know how things turn out.

Would I be better off not using 2" eyepieces at all (or just using a cheap
one, knowing that higher prices don't bring that much improvement)?

Michael,

I have found that some eyepieces are more sensitive to field curvature than
others. The better ones do not compensate for field curvature per se, but
rather minimize their own contribution to the issue. Unfortunately, the
problem is particularly acute with 40mm 2-inch widefield eyepieces and I
have not found one that works well with my Newtonian (even with a Paracorr).
You either have to use something like a 50 - 55mm Plossl or better yet use
the field flattener as you mentioned.

Del Johnson



"Michael A. Covington" wrote
in message ...
I'm noticing appreciable field curvature when using either of two good
40mm
(2" dia.) eyepieces with my 8-inch f/10 SCT. I can't get the center and
the
periphery in focus at once. (My eyes are fairly presbyopic; I can't
compensate by just focusing my eyes.)

Are there eyepieces that compensate for the field curvature?

I know an alternative is to use the f/6.3 flattener and a 25- or 32-mm
eyepiece.

Thanks.

"Michael A. Covington" wrote
in message ...

Would I be better off not using 2" eyepieces at all (or just using a cheap
one, knowing that higher prices don't bring that much improvement)?

The 24mm Panoptic in the C8 at F6.3 yields 53x, 1.23 degrees and a 3.8mm
exit pupil.

The 35mm Panoptic in the C8 at F10 yields 57x 1.11 degrees and a 3.5mm exit
pupil.

The F6.3 R/C can be had new for about $120.

The cost difference between a 35mm Pan ($365) and a 24mm Pan ($295) = $70.

The cost difference between a 1.25" diagonal and a 2" diagonal = (about)
$100.

So you save 50 bucks, you get flat field performance, and you get as wide an
un-vignetted field as you can expect from an 8" SCT, if you go with the
1.25" diagonal, F6.3 R/C and a 24mm Panoptic.

The optical cost is any appreciable loss of light transmission through the
added glass of the R/C. (I don't know what that is, but it doesn't bother
me).

What I have.....
C8 with the R/C in place:
24mm Panoptic = 1.2 degrees at 53x; 3.8mm exit pupil
13mm Nag T6 = 0.8 degrees at 97x; 2.1mm exit pupil
9mm Nag T6 = 0.56 degrees at 140x; 1.4mm exit pupil
7mm Nag T6 = 0.44 degrees at 180x; 1.1mm exit pupil

What I want to add.....
Nagler Zoom:
6mm = 0.23 degrees at 210x; 1.0mm exit pupil
5mm = 0.2 degrees at 252x; 0.8mm exit pupil
4mm = 0.16 degrees at 315x; 0.6mm exit pupil
3mm = 0.12 degrees at 420x; 0.5mm exit pupil

The above eyepiece set seems about perfect for all scopes with 1200 -1300mm
focal lengths
- Stephen Paul

"Michael A. Covington" wrote in message ...
"Rod Mollise" wrote in message
...
My guess is that you're seeing the defects in the EPs, not the SCT,
Michael. Try to borrow a 35mm Pan - you'll be amazed.

Perhaps. BUT, yes, the SCT's field _is_ strongly curved. The most cost
effective cure is a Meade or Celestron f/6.3 reducer/corrector. You won't
be
able to use a 40mm eyepiece with one, but you won't need to.

Right... I have one of those which I use a lot on my old C5. It's slightly
inconvenient to insert and remove it, compared to just changing eyepieces,
which is why I don't use it more.

Over the years I've vacillated about whether 2-inch eyepieces are a Good
Thing, or whether I should just use the reducer ahead of a conventional
diagonal and eyepiece. Right now I have a Meade 2-inch diagonal, on the
ground that it probably has some advantages w.r.t. optical quality (flat
mirror) and absence of vignetting even if I use only 1 1/4" eyepieces with
it.

I think I've secured an opportunity to do the Panoptic vs. Pentax comparison
in my telescope. If it happens, I'll let everyone know how things turn out.

Would I be better off not using 2" eyepieces at all (or just using a cheap
one, knowing that higher prices don't bring that much improvement)?

You did mention an f/10 8" SCT. IIRC the inside diameter of the baffle tube
is 1.5" meaning you'll also experience vignetting using humongous EPs -- look
athe front of your XL and you'll see almost 2" of glass!

There's nothing "wrong" about using a 2" diagonal on an 8" SCT; I do it too
with a Lumicon diagonal and a 1.25" adapter because the assembly provides a
sturdier mounting for some of my larger 1.25" EPs and a bit of extra mass to
help counterbalance a metal dew shield. :-)

On 16 Dec 2003 11:32:40 GMT, Rod Mollise wrote:

Hi:

Perhaps. BUT, yes, the SCT's field _is_ strongly curved. The most cost
effective cure is a Meade or Celestron f/6.3 reducer/corrector. You
won't be
able to use a 40mm eyepiece with one, but you won't need to.

Peace,
Rod Mollise

Rod,

Coming from the DOB world, and admitting to ignorance of the SCT world, I
would like to know why one can't use a 2" 40mm eyepiece with an F/6.3 (via
reducer/corrector) SCT?

Lawrence Sayre