Question about Pentax XL 40mm in C5+

I'm looking for anyone with experience using a longer FL like a Pentax XL
40mm in a Celestron C5+ (125mm, F/10).

I am thinking of expanding my EP collection for wider FOV. I have a 32mm TV
Plossl, which is currently my favourite EP, followed by a 20mm Orion
Expanse.

My concern is that with such a large FOV that a 40mm will offer, will I hit
the FOV limit of my scope?

Should I re-consider and look at a longer FL in a plossl?

Thanks,

Ed L.

Here are the measurements for the inside of all of the [Celestron Primary]
baffle tubes:
5" 25mm
8" 38mm
9.25" 48mm
11" 54mm
14" 54mm

True field of view equals the field stop divided by the focal length of the
telescope times one radian.

Treating the central baffle as the limting field stop (likely correct)......

C5 at F10: 25/1250*57.3 = 1.15 degrees
C8 at F10: 38/2032*57.3 = 1.08 degrees

Assuming (likely wrong) that in the case of the focal reducer the central
baffle is still the limiting field stop........

C5 at F6.3: 25/787.5*57.3 = 1.8 degrees
C8 at F6.3: 38/1280*57.3 = 1.7 degrees

In any case, you can't exceed the 1.15 degrees of the C5 at its native F10,
without vignetting.

-Stephen Paul

I have used a 2-inch back on a C5, and there may be some vignetting, but
it's not objectionable.

However, the 2-inch back will not swing between the back of the tube and the
base.

I suggest an f/6.3 reducer ahead of your 1.25-inch back and normal
eyepieces. That's what I use now when I want wide-field views. Again, the
vignetting is not noticeable (though it's surely there; I wouldn't do
critical variable-star comparisons).

Michael A. Covington wrote:
However, the 2-inch back will not swing between the back of the tube and the
base.

The dovetail plate on the C5+ allows some adjustment for this. I don't
know if that's possible on your vintage orange-tube C5.

Thanks for the little report on the 2-inch back for the C5. I wasn't
aware of anyone who had used it.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

"Brian Tung" wrote in message
...
Michael A. Covington wrote:
However, the 2-inch back will not swing between the back of the tube and
the
base.

The dovetail plate on the C5+ allows some adjustment for this. I don't
know if that's possible on your vintage orange-tube C5.

No...

Thanks for the little report on the 2-inch back for the C5. I wasn't
aware of anyone who had used it.

Glad to be of service. The other problem with the 2-inch back and eyepieces
is that the telescope was very out of balance. The f/6.3 compressor and
1.25-inch back works much better.

My C5 is so old that I had to special-order it with a 1.25-inch back. By
default it came with a 0.965" one.

--
Clear skies,

Michael Covington -- www.covingtoninnovations.com
Author, Astrophotography for the Amateur
and (new) How to Use a Computerized Telescope

Ed L wrote:
I'm looking for anyone with experience using a longer FL like a Pentax XL
40mm in a Celestron C5+ (125mm, F/10).

I am thinking of expanding my EP collection for wider FOV. I have a 32mm TV
Plossl, which is currently my favourite EP, followed by a 20mm Orion
Expanse.

My concern is that with such a large FOV that a 40mm will offer, will I hit
the FOV limit of my scope?

Should I re-consider and look at a longer FL in a plossl?

The C5+ has a 1-1/4-inch visual back. The Pentax XL 40 is, I think, a
2-inch eyepiece. To use the 40 on the C5+ would require the addition of
a 2-inch back (and probably diagonal, to save neck strain). Not only
would this get to be expensive, but I think the vignetting will probably
be fairly severe.

I would, instead, get a focal reducer/corrector. Both Meade and Celestron
sell one, and they're OEM, so get the less expensive one--usually, it's
been the Celestron. This will allow your telescope to work as though it
were a 5-inch f/6.3, giving you around a 1.8-degree TFOV with your 32 mm
Plossl. There's also vignetting with this configuration, but it's not
severe enough to ruin visual work.

Typically around $125 U.S., but shop around.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Brian,

Thanks for the great advice;

I'll keep an eye out for a focal reducer/corrector.

You were right, I don't have a 2 inch back, and I'm not ready to put out
that kind of money.

Stephen, Thanks for the math, I didn't know the details of how to work that
out.

Thanks,

Ed L.

"Brian Tung" wrote in message
...
Ed L wrote:
I'm looking for anyone with experience using a longer FL like a Pentax
XL
40mm in a Celestron C5+ (125mm, F/10).

I am thinking of expanding my EP collection for wider FOV. I have a
32mm TV
Plossl, which is currently my favourite EP, followed by a 20mm Orion
Expanse.

My concern is that with such a large FOV that a 40mm will offer, will I
hit
the FOV limit of my scope?

Should I re-consider and look at a longer FL in a plossl?

The C5+ has a 1-1/4-inch visual back. The Pentax XL 40 is, I think, a
2-inch eyepiece. To use the 40 on the C5+ would require the addition of
a 2-inch back (and probably diagonal, to save neck strain). Not only
would this get to be expensive, but I think the vignetting will probably
be fairly severe.

I would, instead, get a focal reducer/corrector. Both Meade and Celestron
sell one, and they're OEM, so get the less expensive one--usually, it's
been the Celestron. This will allow your telescope to work as though it
were a 5-inch f/6.3, giving you around a 1.8-degree TFOV with your 32 mm
Plossl. There's also vignetting with this configuration, but it's not
severe enough to ruin visual work.

Typically around $125 U.S., but shop around.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt