At 11:02 AM 7/10/2009, you wrote:
>What is the highest focal length eyepiece for an xt10i?
>I recently tried a 40mm celestron eyepiece- good for star clusters, but
>the eye relief wasn't so good.

Bruce,

Which eyepiece was this? Was it a 2" model? I can't imagine that eye relief
was lacking but perhaps you mean that you had trouble with positioning your
eye. Many of the 40mm Plossls tend to be picky about eye placement.

>The exit pupil for a 40mm on this scope is about 8.4. Is the light loss at
>this focal length an issue?

It is correct that the large exit pupil will mean that all the light
gathered by the objective won't hit your retina. But in use the loss of
light is meaningless, at low magnifications you still get images that are
plenty bright.

The problems to be concerned with a large exit pupil are astigmatism from
your eye and reduced sky darkening making for low contrast. As for me in my
XT10 one night I got to compare a 27mm Pan, 31mm Nagler, 35mm Pan and a
40mm Pentax XL. The 40mm XL and to some degree the 35mm Pan showed greatly
reduced contrast due to the sky background not being truly black. I also
found that the 40mm XL seemed not to be sharp to the edges, all the other
eyepieces were. The 31mm Nagler was the easy pick of the bunch but I bought
the 27mm Pan due to its price/performance.

>Suggestions for a decent 40mm or lower eyepiece?

I'm not sure what your goal is going this low in magnification is, but in
my opinion I'd never use a 40mm eyepiece in an f/5 scope. One thing I like
to point out is to compare your lowest magnification eyepiece only on the
basis of its true FoV. The widest TFoV you will get out of an XT10 is
around 2.3 degrees. The 27mm Pan gives about 1.53 degrees which is enough
for me. Many of the 40mm eyepieces on the market have apparent FoV's of 55
degrees or less meaning that they will give TFoV's of 1.8 degrees or less.
This means that you get smaller image scale and reduced sky darkening but
you don't fit much more in the field then you would with the 27mm Pan! For
the lower power pick TFoV is the most important issue.

Clear Skies,

PL

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Phil Lefever - Burnsville, MN
Amateur Radio Callsign - KB0NES EN34jt
C8-SP XT-10 C102 80WV