Prism vs. Mirror Diagonals and an AstroTech 66ED (I did not know that)

I just discovered through experimentation that my 1.25" Celestron Prism
diagonal (standard SCT fare of yore), requires more backfocus than does
my generic 1.25" mirror diagonal.

That's kind of interesting and unexpected, and at the same time,
problematic for my newly aquired AstroTech 66ED which comes to eyepiece
focus out near the end of focuser travel (don't worry, with the mirror
diagonal there is sufficient travel left to focus with a barlow.
Hence, most eyepieces should be good to use).

This is a wonderful little scope by the by, and highly recommended for
both build quality (think TeleVue, not Synta), and for optics. It has
by far the best focuser I have ever seen in a small inexpensive scope
(think feathertouch crayford, not GSO/Hardin/Antares crayford). The
focuser is not perfect, it has some tight spots that you won't find
with a feathertouch, but the weigthed focus knobs and the included 11:1
microfocuser are simply amazing to the touch. I simply can't believe
the build quality here for the price. The optics must have cost them
next to nothing to manufacture, but don't let that statement fool you
into thinking that they aren't any good. Oh, contraire.

Deep sky performance includes seeing M15 as a globular with a trained
eye using a barlowed 12.5mm Ultima setup for 64x (1mm exit pupil).
Stars just hinting of resolution at the edge with averted vision. M35,
36, 37, and 38 are easy to pick out in mag 5+ skies at half that power
and twice that exit pupil, and the "big" clusters, M45 and the Double
Cluster fit easily into the view of a 25mm Plossl, make it a 32mm
Plossl and you get a nice big 4 degree field of view. Turn toward the
Big Bear (Ursa Major) and you'll easily find the bright galaxy pair
M81/82.

In case you can't tell, I am very pleased with the scope, and I think
it's an excellent bargain if you can find one on the used market for
around $250. In my recent bought of downsizing, and taking the hobby
down a few notches to the "casual observer" mode, this little scope is
a wonder. It fits super nice on the UA UniStar Light with the
ultra-light Davis and Sanford Compact Short tripod. I only wish I had a
UA Microstar instead, as this scope is short enough that it doesn't
need the angled back arm of the Unistar. And not having that angle back
piece would save just enough more packing space to make it worthwhile.

Finally, it is easily handled by a lightweight photo-tripod for
terrestrial use, and if you're adept and willing, also for astro use
(although I'd recommend a Microstar or similar...)

Regards,
Steve Paul
Shirley, MA

On 19 Jan 2007 09:23:25 -0800, "Stephen Paul"
wrote:

I just discovered through experimentation that my 1.25" Celestron Prism
diagonal (standard SCT fare of yore), requires more backfocus than does
my generic 1.25" mirror diagonal.

The reason is that the prism diagonal puts about 3 cm of glass in the
optical path. The change in the effective optical path is the
thickness of glass times (the index of refraction minus 1).

Bud
--
The night is just the shadow of the Earth.

Stephen Paul wrote:
I just discovered through experimentation that my 1.25" Celestron Prism
diagonal (standard SCT fare of yore), requires more backfocus than does
my generic 1.25" mirror diagonal.

That's kind of interesting and unexpected, and at the same time,
problematic for my newly aquired AstroTech 66ED which comes to eyepiece
focus out near the end of focuser travel (don't worry, with the mirror
diagonal there is sufficient travel left to focus with a barlow.
Hence, most eyepieces should be good to use).

This is a wonderful little scope by the by, and highly recommended for
both build quality (think TeleVue, not Synta), and for optics. It has
by far the best focuser I have ever seen in a small inexpensive scope
(think feathertouch crayford, not GSO/Hardin/Antares crayford). The
focuser is not perfect, it has some tight spots that you won't find
with a feathertouch, but the weigthed focus knobs and the included 11:1
microfocuser are simply amazing to the touch. I simply can't believe
the build quality here for the price.

The William Optics version is 10% less expensive as well.
If these things came from TeleVue, they'd be $1100/ea.