Thank you. I really appreciate the discussion. It never gets easier, does
it. From that newbie question of what's the best first telescope to the
advanced what's the best telescope for a given purpose. For a purpose as
spcialized as moon viewing, there are still numerous factors at play which
contribute and detract from the view. After all this discussion, I'm leaning
toward a Russian MN or MCT OR a Meade RC (OTA only) 8-10". I can't find any
good details on the Meade RC OTAs. The 10" Meade RC OTA is less costly than
a high end Russian 6"! That's confusing...

Doink


"Dan Mckenna" wrote in message
news:Wbk0g.7089$Qz.4347@fed1read11...
Indeed, loss of light is not an issue. The optical performance is most
sensitive at the entrance pupil i.e. in front or near the primary mirror
in a newt. As you get closer to the image plane it matters less.
Thus you need a high quality glass for an entrance window or corrector. if
it as a slight curve like a sag due to gravity refocusing gets rid of the
error so not a problem. Reflections of the front and back surface can be a
problem. If you are using narrow band filters you can get fringes and even
the clearest glass has some scattering. It's best to put the money in to
the quality of the primary mirror. you must also ventilate the OTA to
reduce the thermal time constant and the front window or glass cools to
the night sky because the glass is black to the long wave thermal
infrared. Long dew cap can reduce this by limiting the "view factor" or
solid angle that the front optic sees of the sky.

I have measured a 6 degree cooling of the front window exposed to sky in
still air at a high altitude site with an infrared thermometer and thats
got to make tube currents.

OTOH to reduce scattering you need to keep the optics clean. would you
rather clean a window/corrector or the mirror ?

myself I have found it less of a problem to pull the primary and wash it
than to demount and clean a corrector. Ok so you can use gloves and all
that. still the old pucker factor for me, is higher cleaning a
transmissive optic because they are thinner.

I bring all this up because it sounds like you want the best performance
and it all counts. (to some degree)

After all Its only a hobby

d.


Doink wrote:
I wouldn't think the added glass would be a problem on such a bright
target. Generally, I would agree but I think, if well made, the
correction would be worth the light loss...lunar observing being the
topic of conversation.

Doink
"Dan Mckenna" wrote in message
news:whj0g.7084$Qz.5691@fed1read11...

Doink wrote:

Greetings...

Thanks for the input on lunar viewing.

I really would love to find an "off the shelf" reflector OTA in the 8"
f/8 range. BUT, Orion sells a 6" f/8 OTA---anyone have any experience
with it? I'd like to get to 250X---I want to do lunar detail
study...think the 6" would do it?

OR, what about Schmidt Newts? An 8-10" SN seems like a great bet....any
thoughts on that? I've never used a SN before.

Doink

Without an atmosphere and perfect optics the resolution is in arc seconds
about (4.5/diameter) in inches.

zo a six inch would have a resolution of 0.76 arc seconds
and an 8 would be about 0.57 arc seconds.

At 250x 0.76 arc sec = 3.1 minutes of arc to your eye
which is just enough to resolve if your eyes are good.

In general a smaller telescope has a better chance for moments of good
seeing.

so I would guess that 6" would just do it at 250x

as far as a SN goes, any time you put glass in the beam you
degrade the performance. I much rather have a spider than a hunk of glass
for high resolution.

d.