Re: Moon Viewing Continued

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" <dmckenna@xxxxxxxxxxxxxx> wrote in message news:whj0g.7084$Qz.5691@xxxxxxxxxxxxx

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.







.

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