Re: FAST Premium 8" F/3.7 Strehl 0.97 mirror on Ebay!

From: Jon Isaacs (jonisaacs_at_aol.com)
Date: 09/01/04 

Date: 01 Sep 2004 20:31:50 GMT

>First of all, only the fast scopes will allow the optimal "spacewalk" views.
>Because the ultrawide 80-deg+ apparent field eyepieces are not available
>longer than about 31mm. An F/6 newt would require a 42mm to achieve the same
true field as a 26mm on the F/3.7 with a full rich-field 7mm exit pupil.

I have some understanding of these issues, I have an F4.06 Newt as well as F5
and F6 Newts. For me, the space walk view is only worthwhile if the edge views
are not distracting. At F4 a bright star at the edge of the FOV is
distracting.
 If your eye opens to 7mm then you might be getting an advantage, but if your
eye only opens to 5mm, quite common, then you are losing aperture with anything
over a 19mm eyepiece.

And if you choose F5, certainly better edge correction by far and not so
demanding of eyepieces, then that 31mm 84 degree eyepiece will provide the same
FOV as the 22 mm.

>Don't belittle the physical size issue.8" f/6 weigh 50% more than an 8"
>f/3.7 and the tube length is unwieldy and needs a SUV to carry it.

I guess then my 1992 Ford Escort sedan much qualify as an SUV <G>. My 8 inch
F6 DOB fits nicely in the trunk. Not too many cars are much smaller than this
one. I certainly don't find the 45 inch Tube length unweildy.

The back packing issue is a real one but I if I wanted to backpack a DOB, I
would choose a Truss design, that would allow me to have an F5 or F6 focal
length while keeping the weight to a minimum and maximizing portability.

>It
>requires a lighter mount and provides steadier views under adverse
>conditions.

Actually it depends on the mount, if it is a DOB mount, then the mount might be
slightly smaller but with a 30 inch OTA length, it is awkward to view with
unless there is a tripod or other way to get the raise the tube to a reasonable
viewing height.

It would allow the use of a somewhat smaller EQ mount, or making the EQ mount
more steady.

--------

So, if size is the issue, then I think scopes around F4 are worth considering.

But in my experience there are large prices to be paid in terms of eyepiece
requirements and edge performance. Normally the size of an F5 or F6 newtonian
does not cause problems with transportation or portability.

F5 and F6, these are the nice focal ratios that provide reasonable well
corrected views across the FOV.

Jon

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