Re: ATM 4.25" Reflector Eyepieces

[The Silent Observer <silent1@ix.netcom.com>]

Jeff Paterson wrote:
> 
> Hello All,
> 
> I'm building my first telescope, a 4.25" - f/10 Reflector. I will be using 1
> 1/4" eyepieces but am confused about magnification limitations at the high
> and low ends of the spectrum.

Assuming an excellent mirror, you should be able to go a bit over 200x
(the rule of thumb is 50x per inch of diameter, though exceptional
optics will support more).  With a 42.5" F.L., that would require about
a 6 mm eyepiece -- but my experience is you'll get better images with a
good 18 mm and 3x Barlow than with a 6 mm, for the same money as a good
6 mm (the shorter they are, the harder it is to get good images), and
get more eye relief into the bargain.

The longest practical eyepiece in 1.25" size is about 32 mm -- though I
think there's at least one 38 or 40 mm out there.  A 32 mm would give
about 34x and a field of close to two degrees (depending on the apparent
field of the eyepiece, of course).  That's large enough to include all
of M31 or M45 in your eyepiece, and pretty close to getting all of M44
or the Hyades.

Assuming you get a good 3x Barlow, a good combination would be 32 mm, 24
mm, and 18 mm, with the barlow then giving effective 11 mm, 8 mm, and 6
mm -- and total eyepiece outlay (if you buy get Rini eyepieces) could be
under $200.  If you throw in a 2x Barlow as well, you'd get 16 mm, 12
mm, and 9 mm, which gives some overlap; in that case, you could probably
get away with just the 32 mm and, say, 20 mm eyepieces to get:

32 mm              34x
24 mm              49x
32 mm + 2x         68x
24 mm + 2x         98x
32 mm + 3x        101x
24 mm + 3x        135x
32 mm + 2x + 3x   202x
24 mm + 2x + 3x   270x (which is pushing things pretty hard on a 4.25")

And there's only one overlap in that group, with every combination
likely accessible with your glasses on (at least if your glasses and
eyepieces are like mine).

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Opinions expressed are my own -- take them for what they're worth
and don't expect them to be perfect.