Re: [ATM] two mirror folded Newtonian

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Peter wrote:
> Here is a scope you should check out. This is an 18" f/8 two mirror folded
> design which is well baffled.
> http://www.johanneswilm.org/mike/telescope/

Yes, this page has been pointed out before.

While I admit that the eyepiece position is lower than a conventional 
Newt., in my mind I can't call that telescope "well baffled", 
especially as shown in the photo.  No matter how long the tube is, 
without extra central obstruction some light reaches the eyepiece 
directly from the sky, though that light comes from a region of the 
sky away from where the telescope is aimed.  Cassegrains have the same 
problem, except that the light comes from near the object being 
viewed.  The problem gets worse with wider field eyepieces.

A conventional Newtonian with a proper light shield (behind the 
secondary mirror) or a tube is better baffled because the eyepiece is 
not pointed at the sky - that is one less source of stray light. 
(This assumes the inside of the cage/tube is darker than the sky.)

There are several false claims on that page (just look at the table of 
telescope characteristics near the bottom and see if you agree).  For 
example, he claims that a folded Newt has a wider field than a 
conventional Newt., which is simply not true.  (Ironically, using a 
wider field eyepiece leads to poorer baffling with this design.)  It 
is claimed that a large secondary makes "1/10th wave optics easier to 
achieve" - also not true.  Finally, the baffle specs (for which no 
analysis or assumptions are shown) claims that the folded Newt has 
40-50 "DBs" of baffling, while the conventional Newt has only 10 
"DBs".  This implies that the folded Newt has up to 100 times better 
baffling (if every 20 dB is a factor of 10, again, no definition) than 
the conventional Newt, which is false.

All these claims and arguments, the intention of which seems to be to 
bias the unsuspecting reader toward the folded Newt., are explained by 
the large table at the bottom: The table contains a price list, and 
lots of buttons labeled "Buy Now" that you can click on to purchase 
various components, like an 'Interferometer 10"' for only $160!

Buyer beware.

	Mike L.

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