[ATM] two mirror folded Newtonian

[Polaraligned <polaraligned@optonline.net>]

I used to be bent on doing a scope with a secondary "other than
45 degrees".  I did a lot of solid modeling in AutoCad and looked
through the "virtual eyepiece" many times.  I believe my biggest
problem was that the focal ratio I was using was too short.  I was
trying to use f/5.  I think an f/8 would be much, much better for
using a  secondary tilt other than 45 degrees.  Without modeling it,
and just based on my experience trying to make a f/5 work, I would
say that if the secondary angle to the optical axis is anywhere between
22 degrees up to 45 degrees you should be able to get it properly baffled.
This probably would require a short extention off of the secondary cage
opposite the eyepiece.
As for the website noted, I  sent several e-mails to this scope builder
and only got back one sentence cryptic answers.  I think his arguments
for the advantages of a folded scope are mostly terrible.  Frankly, I think
his whole folded scope implementation is terrible. His claim that
"1/10 wave is easier to achieve" I do believe to be correct though.
The primary wavefront error is multiplied by (1/cos *  secondary
angle).  This results in a smaller error introduced by the secondary
for such a "folded" system.

 A much nicer design can be seen here:
http://www.irony.com/Ed/astro/18inch/
Unfortunately, the light baffles are not in place on this scope.  It would
be interesting to see how Ed implements them.

I think folded designs can be well implemented and I think that f/8 is a
great focal ratio to do it at.  I also think that the central obstruction 
increase
is nothing to worry about, and it can be kept under 30% in a careful design.
A commercial SCT by comparison is anywhere from the low 30's to the
high 30's in obstruction percentage.  A 12.5" f/8 with fantastic optics will
surely outdue a 6" APO even with a 30% obstruction in the reflector.  Don't
argue with me, argue with the laws of physics......

Scott







>
> 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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