Re: [ATM] computing Yolo

["Bob May" <rmay@nethere.com>]

Optical system generally don't scale up too well.  Shiefs in
particular don't like that at all.  Generally, the spot size
drops (the Airy disk) as the design is scaled up so what may be
acceptable for a small scope where the Airy Disc is large just
isn't so with a larger design wheere the Airy disc is smaller and
the ability of the design to produce the aberrations is much
larger.  For example, it doesn't matter how big you build a
Newtonian, A F4 system will produce an image size of x before the
coma gets large enough to be visible.
Schiefspieglers for example, get their performance because they
have such long focal lengths compared to other designs that it
just doesn't matter that much how bad the optical surface really
is.  To get closer to a small value, there is often some errors
put into the secondary and even the primary to bring the focus to
a better point than the simple two spherical mirror setup.  Even
then, the aberrations are still there when the attempt is done to
get the scope to a shorter focal length.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net

----- Original Message -----
From: Mircea Pteancu <mirceapte@yahoo.com>
To: <atm@atmlist.net>
Sent: Sunday, April 27, 2008 11:13 AM
Subject: [ATM] computing Yolo


> Please help me to understand what I'm doing wrong.I want to
build a small Yolo, in the aperture range of 3'' or 3,5'' .Carl
Anderson kindly done some calculations for me.Then I decided to
treat myself with a tour into  the witchcraft of optical
design.Using ''winspot'' ,my KISS strategy was  this:I will
insert known designs and then I will use the ''scale'' function
of the software.I inserted the specs of Arthur Leonard's 12''
F/15 Yolo but the output of ''winspot'' was a F/25 system with a
spot size bigger than my house compared to the Airy disk.I will
try now the Knabe 3'' schiefspiegler ,recently discussed on the
list.I'm not enough KISS or too much ? Thanks in advance,Mircea
>
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