Re: [ATM] ATM: simplest ever unobstructed scope

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

One of the llittle gotchas is that an eyepiece needs to point at
the center of the axis of the telescope which may not be the
source of the light.  Off-axis telescopes like the Herschelian
have to have the EP point at where the figure of rotation point
is for the mirror.
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: Tuesday, April 29, 2008 11:27 AM
Subject: Re: [ATM] ATM: simplest ever unobstructed scope


> To David Harbour :
>
>   I was reading somewhere,I cannot quote the source,that the
Herschel's was tilting the eyepiece to compensate for
astigmatism.Most of the eyepieces used by Wilhelm Herschel was
singlet plano-convex or biconvex lenses,so ''tilting the eyepiece
done a long way toward compensating astigmatism'' (who i don't
remember,maybe Gunther Buttman ?).John Pierce was still
recomending singlet lens eyepieces  80 years ago ,isn't?Last year
I done some experimenting with an eyepiece made of a biconvex
lens having a focus of 15mm ,stopped down to about 40% of
aperture,used on a normal small Newtonian.The image was so bright
and it had such an optical  ''flavour'',that one of my projects
is to make three such eyepices ,of  40mm,25mm and 15 mm.(But I'm
writing all of this to you  in confidence,please don't unveil
this to any of the ATM guys because,living the days of Naglers
and Panoptics, they will take me to the shrink ,using that
special shirt ...).
>   Enjoy some Herscheliana,especially the eyepieces made by
Johann Alexander Herschel, at the link below:
>
>
http://www.nmm.ac.uk/collections/explore/object.cfm?ID=AST0844
>
>
>   Regards,Mircea
>
>
>
> ''R-101
>
> P.S. How did the Herschels ever cure the astig. and coma, that
they must
> have had in horrendous amounts, in their big scopes? Does
anyone know? There
> are some interesting "legends" about the Herschels- ''
>
>
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