From: Michael Lindner 
To: John Lynch , atm{at}shore.net
Cc: john_p_lynch2003{at}yahoo.com, John.P.Lynch{at}nasa.gov
Reply-To: Michael Lindner 


On Saturday 07 June 2003 04:57 pm, John Lynch wrote:
> Would I be able to use a 48mm Brandon 2" ocular with
> an 8" f/6.25 reflector telescope using a 1.83"
> secondary
> and 2" Crayford focuser.  Would there be any problems
> associated with this combination?

Your exit pupil will be about 7.7mm (48mm / 6.25), which is fairly large.
Thta means the shadow of the secondary will be about (7.7mm x 1.83/8) or
about 1.8mm. If you are using the scope for lunar or solar observing, your
pupil will be contracted, so you may be bothered by the shadow of the
secondary.

I don't know the size of the field stop of the brandon, but assuming it has
an apparent field of view of 40 degrees, then the field stop would be 48mm
x 2 x tan(40 / 2) or about 1.37". According to your illumination
calculations, that would be OK, but if the eyepiece has, say a 50 degree
apparent field, the field stop would be somehting like 48mm x 2 x tan(50/2)
or about 1.76", in which case, you would likely notice vignetting at
the edge of the field.

Both of these issues are subjective, and niether will prevent you from
observing with the eyepiece.

--
Michael Lindner
http://www.starastronomy.org
http://home.att.net/~mikel http://www.atmsite.org
http://home.attbi.com/~dcass/atmfaq/atm-faq.htm

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