Re: [APML]: Astro mount advice

["Jan Saver" <JanSaver@glo.be>]

> Graham Warrellow wrote:
> -snip-
> > Can somebody suggest suitable astro mounts for a 35 mm camera [80 mm,
> > 135 mm, 200 mm lenses] as I'm off on a family holiday soon, hopefully
> > with darker skies than I'm used to!  Something which could be set up
for
> > 5 min to 1 h exposures, for example.  I'd be interested in suggestions
> > covering a range of prices, say, up to 250 UK pounds ($400 US), £250 to
> > £500 UK pounds ($400 to $800), and more than £500 UK pounds (> $800).

> 2) a used GEM
>       Look for a basic, bare Polaris or Super Polaris mount, the type you see
a C8 
> mounted on.  for piggyback photos alone, they probally work okay.  Make a
little 
> platform to palce a camera and maybe a 50mm finder, and I think it might
work nicely. 
>  I do not know how accurate the worm gears are on them, so I would not
try any doing 
> 300mm shots.

The Vixen SP mount is quite good.  RA corrections I do about every 3 to 5
mins, and dec I can leave untouched for well over 15 mins (setted up using
the polar finder scope)  That's when you want to keep the guidestart in, or
at the edge of, a 9mm Meade doublecrosswire eyepiece (giving 133x magn on
my 1200mm f.l. refractor). 1h exposures are unlikely to be guided well,
unless you use lenses < 50mm.

Piggybacking is best done on the counterweight axis : this way your mount
is less stressed.  Balancing is not that important. An unbalanced RA axis
can even squeeze the gears a little tighter so they respond better.

For $800 you should have no problem finding the mount and dual axis drive
(imperative - the manual dec slow motion control is way too coarse for
anything above 300 mm).  And some time ago (I guess it was in 1992, when
the dollar was incredibly low), they sold the SP-mount and a 4"  f=1000mm
achromat refractor for about 36.900 BEF (no drives included :-(, that must
have been something like £650 back then, so if you can get one of those
second hand, you should be able to bargain for a good price.

Good hunting,

Jan Saver

P.S. Feel free to mail me back, there is plenty more to tell about
astrophotography with the SP mount.