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Subject: [M]: ETX as Guidescope questions
From: Emery Hildebrand
Reply To: mapug@shore.net
Date: Wed Sep 10 00:13:19 1997

> I would appreciate anyone who is currently using the ETX with an LX200
to share some knowledge and help.... I would be mounting the ETX on a 10"
LX200. Some pictures I have seen have the ETX mounted on the LX200 with 6
1/2" rings. The outside diameter of the ETX is only about 4", why are such
large rings used? Can you use smaller 5" rings? <

I don't have an ETX, but I do use a 2045 SCT as a guider for my 8" and 12"
LX200s. I use the 6.5" Milburn rings. The extra diameter allows you to
point far enough away from the object you are photographing to locate a
suitably bright guide star. You can use smaller rings with an ETX, but the
larger the rings, the more range you'll have. The larger rings will also
come in handy if you upsize to a C5 guider, which would be a more suitable
guide scope for a 10" working at f/10. With either, you should guide with
at least a 2x barlow but a 3x would be better.


> From my estimation, once mounted the ETX would be about 10" off center
from the center of the LX200 as compared to an OAG which is only about 1"
offcenter. Is there any difference in a photograph taken at prime focus
between the two setups given the large variance in the offcenter distance?
<

The off center distance won't make any difference. You can accurately
guide on stars at least 5 degrees away from the object you are
photographing. Both Losmandy and Milburn rings will be stable enough so
that flexure will not be a problem - as long as you tighten the alignment
screws very tight. I use rubber caps made for wire shelving tips to keep
the screws from marring the guide scope.


> I have read a lot about guidescopes having flexure problems. The ETX
seems fairly light and the dovetail mounting looks fairly snug. Does the
ETX flex at all or is this setup snug enough to eliminate this problem? <

If they are badly mounted they can be a nightmare, but the rings you're
considering won't allow flexure. Some people worry about mirror shift
during exposures, but I've never seen this happen in 30 years of
astrophotography with SCTs. What I have seen people blame on flexure has
always been caused by not using enough magnification in guiding.

My 4" SCT has a 1,000mm focal length. Using a 9mm eyepiece and 2x barlow is
barely adequate for guiding my 8" f/6.3. It isn't nearly enough
magnification for guiding the 12" scope. The ETX has a 1,250mm f/l which
with a 9mm and barlow just might be enough to guide your 10", but it will
be close. The C5 has about the same focal length as the ETX, but it would
be a better guide scope since the guide stars will be much brighter and
easier to track.

I'd also like to hear from those who have successfully used an ETX or
longer setup to guide a 10" or 12" scope. Perhaps stacking several barlows
would help get adequate magnification?

Emery Hildebrand