LXD55 / LXD75 Technical Tips Last updated: 20 August 2006

Subject:	A Guide Scope Pointer Adjuster
Sent:	Thursday, August 17, 2006 13:10:38
From:	Ted Rafferty (raff650@erols.com)
A Guide Scope Pointer Adjuster
 
A means to adjust the pointing of a guide scope independent of the
pointing of the main scope can make auto guiding easier since it
increases the options of finding a suitable guide star. This guide scope
pointer adjuster is both very simple and light weight. I am auto guiding
a Meade LXD75 8-inch f/4 SN, which is close to the weight limit of the
LXD75 mount, so keeping things light weight is important.



My primary imager is a DSI-I and I am using a LPI as the auto guiding
imager. The guide scope is a Japanese 60mm aperture, 700mm focal length,
refractor that I have modified to use the original focuser that came
with the 8-inch (replaced with a motorized JMI focuser) and an Orion
right-angle finder. I use a 3x Barlow with the LPI on the guide scope to
increase its effective focal length. When imaging, I rotate the tube of
the 8-inch so that the focuser/DSI are pointed toward the Dec
counterweights thus decreasing the amount of counterweight needed.
Adjustments of the pointing of the guide scope is made using two 7-inch
long -20 threaded shafts. Each threaded shaft has two supports; one with
a through hole and the other with a tapped hole.



The double nuts on each side of the through-hole support keep the
threaded shaft from sliding. Each support must be allowed to swivel
since the threaded shafts are straight and the resulting adjustments
arcs. I have made each support out of plastic and they are split so that
the bolts that they swivel on can also be used as a clamp for the
threaded shafts. The threaded shaft located towards the back of the
telescope adjusts the pointing of the guide scope in declination.



The Dec through-hole support for the threaded shaft is attached to a
plate that is bolted to the back tube ring for the 8-inch telescope. The
Dec threaded support is attached to the end of a long plate that pivots
on a bolt in the front tube ring for the 8-inch. The threaded shaft
located towards the front of the telescope adjusts the pointing of the
guide scope in right ascension.



The RA through-hole support for that threaded shaft is attached to the
long plate that the Dec threaded shaft can move. And the RA threaded
support is mounted to a metal collar near the front end of the guide
scope. The RA pivot point is located at the backend of the long plate
that the Dec threaded shaft can move. My plans were to start off
building the simplest arrangement and then make modifications as
necessary. I thought having just the -20 threaded shaft supporting the
front end of the guide scope would not be adequate, but it seems to do
pretty well. Due to the bright skies at my location, I am limited to
taking exposures no longer than 4 minutes (using the High Gain option
with the DSI-I allows exposures no longer than 2 minutes before reaching
the sky-fog limit of my location). There is flexure in my setup, but it
is not noticeable in the single exposures. Over a series of 6 to 10
exposures, the imaged field will shift slightly due to flexure (I am not
sure how much of the flexure is in the pointer adjuster for the guide
scope or in the optics of the 8-inch), but such a shift can be very
useful when combining the exposures (provides a "Drizzle" affect). Here
is an image of M27 (the Dumbbell Nebula) made by combining nine 2-minute
exposures.



For this series of exposures, I used Altair as the guide star which is
located about 14 degrees away for M27. Some of the shifting of the field
during the series could be caused by field rotation, which is both a
function of how good the polar alignment is and how far the guide star
is from the field being imaged (the field rotation is centered on the
guide star). I usually use 1-second exposures for the LPI, which seems
to auto guide better with bright stars. I have been using this pointer
adjuster for about a year now and have not had a problem finding a
suitable guide starts for the targets I have imaged.
 
Ted Rafferty
Gaithersburg, Maryland

Return to the top of this page.

Go back to my LXD Home Page.