From: "Tom Krajci" 
To: 
Cc: 
Reply-To: "Tom Krajci" 


>From: "Peter Walton" 

>...Given that one can calculate theoretically exact positions of stars
at >the exact time one looks for them, there would seem to be call for
equally >accurate scaling of altitude and azimuth, which I should
imagine on a home >made dob could be quite difficult to achieve.

See:
http://overton.tamu.edu/aset/krajci/
specifically:
http://overton.tamu.edu/aset/krajci/dobsonian/dobsonian.htm  (the bottom
section on alt-az setting circles)

Excerpt of web page:

"...you will see a protractor on the elevation trunnion, and a tape
measure wrapped around the circular ground board. These were used as alt-az
setting circles, with a 386 laptop converting RA/Dec to alt/az in real
time. (If anyone wants a copy of this free, home made QBASIC software,
please contact me.)"

"...Why use setting circles these days? It was far less expensive than
adding encoders, encoder reader, and other electronics, plus I had an old
(and otherwise obsolete) laptop laying around. Pointing is accurate enough
that I did not need a finder scope after the scope was aligned on a known
object...I could put an object in a low power field of view with the
setting circles alone. In daylight setting circles also come in handy. I've
been able to find and observe Mercury, Venus, Mars, Jupiter and Saturn when
the sun is above the horizon, or in very bright twilight. I did not do this
as a circus stunt. Frequently the seeing is best just before or shortly
after sunset, and gets worse as temperature drops through the night."

If you want to make your telescope with alt-az setting circles, I can help
you with ideas and free software.  Got an old laptop (even 286 class will
run this software) laying around?  If you are careful in mount/setting
circle construction...the pointing performance will surprise you/

>...Why is not greater use made of equatorial mounts, on home made
>reflectors, and wouldn't this make for easier aiming?

I find it easier to make a reasonably well performing alt-az mount than a
reasonably well performing equatorial mount.

Tom Krajci
Tashkent, Uzbekistan



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