Re: Help with AP GTO on Parallax 125


ericj <ericj@...>
 

Hi Mike:

I just recently went through this with my 900 GTO mount so can offer a
couple of suggestions.

This is how I tried to align the mount and scope on Polaris, as suggested in
the AP manual that comes with the mount. After setting up the mount and
setting the Dec setting circle to 90 degrees I would look through the polar
alignment sight hole in the center of the polar axis to align on Polaris.
Then I would add the counterweight shaft and counterweights, and finally the
tube assembly. Each time I did this though I noted that OTA would be offset
slightly to the left or west of Polaris.

At first I thought that I had somehow shifted the mount a little when adding
the counterweights and tube assembly, so would just "pick up" (more like
nudge) the mount and scope until Polaris was aligned through the scope. But
I found that even after using the two star alignment method I would not end
up anywhere near the deep-sky object that I was trying to find using the GTO
feature.

So finally I looked through the polar alignment sight hole and to my
surprise Polaris was no where to be seen, even though it was still centered
in scope. At this point I figured the problem had to be that the rings were
not orthogonal with the mount. So off came the OTA and after checking the
rings realized they were fine. I then removed the ribbed mounting plate and
examined it to see if there could be a problem with it. It was fine too
however.

My next thought was that since I had used the used the scope and rings on my
home made Dob mount for the past four years that somehow I warped the rings
slightly (I rotate the rings on their sides and attach side bearings to
them; a photo of how they look is on my web page at
http://www.metro2000.net/~ericj, and click on Telescopes). I couldn't be
sure if this was a problem or not, and figured the only way to be sure was
to find another pair of AP mounting rings that would not be warped. Since I
have only had my 5.1" AP for less then a year I figured the likelihood that
these rings would be warped as the rings for the 7.1" were was unlikely.

So off came the 7.1" and rings, and on went the 5.1" and rings. Same
problem. Finally, I decided to look down the polar alignment sight hole from
the front to see if it looked aligned correctly. It didn't. With the setting
circle to 90 degrees the Dec axis seemed off to one side. I aligned it by
eye so that it appeared to be aligned correctly, and noted that the Dec
setting circle read 87 degrees. Back on went the 5.1" with rings, and this
time with the mount aligned on Polaris the scope was aligned on Polaris as
well.

Did the two star alignment method, but although the scope slewed closer to
the deep-sky object it still was off. So I finally tried using three star to
align the mount: Polaris, Vega, and Regulus. This time, the scope slewed to
M101 correctly, and with the high contrast of the 5.1" the galaxy was sharp,
well defined and surprisingly bright. Tried a few more objects and the
results were the same.

At this point, even though it was a during the work week, and it was almost
midnight, I couldn't resist the temptation of not using the 7.1". So off
came the 5.1" and on went the 7.1". This time I was even more careful when
aligning the mount, so I got to the point that I just left a 16mm Zeiss
eyepiece in the MaxBright when slewing from object to object and they were
often very close to the center of the FOV, which in the 16mm is around half
a degree or so. I finally got to bed around 2 AM, but it was worth it, even
though I dead tired the next day a work.

So I have a couple of suggestions for you. First, check to make sure that
the rings and OTA is orthogonal with the mount.

Second, check to see if, when looking down the front of the polar alignment
sight hole on your mount, if the Dec axis is centered or not. If not, you
cold have the same problem that I do. I have sent an e-mail to Marj and
mentioned it so she is aware of the situation. Whether yours has the same
problem or not I am not sure, but you can check.

Third, when you are doing you alignment do not use the N-S-E-W buttons to
recenter the star if it is off to one side. Use the altitude and azimuth
adjusters on your mount for this.

Fourth, try using three star rather then two.

Hope this helps. For what it is worth, I have found the 900 to be a very
fine and stable mount. I got into astronomy in the early 1970's, and got
quite use to star hopping to find deep-sky objects. When computer controlled
telescopes first came out I did not think I would ever want one. However,
since I have now had a chance to use the GTO feature I realize that there
are many deep-sky objects, like some of the fainter NGC or IC objects, that
will be much easier to find and observe now. So I look forward to many years
of finding and observing with my AP scopes and mounts.

Clear Skies,

Eric Jamison

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