Pride Goeth Before a Fall

[Tom Droege <droege@wwa.com>]

Well, I was pretty pleased with myself over the design of the RA drive.  I
thought it was a pretty good scheme.  Sigh!  One learns.  OK back to the
beginning.

I have had many problems with the RA drive.  There has been vibration,
there has been stiction, but both have been cured.  However, when I plot
star E-W trail length against RA drive rate, it goes to a minimum of 5
pixels or so, but does not go through "zero" as one would expect.  I
thought this was vibration.  

You may recall my reporting "gull" like star trails as I adjusted the north
orientation of the mount.  In moving the mount so it did not run into the
wall of the tower, I had to align it to north again.  I again noticed the
gull like star trails.  To investigate what this meant, I shortened the
exposure.  The stars turned into little points.  Then I lengthened the
exposure.  Now the star trails turned into little cork screws.  Ah! Ha!  I
am looking at periodic error.  Sigh!  I had designed with the hope of
eliminating the periodic erro of most drives, but there it is.  About 5
pixels worth.  

What to do?  Well, I picked the screw for certain properties.  It is made
by cold forming which makes a very smooth and cheap thread.  Possibly
though, it also allows the rod to go through the die at an angle which
would introduce a periodic error.  I note that the cork screw star trails
were very regular.  They have a 60 second period.  Lots of time to do
something in software to correct it if we know where the screw rotation is
in time.  All that is needed to measure that is to add a limit switch with
a once around flag on the screw.  Now one will know where it is.  There is
already a DAC and a VCO to allow small changes in the stepping rate.  There
should be enough range, I will check that there is.  I think I will leave
this project for one of you to figure out how to code.

Meanwhile, I will investigate other screws.  There is a chance that I just
made an unfortunate choice, and that some other screw will be better.  Yes,
I know you can buy precision lead screws.  When I was trying to build a
cheap (I actually did it - the only sale was to a Japanese company)
photoplotter I investigated precision lead screws.  One for the RA drive
would run about $5000.00 and there would be other expensive parts.  I still
think I have a good scheme, I will just have to make it a little better.  

So I will investigate other screws.  Meanwhile we get star images that are
about 2 x 6 pixels FWHM.  I think we can do a lot with such a system.  I
now have lots of pretty good data taken last night.  

Seems to me that this problem will cost us a mag or so in sensitivity, some
loss in ability to separate stars, and gain us a mag or so before bright
stars saturate.  Comments please.  

I still lean towards charging ahead.  Once we get a lot of systems in the
field then a lot more of you will be able to apply your talents to fixes
for the various problems.  I think as it stands it can do pretty good work.  

Tom Droege