Re: Pride Goeth Before a Fall

[Tom Droege <droege@wwa.com>]

Chris and all, 

At 01:59 PM 3/18/99 -0800, you wrote:
>I took apart my Meade SCT once.  This is exactly what they do.
>There is a screw sticking out of the worm gear.  The screw head
>hits a micro switch every revolution.  Looking at their setup I'd
>guess it cost them all of about $2.00 to implement.  It is very
>simple.

Hmmmm!  Very interesting.

>
>The PC software could update the tracking speed every five seconds
>This would be 12 times per rev.  This could reduce the error to
>one pixel.  Going to a two second update period would not be bad
>if the STAMP could handle it. (Does the motor skip a beat when the
>rate is adjusted? I think you are using a count down rather then
>a VCO so it may) 

One could do it right in the stamp if the stamp is programmed to look 
at the serial port every time it finishes a correction.  I am using both a 
count down (seldom adjusted except for a major speed change of a 
power of 2) and a VCO.  You could change the VCO withoud introducing
any bump.

>
>One problem with this is that the PC will need to exactly know when
>the switch closes.  I'd say to within 0.1 seconds.  We can't

As I say, do it in the STAMP which is much faster.  Just try to get back
to the serial stamp input every second or so to get other commands 
which should be easy.  Just have to worry about how much time is used
looking at the serial port.  But note that the only thing that comes in 
during an exposure is something that stops the exposure, so the time 
is constant and can be calibrated.

>poll that fast using the current interface.  I'd like the PC to 
>know the current phase angle of the lead screw to within about one
>degree.  I see two ways to do this:
>
>1) Modify the way the STAMP talks to the PC so that the STAMP can
>   send unsolistited asynchronous messages to the PC.  The STAMP
>   would have to poll the switch at a fairly high rate.
>2) Run the micro switch leads directly to the control PC, bypassing
>   the stamp board.  I'd go to the printer port.  
>
>Another big problem is finding a way to measure the periodic error.
>We need to know:
> 1) The amplitude of the error
> 2) The phase 
> 3) The shape of the periodic function.  Or do we assume a sin wave?
>Taking images only gives us #1.  Any ideas?

I wish I had saved one of the images of the cork screw stars.  Assuming a
sine wave would reduce the amplitude from 5-6 pixels pead to peak to of
order 1.  Good enough for government work as they say.

>
>This has to be measurable in the field.  If a system is ever disassembled
>there has to be some way to redetermine the periodic error function.
>Normally I think this is done by an autoguider of by manual guiding.

The way you do it is to move the telescope away from north - 5 degrees or
so.  Then you get the cork screw pictures.  OK, that gets the peak to peak
amplitude.  We know the period it is 60 seconds.  So we just have to make a
run stepping through the phase.  Would take about an hour.  Plot star E-W
vs phase.  Pick the minimum.  

But don't start coding yet.  First there are the 4 different lead screws I
have ordered to try.   There is also leather to try.  

Tom Droege
>
>
>
>-- 
>--Chris Albertson
>
>  chris@topdog.logicon.com                Voice:  626-351-0089  X127
>  Logicon RDA, Pasadena California          Fax:  626-351-0699
>
>