Re: Requesting alignment tips for ROB...

[Tom Droege <tdroege2@earthlink.net>]

Rob,

Yep, when you are trying to adjust the first time it is a pain.  First get 
the plane of the base level at your latitude.  You can use the marking on 
the side plates to figure how much it needs to be tilted.  As I said 
before, I think you can re-drill the mounting angles enough to make the 
pole angle correct.

I point at the equator.  Point the side plates N-S as close as you can.  As 
sent, the RA drive should be going at about the right speed.  If you have 
somehow bumped the VCO adjustment pot, then you have a problem if you don't 
have a frequency meter.

Assuming you are close, I would point at the equator.  Run with the VCO 
voltage at +10, +5, 0, -5, and -10.  Beware, the DAC might fold over at the 
limits so you need to read back the DAC setting through the ADC setting to 
make sure.  Possibly you should go +8, +4, 0, -4, and -8.

Now plot the length of the star streaks for each voltage.  If you are lucky 
it will be a V plot against voltage.  This should get you close.  Remember 
that you can't tell whether it is drifting E or W by looking at the streak.

Now take a series of images while tracking the sky.  Just short - say 10 
second exposures.

With my QBasic code, raw images end up with N to the right and E at the top 
of the screen.  They will show up this way on DS9 if you turn of the 
WCS.  If WCS is on then you get what you ask for.

Look for E_W movement of a star in successive images.  Make a small 
adjustment in the RA voltage setting - say 1 volt and see what 
happens.  Knowing the time between images and the number of pixels a star 
moves between images you can figure out a calibration factor.  Volts per 
pixel at the given image spacing in time.  After a few sets of images you 
should be able to get the RA rate close.

Now you will be able to see N-S movement caused by not pointing directly 
South.  Pin the telescope on one corner so you can rotate it.  Put a piece 
of a steel ruler on a convenient place so that you can measure rotation as 
far away from the pinned corner as reasonable.  1/8" is about as close as 
you might need to set it.  1/8 inch is 20 minutes at 24" (about).  I put 
teflon furniture glides under three corners of the angle mounts and a 
matching standoff under the pivot screw.  Now I can easily and smoothly 
rotate the telescope on it's plywood mounting plate.

Now rotate the telescope quite a bit off North/South and take an 
image.  Measure the N/S movementin pixels per second of exposure.  Now 
rotate it even more off N/S and take another.  If you have the sign right, 
the N/S streak should grow.  This gives you a rough calibration of pixels 
vs rotational movement as measured by your scale.  Now you can try for an 
exact position.

BTW, when you are moving NS, it pays to have the RA rate somewhat off.  Now 
you are looking at a streak that you are trying to rotate so that it is 
going due EW.  Once you get it exactly EW then you can reset the RA rate to 
it's correct value.

Now you should be close enough that you can take a series of images.  If 
you always take them at the same time interval, you can derive a 
calibration of how a rotational movement affects the NS movement between 
images and how a RA voltage trim affects the EW movement between 
images.  Just iterate in until you get both as close as you have the patience.

All this means that you have to have some clear sky.  Sigh!  I don't have 
any this week, how about you?

Tom



At 01:20 PM 2/1/03 -0700, you wrote:

>Hey folks,
>
>I'm in the process of aligning ROB, and it's a painful process (for me at 
>least).  So far, my best progress is when DEC is pointing at the pole and 
>RA is stationary and level.  I've then been making mount adjustments based 
>the gradients of the streaking stars (E-W and N-S), trying to get the 
>stationary star in the middle of the frames.
>
>One reason it's taking a long time as I've been having to run all night 
>like this, hoping for breaks in the clouds, before I can make an 
>adjustment for the next night.
>
>Any other methods, pointers, hints that others may care to share?
>
>Thanks,
>Rob