Re: [APML]: barndoor tracker question

["Tom Krajci" <krajcit@det5wg57.barksdale.af.mil>]

> From:          DROWESMI <DROWESMI@aol.com>

> Tom Krajci wrote:
> 
> >300mm lens shots for 25 minutes.
>  
> >20mm lens shots for 60 minutes.
>  
> >I got to take naps during exposures! ;-)
>  
>  Tom:
> 
> This is quite remarkable!  How are the star images on the 300 mm shots? 

Round.  Not super sharp, but I was shooting with the lens wide open 
at the time and it's not a good lens.

> If we could get you to write some C-code I'm frightened to think what might
> happen.<g>

The software isn't hard to set up if you want to look at the QBASIC 
code.  Basically it was a loop that kept track of the elapsed time 
and how far a star moves in R.A. over that time interval.  Then the 
computer keeps count of the number of steps the motor has been 
commanded to make.  The computer knows the length of the driven arm, 
the number of steps per revolution in the stepper motor, the pitch of 
the screw thread.  The computer keeps evaluating "is the driven 
platform lagging the star's R.A. motion by more than the angle that I 
can move in one stepper motor step?  If yes, then step the motor 
once.  If no. . .go back and check again!  The loop exits on a 
keystroke such as "X"

I ran this on a 286 laptop and uncompiled QBASIC. . .still fast 
enough to do the math and check the keyboard for other functions.  
The command for "pulse the stepper" was given through the serial 
cable to my stepper driver board.

I later made a version that used the parallel port to drive two 
steppers, and also read hand paddle/push button inputs in real time 
to slew an equatorial scope.  Worked fine with the 286 also.  And the 
computer still had time to check the keyboard for special characters, 
such as "F" to increase the drive rate by 0.1% every time I pressed 
the F key, or "S" to decrease drive rate by 0.1% every press of the S 
key.

I copped all these ideas (and what I copped was only about 2% of 
what he does) from the dean of computer controlled amateur 
telescopes:  Mel Bartels.  See his page at 
http://www.efn.org/~mbartels/tindex.html    Check out his section on 
compuer controlled dob's and ultralight dobs.  His plywood/truss tube 
20" f/5 scope is well enough made that unguided exposures of two 
minutes can give round images on a cookbook CCD.

And even if you're not into astrophotography. . .it's a computerized 
"go-to" system buildable by a clever amateur. . .more fun than an 
LX-200!

Andy Saulietis has developed a focal plane derotator for big alt az 
scopes too and is stacking exposures of 2-4 minutes duration from a 
big dob and cookbook CCD.

Astrophotography is fun, but with all this cloudy weather I spend 
more time building stuff than observing or shooting!  ;-)

Tom Krajci

Capt Tom Krajci
B-52 Intelligence Officer
"In God we trust, all others we monitor!"
http://spur.barksdale.af.mil