[APML] Re: How to use Pictor 208XT autoguider with Celestar 8

[<jthirsty@charter.net>]

> Message: 101
> Date: Thu, 10 Mar 2005 22:51:22 -0800
> From: Duncan Munro <dmunro@sfu.ca>
> Subject: Re: [APML] How to use Pictor 208XT autoguider with Celestar 8
> 	Deluxe?
> To: Discussion of Film Astrophotography <astro-photo@seds.org>
> Message-ID: <42313FEA.2020703@sfu.ca>
> Content-Type: text/plain; format=flowed; charset=ISO-8859-1
> 
> The 208xt should work without a relay box  Meade and Celestron use the 
> same pinout on the autoguider port. You should recieve a dual ended RJ12 
> cable with the 208xt to connect it directly to your autoguider port. 
> However, the 208xt autoguider relays can only be controlled by the CPU 
> on-board the 208xt camera head. Meade has  released a simple program 
> called "Auoguider" which allows the user to use an external computer to 
> focus the 208xt and monitor the calibration and guiding routines of the 
> CCD, but you must still use the one button interface on the CCD head to 
> set most parameters. The 208xt actually has fairly good stand-alone 
> autoguiding abilities and the one button interface is well thought out 
> and easy to use.
> 
>  Other software, including Pictorview can only use the 208xt to 
> autoguide if you have a computer rs232  to telescope interface.You 
> CANNOT use a computer to guide with a 208xt through the 208xt on-board 
> relays.  For the Celestar this would involve building or buying a relay 
> interface to allow your software controlled rs232 port to issue guiding 
> commands directly to the drive controller. However, the 208xt is a poor 
> choice to use as an autoguider  with a computer as the low speed (56k) 
> serial connection and the slow digitization rate, limit  the frame rate 
> to about 1 frame every 5 or 6 seconds, even with 2x2 binning and minimal 
> sized subframes. Consequently, the maximum correction rate is only about 
> once every 7 or 8 seconds. For reasons unknown the on-board CPU can read 
> the guiding subframes much more rapidly and can issue guiding commands 
> as fast as about 1 per second.
> 
> Another caveat is that the 208xt can issue simultaneous movement 
> commands in RA and Dec. My CI-700 controller will not allow simultaneous 
> movement in both axis, so if the 208xt happens to issue such a command 
> one of the drives will not be activated. This can cause problems, which 
> might cause the tracking to eventually fail. If you have a precise polar 
> alignment there is a good  chance, that the number of simultaneous 
> guiding commands will happen infrequently enough, that it won't be a 
> problem. IIRC, the Celestar and  the CI-700 share the same controller. 
> You can test your controller to see if it will accept simultaneous 
> ra/dec commands by looking through a high power EP and pressing an RA 
> and Dec button at the same time.
> 
> cheers
> 
> Duncan
> 

Thanks for the heads-up Duncan.  I tried operating R.A. and Dec simultaneously; if I press and hold R.A first and then press and hold Dec, not only will it not move in the Dec, it also won't start moving as soon as the R.A. button is released.  Apparently there has to be a period where there is no signal before it will switch between the two motors.  Obviously this could cause a problem if the guide star is drifting and the 208XT locks up the motor controller by constantly sending it simultaneous commands.

So that brings up another question: can programs like Guidedog and K3CCD be configured to not send R.A. and Dec commands simultaneously, or is it built into their telescope databases?

I did a lot of reading and surfing yesterday and decided to try modifying a Quickcam 4000 for my first attempt, rather than blowing most of my budget on the 208XT.  Am I right in thinking that because it uses a USB connection for the guide camera it will be fast enough to prevent the low correction rate described above?  If anyone has any comments about this method, I would be glad to hear them.

thanks,
Jerry

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