Re: Autoguider correction frequency


Kurt Mihalco <mihalco@...>
 

Hi Murray,
First, I don't have an Orion Starshoot autoguide camera, so my
comments could be inaccurate. Their specs show a maximum full res
frame rate of 15 fps. This is based on the time it takes the
hardware to scan out and digitize the imager at full resolution and
transfer it to the PC over the USB. So, irrespective of the
exposure, 15 frames per second is the maximum the camera is capable
of delivering a full frame image to the PC (I don't know if the
camera supports binning or ROI, which could allow for faster frame
rates). The PC sofware (PHDGuide?) then
has to process the frame, compare it to the previous one, and issue
any guiding commands, back through the USB to the camera, which then
closes the solid state switches which command the mount to move a
particular direction (by the SBIG compatible interface). If your
exposure is greater than 1/15 second (66.7 mscec), then the frame
rate the camera delivers images to the PC must slow down
accordingly. For example, if you set a 1/2 second exposure, then the
camera can only deliver 2 frames per second to the PC. The key point
here is that the guide software is what issues the corrections to the
mount (through some hardware in the camera head). The camera itself
only delivers the images to the PC based software which analyzes them
and determines the appropriate corrections. The PC Software cannot
issue guiding corrections any faster than the camera is delivering
images to it. If the camera is taking 2 second exposures, then the
maximum rate the software can issue a correction to the mount is
every 2 seconds. Hope this makes sense.
Regards,
Kurt Mihalco

--- In ap-gto@..., Murray Hammick <mphammick@...> wrote:

I hear what you say and do not doubt the veracity of your comments -
and especially Roland's authority in this matter.

That said - why does the Orion guide camera offer you a choice of
exposure down to .02 sec ? Is it in fact not sending corrections
after
each exposure (assuming less than perfect tracking of course) ?

Murray

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