Re: ASTRO Auto tracking for telescopes

On Mon, 14 Jan 2008 21:06:50 -0800 (PST), "dkelvey@xxxxxxxxxxx"
<dkelvey@xxxxxxxxxxx> wrote:

Resolution: a typical autoguider camera produces a meaningful error
response for a movement of about 1um at the image plane. A mouse has a
sensitivity about 1000 times less than that. So you'd need a very long
focal length guidescope to compensate.

Not sure if it is quite that good for the ones I've seen for
amatuer use. None of the cameras, I've seen, are that good.
Maybe adding a Barlow or two.

I'm talking about the resolution without optics. Typical pixel sizes are
10um, and typical position sensitivity of a star is about 1/10 pixel.
What 1um at the image plane translates to in terms of angular resolution
depends on focal length, but is ordinarily an arcsecond or less.


Typical video cameras are only about 600 dpi. This is about the same
as used in the mouse. Some mice are up to 1600 dpi.
Even my fancy canon camera isn't much better than about 6000 dpi
at the sensor.

You're comparing apples and oranges. Autoguiders (including those using
webcams and ordinary video cameras) have an actual resolution of about
1um, something like 25000 dpi. Optical mice produce around 1000 counts
for an inch of mechanical motion. Those are completely different
measurements. I don't think optical mice are doing any sort of fancy
subpixel calculations; they just look for image correlations between
images. So their actual resolution is probably a lot less than a typical
autoguider.

I'm not sure this is true but would need experimenting. I think the
algorithm is similar to what is use in guide code. In any case, most
have an image dump mode that might be used instead.

I think the algorithms are completely different. An autoguider tracks
the centroid of an isolated stellar image; it doesn't compare one image
to another. A mouse measures distance by applying some sort of
correlation between image pairs. I didn't know that optical mice have
image dump capability. That would be fun to play with.

There are a lot of obsolete computers out there that could do this
without problems. I'd have a PC connected to my CCD camera
anyway.

The question is, can you use the PC for anything else when operating
this way? If you have to treat the guider input as if it is an actual
mouse, I don't think so.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
.

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