Re: How much dec drift, field rot, and periodic error allowable?

On 29 Jan 2006 17:01:22 -0800, "canopus56" <canopus56@xxxxxxxxx> wrote:

>Is it possible to estimate acceptable tolerance limits for declination
>drift, field rotation and periodic error, given a specific detector
>size, focal length and image scale?

My rule is simple: if I can measure it, it's too much. These effects
alter the PSF: they make it broader and possibly asymmetric. I evaluate
my PSF by taking several short exposures (no more than a few seconds)
and measuring the FWHM and elongation. That becomes my baseline-
essentially I'm seeing the best I can get based on my optics and the
seeing.

The actual PSF in the final image will deviate from this ideal based on
the extent to which the various tracking errors are not corrected by
guiding. Most of the time I see very little difference. The larger any
difference is, however, the more likely I am to not use the image
(depending, of course, on what I'm after- photometry and astrometry
aren't affected as badly as simple aesthetic imaging).

In practice, declination drift and field rotation are insignificant with
an actively guided system that is reasonably polar aligned. Periodic
error can easily be too large to guide out completely, as can the errors
caused by wind buffeting. Wind is by far the most significant
contributor to a broadened PSF with my setup. When things are working
well, my FWHM values for long exposures are within a few tenths of an
arcsecond of my reference values.

_________________________________________________

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

Relevant Pages
Quantcast