Re: MK IV astrometry again

[hjohnson@pluto.njcc.com (Herbert R Johnson)]

On Thu, 30 Dec 1999 16:00:11 GMT, andrew.bennett@ns.sympatico.ca (Andrew Bennett) wrote:
*>A while ago I posted preliminary astrometry for a raw
*>CD5 image. I have now repeated this with my crude
*>Dark/Flat processing. Using a quadratic fit, the
*>rms residuals (from Tycho) are
*>
*>              RA         Dec
*>raw         0.00056    0.00038 degrees
*>processed   0.00032    0.00025
*>
*>Just think how much better things will get when we
*>get PROPER Dark/Flat processing!

So what is the method? I presume your Web page will detail this.
A quadratic fit to what? and arc seconds would be a more convenient
term. Is this just for correcting field distortion with declination
or does this also deal with comatic images, and to what amount?

*>Surprisingly, results are quite good for the "Bright"
*>sources ... 
*>              RA         Dec
*>"Star" fit 0.00351    0.00367 degrees
*>New fit    0.00064    0.00040
*>
*>        ... provided one uses an appropriate fit. This is
*>a measure of the extent to which the coma is affecting the
*>very different methods of extracting position. I used the
*>centre of gravity of the comatic image down to a level
*>200 ADU above background for my "Bright" sources. The
*>difference from the "Star" fit is around 3 pixels in the
*>corners of the image. Ouch! 

How does "center of gravity" (I think that is also called the "second
moment" in other fields) correspond to the maximum pixel value?
Frankly I don't know how star makes the fit, it would be useful to
mention it. This is an informative finding! I look forward to reviewing
your Web page again.

Herb Johnson

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