MK IV astrometry again

[andrew.bennett@ns.sympatico.ca (Andrew Bennett)]

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!

I have looked at the proper way of doing the
astrometry that somebody posted recently. Expanding
the formulae in powers of pixel position confirms
the empirical result that the second order approximation
is adequate for our needs unless we can get the scatter
down a lot or unless we choose to observe near the pole.

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! 

I think this scatter (from the new fit) is remarkably
small. Being able to do things with the brighter sources
greatly expands the usable magnitude range for the MK IV.
(Just think what happens when the MK IV gets somewhere
really dark and somebody cranks up the exposure. Mag 10
stars will be saturating and there won't be any non-
saturating Tycho stars on an image, let alone Good 
Tycho stars.)

Andrew Bennett, Avondale Vineyard, Nova Scotia, Canada.