Testing Imaging at f/10 instead of normal f/6.3
Saturday, November 14, 2009 at 5:30PM At my normal focal ratio of f.6.3 images are approximately 20 minutes square which gives a good field of view but means that many galaxies are quite small. Also there does tend to be vignetting of the image. Whilst incorporating a new external focuser to get around image shift problems I thought I would try imaging at f/10 and check the practicality of centring the images (at the smaller field of view) for a galaxy run. On 9th Nov the sky was clear with drifting cloud which eventually became thick cloud - limiting the test. I tried it out with one object - NGC 744 - see the image below.
Clearly it is quite well centred. Here is the Deep Sky Survey Image - courtesy of The STScI Digitized Sky Survey.
The above image was obtained from STScI by completing the table below
The plate solution is shown here - 21 matched stars - RMS 0.08 which is a good astrometric solution.
This (as represented in the first image above) shows that the camera is not North aligned - the Position Angle is given as 347.57 degrees. There was not time to rotate the camera to the 360 or 0 degree position as clouds set in. The matched stars on the image are shown here.
This is the image plotted onto the Sky Chart solution showing that it is indeed NGC 744
and the Object Information Box
and the exact match of the Sky chart to the image. Note the direction of North corresponding to the discussion above.
