January 2001 Image of the Month

Saturn and M51 at same image scale

This is not a trick image, just a composite at equal size. This image of Saturn was obtained on December 27 after trying to answer a potential customer's question regarding using a Centurion 18 for lunar/planetary imaging. This is the first planetary image I have done with a Centurion since I consider this to be a misapplication for a telescope designed primarily to excel in deep sky imaging at f/2.8 and having a large 18" aperture. A PixCel 237 ( ST-237) imager is used for both color images. The seeing that night was about 3 arc seconds. Much can be gained from this comparison.

The full frame size is 640 X 480 pixels which are 7.6 micron square. As can be judged, M 51 which is a medium sized galaxy for amateur telescopes easily spans the entire 640 pixels and yet Saturn is so small in comparison! At the Centurion's 50" prime focus, Saturn's globe which is miniscule 20 arc seconds diameter, covers about 20 pixels or just .012" ( 0.3 mm.). To put this microscopic size into comparison, a pin head is 5 times this diameter and a pin's shaft is still twice this diameter! See below illustration.


Pinhead size versus actual image size on CCD chip


The exposure time for the Saturn tricolor image is .01 seconds per color while the M51 was in tens of minutes. For deep sky imaging, light gathering power by virtue of large aperture and fast speed is much more important than high resolving power. Also the relative brightness of these two objects requires a 50,000X difference in exposure time. This is precisely why it is imperative that one have a site with great air steadiness to obtain good stellar detail over a long exposure in order to do fine deep sky imaging. Note that the obscure edge-on galaxy below Saturn in the composite image has about the same angular size. If one wants to also do planetary CCD imaging, a longer focal length telescope such as a 12 or 14 inch SCT is a more economical consideration.



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