Creede Dark Skies: Moon. Polaris. Sky-Watcher SkyMax 102 MCT.

The 11.8-day moon was only three days past perigee on Friday night and therefore slightly larger (32.45 arcmin) than average. I used this angular size to measure some effective focal lengths for a Sky-Watcher SkyMax 102 MCT (Maksutov-Cassegrain telescope). Like the previously discussed Celestron C6, this scope focuses by changing the mirror spacing, which moves the focal plane and changes the focal length. The nominal focal length is 1300 mm, which corresponds to a focal ratio of 12.75, but that is true for only one specific location of the focal plane.

An Olympus E-M5 camera with T-adapter was used to image the moon directly inserted into a SCT-2" adapter and then inserted into a 2" diagonal. When inserted directly into the SCT adapter, the camera image sensor was 88 mm from the base of the scope and yielded a focal length of 1290 mm. At this focal length the moon image just fits onto the micro-four-thirds sensor.

A 2-inch mirror diagonal plus 5-mm parfocal ring adds 110.5 mm to the back-focus position. The measured focal length for this configuration was 1671 mm. The entire moon no longer fits onto the sensor.

From these two measurements the focal-length differential was determined to be 3.45-mm focal length per mm of extension. This gives a focal length 1526 mm at the exit flange of the 2-inch diagonal, for a focal ratio of f/15. This result is useful for estimating the magnification provided by an eyepiece when used for visual observing.

With the moon measurements completed, the scope was turned toward Polaris in the straight-through (1290 mm FL) configuration, this time with an E-M5iii. The "North Star", Polaris, lies about 2/3-deg from the north celestial pole. Polaris is actually a triple-star system. The magnitude-2 primary Polaris A is a spectroscopic double star. Polaris B, a 9.1-magnitude companion star, is separated by 18 arcsec. This visual pair is a challenge for small telescopes because of the large magnitude difference.

Polaris A and B. E-M5iii, ISO 1600, 1 sec.

The star colors are usually described as yellow(ish) for Polaris A and white for the fainter Polaris B. However, in Burnham's Celestial Handbook, Vol. Three, (Dover, 1978) p.2009, Polaris B is described as "a small companion which seems of a pale bluish tint."