A couple snapshots with the Celestron C6

 Some winter weather has moved in, but last night was still clear and calm.  This provided an opportunity to do some focal-length measurements with a Celestron C6 SCT (Schmidt-Cassegrain telesceope).  This scope has a 6-inch (150 mm) mirror with a nominal system focal length of 1500 mm. This type of scope is focused by changing the spacing between the primary and secondary mirrors, which moves the focal plane.  The actual focal length therefore depends on the specific location of the focal plane 

The picture below shows the telescope with a 2-inch mirror diagonal and a camera inserted into the diagonal.  

 The measured optical path from the exit flange of the telescope to the camera sensor was 165 mm.  Star-field images obained with this arrangement showed that the effective focal length was 1815 mm.  For this particular  optical train, the effective focal ratio was therefore f/12.1.

Previous measurements showed that the effective focal length changes by 3.3 mm for each millimeter of extension.  The optical path to the exit flange of the diagonal, where an eyepiece would be inserted, is 43.5 mm shorter.  At this distance the effective focal length is 1671 mm and the focal ratio is f/11.1

After the star-field focal-length measurements were complete, the telescope was turned to a couple bright objects in Orion for some quick exposures.

Rigel is the brightest star in the constellation Orion.  It is also a visual double star in moderately-sized telescopes.  This image is a single exposure at 1/4 sec at ISO 1600.

The companion star is magnitude 6.8 and is separated from Rigel A by 9.4 arcsec.  It is challenging to separate in small telescopes because of the large brightness difference compared to mag-0.3 Rigel A.  Investigation has shown that this secondary star actually consists of three stars, so Rigel is therefore a quadruple star system.

The Orion Nebula, M42, is a favorite subject for astrophotographers.  This image is a 15-sec exposure at ISO 1600.

The constellation Orion:

credit: SkySafariAstronomy.com