The Celestron C11 Starbright SCT (pre-XLT coatings), weighing nearly 30 lbs and large enough to hold a basketball inside it, is a large telescope. With a native focal length of 2800mm and 280mm (11 inches) of aperture (F/10), it’s a long, slow scope and requires a substantial mount. Its field of view is too narrow for a supermoon to fit inside an APS-C sized sensor, even when reduced to F/6.3 (1760mm-equivalent), so it’s best-suited for shooting globular clusters, galaxies further than Andromeda, and planetary imaging. Below is a summary of tests I’ve performed on this telescope in conjunction with Celestron’s F/6.3 Reducer Corrector.
The following 100%-crop mosaic was made with a Canon EOS70D. No filters were used for this test, although it’s possible to attach 2-inch filters using a Blue Fireball SCT Eyepiece Holder with any 2-inch nosepiece adapter threaded for filters, or in conjunction with the Orion Thin Off-Axis Guider.
Figure 1: Single exposure of Messier 13 taken with a Canon EOS 70D (unmodified), 120 seconds, ISO 400. Several dozen exposures, including this one, were stacked to create the image of the Great Cluster in Hercules in Figure 3 below.
Some color fringing is present, but it isn’t anything that can’t be dealt with in post. What’s disappointing is that the corners and edges exhibit elongation of the stars, so it appears that the field curvature native to the telescope itself is not well-corrected throughout the entire APS-C frame.
Below is a contoured image created from flats produced by a Canon EOS 70D.
Figure 2: Vignetting profile for the Canon EOS 70D.
In the extreme corners and edges, more than 50% of light gathered in the center is lost. The end result is that these extreme edges must be cropped out in most final versions of processed images. The odd shape of the contour in the upper left is caused by the prism of the Orion TOAG just barely sticking into the path of light reaching the 70D’s sensor, between it and the Reducer/Corrector, so this is not a deficiency of the Reducer/Corrector.
All telescopes of Schmidt-Cassegrain design project a curved field of sharp focus, so a field flattener must be used to get decent image quality when using larger sensors. The Celestron F/6.3 Reducer/Corrector provides some level of flattening, but not all the way to the edge of an APS-C sensor. Micro Four Thirds sensors will fare much better where flat fields are concerned. This corrector also may be somewhat soft, although it’s hard to tell if the softness in my images is coming from the corrector, bad seeing, or bad focus (I did use a Bahtinov mask). Perhaps a comparison with either of the available Starizon SCT Correctors is warranted.
Figure 3: Messier 13, taken with a Canon EOS 70D.
Figure 4: Messier 101, taken with a Canon EOS 60Da.
Figure 5: Messier 81, taken with a Canon EOS 60Da.