Experience with a C11

These notes are an attempt to document my experience of switching from a 200mm Newtonian to a 280mm SCT recently; they may be of interest to others who have contemplated a similar change.

The attraction of the 280mm scope, of course, was the aperture, almost exactly twice the light gathering power of the 200mm. The big disadvantage was the cost, but when a used Celestron C11 appeared on advert.ie for a reasonable price I couldn't resist. Early testing of the scope gave me a fright, there was obvious vignetting on one side of the fov when using longer focal length eyepieces. Happily, the origin was a misaligned mirror in the 1.25" diagonal, and reseating the mirror properly solved the problem. With that out of the way, here are some thoughts on using the C11, and how it differs from the 200mm Newt (which I liked very much).

The C11 is heavy, but quite easy to lift and handle - the same would probably not be true of a C14. Mounting it on my CG-5 mount is trickier than mounting the Newt. The large diameter means that I cannot see the dovetail and clamp, but I have figured out how to hold the scope securely while guiding it into the clamp by touch. Nonetheless, clamping it securely is not always straightforward and I continue to hold the scope until I can see that it is properly clamped. Alignment is no problem. I removed the finder scope and replaced it with a Baader red dot finder, and that is accurate enough (though the first star can be a pain if the RDF is not collimated). There is usually no problem locating and aligning the stars using a 8-24mm zoom Meade series 4000 zoom eyepiece. The latter has the advantage of allowing more accurate centering by zooming in. The CG-5 seems to have no problem coping with the weight of the scope and GoTo accuracy is excellent - most targets are placed in the 20 arcminute field of view of the zoom eyepiece at 24mm.

The tripod legs are extended by about 20cm to place the scope at a more comfortable height. The eyepiece position varies far less than with the Newt, and is generally much more comfortable, so I can sit most of the time. The resulting steadiness and comfort allows more careful and accurate observation and is a big advantage over the Newt.

I had hoped to compare the Newt and the SCT side by side, but didn't get an opportunity to do so. However, my impression is that the increased aperture makes a significant difference. More detail is visible on Jupiter, and galaxies are noticeably brighter and reveal more detail. The narrowest double that it has resolved so far is STF 1037 in Gemini which has a separation of 1.1' and was nicely resolved, with a hairline gap between the components, at 350x.

One huge difference, whose impact I did not fully anticipate, is the massive increase in focal length (1000mm f/5 to 2800mm f/10). This has two effects, a corresponding reduction in fov, and increase in magnification. The result is that a 20mm Plossl is now a medium power eyepiece, a 10mm is high power, and a 5mm orthoscopic is super high power. The only lowish power (90x) eyepiece I have is a 31mm Hyperion. I bought a 2" dielectric diagonal to be able to use the 31mm, but even then the field of view is only about 48'. That is adequate for most objects except a few large clusters and nebulae, but I am considering getting one lower power eyepiece to get up to the maximum fov of nearly 1°, to see some objects in a broader context, and to better observe super wide doubles. It is nice not to have to squint through tiny eyepiece lenses any more. Bottom line is that the C11 needs a range of longer focal length eyepieces, ideally with wide fields of view, quite different from the range that worked with the 200mm f/5. Of course the longer focal length is an advantage for small targets, e.g. planetary imaging, and observation (and measurement?) of tight doubles, but I have not had an opportunity to take full advantage yet.

One definite weakness of the C11 is the focuser. It is stiff, and it is very difficult to make fine adjustments. Also, it is tedious to have to fine focus in only one direction (against the weight of the mirror). I considered getting a Crayford dual speed, but eventually opted for a smaller lighter solution that does not increase the optical length too much. I bought a relatively inexpensive helical focuser from ZW Optical, which arrived from China in just three days. It needs an M46 to T2 adapter, also obtained from ZWO, to fit securely onto the T2 thread of a Vixen flip mirror. The flip mirror can be used as an economical 1.25" diagonal, with the option to do imaging without having to switch between eyepiece and camera. It's optical length is short enough for use with a focal reducer,especially if the stock 1.25" receptacle is replaced with a shorter version (or cut short as others have done). The focuser, which is non-rotating, is smooth and has negligible backlash. It is possible to rotate it by only 1 or 2 degrees which corresponds to as little as 0.04mm, so achieving very accurate focus is easy. It as a focus travel of 24mm which is ample, and has graduation marks and a locking screw. It also has a T2 thread at the "top", allowing for direct connection to a camera.

Coarse focusing using the focus knob, followed by fine focusing using the helical works really well. The zoom eyepiece, together with the fine focus, makes a big difference to the ease of use of the scope. With planets and binary stars, it is a breeze to zoom in as far as the seeing will allow, and achieve sharp focus. [strike]I can't seem to attach a picture of the setup, but will try again.[/strike] Picture now attached below.





All in all, I'm very happy with the C11. It has cured my aperture fever (for now at least) and is still portable and easy to set up. With the zoom eyepiece and helical focuser, it is very convenient and comfortable to use, and really suits my style of visual observing.

Thanks for reading.

Mike

Nice report Mike, here's hoping you have many happy years together.

Paul

Thanks for sharing your experience with your new scope. Great to see what others are doing which can be difficult for me at the moment.

Clear skies