Trying to push the envelope a bit. Stuck a Canon 2xII and then a 1.4xII to the Hyperstar and did some crude testing. Results:

The 2xII does not focus properly with the Hyperstar. On-axis, the focus does not pop-in and -out, just sort of very smeared. Off-axis the stars have gull-wings on one side of focus and are tear-drops on the other. There is no focus point where they are round. Note that with the 2x and the small 4.7micron pixel pitch of the Canon 500D (T1i) camera the sampling is at 0.7 arc-sec per pixel. With the camera's LiveView at maximum magnification you can watch the brighter stars actually going squishy-squashy from the seeing. Obviously 0.7 arc-sec per pixel was a rather ambitious image scale for DSO imaging at my site. I do not use autoguiding currently, but at this image scale it looks like autoguiding is indeed required even with one-minute subexposures.

The 1.4x does a much better job over much of the frame as regards the stars, some minor gull-wings along the edges, probably quite easy to get rid of by minor cropping. But again, with the image scale of 1.0 arc-sec/per pixel, autoguiding looks like it will definitely be required with subexposures of more than a minute. The above f/2.7 is an example of what I got with one-minute unguided subs on an AP1200. The crop on the Bubble Nebula was resized down to match the size of the left image shot without any tele-extender. In both cases the Bubble was about a third of the way across the frame, off-axis, since I was framing both the Bubble and M52.

And the conclusion is? It all depends on the pixel pitch of your camera. With my average seeing and 4.7 micron pixel pitch there does not seem to be much point in extending my image scale from the 1.4 arc-sec per pixel available on the Canon 500D and the C14 Hyperstar, i.e. I do not really note extra resolution in the middle image compared to the one on the left (a 1:1 crop at f1.9). If your pixel pitch is, say, 6 or more microns, and you regularly have seeing better than 3 arc-secs then go right ahead and use your Hyperstar with a 1.4x tele-extender. The Canon 1.4xII does a surprisingly good job and pixels larger than 6 microns are not likely to resolve the small gull-wings on the frame edges either. Overall, you may well benefit. A C11 Hyperstar has a shorter focal length compared to a C14. Consequently on a steady night seeing limits may not show up at 1.4x extension, even with the 500D; very likely worth a try. In the olden days of film 2x extenders did a great job. With DSLRs the 2x extenders became an exercise in futility as the pixel sizes got smaller, even in daytime use. With pixels smaller than 5 microns I suspect that even 1.4x extenders may not see much use either, even for daytime. I.e. Earlier DSLRs (2nd generation onwards) have made cropping an image, say, of a bird, more satisfying than an uncropped image using a 2x tele-extender, having also lost 2 f-stops in high-speed flexibility. The current generation of DSLRs with their sub 5 micron pixels seem to make cropping equally more satisfying than using a 1.4x tele-extender. The C14 Hyperstar imaging on these small pixels seems very well matched to 3 arc-sec average seeing. Basically most of the time you will not be under-sampling your seeing. You want to frame a small astro object? Just crop. But if your camera's pixel pitch is significantly larger, then both the seeing and the optics will be under-sampled, and a 1.4x extender may well show up finer detail.

I had to use an Astronomik UHC and a UV/IR Blocker in order to get anything processable. My skyfog was Mag 17.6/sq arc-sec, roughly Visual Limiting Mag 3.5. Note that the shorter integration time with no tele-extender seems to produce a better SNR than the longer integration time with the 1.4x. Focal ratio does seem to matter; perhaps one-minute subs were too short at f2.7 for efficient stacking? If "original" below right is activated (if not, just click on it) you will see the left image at 100% or 1:1. Actually what you see above is just a small fraction of APS-C frame, not even on-axis; indicating that Hyperstar images can indeed be cropped quite tightly for smaller targets. See next slide.