The following are just quickie tests, basically to assess whether the Hyperstar is a worthy instrument, competitive with a premium APO on an APS-sized sensor. It is, text beneath each image.

I try to answer many of the common queries regarding Hyperstars that face prospective buyers and new owners, such as:

How flat is the image field? Very flat for up to an APS-C sensor, Canon DSLR 1.6x crop cameras. Stars are basically round right into the corners.

How rectilinear is the image field? Not. It will not be a good instrument for architecture! There is either barrel or pincushion distortion into the corners (I have not bothered to find out which). To align subs easily they should be shot preferably with more or less the same framing. If, as I did, one set has the object on the side and another set of subs has it in the center, you will need to warp one to match the stars on the second and use multi-point alignment(or use Registar). Two-star registration will be unsatisfactory.

How difficult is it to collimate and focus? Trivially so, same as at f10 or f11.

How small an SCT can one use with a DSLR? Currently a C9.25. This is what you get:
C14: 675mm focal length f1.9; C11: 560mm f2.0, C925: 540mm f2.3

Do I have to settle on only widefield images? No, use a late model DSLR with tiny pixels and you can sample at your seeing limit. Then just crop.

How do the FWHM of stars with a Hyperstar compare with a high-end premium APO? Remarkably, the Hyperstars deliver much tighter stars, as explained on this reference:
http://tech.groups.yahoo.com/group/fastar/message/5661

Can I use 1.4x and 2x tele-extenders? Yes and no. Click on the relevant slide in this gallery. Basically best is to get a tiny-pixel DSLR or astroCCD and forget about extenders.

Are there any negatives? Of course. Bright stars (relative to the rest of your image field) tend to generate a lot of flare and also disconcerting spikes/camera shadows. Many people get nervous about sticking stuff on their corrector plates. No need for nervousness, just do not then crash into your camera or have your camera cables snagged somewhere while slewing. There is strong vignetting and you have to be extra careful when taking your flats (twilight or dawn sky flats are probably the simplest, most accurate solution).

Finally, do not use your Hyperstar as a toy imager. You still need adequate integration time to get decent final images. Yes, you can get away with only a quarter of the integration time required with a pee-wee six-inch APO, but you still need an ample amount. Best solution I have come across till date for a heavily light-polluted site. Instead of 20+ hours integration time when using an APO, you can settle for a mere 5 hours ;-)

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