Owners of some of Celestron's older SCTs, current C14s, and modified/custom-order Meade and Celestron OTAs have a special method of picture taking available beyond piggyback and prime focus—Fastar. In a Fastar set up, the SCT's secondary mirroris removed and replaced by a CCD camera, which takes advantage of the SCT's fast f/2 primary mirror for wide-field imaging with little or no guiding. How can the secondary mirror be removed? Fastar compatible telescopes feature a mirror holder that allows the secondary to be pulled out after unscrewing a threaded ring.
Why isn't every astro-imager using Fastar? One reason is that the camera can't just be mounted in place of the secondary mirror. The steeply curved spherical primary mirror would deliver images that would suffer from multiple sharpness problems such as aberrations. Fastar gets around that by having users install a corrective optics package ahead of the camera. Unfortunately, the prices of these corrective lenses, currently only available from the U. S. vendor Starizona, are relatively high. The company's "Hyperstar" lens for a C11 is nearly $700. Many imagers would rather invest that money in an ED refractor if wide-field imaging is a goal. Hanging a CCD camera off a thin corrector would also seem to invite disaster. One slip of the declination clutch and the camera will contact a fork mount scope's drive base; the probable result will be a broken corrector. This is a real concern and has happened to several astrophotographers at least. Despite these caveats, there is no doubt that Fastar is a path to relatively easy wide-field imaging; at f/2 tracking is forgiving and exposures are very short.
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