From early on, amateur astrophotographers started connecting cameras to Celestron SCTs. Unfortunately, the standard SCT design has a problem here: long focal length. At 2,032 mm, the f/10 focal ratio of the C8 is not well-suited for imaging.
A longer focal ratio means that the light from the target object gets spread out over a larger area of the digital image sensor (or, in the older days, film). This means that a longer exposure is needed than is the case for telescopes with shorter focal ratios.
For example, if you take two telescopes that are otherwise identical but one has twice the focal length of the other and uses identical cameras in them, the shorter focal ratio will capture the same level of signal (brightness) as the longer scope in one-quarter the time. This is why telescopes with shorter focal ratios are often referred to as “fast scopes.” The shorter focal length also leads to a wider field of view, which lessens the demand for precision in tracking.
F/10 is a very long focal ratio for imaging, and not one that’s generally recommended. For some time, Celestron, as well as their main competitor in this market segment, Meade, have made focal reducers—optical elements that act somewhat like reverse-Barlow lenses to effectively decrease focal length—that drop the focal ratio of a typical 8-inch SCT from f/10 to f/6.3, which is much better suited for imaging.
But this still leads to a rather tight field of view and longer exposure times.
For this reason, Celestron introduced the Fastar system in the 1990’s.
For a typical f/10 SCT, the primary mirror has a curvature ground to a focal ratio of f/2, and the secondary to f/5, combining for an f/10 focal ratio.
Celestron’s innovation was to remove the secondary mirror and place a camera at this point. Thus, the telescope remains at f/2—very fast indeed. To accomplish this, they created a secondary mirror mount that allowed the mirror to be easily removed and replaced as desired, and when removed, the socket accepts an adapter for a camera.
The adapter, or Fastar lens, is more than just a connector but also includes optical correcting lenses to perform some of the same corrective functions done by the secondary, as well as further flatten the field.
Unfortunately for Celestron, this feature was a bit ahead of its time. Digital astrophotography hadn’t yet come far enough, and while some amateurs did film astrophotography, the bar was still very high.
In 2005, Celestron stopped making the Fastar lens, but granted permission to a third-party, Starizona based in Tucson, Arizona, to continue the technology. Starizona’s version, dubbed HyperStar, is available for a wide range of Celestron SCTs from the C6 on up.
As digital imaging has advanced since 2005, the HyperStar system has become more and more popular. Though not an inexpensive option, it can capture the same amount of signal in 1 minute that the full f/10 focal ratio would capture in 25 minutes, with a wider field of view and far less stringent requirements for tracking.
Celestron has continued to offer the Fastar/HyperStar secondary assembly on its SCT line, so owners don’t have to worry about a retrofit, just buying the HyperStar lens and a compatible camera. It’s not a perfect system; it does have some critical limitations, but even then, it’s still an excellent option for imaging—that is, if you can afford it.
