What are Fastar and HyperStar? How do they work?
December 19, 2017
Fastar is a system invented by Celestron in the 1990s to quickly and easily convert our Schmidt-Cassegrain telescope (SCT) f/10 optical tube assembly (OTA) into a very short focal length, fast f/2 imager. The Fastar system consists of two parts. One is the secondary mirror cell assembly in the OTA. The second is a two-element lens that replaces the secondary. Fastar is designed with an easily removable secondary mirror holder with an alignment pin. Taking out the secondary is a snap – unscrew the knurled retaining ring and lift out the mirror and its attached holder. Reinsert the mirror by lining up the pin on the holder with the slot on the Fastar cell – no collimation is required.
The heart of the Fastar system is the corrective lens assembly (it corrects for spherical aberration, coma, off-axis astigmatism and field curvature) that goes in place of the secondary mirror. It slips in and is locked down by the knurled ring. Because the secondary’s amplification factor is absent, the scope’s light focused by the lens is directly from the f/2 primary mirror. Now your SCT gathers light 25 times faster than the original optical configuration. It also provides 25 times the sky coverage with the same-size chip (or film). Insert your CCD camera’s nosepiece into the Fastar lens assembly. Digital and film SLRs would usually use a 1-1/4” T-adapter and T-ring to attach to the Fastar. Imaging at f/2 allows very short exposures - 30 seconds to 1 minute - for deep sky objects. You can use an alt-azimuth mount without a wedge and without guiding!
Celestron discontinued the Fastar in 2005. A third party, Starizona, introduced the Fastar-based HyperStar lens assembly about the same time. They are functionally identical. Starizona also makes conversion kits to mount the HyperStar (Fastar) cell to older SCTs. All Celestron EdgeHD OTAs have Fastar cells.