An SCT used for visual observing at f/10	The same SCT with its secondary mirror replaced by Hyperstar and a CCD camera

What is Hyperstar?

Hyperstar is a small lens assembly that replaces the secondary mirror in a Schmidt-Cassegrain Telescope (SCT) to provide a wide, corrected field of view suitable for imaging. It incorporates a mount for CCD cameras and other imaging devices.

Most SCTs have a focal ratio of f/10; standard focal reducers are available to give faster f-ratios such as f/6.3 and f/3.3 but the Hyperstar outperforms them all at around f/2. The exact focal ratio varies a little with telescope model.

A CCD camera and Hyperstar mounted on a Celestron SCT

Why is focal ratio important?

From conventional photography we know that a faster focal ratio (smaller number) gives a shorter shutter speed or exposure time. The same is true when taking images of extended objects such as nebulae and galaxies in the night sky. Shorter exposures mean that telescope tracking requirements can be relaxed and short periods of good weather can be exploited. In some cases it means the telescope doesn't have to be polar aligned which also saves time if you don't have a permanent installation.

This table provided by Starizona shows comparative times for the same amount of exposure at different focal ratios

The other advantage of a fast focal ratio is that you get a wider field of view. Hyperstar is designed to give excellent optical quality over the whole field so really large objects such as the Andromeda Galaxy will fit onto the detector.

Some Background

Large professional research telescopes such as the 2.5-metre Isaac Newton Telescope (INT) and 4.2-metre William Herschel Telescope (WHT) on La Palma are fitted with 'prime focus correctors' when used for wide-field imaging. These operate in exactly the same way as the Hyperstar although they sometimes incorporate other features as well. For example, the INT and WHT each have prime focus correctors with rotating prisms that are adjusted automatically to compensate for atmospheric dispersion, depending on the altitude of the object being observed.

The secondary mirror assembly at the top of the open-truss tube of the William Herschel Telescope can be replaced by a prime focus corrector and wide-field camera in the same way that Hyperstar is used. But note the person on the right-hand platform showing the difference in scale

The INT and WHT prime focus correctors were designed for photography when large photographic plates were in regular use. Compared to these plates, CCD detectors are quite small and mosaics of chips are used to capture the whole field of view.

The image scale is much smaller on amateur telescopes so the detectors can also be smaller. In fact, recent CCD developments mean that the hyperstar field matches the larger CCDs very well.

As far as I'm aware, Celestron was the first company to make a prime forcus corrector as a standard item for the amateur market. They called their system 'Fastar' and made some of their SCTs 'fastar compatible'. Starizona have taken the concept a little further and have improved the design in their Hyperstar. They've also come up with ways of fitting it to some Meade telescopes as well as Celestron.

Hyperstar imaging sounds attractive - can you recommend a telescope package including Hyperstar?

Most Meade and Celestron SCTs from 8 inches to 14 inches can be fitted with Hyperstar but the most popular portable telescope is probably the Celestron CPC800. Large apertures are not essential for high-quality imaging as you'll have seen if you've looked at Steve Loughran's images taken with a 66mm refractor on our web site. The convenience of a telescope that can be handled easily by one person is an important factor, especially if you have your imaging equipment to set up as well. In the UK it is unusual to get weather conditions that allow you to take advantage of the extra resolution that larger apertures offer.

Accurate focusing is key to obtaining good images and the fast focal ratio of Hyperstar requires very accurate focusing. We recommend the use of an electronic focuser, preferably one that can be used with automatic focusing software. A good example is the Feathertouch focuser working with the Microtouch Autofocuser.

Using the Hyperstar with a colour camera is the most convenient way, but if you want to use filters then an extension piece designed to take a 2-inch filter is also needed.

Compatibility

Hyperstars are available for Celestron 8, 11 and 14 inch SCTs and they fit directly onto 'fastar compatible' models. An adaptor that replaces the secondary mirror assembly is available to convert those telescopes that are not fastar compatible.

Hyperstars are available for Meade 10 and 14 inch f/10 SCTs. Some Meades are compatible with hyperstar and some require an adaptor to replace the secondary mirror assembly. The pictures below should help you identify whether you need the adaptor for your Meade telescope.

The Adaptor

The adaptor replaces the secondary mirror assembly. Once fitted, it is a simple job to switch between the hyperstar and secondary mirror.