SBIG FR-237 Focal Reducer
Item No. 757-41000
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Manuf. No. SBIG FR-237
The Focal Reducer lens is mounted in a 1.25" cell that will screw into the 1.25" nosepiece of the STV, ST-237, ST-5C or ST-4. When used with the short 1.25" extension tube the amount of focal reduction is increased. The chart below gives an example of the effective focal ratios one will achieve with different telescopes.
The ST-5C, for instance, uses a relatively small CCD, 3.2 by 2.4 mm, that may be inconvenient when attempting to image extended objects with longer focal length telescopes. For example, the field of view when used with an 8" F/10 telescope is only 4.1 by 5.5 arc-minutes. The FR237 focal reducer is designed to reduce the focal ratio of an F/10 telescope to F/5.95 or F/3.75. At F/3.75 the CCD, with this typical telescope, would image an area of 11 by 14.7 arc-minutes. The result is a sharper appearing image with crisper star images, an increase in field of view of 7X, and an increase in sensitivity to extended and nebular objects by 7X. Since the pixel size is still smaller than a stellar image under typical seeing conditions, no resolution is lost. The image improvement is dramatic with diffuse nebulae and galactic objects! An ST-5C with a focal reducer is nearly equivalent to an ST-6 used without one, for considerably less cost.
Although the STV and ST-237 use a detector that is twice the size of the detector used in the ST-5C, the same increase in field of view and sensitivity apply. The 237 detector has a field of view of approximately 6 by 8 arcminutes (48 arcminutes square) when used at the prime focus of an 8" F10 telescope. With the FR237 operating at F3.75 the field of view of the TC237 detector becomes 16 by 21 arcminutes (336 arcminutes square) and extended objects appear 7 times as bright. This can be particularly important with the STV video camera where one may wish to keep exposure times as short as possible.