ST-5C Focal Reducer New Product Announcement

New Product Release
ST-5C Variable Focal Reducer - $180 Suggested List
June 8, 1998

ST-5C Focal Reducer and Spacer Ring

The ST-5C Imaging Camera (and all CCD cameras based upon the TC255) 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 ST-5C focal reducer is designed to reduce the focal ratio of an F/10 telescope to F/4.7 or F/3.7. At F/3.7 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!

The reducer is mounted to the camera by slipping it into the drawtube mount on the front of the camera, with or without a brass spacer ring that sets the amount of total reduction.

We have found that the increased field of view with the focal reducer makes finding objects and positioning them on the CCD much easier. Focusing is also much easier due to increased sharpness of the image.

The focal reducer, in its nominal configuration without the ring, will reduce the telescope focal length by 2.1 times. An F/15 refractor would be reduced to F/7. Vignetting will not become noticeable until final f-ratios below F/4 are attempted. The focal reducer can be used with a system as fast as F/6.3 without the spacer ring and the images will still be good, with a final F/ratio of about F/3.0. The corners of the image area will show some elongation of star images, though.

An ST-5C with a focal reducer is nearly equivalent to an ST-6 used without one, for considerably less cost.

Assembly of the ST-5C Focal Reducer with Spacer Ring for 2.7x Reduction
(Shown attached to a PixCel 255)

Sample Images taken with ST-5C through 8" SCT

M51 at f/10
5 minute image taken with ST-5C and 8" SCT at f/10 (no focal reducer). Image field of view is approximately 4.0 x 5.4 arc minutes. Nebulosity is faintly recorded and small tracking errors show elongated star image near bottom of frame. The entire galaxy cannot fit into the field of view of the camera.

M51 at f/5
5 minute image taken with ST-5C and 8" SCT at f/5 with focal reducer and no spacer ring. Image field of view is approximately 8.0 x 10.0 arc minutes. Nebulosity is about 4 times as dense as the f/10 image and appears much brighter at f/5. Stars are smaller and crisper looking with good resolution at the edge of the field.

M51 at f/4
5 minute image taken with ST-5C and 8" SCT at f/4 with focal reducer with spacer ring. Image field of view is approximately 9.3 x 12.4 arc minutes. Nebulosity is slightly brighter than f/5 image and the galaxy is nicely framed in the field. Star images are still sharp near the edge of the field.