This report determines the field-or-view (FOV) and associated parameters, for 35-mm photography, of the Celestron Radial Guider ("RG," model #95176) and Celestron Reducer/Corrector Lens ("R/C," model #94175) on a Celestron 5" series Schmidt-Cassegrain telescope (SCT). Note that a spacer ring must be removed for this RG to work with the 5" SCT series. See the Celestron 5" Usage Notes for details.
The RG and R/C Combo
The RG picks off a small section of the light path, by means of an adjustable prism, that is sent to the guiding eyepiece. Without affecting the telescope or camera, the eyepiece and prism can be adjusted 145-degrees radially around the FOV to locate a guide star. A guide star is then centered in the FOV by means of an adjustment screw that tilts the prism perpendicular to the radial motion.
A 35-mm camera attached to the RG is focused using the knob on the telescope. Because they share the same light path, this affects the guiding eyepiece focus. The guiding eyepiece is brought to focus independent of the camera by the, somewhat tedious, process of raising and lowering it in the holder. Thumbscrews must be loosened and tightened to move and hold the eyepiece.
It is wise to use a wide-angle eyepiece (20-mm sees the whole field) to initially locate/center the guide star before employing the guiding eyepiece. Your guide star selection is confined to a small swath of sky near the target to be image. The swath is further limited by the orientation of the camera in relation to the target. As a result you'll find yourself guiding on very dim stars. Use at least a 12-mm eyepiece with guide marks; the illuminated LED type is best. If using a non-illuminated cross-hair you'll find the marks may cover the dim star. In this case defocus the eyepiece so the star forms a blob (doughnut section) that just surrounds the center mark.
The RG and R/C combo was used to image star fields using a 35-mm camera with T-adapter. Linear distances between stars on the resulting negatives were related to arc-distance of these stars from a star atlas. These measurements yielded values for image-scale (millimeters per degree) at film plane. Film frame FOV and telescope focal length were mathematically derived from this.
The guiding eyepiece FOV is much narrower than that at the 35-mm film plane. Using the star-drift method the FOV was measured. The orientation of the eyepiece FOV in relation to the film plane FOV was determined by view a nearby chimney and counting the bricks!
The image scale measurements at the film plane had a standard deviation that was 1% of the average measure. This scale was extrapolated into film plane width and height FOV based on a 36-mm x 24-mm image. In practice your camera may produce a slightly bigger image in the height dimension. The scale drawing here displays the film plane and the guiding eyepiece FOV results.
The image scale at the film plane averaged 15.74-mm/degree. This yielded a 2.29-degree x 1.53-degree FOV on the negative. This image scale corresponds to an optical system with 902-mm focal length. Assuming an 127-mm objective diameter yields an f/7.10 focal ratio for this setup. (Though advertised as an 125-mm objective the 127-mm value is stated on Celestron's website for the NexStar5 and implied from a Sky & Telescope Oct. 1993 test report of the C5+. The same value is employed as the default diameter in Michael Covington's Astrophotography Calculator program for the C5/G5/NexStar.)
The drawing also shows the sky area covered by the radial motion of the guiding eyepiece, approximate overlap between the two FOVs (about a quarter of the smaller) and the relative size of the full Moon. Note that the eyepiece FOV shown is the maximum and will be smaller with less overlap for eyepieces shorter than about 20-mm.
Using the Results
The scale drawing here is most useful when used as on overlay on computer or printed charts to determine camera orientation and choice of available guide stars. A 427-pixel x 330-pixel version of the drawing, with a white background suitable for printing or use as an overlay, is available for download.
If printed at 150-dpi the image scale will be 18.4-mm-per-degree -- the scale used by the Uranometria 2000.0 atlas. Make sure the program you print through does not attempt to scale the image to full screen. If dot-pitch is not controllable on your image program then specify printed graphic width and height as 2.85" x 2.20" (72.5mm x 55mm) to achieve the Uranometria scale. Some word processing programs allow setting height and width for imbedded graphics. Validate your results by matching the ½-degree scale on the drawing to the declination scale on your chart. Check the long and short axis of the scale to confirm that the image maintained scale in both dimensions. Don't bother trying to scale the image for Sky Atlas 2000.0 - the image scale is much too small on that atlas.
For computer charts the drawing must be made partially transparent to view stars through. If your sky chart program can't import the scale drawing then export the chart of interest as a graphic and use a suitable program to overlay the drawing. If this is awkward consider printing the drawing onto transparency and placing it on your computer monitor - scaling the on-screen chart to the drawing scale.