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A 9-Inch F/11 Split Field
Schupmann Medial Telescope

Built by Jim Daley of New Hampshire
1999-Jun-25

The split field Schupmann idea was first suggested by Dr. James Baker and as far as I know the instrument described here is the first working example. Only a small departure in the usual tilted component medial is required. However, it should be pointed out that the split field configuration is limited to Schupmann's that utilize a field lens. This is because baffling in the field mirror type is impractical if not altogether impossible if one attempted to adapt it to split field use.

The main features of this type of medial is the use of untilted optics and the off-axis field center. Because the instrument is used off-axis the field of view is limited by astigmatism. In my version the astigmatism is undetectable over the entire, although small, 12 min. of arc field. This field diameter is more than enough for the work I do, such as CCD planetary imaging and visual micrometric binary star measures. For somewhat larger fields one could safely tilt the objective a tiny amount to correct astigmatism at the field center. Comma remains completely corrected either way.

Optically the system works by imaging the rays from the objective incident on the lower half of the field lens onto it's upper half by way of the, inverting relay, Mangin corrector. The returning rays are intercepted by a small flat just before the field lens and sent to the eyepiece assembly. This flat, positioned very close to focus, divides the full field by it's edge closest to the the axis thus "splitting" the field. Stray light control in this system is complete and utilizes the usual Lyot stop centered over the corrector.

I am very satisfied with this configuration, optically at least, although the tube is longer than an equivalent field mirror system. This I've minimized by making a very short focus corrector. On this system the 9.25-inch objective is corrected for spherical by aspherizing as is the 2-inch corrector which by itself required 10 waves of aspheric work.

Presently perfect images are obtained with the Lyot stop limited to 8-inch effective aperture because of a small defect at the corrector edge. Despite this, the performance is quite good at 8.5-inches for less critical observing. Some refigure work is planned in the near future to bring the working aperture to at least 9-inches. This will improve the resolution slightly and, more importantly, will reduce the exposure time for planetary imaging with the CCD.

THE SCHMIDT CAMERA

In addition to the principle instrument, the 9-inch Schupmann (above article), a 6-inch f/3.2 Schmidt camera was recently completed and tested. As the photos show (Image 1 & 4), it is mounted alone in place of the Schupmann (Image 3). Dedicated saddle plates remain with each instrument making the switch-over an easy job.

The Schmidt was to be a wide field photographic camera utilizing the usual curved film plane. However, before the instrument was completed, I became infected with the pixel virus and changed my plans! Instead the camera was completed with a Newtonian focus feeding the ST-5 CCD head. To focus the camera I built a micrometer driven slide plate which carries the diagonal mirror, spider and CCD along the optical axis (see photo of scope and CCD). This has proven to be stable and precise.

At first thought it seems a Schmidt system is overkill for such small fields (28 min. of arc diagonal field when using the ST-5) however, an equivalent Newtonian will exhibit noticeable coma at the field edges. The second thought that occurred to me concerned the impact on image quality resulting from the curved field of this classical Schmidt. This turned out to be a non problem with only an undetectable (0.00025-inches) defocus across the field. Even an ST-7 with nearly double the field is perfectly satisfactory from a field curvature standpoint.

To wrap this up, it seems that that this little Schmidt camera will be very useful in student programs which require a generous field for supernova search projects (the system detects 18th mag. in 15-min.) as well as the usual study of variable stars, many of which have the comparison stars in the same field. A 6-inch objective prism is underway to be used in student work in spectral survey studies.

I have included a few shots of popular objects taken with the Schmidt. They are displayed in the Astro Image Gallery.

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