Celestron® NexStar 11-GPS
Celestron® Super C8 Plus

The UCSB physics department has four telescopes for use in sky viewing sessions. One is a Celestron® NexStar 11-GPS, and the other three are Celestron® Super C8 Plus telescopes. These all have a Schmidt-Cassegrain (catadioptric) optical configuration. That is, a primary mirror, at the rear of the telescope, reflects incoming light towards a secondary mirror located at the entrance aperture, which reflects the light through a hole in the primary mirror to the eyepiece assembly, which sits at the focus of the primary mirror. On entering the telescope, light passes through a corrector plate (or corrector lens), which compensates for spherical aberation of the primary mirror.

For the NexStar 11-GPS, the clear aperture is 279 mm and the focal length is 2800 mm, and for the Super C8 telescopes, the clear aperture is 200 mm and the focal length is 2000 mm. For eyepieces, the NexStar 11 has a 25-mm SMA (super modified achromatic) wide-angle lens and a 40-mm Plössl lens. Each of the Super C8 Plus telescopes has a 7-mm orthoscopic lens, a 26-mm Plössl lens and a 36-mm Plössl lens. In addition, there are three 10-mm Plössl lenses, two of which are with the Super C8 Plus telescopes, and one of which is with the NexStar 11.

All of these eyepieces fit a standard 1-1/4″ mount, and they are all interchangeable between the Super C8 Plus and the NexStar 11. Each of these telescopes is equipped with a 1-1/4″ mirror star diagonal, which orients the eyepiece perpendicular to the telescope axis, as shown in the photographs above. It also causes the image to be erect along one axis and inverted along the other. That is, with the eyepiece oriented as shown in the photographs above, the image appears right side up, but mirror image from left to right. Similarly, if the star diagonal is rotated so that the eyepiece is horizontal, the image will appear upside down, but correct from left to right.

For wide-angle viewing, we have a Meade 56-mm Super Plössl lens, which has an apparent field of view of 52° and fits a 2″ mount. To accommodate this lens, we have an OPT (Oceanside Photo and Telescope) 2″ mirror diagonal, which fits the rear cell on both the NexStar 11 and the Super C8 Plus. In addition, we have a Celestron reducer/corrector, which shortens the effective focal length of the telescope by 37%, thus increasing the field of view by a factor of 1.6. It fits between the rear cell and the mirror diagonal.

The orthoscopic lenses have a plano-convex singlet eye lens and a biconvex triplet field lens. The SMA, which is a modified Kellner-type lens, has an achromatic doublet eye lens and a singlet field lens. The Plössl lenses are four-element lenses, having a doublet eye lens and doublet field lens. Plössl lenses offer a flatter field of view than the others, and an image that is sharp across the entire field. You can find a useful summary of eyepiece designs and their characteristics on Wikipedia.

According to the NexStar 11 manual, the apparent field of view of the 40-mm Plössl eyepiece is 46°. As best I can tell, the apparent field of view of all the other eyepieces is close to this, except for that of the SMA, which appears to be 52°. The magnification of a telescope is the ratio of its focal length to that of the eyepiece (M = fo/fe). For example, the NexStar 11 with a 25-mm eyepiece gives a magnification of 2800 mm/25 mm = 112. The field of view is the apparent field of view of the eyepiece divided by the magnification (FOV = FOVapp./M). For the example above, assuming that the lens has an apparent field of view of 52 °, the field of view is 52/112 = 0.46°. Assuming an apparent field of view of 46° for all the eyepieces except for the 25-mm SMA, and a 52° apparent field of view for this lens, the available magnifications and corresponding fields of view are:

NexStar 11-GPS and:
M
FOV
FOVcorr
7-mm orthoscopic
400
0.12°
0.18°
10-mm Plössl
280
0.16°
0.26°
25-mm SMA
112
0.46°
0.74°
26-mm Plössl
108
0.43°
0.67°
36-mm Plössl
78
0.59°
0.94°
40-mm Plössl
70
0.66°
1.04°
56-mm Super Plössl
50
1.0°
1.6°
Super C8 Plus and:
M
FOV
FOVcorr
7-mm orthoscopic
286
0.16°
0.26°
10-mm Plössl
200
0.23°
0.36°
25-mm SMA
80
0.65°
1.03°
26-mm Plössl
77
0.60°
0.95°
36-mm Plössl
56
0.83°
1.3°
40-mm Plössl
50
0.92°
1.5°
56-mm Super Plössl
38
1.4°
2.3°

As noted above, using the corrector/reducer would increase the field of view for any of these lenses by a factor of 1.6. The resulting fields of view are given in the colums labeled “FOVcorr”. As of this writing, there is some question as to whether the reducer/corrector can be used with the 56-mm Super Plössl, and it has not yet been tested with the 40-mm Plössl. It should, however, work well with any of the other eyepieces.

For solar viewing, we have two full-aperture filters for the Super C8 Plus telescopes, and one 3″ filter, also for the Super C8 Plus. (This is an 8″-diameter cover with a 3″ filter set in it.)

You can view the manual for the NexStar 11-GPS here, and for the Super C8 Plus here. This version of the manual for the Super C8 Plus is actually not the same as the one that came with the telescopes, but the information it contains should still be useful. A copy of the manual that came with the telescopes is available in room 1630 for anyone who wishes to see it.

Courtesy of Tom Fuller, there is also a condensed, step-by-step set of instructions for how to use the NexStar 11-GPS, which you can view here. This telescope uses the GPS to determine its location (latitude and longitude), and it has an electronic compass to determine in which direction north lies. As of this writing (11/08), using this built-in compass introduces a large error in the alignment of the telescope. This may be due to stray magnetic fields from the telescope tripod, enclosure or hardware, or from the roof deck, or it may be that the compass merely needs to be calibrated. The NexStar 11 has a utility in its menu for performing this calibration. In any case, Tom Fuller’s directions give a manual alignment procedure, which you can perform with the aid of a hand-held compass and a spirit level (both of which are kept with the telescope), and which should work well.