In this report the Celestron 6x30, Long Eye Relief (LER) Finderscope (Model #51602-W, made in China), that was standard issue on later models of the C5+, is disassembled, examined, and measured. This 6x30 straight-through finder is a step-up from the 5x24 type sold with earlier models of this scope. It offers about 7.6-degrees of field-of-view at a magnification of 6x -- a good match when the main scope is configured for low power wide-field observing or object locating. Figure 1 displays this finderscope mounted on its rings, with end-caps on. Note that the Light Block Disk near the eyepiece end is a customized piece of equipment.
How It Was Done
The procedure for this article was to first unscrew the black objective lens cell from the white body tube. Focal length of the lens was measured while in the cell, after ascertaining that a retaining ring was supporting the lens. Next, the retaining ring was unscrewed to allow removal of the objective lens. A pencil mark was put on the front edge of the lens to facilitate correct reassembly. Measurements and observations were made of the lens, ring, and body tube. Figure 2 displays the retaining ring, lens, and objective cell.
The lens is 31-mm in diameter, when measured removed from the cell. Edge thickness is 7-mm. Rough measurement places the focal length, from lens center to focal point, at about 118-mm. Blue reflections from the lens indicate at least one surface is coated. A seam running around the lens circumference indicates it is two lenses glued together -- a doublet. The rear lens is much thicker at the edges than the front lens, where the light enters. These are the properties of an achromatic lens: a compound lens that mitigates chromatic aberration better than a single-element lens.
Retaining Ring and Cell
The objective is held in place on the backside by a 3-mm wide, threaded, retaining ring that screws into the cell barrel. The threading of the barrel creates an indentation that keeps the lens from falling out the front of the cell. The indentation and the retaining ring stop the lens down to at most 30-mm.
Note that the lens retaining-ring has notches on either side of a diameter to facilitate use of a lens spanner wrench. It turned easily when a soft stick (whittled to fit) was pushed against a notch. After examination, it was tightened just enough to keep the lens from rattling. This will make replacing the lens easy if it ever becomes necessary and also prevent lens pinching in cold weather.
The outside of the finder tube has a grove that runs the circumference. When the tube is properly aligned on the finder bracket this grove serves no purpose. The eyepiece assembly at the end of the tube seems firmly attached and I did not unscrew it.
The tube has an interior light stop 40-mm down from the front. Behind it can be seen the crosshair and eyepiece. Some persons suggest removing the light stop in order to obtain a half-magnitude improvement when sighting objects. This is done at the expense of having color fringes (chromatic aberration) around the Moon. This is not a bad trade-off. I have not attempted to remove the stop on my sample to verify this.
This finderscope normally needs no maintenance. Focus "tweaking" can be done by unscrewing the objective lens barrel a bit -- take care it doesn't fall off! If the image appears foggy or dusty: the first remedy is blowing air, or using a soft brush on the lens and eyepiece. If that doesn't work: unscrew the objective lens cell barrel and repeat the procedure on the inside surface of the lens. Due to the crosshair, the inside lens of the eyepiece is hard to reach for cleaning.