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Author Topic: dew shields, C8 front covers, etc.  (Read 1906 times)


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dew shields, C8 front covers, etc.
« on: July 25, 2015, 04:47:55 PM »

Regarding C8, and accessories:

A dew shield was made from foam sheeting available at Michaels ($6.95 for a 3' x 5' sheet).  15' of 2" wide sticky-back Velcro is available at Home Depot for $29.95, which you also can use for many other projects.  You will need some black gaffer's tape to strengthen the foam edges, also available at both Home Depot and Michaels.  From these materials, you can make several dew shields.  You will need instant glue to securely fasten the Velcro, as its own sticky side is quite sticky, but not quite strong enough.  You will also need instant glue to glue two additional thicknesses of the foam around the open end of the shield to give it rigidity so it will stay round.  About three 4-packs of instant glue should suffice (about $6 at Home Depot in the paint department).

A dew heater base was made from 2" wide aluminum duct tape about 30 long".   A tape measure is used to measure the circumference of the C8 front casting.  For the precise length, add 2" for the Velcro closure overlap.   Nichrome wire was used.  It was wrapped around-and-around two posts spaced 40" apart until the overall resistance is about 28 ohms (5 Watts dissipation on a 12 volt source).  This power will raise the corrector plate and surroundings about 2-3° F, and dew will probably not form.  Also using a dew shield along with the heater will almost guarantee dew will not form on the corrector plate.  You will need to make an eyepiece heater to prevent dew.

The multiple strands of nichrome wire are threaded through the thin external vinyl plastic sheath of some small 4-conductor cable (interior wires removed).   The nichrome wire is now insulated.  This assembly is folded into a "U" shape, and the connection wires silver-soldered to the nichrome wires (you can't tin-lead solder to nichrome - it won't stick).  The connections are then insulated with heat shrink tubing.

The aluminum tape is stretched out on a table top with the sticky side up for the bottom side.   The "U" shaped nichrome wire assembly is placed on the sticky side of the aluminum tape in a sinusoidal type pattern starting with the closed end of the "U".   Another layer of aluminum tape is placed on top.  A graphics-art paper roller is used to force the top aluminum tape and insulated wire into a flat assembly.

The side of the heater assembly that will contact the C8 front casting is painted flat black to increase its emissivity.  The back side is left shiny aluminum to internally reflect the heat.  Shiny aluminum has about 4% emissivity (96% reflectivity).   Black paint is about 95% emissive.

1" wide fiberglass packing tape is used to cover both sides of the aluminum tape assembly to give it strength.

Velcro is placed in each end to provide an adjustable closure mechanism.

The dew shield goes over this heater.

The collimation adjusting Allen screws in the secondary mirror housing can be replaced with knurled brass thumbscrews, which are available at Specialty Tool on Aero Camino in Goleta.  Use 6-32 x 3/8".  A set of three will cost about a dollar.  You don't need the big knobs like I made.  The smaller brass thumbscrews are sufficient.  The big knobs are just easier to turn.

I have two C8's.  One made in 1975, which has 8 screws in the corrector plate holder ring, and a shiny orange paint job.  The other was made in 1976 and has 6 screws in the holder ring and a rough orange paint job.  The 1975 model has a spun aluminum front cover.  The 1976 model has a molded plastic front cover.

The 1975 C8 has the secondary mirror housing in the same plane as the front of the front casting, i.e., the spun aluminum cover nearly rests on the secondary mirror housing.  This occurrence is not good.  It takes very little axial force on the secondary to shatter the corrector plate ($800 for Celestron to replace).  With thumbscrews in place for collimation, the spun aluminum front cover won't fit.  It will bump into the thumbscrews.  I made a new front cover out of 1/4" masonite for the disc, and 1/8" masonite for the rim with plenty of interior clearance above the collimation thumbscrews, front mirror and corrector plate.

The disk was made by placing the C8 (face down) on a piece of 10" x 10" x 1/4" tempered masonite and tracing the outside of the C8 front casting onto the masonite.  A series of straight saw cuts were made to create an irregular polygon that was slightly bigger than the circular trace.  A stationary disc sander was used to grind away the masonite up to the circular rim mark.

A piece of 1/8" thick masonite was ripped to a 2" wide strip on a table saw.   The flat strip was made into a circular band by heating with a hot air gun and gently bending it.

This masonite rim strip was test fitted around the circumference of the 1/4" masonite disc, and a cut line marked (a little long so adjustments can be made).  The rim masonite is cut to length.   A wire tie was fashioned to hold the masonite rim to the masonite disc, and it was again test fitted.  Its length is gently modified (by sanding) until it closed completely around the masonite disc.   Instant glue was applied to the masonite disc edge and the mating part of the masonite rim.  The wire tie was tightened, and the glue allowed to set (about 5 minutes).

Three 1" wide x 1-1/2" long pieces of 1/8" masonite were cut, then heated to match the inside radius of the rim (on the 1-1/2" dimension).  They are instant glued on the inside of the rim 120° apart to provide a stop to limit the amount the cover can be inserted.

Thin felt pieces were glued to the inside of the rim to make a snug fit to the telescope front casting.

The masonite edges were sanded and then the assembly painted with spray can flat black on the outside.  The 1/8" masonite I used was already pre-finished white on one side (now the inside).

Bruce M.
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