It looked horrible! The paint was coming off everywhere with heavily corroded aluminum underneath. Most of the screw heads were small balls of rust, with hopefully enough remaining to fit a screwdriver to and remove. For a precision optical instrument this small telescope was not very encouraging.
On the bright side the optics looked to be in decent shape. While there was some dirt and mold on the corrector, the primary looked almost perfect. Not bad considering the condition of the metal parts. Maybe, just maybe, this telescope is salvageable.
The telescope in question is an old orange tube Celestron C8. Thousands of these little telescopes were manufactured in the 70’s and 80’s. It was the C8 that set the standard for amateur telescopes at the time. The C8 is still in production forty years later, but the tubes are no longer painted orange as they were originally. Compact, yet offering decent performance, these telescopes were well regarded and hold a special place in the memories of many amateur astronomers. I have seen these little orange telescopes at dozens of star parties, even bolted to the side of huge professional telescopes for use as finders.
The paint had failed on almost every exposed part of the optical tube, the result of tropical humidity. At first glance one would think that much of the paint was intact with a few bad patches, a few probes reveals that much of the solid appearing paint is completely loose and comes away easily. The aluminum underneath was heavily powdered on the castings and there were notable spots of corrosion on the orange tube itself. Somewhere along the line the tube had been dismantled and flocked with velvet paper, stripping this away revealed even more areas of corrosion underneath.
Dismantling the ‘scope is pretty easy. Just set to with a screwdriver and the corrector comes out along with the secondary mirror. A few taps with a mallet were needed to free the front ring. After the front is gone you can remove the primary, just remove the focus assembly and then remove a clip from around the central tube that retains the mirror. A few more screws and the rear cell comes free. It does not appear the central tube and rear threads can be easily removed from the cell. No problem, leave in place just tape over the for painting. The anodized parts appear as good as new, no need for restoration, just a little cleaning.
With the telescope entirely dismantled it is time to start the real work. A few sweeps of a heavy wire brush on the mirror cell casting produced a cloud of white aluminum oxide powder and paint chips. Not wanting to breathe this and get it all over my garage I moved the effort into the driveway where a stiff trade-wind breeze would keep me from choking on the product of my labor. A couple hours of scraping, brushing, and sanding removed much of the paint from the front and rear castings.
The orange tube took even more effort. First soaking to remove the last of the flocking paper and Goo-Gone to remove the adhesive. I then hit the tube with an vibrational sander loaded with 80 grit paper. Chips of orange paint everywhere! The orange paint had almost no adhesion to the metal underneath, it just flaked away when under the sander. In a matter of five minutes there was a pile of orange underneath my work stand and a shiny tube of bare aluminum ready to be painted. The orange tube was no longer orange.
Now for the question of paint… For the castings I used a tried and tested solution. Rust-Oleum manufactures a textured paint designed for refinished metal surfaces, the hammered finish series. This is a thick paint designed to cover a ragged surface with rust or remnants of a previous finish, exactly what I am doing here. I have used it before on several telescopes, it looks quite good, a rich gloss with a touch of metallic sheen.
The hammered paint looks particularly good on aluminum castings. The chips and flecks of older paint that remain simply become part of the paint texture. Once fully cured it is tough and durable, ideal for a telescope that will see rough handling. It dries to the touch in fifteen minutes, but remains soft for a day or two. I have learned to wait up to a week before assembly as it can remain slightly tacky for days and stick if pressed against another surface.
I thought for a bit on the choice of color for the tube. Blue is right out, this is a Celestron, not a Meade telescope. I did not find anything that would reasonably match the original orange at my little rural Ace hardware store. I considered some form of black, that would look good and resemble a modern Celestron ‘scope. In the end I chose to stick with the Rust-Oleum Hammered finish and use a bright red for the tube. No luck, I can not find the red anywhere, back to the original plan, metallic black it is! This will be an outreach telescope, it needs to look a little sexy.
All of the hardware has been replaced with stainless steel bolts and screws wherever possible. A few of the original screws were stainless, perhaps replaced over the years? I also needed to machine a new focus knob, the original had gone missing.
Cleaning the optics reveals good news. The primary is nearly flawless, no damage and no apparent degradation of the coatings during the three decades since its manufacture. There are a few small spots of coating damage on the secondary, about a millimeter in diameter each. The corrector is in very good shape as well, some minor coating damage near the edge of the secondary. All looks good for making this a working telescope again.
Reassembly of the optics worried me a bit. While I had taken out SCT corrector plates before, mostly for cleaning, I had never stripped an SCT down all the way like this. As I disassembled things I made a few discrete marks here and there for alignment. I anticipated that a complete collimation would be required on reassembly.
A new mounting rail was machined from a bit of aluminum scrap, a solid bit of plate to securely hold the tube. The tube was de-forked many years ago and used with a different mount than it was originally supplied. The original fork mount is present, the aluminum castings in just as bad of shape as the tube. Perhaps restore this later? I have opted to restore the mount that appears to have been used more recently with this ‘scope. A TeleVue Renaissance equatorial mount in the same poor condition as the optical tube.
The mount is taking a bit longer but is well underway. While the optical tube is complete and mount has a few items remaining, it is reassembled and roughly operational. The worm gears need adjusting, one is too loose, the other too tight. A few screws need replacement yet, the best of the rusted ones used for a first assembly. Need to refinish the counterweight shaft and do something about the clutch knobs before I can call the job complete. Rebuilding the mount will be the subject of another post.
I can confirm that the new telescope curse applies to refurbished telescopes as well. I shared a few quick looks at the Moon with a neighbor waiting for dark. It was enough to establish that the optics had taken no major harm during the dis-assembly and reassembly. Indeed the craters were pleasingly sharp! Then the clouds rolled in. By the time it was full dark there was not a star to be seen, the Moon just a glow in the clouds. Collimation and final adjustments will await another evening.
3 thoughts on “Restoring an Orange Tube Celestron C8”
I would have thought you would have zinc coated the allum first?
This is not a marine product Dan! Good primer and paint should do the job.
Great job, Andrew. This telescope looked good and performed nicely at the Keck outreach event last night. It is amazing to see what a mess it was when you received it! Very nicely done!