A lovely little telescope, resurrection and refit.

I answered an ad on one of the astronomy forums I regularly peruse, where someone was getting rid of a badly water-damaged telescope. I figured it would make a nice project for myself so why not reply and see how much it was going to cost me.

A mate of mine picked up the telescope from the owner, and warned me that it appeared to be in relatively bad condition, so I was a little apprehensive.

When I collected the scope and paraphernalia from my friend, I recognised the scope as a Celestron CPC800 – a Schmidt-Cassegrain design with 8″ aperture and focal ration of f/10, with the computer control and GPS-based time and location finding. With the scope came the tripod and spreader bar, the standard 8×50 finder, a prism diagonal with 1.25″ visual back, an older NexStar handset, and the power cable with a generic wallwart adaptor. The tripod showed evidence of corrosion under the black paint on all of the non-chromed parts and a little surface rust on the bolts that hold the scope down. The ports on the fork base were showing heavy corrosion on the contact wires and a lot of green crud at the base of the ports, which did not bode well for the mechanicals at that point.. The tube was showing a lot of corrosion, with lots of the black paint paint on the front dell having been completely flaked off, and the metal underneath turning to a white flaky powder that had dusted all over the corrector plate. The primary mirror itself looked in reasonable condition actually, with a very light coating of dust but pretty much bright and untarnished in any way. The secondary was also appearing to be pristine

I took the scope home, and immediately started to disassemble, at 11pm, in my kitchen..

Attempting to power on the scope with power from the 12v lead acid battery that I use to power the AVX mount with, led to seeing a bright LED on the base lighting up, but absolutely no response from the handset. Knowing that the two AUX ports are parallel with the HC port, I tried the handset in the two AUX ports to have a response from the third port. The handset was alive at least. Motor errors 17 and 16, somewhat expected, showing a lack of communication with the motors. But, the motors would move but not stop. That was a really good sign! Maybe the scope could be resurrected without too much expense.

So, after a little research, I found I could replace the two circuit boards that are attached to the top plate of the fork base for a reasonable cost. The other PCBs around the scope appeared to be in good physical condition at least, dry and free from obvious corrosion. The testing of those would require the two boards in the base to be correct, as it appeared that the errors 17 and 16 were as a result of low power, mostly due to corrosion raising resistances and allowing current leakage between wires.

The replacement boards arrived, (many thanks to Stephen in Ktec Telescopes for a quick turnaround), and the installation of these was trivial enough. The smoke test was passed – nothing let out the magic smoke when power was applied! The handset powered up without issue, and the motors appeared to work perfectly on all axes at all speeds tested. I parked the scope outside on a paving stone in the lawn, and left it powered on for ~30 minutes to see if it could pick up a GPS fix. It did in fact pick up a GPS fix, one that was correct for both time and location, so I knew now that the electrics and electronics were now in fairly good condition.

I connected the serial port from one of my Dell SFF pcs to the handset, and powered on the scope. Using the HCUpdate and MCUpdate programs from the Celestron website, I was able to flash the current firmware on the handset. The motor controllers were at current levels anyway, not that surprising as the current version is unchanged since ~2011 or so.

Now on to the optics and tube. The corrosion on the tube front cell was such that the little steel screws holding on the corrector plate had bound tightly to the aluminium alloy body of the cell. Three screws were not bound and came away with a little bit of brute force and ignorance. One screw simply sheared off under the head, another two would not budge and have needed to be drilled off to allow the corrector plate to be removed for cleaning. The secondary is a Fastar-compatible and could be removed easily while I was trying all of this. It’s rather nerve-wracking to use spinning HSS steel bits that close to the corrector plate.. Eventually I managed to get the black plastic ring with the white writing off, and I could remove the corrector plate from the cell.

There was much brushing and cleaning of the cell as it was very flaky and dropping powder all of the time. I got all of the loose powder and pretty much all of the visible corrosion off, but I know I have another day of work to properly refurbish the cell. The screws and nuts holding the cell to the tube are bound solid in the aluminium cell – I managed to twist one of my screwdriver bits when trying to work it free so it was not a lack of torque at least.

The cleaning of the corrector plate was a fairly tedious process as there were significant marks in the anti-reflection coatings. Clean flowing water with a little dish soap and fingers, isopropanol and cottonwool, careful application of distilled water then to rinse. There are still some faint marks left but I expect that these are going to be pretty much invisible in general use.

The next step for me was to update the spare NexStar+ handset left over from my StarSense update that was performed on my AVX mount. Flashing it with the alt-az firmware via the Celestron CFM software meant that it was now suitable for the CPC mount. Testing the handset showed it to work exactly as was hoped, with the updated catalogues available for use, as well as the more familiar layout of the buttons and functions. Connection from the SkyFi box was also working perfectly.

Putting everything back together, cleaning all of the dust out of the base and re-lubricating the worm and wheels at the same time, I had a partial first light on a cloudy day. Using a Hubble Optics “artificial star” I could attempt a collimation across the length of Lily’s lawn. Using an 8mm eyepiece, and the working motor drives, I could get a pretty symmetrical set of in-focus diffraction rings. It really looks as though the optics are good enough to use!

First light was on a typically broken cloudy Irish evening. Two star align went okay, aligning the finderscope in the process. Given the weather conditions, double stars were the order of the day. Lots of pretty sights, Izar was cleanly split, the double-double Epsilon Lyrae was also cleanly split. Both were with very visible diffraction rings. Intra-focal and extra-focal were reasonable and will require a more comprehensive Roddier analysis in coming weeks.

EAA with the SCB-4000 camera was successful though the long focal length is a hamper. My 0.5x reducer is pretty crappy and gives teardrop aberrations halfway to the edge of view. I’ve got a 6.3 reducer in my hand that should help with the visual field, as I have a 18mm 82 degree eyepiece that should give a nice 1.15 degree FOV up from the 0.7 degree when at f/10, while still giving me an exit pupil of 0.75 with my 4.7mm. 275x is still not commonly usable in Ireland with the seeing we usually get here..

The plan is for this scope to be used by the UL Astronomy Society for member use and public outreach. I will be maintaining the scope and keeping possession of it when not actively used by the AstroSoc. It should mean more availability compared to the hoops needing jumping through to get the Physics Dept 10″ Meade LX10 out for use.

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