Restoring the Cave Astrola

I do enjoy a nice project I can work on, get my hands dirty, and use my skills. A classic telescope in need of restoration? Just the thing.

Restored Cave Astrola 8" f/6
A restored Cave Astrola 8″ f/6 Lightweight Deluxe

This telescope was literally rescued from the trash. Matt, the owner of the excellent Mountain Road Cycles in Waimea found the telescope at the transfer station.

Matt put a message into the folks at the observatory, who represent more than a few of his customers. The info was quickly forwarded to me, as everyone here knows I build and restore telescopes for fun.

Judging by the descriptions and photographs in the 1970 catalog, the telescope is an 8″ f/6 Cave Astrola Lightweight Deluxe. This telescope was one of the most popular of the Cave designs and listed at $495 in 1970, or about $3,122 in 2017 dollars when adjusted for inflation.

Tube Damage
Damage to the optical tube of the 8″ Cave Astrola

The worst of the damage was the optical tube assembly. The tube was shattered around the mirror mount with the mounting screws pulled through and the mirror cell jammed into the tube at an angle. The telescope had clearly taken a hard fall at some point.

Other issues included heavy rust on the shafts, some damage to the setting circles, a missing finder ‘scope, a missing caster, flaking paint, and a lot of dirt and grime.


The optics are the heart of the telescope, if the mirrors are of poor quality or damaged the whole restoration becomes a questionable idea. In this case I can not easily check, the mirror mount damage means I can not use the telescope in its current state. Instead I take a gamble that the optics are serviceable and of decent quality, relying on the Cave optical reputation for good optics. I set about dismantling the telescope and assessing the work it will take to put the telescope back into service.

Cleaning the Flash
Filing down the mold flash on the legs of the Cave Astrola mount.

One of the first things that struck me as I disassembled the telescope was the lack of refined fit and finish. There were a lot of bits that could have been done better. This was particularly apparent on the aluminum castings making up the mount. Little effort was put into cleaning up the mold flash. The basic parts and engineering were good, it was just a lack of effort on the part of the assembly crew.

The rusted shafts are a real problem. Substantial filing and sanding of the rust is needed before the various rings and counterweights can be removed from the shafts. A couple parts defy my attempts to remove them from the shafts, notably the RA worm gear. It takes the use of a hydraulic press to force the gear off the shaft and remove the RA shaft from the bearings. Not much a few tons of force cannot solve.

How Faithful?

When restoring a classic bit of equipment you are faced with an interesting choice. How faithful to the original state of the gear do you want to be? Some collectors are purists, requiring that the restored gear be exactly, or as close to the original as possible.

8" Cave Astrola
The 8″ Cave Astrola as it arrived

This question was the source of some thought for me. A few hours of sanding and scraping leave the mind free to wander, time to consider this question. There were a few places where this telescope could be notably improved, and a few items that would be very difficult to restore exactly as they had been originally when the telescope was built.

In the end I decided that it was more important to create a functional telescope than a faithful restoration. I would be true to the original classic appearance of the telescope where possible, but would make an effort to improve its usablity.

Where did I deviate from the original? As you will find further on the new paint colors would be reasonably close, but not exact matches. I did modify the latitude adjustment substantially. Another major change was the replacement of the original focuser with a modern Crayford 2″ focuser.

The few original parts that were removed from the ‘scope, such as the original focuser, finder rings, and the original labels, have been stored away to be kept with the telescope.

Restoring the Mount

I start with the mount, working from the ground up. The aluminum castings, shafts and bearings are all straightforward issues to deal with. Hours spent working on a folding workbench in front of my garage in the warm sun… Sanding, scraping, filing, and painting the various parts.

The fit and finish issues could easily be addressed. As everything needed to be stripped and painted anyway, it was just a matter of time and effort on my part. A trip to my toolbox to retrieve the files and I set to cleaning up the castings properly, filing away the flash and removing the sharp edges.

Painting the Mount
A batch of painted parts sits drying in the tropical sun.

Aluminum castings are fairly easy to restore. Just a matter of sanding the old finish away where it has failed. Cleaning up any issues with a file, then priming and painting. So often the result looks as good or better than the part may have been originally.

For paint I again used the Rust-Oleum hammered finish spray paint that has served quite well on previous restorations. The light blue shade available is comparable to the original blue-grey finish of the telescope, if now much brighter and glossy metallic. I kept the original paint scheme, light blue and black on the parts that were blue-grey and black before.

The rusted shafts were fairly easily dealt with once I had them completely free. Chucked into the big lathe I lapped the steel with emery cloth until the rust was gone and the steel was once again smooth.

The casters on the legs simply needed to be replaced. One just missing, the remaining two rusted and stiff. I looked about for a suitable replacement… Our local Ace hardware has a good selection of casters in a lot of sizes. A little looking revealed a near identical replacement for just under $10 each. The issue is that these, like the originals have no brakes, how do you park the telescope for use?

I looked again at the casters I have in my own stash… I do have a dozen nice big casters unused from assembling my garage shelving, with the wrong stem, threaded not locking ring. I realized a few layers of heatshrink over the threads would give me something that would slide into the holes on the telescope mount and neatly solve the fit issue. The result? Three big casters with brakes to park the telescope for use.

When I first assembled the mount the result was quite pleasing. The new paint, new casters, re-greased bearings, and cleaned up shafts resulted in a beautiful mount with a solid feel and smooth motion. This is what the Cave Astrolas were renowned for, and this one will be again.

The Clock Drive

One attractive feature of the Astrola telescopes was the standard clock drive, not typically available in that era. The drive could be either single axis, or dual axis in some models. This particular scope has a single axis drive. I am happy with that, I understand the Cave dual axis systems were a bit troublesome.

Clock Drive
The clock drive on the 8″ Cave Astrola

One minor miracle is the motor… The old AC synchronous motor works fine. These motors are essentially irreplaceable, long out of production. Without the motor repairing the drive would require some sort of redesign around a modern stepper or servo motor with new electronics.

With a running AC synchronous motor it was fairly easy to restore the clock drive. A good cleaning was necessary, the grease in the clutch had become more like a glue, nothing that soaking in WD-40 for a week would not solve.

The old power switch is wonderfully retro, with a built indicator lamp in the toggle. The old NE-2 neon bulb was long dead. I replaced it with a red LED by taking the switch apart and making the necessary changes.

A new power cord was also needed, no problem there. The new cord is grounded as per modern safety requirements with the ground properly connected to the telescope frame.

The assembly of the worm gear and clutch assembly onto the shaft is a bit perplexing… To align properly with the worm the gear has to be right up against the heads of the screws that secure the drive to the rest of the mount. Unhappy with the fit I ended up modifying the assembly to cut away some aluminum to create some clearance.

20°N Latitude

Using the mount at 20°N is a bit of an issue, this is right at the end of the latitude adjustment range. When adjusted for 20 there is not much clearance for the RA shaft and counterweights.

The latitude adjustment on the Cave mount is also known for slipping, simply held by friction on the large bolt. I have run across information on other restorations that spell out modifications to improve this issue.

Cave Astrola Pier Top
The top of the pier with the latitude adjustment, latitude locking pin, and a bubble level

Examining the mount I decided on a fairly drastic solution… I would reverse the position of the RA shaft, this would give much more clearance and provide a place to install a pin into the latitude adjustment. The price would be shifting the center of gravity a couple inches and placing the clock dive very close to the pier.

I installed a latitude lock pin by drilling a 1/4″ hole through the adjuster and installing a nice 1/4″ ball lock pin. This solidly locks the adjustment allowing no slippage.

Cave Astrola Clock Drive
The restored Cave Astrola clock drive and right ascension circle

The clock drive clearance I dealt with by machining a 1″ extension to the RA shaft housing, mounting the drive an inch further from the pier. This could be done as the RA shaft is almost exactly an inch longer than originally needed. As a result the drive is now well clear of the pier and angled 25° from the original position. It is turned to make the power switch more accessible.

The Optical Tube

The shattered sections of the optical tube looked pretty bad at first. Further examination provided a plan to dealing with the damage. The tube is not cardboard as it may appear, rather it is fiberglass as sold by Parks Optical.

Repairing the OTA
Repairing the damaged fiberglass OTA with epoxy and sanding

Much of the damage was delamination of the layers. This was addressed by loading a syringe and 18 gauge needle with JB Weld and injecting epoxy into the damaged layers. Clamp the area, allow to cure, repeat the process until the damage is stabilized.

I then simply sanded the areas to reshape the tube. A few voids were revealed in sanding down the damage, a little more epoxy in a skim coat and yet more sanding was needed to complete the process.

Next step was to reinforce the tube around the mirror cell and the spider. To this end I bought a fiberglass repair kit and glassed the interior of the tube at both ends. Not only are the repaired sections substantially reinforced, the front end is strengthened as well. The kit provided two square feet of fiberglass cloth, I used it all.

More than a bit of sanding was needed to smooth out the fiberglass reinforcement. When finished it looked pretty good indeed!

A couple coats of white enamel paint had the OTA looking very nice. I filled the old mirror cell mounting holes with epoxy, thus new holes were needed, that would wait until I got the new focuser and checked the focal length to allow adjustment of the primary mirror position.

Focuser and Telrad
The new focuser and Telrad for the Cave Astrola

The last touch to the OTA was a new focuser. The old rack and pinion focuser is a POS by today’s standards. A new 2″ crayford focuser from ScopeStuff will be a substantial upgrade, particularly with a two speed knob.

The new focuser has a 2.8″ to 4.8″ range of travel, while the original Cave focuser is about 3″ to 5.2″. No modification of the primary mirror position in the tube should have been necessary… I checked anyway… Trouble!

It looks like the old focuser reached focus right at the top of travel, perhaps as the primary mirror is a touch over the specified focal length at 48-5/16″. The new focuser is a touch shorter and will not reach!

To adjust I moved the primary mirror position as far down the tube as possible. Not a problem as this is the repaired area of fiberglass and I needed to re-drill the holes anyway. Focus is still right at the top of the new focuser. Next step was to machine a spacer ring for the focuser base to put focus into the middle of travel.

The Final Word

So how well does the scope perform in the end?

There is some wobble in the mount, not bad, but notable when adjusting position. Better than many inexpensive modern mounts, but certainly not up to the standard of my Losmandy G-11. Vibrations do damp out in a second or two.

Some of the old Cave Astrolas were pretty tall, requiring a boost to get to the eyepiece. I find the f/6 has a very comfortable eyepiece height. Add the rotating rings and the eyepiece is always very conveniently placed.

One bit of damage did not get repaired. It looks like the right ascension setting circle got dragged on the ground at some point. The damage is on the etched scale, no easy repair. You can still read the RA at that point, it just looks bad. All I did to the setting circles was to clean and wax them.

The result of this restoration effort is a telescope that is notably better than it was when it was shipped from Cave back in 1978. A few bits of damage left behind, mostly visible on the setting circles.

Author: Andrew

An electrical engineer, amateur astronomer, and diver, living and working on Mauna Kea, Hawai'i.

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