Getting a Mirror Re-Coated

The classic Cave Astrola telescope has become my roll-out, quick observing session ‘scope, often found in my driveway. I have also used it a few times at darker sites when I expect the weather to be damp or dewy as a Newtonian is more protected.

The restored 8" f/6 Cave Astrola under a dark sky at Kaʻohe
The restored 8″ f/6 Cave Astrola under a dark sky at Kaʻohe

While the restoration job was finished some time ago, I never got around to re-coating the optics. Meanwhile the telescope has seen good service on many occasions as I enjoyed this fun-to-use instrument.

The optics did need some attention… The primary mirror from the Astrola appeared to have not been re-coated since it was made in 1978. Thus the aluminum coating was over 40 years old. While the coating looked bad, it was still serviceable, producing reasonable images.

Still, the loss of light due to the old aluminum coating was probably reducing the effectiveness of this 8″ telescope to something more like a 6″ telescope. I had meant to get it re-coated some time ago, but we know how these things work.

Thus I finally decided to yank the optics out of the telescope, pack them up, and send them off to a coating house. To get the job done I chose to use the services of Paul Zacharias of Spectrum Coatings, the same outfit that coated my 18″ mirror well over a decade ago. His prices are reasonable and I know his quality is good.

The 8" Cave Astrola primary mirror and secondary packed for shipment to the coating house.
The 8″ Cave Astrola primary mirror and secondary packed for shipment to the coating house.

Packing the mirror is fairly simple, just a few layers of foam in a box, the central layer having a circular cutout for the mirror to sit in. The important bit is that there is little to no pressure on the optical front surface of the mirror.

The secondary is slipped into a small pocket cut into the foam in one corner of the box.

A USPS large Priority Flat Rate box just the right size for an 8″ mirror, even a 10″ may fit. USPS is also the most cost effective shipping out of the islands, as even UPS ground was looking to be more than $100 to ship. Priority flat rate is less than $20 with insurance and tracking and generally quite reliable.

Now I just have to wait. Paul may offer good prices, but he quotes a turn around time of about a month. it will take a while, I will even miss a full Moon, though there are a few other telescopes in the garage, no need to suffer photon deprivation.

It indeed took over well over a month for the mirrors to arrive. No reason to worry, Paul was upfront with the expected time and readily answers emails from impatient amateur astronomers like me. Finally the email that the mirrors were on the way appeared.

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

First surprise? The mirror is marked EAL, I got the enhanced aluminum coating for the price of regular. My guess, it went into a batch with other mirrors getting the enhanced coating.

Putting the optics back into a Newtonian is always a bit of an exercise. The actual re-installation takes only a few minutes, then there is the hour of more of collimation, getting the optical alignments right.

Collimation is aided by the neat reference mark added to the center of the mirror. The mark is an extra $10 charge, but one well worth it. A first shot with the laser, then a bit more using the cheshire alignment tool. A final tweak is needed using a star before I lock down the adjustments.

Done! This classic old telescope is back in operation, with some care and new parts in restoration, plus a fresh enhanced aluminum coating, the ‘scope is in better shape than when it was new. Not bad for a forty year old telescope.

Now if the clouds would just go away. The forecast for this coming new Moon weekend sucks. The club star party is scheduled and the permits are on-hand. Does the new telescope curse apply to re-coated optics?

Author: Andrew

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

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