A broken corrector plate

Occasionally, working in an optics shop, we see sad sights. This is one of them.

A customer was recently setting up their telescope - a beautiful EdgeHD 925 - when it fell off the mount.

In a previous life I was employed to analyse incidents like this and make recommendations as to how they might be avoided in the future. In this case, however, we didn't feel the desire to cross-examine a clearly distressed telescope owner.

We took the scope into the warehouse and examined it. The corrector plate at the front of the scope was completely shattered, and some of the glass had fallen into the tube. The primary mirror didn't look damaged though, and apart from some superficial damage on the edge of the tube's front, the rest of the scope seemed - without examining it at least - to be OK.

The corrector plate in a Cassegrain is not a simple piece of plain glass. It's carefully and subtly made to counteract the spherical aberration arising from the primary mirror.

Because of this interaction between the corrector and the primary, they are matched, albeit coarsely, in the Celestron factory. Batches of mirrors and corrector plates are tested, and from each batch, plates and mirrors are selected to produce the best overall optical results.

This means that if the corrector plate is damaged, and simply replaced, the new combination of plate and mirror won't give the same quality results.

In this case, though, the owner wanted to know if an unmatched pair would produce a scope that would be good - enough - for visual use. The scope was intended for photography, but that is now out of the question.

We contacted Celestron Australia, who in turn consulted the US office.

The news was that they could simply replace the corrector plate, but at a price. Sadly, including freight and dollar conversions, that price was more than that of a used replacement. I contacted Celestron in the US myself, only to receive an unexpectedly blunt reiteration of the same.

This resulted in a dilemma for the owners and after a lot of soul-searching they decided to abandon the scope, keeping, of course, any accessories that might be useful in future.

I have no notion of how involved the job of replacing the corrector plate is. However, it's a sad indictment of our economy that scrapping a beautiful and high-precision instrument is cheaper than rehabilitating it.

By way of an epilogue, the tube and front end of the telescope is now serving in a rather ignominious role as waste paper basket in our showroom.