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[Chicago Astronomer - Astro Joe]

Cracked C14 Corrector Plate - Not mine!!!

users.wirelessbeehive.com/~paw/temp/crack.jpg

No, not mine, but on a Yahoo SCT group, some poor soul had his C14 knocked over by a gust of wind...with severe cracks. Still usable, but I would imagine radial distortions on one side of the object.

Apart from the fact the the CP is filthy, it's not the end of his observations with the C14.

Must have been a hellava gust....I say human error.

[neilo]

Ouch!!

[Chicago Astronomer - Astro Joe]

Some of the more interesting replies:

Why not just use some black paper and cover the cracks with pie slices? There will be some spikes on the brighter stars but otherwise the OTA ought to be still quite usable. The crazy glue may or may not be required, and if under the black paper then it should not bother anybody. Alternatively, after the crazy glue one could use a thick black marker pen to cut down on light scatter, instead of pie slices of black paper.

I remembered that when someone fired several bullets into the primary of one of McDonald Observatory's scopes they just painted the divots caused by bullets with black paint.

So in addition to suggesting the crazy glue I added that black paint along the cracks might be a good idea.

come on guys...
that scope is toast! if he was going to implement the paper trick, it's
pointless, as the overall shape of the corrector
has changed dramatically where it's not broken due to stress release inside the
glass.
try drilling a hole into a perfect primary mirror to make a cassegrain. The
shape is shot to hell and the mirror has to be figured again.
it's optics, nanometers matters.

What moron would shoot into a observatory?

Years ago, while operating the Doane Observatory at the Adler, some of the metal on top, near the slit, was hanging loose and binded when I attempted to close the shutter. Going topside and climbing to the damage, it looked to me like someone shot at the Doane from a boat out on the lake....



Clearly, an impact hole is shown.

[RobbW]

Joe, I have to admit that I nearly had a stroke when I first saw that photo. Then I scrolled and saw that it was a C14 that wasn't yours. Huge sigh of relief!!! Sucks for the poor guy, though.

[Chicago Astronomer - Astro Joe]

Another interesting post about Corrector Plates:

This topic is discussed in depth in Robert Piekiel's great book on "Testing and Evaluating the Optics of SCT's" ( [e-mail]). He explains that there is no residual stress in the corrector plate after the fabrication is complete and the vacuum is released, allowing the corrector to spring back to an unstressed state with the correct curve now ground into it. When a corrector is cracked, sometimes the crack propagates without longitudinally displacing the two sections of glass on either side of the crack. This is most likely for radial cracks. In this case, there may be almost no effect on the optical performance except a slight added diffraction effect from the crack itself. In other cases, the glass on either side of the crack may be physically displaced or tilted, which will effectively ruin the optical performance. This is more likely for cracks along a chord section of the corrector. So it may be largely a matt er of luck whether the harm is minor or terminal.

Actually replacing the corrector is a different matter. The secondaries of SCT's are individually hand matched for an individual optics set by testing the entire set on an optical bench and applying very small asymmetrical corrections to the secondary with a petal lap. Difficult and delicate process requiring a trained technician. This is why the correctors and secondaries of a disassembled SCT have to be reinstalled in exactly the same relative orientation. Buying and installing a new corrector plate is unlikely to give good results. The optic set has to be redone and rematched, which is why the cost is very high. Robert gives a very good discussion of this in the book, along with some suggestions about possible mitigating measures. The book is worthwhile for anyone with an interest in SCT's.

[Chicago Astronomer - Astro Joe]

Robb,

From what I have been reading, The CP and optics are a matched set. Not just a simple swap out for a new one.

When I was T-boned last June, I had the C11 in the back seat...and feared it suffered damage...including a possible cracked CP. Hell...even the objectives from my binoculars popped out from the impact!

But it survived nicely, protected by the heavy metal of the Chevelle...which will be worked on starting in April.... ;D