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Mak correctors

Started by yahganlang, 02/20/2006 01:12AM
Posted 02/20/2006 01:12AM Opening Post
Hi. I've read that Mak corrector thickness is the main reason Mak type scopes (of whatever configuration) take so long to equilibrate temperature-wise, and that this can be an issue for the largest ones, especially if temperatures vary significantly during an evening.

I've been looking into making relatively thin, lightweight gradient index optics, and correctors (for Maks as well as others) were one of the earliest astro applications suggested for gradient index techniques (back in the early '70's at least). If successful, thin Mak correctors might go some way towards obviating temperature equilibration problems.

Since I'm new at this, I don't have all the facts (primarily I'm looking at fabrication techniques, and am not that familiar yet with optics particulars).

Are Mak correctors simple one-piece affairs, or are they doublets or triplets to correct for chromatic aberration? Or does the corrector shape eliminate relevant aberration?

Thanks.
Jess Tauber
Posted 02/20/2006 06:20AM #1
I've read that Mak corrector thickness is the main reason Mak type scopes (of whatever configuration) take so long to equilibrate temperature-wise, and that this can be an issue for the largest ones, especially if temperatures vary significantly during an evening.

The corrector, however thick, equilibrates very quickly and is not an issue. It's the stuff in the back of the scope that causes temperature issues. Roland Christen has done a lot of testing and experimentation on Mak temperature management, hopefully he will pop in here.
Posted 02/20/2006 09:04AM #2
Hopefully Roaland will indeed jump in here.

My understanding is that Mak correctors are thick, one piece, and spherical on both surfaces. It also corrects the aberration of a spherical mirror, so the primary mirror is also spherical. I believe that the thickness of the corrector is designed to make this happen. The entirely spherical surfaces are supposed to make them easier to grind and test to close tolerances.

It certainly works well on my little Mak-Newt.
Posted 02/20/2006 09:32AM #3
>>I've read that Mak corrector thickness is the main reason Mak type scopes (of whatever configuration) take so long to equilibrate temperature-wise, >>

Yes, lots of folks seem to believe that and it is total myth. Now that you know this is not true, you can laugh every time you read this kind of statement and dismiss it as pure fiction.

Roland Christen
Posted 02/20/2006 01:16PM #4
>>I've read that Mak corrector thickness is the main reason Mak type scopes (of whatever configuration) take so long to equilibrate temperature-wise>>

Let me put it another way. What you have read is plain wrong. The corrector of a Mak-Cass is not to blame for the thermal problems of some Maks. The reason that you believe this is because you read it somewhere in a post or letter written by another person who read it somewhere, etc. etc.

The letter that you have written here will no doubt be read by another couple of people who will in turn relay that on and on. It will never stop. For every one time that I see something like this written, there will be 1000 posts which will say again that thick Mak correctors are to blame for temperature problems. And the plain fact is that not one of those letters and posts will be based on real information, just hearsay. So, please the next time that you post something about Maks, keep in mind what I have told you, and please mention that the thermal thing of thick Mak correctors is a myth (probably perpretrated by some competing Cassegrains that feel deprived).

Rolando
Posted 02/20/2006 04:03PM | Edited 02/20/2006 06:18PM #5
Stop it Jess. I'm beginning to hear snickering. Do not F--k with Roland!
Posted 05/15/2006 11:40PM | Edited 05/15/2006 11:42PM #6
FWIW, I own Suiter's book, and after studying the star test images I determined that the baffle tube might be causing the aberration I was experiencing. Heat from the LONG baffle tube (my scope is f/14.2) causes a long vertical heat pattern that dances around a bit as it rises to the top of the OTA. Pointing the scope at zenith and letting the heat rise straight up the baffle tube reduced the problem. To prove my theory, I put a 2" eyepiece heater strap around my 2" visual back. That fed heat directly to my baffle tube. The distortion of my star test image came back like gangbusters.

There you have it: the baffle tube has a VERY long cooldown because it is inside the middle of the scope and is always chasing the OTA which cools down the quickest of all because it is thin and directly exposed to air and has a direct radiation path to space at the top (the coolest part of the OTA at night). The slightly warmer baffle tube will always generate a rising heat wave that runs almost the entire length of the OTA as long as any cooling of the OTA exterior tube is taking place due to falling night temps, which almost always happens to be the case.

Case closed as far as I am concerned, and I am back to enjoying my LOMO 150 without needing to see a perfect star test pattern, which is what Suiter tells people to go do if you happen to read that part of his book. The effect is minor after 30 minutes of cooldown on most nights. If you run an eyepiece heater, enough heat will feed through the diagonal and visual back to the baffle tube to keep this phenomenon alive all night long if you go back and check it out. Turn up the heat, and it gets worse.

I once worked in failure analysis in microchips, and finding this phenomenon was a piece of cake for me.

Phil