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Buying a Used Mak

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#1 Peter Natscher

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Posted 12 September 2013 - 09:45 AM

Hi Forum,

I'm considering the purchase of a used 12-year-old medium-aperture Mak-Cass and wondering how I can judge to condition of it's optics? The seller states that it is in "as new" condition. What do I look for in my physical inspection of it? If properly cared for, how long do the reflective surfaces perform to spec? The secondary in this type of Mak-Cass is a reflective spot on the inside of the corrector.

Thanks,
Peter

#2 Eddgie

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Posted 12 September 2013 - 11:00 AM

The question to ask is if there are SiO2 coatings on the mirror.

Silicone Dioxide is used both to improve transmission, and to protect the aluminum from loosing reflectivity.

If it does not have this kind of coating, then at some point in the future you may have a reflectivity issue, but even aluminum can last a long time in a closed tube where it is not usually exposed to weathering. There are 25 year old C8s out there that seem to be working fine.

I would look for SiO2 overcoating on the primary, and for high transmission coatings on the rear of the Meniscus (MgF2, which like SiO2 seals the secondary spot and helps preserve the aluminum finish).

I am sure it will be fine after only 12 years, but in time, it could be an issue and not having these coatings should not appear to be a show stopper if the coatings otherwise look good, but enhanced aluminum is far more durable and has a far better chance of holding reflectivity for a very very very long time.

#3 Peter Natscher

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Posted 12 September 2013 - 05:00 PM

How often have you had to clean the inner optical surfaces inside your C14? I had a Mak-Cass five years ago that acquired a hazy primary mirror surface after five years of regular use. It had rear-side fans that allowed some outside air to get in for cooling. The mirror cleaned easily but I was surprised that it needed cleaning in such a 'short' time frame.

The question to ask is if there are SiO2 coatings on the mirror.

Silicone Dioxide is used both to improve transmission, and to protect the aluminum from loosing reflectivity.

If it does not have this kind of coating, then at some point in the future you may have a reflectivity issue, but even aluminum can last a long time in a closed tube where it is not usually exposed to weathering. There are 25 year old C8s out there that seem to be working fine.

I would look for SiO2 overcoating on the primary, and for high transmission coatings on the rear of the Meniscus (MgF2, which like SiO2 seals the secondary spot and helps preserve the aluminum finish).

I am sure it will be fine after only 12 years, but in time, it could be an issue and not having these coatings should not appear to be a show stopper if the coatings otherwise look good, but enhanced aluminum is far more durable and has a far better chance of holding reflectivity for a very very very long time.



#4 Eddgie

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Posted 12 September 2013 - 05:43 PM

I have yet to have to clean the inside of the corrector or mirror in the C14.

I had to clean the inside of the EdgeHD 8" corrector after a little over one year do to some kind of film that built up. Some kind of outgassing is the theory.

The inside of the C14 though has been fine.

#5 iluxo

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Posted 12 September 2013 - 06:32 PM

Properly cared and/or with little use for the mirror coatings can last 20 years or more. I've had two SCTs longer than that and eventually had one recoated. One of my maks is 14 and while not perfect, I wouldn't recoat it yet.

Their life also depends a lot on other factors:
- whether fan cooling was used (brings contaminants into the OTA),
- the particular coating type, and whether an SiO2 coating was applied.

Simplest is to have a look, if it is in poor shape it will be cloudy (contamination) or show little flecks appearing like grains of sand, which mean its time it was recoated.

Recoating isn't impossible.






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