When to recoat an old C8?
Posted 08 December 2012 - 07:16 PM
My C8 is about 34 years old now, and I've been wondering if there has been significant decline in its coatings. They were standard coatings to begin with, and I don't see any obvious signs of change, but that's just with my old eyeballs.
What are the guidelines for this? And, how successful is recoating these old classics?
Thanks for your thoughts and experiences!
Posted 09 December 2012 - 10:34 AM
Posted 09 December 2012 - 12:06 PM
Posted 09 December 2012 - 12:20 PM
Worry about it IF, and I mean IF, you notice any degradation.
Posted 09 December 2012 - 12:40 PM
Posted 09 December 2012 - 03:27 PM
Posted 09 December 2012 - 08:17 PM
It would be a small job to remove and recoat. It would be a bigger deal to recollimate the primary, as it's glued in place and has to be reglued and aligned. If the corrector plate is not coated, they could put an anti-reflection coating on it and that would get you another 6% or so from that. You'd want to think through how to align the primary. I saw a translucent target with concentric rings which was clever that someone used once to re-glue and collimate a primary mirror.
Posted 10 December 2012 - 03:36 PM
Guys the fact is those coatings are way old and don't have the reflectivity they once had. Too the coatings out today are far superior than standard mfg. The XLT coatings on my celestron have the secondary looking like its levitating without glass support. You guys really ought to consider it. Pete
Sure, modern coatings have higher through put, but recoating an old C8 to even Starbright® levels makes little economic sense. The total transmission of a system is generally calculated by multiplying the reflectance or transmission percentages for each air-to-glass surface. If we assume the standard value of 96% transmission per surface for uncoated glass, and 87% reflectance for standard aluminum coatings, the total transmission for these early telescopes would have been - 0.96 x 0.96 x 0.87 x 0.87 = 0.697 or 69.7%. So 30% of the light is lost, without considering degradation of the optics or the star diagonal and eyepiece.
Celestron announced Starbright coatings on the 1982 Super C8. Starbright coatings included both enhanced aluminum mirrors and Special Coatings® (MgFl) on correctors. Starbright has a theoretical through put of 82% and XLT of 83.8. This is about a 15-16% improvement over the original C8's. I have compared side by side uncoated C8s with Starbright and XLT. The difference in brightness is noticeable with Starbright. There is not any noticeable difference between Starbright and XLT, other than color. The XLT corrector has no iron in the glass so more green light passes. This gives object a yellower cast than usually is seen. Now that digital cameras have a histogram display I'll have to try these comparisions again and see if the camera can record a difference. It is worth noting that at the time that XLT coatings were introduced Celestron claimed that the actual Starbright coated C8 had an average (of all visable wavelengths) through put of only 72%! Guess they wanted XLT to look worth the higher price.
At least as far as amateur SCTs go, there is no truth to the statement that "the coatings out today are far superior than standard mfg." Celestron's 1982 Starbright coating's 82% through put has only been bettered by a couple percent in the last 30 years. Vixen's dielectric mirror coatings on their modified Cassegrains excepted, coating technology has not advanced that much.
Recoating your C8's mirrors to enhanced coatings and MgFl on the corrector's surfaces is going to run at least a couple hundred and then you have recollimate the primary. For that money you can pick up a later XLT C8 or just buy a 99% diagonal.
Posted 10 December 2012 - 07:04 PM
I do want a c-pacific c8 though. I'd baby it, shine it up and make it new.
Posted 10 December 2012 - 08:26 PM
I have a 1975 C8 and did have it recoated by Celestron several years back from original aluminum and bare corrector (non "special coating") due to some oxidation creeping in form edge of primary in a couple of spots. Celestron put Starbight on the original mirrors (after stripping) and put the AR on the corrector. I did not notice much difference in deep sky or limiting magnitude. Contrast visually is what matters most. For Lunar and planetary, I do see favorable increase in contrast, mainly not having the "ring" of light around a bright planet that an uncoated Schmidt corrector shows, but the view is fairly yellow. This is with the original plate glass corrector (same serial number). I liked the more bluish cast of the original coatings better, especially for the Moon. So if you don't have areas of oxidation visible I'd think the scope is good as is.
I also had new motors made back in 2002 from Hansen so I've got a like new 1975 orange tube at this point. But there's a lot more $ tied up in this scope than needed. But it was my first decent scope when I bought it used in 1990.
Posted 11 December 2012 - 02:59 AM
Posted 11 December 2012 - 09:51 AM
Posted 11 December 2012 - 09:57 AM
Pete what is your source of data for the decline of reflectivity? how did they test it?
Guys the fact is those coatings are way old and don't have the reflectivity they once had. Too the coatings out today are far superior than standard mfg. if your waiting for the coatings to peel before you get it done your really hobbling your scope. The XLT coatings on my celestron have the secondary looking like its levitating without glass support. You guys really ought to consider it.
I'd like to check it out. I have heard people "say" there is loss of reflectivity, but no one has ever pointed to a source?
I have heard everybody knows they loose light, but when I have asked for their source it seems to point to hear say.
My mid 70's C8 still shows the same objects as it did decades ago. I would think faint fuzzies would have disappeared over time.
I know salty air will effect coatings, but I'd love to see some written data on the subject. If anybody can point my way I'd be most gratefull.
Posted 11 December 2012 - 11:25 AM
"If the reflective coating of a mirror goes bad, it will start turning gray or cloudy. It may appear mottled in spots, and if you shine a flashlight through such a mirror, [from the back] you will often see light coming through the mirror..."
Bob warns that when you are at this stage it is time to recoat as the oxidized aluminum can attack the glass surface, but also notes he's had several 40 year old Celestrons with mirrors as good as new.
Obviously, your mileage will vary depending on your use and the environment you live in. I had a '90s C11 go through here with a degraded corrector. The owner had left it on is balconey in Chicago for most of its life and the coatings were pitted.
I think most of the talk about the loss of reflectance arises from the day when protective overcoats were not applied. At that time ten years would be good performance for a coating, particularly in the common Newt of the day that was open to the environment. Probably wouldn't get ten years today with all the acid in the air. It takes paint off cars here. Large observatory mirrors are not overcoated and are routinely stripped and recoated. The 100" Discovery Telescope's primary (4" thick) is designed to be demounted and recoated at the observatory in its own vacuum chamber. I think the schedule is 5 years.