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Cellular Mirrors--Just a Fad?

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#1 GeneT

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Posted 04 January 2011 - 11:04 PM

Cellular Mirror blanks have been available for several years. Yet few reflector telescope makers offer cellular mirrors as an option for their telescopes. Why? Is the reason cost of materials? Is the reason that grinding and polishing a cellular mirror blank is more labor intensive and time consuming?

What I like about the possibility of a cellular mirror are these factors:
--They would be lighter;
--They would cool down or reach ambient temperature much more quickly;
--Being lighter, it would be far easier to remove them from the telescope for cleaning, adding a new sling, or making other adjustments;
--The telescope could be redesigned to be both lighter and stronger in relation to the mirror;
--Accessories such as telescope drives would pull less weight which should increase their longevity;
Wouldn't cellular mirrors move ahead telescope design, and in turn the whole hobby?
GeneT

#2 mark cowan

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Posted 05 January 2011 - 12:37 AM

Yet few reflector telescope makers offer cellular mirrors as an option for their telescopes. Why?


Expense, mostly. Uhm, that, and expense.

Other than that, there's really no downside with modern available designed (cast) borosilicate blanks. All the advantages you list are the same as for ultrathin mirrors, with one exception:

With cellular, you can have an ultra-fast f/ratio in large blanks without imposing a penalty for the steep curve. Except expense, of course.

Which means (theoretically anyway, I didn't get a solid answer on the weight target I was looking for with a 28" f/2.5) you can field a lighter mirror using cellular as opposed to even an ultrathin monolithic blank. The other option is a slumped meniscus blank, but that brings its own share of difficulties.

I don't see them moving ahead telescope design or the hobby because they're really not going to become common as long they cost what they do, and they don't really lend themselves to mass production due to mold issues. Neither does better telescope design depend on them, though in larger sizes they certainly are a facilitator.

Best,
Mark

#3 Bob S.

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Posted 05 January 2011 - 07:06 AM

Gene and Mark, I have one of the few Stabilite mirrors that were put into Starmasters back in the late 90's. The front and back plates are 1/2" Pyrex. The 18" mirror cools faster than a 1.25" Pyrex mirror in the same type of Starmaster mirror-box. I really love this mirror and scope.

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#4 mrturtle

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Posted 05 January 2011 - 10:46 AM

Gene,

When i was looking into a 22" UC one of the things that really bothered me was the weight. i know a large mirror is going to be heavy, but i just wish it didn't have to be so heavy. i started looking into options and for a while i was going back and forth with dreamcellular about getting a cellular primary for the scope (mostly for the weight, but for the other reasons you mentioned above as well). they were willing to make it, but of course it would cost more. another problem was that ironically the mirror would weigh less, but then the UC would need to be reconfigured to add additional weight for balance issues (and obsession wasn't willing to modify the scope to accomodate a cellular mirror). i also inquired about getting one made. it was mentioned to me that some mirror makers won't touch them since they can be a bit more delicate then a solid mirror.

I'm sure some day they'll be more common place, but right now this is how all 'newer' technologies develop, slowly. and in my opinion, telescopes are a VERY slowly moving technology. i think it's because very few people are into astronomy compared to other techs like computers, cell phones, etc.... which seem to move by leaps and bounds every few years.

#5 Jarad

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Posted 05 January 2011 - 11:44 AM

I used to have one of the few other 18" Stabilite mirrors from SM. It was a nice mirror, and did equilibrate very quickly.

Some other problems in addition to expense: Mine was figured by Zambuto. He told me that it took a LOT more work to figure than a monolithic mirror because of print-through issues. He stopped offering them because he couldn't price them high enough to justify the amount of labor. Apparently this was a specific downside to the Stabilite rib design.

He later worked with some other blank manufacturers on testing a different cellular design (one with lots of posts rather than the long ribs). There was a series of posts on his Yahoo group about the attempt, but he never offered any mirrors using it, so I presume he still wasn't satisfied with the results in terms of figuring labor and/or print-through or annealing issues.

Cary from OWL Optics was offering some cellular designs (quite difference than the Stabilite design - more like a Royce Conical mirror with some of the excess material removed from the back). There is a page describing them on his website: http://www.opticwave...om/cellular.php, but they don't seem to be available for purchase yet (or at least they aren't listed on the price or products pages - maybe he does them on a custom basis).

I think the extra expense is just too much. Zambuto and others have been moving to much thinner monolithic mirrors instead, and that seems to be "good enough" in terms of cooling and weight savings compared to the old standard 1:6 ratio mirrors without increasing the materials cost or making the figuring process too difficult.

Jarad

#6 mark cowan

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Posted 05 January 2011 - 12:08 PM

Some other problems in addition to expense: Mine was figured by Zambuto. He told me that it took a LOT more work to figure than a monolithic mirror because of print-through issues. He stopped offering them because he couldn't price them high enough to justify the amount of labor. Apparently this was a specific downside to the Stabilite rib design.


Indeed. The only manufacturer I'm aware of that addresses this issue is Dream Cellular (if I can mention them under the CN TOS). They incorporate multiple sizes of ribbing into the design, to both support the face during fabrication and use. Mike L. has said he's worked some of these (maybe quite a few?) and been very satisfied with them. Which is good enough for me. ;)

Best,
Mark

#7 Bob S.

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Posted 05 January 2011 - 01:12 PM

Some other problems in addition to expense: Mine was figured by Zambuto. He told me that it took a LOT more work to figure than a monolithic mirror because of print-through issues. He stopped offering them because he couldn't price them high enough to justify the amount of labor. Apparently this was a specific downside to the Stabilite rib design.


Indeed. The only manufacturer I'm aware of that addresses this issue is Dream Cellular (if I can mention them under the CN TOS). They incorporate multiple sizes of ribbing into the design, to both support the face during fabrication and use. Mike L. has said he's worked some of these (maybe quite a few?) and been very satisfied with them. Which is good enough for me. ;)

Best,
Mark


Jarad, Carl Zambuto ended up only doing one Stabilite mirror. John Hall of Pegasus did about 12-15. The figure on my Pegasus/Stabilite is absolutely superb. John Hall reportedly had a number of bummer Stabilite blanks and when the company went out of business, they left John and others holding the bag. Those Dream Cellular mirrors look beautiful and up-to-date. The ribs on my Stabilite were a sub-optimal design compared to what it looks like Dream and Hubble Optics are offering. Bob

#8 mtb54703

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Posted 05 January 2011 - 02:06 PM

I've got one of the Stabilite mirrors as well. I built it into an AstroSystems Telekit last winter.

Bob S - I'm curious as to what type of mirror support system is used on your scope - would love to see more pictures.

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#9 John Kuhl

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Posted 05 January 2011 - 03:08 PM


What type of cell would you use to mount a Dream mirror? On their website the ribbing looks open on the back. It seems like making a cell for that mirror would be difficult.

Best, John

#10 mtb54703

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Posted 05 January 2011 - 03:19 PM

At least the dream cellular has a solid edge - which would probably end up working well with a double cable sling I would suspect.

Currently on my ribbed cellular I'm using a single cable sling place just in front of the center of gravity - so far I haven't been able to detect any induced image defects, but my then again my shortest EP is 8mm making my maximum magnification about 238x.

#11 John Kuhl

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Posted 05 January 2011 - 03:27 PM


Yes, but with the open ribbing on the back the points on the cell is the problem. It isn't like a solid or Stabilite mirror where you have no trouble with the cell points.

Best, John

Best, John

#12 Mike Lockwood

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Posted 05 January 2011 - 03:54 PM

Oh great, there's that f-word again... fad. It's still coloured and colored.

The only manufacturer I'm aware of that addresses this issue is Dream Cellular (if I can mention them under the CN TOS). They incorporate multiple sizes of ribbing into the design, to both support the face during fabrication and use. Mike L. has said he's worked some of these (maybe quite a few?) and been very satisfied with them. Which is good enough for me. ;)


I have a lot of experience with cast cellular blanks. I have worked from 6" to 28", and I have a 50" project coming up this year (not to be confused with the 50" monolith that I am currently figuring).

Unfortunately, I don't think the TOS will allow me to share most of my information here, but it could be discussed on my Yahoo group.

I can say this, since it has already been mentioned. I have worked on or inspected ~10 blanks from the source mentioned by Mark, and I have found them to be of excellent quality. These blanks are actually engineered - designed to support themselves under gravity and designed to support polishing loads. To my knowledge, no one else is doing this. (If they say they are, ask for proof.)

This is the only source I am currently getting cellular blanks from, due to the quality of their products and prompt delivery times.

My policy has always been to only work on CAST cellular blanks (which are one piece of glass), and not FUSED blanks (which are NOT one piece of glass). I simply don't trust fused joints in glass.

Regarding mirror cells for these blanks, there are many possibilities and methods to use, from conventional support to central supports to the more sophisticated methods utilized in some professional instruments. Another solution is simply to have a blank engineered that can be supported by a conventional type of mirror cell. The possibilities are pretty much endless because these are engineered structures, not simple plates of material.

#13 mark cowan

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Posted 05 January 2011 - 04:08 PM

On their website the ribbing looks open on the back. It seems like making a cell for that mirror would be difficult.


As Mike indicates these are designed to order - they will make them with support points integrated for the support cell you're going to use. That becomes part of the FEM design iteration as well. The same goes for the side support intended to be used, reinforcing can be designed into the blank where it's needed.

Best,
Mark

#14 GeneT

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Posted 05 January 2011 - 05:11 PM

Gene and Mark, I have one of the few Stabilite mirrors that were put into Starmasters back in the late 90's. The front and back plates are 1/2" Pyrex. The 18" mirror cools faster than a 1.25" Pyrex mirror in the same type of Starmaster mirror-box. I really love this mirror and scope.


Looks like a great mirror and telescope.

#15 GeneT

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Posted 05 January 2011 - 05:16 PM

Oh great, there's that f-word again... fad. It's still coloured and colored.


Those darned headline writers always use hyperbola just to get you to read the story. However, the upside is that some good information is being posted on this thread. I have also received some private e-mails that have greatly enlightened me on Cellular mirrors.

#16 Bob S.

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Posted 05 January 2011 - 08:16 PM

"My policy has always been to only work on CAST cellular blanks (which are one piece of glass), and not FUSED blanks (which are NOT one piece of glass). I simply don't trust fused joints in glass." Mike Lockwood

Mike, I have not heard of a Stabilite fused blank failing in the field. I had heard rumors that they could fail at the time of manufacture/figuring. My Stabilite appears to be fully intact and gives beautiful pinpoint views. We compared the cooldown time against a 16.5" 1.25" thick mirror that you made for a buddy's Starmaster and the cellular mirror produced equilibrated images a bit faster than the slightly smaller but monolithic mirror. However, that Lockwood mirror in the 16.5" simply rocks and beats the pants off of any conventional similar sized scope on the field. I think the fact that the Stabilite is a very open design meant that there may be less boundary layer turbulence produced from a faster equilibrating mirror? Bob

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#17 Jarad

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Posted 05 January 2011 - 09:37 PM

Jarad, Carl Zambuto ended up only doing one Stabilite mirror. John Hall of Pegasus did about 12-15. The figure on my Pegasus/Stabilite is absolutely superb.

That's the one I had. Like I said, it was an excellent mirror, and it cooled down very fast (I added a fan that blew into the central hole in the back for even faster cooldown). But he charged and extra $1000 for that mirror, and said that he couldn't afford to make more without increasing that upcharge significantly due to the figuring effort needed.

Those Dream Cellular mirrors look beautiful and up-to-date. The ribs on my Stabilite were a sub-optimal design compared to what it looks like Dream and Hubble Optics are offering. Bob



I have no doubt that these blanks will do what they say - reduce weight and speed cooldown. The question is how much more do they cost, and how many people will be willing to pay it. I am a fan of thin and/or cellular mirrors - I bought that 18" Stabilite, and I also have a 14.7" quartz mirror that is only 0.75" thick from Mark Cowan. Both give fast cooldown and light weight. Both cost more than a standard thickness mirror, but the thin quartz had a smaller upcharge compared to pyrex than the Stabilite did.

For mass market, Zambuto has moved to thinner substrates (his 14.5" mirrors are 1-3/8" now, down from 1.6" a few years ago). The larger mirrors have dropped from 2" to 1.6". Several others have followed suit, because it doesn't really add any cost or labor.

I am sure there will be a small market for the cellular mirrors, particularly in the larger sizes where cooldown and weight become more critical, but I think the extra cost will keep that market small, and is why it hasn't caught on more widely so far.

Jarad

#18 gatorengineer

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Posted 05 January 2011 - 10:08 PM

As far as I know only one vendor makes them currently, so I would call it a botique and not a fad and what a high end boutique it is. It's also a boutique that I would love to be able to shop in. The whole 20 inch scope I am trying to sell wouldn't buy a 14" finished dream cellular mirror. But you could have a sub 20 lb finished OTA.

#19 mark cowan

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Posted 05 January 2011 - 10:36 PM

and I also have a 14.7" quartz mirror that is only 0.75" thick from Mark Cowan. Both give fast cooldown and light weight. Both cost more than a standard thickness mirror, but the thin quartz had a smaller upcharge compared to pyrex than the Stabilite did.


And I'm glad he does! But the thin quartz is tricky to figure - Jarad can attest to the time it took to deliver that particular mirror. A potential advantage to going fused cellular is the reliability of simpler figuring and testing regimes.

Best,
Mark

#20 John Kuhl

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Posted 06 January 2011 - 04:35 PM


Mark, if you had a choice to go with the thin quartz or a cellular for your scope which would it be and why?

Best, John

#21 Mel M

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Posted 06 January 2011 - 04:58 PM

Mark, if you had a choice to go with the thin quartz or a cellular for your scope which would it be and why?

Best, John


I'm also interested in this answer for an 18" or 20" mirror; without considering cost differences.

#22 Mirzam

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Posted 06 January 2011 - 06:40 PM

I have one of Mark's 14-inch quartz mirrors and can attest to its excellent performance and practicality. I don't know if the quartz is readily available in larger sizes and would be interested to learn this.

According to the Optic Wave Labs webpage they offer an 18-inch cellular blank that is designed to be its own mirror cell. That looks interesting too!

http://www.opticwave...om/cellular.php

JimC

#23 Jarad

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Posted 06 January 2011 - 06:59 PM

Since I have had both, here is my take:
If your main goal is weight reduction, go with thin quartz. The single 0.75" thick plate is lighter than a cellular with a 0.5" front plate plus ribs plus a 0.5" back plate.

If your main goal is thermal equilibrium, go with a cellular - the front plate can be thinner than a thin quartz since it has the ribs to add structural strength, so it should cool even faster. This will favor cellular ones more as size increases, since you probably can't keep even a thin quartz mirror below 1" thick as you go above 20" diameter, but you can keep the front face of a cellular thin by properly designing the ribbing.

That's without considering price. But price and availability will be big issues...

Jarad

#24 jayscheuerle

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Posted 07 January 2011 - 09:40 AM

I know Norman Fullum has made a 36" cellular mirror and offers them up to 50" on his site.

I believe he's also responsible for the mirrors Orion is offering for the "Monster Dobs" (haven't heard of one of these being sold yet) - j

#25 Peter Natscher

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Posted 07 January 2011 - 08:23 PM

Some other problems in addition to expense: Mine was figured by Zambuto. He told me that it took a LOT more work to figure than a monolithic mirror because of print-through issues. He stopped offering them because he couldn't price them high enough to justify the amount of labor. Apparently this was a specific downside to the Stabilite rib design.


Indeed. The only manufacturer I'm aware of that addresses this issue is Dream Cellular (if I can mention them under the CN TOS). They incorporate multiple sizes of ribbing into the design, to both support the face during fabrication and use. Mike L. has said he's worked some of these (maybe quite a few?) and been very satisfied with them. Which is good enough for me. ;)

Best,
Mark


Dream Cellular blanks are 50% more expensive than a matching monolithic blank. There are a limited number of mirror size choices for the Dream Cellular blanks and not all of them have cells available. It's a new technology. IIRC, Mike Lockwood told me that there isn't much difference in weight between a monolithic blank and similar aperture blank from Dream Cellular. The total mass per aperture is similar between the two. There's just more surface area in the Dream Cellular blank to release heat more quickly.


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