Re: [ATM] Why not metal mirrors? (was Beryllium)

["Mel Bartels" <mbartels@bbastrodesigns.com>]

...in case I haven't posted this to the atm list in the past - btw, there is 
extensive discussion of aluminum mirrors several times in the archives... 
Mel Bartels


Subj: Tuning a telescope Section: Telescopes/Hardware

To: Mel Bartels, 73422,3270 Tuesday, June 13, 1995 11:04:14 AM

From: THOMAS WAINEO, 70671,3264 #223124

Mel: I have Colen's book. The mirror has never gave a good image due to poor 
figure. I read

that they have placed a second layer of plating. We tried doing second 
layers and never had

success. If anything goes wrong we strip and correct things on the base 
material and replate.

That mirror has a tapered back to make a thin edge. You have a CTE mismatch 
between Al

and Ni. You need to have the edge strong to overcome this mismatch. Al 
mirrors should have

thick edges. Lunar and Planetary Observatory made a comment that they are 
happy with the

images of their 60 inch nirror and are making another. Both are in 1965 Sky 
an& Tel. If you

want the same stiffnes as 6 to 1 in pyrex, you use 10 to 1 diameter 
thickness ratio in Al. For a

10, I would use 1 inch thick 6061-T6 plate. That would make a 980 lb 5 inch 
thick 50 inch

mirror. That much weight may be way too much. A 3 inch thick is 589 lb and a 
2 inch thick 392

lbs. The 2 inch Al would have the stiffness of 3.333 pyrex. You may be happy 
with a pyrex blank

that thick so 2 inch Al would be alright. Anything thicker, you may have to 
hire a rigger everytime

you set up a telescope. That is 25 to 1 or 0.4 inch thick for a 10 inch 
mirror. My feeling is that if

a 1/2 inch plate (4 lbs) works for a 10 inch, then you could go for a 2 inch 
thick 50 incher. The

GOES Next Scan mirror weighs 4 lbs for 20 by 12 by 2 inch thick Beryllium 
plus Ni plate. That

has machined pockets with 1/16 inch walls and 0.1 inch base. The shop made 
one from Al to

check out the programing of the CNC machine. It was Ni plated and polished. 
Al has 1/4 the

stiffness of Be. I was surprised how strong it was when it was polished. 
There was a

company that offered Al blanks for telescope mirrors by claiming that a good 
polish can be

obtained. No one could make it to their claim. I would try to find a copy of 
the book when I am at

the shop. BTW, why angle a diagonal at 45 degrees? You may use a little 
bigger and angle at

30 to get a 60 degree fold. That would be better than two mirrors. Then you 
are angled to look

up and do not need to stand as high on that ladder. - Tom

Subj: celestron or meade? Section: Telescopes/Hardware

To: Mel Bartels, 73422,3270 Tuesday, June 06, 1995 6:04:07 PM

From: THOMAS WAINEO, 70671,3264 #222021

Mel: Use 6061-T6 plate. A 10 to 1 thickness ratio has the stiffness of 6 to 
1 pyrex. Talk

to your local Al supplier, machine shops, plating shops, and heat treating 
shops.

Usuually they know each other. Talk to them about a 50 inch mirror as well 
as a 10 inch.

I want to send you a sample of a electroless nickel plated Al mirror 1 1/2 
diameter by 1/4

inch thick so you can show them what you want done. With the sample, the 
procedure

Applied Optics used for 25 years to stablize Al mirrors, and some 1 micron 
Aluminum

Oxide polishing powder to try. Electroless nickel is used in so many 
industries that you

may have local firms to help you. Can you give me an address? - Tom

Subj: Aluminum Mirrors Section: Telescopes/Hardware

To: Mel Bartels, 73422,3270 Saturday, March 04, 1995 1:04:19 AM

From: THOMAS WAINEO, 70671,3264 #208105

Mel: What you do not want is a temperature gradiant. Metal mirrors are 
designed for

thermal emissivity or absorbtion equally front and back. When there is a 
reflective

coating in front, the back is painted flat black. The back faces the housing 
and air and

gains or loose energy from it. In process for stablization, mirror are 
cycled -100 F and

+200 F with holds at each temp long enough for mirrors to reach those 
temperatures.

With a surface temperature probe, you would know the hold time. A finished 
mirror is

cycled 20 degrees more on high and low then it will ever see in use. The 
should be no

figure change. Military is -60 F and +160 F. With water heating/cooling, you 
would be

doing a overkill. You would find in the cycles how fast the mirrors change 
in

temperatures. GOES NEXT, MIAI tank mirrors, and the Brady Fighting Velicle 
mirrors

have backs painted flat black. The idea is the heat/cool exchange front and 
back are

equal and there is no temperature gradiant. Those mirrors are flats and 
temperature

gradiant causes curvature. You do not have this problem with a focusing 
telescope.

About 20 years ago a firm was making 60 inch Al spherical mirrors with the 
correction for

spherical done by a convex aspheric secondary. I know of another 60 inch in 
Italy. I do

not know of any visual Al scopes. With a high thermal coductivity, you would 
have less a

problem with a metal mirror than glass in regards to unequal temperatures 
through out

the blank. I guess the thing to do is to make 2 ten inch scopes of the same 
quality and

focal length and see if you could tell a differencve in performance. I am 
willing to bet that

you would not. - Tom

Subj: Aluminum Mirrors Section: Telescopes/Hardware

To: Mel Bartels, 73422,3270 Thursday, March 02, 1995 4:18:01 PM

From: THOMAS WAINEO, 70671,3264 #207905

Mel: The metal mirrors I have made are used on a test tunnel optical bench, 
in a vacuum

test chamber and cooled to liquid nitrogen to -270 F, and in space. I do not 
recall any

used in outdoor air except military. The thermal conductivity in BTU/(hr ft 
F) is 99 for AL

and 0.59 for Duran 50 (Scott Glass pyrex eq.). The ratio is 167.8 which 
means that much

faster ratio of heat or cooling. To me, it means that Al will change that 
much faster than

glass. The specific heat in BTU/(lb F) is 0.23 for Al and 0.20 for Duran 50. 
Not much

difference, but it means that they both have 1/5 the thermal mass of water. 
I do not think

you want to use fans, the Al will change so fast that it will go below the 
dew point faster

than glass. You may need the thermal mass to keep it warm and use heaters 
rather

than fans to keep it above the dew point. Does your car get wet from dew 
often in your

area? Since Ni reflects 60%, it would reflect as much as a 41 inch AL + SiO 
coated

mirror. Honest, I do not know why Al mirrors are not considered for 
telescope optics. For

a professional mirror material cost is very minor consideration since the 
rest ot the

scope, shelter, and site are far more. For the amateur the material is 1/3 
or 1/2 the total

cost. - Tom

Subj: Tuning a telescope Section: Telescopes/Hardware

To: Mel Bartels, 73422,3270 Wednesday, June 14, 1995 1:45:10 AM

From: THOMAS WAINEO, 70671,3264 #223218

Mel: Another thing to consider. If 400 lbs bothers you, pockets can be 
milled in the back. As

long as you do not go overboard in lightweighting, you do not loose much 
stiffness. A 400 lb

mirror can be 300 lbs or 75% of solid. So far I have been talking flats. You 
should consider the

volume of the curve. So you would be a little less. How does a 280 lb 50 
inch sound? The flat

should be made from aluminum as well. Make it from 1 1/4 plate. About $180 
for polish and

figure should do it. About 25 years ago, I did a 36 by 2 inch pyrex. It 
bends by waves in a sling

mount. No problem since the mirror was cut up into 8 inch off axis 
sections. - Tom

FROM: TOM WAINEO, 70671,3264

TO: Mel Bartels, 73422,3270

DATE: 6/15/95 6:39 AM

Re: metal mirrors

Mel You can have pockets milled just over floatation triangles so you have

bridges of thick ribs between floatatrion points. It may be best if you have 
a

hole in the mirror so some radial support can be in the hole rather than 
have

the edge do all the mirror radial support. Soft metals do not do well in 
loose

abrasive grinding. Use sandpaper on pitch. Just wet a little with acetone 
and

stick on. The fine sandpaper gives a shine that can be Foucault tested. If

you have a nice spherical base surface, you can polish right after plating.

It polishes so fast that I often figure with sandpaper laps. Great that a

little less diagonal tilt help so much. Do you know anyone that does finite

element moldeling? It takes out a lot of guess work out of mirror structural

design. - Tom

FROM: TOM WAINEO, 70671,3264

TO: Mel Bartels, 73422,3270

DATE: 6/15/95 4:27 PM

Re: metal mirrors

Mel: Most engineering colleges have cources in finite element analysis. If

you can locate a instructor for the course, I am sure he could help. It may

be possible to have a grad student do it for a master's thesis on you 
mirror.

The face plate thickness should be at least 1/10 the maximum pocket gap. A

3/4 base thickness would be 7 1/2 max gap. Better a 6 inch. Now how many

points would that be? The center hole can fit a lawn mower inner tube and 
the

outside sling a adjustable spring to balance the support of center and edge.

You should be spherical to better than 0.0005 overall. A big bar spherometer

can check it. It has to have close to the same reading anywhere you place 
it.

I start with 220 grit sandpaper and finish with 400. Your nickel plating 
would

be 0.005 minimum and 0.007 maximum. On a 10 or 12, I polish that much off in

80 hrs. Know what you are doing and do not play games with it. The plating

polishs with 1 micron at least 3 times faster than glass. You big problem 
with

astigmatism is that they may clamp the mirror too tight doing machining. A 3

ft bar spherometer can detect astigmatism. You never plate if you are not

sure. The shops have big air bearings to rotate a mirror level. Those

indicaters can detect a fringe and plot as you rotate. A 1/10,000 inch

indicater shows 4 waves per tenth and on a 3 ft bar spherometer can assure

you are spherical. All this have been done in the pro shops for many years.

No big deal for them, but for a amateur? - Tom

FROM: TOM WAINEO, 70671,3264

TO: Mel Bartels, 73422,3270

DATE: 6/16/95 7:58 AM

Re: metal mirrors

Mel: Any precision machine shop will do. I have suggested Tom Clark at

Tectron Machining. Just be sure that they have CNC equipment. The 1 micron

alumina is mixed with water and polished with GUGOLZ 64 in a A/C shop. We do

find some astigmatism from all large machined blanks. Most are less than

0.003. You take it out in grinding and finish in polishing. We do try to be

less than 0.0005 overall before plating. - Tom

Mel: A 54 point floatation cell has 18 triangles. 6 covers the inside and 12

outside. A 50 inch F/4 with a 12 inch hole has a 3/4 inch depth which leaves

the center only 1 1/4 thick. A design may work where the center 6 triangles

support a solid area. The outside 18 support a webbed structure equal in

weight per area to each of the 6 inside sectional areas. There are 24 
support

points with ribs connecting to the solid inside area. That sounds simple

enough to make. I see nothing gained by pocketing the inside area and a

floation cell works only when supporting equal weights. - Tom

Mel: When you start a machined Al mirror, you grind all over with 220 grit

sandpaper on pitch to remove the tool marks. You keep changing the sandpaper

when worn out until it is even all over. Then switch to 400 to smoothen and

then 600 to shine it. When you can see your face, then you can foucault

test it. It will scatter lots of light, but you can see the cut off shadows.

Tom Clark has a machine shop big enough to do a 40 inch for Applied Optics.

They have closed, but the key people are now in a new firm called Manasota

Optics. They bought most of Applied Optics equipment, but do not plate at

this time. Vendors are now plating for them. Tom Clark is Tectron telescopes

and the yard scope owner. If the base has a good figure, there will be very

little errors to polish from the plating. The plating is done at about 190

degree F. The mirror is baked at 350 F for 6 hrs to harden the nickel and

strengthen the bond of Al to Ni. We have made mirrors without the bake. I

have not found any difference in hardness. Best that the mirror is 
cycled -100

F to +200 F 5 times as done before plating as well to assure stablility. It 
is

done in computer controled temperature chamber. Nickel can be plated to any

thickness. Most often done 0.005 minimum to 0.007 maximum for mirrors. You

need a plating threaded hole in the side for hanging in a plating tank. We

have made mirrors with 0.001 nickel, but the risk of polish thru is high. I

was suggesting 2 inch thick blank in Al which will have the stiffness of a 3

1/3 inch glass blank. I consider that the absolute minimum. It would be

about the stiffness of a 40 inch of 2 inches thick in glass. If you can do a

thicker blank, so much the better. With a FAX machine, you can get material

quotes from many suppliers. If you FAX a drawing, you got machined prices.

Machine shops can shop around for Al prices as part of the quote to machine 
the

blank. Melissa Evans may ask around at T who does what on the west coast. I

only know this side of USA. Nickel plating costs roughly as much as vacuum

coating. The big advanage is that you can use the mirror as is at 60%

and be absolute curtain the you want to coat to get 88% I am 3 miles

away from Tom Clark's shop. Want me to talk to him for some machinist

feedback infro? - Tom

Mel: Electroless nickel was originally made for corosin resistance

coating inside of huge pipes for oil refinerys. Someone on the west coast 
did

those 60 inch mirrors and the 27 ft mirror that Tinsley made. I read in 
Laser

Focus World that a firm in Redwood City Ca did electrolerss nickel for lamp

reflecters. They article says that they had to be polished to 10 A rms.

Aluminum is made in Washington state usiung cheap electricity from the Grand

Coule Dam. I was thinking of asking Shipley who supplies the plating 
chemicals

who has big tanks for a 50 inch in your area. Have you checked the yellow

pages under plating? You mentioned $7 a pound for Al, I am sure it is less

than half that for big plates. It may come in 4 ft slaps which may limit you

to a 48 inch. You may be surprised to know that I have made 3 or 4 times 
more

metal mirrors than glass. When you need big fast mirrors, it is cheaper in

machined metal than glass by many times. I will stop by and have a chat with

Tom Clark in a week or so. Will let you know what I find out. I know all the

people at Manasota Optics. I can have the 10 inch included in the heat 
treat,

plating, and cycling of other work to save you money. It does not cost them

anything more if there is room for another mirror in the chamber. - Tom

Mel: That is close to $4 a pound and is about right. It makes a 2 inch thick

about $1466. If you use a 54 point floatation, then you can do a 48 by 2 
inch

thick blank. Lightweighting makes the mount more complicated. You have to

balance the weight of each floatation section so all gives the mirror equal

support. Talk to Bill Brady at Capricorn Optics. He has plating tanbks big

enough. Tell him I told you to call. He is north of Boston Ma in Georgetown

Ma 508-352-2699. He is doing big aluminum mirrors over 4 ft diameter. - Tom

FROM: TOM WAINEO, 70671,3264

TO: Mel Bartels, 73422,3270

DATE: 6/19/95 10:01 AM

Re: metal mirror

Mel: Talked to Tom Clark on the phone. He says Al is priced roughly $3.50 a

pound. Stock comes in 2 or 3 inches thick. That means a 3 inch 50 incher

costs about $2000 for 590 lbs and a 2 inch $1400 for 393 lbs. I found a

formula that says the F/4 curve takes out 70 lbs and a 10 inch hole another 
10

lbs which would make the 2 inch blank 313 lbs with hole and 323 without. It

may be best to use a 2 inch solid blank to save on pocketing costs. - Tom

Mel: your best bet is platers near optical industry. The Northeast, around

Tucson Az, and LA anbd SF in Calif. If someone has a ready to go tank, than

it will cost about $1200 or about the price of a vacuum coating. It really

depends on the plating tank economics. The price will have a wide range. The

advanage with nickel plate is that the mirror can be star tested with the 
60%

reflection. - Tom

Dear Mr. Bartels

Try Jim Logston of EMF corp. @ 719-576-7733.

Jim EN plated a 23' diameter aluminium solar simulator mirror for Tinsley

using the mirror as the plating tank. It was an interesting project to say

the least.

Reguards Jim Kennon, Tinsley


Mel: Electroless nickel was originally developed for corrosin resistance in

oil refinery pipes. You would find large tanks in companies who service the

industry. Those actuators work under computer control with feedback from

interferometery. Remember the Sky & Tel article about bending a spherical

mirror by pushing in to the back center while the front edge sits on a O 
ring.

It would work by a vacuum pump as Schmidt did in making correctors. I do 
think

a large sphere can be bent parabolic. I have made toric mirrors by bending 
and

polishing. Ever see how much a big piece of thin window glass bends? I 
wonder

if it is possible to bend from flat to parabolic just by vacuum deformation? 
I

guess it is possible to buy a big thin mirror and bend as a mangin mirror.

Mirrors and windows are made by float and roll and may be flat enough to

make into mirrors. I see it working for one radius and thickness. Any

other radius and/or thickness would be over or undercorrected. You

may need some figuring on the front surface. A 50 inch F/5 has only a 5/8

sagitta. Can that be bent from a 1/4 inch plate mirror without fracture? Is

it worth looking into? If it does work, it may make big scopes

cheap. My bathroom mirror is 4 by 6 ft and 1/4 inch thick. Can that be cut

round for a scope mirror by vacuum deformation? A 2 arc second imaging

mirror has to have slopes no more than one second. That is 1/4 wave per inch

or a wave over 2 inches. I have seen some pretty good plate glass, You could

check it as a flat by pointing a scope at it face up on a O ring. Any 
gravity

sag can be focused out. You would have to be near the normal to not get

astigmatism from curvature. Pretty wild idea? - Tom




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