Telescope Mirror Coating

48 Years Coating Telescope Mirrors
Sacramento Valley Astronomical Society, November 18, 2016

John Dobson and Bob Fies pose for a picture sometime before Nov. 2003.
We first started coating mirrors around 1968.

In the beginning all the work was new mirrors.
Now in 2016 most of the work is re-coating.

A bit of telescope history.
Yerkes Observatory classical refractor in use before photography.

$Lick's Great Refractor$

Reflecting Telescope Invented

The Newtonian telescope is a type of reflecting telescope
invented by the British scientist Sir Isaac Newton (1642–1727),
using a concave primary mirror and a flat diagonal secondary mirror.
Newton's first reflecting telescope was completed in 1668
and is the earliest known functional reflecting telescope.

Reflecting Telescopes in the 1880's era

Mirrors were made of Speculum, a metal alloy.
The mirrors had to be 're-figured' every time they became tarnished.
Short focal length telescopes were not used and not needed because
all observing was done visually at the eyepiece.

Late 1800's and early 1900's

Photography is shown to be a practical way to record telescope images.
Long exposure shows galactic detail only barely observed previously.

1910 to 1920

Long focal length telescopes are not optimal for photography but
chromatic aberration of refractors makes large aperture lenses
impractical.

~1920 to present

Coated glass mirrors come into general use.

Why use a mirror instead of a lens, Refractor vs Reflector. A film, glass plate or CCD has no pupil. That has changed the optimal telescope design.
$Refractor ve Reflector$ half down	Although the size and geometry of the telescope may change the size of one's pupil has a limit of about 6 mm, and then only if in darkness.

No sticky fingers on the coating, please.
The coating is right on top and its very extremely thin.
The aluminum is more soft than glass and susceptible to acid and finger oils.

Dust is raining down all the time.
If you wipe the dust off it will damage the coating.

Mirror Cleaning

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Big mirrors seem to have a fatal attraction to kitchen sinks.
Mirrors are much happier on their back on the lawn, just watch out for the reflection of the sun.
Forget about that lens cleaning kit with the little bottles of stuff. That won't do this job.
Other things that won't work, vinegar, silver polish, and no, never use abrasive cleanser.

We will presume this mirror has been in a flood or a tornado and is covered with grit glued down with plant resins.
So water spray alone will take some of the large grit away.

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The significant items in this picture are two pans of almost hot water, dish washing detergent,
paper towels and semiconductor grade methyl alcohol warmed in the sun.
And no slip shoes and be very careful.
Alternatives:
rubbing alcohol ( 70 percent )
Kimwipes
cotton balls
some types of toilet paper
blotter paper

Put a bit of dish washing liquid in one pan and soak the paper towel until soft.
Wipe the mirror lightly, don't press down.
Move to the rinse pan. That was easy, now the hard part.
Carefully remove the mirror from the pan and pour alcohol over it to get rid of the water.
A bit of pre planning helps.
Tip the mirror up to get rid of excess alcohol.
Before the drops of alcohol can dry drag a dry towel sheet across the mirror to spread out the alcohol.
Don't press down while dragging the towel.
Then lay a dry paper towel over the mirror and take a break.
( Taking a break at this point will avoid that drop of sweat on your brow from falling on the clean mirror. )

The mirror's coating becomes much harder after several years making the cleaning process easier.

Is the mirror ready for re-coating?

This mirror looks pretty bad with lots of pin holes.
Although it would still work in the telescope its probably time for a recoat.
Larger bare spots might be due to acid rain that has dried on the mirror.

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How about this one? Can it be cleaned or is it ready for a de-coat?
If you look though the coating at a bright light and it looks like the background below this printing its ready for a re-coat.
But some mirrors that look dull like this can look like new after cleaning.
If the haze is caused by nicotine or plant resins it might clean easily.

Plan B, Mirror cleaning was not successful.
Mirror de-coating.

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Mirror in FeCl solution

There are many etchants which will attack aluminum.
HCl, swimming pool acid,
FeCl, printed circuit board etchant,
phosphoric acid
Lye dissolves Pyrex, good for cleaning stainless vacuum tank though.

The older, oxidized coatings are much more difficult to de-coat than a new coating.
The SiO over-coating slows the process but is otherwise not a big factor in de-coating.
For older coatings the challenge is to find a chemical in which the mirror can be soaked for days without
damage. I have been using Radio Shack printed circuit board etchant the last few years.
Fifty percent dilution of swimming pool acid will remove new coatings.
FeCl leaves some type of residue on the mirror so the ferric chloride etch is followed by 10 percent
swimming pool acid.
Typical times, 36 hours in FeCl, 12 hours in HCl.

Etching of the old coating is preceded by a water wash then paint thinner with lacquer thinner to remove
tape, sealant, spray paint, spider webs, etc. Then methyl alcohol to remove commercial grade solvents.

Re-coating of the Lick Observatory 120 inch telescope:
http://mtham.ucolick.org/alumcam/alumCamMovies.html

De-coating the 200 inch....


It is possible to de-coat these small Cassigrain secondarys without removing them from the aluminum backing plate by suspending them face down in etchant.

New Mirrors-- Do the test with the pen light, you don't want to be surprised.

If this was the persons first mirror and they were working alone I will usually notice a faint haze at this point.
Usually at the mirror edge.
Reflecting a bright light after coating will make scratches or unpolished really stand out after coating,
even if only 1/2 percent of the light is being scattered.

New Mirrors, and Final Cleaning

New mirrors sometime have a great deal of CeO/pitch around the edge.
This will dissolve in about 10 minutes in paint thinner or the edge can be scraped with a blade.
The mirror next goes into a pan of warm water with about 20 percent swimming pool acid for about 10 minutes.
If the mirror is not warm on a cold rainy day it will be impossible to wipe it dry.
Next the mirror is wiped lightly with methanol and Kimwipe then wiped a second or third time then until dry.
Hard experience has taught me to hold my breath or at least not talk during this process.

Preparing the tank is done before final mirror cleaning.

In this picture I am replacing a 30 x 3 tungsten filament.
The baffle box SiO evaporators are seen at the right.

The loops of pure aluminum wire are crimped onto the tungsten filament.
Each loop weighs about 1 gram and I have been using about 6 loops divided between three filaments.
I learned over time to stop the evaporation with Al remaining on the filament.
This reduces the amount of tungsten that ends up on the mirror and also greatly increases the filament life.
If three loops were used instead of six it would be possible to barely see the sun through the coating.

Needles to say the aluminum is very hot in order not only to melt but also evaporate in the vacuum.
The aluminum will not burn when heated because there is no air.
The 'mean free path' of the aluminum atoms must be greater than the distance to the mirror.
Otherwise a 'shock wave' will form between the filament and the mirror.

I dumped the ash out of the SiO baffle box and I'm re installing it.

Loading the Mirror

Now after a check of filament continuity and
huffing the dust the mirror is lowered into the vacuum tank.
There are four small washers at the edge to keep the mirror from dropping through.
The shadowed area is usually mostly on the mirror bevel.

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Before starting the vacuum the mirror rotator is tested.
The traveling hoist is a great help in loading the mirror safely.
The helium from the balloon is used to purge sealant residue and other organics from the diffusion pump
oil later in the process. Vacuum tube manufacturers learned the advantage of a 'slow leak' years ago.
So far as I know there is no oil in any of the pumps used while making computer chips.

Coating, recent picture shows new cold cathode vacuum gage.

Starting the vacuum ( roughing )
Next I close the bleed valve and open the roughing valve.
The surge tank and the roughing pump can be seen in this picture.

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Pump Down Sequence
Rough the tank down to 100 microns. A mercury barometer shows 760 mm at atmospheric pressure.
                                                                 100 microns is 1/10 of one millimeter of mercury.
Air purge twice up to 5 mm to get rid of alcohol, etc.
Heat the tungsten filament enough to drive off finger oil.
Open the surge tank valve.
At about 60 microns turn on the glow supply. This causes a purple glow in the tank caused by ionized air.
Turn on the helium bleed. If your eye won't pass the 447 nm helium line then the glow will gradually
turn to a baby blue color.
Turn off the helium and close the roughing valve.
Open the interconnect valve.
Crack the poppet valve just a bit then turn on the helium again.
Keep the tank and fore pressure below 200 micron by closing the helium bleed if necessary.
Over a period of about 10 to 15 minutes gradually open the poppet valve while keeping the fore pressure
   below 200 micron.
Air and water that are held to the tank by Van der Waals force will gradually escape.
The diffusion pump can pump 250 cubic feet per minute if the fore pressure can be kept below 200 micron.
The diffusion pump tends to surge and there is only 20 cubic foot per minute available from the mechanical
   pumps and so the need for the surge tank.
With the poppet valve full open take a break for about 10 minutes then close the helium bleed and wait
   10 more minutes.
Heat a preliminary filament which does not coat the mirror to deposit raw aluminum in the bottom
   part of the tank. Deposited aluminum metal actually helps pump the tank.
With the mirror rotating evaporate the three Al filaments onto the mirror in sequence.
Evaporate the SiO from the two baffle boxes in sequence.
Turn off the glow supply.
Turn off the diffusion pump heater.
After baffle boxes cool down close the poppet valve and begin the air bleed slowly.

The pressure in the tank must be about 5 x 10^-4 TORR or less before coating.

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This picture shows the backing pump, the diffusion pump and other components shown in the schematic above.

While evaporating the SiO these Newton rings gradually expand in the sequence:
sheen, straw, red, cyan, magenta, cyan, magenta
Then I stop.

The mirror is the same temperature now as it was when it went in.

Next the date stamp and wrapping.
I also do a Scotch tape test on the viewing porthole and check coating thickness and check for cleaning streaks.

When the mirror is unwrapped be sure to double back the masking tape to make sure it won't stick to the
mirror face.

I mark the mirror center with a permanent marker pen.
Permanent markers seem to dissolve instantly in methanol.
Shown below the ruler is a sheet of Kimwipe from the wrapper.
Placing paper on the mirror allows you to steady your hand there while making the mark.

I never did get that last 3/32 inch at the edge when figuring.
So easier to just mask it off.
The plywood circles and the Lazy Susan bearing are from Orchard Supply.
'C' clamp the plywood to a redwood bench when mirror making.

Attachment of the mirror to the mirror cell

The three bolts are glued to the mirror back disk.
The bolts go through springs and the scope bottom
and have wing nuts for adjustment.
When glueing the mirror to the backing plate for the first time
coins should be used to space the mirror off the backing plate.
Doing this serves two purposes,
it allows for later removal of the mirror if necessary
and also isolates the mirror from flexure in the back plate.


I taped my 8 inch mirror in the telescope for testing between polishing sessions. Now its time to remove the foil and install the mirror 'permanently'.

Much easier to keep the dust off than to remove it.

The wrench under the bucket edge is to let the sealant vapor escape.
In warm weather 24 hours will be long enough on an 8 inch mirror.

With all the optics out of the telescope tube this is the best time to clean out the cobwebs, etc.

Are you done yet?

Tube covers are probably enough if you can store the tube horizontal.
If you have to store straight up for a long time you can make a mirror cover from foil.

Large Adjustable Swing Arm Mirror Holder

Shown is the 18 5/8 inch swing arm adapter.
Without the adapter the tank will hold a 20 inch mirror.
Refer to website http://www.alcoat.net for details.

Small swing arm adapter inside large swing arm adapter with plastic 360 degree camera mirrors wired in.

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Lick Observatory, two tours of the new APF, ( Automated Planet Finder Telescope )

	Discovery of extra solar planets using Dopper shift.
University of California, Lick Observatory The new telescope dedicated to extra solar planet search. It is designed to obtain stellar radial velocities and will be able to operate by remote control from UC Berkeley ( or from anywhere ). It is located just south of the 'Twin Astrograph'.

The dome shutters are in two sections and run up and down rather than side to side.
It is possible to view at any altitude with minimum exposure under windy conditions.

The dome, catwalks, equipment and everything else rotate on large rollers around the dome perimeter.
Shown are the ground to dome cable ducts which wind like a flat spring.

In this multi frame image we are riding on the dome.
Below is the concrete base for the telescope.
The base is anchored as if part of the Mountain.

Installation of the APF Telescope, seen from the Main Building

Coating of the APF primary mirror

The polished and figured 94 inch primary mirror.
The glass is of a type that will not expand and contract with temperature.

Preparation of the vacuum tank.
The back of the mirror is convex.

First view of the coated primary.

Automated Planet Finder, ( APF ) tour in Fall, 2009

APF first floor
Concrete pedestal is at left.
The center ring of floor is hanging from the second floor and rotates with the dome.
The stationary outer building wall is at right.

Standing on the outer first floor.
Steps to the second floor hang from the rotating second floor.

Dome drive

Pedestal / Telescope interface

Mirror Cell

Secondary Mirror

Mirror Cover

As yet unprotected edge gap below cover and above mirror. ( 2009 )

The third mirror reflects light through the altitude axis.

Future location for the precision APF Eschelle Spectrograph. ( Spectrograph was at UC Santa Cruz for 'final' testing. )
Eschelle demo

Dome ( second floor ) and telescope rotate in a sort of close but separate dance.

Computers are never wrong, but just in case these limit switches are backup.

The Shane 120 inch Telescope and its mirror coating equipment


Shown is the Shane 120 inch telescope. The adaptive optics sodium laser is the long black box on the lower side of the telescope. The declination bearing is in the foreground.	Picture is from the Lick Observatory website: https://www.ucolick.org/main/science/research/ao.html Website used to help laser spotters watching for airplanes: https://www.flightradar24.com/37.34,-121.67/8

Shown is the mirror cell and adaptive optics assembly which works with the laser to sharpen star images.
The high sensitivity 'low resolution' spectrograph can also be mounted at this Cassigrain focus.
The high resolution spectrograph is at the Coude focus in the basement,
below the end of the 'right ascension' axis.
This assembly just clears the edge of the telescope mirror elevator seen at the lower right.

Below the main deck is the mirror coating vacuum tank.
The 120 inch mirror is re-coated here.
The 30 inch Fremont Peak telescope mirror has also been re-coated here
together with other mirrors.

The tripod for the 120 inch telescope is at left.

The mechanical vacuum pump or 'fore pump'.

Roots 'blower' and diffusion pumps.

Shown is the tunnel where the 120 inch mirror was tested during mirror figuring.
At the other end of the tunnel, two floors below the dome, can be seen the grinding/polishing
machine with the prime focus cage resting on top of it.
The prime focus cage is only needed during collimation.

The Crossley Telescope and the Main Building

Volunteer Appreciation night tour, fall of 2010.
A warm evening with the dome closed.

Outside the Crossley Telescope looking east.

Shown are the new video displays in the Main Building.
Subjects include history, various astronomical topics and ongoing work at Keck and Lick Observatories.
There is also a visitors gallery at the Shane 3 meter telescope with additional movies. Visitors that arrive after
3 or 4 PM often miss that area due to the 5 PM closing time.

Online visitor information is available at:
http://www.ucolick.org/public/index.html

End

This Presentation is posted at:
http://www.alcoat.net/mirror_coating_lecture/mirror_coating_lecture.htm

There is additional information about coating in the Jan/Feb SVAS Newsletter at:
http://www.alcoat.net/1-OBS-2016-Jan-Feb.pdf

If you ever wanted to build your own telescope there is information at: http://www.alcoat.net/CD_info00.htm#alt_11return

http://www.alcoat.net

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