60-inch Aluminizing - Wrap Up

Here is a photo from earlier this week as the aluminizing chamber was opened up to reveal the newly re-coated 60-inch mirror.


If you look carefully just to the upper left of the mirror you can see that there was something taped to the ring that surrounds the glass. That is one of the four "witness slides" that were placed inside the tank. Since everything on the inside of the chamber gets a coating of aluminum the slides can be used to study the aluminum that was deposited.

Here is a closeup of one of the witness slides where it was aluminized:


And one, with the tape removed:


With the sample and the tape removed it is possible to measure the thickness of the aluminum coating, which is useful to know to try and improve the process for future coatings.

The tape that held the witness slides in place was clear. Like the slides, the mirror and everything inside, the tape was also coated with a thin layer of aluminum.


If everything on the inside of the chamber was coated with aluminum, why isn't everything nice and shiny? It is only the materials that are very smooth, like glass, that give a good reflective coating.

Finally, to close out my coverage of re-coating the 60-inch mirror, here it is as it was being returned to the telescope:

60-inch Aluminizing - Before and After

Aluminizing the 60-inch mirror has been completed. I have a lot of shots that I could post, but first I wanted to show off the before and after:

I think that most people would agree that the Palomar crew does pretty good work.

A Dirty Mirror

Work is progressing on this week's re-aluminumizing of the primary mirror for the Palomar Observatory's 60-inch telescope. Yesterday the mirror was removed from the telescope and today it was transported to the dome of the 200-inch telescope where it will be washed stripped and re-coated. Work on that is taking place as I type.

I have lots of pictures to go through and more to take, but here is a shot of the dirty mirror as it looked just before lunch today.


If you click to enlarge the image you can see various water spots (likely from dripping condensation that dripped off of the dome on to the glass sometime in the last 2 years) on the surface of the glass.

I Melt With You

It is time once again to re-coat the primary mirror of Palomar's 60-inch telescope. This was last done in August 2008. For some pics and commentary on when when this was last done see 60-inch Aluminizing Part I, Part II, Part III, and Part IV from this blog.

The mirror does not get pulled from the 60-inch telescope until Monday, but the process of getting ready for the job began quite a while ago. I thought I would show one piece of that process that I have not documented before. One of the key elements in re-coating a telescope mirror is the preparation of the filaments that go into the aluminizing chamber.

The big task is to melt some of these:

onto thirty two of these:

Specifically 0.165 grams of aluminum (about 3 of the pieces shown above) gets melted on each of the 32 tungsten wire coils. Those coils get placed into the aluminumizing chamber where a total of 5.28 grams of aluminum gets vaporized to provide a thin, even coating of the top surface of the mirror.

To get the wire coils prepared the aluminum pieces are draped on the coils which are then attached to this device which holds just 6 coils:


A glass bell jar with a protective wire cage is lowered over the device and the bell jar is pumped down to a vacuum.


Once a vacuum has been achieved electric current is individually applied to each of the coils in turn.


This is actually done twice. Once to melt the three staple-like pieces of aluminum to the coil. The second application actually causes the melted aluminum to wick out across the wire filament which will later help to provide a more even coating to the glass surface.

The prepared coils are later loaded into the aluminumizing chamber. Next week, once the 60-inch mirror is washed, stripped, and fully clean and dry it will join them as the new coating is applied.

Video Tour pt. 2

Here is part 2 of my tour with Thunderf00t. This one covers aluminizing the 200-inch mirror. You can watch it here or embedded below.



You might also enjoy his video visit to Mt. Wilson Observatory.

200-inch Aluminizing - more photos

Here is a batch of photos from last week's aluminizing of the 200-inch mirror. BIG THANKS to Susan V for capturing these shots while I was away at Corning. She did a super job!

Remember to click on the images for more aluminizing goodness.

The Palomar day crew working under and inside the 200-inch mirror cell prior to the mirror's removal from the telescope.

The 200-inch mirror removed from the telescope, sitting on the aluminizing cart.

That's one dirty mirror.

Tools of the trade for washing telescope mirrors.

Washing the 200-inch mirror.

"You missed a spot."

"Green River" is applied to the surface of the mirror to strip away the thin coating of aluminum. This reveals the mirror's honeycombed backside. Notice that some aluminum still remained on the right side of the disc when this was taken.

The final aluminum is stripped from the mirror.

The mirror is covered overnight to protect the clean surface.

The 17.5-ton bell jar is lifted as the mirror gets a final air cleaning to remove any dust from the surface of the glass.

One of the three glow discharge events prior to aluminizing the mirror.

Two of the 220 coils fire to deposit the thin layer of aluminum to the surface of the mirror.


If you haven't yet, be sure to check the post below for the "official" before and after shots of the mirror.

200-inch Aluminizing - first photos

I have so many photos of the aluminizing I don't know where to start. Here are the dramatic before and after shots of the mirror. As you can see the crew here has done a fantastic job.

Aluminizing Update # 7

I haven't had time for updates lately, but this will catch everyone up.

Late Sunday afternoon the new layer of aluminum was deposited on the mirror. Monday morning atmosphere was slowly re-admitted to the tank and the mirror was examined late Monday morning. All looks very good. The mirror is being returned to the telescope as I type this.

Many photos to follow later in the week.

Aluminizing Update # 6

The crew has finished cleaning the mirror. Just a short while ago they picked up the 17.5-ton top to the bell jar (above) and carefully lowered it around the 200-inch mirror. The mirror is safely inside and the rough pumping has begun.

All of the air must be drawn out in order to put down the new layer of aluminium on the mirror's surface. It typically takes a day or so to achieve the proper vacuum.

Aluminizing Update # 5

Today the mirror was washed and stripped of aluminum. Starting at 7 am PST on Saturday the crew will give the mirror a final alcohol wash and when they are ready they will lift the top to the vacuum chamber over the mirror tomorrow morning and start pumping down to vacuum.

There will be lots of photos to share here on the blog next week of both the aluminizing and of the amazing trip that I have had to the birthplace of the mirror - Corning, NY. Stay tuned.

Aluminizing Update #4

I am still away and haven 't had any reports come my way, but in the picture below (click to enlarge) you can see the crew giving a final push to put the mirror cart intoplace for washing and stripping the mirror.

Aluminizing Update #3

Day 1 finished with the mirror cart being pushed under the telescope. The "knees" were removed and the cart raised to catch the mirror. You can see a really quick movie here. Don't worry, I'll have a much better movie of the activities sometime next week.


The photo shows the mirror as it is now - just lowered from the telescope. That's how it is spending the night. Expect the Palomar Day Crew to push the mirror cart and its precious cargo back to the aluminizing station sometime after 7 am PST on Thursday.

After than the washing and stripping of the old coating will begin.

Aluminizing Update #2

The Cassegrain cage has been removed.

200" Aluminizing Update #1

I am unfortunately travelling today and will have to update the blog from afar. Work on removing the 200-inch mirror is underway. It is a very involved process. First the science instrument is removed from the Cassegrain cage. Then (seemingly) miles of wire and fiber optics must be carefully removed as well. The telescope is fitted with earthquake tie downs, computer racks are removed and then, eventually, the cage is lowered.

At last check of the webcam, the cage is still attached. There is still work to go before they are ready to lower it.
Here's an interesting shot from today's activities. I explained how such images are produced in an earlier post. Two short webcam movies, hosted at HPWREN, of some of this morning's activities are available here and here.

Update: They've opened the dome.

That's something you don't often see in the daytime, but it provides plenty of light & and some much needed warmth for the crew. Don't worry - I expect they will close the dome before the mirror comes out.

Hale Telescope Aluminizing Preview

Here's another YouTube preview of aluminizing the 200-inch mirror.



Remember the action begins on Wednesday which may be the best day to watch via the webcam.

This Week: Journey to Palomar Finally on TV & 200-inch Aluminizing

The Journey to Palomar will finally be showing on PBS stations starting tonight. Sorry, if I have been promoting it too much in the blog.

On Wednesday of this week we will begin re-aluminizing the Hale Telescope's 200-inch mirror. Here's a time-lapse video preview from June 2005.

Prime Focus Instrument Change

Here's a movie of a recent instrument change at Prime Focus. A high-resolution version (131 mb) is available here. That's the Hale Telescope's Large Format Camera heading down and the Wide-field Infrared Camera going up. Instrument changes such as this one are pretty common, happening several times each month. What is uncommon is re-aluminizing the 200-inch primary mirror. We only do that every 2 years or so.

On Wednesday, November 12 be sure to tune in to the Hale Telescope's web cam to catch all the action. The 14.5-ton mirror will be pulled from the telescope that day.

Re-aluminizing Again

If you've been following the blog, you've seen the results of re-aluminizing before when the Palomar crew re-coated the 60-inch mirror back in August. They just completed re-coating the 72-inch mirror for the 48-inch Samuel Oschin Telescope.

Because of the design of a Schmidt Telescope (also called a Schmidt Camera) it has a mirror that is larger than the glass corrector plate at the front. And because the size of a telescope is determined by the opening that lets in the light, this telescope is called a 48-inch telescope even though it has a larger mirror.

All that aside, I give you another beautiful example of before and after photos of a telescope mirror. The crew does good work, don't they?

Practice Does Make Perfect

Yesterday we finally had the chance to open up the bell jar/aluminizing chamber to have a look at how the practice aluminizing went.



Once the top of the chamber was safely removed and placed on the observing floor the glass plates and slides were examined.

The crew checked the glass for any streaks or imperfections and then measured the reflectivity of the samples which check in as high at 91.75%. All-in-all it was a good test of the system. Everyone was happy with the results and the 200-inch mirror is cleared for its recoating in November.

Big thanks go out to Susan for shooting photos for me yesterday while I was off the mountain giving a talk.

Through the Looking Glass

It looks like we had a successful test of the aluminizing system for the 200-inch mirror on Friday. The chamber wont be opened for a while yet though.

Here are some (hopefully) interesting shots for you. The chamber has a porthole-like window on the side and it is possible for the crew to peak in on the action. I was fortunate enough to be able to grab some photos during the action.

The first shows the first of three glow discharges. A small amount of gas is introduced into the chamber and turned into a plasma (ionized gas). The photo was an exposure of over 1 second and doesn't capture the flickering nature of the plasma. It is a beautiful sight.

The second photo looks into the chamber as aluminum is being deposited. Over 200 tungsten wire coils with aluminum melted on to them lie within. Electricity is applied to the coils in pairs. As this happens they are heated in excess of 1,000 degrees and the aluminum is vaporized. Because all of the air has been pumped out of the chamber, creating a vacuum, the aluminum atoms fly about unimpeded. As each hits a surface they stick and gradually lay down a thin layer of aluminum.

In the photo below you can see two of the tungsten coils glowing brightly and another (in the upper right) that has just finished its job but glowing with residual heat.

Below is a movie of the view looking inside the vacuum chamber. I don't think that anyone other than observatory staff has had the opportunity to see something like this before. The photos used to make the movie were all with a hand-held camera and were all taken with the same exposure speed. This allows you to see the coils heat up and fade away as is described in the photo above.

Enjoy!