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Testing Reflectivity in Clean Mirrors

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#1 Alex McConahay

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Posted 30 March 2020 - 08:33 PM

How do I test the reflectivity of a mirror before and after cleaning?

 

In late October, Greg Crinklaw did a show on Theastroimagingchannel:

 

https://www.youtube....h?v=swcastCzHg8

 

He was explaining how he calibrated a program he had written (Sky Tools 4) to help an imager determine proper exposure times. He said he could not get the program to work with reflectors. He could not figure out why  even after months of butting his head against the wall. Then, he stumbled upon factoring in how long it had been since the mirror was new or cleaned. His research showed that in the course of five years, a typical mirror had lost 45 per cent of its reflectivity. I was really struck by that figure. He said once he figured in "time since last cleaning" his projections for proper exposure were pretty much right where they should be. Okay....That is some pretty good empirical proof. (See the video at about the 15 minute mark to see what I am talking about.) 

 

So, I bought myself a mirror cleaning kit, and will be cleaning the mirror. I want to check out the before and after reflectivity of the mirror. 

 

How should I do that?

 

What procedure would you run to determine how reflective a mirror is before and after a cleaning?

 

Alex

 

 



#2 Midnight Dan

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Posted 30 March 2020 - 08:40 PM

If you have a flat panel, you could use that.  Put it in front of the scope, take an exposure that puts the peak about mid-histogram, and look at the mean value for the image.  Do the same, with the same exposure time after cleaning.  Make sure you leave the flat panel on and running for 10 or 15 minutes before taking each exposure so it has had time to warm up and stabilize.  You might want to try it several times initially to determine how much variability there is in the reading.

 

-Dan


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#3 Jon Isaacs

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Posted 30 March 2020 - 08:57 PM

So, I bought myself a mirror cleaning kit, and will be cleaning the mirror. I want to check out the before and after reflectivity of the mirror.

 

Alex:

 

How will you be cleaning your mirror?  I clean my mirrors on a regular basis using this method. 

https://www.youtube....h?v=9Y8xFnXFVGQ

 

Over the years, I have cleaned my mirrors using a variety of techniques, this is the one that works.

 

Edit:  I fixed the broken link.  

 

Jon



#4 sarmen2

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Posted 30 March 2020 - 09:55 PM

The link to youtube on mirror cleaning does not appear to work. Can this be expanded?



#5 TOMDEY

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Posted 30 March 2020 - 10:21 PM

Interesting!

 

I found just the opposite extremum. Took my Enhanced Silver coating (on 29-inch Newt PM mirror, coating on for 5+ years) into work to measure. Did that in the lab, mirror uncleaned, before stripping --- Even though it was pretty dirty and even breaking down on some little spots (like where a moth got crushed onto it etc.) ... the specular spectral reflectivity was the same as when it left the shop six years earlier! There was no indication that the coating had degraded at all!

 

Undoubtedly, we're both right. Just goes to show that coatings vary a lot, depending where and how they were deposited... and what field-abuse they have seen. I was also involved in LDEF (Long Duration Exposure Facility... satellite) which did similar empirical exposure experiments on orbit. Most of our work work was aerospace.

 

To your question... your before and after cleaning specular reflectivities will be virtually the same. That's not easy to measure, probably no point in bothering. I had built a differential spectroradiometer to test mirror coatings in-situ. But that's with lab equipiment all over the place.    Tom

Attached Thumbnails

  • 30 toms reflectivity tester in-situ large telescope mirrors 80.jpg
  • 31 toms reflectivity tester in-situ large telescope mirrors.jpg

Edited by TOMDEY, 30 March 2020 - 10:21 PM.

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#6 freestar8n

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Posted 31 March 2020 - 03:28 AM

I think that if you can take the mirror outside under blue sky and take a picture of the sky reflected in the mirror - and compare it with a direct picture of the blue sky - that would tell you a lot.  You just need a manual exposure and an image format that is linear - i.e. no gamma applied or something.  A DSLR may be fine for this if the image format is truly raw - but an astro camera and fits would be better.  Then measure the difference in ADU between the images.  Ideally take a 'dark' exposure with the same settings so you can subtract off the bias level and measure an actual percentage change.

 

I think the mirror shouldn't have any kind of focusing effect on the extended blue sky.  You might want to try to be directly above the mirror to minimize any angle effect.

 

I would be very surprised if the reflectivity is significantly reduced unless it really looks dirty.

 

Frank



#7 cookjaiii

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Posted 31 March 2020 - 09:16 AM

What is "performance" (Y-axis) graphed in the video?  Are you sure it is reflectivity?  How can that possibly be?

 

Here is a paper showing loss of reflectivity of a silvered mirror over four years, and losses were only a few percent. I would expect overcoated or even naked aluminum would be more robust than silver.

 

https://wp.optics.ar...es-Tutorial.pdf



#8 Alex McConahay

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Posted 31 March 2020 - 10:37 AM

Interesting comments so far. Thanks for the ideas. 

 

I think the first thing I need to say is that I was seriously taken aback by Crinklaw's findings that after five years an unwashed mirror needs as much more exposure as a freshly washed one. (Reflectivity was off some 45%) Not only did he cite some studies, but he also had the proof of the pudding---his program worked when he factored it in, but did not work without it. 

 

My mirrors look good. I mean, they have never been touched. No fingerprints, nothing, Since dew is not generally a problem, and when not in use they are covered with two plastic bags (not perfectly tightly--air can probably get through the overlaps, etc.), and a cotton cloth wrapped around the tube, they look generally clean. I must admit, they do not have that "new telescope" look.  And a five minute exposure now gets me, I think, the same ADU value in the background sky as I got two years ago when I first installed the camera (in general--I mean the nights differ some but in general, I get the same reading). But, I kinda believe Crinklaw has his findings on his side. That, of course, is why I want to test. 

 

Cookjaiii, your tutorial link is interesting, but I think it supports Crinklaw's observations more than not. It seems that (in the tutorial) they are comparing mirrors that are "washed in situ" to the new mirrors. They do not seem to compare new (or newly washed mirrors) to unwashed. One of the graphs (readings for 2011-2012) shows a jump of about three or four per cent from the before to the after washing after one year. If you continue a three or four per cent loss year after year, you lose 20 to 30% from year one to the end of year five. And that is for a mountaintop in Hawaii. Not for my dusty dry desert. And they note that the conditions in Chile (dry, dustier) mean more loss than in Hawaii.  

 

I think, I can do both Frank's and Dan's tests easily enough. (and probably good to have two or three ways to test. I don't think Tom's will work for me, since I am talking about a remote desert observatory, not a place full of reflectivity testers, and meters, and all that (which I do not know how to use anyway).

 

I think Dan's admonition to somehow calibrate the flat panel (to be sure it is putting out the same before and after the cleaning) is important, too. How could I go about that?

 

And, of course, Frank's idea depends on me having the same sky before and after.... How do I go about that? Or how do I expose to correct for that?

 

Keep the ideas coming.

 

Thanks

 

Alex  



#9 Midnight Dan

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Posted 31 March 2020 - 10:46 AM

>> I think Dan's admonition to somehow calibrate the flat panel (to be sure it is putting out the same before and after the cleaning) is important, too. How could I go about that?

 

Not sure you can really do that without some sort of calibrated light meter.

 

I think the most practical thing is, before cleaning, run the test with the light panel multiple times.  Turn it on, give it time to warm up and stabilize, take a few exposures, average them and get a reading.  Then shut it off and let it cool for an hour.  Then run the test again.  Do this several times so that you get an idea of how much variability there is in your light panel, or the overall test procedure, even when nothing has changed in the optics.

 

Then clean the optics and run the test.  If your results are significantly outside the variability of the test, then you probably have a valid result.  If you saw any significant variability before cleaning, you might want to run the test multiple times after cleaning as well.  You can take the average of the before tests and then the average of the after tests and compare the two.  But to be honest, if you're not seeing the after-results outside of the variability of the test, I wouldn't worry about it.  That just says the cleaning didn't make a big difference.

 

-Dan



#10 theskyhound

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Posted 31 March 2020 - 02:06 PM

Hello,

 

I just wanted to add that there are in fact two factors at work. It's not just how clean the mirror is but a degradation in the commercial mirror coating. I tried very hard to find information about the longevity of commercial mirror coatings, but all I was ever able to come up with were hand waving answers about how durable they are. This was clearly not something that those who apply the coatings (that I contacted) wanted to talk about in detail. For the most part the pros don't use these coatings, so as far as I can tell there have been no studies to see how long they last. The one study I did find was never completed, and at the point where they stopped they were not washing the mirrors, so the two effects could not be disentangled.

 

I would love to study this long term to obtain a more definitive answer, especially for the commercial coatings. If I had access to the right telescope, I would obtain data regularly over a period of at least 5 years, and take note of when the mirror is cleaned. Images would be taken of standard star fields every few months. But care must also be taken to remove any effects from the camera, as these can degrade over time as well.

 

Another idea was to develop some sort of handheld device that could measure the reflectivity of a mirror. The idea would be to sample a large number of telescopes at star parties, and gather the date the mirror was last cleaned and last recoated.



#11 Alex McConahay

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Posted 31 March 2020 - 02:28 PM

Yeah, i think the effect of dirt/dust/pollen is greater than that of coatings wearing out themselves. And, I think I was clear to state that the correction Greg put into the program was about "time since cleaning" (or original installation, when it was as clean as it could be). 

 

I can tell you also that my 10 inch Meade Starfinder still looks good after 25 years. Other people (with less benign climates mostly, I suspect) have their coatings wear out much more quickly. 

 

I was thinking that the one tool I have that can act like a light meter is my DSLR. Aim it at the flat panel and get a reading of expected exposure, and see if it is the same just after turn on, and an hour later after warm up and all that. 

 

At any rate, I will wait until I have nothing to do on a cloudy night,  (or day) and go for it. 

 

Keep your ideas coming if you have any. 

 

Alex



#12 Jon Isaacs

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Posted 31 March 2020 - 02:40 PM

The link to youtube on mirror cleaning does not appear to work. Can this be expanded?

 

Thanks for catching that, I will see if I can fix that.

 

This one should work.

 

https://www.youtube....h?v=9Y8xFnXFVGQ

 

The Video is by Cary Chleborad of OWL (Optical Wave Laboratories).  He makes, refigures and coats mirrors.

 

Jon



#13 Jon Isaacs

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Posted 31 March 2020 - 02:54 PM

Yeah, i think the effect of dirt/dust/pollen is greater than that of coatings wearing out themselves. And, I think I was clear to state that the correction Greg put into the program was about "time since cleaning" (or original installation, when it was as clean as it could be).

 

I can tell you also that my 10 inch Meade Starfinder still looks good after 25 years. Other people (with less benign climates mostly, I suspect) have their coatings wear out much more quickly.

 

 

I think coating life varies a great deal and depends on the conditions the coatings face.  Clearly SCT coatings do not degrade 45% in 5 years.  They degrade very little.  There are SCT's that are still very functional with coatings 40 plus years old.  The dry desert in probably an excellent place for a mirror.  

 

As far as coating degradation and coater's, I am not sure if Greg talked to Cary Chleborad of OWL but I remember that he posted some information he had collected on mirrors he'd been sent to recoat. 

 

As I recall, he was doing an informal survey of how dirty the mirrors were and how much cleaning them helped. He tested them for reflectivity and then cleaned them and tested them again.  

 

Jon



#14 theskyhound

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Posted 31 March 2020 - 04:21 PM

For what its worth, I have an 18-inch mirror that is 19 years old, and the degradation in the coating is obvious to the eye. The humidity here in New Mexico is usually pretty low (single digits in the Spring) and I have kept it indoors when not in use.

 

All of my testing was done with iTelescope.net, and their telescopes tend to be very similar to one another. No SCTs were tested, for instance.



#15 freestar8n

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Posted 31 March 2020 - 04:42 PM

Interesting comments so far. Thanks for the ideas. 

 

I think the first thing I need to say is that I was seriously taken aback by Crinklaw's findings that after five years an unwashed mirror needs as much more exposure as a freshly washed one. (Reflectivity was off some 45%) Not only did he cite some studies, but he also had the proof of the pudding---his program worked when he factored it in, but did not work without it. 

 

My mirrors look good. I mean, they have never been touched. No fingerprints, nothing, Since dew is not generally a problem, and when not in use they are covered with two plastic bags (not perfectly tightly--air can probably get through the overlaps, etc.), and a cotton cloth wrapped around the tube, they look generally clean. I must admit, they do not have that "new telescope" look.  And a five minute exposure now gets me, I think, the same ADU value in the background sky as I got two years ago when I first installed the camera (in general--I mean the nights differ some but in general, I get the same reading). But, I kinda believe Crinklaw has his findings on his side. That, of course, is why I want to test. 

 

Cookjaiii, your tutorial link is interesting, but I think it supports Crinklaw's observations more than not. It seems that (in the tutorial) they are comparing mirrors that are "washed in situ" to the new mirrors. They do not seem to compare new (or newly washed mirrors) to unwashed. One of the graphs (readings for 2011-2012) shows a jump of about three or four per cent from the before to the after washing after one year. If you continue a three or four per cent loss year after year, you lose 20 to 30% from year one to the end of year five. And that is for a mountaintop in Hawaii. Not for my dusty dry desert. And they note that the conditions in Chile (dry, dustier) mean more loss than in Hawaii.  

 

I think, I can do both Frank's and Dan's tests easily enough. (and probably good to have two or three ways to test. I don't think Tom's will work for me, since I am talking about a remote desert observatory, not a place full of reflectivity testers, and meters, and all that (which I do not know how to use anyway).

 

I think Dan's admonition to somehow calibrate the flat panel (to be sure it is putting out the same before and after the cleaning) is important, too. How could I go about that?

 

And, of course, Frank's idea depends on me having the same sky before and after.... How do I go about that? Or how do I expose to correct for that?

 

Keep the ideas coming.

 

Thanks

 

Alex  

What I'm describing involves directly measuring the absolute reflectivity before and after cleaning.  The sky could be different and the measurements could be weeks apart - but each measurement gives you the absolute reflectivity at the time - from two exposures.  You end up with a total of 4 exposures - plus a dark frame.

 

You put dirty mirror on table under clear blue sky with sun low in sky and take picture of reflected blue sky in mirror.  Then aim camera at roughly same patch of sky directly and take picture.  The sky shouldn't change much in those few seconds - and you should have an idea of what part of the sky is reflected so you can capture the same rough part of the sky in both cases.  Use the same exposure and subtract a dark from each.  The ratio of ADU after dark subtract will tell you the absolute reflectivity of the mirror.

 

Then do the same again after cleaning the mirror - days or weeks later.  Just have a uniform blue sky to work with.  I would use exact same f-stop and other manual settings so you avoid introducing other variables.

 

Frank



#16 Alex McConahay

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Posted 31 March 2020 - 05:30 PM

Okay, I get it now, Frank.....

 

It does not matter how bright the sky is (within reason) because you are not measuring the sky. You are measuring how bright the sky is compared to the mirror version of it, at two different times. If there is no difference due to the cleaning, the relative brightness of the sky and mirror image will not change. If there is a change in the ratios, the cleaning did something. 

 

Alex



#17 freestar8n

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Posted 31 March 2020 - 06:58 PM

Yep. That’s the idea. I think it would work ok. You may be able to us a flat panel or uniform white wall instead of blue sky. But it needs to be uniform.

Frank


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