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Does a mirror diagonal degrade after X number of years?

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#51 Jon Isaacs

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Posted 29 October 2021 - 05:50 PM

And just to be clear, a lower reading means brighter sky/greater transmission? So the newer Everbrite has slightly better transmission than the older unlabeled one?

Scott

 

In my first, crude test, yes, the dielectric diagonal was about 0.1 magnitudes = 10%  brighter.

 

But I need to spend some time doing it carefully.

 

And I'm not sure which diagonal is older, the Everbrite I happened to use had Everbrite written in Cursive..

 

Jon



#52 Piero DP

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Posted 30 October 2021 - 03:07 AM

In my first, crude test, yes, the dielectric diagonal was about 0.1 magnitudes = 10% brighter.

But I need to spend some time doing it carefully.

And I'm not sure which diagonal is older, the Everbrite I happened to use had Everbrite written in Cursive..

Jon


Jon, that's very interesting.

Are your numbers averages or single measurements? As the difference is quite small, taking multiple technical repeats seems essential to me.

Given the non linear scale and the way these measurements are taken, I would trust the median and standard deviation of the median more than the mean+/-sd.

Having said this, a difference in 0.1 mag is unlikely to be detectable to our eye.

#53 Jon Isaacs

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Posted 30 October 2021 - 04:34 AM

Jon, that's very interesting.

Are your numbers averages or single measurements? As the difference is quite small, taking multiple technical repeats seems essential to me.

Given the non linear scale and the way these measurements are taken, I would trust the median and standard deviation of the median more than the mean+/-sd.

Having said this, a difference in 0.1 mag is unlikely to be detectable to our eye.

 

Here's the thing:

 

When you're trying to read the difference between 21.31mpsas and 21.35 mpsas you're working with very little light and working down close to the noise floor. The readings vary.

 

When you're at 13.3 mpsas, there's 1500 times as much light. The thing seems to nail it, same number everytime +/- 0.01 or less.

 

I agree that the eye probably can't easily detect 10% difference. I think there's lots of problems to work out but my real goal is to try to develop a scheme to use with telescope primary and secondary mirrors.

 

If with further efforts. I'm confident that the difference between the two mirrors is 0.1 magnitudes, that tells me the non dielectric mirror's coatings are still good.

 

Jon


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#54 sanbai

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Posted 30 October 2021 - 09:04 AM

SQM meters are a good idea. Quality photometers for photography could be also a good tool (I have one for the old 6x6 cameras I bought few years ago). Ideally in "spot" mode. I guess the internals/ principles of both tools are very similar.

Taking a picture through it it's also a way, but it would need to be in fully manual camera settings and raw format. Other than that the camera automatic adjustments and processing can fool you.

#55 SeattleScott

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Posted 30 October 2021 - 09:40 AM

Supposedly 10% change in brightness is on the threshold of being able to visually detect the difference.

That being said, if I got another car that was 10% more fuel efficient, the difference might not jump out at me at the gas pump. But I would still want the more fuel efficient car. 10% improvement is 10% improvement, regardless of how detectable it is. Now how much is it worth to get that last 10%? I don’t know that I would bother recoating a primary for such a subtle difference. But diagonal generally isn’t that expensive, and is super easy, and can be used with multiple scopes. So even a 10% increase in transmission might be worth it.

Scott
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#56 Starman1

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Posted 30 October 2021 - 10:10 AM

Supposedly 10% change in brightness is on the threshold of being able to visually detect the difference.

That being said, if I got another car that was 10% more fuel efficient, the difference might not jump out at me at the gas pump. But I would still want the more fuel efficient car. 10% improvement is 10% improvement, regardless of how detectable it is. Now how much is it worth to get that last 10%? I don’t know that I would bother recoating a primary for such a subtle difference. But diagonal generally isn’t that expensive, and is super easy, and can be used with multiple scopes. So even a 10% increase in transmission might be worth it.

Scott

Perhaps a reason to go with a dielectric coating.

But bear in mind there is a spectrum of transmission.

People studying carbon stars, red giants, reddish features on Mars or Jupiter, might prefer a silvered diagonal due to the higher transmission from 600nm up.

People studying CaK features on the sun might prefer a UV-enhanced aluminum coating on the mirror.

People doing infrared studies might want a gold coating, which is very flat well into the infrared.

For normal 450-600nm viewing with scotopic vision at night, though, it is hard to beat the dielectric coating for transmission.

 

The main caveat is that coatings don't influence sharpness of image, usually.  It's the polish on the mirror that is the main determinant.

And that is often related to the price.

 

The second main issue for image quality is the collimation of the diagonal itself.  This is something that is a pain to accomplish, but IS possible to adjust with a lot of trial and error.


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#57 leonard

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Posted 30 October 2021 - 10:47 AM

Hi all,

 

I've had a 2 inch mirror diagonal from Tele Vue for about 30 years now. Should I ever get it re-mirrored? Is it not worth the hassle, and just run out and get a new one for $200 or so? Is their Everbright ™ really for ever?

 

I also wonder about how flat the flat should be. Some advertise 1/10th wave or better.

     Like many things with telescopes , this is a small but important thing . 30 years of age on standard coatings will have

a degrading effect on reflection . It’s not worth putting a lot of though into . Call TeleVie they will sell you a new Everbright

mirror to replace your old mirror diagonal . Just remove the old one from its casing and put the new one in .

Thats what I did , was easy and the issue long forgotten .

Yes as has been pointed out surface flatness is important .

 


Edited by leonard, 30 October 2021 - 10:48 AM.

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#58 LDW47

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Posted 30 October 2021 - 12:21 PM

Supposedly 10% change in brightness is on the threshold of being able to visually detect the difference.

That being said, if I got another car that was 10% more fuel efficient, the difference might not jump out at me at the gas pump. But I would still want the more fuel efficient car. 10% improvement is 10% improvement, regardless of how detectable it is. Now how much is it worth to get that last 10%? I don’t know that I would bother recoating a primary for such a subtle difference. But diagonal generally isn’t that expensive, and is super easy, and can be used with multiple scopes. So even a 10% increase in transmission might be worth it.

Scott

How can you compare fuel efficency to what this issue concerns, are you ..........



#59 LDW47

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Posted 30 October 2021 - 12:24 PM

Some of this is getting a little bit .........., think about it, the deep technical .......



#60 Jon Isaacs

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Posted 30 October 2021 - 12:56 PM

Supposedly 10% change in brightness is on the threshold of being able to visually detect the difference.

That being said, if I got another car that was 10% more fuel efficient, the difference might not jump out at me at the gas pump. But I would still want the more fuel efficient car. 10% improvement is 10% improvement, regardless of how detectable it is. Now how much is it worth to get that last 10%? I don’t know that I would bother recoating a primary for such a subtle difference. But diagonal generally isn’t that expensive, and is super easy, and can be used with multiple scopes. So even a 10% increase in transmission might be worth it.

Scott

 It's unlikely you or I could see it.  Skilled variable star observers can see it, that is where the 10% number comes from.  

 

The second main issue for image quality is the collimation of the diagonal itself.  This is something that is a pain to accomplish, but IS possible to adjust with a lot of trial and error.

 

 

The current diagonal is a TeleVue 2 inch. They are machined from a single piece of aluminum.  In my experience, collimation is not an issue.  With other diagonals, it can be. 

 

Jon



#61 Starman1

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Posted 30 October 2021 - 01:54 PM

 It's unlikely you or I could see it.  Skilled variable star observers can see it, that is where the 10% number comes from.  

 

 

The current diagonal is a TeleVue 2 inch. They are machined from a single piece of aluminum.  In my experience, collimation is not an issue.  With other diagonals, it can be. 

 

Jon

It isn't how the diagonal is made, but how the mirror is held in place.

And I have seen, over the years, misaligned diagonals from every company in the market.



#62 Jon Isaacs

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Posted 30 October 2021 - 02:28 PM

It isn't how the diagonal is made, but how the mirror is held in place.

And I have seen, over the years, misaligned diagonals from every company in the market.

 

In the case of the 2 inch TeleVue Diagonals it is how the diagonal is made.  The reflective face of the mirror outside the field of view fits against a thin fiber gasket/shim that fits against the machined 45 degree flat of the diagonal. It is held in place by foam on the backside.  The only way it can be misaligned is if it is machined improperly or the foam is gone. 

 

I have seen 1.25 inch TeleVue diagonals that were out of collimation, these were not Everbrite one piece body diagonals but the Japanese models that were like those Orion sold.  

 

Jon



#63 LDW47

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Posted 30 October 2021 - 03:14 PM

In the case of the 2 inch TeleVue Diagonals it is how the diagonal is made.  The reflective face of the mirror outside the field of view fits against a thin fiber gasket/shim that fits against the machined 45 degree flat of the diagonal. It is held in place by foam on the backside.  The only way it can be misaligned is if it is machined improperly or the foam is gone. 

 

I have seen 1.25 inch TeleVue diagonals that were out of collimation, these were not Everbrite one piece body diagonals but the Japanese models that were like those Orion sold.  

 

Jon

Are these last few post not completely different than the original questions, how do these fit with those



#64 hoof

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Posted 30 October 2021 - 10:11 PM

To test the degradation of my 12 year old 15” dob, I used my dslr, and photo processing software (shot the photos in raw). Had a white wall, took a photo of it. Aimed the dob at the same spot, took a photo through the focuser drawtube. I needed to brighten the 2nd image to match the two photo’s brightness at the aim point. I can’t remember how much, but I do remember that the combined reflectivity of the two mirrors was above 85%, indicating very little degradation.

I had cleaned the mirrors prior to the test. Hopefully the coatings will continue to remain that good for a while :)
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#65 David L

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Posted 28 November 2021 - 08:49 PM

I was just wondering if a damaged mirror could be replaced in a star diagonal.  My current mirror is 2 7/8 " X 2 1/8" X 3/8 " thick with beveled corners.  



#66 Starman1

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Posted 29 November 2021 - 01:32 AM

I was just wondering if a damaged mirror could be replaced in a star diagonal.  My current mirror is 2 7/8 " X 2 1/8" X 3/8 " thick with beveled corners.  

They are usually rectangular.  Finding an exact replacement could be more expensive than a new star diagonal.

An oval mirror (like a newtonian secondary) could also fit.

With good quality diagonals fairly inexpensive, replacing the mirror might only be appropriate for a high-end diagonal like Baader, TeleVue or AstroPhysics et.al.




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