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Equipment Discussions >> Reflectors

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Pinbout
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Reged: 02/22/10

Loc: nj
Re: What's on this secondary? new [Re: robininni]
      #5923066 - 06/15/13 11:59 PM

I like the 4in with the 25% transmission 2way mirror effect. I'd like to make a robo collimation system that would continually adjust itself and a partially silvered mirror would be perfect for some inferred leds and phototransistors.

Actually at stellfane this last year, there was a kid who did something similar to keep his solar newt to track the sun. Very cool stuff.


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Vic Menard
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Re: What's on this secondary? new [Re: robininni]
      #5923430 - 06/16/13 09:13 AM

Quote:

...I just heard back from an optician (Lockwood) whom I emailed my same question to as I was not getting any conclusive answer on here, and he looked at the pictures and said the secondary needs recoating.



Rob,
When someone starts inspecting their coatings with a flashlight (front or back), I generally try to stay out of the discussion, for the obvious reasons exposed in this thread. I'm also puzzled that when you sent your optics off to be refigured/recoated, you didn't send the secondary mirror along with the primary mirror, especially considering your initial complaint. I always suggest evaluating coatings when a user begins a thread the way you did back in March. I assumed (apparently incorrectly) that both mirrors were evaluated by OMI and Lockwood.

(In your original thread, "Obsession 25" f4 vs Orion 10" f4.7 expectations", in my first response to your complaint on March 15, I commented, "I wonder if you've checked the mirror coatings? Both primary and secondary mirrors.")

With regards to your flashlight test--I'll defer to Mike Lockwood's advice. But I would still caution readers of this thread that using a flashlight to test secondary mirror coatings (especially conducted with such a powerful beam) will likely reveal pinhole and thin coating defects that may have only minimal impact on visual performance. Some secondary coatings are applied at 45-degrees to maximize reflectivity when the secondary mirror is aimed at the primary mirror. Have you tried redirecting your flashlight beam with the secondary mirror this way, and evaluating brightness against a wall with and without the secondary. I know the result isn't photometric, but if the beam is obviously dimmer with the secondary than without, I think that may be a better test...

And I wonder who did the coatings on both secondary mirrors, what kind of coatings they were, and who Mike Lockwood would suggest for recoating the mirrors? I assume the 4-inch coatings are several years old? I wonder how old the 4.5-inch coatings are?


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robininni
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Reged: 04/18/11

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Re: What's on this secondary? new [Re: Vic Menard]
      #5923564 - 06/16/13 11:05 AM

Quote:

Quote:

...I just heard back from an optician (Lockwood) whom I emailed my same question to as I was not getting any conclusive answer on here, and he looked at the pictures and said the secondary needs recoating.



Rob,
When someone starts inspecting their coatings with a flashlight (front or back), I generally try to stay out of the discussion, for the obvious reasons exposed in this thread. I'm also puzzled that when you sent your optics off to be refigured/recoated, you didn't send the secondary mirror along with the primary mirror, especially considering your initial complaint. I always suggest evaluating coatings when a user begins a thread the way you did back in March. I assumed (apparently incorrectly) that both mirrors were evaluated by OMI and Lockwood.

(In your original thread, "Obsession 25" f4 vs Orion 10" f4.7 expectations", in my first response to your complaint on March 15, I commented, "I wonder if you've checked the mirror coatings? Both primary and secondary mirrors.")

With regards to your flashlight test--I'll defer to Mike Lockwood's advice. But I would still caution readers of this thread that using a flashlight to test secondary mirror coatings (especially conducted with such a powerful beam) will likely reveal pinhole and thin coating defects that may have only minimal impact on visual performance. Some secondary coatings are applied at 45-degrees to maximize reflectivity when the secondary mirror is aimed at the primary mirror. Have you tried redirecting your flashlight beam with the secondary mirror this way, and evaluating brightness against a wall with and without the secondary. I know the result isn't photometric, but if the beam is obviously dimmer with the secondary than without, I think that may be a better test...

And I wonder who did the coatings on both secondary mirrors, what kind of coatings they were, and who Mike Lockwood would suggest for recoating the mirrors? I assume the 4-inch coatings are several years old? I wonder how old the 4.5-inch coatings are?




Hi Vic,

I don't know the age of the coatings on the 4" secondary. It came with the Obsession which was bought in October 2010. The 4.5" secondary, I just purchased, and it was coated by OMI about 4 weeks ago.

I didn't send the 4" secondary along with my primary for testing because I was already going to buy a 4.5" secondary just due to wanting the proper sizing (how ever little difference this may make in *detectable* illumination). Therefore I didn't send it along as I wasn't going to be using it anyway. I intended on selling it.

You bring up good points. I tried the "flashlight on the wall test" with and without the secondaries involved and I honestly can't tell a difference in the brightness. I did the same with a green laser pointer and again I can't really tell. I used both secondaries at 90 degrees and compared with a direct beam to the wall.

Even so, I still trust that the 4" could benefit from recoating based upon it passing a fair amount of light across its whole surface rather than reflecting it all. This unequivocally means lost reflectivity compared to the 4.5" coating where it only passes light through a hundred pin *BLEEP* spots. Is it enough to hurt views? I don't know. I find it hard (with only one scope) to make good comparisons in actual use as by the time parts are switched out, I really can't remember the differences I see or don't. I know we all want to get the most out of our scopes and at some point, you have to make judgments about when to repair or replace something and I prefer to go by data gathered outside of the eyepiece. I feel like astronomy should be no different than other fields in that you can separate out components and measure their quality independently of the assembled package's performance.

I'm into RC aircraft and to give you an example, let's look at heli blades. I can put my blades on a balance and tell whether they are in good balance or not. If they aren't, I don't have to assemble them back onto the heli and take off and hover to predict the flying will not be as good or that problems could occur in flight. Depending upon the amount of imbalance, maybe it won't be terrible? Maybe, maybe not. The vibration induced by the imbalance can cause other problems with loosening parts in flight even if the heli is controllable in flight. If I know they need balancing though, why try it without doing so?

Rob


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Howie Glatter
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Re: What's on this secondary? new [Re: Vic Menard]
      #5923604 - 06/16/13 11:36 AM

"Some secondary coatings are applied at 45-degrees to maximize reflectivity when the secondary mirror is aimed at the primary mirror."

I think Obsession supplied many secondaries with multilayer broadband reflective coatings. I believe I have one. They are very durable, but I have heard the coating is impossible to remove for re-coating without destroying the figure. It has some blueish transmission like that in the photo when viewed normally, but very little at 45 degree incidence, as I recall. Multilayer coatings are sensitive to incidence angle, so the diagonal coatings are designed for 45 degrees.
I suspect that is what the o.p. has.


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Vic Menard
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Re: What's on this secondary? new [Re: robininni]
      #5923611 - 06/16/13 11:41 AM

Quote:

...I don't know the age of the coatings on the 4" secondary. It came with the Obsession which was bought in October 2010. The 4.5" secondary, I just purchased, and it was coated by OMI about 4 weeks ago.



At least to my eye (and I'm no expert), the coatings look different to me. The 4-inch coatings appear to be very thin with minimal pin holes. The 4.5-inch coatings appear to be thicker with numerous pin holes. I know that Antares offers enhanced aluminum and silver, while Spectrum offers IAD coatings optimized for 45-degree reflectivity. But I have no idea what your coatings are or how to determine longevity or performance.

Quote:

I didn't send the 4" secondary along with my primary for testing because I was already going to buy a 4.5" secondary just due to wanting the proper sizing (how ever little difference this may make in *detectable* illumination). Therefore I didn't send it along as I wasn't going to be using it anyway. I intended on selling it.



Considering the issues you were having with image brightness--I just assumed...

Quote:

Even so, I still trust that the 4" could benefit from recoating based upon it passing a fair amount of light across its whole surface rather than reflecting it all.



While it obviously is passing light perpendicularly through the coatings, did you try an image with the secondary mirror tilted 45-degrees? I have to admit that I was stunned with the transparency of the coatings using your high output flashlight, but with new coating technologies optimized for 45-degree surfaces, I didn't feel qualified to comment.

I know that backlighting can be a severe test, and I also know that age and exposure to dew can impact coating performance. That's why I originally suggested evaluating both mirrors...

Quote:

I feel like astronomy should be no different than other fields in that you can separate out components and measure their quality independently of the assembled package's performance.

I'm into RC aircraft and to give you an example, let's look at heli blades.



I'm not sure that's a good analogy. What if you were into parachuting?


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: What's on this secondary? new [Re: Vic Menard]
      #5923663 - 06/16/13 12:12 PM

Thanks Howie. That's good information. I admit I was hesitant to say the coating was bad because if you just look at the mirror surface, you don't detect anything wrong at all. It reflects just like the newer secondary just staring into it.

It's just I noticed that the collimation laser light was easier to see on the 4" secondary compared to my new 4.5" and then I started wondering why? Following up with noting the blue haze when shining a light at the front of the mirror, I shined it at the back and then noticed all the transmitted light (which you can see when looking at the back and shining a light at the mirror side as well). I had read this is how you test mirror coatings to determine when it's time to replace the coating based upon getting a lot of light shining through.

Interestingly as I believe Vic pointed out, the 'see through' coating doesn't have a lot (hardly any) pin *BLEEP* spots of really bright light. However, it could be that they just don't stand out because the whole mirror surface passes light, I'm not sure.

I wondered initially if this was just another type of coating and this is inherently how it looks under scrutiny versus a coating that is going bad or worn out. We're just left to guess at so much with amateur astronomy equipment. This seems particularly true with optics. It would be nice if our equipment came with a thorough owners manual that covered the products in super detail and discussed things you may notice or that could happen and why they are okay and exactly what isn't okay. A manual that explained not just how to set up your equipment, but really gave details about the inherent qualities of the equipment would be great. Most appliances I have bought have a 2 or 3 page troubleshooting section with a bunch of what-ifs and how to resolve them *if necessary*. Several of the what-ifs have an answer of 'do nothing, your XXX is working properly and this XXX is normal."




Quote:

Quote:

I feel like astronomy should be no different than other fields in that you can separate out components and measure their quality independently of the assembled package's performance.

I'm into RC aircraft and to give you an example, let's look at heli blades.



I'm not sure that's a good analogy. What if you were into parachuting?




Vic, parachuting would be no different. Someone should be able to independently evaluate the chute, the ripcord, and the harness. I don't think anyone would say don't worry about whether the ripcord tests high enough in tensile strength, just put it all together and if the chute opens, we're good!

Rob


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Vic Menard
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Re: What's on this secondary? new [Re: robininni]
      #5923693 - 06/16/13 12:25 PM

Quote:

Quote:

Quote:

I feel like astronomy should be no different than other fields in that you can separate out components and measure their quality independently of the assembled package's performance.

I'm into RC aircraft and to give you an example, let's look at heli blades.



I'm not sure that's a good analogy. What if you were into parachuting?




Vic, parachuting would be no different. Someone should be able to independently evaluate the chute, the ripcord, and the harness. I don't think anyone would say don't worry about whether the ripcord tests high enough in tensile strength, just put it all together and if the chute opens, we're good!



I kind of meant my response as a joke.

What I was trying to say (without saying) is while we can individually test optical components for performance, there's no reason we can't try an unknown quantity as part of the system to evaluate overall performance--it won't destroy the telescope (bad heli blades) or kill the observer (bad ripcord). Or, we can confine our system to known quantities (where the quantities are actually know-able) and then assume any performance issues are extraneous factors.


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Mike Lockwood
Vendor, Lockwood Custom Optics
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Reged: 10/01/07

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Re: What's on this secondary? new [Re: robininni]
      #5923703 - 06/16/13 12:32 PM

Keep in mind, I suggested recoating the 4" because the owner wants to sell it. It certainly could continue being used if it were big enough.

That said, the flashlight used to check it was quite bright, probably brighter than I thought, so the condition may be better than I thought.

New mirror coatings have pinholes. That's just how it is. They still have very high reflectivity.

The partial transparency and haze seen on the 4" flat are normal for a coating that has aged a bit. There is no way to tell just how old it actually is, where it came from, how long it sat around before being put into a scope, or what it has been through.

Yes, some coatings are designed for 45-degree incidence. These can be either a modified aluminum or enhanced aluminum coating, with thickness altered to take angle into account, or a more complicated dielectric coating. The latter may not be strippable, or the former may be applied over a chrome undercoat, which itself is not easily strippable.

There's no way to tell unless the secondary has some documetation with it.


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: What's on this secondary? new [Re: Mike Lockwood]
      #5923768 - 06/16/13 01:14 PM

Since I see the brightness of the light is being called into question, for comparison, I took a photo of light behind the mirrors again but this time using my Hotech flashlight on its brightest red light setting rather than my 130 lumen high power LED spot light. The light you see in these pictures comes from a single red LED diode (which I included a picture of too). Here's what that looks like:

'Old' 4" secondary:

'New' 4.5" secondary:


I don't know which side this supports (recoat or not), I'm just including it for more complete information.

Now, if I may rant a little, please note, I am NOT making any statements about the secondary being coated improperly or suggesting that whoever did it did a poor job or anything of that nature. It is as Mike posted above, that I am going to sell it and I don't want to sell the mirror to someone if it is in need of a new coating without either taking care of that first or letting them know it needs this done. That is all.

It seems that it is a tough judgement call by those who have read the thread as to whether a recoat is indicated or not. No one seems to want to fully commit to a definite yes or no. Again, this is the side of amateur astronomy I find frustrating, especially with as many people as are apparently involved with CN that these types of issues are so vague and not cut and dry.

I surmise it is this way because any question like this is somewhat linked to the pervious work of someone or some company so if someone says, 'yes, that needs to be recoated' then some might wrongly extrapolate that statement to infer the original work is suggested to be poor because the coating is likely (not surely) only a few years old. This would be complete fabrication though to assume this. In this secondary's particular case the scope did reside in Houston for 2-3 years though so I imagine the humidity and proximity of Houston to the Ocean could affect the longevity of mirror coatings.

If we were discussing collimation techniques, there would be an endless supply of offerings from the community as collimation is a process and not a product and so no one feels threatened or that they might be threatening someone else with their advice or judgement about how to do it.

It's rather silly I believe to behave this way. I guess its all about today's culture of political correctness which supercedes truth. If you feel I have read way too much into the responses I have seen in this and numerous other threads that seem to be treated similarly, my apologies. I don't think so though.



Rob

Edited by robininni (06/16/13 01:42 PM)


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KerryR
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Re: What's on this secondary? new [Re: robininni]
      #5923833 - 06/16/13 01:56 PM

I've read on several non-professional sources that one reason for thinner coatings is that really thick (and therefore more durable) coatings can affect the surface quality. Low end scopes, like my ancient Meade 10" f4.5 have coatings that seem to last decades. The flashlight 'test' from the rear shows thicker coatings with fewer pinholes. My re-figured 8" f6 had coatings that were far thinner and showed more pinholes. It needed recoating after 5 or so years. It seems it's common for premium optics to have thinner coatings that 'fail' the rear-light test. Perhaps this is intentional, to preserve surface quality at the cost of needing re-coating more often.

Can any professional opticians and/or coaters comment?


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Jason D
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Re: What's on this secondary? new [Re: robininni]
      #5923858 - 06/16/13 02:19 PM

Quote:

No one seems to want to fully commit to a definite yes or no. Again, this is the side of amateur astronomy I find frustrating, especially with as many people as are apparently involved with CN that these types of issues are so vague and not cut and dry.

I surmise it is this way because any question like this is somewhat linked to the pervious work of someone or some company so if someone says, 'yes, that needs to be recoated' then some might wrongly extrapolate that statement to infer the original work is suggested to be poor because the coating is likely (not surely) only a few years old.




Rob, I believe the reason you are not getting a consensus and a definite answer is because we simply don’t know the answer. It is not that we know the right answer but we are afraid to offend some vendor by sharing it. All we can do is share opinions but the decision is yours. If you are unsure then recoat to be on the safe side though it will cost you.
Jason


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Starman1
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Re: What's on this secondary? new [Re: Jason D]
      #5924286 - 06/16/13 07:31 PM

1) New coatings sometimes have pinholes. They should be small and few in number. They are caused by aluminum not adhering to small spots on the glass.
2) magnesium fluoride is often used as one of the layers in "enhanced" coatings, along with titanium dioxide, silicon monoxide, etc. The Meade LightBridge mirrors use this as a coating layer. If you see a blue coloration, perpendicular to the glass, that is likely to be one of the coating layers on your mirror.
3) At a 45 degree angle, the one it's used in, you probably don't see blue in the reflection.
Many overcoat layers reflect differently when examined at angles far from the angles of use. Try a nebula filter (designed to be used with rays perpendicular to the glass) at a 45 degree angle to see what I mean. Or look at a dielectric-coated diagonal mirror at an angle nearly perpendicular to the glass.

So what's the verdict? If it's nicely reflective at a 45 degree angle, without coloration, and the number of pinholes is small, then your mirror is likely to be just fine. If it has been less than ten years since coating, I'd just use it. Dirt is the main killer of reflectivity. And blue does not indicate a failing coating. Massive numbers of pinholes or gray spots in the coating do indicate that.


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robininni
scholastic sledgehammer


Reged: 04/18/11

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Re: What's on this secondary? new [Re: Starman1]
      #5924295 - 06/16/13 07:38 PM

Quote:

1) New coatings sometimes have pinholes. They should be small and few in number. They are caused by aluminum not adhering to small spots on the glass.
2) magnesium fluoride is often used as one of the layers in "enhanced" coatings, along with titanium dioxide, silicon monoxide, etc. The Meade LightBridge mirrors use this as a coating layer. If you see a blue coloration, perpendicular to the glass, that is likely to be one of the coating layers on your mirror.
3) At a 45 degree angle, the one it's used in, you probably don't see blue in the reflection.
Many overcoat layers reflect differently when examined at angles far from the angles of use. Try a nebula filter (designed to be used with rays perpendicular to the glass) at a 45 degree angle to see what I mean. Or look at a dielectric-coated diagonal mirror at an angle nearly perpendicular to the glass.

So what's the verdict? If it's nicely reflective at a 45 degree angle, without coloration, and the number of pinholes is small, then your mirror is likely to be just fine. If it has been lass than ten years since coating, I'd just use it. Dirt is the main killer of reflectivity. And blue does not indicate a failing coating. Massive numbers of pinholes or gray spots in the coating do indicate that.




Wow, thanks, Don. That was clear and concise. Much appreciated.

Rob


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amicus sidera
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Re: What's on this secondary? new [Re: Howie Glatter]
      #5924340 - 06/16/13 08:11 PM

Quote:

"Some secondary coatings are applied at 45-degrees to maximize reflectivity when the secondary mirror is aimed at the primary mirror."

I think Obsession supplied many secondaries with multilayer broadband reflective coatings. I believe I have one. They are very durable, but I have heard the coating is impossible to remove for re-coating without destroying the figure. It has some blueish transmission like that in the photo when viewed normally, but very little at 45 degree incidence, as I recall. Multilayer coatings are sensitive to incidence angle, so the diagonal coatings are designed for 45 degrees.
I suspect that is what the o.p. has.




Those certainly look like multicoatings to me, as well. I've seen a lot of failed mirror coatings in my time, and I've never seen one go blue... spotty, black, peeling... but never blue. The giveaway may be the mottled crosshatch pattern that was visible on one photo; might be an interference pattern, due to the presence of different layers.

Just my 2 cents, worth what you paid for it...

Fred


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GlennLeDrew
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Re: What's on this secondary? new [Re: amicus sidera]
      #5924391 - 06/16/13 08:45 PM

That cross-hatched pattern in an early photo was, I thought, the reflected image of a chair back... (?)

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Mike Lockwood
Vendor, Lockwood Custom Optics
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Re: What's on this secondary? new [Re: Starman1]
      #5924467 - 06/16/13 09:35 PM

Quote:

2) magnesium fluoride is often used as one of the layers in "enhanced" coatings, along with titanium dioxide, silicon monoxide, etc. .... If you see a blue coloration, perpendicular to the glass, that is likely to be one of the coating layers on your mirror.



The blue color is only an indication that a layer (of material or of residue on top of the coating) reflects or scatters more blue light than it does other colors.

The apperance of color is a characteristic of the thickness of a layer, not the material type.

So, it's not so easy to figure out what a coating is just by its appearance.

Also, SiO and MgFl have vastly different indices of refraction (2.29 vs 1.37), so their uses in thin films applications will be quite different. They are not interchangeable.


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: What's on this secondary? new [Re: GlennLeDrew]
      #5924513 - 06/16/13 10:07 PM

Quote:

That cross-hatched pattern in an early photo was, I thought, the reflected image of a chair back... (?)




Correct, Glenn. Sorry, I almost pointed that out in an earlier post.

Rob


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amicus sidera
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Reged: 10/14/11

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Re: What's on this secondary? new [Re: GlennLeDrew]
      #5924995 - 06/17/13 08:16 AM

Quote:

That cross-hatched pattern in an early photo was, I thought, the reflected image of a chair back... (?)




Notice the artful use of the words "may" and "might" in my post... wiggle room is a good thing!

Fred


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Starman1
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Reged: 06/24/03

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Re: What's on this secondary? new [Re: Mike Lockwood]
      #5925157 - 06/17/13 10:19 AM

Quote:

Quote:

2) magnesium fluoride is often used as one of the layers in "enhanced" coatings, along with titanium dioxide, silicon monoxide, etc. .... If you see a blue coloration, perpendicular to the glass, that is likely to be one of the coating layers on your mirror.



The blue color is only an indication that a layer (of material or of residue on top of the coating) reflects or scatters more blue light than it does other colors.

The appearance of color is a characteristic of the thickness of a layer, not the material type.

So, it's not so easy to figure out what a coating is just by its appearance.

Also, SiO and MgFl have vastly different indices of refraction (2.29 vs 1.37), so their uses in thin films applications will be quite different. They are not interchangeable.



Not interchangeable, yes.
But MgFl2 is often used as an enhanced coating material, and I have seen it often as a blue-tinting in the reflections from enhanced coatings.
You're correct to point out it could be another material.


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howard929
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Re: What's on this secondary? new [Re: Starman1]
      #5925198 - 06/17/13 10:47 AM

Hey Don,

Since you're here and I know you to be quite knowledgeable, I'll ask you a question.

I removed some milk jug washers that were behind my secondary mirror holder since they were too rutted out to be of much use. Should I PM you a photo of them or just use my telescope to view the heavens?

(sorry ALL, i just had to...)


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