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What's on this secondary?

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#1 robininni

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Posted 14 June 2013 - 06:25 PM

I noticed that my old 4" secondary compared to my new 4.5" secondary reflects the laser collimator light differently. What I mean is the laser beam is very difficult to see on the new secondary where it hits it and it is VERY easy to see it on the old secondary.

I then noticed upon shining a flash light at the 4" secondary it is covered with an opaque like blue tint but this isn't visible without artificial light shining on it and it helps to show up if the light is shined and an angle other than perpendicular to the mirror.

I have included some pics of both mirrors: side by side, then up close with the flash light showing the blue on the 4" and the absence of it on the 4.5".

Is this cleaning residue? Is this a failing coating? What's going on here?

Thanks,

Rob

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#2 GlennLeDrew

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Posted 14 June 2013 - 06:38 PM

There would seem to be more scattering from the older surface; the red of the laser and the blue from (I presume) your while LED flashlight. How do the surfaces vompare when sunlight falls on the surface?

#3 robininni

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Posted 14 June 2013 - 06:49 PM

Glenn,

The 4.5" is installed back in the scope so I couldn't take it into the sunlight but I did take the 4" outside just now and I don't notice anything unusual with sunlight on the mirror surface.

Also, yes, your presumption is correct about a white LED flashlight.

Rob

#4 robininni

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Posted 15 June 2013 - 03:50 PM

So does anyone really know? My first thought was cleaning residue (soap), but I didn't think that would be so uniformly distributed. I got the idea of soap because somewhere I read someone asking something similar of a primary, although it was only a spot here and there and cleaning residue was a given answer.

My primary, before getting refigured and recoated, had some areas and streaks that looked like this in white light from a flash light, and in one of these areas, I could see a weird honey comb like pattern. Again, I wasn't sure what it would be having no experience with reflector mirrors but now that I think about it it doesn't make sense that you cannot see this just looking at the mirror in daylight if it was on rather than 'in' the coating. What was the honey comb pattern? Is this a coating getting thin with time?

And so back to the secondary... A failing coat, a worn coating? What say you? This secondary is roughly 2 1/2 years old.

Rob

#5 Darren Drake

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Posted 15 June 2013 - 04:28 PM

Did you clean the old secondary. Unseen residue could cause this.

#6 Howie Glatter

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Posted 15 June 2013 - 04:38 PM

". . the laser beam is very difficult to see on the new secondary where it hits it and it is VERY easy to see it on the old secondary."

The dirtier the mirror, the easier to see the beam impact.

"I then noticed upon shining a flash light at the 4" secondary it is covered with an opaque like blue tint . . and it helps to show up if the light is shined and an angle other than perpendicular to the mirror."

I think that observation might indicate a multilayer dielectric reflective coating. Do you know what the coatings are on the mirrors ?

#7 Jason D

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Posted 15 June 2013 - 04:55 PM

Rob,
What is your concern? Are you concerned you did not get your money's worth with your recent secondary mirror purchase or are you concerned about the view at the eyepiece?
Most of us will see unpleasant views when shining a light on our mirrors yet that has little to no impact on our views.
With all the discussion in the other thread about star testing, yesterday I took out my StarBlast 6" which I have not used for years and which I knew did not have perfect optics. The outer defocus star was sharp and bright. The inner defocus star was dim and mushy -- a classic case of bad SA. Yet, when I pointed it at Saturn, I did get a nice view of Cassini division. Using my experienced eye, I did see differences when compared the view to my 10" premium optics but only an experienced eye can see the differences I am referring to. The point here is even for a scope with terrible star test result good views are attainable.
Jason

#8 robininni

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Posted 15 June 2013 - 05:09 PM

Rob,
What is your concern? Are you concerned you did not get your money's worth with your recent secondary mirror purchase or are you concerned about the view at the eyepiece?
Most of us will see unpleasant views when shining a light on our mirrors yet that has little to no impact on our views.
With all the discussion in the other thread about star testing...

Jason


Whoa. Just asking a question. I thought this was a place to learn. I have no ulterior motive with my question here. No need to second guess me.

Rob

#9 GlennLeDrew

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Posted 15 June 2013 - 06:04 PM

Might a careful cleaning reduce the scatter?

Upon further rummaging through the memory banks, I recall seeing a blue reflection from one or two mirrors in high end 2" star diagonals... It might have to do with the overcoating characteristics?

#10 robininni

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Posted 15 June 2013 - 07:43 PM

Glenn,

I cleaned the secondary in question with a distilled water/soap concoction (very little dish soap) and then rinsed with distilled water. It didn't go away. I then cleaned with acetone and it didn't go away (although I was able to remove some crud that was on the edge which was hiding under the lip of the secondary housing). I then tried denatured alcohol. I didn't removed the hazy look seen when using a flashlight which my new secondary, freshly coated, does not have.

So having an extremely clean secondary mirror out of the housing, I decided to aim the flashlight at the rear of it and this is what I saw:
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You can see the shadow of the dew heater and and basically see through the entire secondary. The 'haze' is also visible.

I assume the 'haze' is actually the surface of the mirror under the coating and is somewhat visible because the coating is old, worn, and needs to be redone.

Is this a fair deduction for this discovery? Should a flashlight be able to penetrate through the coating from the rear of the mirror? I assume not, as this is how folks check for failing coatings so I read.

The only thing left to do is see if my flashlight will penetrate through my new secondary. I didn't do this yet because it is installed in my scope and I wanted to see if what I have posted here is enough for you to confirm my suspicions.

Rob

#11 GlennLeDrew

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Posted 15 June 2013 - 07:58 PM

A good look-see with an inexpensive (plastic) 30X pocket microscope might prove instructive. I *slightly* wonder now if the surface had been fully polished out. This is only a germ of a suspicion, but worth allaying (or confirming.) It would be surprising if the diagonal was passed with sufficient pitting to generate a fine haze, but you never know...

#12 robininni

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Posted 15 June 2013 - 08:10 PM

I took a couple more pictures. I wanted to get a close up to see the haze detail better. The square you see is the secondary string attachment Obsession puts on mirrors as a fail safe for it dropping from the UTA.

I did shine the same flashlight at the back of the newly coated primary and I see no light at all shining through. Granted, it is 2" thick but I think that doesn't matter since it is glass. I believe it is the coating only that stops the light.

Thanks,

Rob


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#13 Jason D

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Posted 15 June 2013 - 08:54 PM

Whoa. Just asking a question. I thought this was a place to learn. I have no ulterior motive with my question here. No need to second guess me.

Rob


Rob, you missed my point. You are driving yourself crazy with questioning every imperfection. I would argue every scope and optics will have some sort of imperfection. Let the views at the eyepiece be your final judge. The example I described in my last post based on my first hand experience with imperfect optics and terrible star test result yet the scope gave pleasing views. Are you seeing bluish haze at the eyepiece? If not and the views are good then I suggest you do not touch your secondary mirror. The more you touch it the greater the chance for a scratch or a drop. It is your scope and it is your money and you are free to do whatever you want. I am just providing a friendly advice.

Jason

#14 robininni

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Posted 15 June 2013 - 09:15 PM

Whoa. Just asking a question. I thought this was a place to learn. I have no ulterior motive with my question here. No need to second guess me.

Rob


Rob, you missed my point. You are driving yourself crazy with questioning every imperfection. I would argue every scope and optics will have some sort of imperfection. Let the views at the eyepiece be your final judge. The example I described in my last post based on my first hand experience with imperfect optics and terrible star test result yet the scope gave pleasing views. Are you seeing bluish haze at the eyepiece? If not and the views are good then I suggest you do not touch your secondary mirror. The more you touch it the greater the chance for a scratch or a drop. It is your scope and it is your money and you are free to do whatever you want. I am just providing a friendly advice.

Jason


Jason,

My point is that this has nothing to do with views or using it. I am just asking, as a matter of fact, why this secondary reflects light this way. I assume you read my further down posts in this thread with more pictures showing substantial light shining through from the backside of the mirror. Should this happen or does this signify a mirror needing a new coating? You said I am driving myself crazy with every imperfection, but I would say a failed coating is something one might want to bother themselves with, don't you think?

While not directed totally at you, I get amazed (and frustrated) that there are so many seemingly 'experienced' people who post many replies to all sorts of threads with great authority here on CN, and yet it seems no one can answer my seemingly simple question (for an experienced person) about the shape of this secondary's coating.

I answered for myself why I see the haze shining the flashlight at the mirror face and as I now know it can be seen from shining a light through the rear of the mirror. Obviously, the coating is super thin as you can't see any light shining through the freshly recoated primary I just got back. So now I am just asking a simple question, that again, I would think a great multitude of you should be able to answer based on being experience with reflectors: Does this mirror need a new coating? Is this a sign of a coating past its prime or a coating that for whatever reason was too thin? Don't any of you know? Sorry to rant, but I just find the lack of concise answers ironic with so many authoritarian types on here.

Rob

#15 Pinbout

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Posted 15 June 2013 - 09:31 PM

most coatings will show ugly stuff, doesn't matter if its new, old, when you shine flash lights on them and especially from behind.

what you show is normal, so you can chillax. :grin:

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#16 GlennLeDrew

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Posted 15 June 2013 - 09:35 PM

That last close-up shows a very large number of bright specks, more readily seen in the transition between bright and dark. I find that a tad concerning. It would be worthwhile to examine them under magnification, to settle the question of their being on the sure tests or in the coating. The more I look at these pics, the more inclined I am to think a re-coating is in order--as long as the mirror is pit-free, of course.

#17 robininni

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Posted 15 June 2013 - 09:42 PM

most coatings will show ugly stuff, doesn't matter if its new, old, when you shine flash lights on them and especially from behind.

what you show is normal, so you can chillax. :grin:


Thanks for your input. Although I have bad news for you and your secondary. 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

#18 okieav8r

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Posted 15 June 2013 - 09:52 PM

Rob, I'm no mirror expert, and won't pretend to be, but I do know that the thickness of mirror coatings are on the molecular level, only a few angstroms thick, and that being able to see the light of a flashlight shined through the back of a telescope mirror is pretty common. One thing you'll read in articles about telescope mirrors and cleaning them is, don't shine a flashlight on the mirror, because it will look dirtier than it really is, and don't shine it on the mirror through the back, because you will likely be able to see through it. In other words, it will just cause undue worry.

I was reading an article awhile back about one of the big observatory mirrors, and it said that it took something like only a cubic centimeters worth of aluminum oxide to coat the entire mirror. I wish I could remember where I read it.

#19 Pinbout

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Posted 15 June 2013 - 09:59 PM

don't hate the player, hate the game.

at $10 an inch to get it recoated, its not going to cost you that much. and are you going to get enhanced coatings, I've heard them fail quickly, and that will be more than $10 an inch for only a couple of percentages of reflectivity.

and if you have it recoated you can have it tested to make sure you got what you wanted in regards to surface spec.

a friend just recently told me his story about antres optical and how they state they can gives all these different wave specs but what he got wasn't close to his desired spec, but then he knows how to test it with his own 1/20~ flat.

#20 robininni

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Posted 15 June 2013 - 10:11 PM

Well, since I was investigating mirror coatings and I do have that brand new 4.5" secondary just back from OMI coating chamber (along with the 25" primary) I decide to get unlazy and take it off the Obsession and do the same flashlight test to show that light does not shine through a good coating.

I got a little surprise in that it has lots of pin pricks where light DOES shine through. However, it is otherwise totally opaque to light. By the way, I'm not using some dinky flashlight either. I'm using a 630 Lumen LED Fenix TK30.

So here's the pic of the new secondary with said flashlight shining at its rear:
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For reference, here is the same treatment of the 4" secondary in question:
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I guess I now have to ask (you knew I would), should the brand new coating have the pin pricks of light shining through? Well, actually I already did ask the optician again, and he said this is normal for a new coating and as it wears, it starts to appear like the 4" secondary coating... or worse with more time and depending on what it has been put through.

So, I hope these posts will help others who may ask the same questions I have when discovering similar things with their primaries or secondaries. :)

Rob

#21 Pinbout

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Posted 15 June 2013 - 10:59 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.

#22 Vic Menard

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Posted 16 June 2013 - 08:13 AM

...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?

#23 robininni

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Posted 16 June 2013 - 10:05 AM

...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

#24 Howie Glatter

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Posted 16 June 2013 - 10:36 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.

#25 Vic Menard

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Posted 16 June 2013 - 10:41 AM

...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.

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...

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...

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? :foreheadslap:






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