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Equipment Discussions >> Reflectors

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robininni
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Reged: 04/18/11

Loc: Stephenville, TX
What's on this secondary?
      #5921118 - 06/14/13 07:25 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







Edited by robininni (06/14/13 07:29 PM)


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GlennLeDrew
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Re: What's on this secondary? new [Re: robininni]
      #5921134 - 06/14/13 07:38 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?

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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: What's on this secondary? new [Re: GlennLeDrew]
      #5921147 - 06/14/13 07:49 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

Edited by robininni (06/14/13 07:50 PM)


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: What's on this secondary? new [Re: robininni]
      #5922534 - 06/15/13 04:50 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


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Darren Drake
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Re: What's on this secondary? new [Re: robininni]
      #5922581 - 06/15/13 05:28 PM

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

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Howie Glatter
Vendor


Reged: 07/04/06

Re: What's on this secondary? new [Re: robininni]
      #5922593 - 06/15/13 05:38 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 ?


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Jason D
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Re: What's on this secondary? new [Re: robininni]
      #5922624 - 06/15/13 05:55 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


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: What's on this secondary? new [Re: Jason D]
      #5922648 - 06/15/13 06:09 PM

Quote:

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


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GlennLeDrew
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Re: What's on this secondary? new [Re: robininni]
      #5922710 - 06/15/13 07:04 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?


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: What's on this secondary? new [Re: GlennLeDrew]
      #5922827 - 06/15/13 08:43 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:


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


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GlennLeDrew
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Re: What's on this secondary? new [Re: robininni]
      #5922846 - 06/15/13 08:58 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...

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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: What's on this secondary? new [Re: GlennLeDrew]
      #5922858 - 06/15/13 09:10 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





Edited by robininni (06/15/13 09:15 PM)


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Jason D
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Re: What's on this secondary? new [Re: robininni]
      #5922909 - 06/15/13 09:54 PM

Quote:

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


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: What's on this secondary? new [Re: Jason D]
      #5922945 - 06/15/13 10:15 PM

Quote:

Quote:

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

Edited by robininni (06/15/13 10:27 PM)


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Pinbout
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Reged: 02/22/10

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Re: What's on this secondary? new [Re: robininni]
      #5922960 - 06/15/13 10:31 PM Attachment (16 downloads)

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.


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GlennLeDrew
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Re: What's on this secondary? new [Re: robininni]
      #5922964 - 06/15/13 10:35 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.

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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: What's on this secondary? new [Re: Pinbout]
      #5922975 - 06/15/13 10:42 PM

Quote:

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.




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


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okieav8rModerator
I'd rather be flying!
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Reged: 03/01/09

Loc: Oklahoma!
Re: What's on this secondary? new [Re: robininni]
      #5922989 - 06/15/13 10:52 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.


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Pinbout
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Re: What's on this secondary? new [Re: robininni]
      #5923000 - 06/15/13 10:59 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.


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: What's on this secondary? new [Re: Pinbout]
      #5923017 - 06/15/13 11:11 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:


For reference, here is the same treatment of the 4" secondary in question:


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

Edited by robininni (06/15/13 11:31 PM)


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Pinbout
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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
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
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
Vendor


Reged: 07/04/06

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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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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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
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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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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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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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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
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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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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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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
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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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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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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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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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Re: What's on this secondary? new [Re: howard929]
      #5925244 - 06/17/13 11:06 AM

Can anyone tell me just how many pinholes can be on a mirror before it is necessary for a recoat? I have a 10 inch mirror with several dozen pinholes that show from the back with a bright light. It is berol coated,about 8 to 10 years old,but rearly used. Thanks Bill

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Starman1
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Re: What's on this secondary? new [Re: howard929]
      #5925278 - 06/17/13 11:23 AM

Quote:

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...)



Well, actually, you have to use two-part epoxy to put them back in the milk jugs they came out of before the milk leaks out. Make sure, though, that the grain of the plastic lines up.


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robininni
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Re: What's on this secondary? new [Re: BillShort]
      #5925335 - 06/17/13 11:54 AM

Quote:

Can anyone tell me just how many pinholes can be on a mirror before it is necessary for a recoat? I have a 10 inch mirror with several dozen pinholes that show from the back with a bright light. It is berol coated,about 8 to 10 years old,but rearly used. Thanks Bill




That's a good question. Thanks for asking as I'm interested in the answer to. But I bet the answer is "it depends on how bad you think the view is degraded by the missing reflective material." Several dozen 'pin' holes probably doesn't qualify as bad as my new secondary, freshly coated, has a lot more than several dozen across its face and I was told this is normal.

You could post a picture of it for better clarity but you may be ridiculed for asking such a question as this forum isn't intended for that sort of learning which could be construed as questioning the optical quality of something. As Howard suggests, just use your scope to look at the heavens and don't ask questions.

Rob

Edited by robininni (06/17/13 11:56 AM)


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KerryR
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Re: What's on this secondary? new [Re: robininni]
      #5925494 - 06/17/13 01:01 PM

I'm not sure things like "spend some time enjoying your scope and watch out for falling prey to overly obsessive testing/worrying, particularly while developing observing skills" qualify as ridiculing. I thought the participants in your recent threads have been quite supportive and respectful, mostly intended to reduce unnecessary anxiety.

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robininni
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Re: What's on this secondary? new [Re: KerryR]
      #5925612 - 06/17/13 01:57 PM

Quote:

I'm not sure things like "spend some time enjoying your scope and watch out for falling prey to overly obsessive testing/worrying, particularly while developing observing skills" qualify as ridiculing. I thought the participants in your recent threads have been quite supportive and respectful, mostly intended to reduce unnecessary anxiety.




I agree, the advice you listed in quotes doesn't sound like ridicule to me either. But I guess you accidentally skipped over Howard's smart *BLEEP* post a few replies back? Read that one and I think you'll find the ridicule I mentioned.

Many of you have great scopes and optics and some of you don't. Perhaps most of you have not found reason to investigate any optical issue and therefore don't care much for the intricacies of the subject and would rather just use the telescope. Others, like me, either have had reason to investigate the subject further or are just plain interested in the details.

I am learning as much as I can about telescope optics as they pertain to me, mainly reflectors. I am learning the answers to the questions not given in the owners manuals. Some of the information is cut and dry, based upon objective measurements and characteristics (Strehl Ratios, Transverse error, mirror vs ambient temperature, center sticker placement). I find it interesting how this objective information is looked at subjectively by some therefore watering down its usefulness. However, there certainly is some subjective material with optics, based upon perception or a 'judgement call' because there isn't a good way to measure the issue (such as when to recoat optics--how many pinholes are too many, how much light should shine through the back, how can one easily measure how much light is reflected vs absorbed?).

I suspect others are on the same quest I am to really understand the characteristics of the instruments. It doesn't take years of observing to start learning this. Asking questions, reading, measuring things and making comparisons--this will facilitate learning along side observing. One thing that will not facilitate learning is not being critical of the subject.

I am currently reading Star Testing Astronomical Telescopes, 2nd Edition. So far it is very good and very enlightening. At the end of the foreword, Richard Berry states "If your telescope has properly adjusted and excellent optics, the star test will confirm that fact--and you are free to turn your attention to observing the splendors of the heavens." Sounds to me like he thinks it is pretty important to be focused and concerned about your optics and then, after all is well, observe the heavens.

Rob


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Re: What's on this secondary? new [Re: robininni]
      #5925663 - 06/17/13 02:21 PM

I didn't accidentally or deliberately skip it, I just didn't understand what he meant. I'm still not really sure. Even if that post was intended to be malicious, that's one post out of many, and certainly doesn't indicate that the general atmosphere on CN is one of ridicule.

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Re: What's on this secondary? new [Re: robininni]
      #5925673 - 06/17/13 02:30 PM

If I had to pin down a number for an acceptable number of pinholes in an aluminized coating, I might go with 10 per square inch (2 per square centimeter.) It would be worth knowing just what number of pinholes within the pupil start to become problematic...

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Re: What's on this secondary? new [Re: GlennLeDrew]
      #5925815 - 06/17/13 03:51 PM

What about residue from if the scope really dewed over, SCTs can get a weird residue if they dew over and there is enough dust or the right kind kind of dust.

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Re: What's on this secondary? new [Re: robininni]
      #5925824 - 06/17/13 03:56 PM

Quote:

...I am learning as much as I can about telescope optics...Some of the information is cut and dry, based upon objective measurements and characteristics (Strehl Ratios, Transverse error, mirror vs ambient temperature, center sticker placement).



Cut and dry?

Let's see--Strehl ratio and transverse error. How about a quote from Mike Lockwood, "Results from an interferometer are only as honest as the optician/supplier allows them to be."
Mirror vs. ambient? How close is close enough? What temperature differential is required to cause a change in correction? How significant are boundary layer effects relative to aperture? When will a smaller mirror outperform a larger mirror?
Center sticker placement? How can you tell if the mechanical center spot placement does not coincide with the optical center?

I don't think any of these are "cut and dry".

Quote:

...I find it interesting how this objective information is looked at subjectively by some therefore watering down its usefulness.



I disagree. While I think it's important to remember that the information is subjective, it is data and data can provide a useful metric. While the Strehl ratio of a mirror may vary from one manufacturer to the next, the number is useful when evaluating the individual manufacturers finished product (average, better, best). And while there's no argument with regard to managing temperature differentials, unless equilibrium is achieved, predicting image impact with single digit swings has so many variables that it's probably best to experiment and see what you get. And mechanical vs. optical centering is a lot less problematic if you keep the focal ratio above, here's a good guess, about f/5.

Quote:

...there certainly is some subjective material with optics, based upon perception or a 'judgement call' because there isn't a good way to measure the issue (such as when to recoat optics--how many pinholes are too many, how much light should shine through the back, how can one easily measure how much light is reflected vs absorbed?).



You forgot to include the effects of airborne contaminants and aging on mirror coatings. Or how about when it's safe to clean a new coating? Or how often you should clean your optics? Or when they should be replaced? And with what type of coating(s)?

Quote:

I suspect others are on the same quest I am to really understand the characteristics of the instruments. It doesn't take years of observing to start learning this. Asking questions, reading, measuring things and making comparisons--this will facilitate learning along side observing. One thing that will not facilitate learning is not being critical of the subject.



It's OK to ask questions. But when you're in the deep end, you should expect a variety of answers. You may even hear one that fits your own sensibility and personal understanding of the problem. But rest assured, it's probably not going to be the best answer for every situation or everyone's sensibility and understanding.

Quote:

I am currently reading Star Testing Astronomical Telescopes, 2nd Edition. So far it is very good and very enlightening. At the end of the foreword, Richard Berry states "If your telescope has properly adjusted and excellent optics, the star test will confirm that fact--and you are free to turn your attention to observing the splendors of the heavens."



It's on my bookshelf--I reference it often. I've enjoyed many hours of discussion with Suiter and Berry (mostly on collimation and its impact on image performance). At this point in your 24-inch Obsession exposé, I think Berry would have cautioned you to be wary of reading too much into anomalous/distorted star test results generated by a thermally unsettled 24-inch f/4 Newtonian in average (or even good) seeing. In fact, I think he would encourage you to, "...turn your attention to observing the splendors of the heavens". I'm not trying to ridicule or dismiss your concerns. But I think you'll gain valuable insight under the stars at the eyepiece.

Sorry for the long post.


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robininni
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Re: What's on this secondary? new [Re: Vic Menard]
      #5926121 - 06/17/13 06:44 PM

Okay, Vic, I want to do battle with you a bit with some of what you said .

Quote:

Quote:

...I am learning as much as I can about telescope optics...Some of the information is cut and dry, based upon objective measurements and characteristics (Strehl Ratios, Transverse error, mirror vs ambient temperature, center sticker placement).



Cut and dry?

Let's see--Strehl ratio and transverse error. How about a quote from Mike Lockwood, "Results from an interferometer are only as honest as the optician/supplier allows them to be."
Mirror vs. ambient? How close is close enough? What temperature differential is required to cause a change in correction? How significant are boundary layer effects relative to aperture? When will a smaller mirror outperform a larger mirror?
Center sticker placement? How can you tell if the mechanical center spot placement does not coincide with the optical center?

I don't think any of these are "cut and dry".




Well, the mirror values are objective. They are cut and dry, but I suppose the amount of value you should place on them depends on who did the measuring. I'm sure some opticians would take offense to you suggesting their measurements are subjective and not cut and dry--the honest ones anyway.

Mirror vs ambient? I was told with 2 degrees Fahrenheit was close enough. If the mirror is then close enough to ambient temperature, boundary layers don't form causing issues. And of equal quality, a smaller mirror never outperforms a larger mirror.


Quote:

While I think it's important to remember that the information is subjective, it is data and data can provide a useful metric. While the Strehl ratio of a mirror may vary from one manufacturer to the next, the number is useful when evaluating the individual manufacturers finished product (average, better, best).




No. Measured data by definition cannot be subjective. What you do with the data can be subjective. The Strehl of a mirror is the strehl of that mirror (given a reasonable margin or error). There is only one accurate Strehl ratio. Whether all manufacturers are equally adept at measuring and calculating it doesn't change this fact. If you passed the same mirror from optician to optician and had them test it, you would find a repeating close range and then some outliers. The outliers are wrong. I don't know what an acceptable margin of error is, but I bet any good optician could tell us. The Strehl ratio is not only useful to the individual manufacturer if he or she does accurate testing following accepted principles.


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...there certainly is some subjective material with optics, based upon perception or a 'judgement call' because there isn't a good way to measure the issue (such as when to recoat optics--how many pinholes are too many, how much light should shine through the back, how can one easily measure how much light is reflected vs absorbed?).




You forgot to include the effects of airborne contaminants and aging on mirror coatings. Or how about when it's safe to clean a new coating? Or how often you should clean your optics? Or when they should be replaced? And with what type of coating(s)?




When it is safe to clean you new coatings should not be subjective. The coater should be able to tell us this. As, I have recently learned, when to *recoat* them is subjective depending on how much image degradation you are okay with. I would just pic your favorite optician or telescope manufacturer and go with what they say is their guideline.

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I suspect others are on the same quest I am to really understand the characteristics of the instruments. It doesn't take years of observing to start learning this. Asking questions, reading, measuring things and making comparisons--this will facilitate learning along side observing. One thing that will not facilitate learning is not being critical of the subject.



It's OK to ask questions. But when you're in the deep end, you should expect a variety of answers. You may even hear one that fits your own sensibility and personal understanding of the problem. But rest assured, it's probably not going to be the best answer for every situation or everyone's sensibility and understanding.




This makes sense .

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I am currently reading Star Testing Astronomical Telescopes, 2nd Edition. So far it is very good and very enlightening. At the end of the foreword, Richard Berry states "If your telescope has properly adjusted and excellent optics, the star test will confirm that fact--and you are free to turn your attention to observing the splendors of the heavens."



It's on my bookshelf--I reference it often. I've enjoyed many hours of discussion with Suiter and Berry (mostly on collimation and its impact on image performance). At this point in your 24-inch Obsession exposé, I think Berry would have cautioned you to be wary of reading too much into anomalous/distorted star test results generated by a thermally unsettled 24-inch f/4 Newtonian in average (or even good) seeing. In fact, I think he would encourage you to, "...turn your attention to observing the splendors of the heavens". I'm not trying to ridicule or dismiss your concerns. But I think you'll gain valuable insight under the stars at the eyepiece.




How do you know my scope has been thermally unsettled for all star tests? I've had it at 1-2 degrees from ambient temperature the last few nights I've used it after I changed my cool-down routine. I've had similar star test results, slightly easier to read on the inside of focus, since doing so. Your statement exemplifies something I see which I have come to dislike on CN. Experienced (in your case well respected as well) people making statements put forth as a matter of fact which actually contain assumptions (in your case, a critical one, that my mirror was not equilibrated well) but without acknowledging such premises. These statement seem to be directed at those perceived as inexperienced. Is it impossible that I have learned how to cool my big mirror down?



Rob

Edited by robininni (06/17/13 07:21 PM)


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Vic Menard
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Re: What's on this secondary? new [Re: robininni]
      #5926249 - 06/17/13 08:03 PM

Quote:

...Well, the mirror values are objective.



I humbly disagree. I think you would be surprised how they are derived and how much they vary from one optician to another. I suspect this is the reason some of the best refuse to put a number on their finished product.

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Mirror vs ambient? I was told with 2 degrees Fahrenheit was close enough. If the mirror is then close enough to ambient temperature, boundary layers don't form causing issues. And of equal quality, a smaller mirror never outperforms a larger mirror.



The number of texts describing the efforts to scrub boundary layers and rapidly equilibrate and track moving temperatures in these forums alone is enough to fill a text. And that's just dealing with thermals, not correction. There's much left to be written.

Similarly, a smaller, more responsive aperture may indeed outperform a larger aperture struggling with rapid temperature changes. As I mentioned earlier, more resolving power makes larger apertures better detectors of thermal instability.

Quote:

...No. Measured data by definition cannot be subjective.



Once again, it depends on how the data is derived, how it's being measured, how many data points are being used, how the "outlier"s are handled, etc.

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...The Strehl of a mirror is the Strehl of that mirror (given a reasonable margin or error).



Even Zambuto would tell you it's fluid, and although he doesn't use an interferometer to quantify his work, Wolfgang Rohr (considered by some an expert with an interferometer) has found his work to be better than most, while showing wild disagreement with IF certified mirrors sent for evaluation. Changing the way a large, thin mirror is supported (for example, changing the altitude of the telescope), measurably changes the mirror surface--before cable slings and better edge supports, as much as a wave deformation!

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...I don't know what an acceptable margin of error is, but I bet any good optician could tell us.



I bet most "good" opticians are reticent to have their work evaluated by someone else unless they have thoroughly vetted the tester, the equipment, the method, and the reliability. Even then, I think they would prefer having a "ringside seat" to make sure their metric is either supported, or closely in line with the results of an outside testing facility. If you're looking for a "bureau of standards" reference for mirror testing reliability, you're probably not going to be able to afford it. Similarly, optical flat testing can be performed with a reference sphere or a reference flat, which even then, is dependent on the reference accuracy and the number of data points, and here we go again.

Don't get me wrong, there's a technological shift occurring with relatively inexpensive IFs (there's a thread in the ATM forum right now). All that data and number crunching can be a good thing or a bad thing...

Quote:

...When it is safe to clean you new coatings should not be subjective. The coater should be able to tell us this. As, I have recently learned, when to *recoat* them is subjective depending on how much image degradation you are okay with. I would just pic your favorite optician or telescope manufacturer and go with what they say is their guideline.



I have certain "rules of thumb" I adhere to, but, it's all subjective and dependent on many outside factors.

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...How do you know my scope has been thermally unsettled for all star tests?



I didn't say "for all star tests", but you described your first tests that way in this thread, IIRC.

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I've had it at 1-2 degrees from ambient temperature the last few nights I've used it after I changed my cool-down routine. I've had similar star test results, slightly easier to read on the inside of focus, since doing so.



Perhaps it's just atmospheric turbulence. As I mentioned in an earlier post, the scintillation you've described in your drawings is indicative of this.

Quote:

Your statement exemplifies what I have come to detest on CN. Experienced (in your case well respected as well) people making statements put forth as a matter of fact which actually contain assumptions...



When I make a comment based on an assumption, I usually note that it is, indeed an assumption. But when someone calls me out and says they "detest" my commentary, I do my best not to take it personally. I have tried to offer my insight and I've tried to be supportive of your journey. If there was a way I could be there to help, I would very much like that.

Quote:

...Is it impossible that I have learned how to cool my big mirror down?



I don't know. When my 22 is reasonably stable and seeing is as perfect as I can hope for, Sirius at 550X with full aperture is a brilliant pinpoint with steady gray ribbons defining the diffraction pattern (not a flicker). On one occasion I observed the Pup embedded in one of these ribbons, with a dark minima around it. But I'm sorry to tell you that I don't know what the temperature differential was, or just how good the seeing actually was. It was an exceptionally steady night and a serendipitous alignment. I've had similar observing moments over the past several years (sparkling stars embedded in M82 and NGC4449, surface detail on Mars during the last opposition, following Porimma through periastron, both components of the Twin Lensing Quasar, etc.). Those observations are the data points I use to define my scope's performance.


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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: robininni]
      #5926292 - 06/17/13 08:32 PM

Okay, Vic, thanks for hanging in with me. I appreciate you. I also changed the 'strong' word detest to dislike, but you were too quick on the draw and quote-replied before I submitted my change.

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Vic Menard
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Reged: 07/21/04

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Re: What's on this secondary? new [Re: robininni]
      #5926339 - 06/17/13 08:56 PM

Rob,
I do appreciate your dilemma.
Hopefully, your solution will provide a clear path from this unfortunate situation.
As Tippy D'Auria would say, "Soon it will be time, to commit astronomy!"
And then there's Plato, "He was a wise man who invented beer."

My best,

Vic Menard


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Joe G
scholastic sledgehammer


Reged: 01/10/07

Loc: Southern California
Re: What's on this secondary? new [Re: Vic Menard]
      #5926440 - 06/17/13 10:02 PM

Rob,

I have been following your various threads from the beginning. As many have pointed out, there is not always a "perfect" answer to your questions. With all dew (astro joke) respect, please consider that you are getting many answers from some of the most respected amateur/professional telescope gurus in this field. Vic, Jason, Howie, Mike, Don, etc have posted literally thousands of helpful replies for many of us asking similar questions over and over.

As you are finding out, there are a myriad of factors that affect telescopes and their performance in the real world. As many have suggested enjoy your telescope(s) and keep asking questions. But lighten up a little. The folks responding in this thread (and your many others) mean no harm and have been very helpful in ramping you up the learning curve. Imagine what your learning curve would be like if there was no Cloudy Nights?

Also, there have been relatively few vendors that have not been given a hard time on CNs. There is no plot to protect any vendor at the expense of sharing knowledge, both good and bad. Personally, I believe that the majority of the folks responding in this thread are highly objective. Learn from them.

Three nights ago, I set up my 16 inch reflector for some friends on a weekend get away. I wanted to show them Saturn. My mirror is a refigured 16 inch Lightbridge. It performed much better after the refigure (based upon a side-by-side comparison with my 12 inch Orion reflector). Much to my surprise, Saturn looked okay. No one complained about it. But stars were oblong rectangles. Of course I was frustrated and tweaked the collimation with Catseye tools, my Glatter laser, etc. Nothing changed. I had a fan blowing on the mirror for hours.

The next day I loosened the mirror clips. Voila, later that evening it made a major difference. Without CNs, I would never have known what to do or what to expect. I would not have known where or how to get my mirror refigured or recoated. I would not have known that a Glatter laser is a huge improvement over an Orion laser. I would not have purchased Vic's fine book on collimation. Nor seen Jason's many descriptions and charts and how he improved the Catseye autocollimator.

Keep asking questions. But remember these people are just trying to help.

Joe


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okieav8rModerator
I'd rather be flying!
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Re: What's on this secondary? new [Re: Joe G]
      #5926539 - 06/17/13 11:01 PM

Quote:

Rob,

Keep asking questions. But remember these people are just trying to help.

Joe




Also remember, that at some point, you just have to let go and have fun with it. After all, it was in interest in astronomy that got you into this endeavor in the first place.


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frito
Pooh-Bah


Reged: 10/05/12

Loc: Fremont, CA
Re: What's on this secondary? new [Re: robininni]
      #5926596 - 06/17/13 11:23 PM

man you don't want to know about the pinholes you see in coatings on mirrors when you do this to them.

its normal, carry on enjoy the views.

optics are never perfect, glass is never perfect. we live in an imperfect world, scrutinizing things to this level will just drive you crazy.


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frito
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Reged: 10/05/12

Loc: Fremont, CA
Re: What's on this secondary? new [Re: frito]
      #5926630 - 06/17/13 11:40 PM

also Rob, i work in the glass industry.

specifically i run a ceramic ink jet printer that prints directly onto glass, we have the same problem with ceramic inks, a substance that is compatible with glass as its the same material and it is fused into the glass surface via heating the glass to high temperature via our tempering furnace. its extremely rare that i get a print on even small sample sized glass (12" x 12") that is completely free of pinholes.

the coating process used to put aluminum and many other materials onto the surface of glass can and will suffer the same problems if not worse ones than our process simply because those coating processes are not actually putting anything into the glass surface, they are simply layering on top of the surface via vacuum deposition

see http://en.wikipedia.org/wiki/Sputter_deposition for more info.

this process allows the coating of otherwise incompatible materials but the problem with it is durability. its durability is very low compared to a process that is fused into the glass like say our ceramic inks or pyrolytic process used in hard coat low-e's and other coating types like reflective coatings on architectural flat glass.

http://www.pilkington.com/north-america/usa/english/building+products/trade/t...

if it helps you at all to know this, minor surface defects like these effect the image extremely little, in total its probably a fraction of a percentage of light lost due to it and i would wager that one could search the world and never find a aluminum coated mirror that is free of a defect. its not worth loosing sleep over. just like in your brand new multi-thousand dollar heavy glass shower door you just bought has tons of surface defects known as pitts from the tempering process they must go through for safety requirements that the average person will never even see but if one is as picky as you are they will see them and raise holy hell over it and it has no real impact on anything ever.


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Vic Menard
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Re: What's on this secondary? new [Re: frito]
      #5927184 - 06/18/13 10:30 AM

The advice that Rob was/is seeking was not unsolicited, and while some responses may have seemed unwarranted, in retrospect, his enthusiastic curiosity (and subsequent angst) is both genuine and "normal" considering the step up in aperture (10 to 25) and the unforeseen detours along the way.

Many of us have encouraged Rob to get in some serious observing time to get a better idea of his scope's performance. (Sometimes, not "knowing" can make a big difference, but in this case, Pandora's box has already been opened.)

Tom Clark is a good friend and an old observing buddy from Chiefland, Florida (he's since moved to New Mexico for even darker skies). I often describe Tom's observing method as "brute force" astronomy. His 42-inch f/4 GoTo Dobsonian is the largest scope I've looked through, and for the 10 years he had the scope in Chiefland, he invited an awful lot of folks to climb the movable stairwell to the eyepiece for the brilliant views of many deep sky "showcase" objects. For reference, "low" power (31mm Nagler and Paracorr = about 7mm exit pupil) on this scope is 158X (about 0.5-degree tfov). IIRC, Tom's pretty fond of his new ES20 (245X at 0.4-degree tfov--still less than 6X per inch of aperture). On their way to New Mexico, his 42-inch mirror and 7-inch m.a. secondary spent some time at Mike Lockwood's shop. The story is here.

This isn't intended to admonish Rob for expecting to receive what he thought he purchased! While Tom was pretty sure his optics weren't Mike Lockwood class, I think if he had known how bad they really were it could have spoiled those 10 years of observing fun in Chiefland.

Of course, now Tom's optics are Mike Lockwood class, and soon, the comparisons will begin. But the optics have a new home now too, so will Mike get all the credit, or will Tom attribute part of the performance to New Mexico skies? I know--we'll ask Jeannie!


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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: What's on this secondary? new [Re: Vic Menard]
      #5928617 - 06/19/13 02:15 AM

Quote:

This isn't intended to admonish Rob for expecting to receive what he thought he purchased! While Tom was pretty sure his optics weren't Mike Lockwood class, I think if he had known how bad they really were it could have spoiled those 10 years of observing fun in Chiefland.




Maybe if he had done what Rob is doing he would have had the optics refigured ~ 10 years ago and would have enjoyed his scope even more over that time?

Dave


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Vic Menard
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Reged: 07/21/04

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Re: What's on this secondary? new [Re: dave brock]
      #5928828 - 06/19/13 09:00 AM

Quote:

...Maybe if he had done what Rob is doing he would have had the optics refigured ~ 10 years ago and would have enjoyed his scope even more over that time?



I'm not so sure it was possible back then--42-inch aperture optical work had limited takers--and that's not even considering pickup and delivery!

But as I commented above, let the comparisons begin...


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davidpitre
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Re: What's on this secondary? new [Re: dave brock]
      #5929712 - 06/19/13 05:15 PM

Quote:



Maybe if he had done what Rob is doing he would have had the optics refigured ~ 10 years ago and would have enjoyed his scope even more over that time?

Dave



I'm guessing he would not have enjoyed his time spent any more than he did. I have a couple of Zambuto mirror scopes and a couple of so-so Chinese mirrors and some in between (Russian, Japanese). I generally enjoy my time with my lesser quality scopes just as much as the top notch.


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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: What's on this secondary? new [Re: davidpitre]
      #5930481 - 06/20/13 01:09 AM

Quote:

I generally enjoy my time with my lesser quality scopes just as much as the top notch.




So if for some reason you had to sell one of them, you'd toss a coin?

Dave


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jpcannavo
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Reged: 02/21/05

Loc: Ex NYCer, Now in Denver CO!
Re: What's on this secondary? new [Re: dave brock]
      #5930662 - 06/20/13 06:32 AM

Rob
There is one more point worth making here. I understand that you are looking for some sort reasonably definitive input/data/content to judge the quality of said coating. Quite understandable. But part of the problem here is that while we bench test, star test - and "interferometize" mirrors to get analogous handles on their figures, we don't routinely make use of a parallel methodology to get a handle on the spectral reflectance of their coatings. As such, we judge by a combination of hard won experience, inference and leap of faith. There are however methods of direct measurement of "specular reflectance" out there: reflectance

Now to be clear, I am not saying that this is what you, I or anyone should/should not be looking into. I am just offering an observation about the state of the art. BTW, if you want to open another can of worms (that I myself have spent too much time with) start wondering about how much scattering there is from a coating. BTW, and as you likely know, the harder it is to see the laser on the secondary, the less light (ceteris paribus) it is scattering.


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