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Spray on Silver Coating/Overcoating (Oregon Scope Werks method)

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#1 Alan Brunelle

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Posted 22 June 2021 - 12:16 AM

I was curious if anyone has tried the spray method of silver coating on a dielectric coated mirror.  Meaning doing so without the removal of the dielectric coating.  I am interested in a scope that comes only with a dielectric coated mirror and this is not usable for me.  I understand that dielectric coatings cannot be removed without grinding off the coating and re-polishing, which means re-figuring, which likely means ruining the scope.

 

A mirror coating service, which does electro deposition of metal coatings states that they would be worried that their method of coating would have adhesion issues if overcoated onto dielectrics.  However the true deposition of silver via the chemical method is a completely different process.  Also, since the method onto bare glass has always been assumed to be a no-clean, no touch surface, then maybe adhesion is only an issue of expectation of the commercial guys.  I am thinking that the surface of the dielectric is basically a glass of sorts, right?  Just consisting of elements different than silicon dioxide (though I do understand that some layers can be silicon dioxide).  Other than if this can in principal work, i.e. there is not chemical inhibition from the dielectrics that would prevent the reduction and deposition of the silver, is the dielectric layer not smooth enough?  That the laying down of dielectrics may add a bit of defect that each time slowly builds up deformities that are not an issue with the dielectric alone, but would serve as a less than perfectly smooth layer to which one would want to then layer on silver?  Confused yet?

 

So back to the question at hand, anyone try it?  If it worked, I would stop worrying about a lot of things and buy the new scope, then just coat the **** thing.  Actually if I left the dielectric layer on and failed at to get good silvering, I would easily be able to remove the silver and get back to the original dielectric coated mirror.  Then I would be left with an expensive telescope that satisfied only a fraction of what I have planned for it!

 

I do not know if any of the Oregon Scope Werks people still frequent CN, but it would be great to talk with them.  I live in Oregon, so you would think that it would be simple to reach them through the one of the local Oregon Astronomy Clubs such as Eugene, but with Covid, there have been no meetings.  I went to one just before things shut down...

 

Thanks,

Alan



#2 astrokeith

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Posted 22 June 2021 - 03:46 AM

There may or may not be a problem.

 

dielectric coatings are now commonly used as 'overcoats' on what is marketed as 'enhanced aluminium'. They are relatively easy and routinely removed.

 

The really tough one was chromium which was or is sometimes used as a base coat.



#3 Oregon-raybender

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Posted 22 June 2021 - 03:26 PM

The sprayed silver coating requires a bare glass surface to allow the coating to take, it has to be very clean. A dielectric coating is a series of stacked layers of materials to get the designed result. It may worth a try, but you need to clean the surface with chalk first to remove oils and such to the mono layer. So in that process it may degrade the dielectric coating to cause sleeks or other types of defects that will appear in the silver coating. Think of the glass surface as one layer, super clean, were as the dielectric as many layers like a cake with textures. I have silver coated hundreds of optics as an optician for testing.

 

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#4 Steve Dodds

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Posted 22 June 2021 - 03:50 PM

I have had good luck overcoating dielectric coated surfaces with aluminum.  Silver should work too. 



#5 Oregon-raybender

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Posted 22 June 2021 - 04:24 PM

True Steve. I have coated AL on MgF2, but spray silver is a water based process, wet. Coated AL is just metal in vacuum. The spray silver with not stick, it may just run off, like feathers on a duck. It's the structural layers of the dielectric that will interfere with it. It (spray) silver requires a very clean single surface. But I always say, it's worth a try and see what happens, that is what we do, run a test. Here is a paper on silver (vacuum) coating.

 

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https://www.spiedigi...4172.full?SSO=1


Edited by Oregon-raybender, 22 June 2021 - 04:25 PM.

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#6 Alan Brunelle

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Posted 22 June 2021 - 04:42 PM

I was writing my response when raybender sent an additional reply.  So I will add my response to the latest after my first:

 

Thanks Steve.  Those are very hopeful words.

 

Keith, yes it sounds like the enhanced and protected aluminum surfaces are routinely used for stripping and recoating in the amateur world.  One or the other is a bit more resistant to the stripping agent, but with a little extra time, either is fully strippable.  The devil is in the details with the mirrors I want to work with.  And that comes down to the manufacturer and the vendors' offering little confidence in their statements of fact on the optics they are selling.  All scopes are said to come from one manufacturer, are marketed by at least three different vendors (and when you see the scope they all look identical or nearly so).  Most vendors state that the optics are dielectric coated with very high reflectivity.  Higher than expected for typical enhanced aluminum, so it appears that these statements align.  If so, it is my understanding that these are essentially impossible to strip by simple chemical means.  They are supposed to be very tough to mechanical handling, so the hope would be that the surface would be consistently the same layer and resistant to the necessary cleaning that raybender rightly brings up.  The vendors' of this optic will not give out the reflectance profile of primary or secondary and state that the manufacturer has refused to do so over the decade or so of requests made to them.  If the optics are truely pure dielectric, I would have serious doubts that these reflect well in the region of the NIR that I want to use this for.  Other reputable sellers clearly state that the dielectric coatings they offer are for visible and poor in the NIR and IR.  Without a spectrometric profile, I am not willing to plop down several grand for a wish.  If their are misstating what their coating is and it really is just an enhanced aluminum (i.e. with some dielectric layers for that purpose) then that is a different matter.  Reason being is that enhanced aluminum is rather decent in the NIR and IR.  But there is little trust and I don't have a reflective spectrometer!  One vendor states that they do use the enhanced aluminum for their version, but it looks like it comes from the same manufacturer in China and I have not seen a reflectance profile to prove it.

 

So the potential pitfalls of doing this has me a bit scared, hence I am relying on the hope that someone who has done this can offer me real evidence of likely success.  So Steve's comments are very welcome.  While it seems crazy to think someone would want to do this, I am very aware that amateur astronomers get their hands on old optics all the time and re-purpose them.  This, I assume would be dielectric coated primaries, though these may be much less common.  If someone were to get their hands on a dielectric primary knowing full well that the coating is harder than the glass itself, why would one not try to just coat it if it met criteria, such as surface smoothness.  Certainly if it were scratched, or the reflectivity were mottle, then the coating itself would be suspect.  So I am hoping that people out there who have done it will be willing to speak up, such as Steve.  Also, anyone who attempted it and failed!  And the reason they failed.

 

To Raybender's latest:  I am a biochemist by training and so not completely a novice to such things.  I do respect your saying that it is worth a try.  But not sure the several thousand bucks will make me feel like it was worth it if it fails!  Hence I need a ray of light.  BTW, I am certainly open to vacuum deposition coatings and have inquiries into a couples coaters on the topic.  

 

I do have to question your reasoning that the spray on silver will not stick.  Unless you are aware that any of the materials used in any of the layers is repellant to the silver upon deposition.  I do know that the method uses an activator treatment prior to the spraying of silver and reducer solutions, but you haven't made the chemistry clear to me.  Given a pristine hard dielectic, I would hope that even with cleaning, using the recommended cleaners (talc, etc.) would leave the original exposed coat as a continuous layer upon which the silver will lay.  I don't see how the many layers deposited below will matter.  The talc recommended is stated to be too soft to etch or alter glass when used in the normal process.  How then could it alter that layer of a harder dielectric? I am questioning only because I want to know.  It is also my understanding that the spray on method is directly derived from a method used to gild any number of different surfaces with silver.  So again, I would like to hear from those who may have tried this and succeeded and for sure, those who failed and why.

 

As I said, I am certainly open to vacuum deposition, assuming I can find someone who would try doing it over the dielectric.  For my purposes, silver is the tops in reflectivity throughout the range I am interested in.  I could live with spray coating silver once a year if need be.  But enhanced Al would be fine and save me a day or so and lots of ugly collimation!  


Edited by Alan Brunelle, 22 June 2021 - 04:47 PM.


#7 Alan Brunelle

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Posted 22 June 2021 - 04:56 PM

I would like to add that vacuum deposition coatings can be made with Silicon dioxide overcoats for various reasons.  Well pure silicon dioxide is basically quartz glass and I know that quartz optics can take all the coatings we are discussing. 

 

I am not sure why one laser optics company was concerned when they told me they would be worried about a vacuum deposited metal layer would adhere to a dielectrically coated mirror.  Reason being is if they were, then the logical thing to do first would be to vacuum deposit a silicon dioxide layer and the the metal on top of that.  Is my logic flawed?

 

What I am sure of, is that human nature is to mostly always do only that which you know how to do.  And my guess is that is where a company such as this stands.  Also, they would rather not try and fail for a paying customer.  But I promise that if I do go down this path uncharted, I will certainly post what if find out.


Edited by Alan Brunelle, 22 June 2021 - 04:58 PM.


#8 Steve Dodds

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Posted 22 June 2021 - 05:12 PM

I would like to add that vacuum deposition coatings can be made with Silicon dioxide overcoats for various reasons.  Well pure silicon dioxide is basically quartz glass and I know that quartz optics can take all the coatings we are discussing. 

 

I am not sure why one laser optics company was concerned when they told me they would be worried about a vacuum deposited metal layer would adhere to a dielectrically coated mirror.  Reason being is if they were, then the logical thing to do first would be to vacuum deposit a silicon dioxide layer and the the metal on top of that.  Is my logic flawed?

 

What I am sure of, is that human nature is to mostly always do only that which you know how to do.  And my guess is that is where a company such as this stands.  Also, they would rather not try and fail for a paying customer.  But I promise that if I do go down this path uncharted, I will certainly post what if find out.

Most dielectric coatings have at least one layer of SiO2, and all layers act as glass.


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#9 Alan Brunelle

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Posted 22 June 2021 - 10:30 PM

Most dielectric coatings have at least one layer of SiO2, and all layers act as glass.

Thanks Steve,

 

I think I will be moving forward.  Nothing ventured, nothing gained...  It may take some months, given the dearth of inventory right now.  If and when I resolve some of these issues, I will update on this thread.

 

Also, thanks to Ray-bender and Keith for your helpful input.



#10 hbanich

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Posted 23 June 2021 - 01:38 AM

If the spray silver coating doesn't fully adhere it would be easily removed with Angel Gilding silver remover, which only removes silver. I agree with Oregon-Raybender - it's worth a try.



#11 Oregon-raybender

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Posted 23 June 2021 - 11:07 AM

Glad to be of help. I been the optical industry for 50 years (retired, some what)  Worked in R/D for 18 years, the rest in

production (Optician) and QA management. Coming from my knowledge working for a world class coating company in the R/D department. I always was willing to give a idea a chance ( out of the lab or box thinking). I only offered my concern based on the the two different processes, one in a vacuum and the spray wet. When you look at a dielectric coating under a electron microscope, the layering is of stacked layers with underlining valleys and pits, these do not "fill in" with each layer, in fact they grow. (there has been improvement, but..) The last layer, which you will be spraying on to, maybe have voids that are not visible, the main reason why it may not stick, or adhere, the appearance may look like a underpolished surface (this also depends on the coating design and coater process, no way of telling) A fresh glass surface (bare glass) is a single layer minus these voids. I have direct experience with overcoating with AL in a vacuum. But spray silver I do not on a dielectric or AR coat. Had no reason to.  I used my knowledge of precision optical coatings to issue a caution of what the result maybe. Coming from a R/D direction, I feel as long as the test is performed under standard norms, IE. following the normal process for spray silver, cleaning and using the correct chemicals it's worth a try. I would suggest, maybe trying it on a small sample first, without running the risk of the larger optic.

 

Good Luck and let us know the result.

 

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#12 Alan Brunelle

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Posted 23 June 2021 - 12:56 PM

Glad to be of help. I been the optical industry for 50 years (retired, some what)  Worked in R/D for 18 years, the rest in

production (Optician) and QA management. Coming from my knowledge working for a world class coating company in the R/D department. I always was willing to give a idea a chance ( out of the lab or box thinking). I only offered my concern based on the the two different processes, one in a vacuum and the spray wet. When you look at a dielectric coating under a electron microscope, the layering is of stacked layers with underlining valleys and pits, these do not "fill in" with each layer, in fact they grow. (there has been improvement, but..) The last layer, which you will be spraying on to, maybe have voids that are not visible, the main reason why it may not stick, or adhere, the appearance may look like a underpolished surface (this also depends on the coating design and coater process, no way of telling) A fresh glass surface (bare glass) is a single layer minus these voids. I have direct experience with overcoating with AL in a vacuum. But spray silver I do not on a dielectric or AR coat. Had no reason to.  I used my knowledge of precision optical coatings to issue a caution of what the result maybe. Coming from a R/D direction, I feel as long as the test is performed under standard norms, IE. following the normal process for spray silver, cleaning and using the correct chemicals it's worth a try. I would suggest, maybe trying it on a small sample first, without running the risk of the larger optic.

 

Good Luck and let us know the result.

 

Starry Nightswaytogo.gif

This is very useful information.  As a scientist myself, I do like details in the reasoning.  Actually, one of my biggest concerns about overcoating had to do with the compounding effect of defect accumulation, both within the dielectric coating and then any subsequent layering on top.  I am not surprised that none of these coating methods is defect free.  I have to assume that the defects in the many layers in the dielectric coating ultimately fall below a size that appreciably impacts the performance of the coating.  These would seem to be both the ability of the layering, as designed to reflect a certain spectral range, and the possibility that the defects are large enough to just create a general light scattering, similar to what pitting might do.  Given the state of the art, I assume that the defects present in a modern coating are too small to impact the image to a high performance.  I am talking just about the an given commercial dielectric coating.  Am I correct?  (I also assume that different coaters have widely variable quality, just like any other industry!)  (As an edit, I later see that you already answered this, but will leave it here anyway:  When laying down dielectric layers, is it typical that a defect, such as a pit, get preserved and amplified, or is there some "healing" that goes on.  I.e. can the next layer actually fill in the defect to some extent.)  If the effect is always only negative, then I would assume that increasing layers would always be a diminishing return on investment in the process.  But, one scope I am looking to acquire clearly states full dielectric coating, at 99% reflectivity.  Question I have is do the reflected photons go where they are supposed too!

 

My understanding is that the goal of polishing any glass surface with abrasives, such as polishing rouge, is not to create a perfect surface, but that the gouge lines cut by the polishing agent are smaller than the wavelength of the light expected to be imaged.  And that the totality of the peaks and valleys from the polishing create a surface that is smooth, again, relative to the wavelengths to be imaged.

 

I really want this scope to work as a general imaging instrument within the visible, so in that regard I really do not want to create a defective coating relative to visible light.  But my current thinking is that I may mostly use it for NIR imaging very close to 1 micron wavelength, and in that, the requirements may not be as stringent.

 

I would prefer a more robust coating as a final solution for this scope, so will likely be looking to enhanced aluminum at some point.  But from what I see, the spray on silvering method uses non-aggressive chemicals, either in coating or if need be in stripping.  So I think it will be the safest way to learn if overcoating can work without destroying the dielectric coating.  If that works, then it may justify sending it off to a vacuum coater.  For the desired broad band from vis through 2 micron, I sure wish that silver could be made more durable as a vacuum coat.  So that may have to wait for when I move from Oregon to NM.

 

I very much appreciate the time you have given me to share your expertise and advice.  I have a line on a telescope and if that plays out, then maybe by this fall I can report back on my experience with this.

 

Clear Skies!

Alan 


Edited by Alan Brunelle, 23 June 2021 - 01:01 PM.


#13 Bob4BVM

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Posted 23 June 2021 - 01:43 PM

Hello Alan,

Interesting discussion here !

I have to ask, have you looked at the body of work done by Howard and his group at OSW up in Ptld ?

Their papers are literally indispensable to anyone considering the spray-silvering process. They have documented each step of the process over their years of work, including all successes and failures.

 

I will (hopefully soon) be silvering the optics for my binoscope and following their extensive advice.

 

I am interested in how your experiments turn out, please keep us posted.  BTW, I am nearby, east of Albany.

 

CS

Bob


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#14 Alan Brunelle

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Posted 23 June 2021 - 04:00 PM

Hello Alan,

Interesting discussion here !

I have to ask, have you looked at the body of work done by Howard and his group at OSW up in Ptld ?

Their papers are literally indispensable to anyone considering the spray-silvering process. They have documented each step of the process over their years of work, including all successes and failures.

 

I will (hopefully soon) be silvering the optics for my binoscope and following their extensive advice.

 

I am interested in how your experiments turn out, please keep us posted.  BTW, I am nearby, east of Albany.

 

CS

Bob

Hi Bob, I can say that I have read a lot about the technique that they wrote up.  I also saw some of their stuff that included failures, reasons, etc.  But not sure I have seen all of the work.  I actually was hoping someone from that group would have been able to answer some of the questions, in particular the ones with direct experience spray coating over other surfaces such as dielectrics.  I know it is a long shot for obvious reasons.  But as an informal group, there was no way to contact them and local Astro groups are not meeting, so no way to ask around.  As you can tell, I am profoundly unconnected to the community!

 

Is the complete work available in a single simple location on the net?  In other words, I know what I have seen, but can't know what I may have missed.

 

Best of luck with the Binoscope!  I would love to use one at some point.  I have heard that bringing two eyes into the observing action greatly improves the experience.  I was at a Eugene meeting of the Astronomical Society (my first) and someone there showed off a bino that they were designing for commercial sale, eventually.  It was nice.  Then COVID shut is all down.  I would hope that things should open up soon.  Will this be your first silvering using the spray method?  Will you use the tarnish shield product?  I would like to know what your results are and the lifetime of the coating in our area.

 

I will keep you posted on my project.  Just need to get some optics in hand and get it set up.

 

CS

Alan


Edited by Alan Brunelle, 23 June 2021 - 04:01 PM.


#15 Dale Eason

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Posted 23 June 2021 - 06:42 PM

I don't think any of that group has done silvering over anything but glass.

 

Here is their web site https://sites.google...opemirrors/home


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#16 Alan Brunelle

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Posted 23 June 2021 - 06:47 PM

I don't think any of that group has done silvering over anything but glass.

 

Here is their web site https://sites.google...opemirrors/home

Thanks Dale.  That is what I thought.  Again, not surprising.



#17 Steve Dodds

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Posted 23 June 2021 - 07:01 PM

I don't think any of that group has done silvering over anything but glass.

 

Here is their web site https://sites.google...opemirrors/home

The Angel Guilding people have been coating ceramics and plastic.



#18 Bob4BVM

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Posted 23 June 2021 - 11:16 PM

Hi Bob, I can say that I have read a lot about the technique that they wrote up.  I also saw some of their stuff that included failures, reasons, etc.  But not sure I have seen all of the work.  I actually was hoping someone from that group would have been able to answer some of the questions, in particular the ones with direct experience spray coating over other surfaces such as dielectrics.  I know it is a long shot for obvious reasons.  But as an informal group, there was no way to contact them and local Astro groups are not meeting, so no way to ask around.  As you can tell, I am profoundly unconnected to the community!

 

Is the complete work available in a single simple location on the net?  In other words, I know what I have seen, but can't know what I may have missed.

 

Best of luck with the Binoscope!  I would love to use one at some point.  I have heard that bringing two eyes into the observing action greatly improves the experience.  I was at a Eugene meeting of the Astronomical Society (my first) and someone there showed off a bino that they were designing for commercial sale, eventually.  It was nice.  Then COVID shut is all down.  I would hope that things should open up soon.  Will this be your first silvering using the spray method?  Will you use the tarnish shield product?  I would like to know what your results are and the lifetime of the coating in our area.

 

I will keep you posted on my project.  Just need to get some optics in hand and get it set up.

 

CS

Alan

 

Alan.

Dale posted the site below. It is all well organized and collected there.

For astro, don't know why anyone would coat over previous coating, since people normally only recoat when the old coat is deteriorated. So doubt you would hear from anyone in that regard.

 

I have not silvered yet as i am getting my system running mechanically first, then Steve Dodds is refiguring for me, then i will coat, Based on what i have learned from the OSW work, i am confident enough in the process. I am hoping for a year+ life, but no biggie for me as it appears not much more hassle that a good washing which i do yearly anyway.  I plan to try the Midas overcoat but also will eventually try thinned lacquer overcoat as well as described in the Scientific American ATM volumes.

 

The bino gets closer day by day, hope to be up this summer sometime !  Yes bino is a whole 'nother level from cyclops viewing. My memory of viewing thru a large bino years ago sticks with me and has been the driver for my project.

 

CS
Bob


Edited by Bob4BVM, 23 June 2021 - 11:18 PM.

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#19 Oregon-raybender

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Posted 24 June 2021 - 01:31 PM

As layers are laid down, the defect is enhanced and not healed, but grows. That is why the surface has to be very clean. Also

if there is a hiccup during coating, the coating fails and looks like underpolish, scattering of particles, they look like 

very small ball bearings, spherical. There a number of causes, the list is long, a ~20% failure rate is not uncommon.

One of my many hats in R/D, was to review and test the results of failed coatings and report. This were my caution comes in.

 

I can't answer all the questions here. I would suggest to anyone look into past coating papers and books from SPIE or OSA societies.

I attended many conferences and taken a number of classes on coating processing. If you want the real scope on coatings take a offered class at a conference. Since you want to work in the NIR, these are different set of coatings involved. Maybe so far as going to gold, but silver may do.

 

Starry Nightswaytogo.gif


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#20 Alan Brunelle

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Posted 25 June 2021 - 11:24 PM

As layers are laid down, the defect is enhanced and not healed, but grows. That is why the surface has to be very clean. Also

if there is a hiccup during coating, the coating fails and looks like underpolish, scattering of particles, they look like 

very small ball bearings, spherical. There a number of causes, the list is long, a ~20% failure rate is not uncommon.

One of my many hats in R/D, was to review and test the results of failed coatings and report. This were my caution comes in.

 

I can't answer all the questions here. I would suggest to anyone look into past coating papers and books from SPIE or OSA societies.

I attended many conferences and taken a number of classes on coating processing. If you want the real scope on coatings take a offered class at a conference. Since you want to work in the NIR, these are different set of coatings involved. Maybe so far as going to gold, but silver may do.

 

Starry Nightswaytogo.gif

Thanks so much for your input.  I will certainly look into it from where I sit at my computer.  I would love to attend a conference though as things open up again.  There, I would have a better chance of getting clarity as questions come up. 

 

I really like what I see in silver's capability.  But not thrilled with the life of the coating, even for the protected or enhanced silvers.  This, especially in the western Oregon region where I live.  I am trying to work out a move to NM, and there I may feel more brave in using the silver coating.  Otherwise, enhanced aluminum makes more sense.  Yes, I am aware of the ability of gold in the IR.  But I hope to do double duty with this telescope.  Routine photometry on multiple targets, then when I take a break from that, use it as a long fl scope to image whatever with a regular larger frame camera.  For that, I need the wider band performance of silver or aluminum.  

 

Still searching for a scope at this point.



#21 Alan Brunelle

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Posted 25 June 2021 - 11:27 PM

 My memory of viewing thru a large bino years ago sticks with me and has been the driver for my project.

 

CS
Bob

Maybe I had better stay away for binos for the time being if there are ever any star parties again!




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