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.