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Advice requested on a repair/refit of a mysterious charming old handmade Dob

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#1 Matt78

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Posted 15 April 2024 - 11:17 AM

Hi there,

 

I want to start by saying I know nearly nothing about ATM. A couple of years ago, I was given a handmade very nice 6" Dob by the dept admin for the science dept at the university where I work. The story is that she was given this by an astronomy professor when he retired. I believe he has since passed away. This professor told her that it was given to him by a favorite student, who made it in either the 1970s or 1980s. He told her that it won an award of some type at Stellafane one year. The original maker is unknown to anyone currently at the dept.

 

IMG 3631
Album: Mysterious Vintage Dob
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It sat in my garage under a trash bag for a couple of years, and recently I decided this might be a nice time to take it out. I very much admire its craftsmanship. There may be a few awkward aspects to the build, but I think they add wabi sabi to it. I particularly like how it sits a bit high, so that you can use a chair.

 

Unfortunately, I made a dumb mistake. The mirror was perfectly clean, but the secondary was probably at about 10% reflectivity from grime and dust. Being a SCT and refractor guy, I assume you can clean a mirror with some alcohol and a clean soft cloth. The dust came off, but it took a tiny bit of alumizing with it. I thought ohh, there was a big spot of grime, and rubbed harder. You can see for yourself the result.

 

I would like to fix the mirror. Not because I have a deep need for a 6" f/4.5 (based on rough measurements), but because this scope has character, and I hate to think of it going to a junk pile. I measured the minor and major axes, and found something from GSO for $40 that matches. While I know nothing of ATM, I am very handy, doing knife-making and silver-smithing. I believe I have the chops to cut off the epoxy that was used for this, and carefully sand the disk it was mounted to flat.

 

What I do have is a question about that secondary mount. Is it me, or does that single-vane secondary mount look like it will be a bear during collimation? If I ordered a suitable 4-vane secondary holder, do you think it will be easier to collimate going forward?

 

If I do that, do people think I'll be ruining the spirit of the maker's intention? I realize that the answer to this is a personal question only I (or the maker) can answer. I'm still curious people's thoughts. Also bear in mind that the focuser works by moving the secondary further or closer to the primary, so I'd need to install a focuser as well as a new secondary mount. I did test the focuser, and it works a lot better than I'd have guessed from looking at it (the blind spots in the center of the image are a whole other matter).

 

My personal feeling, as someone who has made other things, is that if someone, some decades later, were to modify something I had made, I would mostly be glad that someone still cared enough about it do that. However, if they did a crap job, or maybe if they significantly modified it (for example trying to turn a bushcraft knife into a paring knife), I might be mildly miffed. Other's thoughts?

 

 


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#2 Dale Eason

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Posted 15 April 2024 - 11:35 AM

That scope uses a sliding focuser when means the secondary has to move with the focuser.  You can not replace it with a spider unless you change the focuser to a none sliding type or somehow make the new spider slide with the focuser.

 

It looks like the focuser stalk is made of carbon fiber.  If so that is a rather modern thing and not from the 60's in my opinion.

 

Very nice looking scope.  One might be able to find the maker by going back through Stellefane awards photos from each star party.


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#3 Matt78

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Posted 15 April 2024 - 12:08 PM

That scope uses a sliding focuser when means the secondary has to move with the focuser.  You can not replace it with a spider unless you change the focuser to a none sliding type or somehow make the new spider slide with the focuser.

 

It looks like the focuser stalk is made of carbon fiber.  If so that is a rather modern thing and not from the 60's in my opinion.

 

Very nice looking scope.  One might be able to find the maker by going back through Stellefane awards photos from each star party.

I think I didn't explain very well, but I was aware I'd need to permanently attach a spider to the tube and put in a new focuser.

 

I thought at first glance it was carbon fiber, but in my opinion it's actually fiberglass that was made into a composite by the maker with some epoxy. I don't know about ATM, but for other things people used to make their own fiberglass composites by draping the fabric onto a mold, and probably still do. You can see that it wasn't quite sanded fully flush if you look carefully. The maker also made a sort of clip for the scope tube in the same fashion, but it wasn't in the photos.

 

I will take your advice and see what I can find for old photos. Thanks for your response!



#4 Dale Eason

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Posted 15 April 2024 - 12:13 PM

I would not replace that focuser and diagonal holder.  Just remove the diagonal from the holder and get it recoated.  Then reattach it.  It looks to me like it has all the adjustments necessary for collimation.  It looks like it can be rotated and tilted which I think is all that is needed since it can also slide.

 

Yes I decided that was fiber glass as well.


Edited by Dale Eason, 15 April 2024 - 12:14 PM.

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#5 Matt78

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Posted 15 April 2024 - 12:21 PM

I would not replace that focuser and diagonal holder.  Just remove the diagonal from the holder and get it recoated.  Then reattach it.  It looks to me like it has all the adjustments necessary for collimation.  It looks like it can be rotated and tilted which I think is all that is needed since it can also slide.

 

Yes I decided that was fiber glass as well.

I can definitely be convinced to keep the focuser and diagonal. I'm curious though, why recoat rather than replace? I'm all for keeping the (presumably) hand-ground secondary, but wouldn't it be considerably more affordable to put in a premade equivalent? I'm actually not very up on the costs of getting a mirror coated, so I may be way off base.



#6 davidc135

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Posted 15 April 2024 - 01:34 PM

The secondary with any residue epoxy removed can be sent to the coaters who will remove the old aluminium before recoating. Or you can remove it yourself with ferric chloride + HCl solution if you wish to test it for flatness. Which is easier said than done without an optical flat. But you may be able to borrow one from the university? Removing the coating yourself enables you also to check that the glass substrate isn't scratched.

But $40 for a new one isn't very much or probably pay a bit more to be sure of quality.

 

To refix I'd use 3x 2mm thick blobs of silicone rubber rather than hard epoxy.

 

David


Edited by davidc135, 15 April 2024 - 01:35 PM.

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#7 Matt78

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Posted 15 April 2024 - 01:40 PM

The secondary with any residue epoxy removed can be sent to the coaters who will remove the old aluminium before recoating. Or you can remove it yourself with ferric chloride + HCl solution if you wish to test it for flatness. Which is easier said than done without an optical flat. But you may be able to borrow one from the university? Removing the coating yourself enables you also to check that the glass substrate isn't scratched.
But $40 for a new one isn't very much or probably pay a bit more to be sure of quality.

To refix I'd use 3x 2mm thick blobs of silicone rubber rather than hard epoxy.

David

Interesting. Like the silicone they sell in tubes at the hardware stores?

As for borrowing a flat, I suppose I could ask if the physics people have one, but it’s a pretty small science department. On the other hand they’ve been accumulating stuff for many decades, so it would be worth an ask.

Edited by Matt78, 15 April 2024 - 01:42 PM.


#8 Dale Eason

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Posted 15 April 2024 - 02:51 PM

Most of the time the diagonal gets coated along with the main mirror so it is not very expensive.  When you buy one that is already coated it could be cheaper than getting yours recoated by itself but you just have to find out.  Then in addition yours even if homemade could be a very good one.  If the scope won any optical awards then it is a good one.

 

To begin with use it as is to get a feel for it and the scope.


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#9 PYeomans

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Posted 15 April 2024 - 07:00 PM

I remember this scope being shown in pictures from Stellafane in Amateur Astronomy magazine many years ago. I'll have to thumb through my old copies and see if I can find the builder's name. I wouldn't change much on it, but a new diagonal mirror would be nice.

I just finished rehabbing my 25 year old 14.5 Dob after years of toting it around. Finish needed redoing, new hardware where needed, upgraded electrical system, etc. If you're handy there shouldn't be problems. Nice older scope by the way.


Edited by PYeomans, 15 April 2024 - 07:05 PM.

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#10 hamishbarker

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Posted 15 April 2024 - 09:05 PM

Gorgeous scope. Don't change a thing!!

 

if you still know anyone at the school, particularly the chemistry department, you could silver the mirror without removing it from the stalk. You still need to clean it properly (see the other silvering threads here on CN) but it's so small that it could easily be done by immersion in the Tollen's reagent, particularly since it has the stalk on it. The Tollen's reagent "silver mirror test for aldehydes" is a classic and pretty chemistry demonstration.  The downside is that silvering isn't as durable as aluminizing, but it might get you going until it is time to coat both the primary and secondary again.

 

Are you certain that it is glued to the stalk with epoxy? Usually silicone glue is used as it remains flexible so that when the (FRP) backing plate changes size (plastic has a much bigger coefficient of thermal expansion than glass or pyrex), the flexible glue is compliant enough to take up the difference without imposing any stress on the mirror.

 

If epoxy has been used to glue the mirror, it will be quite hard to remove it. 

 

If the primary anyway needs aluminizing, then yes, get them both done at the same time. You could probably post the mirror to a coater and I doubt they would charge much to do it as part of someone else's mirror coating job. I have an aluminizer and know that a little mirror like that is only a couple of minutes extra work on the cleaning compared to a typical 8" primary, and as long as the coater has a stand to put it in the chamber, could be done during the same evac/coating cycle when a bigger mirror is being coated.  If a turbomolecular pump (rather than an oil vapour diffusion pump) then I suspect the old epoxy and FRP may outgas little enough that recoating might be possible without removal from the stalk. Most coaters are rather cautious about introducing anything other than clean glass or metal into their coating systems, for fear of contamination leading to problems for them later on (although personally I would try with my machine.


Edited by hamishbarker, 15 April 2024 - 09:11 PM.

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#11 Oregon-raybender

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Posted 15 April 2024 - 10:02 PM

I would keep it together as much as possible. Replace the secondray

with a new one. It will be cheaper than getting it recoated, unless the

primary needs it, coat together.

 

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#12 Matt78

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Posted 16 April 2024 - 06:55 AM

I remember this scope being shown in pictures from Stellafane in Amateur Astronomy magazine many years ago. I'll have to thumb through my old copies and see if I can find the builder's name. I wouldn't change much on it, but a new diagonal mirror would be nice.

I just finished rehabbing my 25 year old 14.5 Dob after years of toting it around. Finish needed redoing, new hardware where needed, upgraded electrical system, etc. If you're handy there shouldn't be problems. Nice older scope by the way.

I'm actually amazed to have found someone who remembers it, that's so cool! Let me know what you find! I emailed Stellafane, and they didn't have any photo archives, but they're putting the pic on their mailing list to see if anyone else might remember.



#13 Matt78

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Posted 16 April 2024 - 07:16 AM

 

If you still know anyone at the school, particularly the chemistry department, you could silver the mirror without removing it from the stalk. You still need to clean it properly (see the other silvering threads here on CN) but it's so small that it could easily be done by immersion in the Tollen's reagent, particularly since it has the stalk on it. The Tollen's reagent "silver mirror test for aldehydes" is a classic and pretty chemistry demonstration.  The downside is that silvering isn't as durable as aluminizing, but it might get you going until it is time to coat both the primary and secondary again.

We never used that one in my chem classes, but it sounds pretty cool. I could possibly see if the current astronomy professor (whom I know) could put in a good word with one of the chem professors for me. Would it be advised to remove the existing coating, and if so is there a preferred method for that?

 

Are you certain that it is glued to the stalk with epoxy? Usually silicone glue is used as it remains flexible so that when the (FRP) backing plate changes size (plastic has a much bigger coefficient of thermal expansion than glass or pyrex), the flexible glue is compliant enough to take up the difference without imposing any stress on the mirror.

I am not certain at all. It looked like epoxy to my eye, but I didn't actually try to verify that by checking its hardness. I know nothing of ATM, so it was just a guess. If silicone is the usual method, I'd assume that's what it is.

 

If the primary anyway needs aluminizing, then yes, get them both done at the same time. You could probably post the mirror to a coater and I doubt they would charge much to do it as part of someone else's mirror coating job. I have an aluminizer and know that a little mirror like that is only a couple of minutes extra work on the cleaning compared to a typical 8" primary, and as long as the coater has a stand to put it in the chamber, could be done during the same evac/coating cycle when a bigger mirror is being coated.  If a turbomolecular pump (rather than an oil vapour diffusion pump) then I suspect the old epoxy and FRP may outgas little enough that recoating might be possible without removal from the stalk. Most coaters are rather cautious about introducing anything other than clean glass or metal into their coating systems, for fear of contamination leading to problems for them later on (although personally I would try with my machine.

As it currently stands, it looks just fine on the primary. I haven't removed the cell to look closely, but the flashlight down the tube didn't reveal anything missing.
 



#14 Matt78

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Posted 16 April 2024 - 07:23 AM

I think that people here have convinced me that this scope is special (honestly any home-made scope is special), and I'll be doing my best to preserve it as is. Perhaps I'll apply a bit of Renaissance wax to the wood, but that's about it.


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#15 Dale Eason

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Posted 16 April 2024 - 06:42 PM

I once stumbled upon a site that showed many past Stellafane star parties and pictures of each of the scope winners.  So they are someplace.  I just don't remember where I saw them.  They went all the way back into the late sixties IIRC.  I thought I got there through their site. 



#16 hamishbarker

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Posted 16 April 2024 - 07:32 PM

We never used that one in my chem classes, but it sounds pretty cool. I could possibly see if the current astronomy professor (whom I know) could put in a good word with one of the chem professors for me. Would it be advised to remove the existing coating, and if so is there a preferred method for that?

 

I am not certain at all. It looked like epoxy to my eye, but I didn't actually try to verify that by checking its hardness. I know nothing of ATM, so it was just a guess. If silicone is the usual method, I'd assume that's what it is.

 

As it currently stands, it looks just fine on the primary. I haven't removed the cell to look closely, but the flashlight down the tube didn't reveal anything missing.
 

 

Removing coating: 

DO NOT USE CAUSTIC SODA!!! (it rapidly damages optical glass surfaces)

 

Safe (for the optical surface) options:

-  Ferric Chloride solution (readily available for etching circuit boards. Watch out, it stains and is somewhat acidic, don't get it on your skin!)

- "green river" solution (dilute HCl + CuSO4). Google the recipe. Used to strip coatings from some professional observatory mirrors. (also not good on skin, obviously).

 

That gets the coating off.

 

Then there is the cleaning: precipitated, pharma grade calcium carbonate (chalk). NOT magnesium carbonate (gymnast chalk)!! See the instructions in the various silvering threads here on CN or at the oregon scope werks google webpage:

 

https://sites.google...elescope-mirror



#17 Matt78

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Posted 16 April 2024 - 07:34 PM

Removing coating: 

DO NOT USE CAUSTIC SODA!!! (it rapidly damages optical glass surfaces)

 

Safe (for the optical surface) options:

-  Ferric Chloride solution (readily available for etching circuit boards. Watch out, it stains and is somewhat acidic, don't get it on your skin!)

- "green river" solution (dilute HCl + CuSO4). Google the recipe. Used to strip coatings from some professional observatory mirrors. (also not good on skin, obviously).

 

That gets the coating off.

 

Then there is the cleaning: precipitated, pharma grade calcium carbonate (chalk). NOT magnesium carbonate (gymnast chalk)!! See the instructions in the various silvering threads here on CN or at the oregon scope werks google webpage:

 

https://sites.google...elescope-mirror

I think I may have some ferric chloride right here from when I was doing cyanotypes actually. Thanks for the pointers.

 

Update: That's ferric ammonium citrate. Alas. I'm sure they'll have it in the chem dept though.
 


Edited by Matt78, 16 April 2024 - 07:36 PM.


#18 Dale Eason

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Posted 17 April 2024 - 12:39 AM

Usually you don't need much ferric chloride either.  Just wet a paper towel laid over the mirror.


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#19 Matt78

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Posted 01 May 2024 - 08:40 AM

Well the mystery is solved. The scope is considerably newer than I had been led to believe. I ran into the woman who gave it to me, and she said she now remembers the name of the man who made it. First name at least: Terry. I looked over the Stellafane winners again, and found a Terry Ryan, from Maine no less, who won in 2002 for 6” f/5 scope with a magnetically attached focuser. I found a photo gallery from the 2002 event, and eventually found a photo of someone, I assume Terry, standing next to the scope. Now I need to, carefully, try to figure out where the magnets come into it. I’ll try to post a photo of him with the scope later.
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