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franzmetcalf
member

Reged: 06/21/11

Loc: Washed out Los Angeles

Finishing a 12" Dob from 1971?
#5419245 - 09/13/12 01:43 PM

Hello everyone,

Strange subject line, eh? Well, I grew up in San Francisco and learned about the Sidewalk Astronomers pretty young. My dad and I got interested and went to a star party and met John Dobson. Well, that's perhaps enough said, right there. We got the astronomy bug straight from the source. When I was, I think, 11, my dad and I took a class taught by John at the California Academy of Sciences where all the students made their own, quite literally, Dobsonian reflectors. Well, almost. We actually didn't quite finish ours. In the last class, John looked through the various scopes to evaluate the mirrors and told us ours needed some fine correcting. Somehow that never happened. I do recall looking through it at Saturn one evening and being able to see the rings—despite the fact the mirror was unsilvered! Ah, the beauty of a big light bucket!

My father passed away a few years ago and I inherited all the junk in his basement, including the components of this scope. So now, forty years later, I'm wondering if there's any way to, at last, finish this monster. I don't know if Dobsonian tubes and mounts have evolved much in 40 years, but this thing is HEAVY. I mean, it takes two people to lift the mount. But the mount can easily be remade or a new one purchased. The heart of a Dobsonian is the mirror.

Can this mirror be saved? It's still on its plywood grinding base, but I'm not sure I still have the grinding blank. Hey, it's been 40 years! I would dearly love it if this darn near historic artifact could finally be finished and used. It would make my father proud.

Any words are welcome, of course, even bad news. Oh, and I can post photos, if that would be helpful.

Thanks,

Franz

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killdabuddha
Pooh-Bah

Reged: 08/26/11

Re: Finishing a 12" Dob from 1971? new

new

[Re: franzmetcalf]
#5419318 - 09/13/12 02:18 PM

Yes, definitely send pics. It's never been mirrored? Has it been covered? No doubt you'll wanna refigure it, if only to finish the original work. But if it's gotten pitted...? But post a pic and others will steer you rite

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franzmetcalf
member

Reged: 06/21/11

Loc: Washed out Los Angeles

Re: Finishing a 12" Dob from 1971? new

new

[Re: killdabuddha]
#5419471 - 09/13/12 04:05 PM

Hi, I'm back with photos, having uploaded them to flickr. To answer killdabuddha's questions, it's not been covered (unless you count dust), but it's also not really been disturbed much. It seems pretty clean to me. I brought it out of the basement and into the light where I washed it. I tried to take some shots that showed the condition of the glass. There must be tricks to that, that I don't know. I don't see pitting, nothing obvious to my eyes, anyway. But here's what I've got for you, for now:

The mount:

12.5" mount by franzmetcalf, on Flickr

The tube:

12.5" tube by franzmetcalf, on Flickr

The grinding board:

Mirror and resin by franzmetcalf, on Flickr

The mirror, 1 of 2:

12.5" mirror reflecting tea kettle by franzmetcalf, on Flickr

The mirror, 2 of 2:

12.5" mirror reflections by franzmetcalf, on Flickr

Perhaps those last two show how nearly done we got, way back when. Anyway, looking at some posts here, I am cautiously optimistic.

Franz

Edited by franzmetcalf (09/13/12 04:12 PM)

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rboe

Reged: 03/16/02

Loc: Phx, AZ

Re: Finishing a 12" Dob from 1971? new

new

[Re: franzmetcalf]
#5419481 - 09/13/12 04:12 PM

Find someone fairly local that is willing to do the final corrections and get it coated. Then build yourself the chassis to toss it in.

As far as the chassis goes, there are a ton, way too many, of good ones out there. Astro-systems.biz sells kits that are pretty slick. Not cutting edge technology but years ahead of what you did in 71. That would be about the easiest way to go assuming you have no tools to build one from scratch.

You have a very nice project in front of you.

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Pinbout
Post Laureate

Reged: 02/22/10

Loc: Montclair

Re: Finishing a 12" Dob from 1971? new

new

[Re: franzmetcalf]
#5419488 - 09/13/12 04:19 PM

that mirror needs a beveled edge, a diamond hone and water...stroking down and away from the optical surface will help protect it from being chipped badly.

that mirror is as old as my wife...

i mean "that mirror is as old as my wife"

no I mean "that mirror is as old as my wife"

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killdabuddha
Pooh-Bah

Reged: 08/26/11

Re: Finishing a 12" Dob from 1971? new

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[Re: rboe]
#5419502 - 09/13/12 04:26 PM

Thatsa very thin mirror, maybe/maybe not annealed glass, and fast judgin by the shortness of the tube. Probly difficult to have gotten right for a 1st effort.....Which is all to say that my advice would be to get a figure on it--from sum1 local if possible and at little cost--and affordably coated (maybe Newport Glass for this). I'd also finish the original build, both cuz it's sufficient, and for sentiment/nostalgia sake. Also, I'd use a flat cell rather than a floating (6-point, 12-point, etc) cell, cuz of its thinness (sum good quality gap-less 3/4" ply).

Sweet. I envy you. Keep us posted?

Pinbout, you kill me

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laserrain
member

Reged: 09/07/12

Re: Finishing a 12" Dob from 1971? new

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[Re: killdabuddha]
#5419528 - 09/13/12 04:44 PM

Interesting question, refiguring a mirror after 40 years.
Once I "restyled" a 5", after I had learned more about testing. It wasn't an easy job pouring another pitch pot after five years, but in the end I was lucky.
Besides, in the middle of '80s, I bought a 10" and, after I had mounted it in a PVC tube, I waited sixteen years before assembling a truss dobsonian...How many blueprints had crowded my shelves...

Back to your mirror, I'd try testing it on my own or under a friend's assistance, collecting a large amount of data. Then I'd plan a refiguring strategy. But first you need a clear idea of what your mirror's surface looks like. Nice idea posting pictures of the old telescope parts...

Quote:

that mirror needs a beveled edge, a diamond hone and water...stroking down and away from the optical surface will help protect it from being chipped badly.

I do agree, you've to be careful.
The glass is really very thin. Again, pay attention in collecting your data. Best an interferometer test for detecting astigmatism, before you start refiguring - I mean, find someone can test your mirror without pinching it...
Greetings
Anthony

Edited by laserrain (09/13/12 05:04 PM)

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franzmetcalf
member

Reged: 06/21/11

Loc: Washed out Los Angeles

Re: Finishing a 12" Dob from 1971? new

new

[Re: laserrain]
#5419625 - 09/13/12 05:42 PM

Thanks, guys. Well, I guess it's good I'm in Los Angeles, where there are surely plenty of ATMs who can help me refigure and coat this mirror. I'll look around for a contact. I know that John himself used to stay at the Hollywood Vedanta Society in the winters (maybe he still does). It's actually walkable from my house. I think the LA Sidewalk Astronomers meet there, sometimes. I'll do some googling.

Bottom line: wow, I really can save this telescope! This is great! (Just don't tell my wife.)

Cheers,

Franz

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rboe

Reged: 03/16/02

Loc: Phx, AZ

Re: Finishing a 12" Dob from 1971? new

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[Re: franzmetcalf]
#5419700 - 09/13/12 06:21 PM

Using the old chassis for sentimental reasons is a good idea but putting it into a much lighter chassis will encourage you to actually use it.

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cheapersleeper
Carpal Tunnel

Reged: 01/22/10

Loc: Sachse TX

Re: Finishing a 12" Dob from 1971? new

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[Re: rboe]
#5419828 - 09/13/12 07:41 PM

I vote for a compromise that uses some newer techniques but that results in a structure that is an homage to old school dobs.

http://www.garyseronik.com/?q=node/76

The above linked project has all the old school looks but with some modern ideas that make it more portable and more functional than the old design.

Regards,
Brad

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franzmetcalf
member

Reged: 06/21/11

Loc: Washed out Los Angeles

Re: Finishing a 12" Dob from 1971? new

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[Re: cheapersleeper]
#5419850 - 09/13/12 07:59 PM

Wow, I loved the photo of the telescope *before* Gary redid it. I wonder why he didn't actually use it much in that configuration. Portability is such an issue when you live in Los Angeles and can only look at solar system objects unless you pack up the telescope and drive out of town. But perhaps this Dob is just destined to be the in-town scope, large enough to defeat the light pollution of LA. Is that possible?

Cheers,

Franz

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cheapersleeper
Carpal Tunnel

Reged: 01/22/10

Loc: Sachse TX

Re: Finishing a 12" Dob from 1971? new

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[Re: franzmetcalf]
#5419868 - 09/13/12 08:10 PM

There are compromises in any design. If it needs to store and travel in a small footprint, by all means build something that will do those things. As mentioned earlier, there are kits and plenty of information on doing that. I just happen to like having a tube and relatively closed structure around my mirrors and having something that looks like a traditional Dob tickles my fancy.

B

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Pinbout
Post Laureate

Reged: 02/22/10

Loc: Montclair

Re: Finishing a 12" Dob from 1971? new

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[Re: franzmetcalf]
#5419871 - 09/13/12 08:11 PM

finish the scope, use it. you can decide later, after a season if you want to make something a little more transportable.

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franzmetcalf
member

Reged: 06/21/11

Loc: Washed out Los Angeles

Re: Finishing a 12" Dob from 1971? new

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[Re: Pinbout]
#5419917 - 09/13/12 08:41 PM

@Danny: That seems awfully wise and rational advice. Are we allowed to think that way in a forum like this?

Franz

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cheapersleeper
Carpal Tunnel

Reged: 01/22/10

Loc: Sachse TX

Re: Finishing a 12" Dob from 1971? new

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[Re: franzmetcalf]
#5419979 - 09/13/12 09:11 PM

Quote:

@Danny: That seems awfully wise and rational advice. Are we allowed to think that way in a forum like this?

Franz

If we were wise and rational, we wouldn't be building telescopes.

B

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Pinbout
Post Laureate

Reged: 02/22/10

Loc: Montclair

Re: Finishing a 12" Dob from 1971? new

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[Re: franzmetcalf]
#5420116 - 09/13/12 10:30 PM

Quote:

@Danny: That seems awfully wise and rational advice. Are we allowed to think that way in a forum like this?

Franz

well thanks for the compliment but I'm more like an idiot savant without the savant.

but out of respect for your time spent with your dad in 71, why not.

they look down on us you know.

ok, now I'm gonna cry.

ps my son is 11, but he'll only briefly come out to look at something like the m13 or m42 or whatever else I have to threaten him with no ninetendo ds time if he doesn't look.

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cheapersleeper
Carpal Tunnel

Reged: 01/22/10

Loc: Sachse TX

Re: Finishing a 12" Dob from 1971? new

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[Re: Pinbout]
#5420199 - 09/13/12 11:16 PM

Quote:

Quote:

@Danny: That seems awfully wise and rational advice. Are we allowed to think that way in a forum like this?

Franz

Danny,

Very small sample size here, but it seems that perhaps ATM makes kid's especially disinterested in astronomy? I too have to threaten the DS and my kid usually takes a cursory glance and says "wow, that's great dad I'm going back inside."

B

well thanks for the compliment but I'm more like an idiot savant without the savant.

but out of respect for your time spent with your dad in 71, why not.

they look down on us you know.

ok, now I'm gonna cry.

ps my son is 11, but he'll only briefly come out to look at something like the m13 or m42 or whatever else I have to threaten him with no ninetendo ds time if he doesn't look.

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franzmetcalf
member

Reged: 06/21/11

Loc: Washed out Los Angeles

Re: Finishing a 12" Dob from 1971? new

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[Re: cheapersleeper]
#5420235 - 09/13/12 11:42 PM

@Danny: Still, he loves you. They just express it differently in this generation. But, excuse me, my daughter is yelling from the other room that my "two minute warning is up!" and I need to read to her.

Wow, what's harder, understanding the heavens or understanding our children?

Franz

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laserrain
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Reged: 09/07/12

Re: Finishing a 12" Dob from 1971? new

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[Re: Pinbout]
#5420339 - 09/14/12 02:02 AM

Quote:

finish the scope, use it. you can decide later, after a season if you want to make something a little more transportable.

Sure, finish it, use it, as soon as you can...

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franzmetcalf
member

Reged: 06/21/11

Loc: Washed out Los Angeles

Re: Finishing a 12" Dob from 1971? new

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[Re: laserrain]
#5420359 - 09/14/12 02:46 AM

So, my wife comes home tonight and tells me that John Dobson was on NPR tonight. He celebrates his 97th birthday tomorrow (well, today for you reading this). He's staying at the Vedanta Temple now. I have to go see him.

Cheers to the whole forum!

Franz

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