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What To Do With An Old Dob (~10") and 16" Porthole Blank

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#1 sigma5678

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Posted 13 June 2021 - 01:59 PM

Hey Folks,

 

Some friends of the family were cleaning out their garage and wanted me to take their father's hand made 10" Dob (which I was more than happy to do!). I'm in the SF Bay Area and this scope was built in one of Dobson's classes in the 80s or 90s. The primary mirror is a 10" but due to some issues when they were grinding it the edges were deemed not useable so they masked off the outer perimeter. So in reality this is a 9" scope. 

 

I got this all setup last night and after getting the primary decently aligned, took a gander through it with a 26mm eyepiece. It was quite nice! 

 

Now generally I prefer doing AP vs visual observing. I have a dedicated DSO rig and a solar rig as well (see my signature). I thought that perhaps just for kicks I could try to do some Planetary imaging since I already have some fast cams (e.g. ZWO 174MM). This got me thinking that maybe a simple(ish) project might be to make a low cost equatorial platform to keep the planets in the fov while I'm mucking with my computer or camera, fighting the inevitable USB demons.... There seem to be some plans out there for low cost wooden eq mounts with DC motors. Any favorite designs people have? I probably will not travel with this scope so backyard observing/imaging is the name of the game (implying that the designs don't have to be super light weight and could even run off of AC). 

Thoughts on this? Any other mods people would do to such an old scope? I wasn't planning on any major re-designs but just looking for some ways to have some fun with some simple projects.

I was also given a 16" (probably 1" thick) porthole blank. I have never pushed glass and so the idea of doing anything with this other than adorning a shelf in my garage is terrifying. Really more from a time commitment perspective but I might get brave at some point (though I know I should probably just start with an 8" to see what I'm getting myself into!).... I also have read that I should check the blank to see if it has been annealed which I haven't done yet (just got it last night).  

 

Thanks for any ideas!

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#2 MeridianStarGazer

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Posted 13 June 2021 - 02:09 PM

The portal blank is likely worth close to $200 if you want to post it on the classifieds. I'm not sure what current glass prices are. I do know Swift glass sells individual blanks for a significantly higher price than when in groups of 5, and that their blank prices have gone up significantly a year ago. Maybe even start at $250 and then gradually lower it to $150 till it sells. Someone will buy it. People will want to know if it was strain tested and how good the anneal is.

 

 

 

Sounds like you like the 10". I recommend you use the money from the 16" to buy materials for an equatorial platform for the 10". Much easier to use than an GEM. Few places build equatorial platforms, and those that do charge a lot. The design is actually much simpler than a GEM, with a much higher ease of building a good one, though you will need a router if you want the smoothest path. Otherwise, if just for visual, if you are fine with the planet slowly moving around in the view a bit, you could use other means to cut the arc.


Edited by stargazer193857, 13 June 2021 - 02:14 PM.


#3 Pezdragon

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Posted 13 June 2021 - 02:40 PM

It’s good to see other old alumni from John’s classes. I too live in the Bay Area and built mine in 1968 at one of the first classes. There’s something to said for the keep it simple principle as It promotes more use. If you’re set on tracking a platform tracker is a good way to go and can be removed when you want to keep it simple. Thanks for the a look at an old school Dob! Here’s a few pics of mine back in 1969 and 50 years later…

 

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#4 Mirzam

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Posted 13 June 2021 - 03:59 PM

That’s an awesome focuser you have there ^.

 

JimC



#5 sigma5678

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Posted 13 June 2021 - 04:12 PM

I did a quick check on the mirror 16" blank and it looks pretty bad. This is the first time I've tried this so just wanna post some pics of the process to make sure I did it correctly. I have an LED monitor that I looked at with a pair of sun glasses. Rotating the sun glasses by 45 degrees seemed to null the signal from the screen so I assume the screen is linearly polarized. I put the blank in between and could see a waffle pattern. Rotating the sun glasses shows some pretty nasty dark lines running through this.....

So I assume this piece of glass isn't suitable for mirror making. Let me know if there is anything else to be gleamed from this picture or if my process was flawed. I just searched the forums for a bit and tried this based on what others seemed to have done. 

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Edited by sigma5678, 13 June 2021 - 06:00 PM.

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#6 sigma5678

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Posted 13 June 2021 - 04:15 PM

It’s good to see other old alumni from John’s classes. I too live in the Bay Area and built mine in 1968 at one of the first classes. There’s something to said for the keep it simple principle as It promotes more use. If you’re set on tracking a platform tracker is a good way to go and can be removed when you want to keep it simple. Thanks for the a look at an old school Dob! Here’s a few pics of mine back in 1969 and 50 years later…

Wow that's a great looking scope (both your current and original!). It seems astronomy has been a life long hobby and that's wonderful. I hope that my burgeoning interest in this hobby continues and that my son will also come to enjoy it. Though at 2 yrs old his bedtime doesn't really jive well with when its dark out....especially during summer haha.  


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#7 dogbiscuit

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Posted 13 June 2021 - 05:41 PM

I did a quick check on the mirror and it looks pretty bad. This is the first time I've tried this so just wanna post some pics of the process to make sure I did it correctly. I have an LED monitor that I looked at with a pair of sun glasses. Rotating the sun glasses by 45 degrees seemed to null the signal from the screen so I assume the screen is linearly polarized. I put the blank in between and could see a waffle pattern. Rotating the sun glasses shows some pretty nasty dark lines running through this.....

So I assume this piece of glass isn't suitable for mirror making. Let me know if there is anything else to be gleamed from this picture or if my process was flawed. I just searched the forums for a bit and tried this based on what others seemed to have done. 

Is that the 10" mirror or the 16" blank?



#8 sigma5678

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Posted 13 June 2021 - 06:01 PM

Is that the 10" mirror or the 16" blank?

Whoops sorry. I edited the above post to be clear. That is the 16" blank.  



#9 MeridianStarGazer

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Posted 13 June 2021 - 07:39 PM

I did a quick check on the mirror 16" blank and it looks pretty bad. This is the first time I've tried this so just wanna post some pics of the process to make sure I did it correctly. I have an LED monitor that I looked at with a pair of sun glasses. Rotating the sun glasses by 45 degrees seemed to null the signal from the screen so I assume the screen is linearly polarized. I put the blank in between and could see a waffle pattern. Rotating the sun glasses shows some pretty nasty dark lines running through this.....

So I assume this piece of glass isn't suitable for mirror making. Let me know if there is anything else to be gleamed from this picture or if my process was flawed. I just searched the forums for a bit and tried this based on what others seemed to have done. 

If you try to hog that, it may explode in your face. It even has some color, which is worse. Augustus got glass shards all over him when a blank exploded on him.

 

 

The blank is still good as a flat table top to put smaller blanks on, with padding between them, in case someone does not have a flat surface.

 

 

As for the waffle pattern, I'm curious what causes that. Was the kiln shelf it sat on waffle patterned to prevent vacuum attachment? And for those with maltese cross, is that from a square shaped kiln just big enough for the blank? I'm curious how these patterns happen. I bet the random ones are caused by uneven mineral composition of the glass from poor mixing.
 


Edited by stargazer193857, 13 June 2021 - 07:50 PM.

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#10 sigma5678

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Posted 13 June 2021 - 08:33 PM

If you try to hog that, it may explode in your face. It even has some color, which is worse. Augustus got glass shards all over him when a blank exploded on him.


The blank is still good as a flat table top to put smaller blanks on, with padding between them, in case someone does not have a flat surface.


As for the waffle pattern, I'm curious what causes that. Was the kiln shelf it sat on waffle patterned to prevent vacuum attachment? And for those with maltese cross, is that from a square shaped kiln just big enough for the blank? I'm curious how these patterns happen. I bet the random ones are caused by uneven mineral composition of the glass from poor mixing.


Indeed I believe I read the thread where that was described. Pretty scary.

I've read that the pattern may be a function of how the cooling nozzles are arrayed when undergoing tempering. I found a decently interesting discussion on anistropic stresses in glass here:
https://www.glassonw...-tempered-glass
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#11 davidc135

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Posted 13 June 2021 - 09:49 PM

I did a quick check on the mirror 16" blank and it looks pretty bad. This is the first time I've tried this so just wanna post some pics of the process to make sure I did it correctly. I have an LED monitor that I looked at with a pair of sun glasses. Rotating the sun glasses by 45 degrees seemed to null the signal from the screen so I assume the screen is linearly polarized. I put the blank in between and could see a waffle pattern. Rotating the sun glasses shows some pretty nasty dark lines running through this.....

So I assume this piece of glass isn't suitable for mirror making. Let me know if there is anything else to be gleamed from this picture or if my process was flawed. I just searched the forums for a bit and tried this based on what others seemed to have done. 

Great photos illustrating strain.  David


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#12 MeridianStarGazer

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Posted 13 June 2021 - 10:21 PM

Indeed I believe I read the thread where that was described. Pretty scary.

I've read that the pattern may be a function of how the cooling nozzles are arrayed when undergoing tempering. I found a decently interesting discussion on anistropic stresses in glass here:
https://www.glassonw...-tempered-glass

I'm planning to build a kiln with multiple molds. I was going to have it double decker to get much more blanks for the same amount of heat. Then I realized the vertical heat differential might make that difficult. Not impossible, but very good data and design needed, more than I wanted to deal with. So I was going to have 2 in a square pattern. Then might be too heavy, so 1x2. Now I'm thinking of the cooling process, whether the edges near each other might cool slower than the edges near the walls.

 

Seeing all these strain patterns makes me wonder if I need to really avoid messing around. One controller for a single mold sounds like a waste. I wonder how many square shaped kilns there are that are making some of these patterns. Square is easier to build and easier to have a side door.

 

This patterns have too much pattern to not be systematic. I just really wonder what happened.



#13 sigma5678

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Posted 14 June 2021 - 11:01 AM

I took the scope out for a spin last night with my astrocam to see what I could learn.

First you can see below an afocal shot, imaged with my phone through a 26mm eye piece. The moon took up a nice portion of the FoV and it was decently easy to get it centered once I tweaked the guidescope (wasn't very well aligned to the mainscope). The azimuth rotation is pretty stiff and I noticed that the tube must be loose in its cradle because when I push or pull it the tube wobbles and the mount doesn't move. So that needs to be looked into.

When I mounted the barlow and camera onto the eyepiece holder the CG started to move just enough to make the scope slip a bit in altitude. I have some dumbbell weights I can lash to the back of the tube to counterbalance but not the most elegant solution. Also, when I remove the imaging train then scope is at risk for being too heavy in the back. Thankfully the ZWO 174MM isn''t that heavy so the adjustments in CG compensation were small. I had a fools hope of using my DSLR which was not gonna happen from a weight distribution perspective. 

 

You can see in another pic my camera connected to a Televue 2.5x barlow. It was hard to reach focus with the camera alone (not enough in-travel aka I couldn't get the sensor close enough to the secondary). With a barlow it was possible to get focus but man was that hard to achieve! The focuser is a threaded screw type which is not super easy to fiddle with at high magnifications especially when the target slips by so fast! I would love even a single speed rack type focuser. The telescope actually came with one but it was so large that it pushed the eye pieces too far way from the sensor and made my in-travel issues worse. 

 

I took a few pics of the moon and attached an unprocessed one. You can see it is soft but not too bad otherwise. Definitely a lot of dirt and such on the primary I haven't bothered to clean off yet (and theoretically could flat field some of that out). 

So I guess I'm looking at:

1. Really "needing" and eq platform especially if I want to image Jupiter/Saturn. Using the 2.5X barlow is exactly what I would want to do get increase my pixel scale but achieving focus or even setting up is a pain when the target moves out of the fov so fast. Though I assume I would have more time with the planets since they are much smaller and therefore would take longer to drift across the fov....

2. Need to improve the main tube's attachment to its cradle. Wobbles too much and makes push/pulling tough

3. Azimuth rotation is sticky so I need to see if it just needs some bolt loosening and a bit of cleaning or if this needs a re-design.

4. Not sure if I want to try to find a more low profile focuser.....any ideas?

5. I suppose that now that I am getting started looking at this there seems to be a lot I could do to modernize various aspects or just rebuilt in the same fashion but with new material. I have Berry's book and could follow the 10" f/6 plans. I suppose I could also start from scratch somewhat and re-measure the focal length, drop the secondary down a bit towards the primary and increase the amount of in vs out travel I would have to play with for any focuser I do choose.....That is tempting because I could just cut the top of the tub down and re-use what I have (or leave it as a baffle!). 

No wrong decisions here I guess. Just things to play with. :)    

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