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In Search of an 8-in. Mirror

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#1 Mikeson

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Posted 21 May 2024 - 09:12 AM

Good morning,

 

I'm wondering if anyone has any leads on where I might find an 8-in. F6 mirror I could use in this GSO cel: 

 

https://agenaastro.c...e0aAgbfEALw_wcB

 

This cel can mount a mirror with max thickness of between 22 - 27mm. Max width is 200mm.

 

I've been waiting for the GSO mirrors to come back in stock at Agena for months now. The date keeps getting pushed. I have found other sources for mirrors but they are generally too thick for this cel. 

 

Figured I'd see if anyone had any ideas. I've been searching for months -- to no avail.

 

Best,

 

-Ed

 


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#2 Mikeson

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Posted 21 May 2024 - 09:21 AM

Conversely, I may just see if Agena would be willing to take the cell back in trade for some other supplies I require. Then I'd be freed up to simply make my own cell and mount whatever mirror I can find in stock presently. Just emailed them. We shall see. These mirrors seem to be difficult to come by--yet I'm too lazy to grind my own. lol.gif


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#3 MeridianStarGazer

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Posted 21 May 2024 - 09:51 AM

I would not say you are lazy for not wanting to grind your own. There are start up costs, time costs, and uncertainty of quality. You might even find a shortage of blanks, at least ones without chips.

Pinbout likely can make you an 8" f6, if he is still doing ATM. He might even have an old one laying around.

#4 Mikeson

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Posted 21 May 2024 - 09:59 AM

I would not say you are lazy for not wanting to grind your own. There are start up costs, time costs, and uncertainty of quality. You might even find a shortage of blanks, at least ones without chips.

Pinbout likely can make you an 8" f6, if he is still doing ATM. He might even have an old one laying around.

Fair point on startup costs. I live in CT and my parents are down in Florida. When my father passed in August, he left behind several blanks but I'm unsure as to what state any of them are in and it's not easy for me to get down and test/measure them all. Otherwise, I'd pounce on one of those. LOL. 

 

At this point, I'm thinking strongly of just making my own cell that can accommodate the slightly thicker mirrors I do see available. Hoping Agena would give me a credit for the cell I now likely can't use. Still have the original box but I ordered it all the way back in March, thinking mirrors would be available by now. We shall see:)



#5 MeridianStarGazer

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Posted 21 May 2024 - 10:10 AM

Make sure you tell them why you are returning it, that fair is fair, that they gave dates, and they can still save the sale if they sell you a mirror. I suspect they will be reasonable.

#6 Mikeson

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Posted 21 May 2024 - 10:40 AM

Make sure you tell them why you are returning it, that fair is fair, that they gave dates, and they can still save the sale if they sell you a mirror. I suspect they will be reasonable.

yeah, they're good folks, I have found. 



#7 Sincos

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Posted 21 May 2024 - 11:20 AM

Fair point on startup costs. I live in CT and my parents are down in Florida. When my father passed in August, he left behind several blanks but I'm unsure as to what state any of them are in and it's not easy for me to get down and test/measure them all. Otherwise, I'd pounce on one of those. LOL. 

 

 

Why are you seeing a problem , get one of your father’s blanks shipped up to you and begin . It does not matter what stage it is in , your dad picked it already so finish it for him and posterity’s sake . Besides you will be helping mom distribute fond memories and making new ones . Finding out what state they are in is part of the learning to push glass . You can begin to make a knife edge/ Foucault tester while waiting for them to be shipped from Florida .

 Good Luck and Clear Skies


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#8 dan_h

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Posted 21 May 2024 - 11:40 AM

I have a GSO 8" f6.  I just measured it:  25mm thick at the edge,  205mm diameter.   

 

The diameter would seem to be the difficulty. Not many 7.87 inch mirrors around. 

 

dan



#9 Mikeson

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Posted 21 May 2024 - 12:44 PM

I think I'm actually going to return the cell and make this:

 

https://garyseronik....your-reflector/

 

Simple and elegant. I love the acorn nut on the fender washer. Brilliant pivot point. 

 

The folks at Agena are very kind. No problem if I wish to exchange the cell - just need to subtract their shipping costs (perfectly understandable).


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#10 Mikeson

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Posted 21 May 2024 - 12:45 PM

Why are you seeing a problem , get one of your father’s blanks shipped up to you and begin . It does not matter what stage it is in , your dad picked it already so finish it for him and posterity’s sake . Besides you will be helping mom distribute fond memories and making new ones . Finding out what state they are in is part of the learning to push glass . You can begin to make a knife edge/ Foucault tester while waiting for them to be shipped from Florida .

 Good Luck and Clear Skies

Totally agree but getting my mother to go through his stuff to find and ship all these blanks is tough. As it is, I AM using his 8-in. fiberglass tube for this project. 

 

The next project is going to be an astrograph that I can mount to his Byers 85. Next time I'm in Florida, the plan is to build a small crate for it and bring it back on the airplane with me:)


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#11 Sincos

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Posted 21 May 2024 - 02:02 PM

The next project sounds exciting. In the meantime if it is only an 8” mirror almost any support cell will work . In this model the mirror is sitting on a wooden ring with furniture pads to float on . No springs to collimate but rather used super bounce balls cut in half .

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#12 Mikeson

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Posted 21 May 2024 - 02:09 PM

The next project sounds exciting. In the meantime if it is only an 8” mirror almost any support cell will work . In this model the mirror is sitting on a wooden ring with furniture pads to float on . No springs to collimate but rather used super bounce balls cut in half .

This is what I find so fun about this hobby...there are a million ways to do things. Some more elegant than others, and every time I go looking for ideas, I find something new. 

 

Neat!



#13 MeridianStarGazer

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Posted 21 May 2024 - 02:35 PM

What color are you going to make the base? Or you doing baltic birch?

#14 Mikeson

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Posted 21 May 2024 - 02:44 PM

What color are you going to make the base? Or you doing baltic birch?

I was thinking of using 3/4-in. plywood I have laying around, but then coating it with pickup truck spray bed liner for a solid bullet-proof coating. I'd probably avoid spraying on bearing areas.....I had once thought of using phenolic plywood - which is super nice - but super expensive.


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#15 MeridianStarGazer

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Posted 21 May 2024 - 03:08 PM

I was thinking of using 3/4-in. plywood I have laying around, but then coating it with pickup truck spray bed liner for a solid bullet-proof coating. I'd probably avoid spraying on bearing areas.....I had once thought of using phenolic plywood - which is super nice - but super expensive.

As soon as I read 3/4", I was going to suggest 1/2" for lighter weight and lower price. But then you said you have the 3/4" laying around. Yeah, definitely go with that. And if you have experience applying bed liner, I recommend doing that and showing off a unique scope.

We have so many skilled groups. Some weild steel mirror cells. Others use aluminum tube and bolts or glue. Some use wood. Others cast aluminum. Pipe fitting focusers. Each bringing their trade insights into the construction. Bring them all.

Edited by MeridianStarGazer, 21 May 2024 - 04:35 PM.

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#16 Tom Stock

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Posted 21 May 2024 - 08:39 PM

You wouldn't really need to return the cell. Just use some nylon spacers and longer screws for the mirror clips.  Most 8" mirrors are going to be 1" thick or less which is what the cell is designed for.

 

Have you tried posting a wanted ad in the classifieds?

 

You may also want to check astromart.


Edited by Tom Stock, 21 May 2024 - 08:42 PM.


#17 Mikeson

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Posted 22 May 2024 - 08:36 AM

OK folks, I was able to source an 8-in. F/6 mirror finally. Just gotta buy a 2-in. (minor axis) secondary to go along with it and I should be good to go. Last night I was in my workshop making a circle-cutting jig for the bandsaw with which I can make my custom cell. Fun. I have a background in woodworking which helps--spent several years at a woodworking magazine and loved every second of it. 

 

Thank you again for all of your kind comments and suggestions. As this project takes on steam, I'll be sure to upload some images for more suggestions from ya'll.

 

Cheers and be well.

 

-Ed


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#18 Mikeson

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Posted 22 May 2024 - 09:08 AM

You wouldn't really need to return the cell. Just use some nylon spacers and longer screws for the mirror clips.  Most 8" mirrors are going to be 1" thick or less which is what the cell is designed for.

 

Have you tried posting a wanted ad in the classifieds?

 

You may also want to check astromart.

I decided finally to build my own cell. Reason being that the cell I had purchased didn't quite fit inside my tube. I began to sand back the interior of the fiberglass tube a bit (like 1/32-in.) but I feared sanding too much -- through the gel coat and into the actual fiberglass. So I stopped and thought it would be more fun to build my own that fits like a glove. 

 

Another issue with the store-bought cell is the mounting screw placement on the cell. In a nutshell: the screw holes would be within a 1/4-in. of the edge of the telescope tube. This set off alarm bells for me. I could see that fiberglass cracking with holes so close to the edge. 

 

In the end, I'll learn more by doing it myself, anyhow. 


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#19 Tom Stock

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Posted 22 May 2024 - 03:58 PM

I decided finally to build my own cell. Reason being that the cell I had purchased didn't quite fit inside my tube. I began to sand back the interior of the fiberglass tube a bit (like 1/32-in.) but I feared sanding too much -- through the gel coat and into the actual fiberglass. So I stopped and thought it would be more fun to build my own that fits like a glove. 

 

Another issue with the store-bought cell is the mounting screw placement on the cell. In a nutshell: the screw holes would be within a 1/4-in. of the edge of the telescope tube. This set off alarm bells for me. I could see that fiberglass cracking with holes so close to the edge. 

 

In the end, I'll learn more by doing it myself, anyhow. 

Great! The GSO cell isnt all tbat great anyway and is desinged to fit their steel tubes which is why it had weird screw hole positions.


Edited by Tom Stock, 22 May 2024 - 03:59 PM.

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#20 Mikeson

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Posted 23 May 2024 - 08:55 AM

UPDATE:  Agena just put on line, some new 8-in. quartz mirrors. Available here:

 

https://agenaastro.c...080-quartz.html

 

And the price on all of there mirrors has gone down. At $275, the prices of the quartz mirror is less than what they were charging for pyrex at 325 several months ago. Apparently the supply issues dating back to the COVID era have been solved and prices have eased.

 

Best,

 

-Ed


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#21 MeridianStarGazer

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Posted 23 May 2024 - 09:46 AM

Go for it!

 

GSO was smart. First they sell soda lime till that saturates, then BK7, now Quartz. They sell more that way, since some of the people some people will replace their mirrors with ones of the new substrate.

 

Quartz density is 2.65 vs 2.5 soda lime

Quartz heat capacity is 89% that of soda lime on a volume basis.

Quartz conductivity is much higher

CTE 5.5 x 10^(-7) is 1/16 that of soda lime

 

CTE for quartz is the clear win, though does not kick in till the glass cools enough to not give off thermals, after which the other glasses returned to normal shape. Sky and Telescope had an article on that.

 

Many here think high conductivity is good, but I believe conductivity being high is actually bad. It means it bleeds a lot more heat early on. But even after the heat is bled, I think it will keep up bleeding more than soda lime and pyrex would at that temperature being cooler only because it can keep drawing heat from the center. Heat flow from center is proportional to temperature gradient x conductivity. So it does not win by being cooler but instead just has worse thermals in the beginning. I am comparing thermals, though. As for the figure, yes, higher conductivity will get the mirror back to design shape faster, though that is not needed with such a low CTE.

 

Zerodur is all win and no tradeoffs, but is expensive. Pyrex is also all win against soda lime, but again expensive material and tougher so takes longer grind and polish. A thinner soda lime mirror can be polished as fast as the pyrex and cool faster. I think thinner soda lime is the all around win for cost, cooling rate, manufacturing ease, and fewer thermals. GSO timed it right though, with the market primed wanting quartz, they are giving them quarts. Same thing happened with BK4 vs BK7 prisms, for refractors. Above f4, BK7 is the clear winner and bubble free, yet the market associates BK4 as the higher quality, and manufacturers often make it to flatter tolerances and alignment just because that is who is paying.


Edited by MeridianStarGazer, 23 May 2024 - 10:36 AM.

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#22 jhwilmes

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Posted 24 May 2024 - 10:11 AM

 

Quartz density is 2.65 vs 2.5 soda lime

Quartz heat capacity is 89% that of soda lime on a volume basis.

Quartz conductivity is much higher

CTE 5.5 x 10^(-7) is 1/16 that of soda lime

 

How well does each material figure?  I.e., I have seen some reports of mirrors made out of some glasses (I forget which ones, but the complaints were focused at mass-produced mirrors like those made by GSO) being too soft to refigure later if a problem emerges, or the softness of the material causing it to have microripples.  I have heard quartz is less likely to have these issues - so is a quartz mirror more likely to have a better figure (assuming identical polishing errors) than say a soda-lime mirror?



#23 MeridianStarGazer

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Posted 24 May 2024 - 10:33 AM

How well does each material figure? I.e., I have seen some reports of mirrors made out of some glasses (I forget which ones, but the complaints were focused at mass-produced mirrors like those made by GSO) being too soft to refigure later if a problem emerges, or the softness of the material causing it to have microripples. I have heard quartz is less likely to have these issues - so is a quartz mirror more likely to have a better figure (assuming identical polishing errors) than say a soda-lime mirror?

I've not heard of those problems, even heard of soda lime taking a good polish.

Zambuto says speed polishing causes nonsmoothness, and that slow, light polishinging with a soft lap and dilute cerium gives a much smoother finish.

I have read that a "super polish" can be twice as fine with fused silica vs pyrex, but that is a pH controlled technique that is completely different. Also quartz is different.

Hand polishinging several strokes in one direction and then immediately switching to a different direction can cause dog biscuit, and leave astigmatism.

I've not heard about quartz and soda lime differences though. If so, I'd question the techniques rather than the glass.

I have read that fused silica is a pleasure to figure because they can test right away. Quartz is likely the same there.

For soda lime, a fixed post polisher and plenty of water should reduce uneven heating to a level that dog biscuit is not an issue. Soda lime grinds easier, so requires less weight and less heat to polish. Even if quartz does not change shape, it will store the extra heat from the extra force needed to abraid it, and those thermals will be in the foucault image. Thin soda lime will be free of thermals sooner.

...


Go ahead and buy the quartz mirror, especially if the price is right. But don't get caught up in all the hype. Mirror makers know that for the same money, you get the same quality regardless of glass type. Tougher glass takes longer to polish. Pyrex is bouncier than soda lime. Quartz is heavier. Thick mirrors only need 3 support points.

Edited by MeridianStarGazer, 25 May 2024 - 08:59 AM.

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#24 Mikeson

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Posted 24 May 2024 - 08:18 PM

Largely finished the primary cell today. Just need to paint it black and mount the fan. Pics to come. Curious where some of you mount the AA battery packs to power fan. On the tube or on the base (with a longer cable that doesn’t impede altitude adjustment)?
Cheers,
-Ed
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#25 Mikeson

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Posted 24 May 2024 - 08:21 PM

I do however need to shorten my springs a bit. They begin to get decent tension when compressed from 2 inches down to about 1.5 /1.25. Way too much distance between the two plywood plates. That said, the mirror plate seems to adjust nicely with the three star knobs I put on. So far so good.
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