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What is the best source of 4.5" secondary mirrors?

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25 replies to this topic

#1 stargazer193857

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Posted 22 November 2019 - 01:06 PM

I'm sure this was asked before, but a 2 minute search did not find it.
I know Agenaastro has 104mm and 120mm. The 120mm is quartz, and unknown thickness or 3rd party confirmed quality.

Antares has 4" as their advertised largest. Very impressive claim of using low thermal expansion glass, 96-98% reflectivity, tested optically before and after aluminizing. They admit that not all come out right, and that they reject those, and that's why they don't just do statistical testing. But no 4.5" mirrors advertised.

Do people make their own?

#2 Lukes1040

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Posted 22 November 2019 - 01:42 PM

https://www.astrosystems.biz/secm1.htm

 

Astrosystems sells them.



#3 ngc7319_20

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Posted 22 November 2019 - 01:42 PM

No experience with these, but...

 

https://www.astrosystems.biz/secm1.htm



#4 petert913

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Posted 22 November 2019 - 02:01 PM

Wow, almost $600 for a secondary flat mirror ?  Ouch.



#5 stargazer193857

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Posted 22 November 2019 - 02:24 PM

Thanks for the link. And the $600 4.5" is 1/10 wave, too. Makes me wonder how good the Agena $230 120mm one is. I'm guessing they test every 10th one.

I read you can't see a difference between 1/20 and 1/30 and can't even be sure in a shop.

Someone can make their own, but it might cost more. To save money, you need to make more than one.


When secondary coaters quote prices by MA size, for example MA less than 3.1" is coated for $15, does MA means major axis or minor axis? They say 3.1" is $35, and greater than 3.1" needs a quote. I think ma is minor axis, but I've seen MA used both ways.

#6 stargazer193857

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Posted 22 November 2019 - 02:39 PM

Some places are not measuring edge to edge when they quote flatness.
An oversized mirror even insulated the center from thermal effects.

#7 Steve Dodds

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Posted 22 November 2019 - 02:52 PM

Astrosystems doesn't make diagonals, they resell them from here  https://www.ostahowskioptics.com/  Go to the source they will probably be cheaper.


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#8 Kunama

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Posted 22 November 2019 - 03:30 PM

Thanks for the link. And the $600 4.5" is 1/10 wave, too. Makes me wonder how good the Agena $230 120mm one is. I'm guessing they test every 10th one.

I read you can't see a difference between 1/20 and 1/30 and can't even be sure in a shop.

Someone can make their own, but it might cost more. To save money, you need to make more than one.


When secondary coaters quote prices by MA size, for example MA less than 3.1" is coated for $15, does MA means major axis or minor axis? They say 3.1" is $35, and greater than 3.1" needs a quote. I think ma is minor axis, but I've seen MA used both ways.

 

These Ostahowski 4.5" flats are guaranteed to be minimum 1/10 wave.  I have one that was recently tested and was actually quite a bit better than that.

Reflectance after 5 years is still 95% ...... nice flats.  They are making some quartz one at the moment $750 ....


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#9 coinboy1

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Posted 22 November 2019 - 03:38 PM

The Agena Astro / GSO 120mm quartz diagonal is very nice. I had one for my 14.5" F/4 primary for newtonian configuration and sold it to Zane for his F/2.9 Dob project. The thickness is .75" thick. The image quality was very good. The price point unbeatable. 


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#10 stargazer193857

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Posted 22 November 2019 - 05:22 PM

Ostahowskioptics boasts that their diagonal layer thicknesses are optimized for 45 degrees. I sure hope all diagonal makers do that with their enhanced coatings, but why don't they say so.

If you ever order a low rider secondary, I wonder how much it costs to have the one secondary in there at a custom layer thickness. Maybe not much compared to a 50" primary.

#11 stargazer193857

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Posted 22 November 2019 - 05:27 PM

Wow, almost $600 for a secondary flat mirror ? Ouch.


That is an estimate for a custom order. Their supplier says $750.

I'd like to buy a 1/10 wave reference flat just so I can know I'm not getting a lemon, and I think I'd go with the GSO. A 30% CO illuminated more field.

#12 Kunama

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Posted 22 November 2019 - 08:47 PM

Antares can do larger diagonals than 4" but at a price $$$.

 

What scope do you have that requires a 4.5" diagonal? Have you accounted for the lip of a holder, FIF, etc



#13 stargazer193857

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Posted 22 November 2019 - 10:46 PM

Antares can do larger diagonals than 4" but at a price $$$.

What scope do you have that requires a 4.5" diagonal? Have you accounted for the lip of a holder, FIF, etc

I'm thinking 16" f3.5, 1" gap between mirror and truss, and extra 4.5" of GSO focuser. 13.5"/3.5"+0.5"= 4.5".

Is a holder really necessary?
I guess it removes the question of silicon coming undone or silicon stressing the mirror or doing its own flex. Even insulated the edge a bit, and allows easy swap out.

...
I don't know how the lip is made or how there is enough contact to hold the mirror but not so much there is pinched optics.

Edited by stargazer193857, 22 November 2019 - 10:53 PM.


#14 Kunama

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Posted 22 November 2019 - 11:09 PM

I'm thinking 16" f3.5, 1" gap between mirror and truss, and extra 4.5" of GSO focuser. 13.5"/3.5"+0.5"= 4.5".

Is a holder really necessary?
I guess it removes the question of silicon coming undone or silicon stressing the mirror or doing its own flex. Even insulated the edge a bit, and allows easy swap out.

...
I don't know how the lip is made or how there is enough contact to hold the mirror but not so much there is pinched optics.

I use the Astrosystems holder rather than a 'glued to a stalk' method.  Some of the holders have a rather wide lip of about 3mm.  Mine has the lip only 1.5mm therefore not obscuring the mirror too much.

I think the key is not to pack the back of the mirror too much.  On mine I have about 1cm of dacron wadding behind the glass, the rest of the holder is filled with a styrene foam plug cut at 45ยบ so together they prevent the mirror shifting. It is still loose enough that it 'rattles' if shaken.... (not stirred!)

 

For the nervous types a lanyard can be siliconed to the back as an added safety mech.  



#15 stargazer193857

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Posted 22 November 2019 - 11:33 PM

That makes sense. It only needs to be strong enough to resist a fraction of gravity, and at multiple temperatures. But give the mass some inertia, and don't be worried about the rattle.

...

The 120mm would work great in an 18" at f3.5.

....
15" f3.6 loves 4".
16" f3.375 loves 4.5".
And 18" f3.5 is fine with 120mm.
This is with a GSO focuser, focal plane 5.5" from the on axis aperture.

Edited by stargazer193857, 23 November 2019 - 12:35 AM.


#16 coinboy1

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Posted 23 November 2019 - 07:22 AM

I 3d printed my secondary assembly out of PETG versus going with a commercial system for the 120mm GSO quartz diagonal. The secondary shroud includes a lip so the mirror wont slide out, but I also siliconed the mirror to the secondary backing with 6 equally spaced dots for piece of mind. 

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Edited by coinboy1, 23 November 2019 - 07:23 AM.

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#17 stargazer193857

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Posted 23 November 2019 - 10:09 AM

3D printing. Great idea. I wonder how much scaffolding the lip took. It is allowed a 45 degree underhang.

I just realized u could make an elliptical lip and glue it to the tube, but only if the glue is much stronger than silicon. A scary aspect of silicon is how strong it might be in 120+ deg heat, and whether it creeps.

#18 coinboy1

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Posted 23 November 2019 - 10:36 AM

The lip was printed with the shroud. No supports needed.

#19 stargazer193857

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Posted 23 November 2019 - 10:54 AM

Does anyone know typical aluminizing prices for 4.5" minor axis secondaries at places like Majestic?

#20 vehnae

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Posted 23 November 2019 - 05:22 PM

I 3d printed my secondary assembly out of PETG versus going with a commercial system for the 120mm GSO quartz diagonal. The secondary shroud includes a lip so the mirror wont slide out, but I also siliconed the mirror to the secondary backing with 6 equally spaced dots for piece of mind. 

Interesting! How thick walls did you use for the shell? Or is the point of the red plug to also keep the shell in its form? How are they held together?

 

 ++ Jari



#21 pancho61

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Posted 23 November 2019 - 06:37 PM

Hello,

 

In Europe you have this provider. Locate in France.

 

http://www.mirro-sph...-ellptique.html

 

He is one of the best for primary mirror and he did secondary too.

 

Clear skies



#22 Peter Nance

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Posted 25 November 2019 - 11:42 AM

$600 is just about the blank cost depending on size and material. Unless you want a mirror made from plate glass.

Then you need to find someone to edge it into an ellipse. Treepanning something that big would be an interesting setup.

If you search in the vendor forum you might find someone who just announced making large diagonals. waytogo.gif



#23 JohnH

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Posted 25 November 2019 - 01:13 PM

In the course of looking through my old ATM stuff, I found a box containing three year-old secondaries and one new one from I believe that Optical supplier Who Shall Not Be Named because he caused such controversy on here. But anyway in pursuit of my goal of doing a 24 in corrected Dell Kirkham with constant eyepiece height I acquired a huge 5-inch secondary that is uncoated. And will storing all this away I found the box I had my spare parts in and sure enough there is a four and a half inch in there. It does have damage Coatings but it doesn't look like they actually went through and damaged glass so....

#24 stargazer193857

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Posted 30 November 2019 - 10:02 AM

This is another source. They are at least as expensive as Allied, but they offer Zerodur and Fuused Silica. I think AgenaAstro is hard to beat.

#25 Oberon

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Posted 04 December 2019 - 04:10 AM

There is always Orion Optics. Up to 170mm.

 

https://www.orionopt...ticalflats.html




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