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How much better is a secondary holder than silicone?

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#1 MeridianStarGazer

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Posted 14 May 2022 - 08:56 PM

I read the outer 10% of secondary mirrors often has a turned down edge, and that a holder lip covers that up.

In climates of no dew, do you notice an improvement with a secondary mask? Do all secodary brands benefit, or just the low end ones?

#2 MeridianStarGazer

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Posted 14 May 2022 - 11:21 PM

I bet it depends on several factors, such as the magnification and how the secondary is already sized compared to the scope.

A paper elliptical cover is the best way to test. It would darken the view though, which would need to be accounted for.

Done right on a siliconed secondary, an elliptical face shield would also block light glancing off the side of the secondary, though there is far more of this on the spider.

Edited by MeridianStarGazer, 14 May 2022 - 11:27 PM.


#3 Starman1

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Posted 16 May 2022 - 09:30 AM

The best reasons for a shrouded secondary holder, like on the Astrosystems secondary holder, are several:

1) safety.  The mirror cannot break free and fall and crash on the primary.

2) security for big secondaries--important on larger dobs

3) lack of reflection from the side of the mirror, and if you paint that surface, the mirror may not be recoatable.

4) thermal inertia.  Secondary mirrors dew up easier if exposed.  The shroud holder reduces that tendency.

5) the ability to reach in and rotate the holder during collimation without touching the mirror or running the risk of breaking the glue bond

6) the shroud holder is less likely to yield astigmatism from pressure on the secondary mirror.

7) the lip of the holder covers the least accurate part of the mirror


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#4 BradFran

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Posted 16 May 2022 - 03:54 PM

6) the shroud holder is less likely to yield astigmatism from pressure on the secondary mirror.

This is a big one. The mirror expands and contracts a bit with temperature changes. The silicone applied at room temp could cause strain leading to astigmatism on a cold night. It's possible to glue it on with three globs just right to avoid this, but is tricky. A holder eliminates this when the mirror is properly installed as it has room to expand, contract and settle.

 

Downside to the holder is extra weight and a tiny bit more diffraction from added diameter and potentially needing a beefier spider. But I think it's worth it and you then avoid having to blacken the mirror edge and bevel.



#5 ausastronomer

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Posted 16 May 2022 - 05:55 PM

The best reasons for a shrouded secondary holder, like on the Astrosystems secondary holder, are several:

1) safety.  The mirror cannot break free and fall and crash on the primary.

2) security for big secondaries--important on larger dobs

3) lack of reflection from the side of the mirror, and if you paint that surface, the mirror may not be recoatable.

4) thermal inertia.  Secondary mirrors dew up easier if exposed.  The shroud holder reduces that tendency.

5) the ability to reach in and rotate the holder during collimation without touching the mirror or running the risk of breaking the glue bond

6) the shroud holder is less likely to yield astigmatism from pressure on the secondary mirror.

7) the lip of the holder covers the least accurate part of the mirror

 

All of those reasons and I consider them all pretty important, which is the reason I wouldn't ever have a scope with a secondary stuck on with silicone.

 

The other one that is important to me is the ability to install an internal dew heater with a proper secondary holder.  While some people live in fairly dew free areas the last thing you want to do is to travel a long distance to a star party, or dark site, and not be able to use the scope.

 

The Astrosystems Secondary Holder comes with a dew heater that works pretty well.  You need to use one of the spider vanes as a ground and run a thin insulated wire along the top for the +ve.  The Protostar Heated Secondary holder is the best available.  Mechanically it's the best holder and for the heater you just run the wires to the external heads of the mounting bolts for 2 of the 4 spider vanes.  The vanes are internally insulated and one carries +ve and the other ground or -ve.  I have 2 x Protostar Heated Secondary Holders and an Astrosystems Heated Secondary holder.  The Protostar Secondary Holders are without question "better". Like most things in life, "You get what you pay for".  Unfortunately supply from Bryan Greer at Protostar can be somewhat sporadic and lengthy these days.  It took over a year to get the heated secondary holder and quartz secondary from Protostar when we were building my 10" scope in 2010.  I was happy to wait because I wanted the best.  

 

Cheers


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#6 Jon Isaacs

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Posted 16 May 2022 - 06:18 PM

The Protostar Secondary Holders are without question "better".

 

 

The Protostar secondary center bolt is nylon.  From my perspective as a research engineer who operated a materials science research laboratory for nearly 30 years, a threaded nylon rod is a poor choice for an application that involves both bending and tension. There's a nice stress riser at the root of the thread and it is possible to cause it to fracture and the secondary holder and secondary go flying.. 

 

This happened to me with a scope I purchased secondhand. The bolt was black and I assumed it was steel, I snugged it up with the thumbscrews..  

 

I know now there's a warning somewhere in the literature, but as an engineer, it's just common sense that tightening, even over-tightening by hand, a thumbscrew should not result in catastrophic failure. 

 

Fortunately, the scope was an open truss and the secondary assembly hit the floor in instead of the primary mirror.

 

Jon



#7 ausastronomer

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Posted 16 May 2022 - 07:39 PM

The Protostar secondary center bolt is nylon.  From my perspective as a research engineer who operated a materials science research laboratory for nearly 30 years, a threaded nylon rod is a poor choice for an application that involves both bending and tension. There's a nice stress riser at the root of the thread and it is possible to cause it to fracture and the secondary holder and secondary go flying.. 

 

This happened to me with a scope I purchased secondhand. The bolt was black and I assumed it was steel, I snugged it up with the thumbscrews..  

 

I know now there's a warning somewhere in the literature, but as an engineer, it's just common sense that tightening, even over-tightening by hand, a thumbscrew should not result in catastrophic failure. 

 

Fortunately, the scope was an open truss and the secondary assembly hit the floor in instead of the primary mirror.

 

Jon

 

Jon,

 

I can only assume that someone prior to your ownership absolutely "coybowed" it.  It's a 1/2" diameter nylon (maybe its Delrin) bolt holding a piddly little secondary holder. I have 10mm Nylon bolts securing the marine batteries in my boat that weigh about 40kg each and those battery carriers are subject to a lot of load when you're crashing and banging through heavy seas. Way more load than a secondary mirror and holder weighing well under 1kg is ever going to impose, or that you could impose with the thumbscrews under "NORMAL" use..

 

In all seriousness, you only have to do a quick comparison with a Protostar Holder and an Astrosystems holder to know that one is A-grade and one is B grade.  For instance the outer shroud on the prostar holder is black carbon composite, on the astrosystems holder its a thin bit of pressed steel with black paint on it which falls off onto your mirror after a few years. While Bryan Greer leaves a lot to be desired in terms of his delivery promptness and sales support, I can't fault his design and construction quality. I can guarantee you that I am never likely to have a problem with my Protostar secondary holders.  Yours is the first one I have ever heard of failing and you don't know what it was subjected to prior to it failing.  I am pretty sure that Bryan Greer is in fact an engineer in a former life.

 

Cheers



#8 Jon Isaacs

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Posted 16 May 2022 - 10:07 PM

Jon,

 

I can only assume that someone prior to your ownership absolutely "coybowed" it.  It's a 1/2" diameter nylon (maybe its Delrin) bolt holding a piddly little secondary holder. I have 10mm Nylon bolts securing the marine batteries in my boat that weigh about 40kg each and those battery carriers are subject to a lot of load when you're crashing and banging through heavy seas. Way more load than a secondary mirror and holder weighing well under 1kg is ever going to impose, or that you could impose with the thumbscrews under "NORMAL" use..

 

In all seriousness, you only have to do a quick comparison with a Protostar Holder and an Astrosystems holder to know that one is A-grade and one is B grade.  For instance the outer shroud on the prostar holder is black carbon composite, on the astrosystems holder its a thin bit of pressed steel with black paint on it which falls off onto your mirror after a few years. While Bryan Greer leaves a lot to be desired in terms of his delivery promptness and sales support, I can't fault his design and construction quality. I can guarantee you that I am never likely to have a problem with my Protostar secondary holders.  Yours is the first one I have ever heard of failing and you don't know what it was subjected to prior to it failing.  I am pretty sure that Bryan Greer is in fact an engineer in a former life.

 

Cheers

 

This was an 8 inch scope, the center bolt was not 1/2 inch, it was probably 1/4 inch.

 

If anyone manhandled it, it was me, I assumed the bolt was steel and not nylon.

 

It is good to understand the state of stress in the bolt. Screws put significant loads on the nylon bolt, screws have a large mechanical advantage so while it doesn't take much torque to tighten then, the applied force is much higher.

 

The center bolt is in both tension and bending because the Protostar design uses the bending of the center bolt to provide the tilting of the secondary . The bottom of the thread is a stress riser, a location where a crack can initiate. Tightening the thumbscrews puts both tension and bending, you are opening the potential crack tip.  If too much force is applied, which is what I did, it fractures.

 

 Realize, this happened to me, it could happen to someone else.  You may think it's an OK design, I have designed literally millions of dollars worth of scientific research equipment, it's not a design I find acceptable.  

 

Jon



#9 havasman

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Posted 16 May 2022 - 10:20 PM

The other one that is important to me is the ability to install an internal dew heater with a proper secondary holder.  While some people live in fairly dew free areas the last thing you want to do is to travel a long distance to a star party, or dark site, and not be able to use the scope.

waytogo.gif

 

That's what prompted me to replace the upper end of my old XT10i with a complete Astrosystems system: spider, secondary holder, secondary and dew heater. The weight's easily dealt with on a solid metal tube Dob. And now all my Dobs have Astrosystems complete top ends. I never lose any observing time to dew even though my primary site and most others I visit are conducive to dew.

 

The visual performance improvement in the Orion scope was significant and I laid that entirely to the new 1/16th wave secondary being much better than the OE secondary..


Edited by havasman, 16 May 2022 - 10:28 PM.

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#10 MitchAlsup

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Posted 17 May 2022 - 09:34 PM

The best reasons for a shrouded secondary holder, like on the Astrosystems secondary holder, are several:

 

5) the ability to reach in and rotate the holder during collimation without touching the mirror or running the risk of breaking the glue bond

If the Upper assembly is properly constructed, there is NO need to rotate the secondary ! That is:: If the holes where the vanes are tightened are positioned properly, the secondary will need no rotation other than what is available in tilt-tip.

 

My 13"F/3 (my old 20" F/4 and my upcoming 20" F/3) has no provision for rotating the secondary, and has no <particular> problem with getting the secondary collimated. What little rotation might be necessary due to construction maladies can be managed with the tip-tilt collimation screws.

So, I question the need for #5



#11 Starman1

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Posted 18 May 2022 - 01:04 AM

If the Upper assembly is properly constructed, there is NO need to rotate the secondary ! That is:: If the holes where the vanes are tightened are positioned properly, the secondary will need no rotation other than what is available in tilt-tip.

 

My 13"F/3 (my old 20" F/4 and my upcoming 20" F/3) has no provision for rotating the secondary, and has no <particular> problem with getting the secondary collimated. What little rotation might be necessary due to construction maladies can be managed with the tip-tilt collimation screws.

So, I question the need for #5

Would that this were the case in the average commercial scope.

90% of the collimation threads on CN would disappear, as would all the mentions of "milk jug washers".

But mis-rotation of the secondary is as common as fleas.

And rotation of the secondary necessary on most scopes.

It is true a secondary holder can be constructed so the mirror need not rotate on a center bolt axis (ES truss dobs, for one), but rarely is.

In most scopes, the secondary freely rotates and is mis-rotated on most as the scopes come out of the box.

You know that, Mitch.


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#12 MeridianStarGazer

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Posted 18 May 2022 - 08:09 AM

Would that this were the case in the average commercial scope.
90% of the collimation threads on CN would disappear, as would all the mentions of "milk jug washers".
But mis-rotation of the secondary is as common as fleas.
And rotation of the secondary necessary on most scopes.
It is true a secondary holder can be constructed so the mirror need not rotate on a center bolt axis (ES truss dobs, for one), but rarely is.
In most scopes, the secondary freely rotates and is mis-rotated on most as the scopes come out of the box.
You know that, Mitch.


One can grab the stalk though. They don't need to grab the mirror itself.

#13 Don H

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Posted 18 May 2022 - 05:20 PM

...

 Realize, this happened to me, it could happen to someone else.  You may think it's an OK design, I have designed literally millions of dollars worth of scientific research equipment, it's not a design I find acceptable.  

 

Jon

I had a Protostar center bolt break on an 8" UTA years ago, too. I much prefer AstroSystems for spiders and secondary assemblies. One note of caution, I have found the tiny screws that hold the sleeve somehow come loose over the years. I periodically check them to be sure they are tight. Only once in a great while do they get loose, but I never had one fall out yet. I suppose a little Loctite would solve that, but I prefer not to have anything on the threads, so I can remove the flat easily when I want. I like to check all my screws on blocks, clamps, cells, spiders, etc every year or so just to prevent any looseness that might occur from wood drying, vibrations during transport, or repeated assembly.


Edited by Don H, 18 May 2022 - 05:21 PM.

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#14 MitchAlsup

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Posted 19 May 2022 - 12:55 PM

Would that this were the case in the average commercial scope.

90% of the collimation threads on CN would disappear, as would all the mentions of "milk jug washers".

But mis-rotation of the secondary is as common as fleas.

Common because the secondary mount allows for rotation.

If it did not, the rest of the problem would fall in line.

 

And rotation of the secondary necessary on most scopes.

Here is where I disagree.

If secondary mount manufactures did not provide for rotation, then telescope assembly houses would have to put the holes in the right places, and the downstream customer would not have to worry about it--over the entire life of the telescope.

 

It is true a secondary holder can be constructed so the mirror need not rotate on a center bolt axis (ES truss dobs, for one), but rarely is.

In most scopes, the secondary freely rotates and is mis-rotated on most as the scopes come out of the box.

You know that, Mitch.

Which is why I think that the problem is the ABILITY to rotate--because it is so often misused, misunderstood, and does harm to the average joe buying a first Newtonian.


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#15 Starman1

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Posted 19 May 2022 - 01:01 PM

Common because the secondary mount allows for rotation.
If it did not, the rest of the problem would fall in line.

Here is where I disagree.
If secondary mount manufactures did not provide for rotation, then telescope assembly houses would have to put the holes in the right places, and the downstream customer would not have to worry about it--over the entire life of the telescope.

Which is why I think that the problem is the ABILITY to rotate--because it is so often misused, misunderstood, and does harm to the average joe buying a first Newtonian.

I think we agree.
I've seen a few attempts to fix that.
Let's hope that continues.
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#16 a__l

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Posted 19 May 2022 - 05:25 PM

My opinion, never glue the secondary on silicone. Make a mechanical.
For many reasons, which are mainly listed above.
Silicone is an easy solution with a very high potential for bad consequences. Mainly used by those who don't want to think about good mechanics for attaching the secondary.



#17 MeridianStarGazer

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Posted 19 May 2022 - 05:34 PM

Even for a 2.6" or 2.14" secondary?

#18 a__l

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Posted 19 May 2022 - 06:03 PM

Many nuances. Astigmatism can be for any size. You will not be able to check this on an interference stand in the cold. Chemistry may not work on any size (over a wide range of temperatures). Etc.

 

The main thing is that you can not control the adhesive connection in any way. The mechanical can be disassembled and reassembled.


Edited by a__l, 19 May 2022 - 06:07 PM.


#19 MeridianStarGazer

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Posted 19 May 2022 - 08:22 PM

One advantage of a holder instead of silicone is it is easier for customers to install the mirror themselves. They get the certainty of buying directly from Antares, and the seller has less cost to front, assuming the holder costs less. It would, vs 1/30 wave.

#20 Oberon

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Posted 20 May 2022 - 06:25 AM

I like silicon because with 3 points I can ensure there is no undefined stress in the support.


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#21 Jon Isaacs

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Posted 20 May 2022 - 06:45 AM

I like silicon because with 3 points I can ensure there is no undefined stress in the support.

 

But there can be stress in the secondary.

 

Jon



#22 MeridianStarGazer

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Posted 20 May 2022 - 08:19 AM

I like silicon because with 3 points I can ensure there is no undefined stress in the support.

Nice to hear differing views to more accurately represent the spectrum.

Every current holder I've heard of does have undefined stess. There is a ring lip that gives 3 random points of contact instead of 3 known points. And there is a cotton pressure that pushes in the worst place, the center, instead of just behind the 3 points.

I would have 3 thick prongs that push behind 3 slightly raised points in the lip, and then cotton push the prongs down.

Even then, 3 silicone dots hold the mirror more optimally as far as gravitational stresses, but less optimally thermally. Which is best likely depends on mirror size and where the viewing is done. Attaching at 40% instead of 50% should reduce some thermal stress.

Dew can also be fought with a baffle the size of the secondary. One would also do the same as the primary and have 3 safety clips that don't touch the secondary, or a ring that does not touch it, using silicon to do the heavy lifting.

Edited by MeridianStarGazer, 20 May 2022 - 08:25 AM.


#23 Pinbout

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Posted 20 May 2022 - 08:27 AM

 

The Protostar secondary center bolt is nylon.  From my perspective as a research engineer who operated a materials science research laboratory for nearly 30 years, a threaded nylon rod is a poor choice for an application that involves both bending and tension.

they chose nylon cause its compliant... you don't need to have a pivot inside the holder then.

 

good choice when you don't think binary.


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#24 MeridianStarGazer

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Posted 20 May 2022 - 08:38 AM

One thing I don't understand is how the central bolt holds the plates rigidly enough. Are the threads cemented? Most threads have clearance in normal nuts and bolts.

The single bolt extending from the spider, to a plate, and putting the adjusters then in the plate, sure does put a lot of stock in that stalk.

Also, any pressure at all, even from cotton, concerns me. Maybe I'm way over worrying what has already been measured to be within tolerances. But I'm liking silicone attachments with a safety shroud that does not touch.

If you really want to fight dew, I think silver facing the back of the mirror would do more than a plastic wall.

#25 Starman1

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Posted 20 May 2022 - 08:52 AM

If you want uniform pressure on the secondary, a cylindrical piece of EPS (Styrofoam), cut at a 45° angle, can easily be sanded to exactly the right length to fill the shroud behind the mirror, exert

an extremely minor, and uniform, pressure on the secondary to hold it against the lip and not allow wiggling, can replace the cotton batting usually used in the holder.  It has the additional advantage

of insulating the entire back of the secondary mirror, slowing the cooling of the mirror, so its thermal characteristics are superior to foam rubber or cotton batting.  It's also lighter, which is not bad for balance.

I'm surprised more makers don't use it.  Maybe it has to do with the availability of styrofoam cylinders of various widths and the need to sand to fit.




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