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2" Diagonals clear aperture measurement reference


badhex

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7 hours ago, Louis D said:

Good sleuthing.  They're also sold as TPO brand by OPT (get it?) in the US.  I think one of mine has this branding if I were to check it.

Yes, seems there are a few brands using this GSO design which is good news. Looks like I'll be able to pick one up from TS for about €145 which is certainly more reasonable than the Baader at 2-3 times that. I do like the clicklock, though. 

Oh BTW, the quartz version of this is also available from TS, and is about €15 more expensive. 

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2 hours ago, wookie1965 said:

Right I've just measured the wall thickness on the telescope side of both my diagonals one a Altair and the I think is the skywatcher one. As you can see from the pictures I'm losing 9mm in the Altair and 7.8mm in the skywatcher. 

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Thanks wookie, this matches my findings as well. Seems that these (synta?) designs are to be avoided for those who want the full aperture of their largest EPs! 

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One more thing: in the course of my investigations, I came across this article from S@N in 2019, which covers "six of the best" 2" diagonals, two or three of which appear to be clones (Skywatcher, Omegon, perhaps the WO Durabright) and zero mention of clear aperture, only a note that the first two of the those three seem to be less contrasty which would possibly make sense. Poor show. 

https://www.skyatnightmagazine.com/reviews/six-of-the-best-2-inch-dielectric-diagonals-99-200/

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On 09/04/2022 at 14:17, Louis D said:

About $14 at today's pricing here in the States (see added links above).  Less facetiously, I quote from the quartz diagonal's description linked above:

High quality diagonal mirrors are usually made of BK7 glass. Quartz glass goes a notch higher - it offers greater thermal stability, resulting in minimal thermal expansion and contraction as compared to other glass types. The result is a minimal, if any, shift in focus due to temperature changes over long observation sessions, making this quartz diagonal ideal for photographic and imaging applications.

I can't say that I've ever noticed the difference.  I just picked up the used quartz version because it was being offered for the same price as the used regular glass version.  I wouldn't have picked it up otherwise.

Who would image through a diagonal?  Who observes long enough at one focus setting to notice thermal shift?  If the difference is only $14 at the retailer, it's probably much less at the manufacturer and could become the standard if used in all diagonals which would further drive down the price differential through economies of scale.  However, it's a case that no one would ever notice the difference; so why drive up the price, even slightly, for every diagonal just to have quartz in it?  It's just a marketing ploy in my opinion.

In theory, though it might vary according to who makes it, quartz can be made smoother than glass because it spalls a smaller chip when it is polished.

In practice, I doubt there is a visible difference unless it is a LOT more expensive.

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1 hour ago, Don Pensack said:

In theory, though it might vary according to who makes it, quartz can be made smoother than glass because it spalls a smaller chip when it is polished.

In practice, I doubt there is a visible difference unless it is a LOT more expensive.

If the quartz is laid down via vapor deposition on a substrate like a coating, it can be flat to the atomic level because it is crystalline, but I doubt that's how it would have been manufactured.  I think it just means it's pure silica amorphous glass.

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4 hours ago, Louis D said:

If the quartz is laid down via vapor deposition on a substrate like a coating, it can be flat to the atomic level because it is crystalline, but I doubt that's how it would have been manufactured.  I think it just means it's pure silica amorphous glass.

Yeah, what we call "quartz" is really "fused silica".

Even then, it can be polished smoother than glass:

example:  afm1.jpg

Fused silica is harder than almost any other substrate material. Typically, it will polish 2 to 4 times smoother than Pyrex glass. The atomic force microscope (AFM), with its sub-angstrom resolution capability, reveals the difference. These AFM scans were produced for Protostar by Charles Evans & Associates (Sunnyvale, California), and are of actual Protostar production samples.

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  • 2 weeks later...

I got both in the end. Hard to take a photo but even visually the aperture difference between this and the (synta) ones is significant. 

I hadlve had first light with it (last night) and so far it seems very well built piece of kit. One minor niggle is that the 1.25" EP adaptor gets a bit stuck sometimes on insertion, and a couple of times I nudged the scope enough to lose my current target. Hopefully with use it will insert a bit more smoothly. 

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1 hour ago, badhex said:

One minor niggle is that the 1.25" EP adaptor gets a bit stuck sometimes on insertion, and a couple of times I nudged the scope enough to lose my current target.

Yeah, GSO machines their eyepiece holders to a close tolerance.  You also have to watch out for the compression ring trying to ride up and jamming on an eyepiece safety undercut.  It hasn't happened to me yet, but I've never tried forcing an eyepiece up and out to precipitate it happening, either.  If it snags, I loosen the set screw further and jiggle the eyepiece a bit to reseat the compression ring before pulling again.  After it's out, I inspect the ring by running my finger around it to make sure it's fully seated before putting another eyepiece in.

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I had the iOptron 1.25" version. It was really well built but I sold it as I prefer using the 2".

Undercuts are a problem. It's the only negative against my LVWs. Sometimes they are tricky to get out.

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1 hour ago, Louis D said:

Yeah, GSO machines their eyepiece holders to a close tolerance.  You also have to watch out for the compression ring trying to ride up and jamming on an eyepiece safety undercut.  It hasn't happened to me yet, but I've never tried forcing an eyepiece up and out to precipitate it happening, either.  If it snags, I loosen the set screw further and jiggle the eyepiece a bit to reseat the compression ring before pulling again.  After it's out, I inspect the ring by running my finger around it to make sure it's fully seated before putting another eyepiece in.

 

Thanks for the tip. Thankfully only two of my A-team EPs have undercuts so I will take care whe using them. 

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1 hour ago, Mr Spock said:

I had the iOptron 1.25" version. It was really well built but I sold it as I prefer using the 2".

Undercuts are a problem. It's the only negative against my LVWs. Sometimes they are tricky to get out.

Do you find the 1.25" adapter gets stuck in your GSO 2"? Has it got better with use? 

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4 hours ago, badhex said:

I got both in the end. Hard to take a photo but even visually the aperture difference between this and the (synta) ones is significant. 

I hadlve had first light with it (last night) and so far it seems very well built piece of kit. One minor niggle is that the 1.25" EP adaptor gets a bit stuck sometimes on insertion, and a couple of times I nudged the scope enough to lose my current target. Hopefully with use it will insert a bit more smoothly. 

20220514_132352.thumb.jpg.b30fce6e969320347b932c3bfe271154.jpg

20220514_133120.thumb.jpg.ed1df4914ca287e1bf0f3c0f4c1ccc44.jpg

20220514_133140.thumb.jpg.e957053e597ef1cc44c7423725f28dd3.jpg

20220514_133315.thumb.jpg.3a1c69fa37111b1024919266c501f3fc.jpg

Looks like the one on the left in the bottom photo has a noticeably larger clear aperture.

Edited by Don Pensack
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55 minutes ago, Don Pensack said:

Looks like the one on the left in the bottom photo has a noticeably larger clear aperture.

Yes that rightmost one is the standard one supplied with some Skywatcher scopes like the Evostar, as well as being functionally identical to the Skywatcher Deluxe (not sure what makes it deluxe, it's the same to all intents and purposes!) Opticron, William Optics ones. I have both an Opticron and the standard Skywatcher, per discussion earlier in this thread the guess is the manufacturer might be Synta. 

Edited by badhex
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2 hours ago, Mr Spock said:

No, works just fine.

Interesting. Sometimes mine slots in perfectly, but a couple of times in the dark I guess I wasn't quite square when replacing it and it got a bit caught. To be fair, it was already attached to an EP so maybe I'll remove the EP first when inserting it. 

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  • 1 month later...
2 hours ago, pete_gamby said:

The Opticron diagonals were manufactured by Long Perng, as I suspect are many of the similar looking versions out there.

https://www.longperng.com.tw/goods.php?act=view&no=81

HTH

Cheers, Pete

 

Good sleuthing, bit surprised that a company known for high quality gear like Long Perng would have come up with a design that has such restrictions, but there you go. I may have said earlier in this thread but the 1.25" version of this is basically the same but scaled down, and therefore has the same potential for vignetting issues. Can we assume then that Long Perng made those as well? 

Edited by badhex
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  • 1 month later...
27 minutes ago, ED Splitter said:

I have a 100ED with standard diagonal. Would I likely get an improved view if this was upgraded or is it only restricting maximum field of view in a wide FOV eyepiece? 
 

It will only vignette for EPs with a field stop of more than 41.5mm, so you'd need to check that measurement - not always easy to find. Anything smaller than that and you're fine. 

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  • 1 year later...
On 09/04/2022 at 22:17, Louis D said:

About $14 at today's pricing here in the States (see added links above).  Less facetiously, I quote from the quartz diagonal's description linked above:

High quality diagonal mirrors are usually made of BK7 glass. Quartz glass goes a notch higher - it offers greater thermal stability, resulting in minimal thermal expansion and contraction as compared to other glass types. The result is a minimal, if any, shift in focus due to temperature changes over long observation sessions, making this quartz diagonal ideal for photographic and imaging applications.

I can't say that I've ever noticed the difference.  I just picked up the used quartz version because it was being offered for the same price as the used regular glass version.  I wouldn't have picked it up otherwise.

Who would image through a diagonal?  Who observes long enough at one focus setting to notice thermal shift?  If the difference is only $14 at the retailer, it's probably much less at the manufacturer and could become the standard if used in all diagonals which would further drive down the price differential through economies of scale.  However, it's a case that no one would ever notice the difference; so why drive up the price, even slightly, for every diagonal just to have quartz in it?  It's just a marketing ploy in my opinion.

Bit of an off-topic revival of this to say that when searching through the case for my recently reacquired Skymax 102, I found a Revelation branded version of the 1.25" GSO dialectric diagonal which I had clearly forgotten that I'd purchased, that went into storage in 2015. Seems like I was suckered into the "quartz" one as well 🙂

When I eventually get back to actually doing some observing, I'll have to try both this and the non-quartz version out side by side but I would be very surprised if I can detect any difference, as we have discussed. 

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