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2" Diagonals clear aperture measurement reference


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Hello all,

after a brief discussion on the postie thread about the clear aperture of 2" diagonals, specifically the Baader 2" BBHS, I remembered my own two have features which probably reduce the somewhat aperture more than is ideal. 

 

 

I did a bit of digging for measurements on the two I own but was unable to find any answers, so I decided to buy a digital caliper (or pair of calipers, if you like the plurale tantum 🙂) and measure them myself, sadly with somewhat disappointing results. 

 

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Standard Skywatcher 2" diagonal as supplied with the Evostar and other series

  • Inner threaded sleeve on the telescope side measuring 43.3mm
  • Lip quite far inside on the eyepiece side measuring only 41.5mm 

 

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GSO / Revelation / Skywatcher Deluxe 2" / other brands

  • Essentially identical to the standard SW one despite cosmetic differences.
  • Threaded sleeve on telescope side 43.3mm
  • Lip on EP side 41.5mm

(EDIT: This one is not GSO/Revelation, I was mistaken. It's actually Opticron.)

I know that the Baader BBHS 2" diagonal has a 47.5mm clear aperture as this is what started the conversation, and it is listed by Baader. This means there is a 6mm difference between the Baader and these two models, so presumably some vignetting must occur with these two for heavyweight 2" EPs with larger field stops, such as the Panoptic 41mm with a field stop of 46mm (I believe this is the largest FOV possible but happy to be corrected). 

 

Perhaps if other people know measurements for other brands/models of 2" diagonals, they could be added to this thread for reference. 

Edited by badhex
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1 hour ago, wookie1965 said:

Just measured one side of my Altair lightwave on the Telescope side its 47mm but the eyepiece side I cannot get my gauge in to measure. 

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I had the same issue on the EP side. I tried to unscrew the housing but didn't want to force it, so I cut a piece a stiff paper in tiny 0.5mm increments until it fit through the bottom lip part, then measured the paper. 

It looks like the Altair also has the same lip at the bottom on the EP side, I guess maybe the intention is to stop EPs with a long barrel from crashing into the mirror? If so, they could achieve the same with a much narrower lip (at least on the SW ones). 

 

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16 minutes ago, Don Pensack said:

Largest 2" eyepiece field stop 46.5mm

TeleVue 2" diagonals--47mm on the scope side, 45.5mm eyepiece barrel stop on the eyepiece side.

Ah, yeah I guessed it was an EP barrel stop, hadn't refreshed to see your post.

So the Baader still wins at the moment then. I suspect it's perhaps quite difficult mechanically (and in fact pointless if largest field stop is 46.5mm) to go any larger. 

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1 hour ago, Don Pensack said:

Largest 2" eyepiece field stop 46.5mm

Thanks for the confirmation, that's the Pentax XW40 I guess 🤔

Hence why I got the BBHS 2" diagonal, to try and use as much of that field stop as possible. Good to know the TeleVue is also maximised too 👍

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Edited by HollyHound
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39 minutes ago, badhex said:

I had the same issue on the EP side. I tried to unscrew the housing but didn't want to force it, so I cut a piece a stiff paper in tiny 0.5mm increments until it fit through the bottom lip part, then measured the paper. 

It looks like the Altair also has the same lip at the bottom on the EP side, I guess maybe the intention is to stop EPs with a long barrel from crashing into the mirror? If so, they could achieve the same with a much narrower lip (at least on the SW ones). 

 

Did the card trick EP side is 41mm and as I measured before the scope side is 47mm quite a difference, Ep side is 1 and 5/8 and scope is 1 and 7/8 both not 2" as advertised. So it seems none of the 2" diagonals come  close to the 50mm they should be.

 

Edited by wookie1965
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Posted (edited)
On 08/04/2022 at 16:46, Mr Spock said:

My Revelation measures 47.5mm on the EP side and 48.0mm on the scope side.

Interesting @Mr Spock - which Revelation is it? Does it have an EP barrel stop lip on the EP side near where the tube joins the mirror housing?

The black one I posted above is also Revelation, but it has the inner sleeve a few mm into the telescope side and EP barrel stop.

Would be great if there is an affordable Revelation option which maximises clear aperture!

Edited by badhex
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55 minutes ago, wookie1965 said:

Did the card trick EP side is 41mm and as I measured before the scope side is 47mm quite a difference, Ep side is 1 and 5/8 and scope is 1 and 7/8 both not 2" as advertised. So it seems none of the 2" diagonals come  close to the 50mm they should be.

 

This is essentially the same as the SW ones then - stopped down by the EP barrel stop. I guess you will always have to lose at least a few mm due to the telescope side which has to be 2" outer diameter by necessity, but 41mm seems like quite a step down. 

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Great quality too. I've had a few diagonals over the years and this is the best of them all.

I had a William Optics a few years ago. That vignetted a 22mm T4 Nagler :ohmy: I got rid of it.

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1 hour ago, Mr Spock said:

As you can see from the image, there are no restrictions inside the tube. It is as the size stated 👍

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Lovely, thanks for the images too, looks like a nice piece of kit as you say. Good to know that there is max aperture option that doesn't cost €400!

RE the William Optics one (if you mean the non-rotolock one), many years ago when I bought the Revelation above, I came to the conclusion that the WO was a branded version of the same thing. It certainly looks pretty much identical. Given the 41mm aperture it would explain the vignetting!

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10 hours ago, Mr Spock said:

As you can see from the image, there are no restrictions inside the tube. It is as the size stated 👍

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The inside of the housing needs to have the same blackening the insides of the barrels do.

That is really reflective.

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6 hours ago, Don Pensack said:

The inside of the housing needs to have the same blackening the insides of the barrels do.

That is really reflective.

I have the same diagonal in both regular and quartz, and I have never had any issues with stray reflections on bright objects causing odd halos or other artifacts as they enter and leave the field of view.  On the other hand, I do need to flock the rear baffle tube of my 127 Mak because I do get odd halo inducing reflections off of it with a 2" diagonal as bright objects pass the edge of it.

Maybe you'll say the surfaces cause some amount of contrast decreasing scattered light, but so do the shiny filter threads on many eyepieces, and I've never heard of anyone calling for a ban on shiny filter threads.  Some others start out blackened, but it grinds off pretty quickly using filters regularly.

I'll take my chances with stray light with these GSO diagonals over having a restricted light path with my widest field eyepieces in those other diagonals of unknown manufacturer (Synta?).

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I think manufacturers, if pressed, would say that they have produced diagonals that accept 2" fit and 1.25" fit standard eyepieces and that is what the description is based on and not necessarily the clear aperture.  Eyepieces are described by their fit standard.    🙂

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1 hour ago, Peter Drew said:

I think manufacturers, if pressed, would say that they have produced diagonals that accept 2" fit and 1.25" fit standard eyepieces and that is what the description is based on and not necessarily the clear aperture.  Eyepieces are described by their fit standard.    🙂

Yes agreed Peter, the fit is what you are buying and most manufacturers don't make claims to the clear aperture, excepting maybe a handful. 

I guess though there's also perhaps an unspoken expectation that the aperture you are getting is not unnecessarily and significantly stopped down.

Obviously we know that a cat or a newt loses some aperture due to central obstructions, but unless they are designed badly that loss of aperture is unavoidable. On the other hand the EP barrel stop lip inside the SW diagonals really only needs to be at most a couple of mm to be functional and minimise aperture loss instead of a pretty significant 5mm meaning 10mm total aperture loss. Although it's not as bad, the inner threaded sleeve on the telescope side also seems completely perplexing - what is it even there for?

Once I started to think about the whole thing it just seems pretty unnecessary to have these 'features'. 

Obviously these are just my musings and I don't have the technical and manufacturing knowledge or rationale behind the decisions, and maybe they are all for very good reasons!

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4 hours ago, Louis D said:

I have the same diagonal in both regular and quartz, and I have never had any issues with stray reflections on bright objects causing odd halos or other artifacts as they enter and leave the field of view.  On the other hand, I do need to flock the rear baffle tube of my 127 Mak because I do get odd halo inducing reflections off of it with a 2" diagonal as bright objects pass the edge of it.

Maybe you'll say the surfaces cause some amount of contrast decreasing scattered light, but so do the shiny filter threads on many eyepieces, and I've never heard of anyone calling for a ban on shiny filter threads.  Some others start out blackened, but it grinds off pretty quickly using filters regularly.

I'll take my chances with stray light with these GSO diagonals over having a restricted light path with my widest field eyepieces in those other diagonals of unknown manufacturer (Synta?).

What's the difference between the regular and quartz Louis? 

BTW regarding manufacturer @Mr Spock, I had been under the impression that the black-bodied one of the two I had posted was Revelation but I've just had a dig through for the order confirmation and it turns out it was actually Opticron. Given that it's almost identical to the SW one I have, I would assume they just got a white label one from Synta as @Louis D said. 

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4 hours ago, badhex said:

Yes agreed Peter, the fit is what you are buying and most manufacturers don't make claims to the clear aperture, excepting maybe a handful. 

I guess though there's also perhaps an unspoken expectation that the aperture you are getting is not unnecessarily and significantly stopped down.

Obviously we know that a cat or a newt loses some aperture due to central obstructions, but unless they are designed badly that loss of aperture is unavoidable. On the other hand the EP barrel stop lip inside the SW diagonals really only needs to be at most a couple of mm to be functional and minimise aperture loss instead of a pretty significant 5mm meaning 10mm total aperture loss. Although it's not as bad, the inner threaded sleeve on the telescope side also seems completely perplexing - what is it even there for?

Once I started to think about the whole thing it just seems pretty unnecessary to have these 'features'. 

Obviously these are just my musings and I don't have the technical and manufacturing knowledge or rationale behind the decisions, and maybe they are all for very good reasons!

2" O.D. is 50.8mm.  A 48mm filter thread reduces wall thickness to 1.4mm.

That could be enough (marginal) to support a 2.5 lb eyepiece plus a 1.1 lb Barlow, but not if the insertion tube of the diagonal has an undercut say, 0.5mm, deep, reducing wall thickness to 0.9mm.

So the interior step up to a 47mm I.D. adds the depth of the undercut back to the wall thickness.

 

The thickness of the ring/stop for the eyepiece likewise would seem to be adequate if equal to eyepiece barrel thickness, even with a beveled lip at the bottom of the eyepiece.

So any justification for a smaller opening than 48mm is a bit hard to swallow.  Of course that assumes the external threads that attach the piece have at least the same thickness as the wall of the eyepiece tube itself, and on some diagonals,

that thread is the 2" SCT thread, which means, without a safe wall thickness the wall will be less than 1.4mm with a 48mm I.D.  For safety, increasing thickness at the threads and reducing I.D. slightly is prudent.

The older Lumicons got around this by making body and inserted tube in one piece with a through aperture of 48mm and having the eyepiece tube have an outward facing flange on its bottom end that was bolted to the top of the diagonal body.

Originally, this was 50.8mm clear aperture all the way to the mirror, but in later years they added a ring stop (don't know the I.D.).  The Lumicon also had just about the shortest light path through a 2" diagonal I've seen, though the eyepiece tube

was shorter than some 2" barrels.

 

In today's 3-piece diagonal construction (not counting mirror), I.D. has to be a tad smaller to account for larger heavier eyepieces without breaking a tube off the body of the diagonal and to account for those accursed undercut grooves.

 

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4 hours ago, badhex said:

What's the difference between the regular and quartz Louis? 

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.

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  • 3 weeks later...

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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