Reducing optical length of William Optics 2" dielectric diagonal

[Victor Boesen]

So, I've had this diagonal for a long time now, and have used it with both my fracs for some beautiful wide field sweeping with my 24mm ES82 degree eyepiece. Some time ago I replaced the 2" eyepiece holder in my Evostar 72 with a Baader clicklock to avoid the locking screws damaging the diagonal barrel. However, this means I am just outside of focus with my 24mm ES eyepiece. I don't recall by how much exactly, but it's very little. Today I discovered that I could reduce the optical length by about 1.5mm (yeah, sorry if you expected more😅).

As visible in the images above, the 2" nosepiece has a small ring platform on it. I was never aware of this before, but I measured the thread size, M48, and found a nosepiece from my Lacerta wedge and replaced it with that.

This will leave the 2" clicklock flush with the diagonal instead of leaving behind the 1.5mm from the stock nosepiece. Not sure why this was done by WO, but perhaps it's for avoiding wear on the diagonal housing itself. Comparing the two nosepieces mounted in the clicklock yields the results below.

I'm looking forward to test this as it'd be great if I can leave the clicklock in the focuser instead of switching back and forth between the clicklock and the stock 2" holder. Hopefully this will be useful for anyone else with the WO 2" diagonal lacking some in-focus.

Victor

[badhex]

Nice modification Victor. One thing worth noting, after a previous thread regarding clear aperture of 2" diagonals we discovered that quite a few have less than 46.5mm clear aperture - i.e. max field stop of 2" EPs - so I replaced mine with a GSO manufactured 2" diagonal. Mine was TS but it's also available as Revelation and others. 

As a bonus (and relevant to this thread!), I found that the light path is quite a lot shorter with the GSO/TS than my existing Skywatcher one. The SW appear to be the same design as the WO ones. I'll measure it later today for you, but as I recall it was in the region of 10mm.

[Victor Boesen]

4 minutes ago, badhex said:

Nice modification Victor. One thing worth noting, after a previous thread regarding clear aperture of 2" diagonals we discovered that quite a few have less than 46.5mm clear aperture - i.e. max field stop of 2" EPs - so I replaced mine with a GSO manufactured 2" diagonal. Mine was TS but it's also available as Revelation and others. 

As a bonus (and relevant to this thread!), I found that the light path is quite a lot shorter with the GSO/TS than my existing Skywatcher one. The SW appear to be the same design as the WO ones. I'll measure it later today for you, but as I recall it was in the region of 10mm.

That's quite a difference!! With regards to clear aperture, as you say, there's an obvious field stop in the bottom of the 2" eyepiece holder. Perhaps that could be switched out for another EP holder, not sure. But at that point one would probably be better off getting something like the TS diagonal you got. Please get back with optical length measurements! Would be very interesting since there isn't really a number to be found online for the WO diagonal.

[badhex]

I've just done a few measurements, and some of the results are... odd. I'll take a couple of mins to write up plus what I think is likely to cause some of the weird outliers. 

 

[badhex]

Okay here's the data.

I ended up measuring most of my main EPs to get more accurate results, and measured the two Morpheus EPs 2" mode as well. I tried to be as accurate with the markings as possible, but only went for nearest whole mm, and bear in mind that this is not at infinity but a chimney in the distance. It was also a bit tricky to focus at higher powers due to some shimmer/heat haze.

The figure in each 'diagonal' column is the marking on the focuser, so the 'difference' column shows how much shorter the diagonal light path is for the GSO vs Synta. I would have expected the difference to be the same for each EP, so I out the variance down to measurement and/or focus errors. 

One thing that is obvious immediately is that the difference when using the 1.25" is huge! This must be what I noticed previously when observing, and unfortunately not that helpful for your use case. This is due to the design of the adaptor, which is much lower profile in the GSO (right). It also has an inset which some EPs fit into, some don't. 

In summary, we can say that you will gain ~2-3mm in 2" mode, and a whopping ~10mm in 1.25" mode. 

Sorry, quite long and waffley but hopefully helpful!

Edited May 31, 2022 by badhex

[wookie1965]

I had to buy one of these and butcher my diagonal to make it fit. 

This was because I could not get focus with any eyepiece below 6mm in my Tal 100rs. 

https://www.firstlightoptics.com/adapters/baader-low-profile-eyepiece-adapter-for-steeltrack.html

[badhex]

1 minute ago, wookie1965 said:

I had to buy one of these and butcher my diagonal to make it fit. 

This was because I could not get focus with any eyepiece below 6mm in my Tal 100rs. 

https://www.firstlightoptics.com/adapters/baader-low-profile-eyepiece-adapter-for-steeltrack.html

Ah yes, very similar to the one that comes with the GSO. 

[badhex]

Oh - I almost forgot - for clarity I made these measurements using my TS102 F7 ED, as I don't have a SW 72ED. Thus the in-out focus numbers are obviously irrelevant to your scope, but the light path difference in the diagonal should hopefully be quite similar, if not slightly better at F7.5. Someone with a better understanding of f-ratio and focus distances may be able to offer more info if I am mistaken!

Edited May 31, 2022 by badhex

[Victor Boesen]

Those are very useful measurements @badhex!! As you say, the improvement in optical length for 1.25" eyepieces is probably mostly due to the clever adapter design. Am I correct in my understanding that you have this diagonal?

If the WO dielectric is the same as the Synta, then clearly one should get roughly an additional 1mm of focus range even with the nosepiece modification on the WO. How do you like yours compared to other diagonals you've tried? I know you haven't had the change to compare on planets yet but how about lunar, double stars and etc?

Victor

[badhex]

1 hour ago, Victor Boesen said:

Those are very useful measurements @badhex!! As you say, the improvement in optical length for 1.25" eyepieces is probably mostly due to the clever adapter design. Am I correct in my understanding that you have this diagonal?

If the WO dielectric is the same as the Synta, then clearly one should get roughly an additional 1mm of focus range even with the nosepiece modification on the WO. How do you like yours compared to other diagonals you've tried? I know you haven't had the change to compare on planets yet but how about lunar, double stars and etc?

Victor

I strongly suspect the WO is the same as the Synta in terms of the body construction. Possibly the mirror is better in the WO. (I say Synta, I have no proof of this except that SW provide it with their scopes and sell them separately as well). 

Re the GSO, I have the none-quartz version as consensus seems it's probably marketing more than optically significant. It's a well built unit and only about 100g heavier than the Synta, only had a couple of outings but very good optically from my experiences so far. 

A couple of very minor issues: the placement of the 2" screw and the recessed 1.25" adapter means tiny EPs like BCOs are a bit of a nightmare as I'm left eyed so I have to press my nose into the screw somewhat! I also find that the machined fit is very close with the adaptor, and once or twice in the dark the adapter has become momentarily stuck in the 2" fitting due to me not inserting/removing it squarely. I suspect this is partly because it's new and will get better with use, and also that I had rather chunky EPs in the adapter at the time. 

Honestly, I'd love a Baader BBHS clicklock and will probably eventually get one, but you can get two of these and a 1.25" version for the same cost as the BBHS 😱

[Victor Boesen]

3 hours ago, badhex said:

Honestly, I'd love a Baader BBHS clicklock and will probably eventually get one, but you can get two of these and a 1.25" version for the same cost as the BBHS 😱

I'm with you on this one😅 My WO diagonal is and has always been great. I hope this simple modification Will allow me to use my 24mm eyepiece in the Evostar with the click lock. If not, I'll need to figure out something else. Will probably keep the WO until I can afford the BBHS mirror.

Edited May 31, 2022 by Victor Boesen

[badhex]

In the grand scheme of things, with the Baader BBHS you're probably really only paying +250eur for the click lock and a fancy sounding mirror, the actual optical gains over the GSO will likely be marginal. But it's nice to have no compromises! 

[Victor Boesen]

10 hours ago, badhex said:

In the grand scheme of things, with the Baader BBHS you're probably really only paying +250eur for the click lock and a fancy sounding mirror, the actual optical gains over the GSO will likely be marginal. But it's nice to have no compromises! 

Yes, very true.

The sun was out for a brief moment just now, and I tried using my 24mm with the diagonal in white light. It was hard to tell due to partly cloudy conditions and a low magnification so preferably I should have to test it under the stars to determine whether or not I'm reaching focus. However, I was able to spot all sunspots from SDO so results are promising. If I won't reach perfect focus under the stars, I'll be very close to the point where it's close enough I think.

I could separate AR3023 into both spots. This should tell something about how close I am😅

Victor

[badhex]

7 minutes ago, Victor Boesen said:

Yes, very true.

The sun was out for a brief moment just now, and I tried using my 24mm with the diagonal in white light. It was hard to tell due to partly cloudy conditions and a low magnification so preferably I should have to test it under the stars to determine whether or not I'm reaching focus. However, I was able to spot all sunspots from SDO so results are promising. If I won't reach perfect focus under the stars, I'll be very close to the point where it's close enough I think.

I could separate AR3023 into both spots. This should tell something about how close I am😅

Victor

Good news. Actually yesterday when I was doing those measurements I repeated some of them a couple of times because they seemed unusual, and I could swear that my eye was doing some of the focusing as well - i.e. I was able to move focus in or out by ~1mm and my eye seemed to adjust to keep it in focus. Not something I've experienced before consciously, but then I've never sat and measured focus so intently before either. Maybe it was just the shimmer playing tricks on me? Or, maybe your eye can make up the difference.

[Victor Boesen]

2 minutes ago, badhex said:

Good news. Actually yesterday when I was doing those measurements I repeated some of them a couple of times because they seemed unusual, and I could swear that my eye was doing some of the focusing as well - i.e. I was able to move focus in or out by ~1mm and my eye seemed to adjust to keep it in focus. Not something I've experienced before consciously, but then I've never sat and measured focus so intently before either. Maybe it was just the shimmer playing tricks on me? Or, maybe your eye can make up the difference.

I have experienced the same. Of course there's a position where the eye is more comfortable, however, I can also shift focus slightly. The weekend looks clear so hopefully I can test it there:)

[Victor Boesen]

An update to this thread: Success!! But just barely😅 Racking the focuser as far in as it could focused the moon nicely with the 24mm eyepiece. However, I somehow completely forgot that my 1.25" eyepieces will not reach focus, when the clicklock is used So I will probably need to figure out another solution to this, or just use my T2 Baader prism

Victor

[badhex]

On 05/06/2022 at 16:00, Victor Boesen said:

An update to this thread: Success!! But just barely😅 Racking the focuser as far in as it could focused the moon nicely with the 24mm eyepiece. However, I somehow completely forgot that my 1.25" eyepieces will not reach focus, when the clicklock is used So I will probably need to figure out another solution to this, or just use my T2 Baader prism

Victor

Sorry Victor, I meant to reply here and forgot. Glad you got it figured out but a bit annoying about the 1. 25" issue. Maybe you'll have to get the GSO after all 😜

[Victor Boesen]

11 hours ago, badhex said:

Sorry Victor, I meant to reply here and forgot. Glad you got it figured out but a bit annoying about the 1. 25" issue. Maybe you'll have to get the GSO after all 😜

No worries  Hah, you may be right about the GSO, but I actually doubt that would fix it (was lacking quite a lot of in-focus). Going to revisit this problem at some other time. At least I can just unscrew the click lock and replace it with the old 2" holder for now. Wallet currently needs some time to heal😅