Which diagonal

[reddish75]

Having bought a Williams Zenithstar 73iii from the classified I'm looking for a diagonal for the odd occasion that we'll use it to stargaze. I've seen the William ones but am not sure on them really want to know best options price vs quality and which size best to go for 2" or 1.25"?

[cajen2]

It depends on what eyepieces you have or are likely to buy. It also depends whether you need to be ultra weight conscious. If you can and don't need to worry about weight, get the 2" and a 1.25" adaptor. It works better than the other way around. Or if course you could buy one of each size, like me....😆

I have the Williams Optics 2" di-electric and it's superb but heavy. I also have a Takahashi 1.25" prism diagonal - also very good. Just don't buy the cheapest available: a good diagonal will help give you excellent views.

The StellaLyra range look good but I can't recommend them as I've never tried one.

Edited October 27, 2023 by cajen2

[bosun21]

With the focal ratio of 5.9 I would go for a mirror diagonal, and one with a dielectric coated mirror. I would stick to one from a well known manufacturer. The choice of whether you go for either a 2" or a 1.25" will really depend on your preference and want you want to observe. For really wide field views I would opt for the 2". For everything else a 1.25 would suffice. You should also take into consideration which mount you have as a 2" weighs considerably more than a 1.25".

[glafnazur]

Baader diagonal also are very good, I've got the 2" Dielectric and it's the best diagonal I've used.

[The Lapwing]

WO diagonals in either format will serve you well. Excellent build quality too and not huge money either 

[Mr Spock]

Just be aware the WO has a restricted field of view which will vignette with wide field eyepieces (there's a thread on it here somewhere - I aught to bookmark that the number of times the topic comes up). 2" Diagonals which don't vignette:

Inexpensive: Revelation 2" Di-electric Quartz (About £100 - Harrisons used to stock these)
Expensive: Baader ClickLock 2" DiElectric Star Diagonal (£234 from FLO) but you will also need a 2"-1.25" adaptor (£70). IF you can stretch to one of these, they are worth it.

Both are very good. However, ClickLocks once used are addictive!

 

[The Lapwing]

46 minutes ago, Mr Spock said:

Just be aware the WO has a restricted field of view which will vignette with wide field eyepieces (there's a thread on it here somewhere - I aught to bookmark that the number of times the topic comes up). 2" Diagonals which don't vignette:

Inexpensive: Revelation 2" Di-electric Quartz (About £100 - Harrisons used to stock these)
Expensive: Baader ClickLock 2" DiElectric Star Diagonal (£234 from FLO) but you will also need a 2"-1.25" adaptor (£70). IF you can stretch to one of these, they are worth it.

Both are very good. However, ClickLocks once used are addictive!

 

Some wide field, but not all by any means. I used ES 2 inch eyepieces without issue at the time. 

[bosun21]

Exactly there are very few eyepieces with a field stop large enough to cause vignetting with a WO 2" diagonal. One that springs to mind is the Nagler 31mm.

[John]

That restricted field of view issue is a bit more pronounced in the 1.25 inch version of the William Optics diagonals and their clones. I found that a 32mm plossl showed noticeable vingetting in that type of diagonal. A 30mm Vixen NPL plossl though was OK. 

The GSO / Revelation diagonals are good for their cost I feel (both the 2 inch and the 1.25 inch sizes) and they do not vignette any eyepieces as far as I'm aware.

 

 

[Stephenstargazer]

ZS 73 and 1 1/4" eyepiece can see up to 3.7 deg of sky, so why  consider a 2" diagonal ?  Seems like unwise expenditure and bulk to me, considering you mention occasional use only. For  wider fields a binocular is probably more useful. 

 

[Mr Spock]

This is the Revelation and the clear aperture I was talking about. The Revelation manages 47mm - the most you can get in 2".

A 1.25" option, also sourced from GSO.

[Roy Challen]

2 hours ago, John said:

That restricted field of view issue is a bit more pronounced in the 1.25 inch version of the William Optics diagonals and their clones. I found that a 32mm plossl showed noticeable vingetting in that type of diagonal. A 30mm Vixen NPL plossl though was OK. 

The GSO / Revelation diagonals are good for their cost I feel (both the 2 inch and the 1.25 inch sizes) and they do not vignette any eyepieces as far as I'm aware.

 

 

I just tried my WO durabright 1.25" and StellaMira 1.25" with a Baader 32mm plossl. Can't say I saw any vignetting in the WO. Focus point is further out with the WO though 

[Elara]

Lots of great comments for you so far…

I’m certain this article is still applicable - there may be an update to it somewhere of course. I found it helpful and hopefully you do too. As mentioned by  @bosun21 and advice I’ve received in the past from the team at FLO : dialelectric mirror diagonal for your scope focal ratio of f/5.9; I think I’m right in saying that at under f/7, prism diagonals can introduce a tiny bit of false colour ( happy to be corrected of course! ).

 https://www.cloudynights.com/articles/cat/articles/mirror-vs-dielectric-vs-prism-diagonal-comparison-r2877

[John]

2 hours ago, Roy Challen said:

I just tried my WO durabright 1.25" and StellaMira 1.25" with a Baader 32mm plossl. Can't say I saw any vignetting in the WO. Focus point is further out with the WO though 

The Baader 32mm plossl has a smaller field stop than many of that focal length. It's AFoV is around 45 degrees. Hence the lack of vignetting.

 

[bosun21]

29 minutes ago, John said:

The Baader 32mm plossl has a smaller field stop than many of that focal length. It's AFoV is around 45 degrees. Hence the lack of vignetting.

 

There’s no vignetting with my SL 30mm UFF either which is 70 degrees. There’s very few eyepieces with a field stop large enough to cause vignetting. I have only owned the 2” version though.

Edited October 28, 2023 by bosun21

[reddish75]

I've just bought this one, I take it this is the same as the Revelation one? Didn't want to spend much as this is going to be a. Astrophotography rig and I'm determined to keep prices sensible

[John]

5 hours ago, reddish75 said:

I've just bought this one, I take it this is the same as the Revelation one? Didn't want to spend much as this is going to be a. Astrophotography rig and I'm determined to keep prices sensible

Well it's a good diagonal and the same as the GSO / Revelation but the one pictured has an SCT fitting. I thought you were intending to use it with a WO Zenithstar III refractor ?

[John]

6 hours ago, bosun21 said:

There’s no vignetting with my SL 30mm UFF either which is 70 degrees. There’s very few eyepieces with a field stop large enough to cause vignetting. I have only owned the 2” version though.

No there wouldn't be with the 30mm UFF. It's field stop is around 36.7mm I think. There are a number of 30mm and longer focal length eyepieces that have larger field stops and wider AFoV's. 

It's only annoying if you are seeking the widest true field for the 1.25 inch or 2 inch eyepiece format of course. 

 

Edited October 29, 2023 by John

[globular]

On 28/10/2023 at 20:20, bosun21 said:

There’s very few eyepieces with a field stop large enough to cause vignetting.

Doesn’t the 2” WO have a 43mm clear aperture? 
Don’s 2023 eyepiece guide has 253 2” eyepieces listed and 51 of them would vignette in this diagonal.
Provided all your 2” EPs are one of the 202 that don’t then you won’t notice or care about the restriction. Have any of the 51 that do and you’ll want a different diagonal.

I have 3 EPs with a field stop greater than 43, so I have a baader diagonal with 47.5mm clear aperture.

 

edit: apparently the clear aperture is 41.5 not 43. This increases the vignetting EP count to 70 not 51.

Edited October 30, 2023 by globular
corrected clear aperture

[bosun21]

2 hours ago, globular said:

Doesn’t the 2” WO have a 43mm clear aperture? 
Don’s 2023 eyepiece guide has 253 2” eyepieces listed and 51 of them would vignette in this diagonal.
Provided all your 2” EPs are one of the 202 that don’t then you won’t notice or care about the restriction. Have any of the 51 that do and you’ll want a different diagonal.

I have 3 EPs with a field stop greater than 43, so I have a baader diagonal with 47.5mm clear aperture.

Thankfully  all my 2" eyepieces fall into your latter category and work just fine for me. I didn't recommend a WO diagonal to the OP. I was referring to my own experience with them once they were mentioned. In hindsight if I were to buy another 2" diagonal I would most definitely now buy one with a larger clear aperture. The WO however with its 1/12th lambda mirror does provide very sharp views.

[John]

Nothing wrong with the WO's (and clones) optically and they are well put together.

 

[globular]

9 minutes ago, bosun21 said:

Thankfully  all my 2" eyepieces fall into your latter category and work just fine for me. I didn't recommend a WO diagonal to the OP. I was referring to my own experience with them once they were mentioned. In hindsight if I were to buy another 2" diagonal I would most definitely now buy one with a larger clear aperture. The WO however with its 1/12th lambda mirror does provide very sharp views.

Agree 100%. I was just putting some figures to your ‘few’ for anyone stumbling on this in the future.

[badhex]

Here you go.

The WO, Skywatcher/Skywatcher Deluxe, Opticron , Stellamira and others with the same shared design have a clear aperture of 41.5mm, so may cause some vignetting when going for maximum TFOV in 2", which is 41mm FL at 68˚ / 46mm field stop (or of course, shorter FLs with a wider AFOV). Anything with a field stop of 41.5mm or less will be fine. Whether this bothers an individual or even whether they will detect the vignetting or not, is another matter of course and somewhat personal - and it doesn't (ahem) reflect on the rest of the build quality, they are very solid.

I would imagine that it's more of an issue with the 1.25" version (which is the same design scaled down and therefore has a reduced clear aperture and potential for vignetting), primarily because I assume way more people own a max TFOV EP in 1.25" format than 2". For example, it would affect the 24mm APM/Stellamira UFF and its cousins, ES 24mm 68˚, Televue Panoptic 24mm and so on, all of which are pretty popular EPS. BTW, I haven't actually measured the clear aperture on the 1.25" despite owning one, but I can tell you it's definitely reduced compared to the GSO 1.25" (again a scaled down version of its own 2" big brother).

 

 

Edited October 30, 2023 by badhex

[Mr Spock]

👍 That's the thread I was referring to.

[badhex]

11 minutes ago, Mr Spock said:

👍 That's the thread I was referring to.

I had a feeling it might be 🙂 I also end up referring back to it quite a lot so it's always in my recents bar 😂