In the market for a new 1.25 inch diagonal

[MercianDabbler]

The 30 year old diagonal supplied with my SP102 has developed a crack in the plastic so the telescope tube fitting is not being held securely in its thread. I cant grumble given its age but it is definitely not safe now with heavy eyepieces.

Requirement 1... the new one needs a metal body. I don't want the same thing happening again.

Requirement 2... I've read some past threads on here and am keen to avoid the constriction built in to some types. I use a 32mm Plossl and dont want any loss of field.

I'm unsure whether to go for a prism or a mirror one. Dialectric mirrors seem to be all the rage now and would give me more light (which I may or may not notice).

Are there downsides to a mirror? - for example do they deteriorate over time? Are they more or less difficult to keep dust free?

Choice is also constrained by availability just now.

Two that I've found so far which I think meet the requirements are

https://www.altairastro.com/125in-lightwave-premium-dielectric-diagonal-169-p.asp - the spec looks decent but I'm not clear if this has a constriction or not. I don't think I've seen any reports on it here either... nether good nor bad.

https://www.firstlightoptics.com/diagonals/takahashi-125-diagonal.html - a good brand but a prism. Would I just be paying more money for less light?

Other suggestions are welcome. Thanks in advance.

2 inch is not an option BTW. I dont feel the need to upgrade the focuser.

[johninderby]

The Baader prisms are excellent and far more adapable  than the Tak prism. Thregular Baader prism is 32mm clear aperture and the Zeiss version 34mm clear aperture. As they use T2 threads you can fit 1.25” or 2” eyepiece holders and nosepieces.

https://www.baader-planetarium.com/en/baader-prism-diagonal-t-290°-32mm.html

https://www.baader-planetarium.com/en/baader-t-2-stardiagonal-(zeiss)-prism-with-bbhs-r-coating-(t-2-part-01b).html

In regular dielectric mirror diagonals I like the WO one but does have a slightly reduced clear aperture as does the SW.

https://www.firstlightoptics.com/diagonals/william-optics-125-dura-bright-dielectric-diagonal.html

However the SW one is pretty good.

https://www.firstlightoptics.com/diagonals/SWdiagonal-20967.html

[johninderby]

Picked up one of these cheap diagonals on an Amazon promotion for £15.85 the other week and it doesn’t have any restriction. Amazing buy.

https://www.amazon.co.uk/gp/product/B07WCQ3C1G/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&psc=1

[MercianDabbler]

1 hour ago, johninderby said:

The Baader prisms are excellent and far more adapable  than the Tak prism. Thregular Baader prism is 32mm clear aperture and the Zeiss version 34mm clear aperture. As they use T2 threads you can fit 1.25” or 2” eyepiece holders and nosepieces.

https://www.baader-planetarium.com/en/baader-prism-diagonal-t-290°-32mm.html

https://www.baader-planetarium.com/en/baader-t-2-stardiagonal-(zeiss)-prism-with-bbhs-r-coating-(t-2-part-01b).html

In regular dielectric mirror diagonals I like the WO one but does have a slightly reduced clear aperture as does the SW.

https://www.firstlightoptics.com/diagonals/william-optics-125-dura-bright-dielectric-diagonal.html

However the SW one is pretty good.

https://www.firstlightoptics.com/diagonals/SWdiagonal-20967.html

Thanks John.

I confess that I've never thought of a diagonal as something that can be adaptable or not. I get that there are different ways to grip the eyepiece nowadays but other than that one end goes in the focusser and the other end has an eyepiece inserted.  Am I missing something?

I guess that I do have a mix of EPs some of which have an undercut and others which don't. TBH I've never had a big issue with the old school approach - plain barrel on the EP and a thumb screw on the holder. My eyepieces are there to be used, not to be collectables but I don't rule out 'non marking' alternative holding approaches.

Do those Baader ones need one to buy extra bits to connect to the scope and the EP?

Sorry but reduced aperture is a no-no for me - it just seems like a backwards step even from my 30 year old diagonal.

[johninderby]

The Baader can use any T2 nosepieces and eyepiece holders so it can be configured to take 2” eyepieces or 1.25” ones.

I use a 2” nosepiece on mine so I don't need to use a 1.25” to 2”:adaptor in the focuser but use a 1.25” eyepiece holder. 

[Cosmic Geoff]

It's worth tracking down a comparative analysis of various diagonal types. IIRC, With mirror diagonals, the coating has a bearing on the % relflectivity and the coating life.

Prism diagonals do not deteriorate, but are not suited for short focal ratio telecopes. Somewhat surprisingly, the optical accuracy of the diagonal is rarely an issue, whether cheap or expensive.

[AdeKing]

1 minute ago, johninderby said:

The Baader can use any T2 nosepieces and eyepiece holders so it can be configured to take 2” eyepieces or 1.25” ones.

I use a 2” nosepiece on mine so I don't need to use a 1.25” to 2”:adaptor in the focuser but use a 1.25” eyepiece holder. 

I do the same as John, 2" Nosepiece on the scope side and 1.25" on the eyepiece side but Baader do sell the basic 32mm prism diagonal complete with 1.25" nosepiece and 1.25" helical focusing eyepiece holder as well as the bare prism John linked to above.

https://www.baader-planetarium.co.uk/shop/baader-t-2-90-zenith-prism-diagonal-with-32mm-prism-t-2-part-14/

[Louis D]

There's the GSO diagonals sold under the Revelation and iOptron brands in the UK as well.  I really like their 2" diagonals, but have no direct experience with their 1.25" diagonals.

One advantage to prism diagonals over dielectric diagonals is lower scatter.

[vlaiv]

I have both 1.25" and 2" GSO 99% dielectric diagonal - and both are quite fine - though I don't have much to compare them against except cheap 91% Skywatcher diagonal - one that comes with scopes as accessory.

[AllanK]

I’m certainly not an expert, but I’ve recently bought this....https://www.amazon.co.uk/Svbony-Dielectric-Aluminium-Telescope-Accessories/dp/B08QZ873PC

 

It seems to be good (seems well made and gives good clear images) but not sure how it compares to the 2 you’ve identified already.

[MercianDabbler]

Having read a bit more elsewhere it seems that a mirror offers little advantage on an F10 refractor and that prisms were preferred in the days when more people had long refractors. A mirror would be better on my ST80 but that's a secondary consideration.

The Tak Prism diagonal is out of stock at FLO (not sure if it was when I first looked) so that's not an option now. Perusing RVO I found the Baader one that is ready to use without buying extra bits...

https://www.rothervalleyoptics.co.uk/baader-prism-diagonal-t-2-90-with-focusing-eyepiece-holder-and-125-nosepiece.html

The twist focusser might come in handy too.

I hadn't really settled on a budget but this is probably fairly near to the mark. Hopefully that will buy me a decent quality diagonal without causing too many raised eyebrows from elsewhere in the household.

I haven't made a final decision yet. I've also sent a query to Altair regarding the constriction question.

I divorced Amazon some time back BTW after they signed me up for a 'free' Prime trial a second time against my wishes. They won't be doing it again.

[Second Time Around]

Here's an extract from a comprehensive review by author Bill Paolini on diagonals.  Here he's comparing various prism and mirror diagonals at f/6.25, where the received wisdom is that mirrors will do better.  Quite illuminating!

"..........After completing observations of Jupiter in the Celestron 80mm APO with the prisms, the various aluminium, silver, and dielectric mirror diagonals were tested.  First impression when moving from observing with the prisms to the mirrors was, "wow...more scatter!"  I felt this was a rather obvious tell.  I was also surprised that the details on Jupiter were definitely softer through the mirror diagonals than they were when using the prism diagonals. As example, the NNTB was not showing though the mirror diagonals, and any structure within NEB and SEB was only hinted at as a nondescript albedo differences.  Changing out the mirror diagonals to prism diagonals, and all the NEB and SEB crisp definition and structure reappeared, as well as the ethereal NNTB..........."

For the complete article go to https://www.cloudyni...omparison-r2877

[Louis D]

32 minutes ago, Second Time Around said:

Here's an extract from a comprehensive review by author Bill Paolini on diagonals.  Here he's comparing various prism and mirror diagonals at f/6.25, where the received wisdom is that mirrors will do better.  Quite illuminating!

"..........After completing observations of Jupiter in the Celestron 80mm APO with the prisms, the various aluminium, silver, and dielectric mirror diagonals were tested.  First impression when moving from observing with the prisms to the mirrors was, "wow...more scatter!"  I felt this was a rather obvious tell.  I was also surprised that the details on Jupiter were definitely softer through the mirror diagonals than they were when using the prism diagonals. As example, the NNTB was not showing though the mirror diagonals, and any structure within NEB and SEB was only hinted at as a nondescript albedo differences.  Changing out the mirror diagonals to prism diagonals, and all the NEB and SEB crisp definition and structure reappeared, as well as the ethereal NNTB..........."

For the complete article go to https://www.cloudyni...omparison-r2877

Of course, in Japan, this might be one reason why straight through viewing is popular.

[MercianDabbler]

46 minutes ago, Second Time Around said:

Here's an extract from a comprehensive review by author Bill Paolini on diagonals.  Here he's comparing various prism and mirror diagonals at f/6.25, where the received wisdom is that mirrors will do better.  Quite illuminating!

 

Thanks. That was one of the articles that I read. At F10 it seems like a prism has advantages in terms of lack of scatter. There is usually a tradeoff in all things but with diagonals the advantage for mirrors seems to be on fast scopes... which mine isn't.

14 minutes ago, Louis D said:

Of course, in Japan, this might be one reason why straight through viewing is popular.

Tried that. My neck isn't that flexible.

[MercianDabbler]

Hmm... my purchase is not going so well. It turns out that RVO don't actually have the Baader diagonal in stock unless I want the one without the attachments for the scope and eyepiece.

I've had a reply from a retailer to my query about the Altair one but theirs are all sealed so they can't answer my question about the constriction/step. Looking again at photos of the Altair one it starts to look a lot like the Stella Mira/Skywatcher and William Optics ones... I get the feeling they are all variations on a theme with different branding on the triangular side panel. Maybe I'm wrong but just now I'm feeling that I'm not going to rush out and buy the Altair one.

Mulling the problem over I started wondering if I could keep the crack closed by putting a toolmakers clamp around the housing. I think the body is a bit too wide for my clamps so instead I spent some time in the shed this evening making something that works on a similar principle but is made to measure. It's made from whatever bits of steel came to hand and isn't pretty but it should prevent the crack (and therefore the threads) opening up and ensure that the barrel stays put.

Hopefully astro gear will become less like hen's teeth over the summer as lockdown eases and darkness is in short supply.