2" vs 1.25" diagonal

[jambouk]

Are there any advantages of using a 2" diagonal with a 1.25" eye piece vs a 1.25" diagonal and 1.25" EP, assuming both are of similar build quality?

[Someone asked me this and I gave them my answer, but I wonder what others think].

James

 

[alanjgreen]

Nope. You are increasing the light path by using 2" which  could be a disadvantage, with binoviewer for example.

I guess if you were spending £100 to replace a poor stock diagonal, then you may as well get a 2", you're probably going to buy one eventually! (unless your scope is smaller than 8"  - newt or SCT - then there is no need of 2" eyepieces anyway)

 

[Knighty2112]

Apart from been able to use 2" EP's in a 2" diagonal I don't see any difference between my Altair Astro 2" diagonal, or my Baader 1.25" diagonal, or any of my other 1.25" dagonals really. 

[John]

I can see that the larger sized mirror in a 2" diagonal might have some advantages. This article by Roland Christen of Astro Physics Inc explains:

http://www.csun.edu/~rprovin/roland/dielectric.html

My personal reason for preferring 2" diagonals is they they provide a secure platform for large and long eyepieces, which mine are, even the 1.25" ones.

 

[Moonshane]

As always it depends.  I use the baader t2 system which allows 2" but I only use 1.25" in fracs.  I use binoviewers though so a short light path is an advantage.  another possible advantage is the heavier 2" diagonal allows the scope to be mounted higher in the rings so the eyepiece is higher off the ground allowing easier zenith observing.

[Louis D]

Technically, you might get light cone cutoff using a 2" diagonal because its longer light path means you have to move the focuser tube farther forward to reach focus.  This would reduce your effective aperture somewhat.

[Ben the Ignorant]

Herr Rohr made these interesting measurements:

http://r2.astro-foren.com/index.php/de/11-beitraege/03-newton-systeme-und-verwandte-fragen/755-c093-kalkulierte-genauigkeit-bei-unterschiedlichen-planspiegeln

The 3-inch dielectric diagonal has a stupendous flatness in the small (13mm across) central area, still a very good surface over a 35mm (by semi-major axis) area, but a decadent one over the whole 70mm. The higher the mag, the smaller area you're using, so you might not be too annoyed.

Luckily for us average users, the two 2" mirrors are excellent. One is a GSO, I can tell from the frame's shape, the other I'm not sure. The 2" prism causes several defects, it's a bit astigmatic but its Strehl quality ratio remains tolerable.

I've bought three 1.25" GSO diagonals and one 2" GSO, all are excellent in brightness as well as sharpness, I could not see any difference in image quality between them. By the way, GSO promises 1/12th wave flatness, and does even better with 1/14th. That quality is in line with the more expensive Tele Vue:

http://r2.astro-foren.com/index.php/de/12-beitraege/04-zweispiegel-systeme-astrofotografie/742-d052b-zenit-spiegel-der-feine-unterschied

[John]

20 minutes ago, Louis D said:

Technically, you might get light cone cutoff using a 2" diagonal because its longer light path means you have to move the focuser tube farther forward to reach focus.  This would reduce your effective aperture somewhat.

First I've heard of this concern

I'm trying to think of circumstances / scope specs where this might be an issue ?

 

[John]

16 minutes ago, Ben the Ignorant said:

Herr Rohr made these interesting measurements:

http://r2.astro-foren.com/index.php/de/11-beitraege/03-newton-systeme-und-verwandte-fragen/755-c093-kalkulierte-genauigkeit-bei-unterschiedlichen-planspiegeln

The 3-inch dielectric diagonal has a stupendous flatness in the small (13mm across) central area, still a very good surface over a 35mm (by semi-major axis) area, but a decadent one over the whole 70mm. The higher the mag, the smaller area you're using, so you might not be too annoyed.

Luckily for us average users, the two 2" mirrors are excellent. One is a GSO, I can tell from the frame's shape, the other I'm not sure. The 2" prism causes several defects, it's a bit astigmatic but its Strehl quality ratio remains tolerable.

I've bought three 1.25" GSO diagonals and one 2" GSO, all are excellent in brightness as well as sharpness, I could not see any difference in image quality between them. By the way, GSO promises 1/12th wave flatness, and does even better with 1/14th. That quality is in line with the more expensive Tele Vue:

http://r2.astro-foren.com/index.php/de/12-beitraege/04-zweispiegel-systeme-astrofotografie/742-d052b-zenit-spiegel-der-feine-unterschied

I think "C" is an Astro Physics. It looks just the same as the AP one that I have.

The other major test for a diagonal is how accurately the mirror or prism is held by the housing. I've read of several cases where the mirrors had to be shimmed to put them at the correct angle to the optical axis.

 

[spaceboy]

12 minutes ago, John said:

First I've heard of this concern

I'm trying to think of circumstances / scope specs where this might be an issue ?

 

I think he is referring to the above post ??

[John]

6 minutes ago, spaceboy said:

I think he is referring to the above post ??

I did wonder if that was what was alluded to by Louis.

[mikeDnight]

To get around the potential problem with dielectric 1.25" diagonals as highlighted by John, and if you'd prefere to use 1.25", you could choose a high quality prism diagonal. I use a 1.25" Takahashi prism and it really is superb!

Mike

[Louis D]

4 hours ago, John said:

First I've heard of this concern

I'm trying to think of circumstances / scope specs where this might be an issue ?

 

Just search on drawtube light cone intrusion.  There have been several examples over the years including the ST120.  Here's one post and another on the subject.  Basically, poorly designed fast refractors with steep light cones can have some intrusion of the front edge of the focuser drawtube when racked far inward (think binoviewing).

[Louis D]

4 hours ago, spaceboy said:

I think he is referring to the above post ??

Yes, that's another post on the same issue with the same telescope.

[John]

16 minutes ago, Louis D said:

.....  Basically, poorly designed fast refractors with steep light cones .....

Ahh, yes I understand now.

Frustrating though. Just the sort of scopes that would benefit from 2" eyepieces to maximise the true FoV and the designer sticks in an overlong drawtube

 

[RobertI]

When I use the 2" diagonal and 1.25" eyepices on my Tal I run out of inward focus travel and cannot reach focus. Fortunately I have the Baader Hyperions which also have a 2" barrel and when used, sits lower in the diagonal and can reach focus. Otherwise I would have to use a 1.25" diagonal. I guess this is a well known issue? Otherwise I like 2" diagonals as the bigger barrel seem to attach to the scope more securely.

[mikeDnight]

Here are two ways of resolving the problem of not having enough inward travel when using 1.25" eyepieces in a 2" diagonal.

Use a low profile diagonal.

 Or buy a low profile 2" to 1.25" adapter from Sky Watcher (they are superb!)

Mike