Should I get a new diagonal?

[Ratlet]

I'm planning on picking up a Starfield 102 fpl-53 in the future and considering what additional kit I'll need.

At the moment I've got a circle T prism diagonal which (to my eyes) works well with my ST80 but wondering if a dielectric would work better with the bigger scope?  I'm currently on a 1.25" diagonal so a 2" might be useful, but most of my eyepieces are 1.25".

[bosun21]

I personally use a Williams optics 2" dielectric diagonal with my Starfield which comes with a 2" to 1.25" adapter. I actually use the Baader 2" to 1.25" clicklock instead of the adapter. I like the 2" as for wide field I can use my 30mm UFF. At the end of the day it's what suits you personally.

[The60mmKid]

At f/7, I don't think prism vs. mirror is too important a consideration. But if purchasing a 2" diagonal (which I would recommend since it would allow you to use eyepieces that harness the wide-field potential of your scope), then I'd opt for a dielectric since they're typically much less expensive. Even if you use only one 2" eyepiece, that still allows that refractor to perform as a nice rich/wide-field telescope.

[stormioV]

I use Antares 2" 99% Dielectric Twist Lock Mirror Diagonal with 1.25 adaptor. Big improvement on what I upgraded from. I like the twist lock security.

[Louis D]

Using a 2" diagonal also allows for adding a TSFLAT2 ahead of the diagonal to flatten wide field views for edge to edge sharpness.  In both my 432mm and 600mm ED/APO refractors, it makes quite a difference at lowest powers.  However, I have to remove it at high powers to avoid the introduced spherical aberration from the flattener that is not visible at lower powers.

[Mr Spock]

Perfect match for a Starfield 

[The60mmKid]

23 minutes ago, Mr Spock said:

Perfect match for a Starfield 

Those are very nice. Once you go clicklock, it's hard to mess with set screws again.

[Ratlet]

1 hour ago, Mr Spock said:

Perfect match for a Starfield 

How do you use the eq5 with the scope?  I'm going to migrate to Starsense and trying to figure out a way of doing manual aiming with tracking when on target.

[bosun21]

You ideally need to use the starsense on an Alt/Az mount. I tried it on my EQ mount and i couldn’t contort myself enough to even see if it was working not being on the level. I hastily abandoned the idea and put it back on my dobsonian.

 

 

Edited August 6, 2023 by bosun21

[Zermelo]

The other advantage of a 2" diagonal, even if you don't have many 2" eyepieces, is robustness.

My heavier EPs feel safer in my 2" than in my 1.25" tak prism.  The larger diagonal is significantly heavier, so I do need to think about balance and mount capacity.

[John]

I do use a Baader T2 prism diagonal in 1.25 inch mode my F/9 100mm refractor and it works well but I use 2 inch diagonals (AP and TV) with my other refractors even though I mostly use 1.25 inch eyepieces with them. Quite a few of my eyepieces are on the large side so the security of the 2 inch format diagonals is good to have I feel. The TV's are made from a single block of alloy which seems particularly robust and secure.

[JeremyS]

I use a Tak 1.25-inch diagonal on my scopes up to 100mm. And a 2-inch diagonal from 100mm uP. I.e. sometimes, in fact mostly, I use the 100mm in 1.25-inch mode to keep the weight down when using a lighter mount. With a heavier, more capable, mount I use it in 2-inch mode so I can use 2-inch EPs.

I haven’t noticed a particular benefit in terms of robustness that would mean I would go for a 2-inch diagonal with 1.25-inch EPs.

[Franklin]

I have the Baader 2" dielectric clicklock like others above and consider it my best diagonal. Baader 32mm T2 prism for binoviewing, Ha with quark and for when using 1.25" only, trying to keep the weight down. I have several vintage Vixen diagonals, both 1.25" prisms and 2" mirrors and 45deg erect image ones as well, they all do the job but the Baaders are in a different league in my opinion. I think it's important not to skimp on the diagonal as it plays a key role in the image plane.

[Zermelo]

Yes, I think if I had my time (and money) again, I'd get a 2" Clicklock dielectric.
My Revelation diagonal is very decent, but I like the look of the Baader mechanism.
I believe Baader do a Clicklock 2" to 1.25" adapter, so I'm thinking of getting one to put in the Revelation.

[Ratlet]

I suspect that at least on the short term, a 1.25" will do.  Is there likely to be a difference between a prism and a dielectric?

1 hour ago, bosun21 said:

You ideally need to use the starsense on an Alt/Az mount. I tried it on my EQ mount and i couldn’t contort myself enough to even see if it was working not being on the level. I hastily abandoned the idea and put it back on my dobsonian.

 

That's a good point I hadn't thought about!  

Edited July 31, 2023 by Ratlet

[Zermelo]

4 minutes ago, Ratlet said:

Is there likely to be a difference between a prism and a dielectric?

Assuming that you mean examples of similar quality, then the received wisdom is that it's about neutral at F/7:

https://stargazerslounge.com/topic/328670-mirror-vs-prism-diagonal-for-fast-refractor/

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

[The60mmKid]

4 minutes ago, Ratlet said:

Is there likely to be a difference between a prism and a dielectric?  

I do not think you'll see a difference at f/7 between prism and mirror diagonals of comparable quality.

[John]

I use the Baader 1.25 inch twist lock adapter with my Baader T2 prism and it works exceptionally well I feel. No snagging on eyepiece undercuts because of it's longitudinal nylon tension rods.

[Mr Spock]

I have a Revelation 2" which is almost as good as the Baader for a lot less money.

It's worth noting that some 2" diagonals, such as the WO, have a restricted opening at the bottom of the eyepiece holder. This can create some vignetting with very wide field low power eyepieces. There's a thread on the subject here somewhere.

[cajen2]

Identical to @bosun21. Same 102 scope, same WO 2" dialectric diagonal, same Baader Clicklock 2-1.25". All great. Who is copying whom (or if it's just an amazing coincidence), I have no idea! 😉

[badhex]

Another vote here for the Baader clicklock 2" as a premium option, or the Revelation dialectric as a somewhat cheaper but still excellent second choice. 

One thing that might be worth noting is that a surprising amount of 2" diagonals have somewhat less than the clear aperture available in both the Baader and Revelation, which is around 47mm and the largest clear aperture you can reasonably have in a 2" fitting. There's a thread here with more info: 

 

[badhex]

Whoops sorry, missed @Mr Spocksaying the same thing! 

[bosun21]

1 hour ago, cajen2 said:

Identical to @bosun21. Same 102 scope, same WO 2" dialectric diagonal, same Baader Clicklock 2-1.25". All great. Who is copying whom (or if it's just an amazing coincidence), I have no idea! 😉

Haha 😂 I had this same setup with my previous Starfield as well. Works great for me.

[John]

This group test by Bill Paolini is quite often posted but it is relevant so here it is again if you have not seen it:

Mirror vs. Dielectric vs. Prism Diagonal Comparison - Articles - Articles - Articles - Cloudy Nights

It also highlights the importance of effective baffling / blackening within the diagonal body. 

 

[badhex]

47 minutes ago, John said:

This group test by Bill Paolini is quite often posted but it is relevant so here it is again if you have not seen it:

Mirror vs. Dielectric vs. Prism Diagonal Comparison - Articles - Articles - Articles - Cloudy Nights

It also highlights the importance of effective baffling / blackening within the diagonal body. 

 

Thanks @John, hadn't seen this - looks very helpful!