Diagonals - expensive vs mid range

[Don Pensack]

5 hours ago, Paz said:

Something to consider with diagonals and vignetting is the light cone and how wide it is as its path goes through the diagonal (or any particular thing you might have in the optical train). There will be a point somewhere in the light path where the light cone comes to a focus and has no width, and either side of that the light cone will expand at a rate determined by the focal ratio of the scope. So depending on the light cone you can put things upstream of an eyepiece that may be smaller than the field stop of the eyepiece and get away with it.

The focal plane is only a point for the very center of the field.  If the field of view is wider than a point, the final focal plane has a width.

Technically, the center 10-13mm will be fully illuminated and illumination will fall outside of that small circle, until, when it gets off-axis to the degree found in your lowest power/largest field stop eyepiece, it will be at or about 70% illuminated.

So if you are graphing what happens to the light in a telescope, the final focal plane should be a circle with the diameter of your lowest power eyepiece to determine if the diagonal or filter, or focuser drawtube, cuts into the field of view.

 

[hal9550]

I did some browsing last night and found several Ali-Express options in the value V Quality field

https://www.aliexpress.com/item/1005006787690985.html?spm=a2g0o.detail.0.0.1927iT19iT19NN&mp=1

https://www.aliexpress.com/item/32859098070.html?spm=a2g0o.detail.0.0.4c41r4jar4jaCE&mp=1

 

https://www.aliexpress.com/item/32906056763.html?pdp_npi=4%40dis!EUR!€ 123%2C23!€ 80%2C10!!!134.32!87.31!%40211b8f9b17269972904435200eabf3!65824907242!sh!IE!0!X&spm=a2g0o.store_pc_allItems_or_groupList.new_all_items_2007586209452.32906056763

They all seem to be well specified, for the price. They also seem to have numerous brand siblings available locally in europe, with a 40% odd markup - 

giving serious consideration to taking a shot at one of these

[Don Pensack]

The Celestron one is available with both the thread-on SCT nosepiece and the standard 2" tube in the box, for you to choose which to use.

The advantages of the 2" standard tube nosepiece:

--larger clear aperture

--threaded for filters

--works in SCT, MCT, refractor, and CC scopes

--easier to rotate to the side when the scope points low

--easier to re-sell if need be.

The advantage of the threaded-on style:

--doesn't need 2" visual back adapter (SCT/MCT/CC)

--will have greater clearance to swing through fork arms

Edited Sunday at 19:31 by Don Pensack

[hal9550]

5 hours ago, Don Pensack said:

The Celestron one is available with both the thread-on SCT nosepiece and the standard 2" tube in the box, for you to choose which to use.

The advantages of the 2" standard tube nosepiece:

--larger clear aperture

--threaded for filters

--works in SCT, MCT, refractor, and CC scopes

--easier to rotate to the side when the scope points low

--easier to re-sell if need be.

The advantage of the threaded-on style:

--doesn't need 2" visual back adapter (SCT/MCT/CC)

--will have greater clearance to swing through fork arms

Yes the Celestron one is the most promising - its not listed as dielectric, which is why i was considering the others - but there is some saving buying via Aliexpress - 

I could not find any Prism Based 2" Diagonals on Aliexpress at all - curious if anyone has found one - 

[Don Pensack]

Only the 2" Amici correct-image diagonals, both 90° and 45°.

So far, China does not seem to have gotten into 2" 90° standard prism diagonals.

[hal9550]

18 hours ago, Don Pensack said:

Only the 2" Amici correct-image diagonals, both 90° and 45°.

So far, China does not seem to have gotten into 2" 90° standard prism diagonals.

Oddly enough there seems to be some price reductions on Astroshop and there is now an AMICI 2" for under 180 euro - i may genuinely buy this. More than a few reductions so, and as i was planning a BIG BUY - might be time to do it

 

https://www.astroshop.eu/amici-prisms-90d/omegon-amici-prism-90d-2-/p,74413

 

Unless someone reckons this is not a good buy of course!