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Roofprismbinoculars

I have a Celestron (bought on here) purely for use in the 60mm for birdwatching. It works just fine with my LVWs and the views are much better than actual birdwatching scopes! (well, it is a Tak )

The StellaMira is definitely a lot more solid and of metal construction with a compression ring, but the mirrored version has a lip preventing full use of clear aperture as mentioned on other threads. Since the body of this is the same construction one might assume it has the same issue.

Amici prismdiagonal

I have the non-Amici version of the Baader and it is very good and solidly built, no issues with clear aperture (35mm on the Amici version) but naturally you pay significantly more for Baader, especially since the Amici version does not come with EP holder or nosepiece if needed.

Opticalprism

A stack of glass or apertures which does not produce such an image is not an objective lens. In fact, without a curved surface, you don't really have a lens at all, let alone an objective lens (and the term "objective" in this context is really just short for "objective lens").

Abbeprism

Doveprism

The astro essentials appears to be the same as the synta prism that spear under SW and Celestron branding. They are not particularly high quality as @Louis D says, a pretty tiny clear aperture, and only have the one set screw so will mark your EPs, if that kind of thing bothers you.

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I mean, would you still call it an objective even when there are no lenses in it? I know this example might not make sense. Just imagine an "objective" has several apertures and/or filters.

Porroprism

By definition, an objective lens is a lens which converges light to produce what's called a "real image" — a projection that can be observed on a screen, or (as in the case of camera) recorded on film or a sensor.

I am thinking about getting an Amici diagonal for certain kinds of star hopping (or, in my case, star hoping). But which one? They will all show the same diffracting pattern so is it worth spending more?

Remember, though, the clear aperture on these lower cost Amici prisms is somewhat less than 27mm, so you will probably see vignetting with widest true field 1.25" eyepieces.  Also, they're plastic bodied, so I wouldn't put a heavy, expensive 1.25" eyepiece in them.

Prismdiffraction

The Baader is however attractive as I guess it really is better and the T2 connection means I can use it for short light path applications (my Askar FMA135).

I had the Baader prism you mention, several years ago now, but I recall it being very good. I had a Tak prism at the same time and there was no difference visually. The Baader is considerably better built and easier to use, but way more expensive. Shouldn't have sold that one...

For low power star hopping, I would stick to the lowest cost option available.  The Baader would probably shine at high powers, but who star hops at high powers?

DoubleAmici prism

I only use a 2" Amici prism (William Optics made, by the look of it, but Orion USA branded). It works well at lower magnification, and plays nicely with the 2" Naglers I have. Birdwatching with an APM 80mm F/6 and Nagler 22mm T4, at about 22x magnification with a 3.75 deg FOV is just brilliant. I have used a 45 deg 1.25" Amici prism, but that was a cheap plastic affair that came with a little 70mm F/5 scope I converted into a finder scope. I quickly replaced that horrible plastic contraption with a TS 90 deg 1.25" Amici prism which is far more solid, and works well with heavier EPs. The prism can be seen here:

Binoculars and microscopes have objective lenses which converge the light and then also eyepiece lenses which diverge it again.

The Baader BBHS T2 is pretty remarkable. I can only just perceive a tiny ‘spike’ with a bright artificial star. For lunar observing at high powers it’s top notch.

In conversational English in the context of photography, we generally call the thing that screws to the front of a camera body a "lens", not an "objective". We even do that when the "lens" in question contains mirrors — we say "a mirror lens". Or, even though it's not technically a lens at all, we say "pinhole lens".