"Enhanced" Star Diagonals

[Alfian]

I'm window shopping  with view to eventually acquiring a 2" star diagonal for a achromaric refractor. After the usual trawl through on-line articles and posts I had pretty much come to the conclusion that paying the extra for a dielectric was probably the way to go. Then I came across a couple of "enhanced" diagonals that gave 94-95% reflectivity. A little further digging to gain information on what "enhanced" meant brought up this short article by Gary Hand.

http://www.cloudynights.com/page/articles/cat/articles/optical-theory/diagonals-prisms-vs-mirrors-r79

Dielectric certainly seems to win the day in terms of durability but I'm now left wondering whether the difference between 95% and 99% reflectivity is worth thinking  about. 

[John]

I have an enhanced diagonal by Tele Vue. I can't see any difference in use between it and the Tele Vue dielectric that I also have. The dielectric coatings are reputed to be harder wearing than other coating types. I guess I prefer the dielectric on the basis "every little helps" but I'm possibly kidding myself

I have used prism diagonals but not the premium units so I can't really comment on those.

[Alfian]

Thanks John, I am getting the feeling that as long as I go with something of known quality in terms of sound construction and optics, I should be OK, at least for my purposes. in that sense it could be that it will wind up being a dielectric anyway.

[John]

Thanks John, I am getting the feeling that as long as I go with something of known quality in terms of sound construction and optics, I should be OK, at least for my purposes. in that sense it could be that it will wind up being a dielectric anyway.

That sounds a sensible approach. You don't see too many enhanced diagonals for sale. Mostly they seem to go from "regular" to dielectic with nothing in between. The dielectric ones are quite reasonable now so there is no reason not to get one really.

[Ruud]

Some considerations:

The gain from 99% over a 95% reflectivity is 0.045 magnitudes.

Enhanced aluminium slowly degrades and needs to be cleaned carefully.

A prism has three surfaces that need to be at the proper angles to each other (that's difficult!) as well as completely flat and super smooth.

Micro bubbles inside a prism will scatter light, other irregularities will do the same.

Micro scratches on a mirror will scatter light.

The light cone from a star hits only a fraction of the surface of a diagonal (the beam is already quite narrow there), which means that a 1/10th wave surface functions easily as well as a fully illuminated 1/40th wave surface.

The many layers of coatings on a dielectric mirror give the reflective surface a turned edge, like snow on a roof. A dielectric mirror needs to be oversized by a few mm  for that reason. Relatively, that's a little bit for a 2" mirror, and of course a bit more for a 1.25".

Bottom line:  I replaced a 20y old TV enhanced aluminium diagonal with a WO dielectric one and can't see any difference. In spite of the turned edges I also got two 1.25" dielectric diagonals which seem to work perfectly fine. Dielectric is nice mostly for cleaning: you don't have to worry that you'll dull the surface over time. Diagonals tend to gather dust and do require regular cleaning.

[Moonshane]

I think your logic is sound. It won't make any real difference optically that you'll see. Buy on the basis of function - security of fittings, compression rings, whether you want an undercut etc etc.

[Brian Nordstrom]

I Agree with  the comments here on coatings , I have never really seen a difference between diagonals and more than once we have done blind testing of some 2 inch diagonals , AP ;s , TV's , WO's , SW's  of dielectric and aluminium coatings and to be honest we could never pick a winner , its more about build quality , status , and telescope quality and seeing conditions .

Any mid range dielectric one will be fine .

Brian.

[Alfian]

Thanks guys for the knowledgeable feedback, think I will be looking at a GSO/Revelation or maybe an Altair dielectric. Much will depend on whether Santa thinks I've been a good boy - I'm not holding my breath on that one!

[triton1]

Just make sure you've been very very good!

[Alfian]

I 'll try, but you know how it is sometimes!