New Diagonal from Burgess ???

[John]

I found this on the SCS Astro web site under new products:

http://www.scsastro.co.uk/it020005.htm

Burgess are making some big claims for this diagonal and yet in the past the general opinion here has been that spending big bucks on a diagonal is not really necessary. I wonder if this one will live up to the claims - especially those relating to reducing CA in refractors ???.

John

[Doc]

As you say they are making big claims and with a price tag of £154 this should be pretty good.

I've always associated Burgess with the cheaper end of astronomy gear.

Maybe I'm wrong.

[GazOC]

I assume its a prism? I've seen reports of prism diagonals being used with the old WO Megrez 110mm and improving colour correction but never seen anything mentioned in advertising or sales spiel.

[Ben Ritchie]

Prism diagonals do have a (small) effect on refractor CA in my experience, but my guess here is that the dielectric coatings have a reflectivity that varies as a function of wavelength and therefore reduce visible CA by reflecting less in the far red/violet. Another possible that springs to mind is that there's a polarizing filter effect built-in, to reduce scatter. Dunno though, just a guess.

Interesting idea anyway, but certainly my view up to now is to agree with John - having used a 'big bucks' diagonal side by side with a reaonably priced one for a while there's precious little difference that I can see...

[AndyH]

Here are some sketches a chap called Sol Robbins did with a std diagonal, the cd1 and cd2.

Impressive results methinks.

It is a std looking 2" mirror diagonal, so I guess the coatings are the thing.

Cheers,

Andy.

[AndyH]

There is an ongoing thread regarding these diagonals in 'cloudy nights',

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/2821493/page/1/view/collapsed/sb/5/o/all/fpart/2#Post2933387

The newly posted drawing of Saturn is , well, rather good. Wish I could sketch like that. Wow.

Cheers,

Andy.

[FLO]

Is this not simply where one (the prism) glass type is chosen to counter aberrations in another (the objective)? Similar to the way the crown element in an APO refractor is chosen to complement the APO element so together they deliver the desired result. Or perhaps there are phase-correcting coatings similar to those applied to roof-prism binoculars. (Does light go out of phase when passed through an achromat objective?)

[Whippy]

Just a thought, but I'm sure I read somewhere on CN that there's a few ED scopes about (Certain 110mm and 102mm Stellarvue and Orion models IIRC) with some kind of 'special' coating on the lens that reduces false colour. I'm guessing this diagonal might have the same coating?

Tony..