prism versus diagonal mirror

Is there a big difference in optical quality in moving from a standard prism diagonal to a dielectric mirror? I saw some reflected images in my views of saturn last night which I presume are due to reflection off the prism surfaces (came with the scope-fairly basic one). I am looking at one of the revelation dielectic mirror diagonals (2"). It claims 99% reflectivity and 10th wave optics.

John

Most supplied diagonals seem to be in the region of 89-91% reflectivity and the cheaper ones won't be anything like 1/10 wave. I recently bought a 2" diagonal with dielectric coatings from Teleskop-Service and I can certainly see a difference between it and the standard 1.25" mirror and amici prism models I already had.

Until relatively recently, many would have say that prism were the way to go (at least that's the perception that I get from some people), but like John said above, badly coated ones will cause reflections,
More recently, with the advent of excellent coatings, mirrors have taken over completely.

Is there a big difference in optical quality in moving from a standard prism diagonal to a dielectric mirror? I saw some reflected images in my views of saturn last night which I presume are due to reflection off the prism surfaces (came with the scope-fairly basic one). I am looking at one of the revelation dielectic mirror diagonals (2"). It claims 99% reflectivity and 10th wave optics.

John

You need to read those figures (99% 1/10 wave) in context. The advertising standards rules mean that the figures are probably correct/true, but need to be qualified.

Standard Aluminium is usually about 86% reflective in the main visual part of the spectrum. Enhanced/dielectric coatings go up to about 95% or there abouts - again in the main part of the visual spectrum. However, even ordinary aluminium can be said to be nearly 99% reflective... in other parts of the spectrum, but which are not much used by amateurs (IR, NIR, UV, etc.) There are unscrupulous (though strictly accurate) advertisers out there!

1/10th wave optics: This is an elephant trap! A 1/4 wave (on the surface) mirror can also be accurately stated to be 1/8th wave (on the wave front) 1/16th wave P-V or 1/32 wave RMS! I may have that slightly skew, but you get the general idea. You have to qualify the quantity to understand what it means. Reams have been written about this! You should only be interested in surface quality, and a 1/4 wave surface is diffraction limited. What is more important than surface accuracy is surface smoothness (lack of primary- or micro-ripple) which dictates how much light gets scattered. It's difficult to explain the distinction easily, but the bottom line is that flat mirrors don't actually need to be absolutely flat - they can be several waves concave or convex and not affect the image. They do need to be ultra smooth!

Search the ATM mailing list/website for more info about this.

Phil.

Has anyone got one of the revelation 2" dielectric diagonals from BC&F. The "high quality" diagonal advertised by telescope-service I believe is the exactly the same (same spec and price €149).

John

1/10th wave optics: This is an elephant trap! A 1/4 wave (on the surface) mirror can also be accurately stated to be 1/8th wave (on the wave front) 1/16th wave P-V or 1/32 wave RMS! I may have that slightly skew, but you get the general idea. You have to qualify the quantity to understand what it means. Reams have been written about this! You should only be interested in surface quality, and a 1/4 wave surface is diffraction limited. What is more important than surface accuracy is surface smoothness (lack of primary- or micro-ripple) which dictates how much light gets scattered. It's difficult to explain the distinction easily, but the bottom line is that flat mirrors don't actually need to be absolutely flat - they can be several waves concave or convex and not affect the image. They do need to be ultra smooth!

Search the ATM mailing list/website for more info about this.

Phil.

I agree with alot of what you say there Phil, saying something is 1/4 wave accurate doesnt tell you alot about the mirror. This was all revealed to me when I got the new 20" mirror and I finally figured what its all about, to give the example I have with the 20",
The worse Peak-Valley (P-V) on the mirror is 0.171, which works out as slightly better then 1/5 wave optics, but its the RMS value that tells you how smooth the mirror is. What they do is measure 2819 points on the mirror and measure its wave error at that point and get the RMS value of all those points, I'd imagine if the optics were highly rippled or badly deformed, then this RMS value would be high (as you'd have high errors all over the mirror), if the optics were generally smooth with a number of discrete errors here and there, then it would be low, so is the RMS value not the average wavefront error of the mirror ? (I'm ready to be corrected on this :oops: ). sorry for going off topic John.