ED vs SD

[FOOTPRINT]

Hi All, Can someone let me know what "SD" means as regards Objective Glass, "ED" and Fluorite are common enough, but I notice one of the WO 66 Scopes, an APO has "SD" glass, is this somewhre between ED and Fluorite ?.Thanks......cheers............... ...Jim

[Dennis]

A long time ago, I read that the generic term of SD stood for "Special Dispersion" and in that context, it was a poor man's ED and a very, very poor man's fluorite.

So, the "quality" line appeared to go something like Achromat -> SD -> ED -> Fluorite.

I'm not sure if SD was a marketing ploy to give the consumer a warm feeling that he/she was getting something better than an achromat and closer to ED?

Cheers

Dennis

EDIT:
I might just add that this was from reading UK photography magazines in the early 1990’s. It seems that Televue places SD as you have written, closer to Fluorite.

[g__day (Matthew)]

Dennis,

I guess you're right, but I'm wondering if the quality line doesn't finish on a Fluroite Triplet lens rather than a doublet made of Flourite?

I can't see how any doublet lens commercially available would stack up against a high quality triplet?

[casstony]

I don't pay much attention to these terms anymore as they've become corrupted by deceptive advertising. You really need an abbe number or glass type to know what glass is used. Even then you need reliable reviews to see if the design has been well executed. For example, Skywatcher claims their ED80 uses FPL53 and reviews indicate that it is a very nice lens.

[Kal (Andrew)]

I've read a couple of times that ED glass generally has an abbe number greater than 80, for example FPL-51, while SD glass has an abbe number greater than 90, for example FPL-52 FPL-53 or OK-4. I wouldn't trust this to be true in all cases though, since they are loosely used as marketing terms (just like how loose the term APO is becoming nowdays).

[wavelandscott (Scott)]

I am reading this thread with interest...as noted the lines between different glass types are getting pretty blurry...APO, semi-APO, near semi-APO (okay I just made that one up) but you get the point...

Cheers!

[FOOTPRINT]

Hi All, Many thanks for all your replys, I went and looked at the William-Optics wedpage to compare the Zenithstar ZS66D-SD vs the ZS66P-ED the first is claimed to be an APO, an SD doublet, and the Longitudinal Abberation graph shown makes it look fantastic, no graph is given for the ED Scope to compare it with, I gather that the larger the objective the more difficult it is to get all spectra to focus together, as I looked at the ZenithStar Fluorite doublet Long. Abb. Graph and its way out by comparison. So on a small scope of 66mm Obj. at say F-4 to F-6 (wide Field) is there going to be that much difference between an SD scope and a Fluorite doublet/triplet for Astrophotography, I can see the need on a large refractor for Fluorite Glass and even a triplet (big bucks but nice), your thoughts please.cheers.....Jim

[Kal (Andrew)]

I find the best way to judge these scopes for their astrophotography potential would be to look at actual results from people that have used them, and I think the WO website has a few samples in their gallery you could see. If not, search around, I've seen a few people get good results out of the SD doublet, although you will probably want the field flattener with it if you image with a digital SLR.

Also, you can read a review comparing the 66SD with the 66 Triplet at cloudynights. The ED model is a petzvel design (4 pieces of glass) and is only rated as semi-apo.

[FOOTPRINT]

Hi Andrew, Many thanks for that information, ive followed it up and there is a wealth of information there on these scopes and types of glass used in scopes, followed up on Fluorite glass also, seems its a bit fragile in some ways, exposure to the wrong climatic conditions Etc. can damage them, ms thats why Canon and otheres went for ED glass for their Camera Lenses generally, quite a bit on Triplets vs. Doublets also, as you say " the proof of the pudding is in the eating", thanks again.cheers....Jim