Quote:Greg, Yes - but that ED scope is still an Achromatic instrument. While better in many respects to other Achro's, it's still an Achro. Many folks have fallen into the trap of believing an ED scope is color corrected.None of what I'm saying means that this and others like it aren't wonderful instruments. They're just not Apo's. Joe
Quote:Then how come if a ED doublet isn,t an APO, then a TeleVue doublet is called an Apo??, to be honest myself I find this constant argument tireing, no offence to the posters intended, all scope designs have compromises, be it cooldown time, narrow fields of view,defraction spikes etc, some of my most memorable views have been through well made achromats and in the same breath I seen some very mediocre APO,s. DA.
Quote:Greg, Yes - but that ED scope is still an Achromatic instrument. While better in many respects to other Achro's, it's still an Achro. Many folks have fallen into the trap of believing an ED scope is color corrected.
None of what I'm saying means that this and others like it aren't wonderful instruments. They're just not Apo's.
Quote:Very good summary and examples Greg. Also very accurate and faithful to what I was through my old 102 f/7 ED. Not true APO and not one I'd choose if I were going to do AP work. But I always felt these scopes excelled for visual work and especially their quick cooldown times for a quality grab-n-go. Also their cost at the time they were on the market made them a good choice for visual observers. Thanks.
Clear skies - Jim Crazy PNW imager !
A-P Mach1 on 12" concrete pier - ROR structure.
Tak FSQ85-EDX, Tak FS-128. Orion 8" f/3.9 w/MoonLite motorized - MPCC MkIII CCD:
SBIG 8300M/FW8, Astrodon 36mm LRGB, 5nm Ha, 3nm SII, OIII - Canon EOS 6D unmod SSI3, SSAG,
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Orion XX14g -for visual- diags, ep's, accy tubes, Binocs .
Quote:Greg - nice images and interesting write up.However,"ED" is not a type of refractor. I understand that many manufacturers/resellers brand their scopes with the "ED" designation and I think that has begun to confuse people. There is no such thing as an ED scope. There IS ED glass. And their are both achromats and apochromats that make use of an ED element. But "ED refractor" means nothing other than that it uses one piece of this type of glass - and the level of correction between all available scopes using ED and carrying that label on the tube varies quite a bit.Don't get me wrong - I think you put up a nice post, it is informative, and I appreciate seeing it. I just hate seeing scopes referred to as an ED refractor when that term says nothing at all about how well the scope will perform.Anyway - just my two cents.Good job, nice post.
Quote:There is in fact a need for a designation that describes in-between triplet apo and standard achromat.
Quote:Quote:There is in fact a need for a designation that describes in-between triplet apo and standard achromat. I'm not positive but I think I disagree.As far as I know, the primary distinction between achromat and apochromat is that achromats bring two colors of light to focus at the same point and apochromats bring three to focus at the same point (as well as further control of other abberations).It is impossible to bring 2 1/2 colors to focus and thereby be "semi-apo" or to say that somehow "ED" means this middle ground.How are you going to define and set performance criteria? What specific level of correction for which specific abberations are you going to establish that a scope must meet in order to be "semi-apo" or "ED"?If as you suggest we are going to establish a new classification for a refractor, then you must set a specific quantifiable definition before that classification can possibly have any meaning. Short of that its simply arbitrary. And "ED" doesn't define anything about performance or level of correction.I'm not sure if I agree that even needs to be this designation. What's wrong with saying you have a well corrected achromat?Maybe someone can better explain this to me...
Quote:Alan I'm aware of the text. I was referring to scope like this using ED in the name and model number. One other note. What the eye and camera see is different. So your virtual color free achro may be god awful in the eyes of the CCD... Joe
Quote: One could pass a law requiring spot diagram decals to be on every refractor.
Quote:Here is where the mud comes from Alan...http://www.vixenoptics.com/refractors/ed100sf.htmED Achro... It's confuses folks that don't fully understand what they are getting. Sorry but I've no intension of backing off on the point. There are good makers and misleading makers. Good and bad scopes. I'm not BTW saying Vixen is bad. Sorry but this scope wasn't in the Berry text. There are 100's more like this. You miss the point...that a 200 foot long achro is still and achro.. And again....that's not bad - it just is. Best, Joe
Quote:Quote: One could pass a law requiring spot diagram decals to be on every refractor.Man after my own heart! I know you are saying this in jest, but at least if you were to have objective information included with your scope then you would know something for certain about the performance of what you are buying.
Quote:Yes, that is not a misleading ad at all. I just wanted to show a prominent maker using ED in the model number, an ED glass element and identifying it as an achro.. Joe
Quote:Quote:Yes, that is not a misleading ad at all. I just wanted to show a prominent maker using ED in the model number, an ED glass element and identifying it as an achro.. Joe Joe,I don't see the word "achormat" anywhere on that web page, and "Edit/Find" didn't either. So I am a bit confused now.Clear skies, Alan
Quote:Your right Alan, I'm loosing it. Try this Skywatcher:http://www.australiantelescopes.com.au/100mm-ed-achromatic-telescope-gold-tube-ota-f-6-skywatcher.htmlJust quick googles and they pop up. Joe