I am a relative newbe to the ED and APO refractor game. My experience (30+ years)has been with traditional long focus acromats (f12-15) from 3 to 11 inches in aperature for double star micrometry. I recently purchased a small 66 mm ED F5.9 refractor as a spur of the moment grab and go scope. What I observed is a blue "haze" around brighter stars that is completely absent in my acromats. Is this an effect of the ED glass or is it something more related to the focal length or both? Thanks for any input. Tim
ED Lens
Started by thrutkay, 02/19/2007 02:10AM
Posted 02/19/2007 02:10AM
Opening Post
Posted 02/19/2007 04:49AM
#2
An ED is not an APO, plus at f/5.9 it is fairly fast too. This is likely to result in a spread of the Blue-Purple wavelengths especially on a bright star. My ED at f/9 also shows some blue-purple on the brightest stars where the APO does not at f/6.
I would think that most any doublet will show some color.
I would think that most any doublet will show some color.
[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue
Amateur Imager
Posted 02/19/2007 05:00AM
#3
>>>I recently purchased a small 66 mm ED F5.9 refractor as a spur of the moment grab and go scope. What I observed is a blue "haze" around brighter stars that is completely absent in my acromats.
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With any normal eyepiece, you should not be seeing a purple haze with any of the various FPL-51 based 66mm F/5.9 refractors. I have one of these and have used it with Plossls, orthos as well as various widefield designs on targets like Rigel, Vega and Venus with no apparent false color at and beyond the 50/inch limit. In comparison, I can see a significant amount of purple haze around Rigel and Venus in a well made 80mm F/11 refractor.
To me, a couple of possibilities occur:
1. The scope is one of the 66mm Petzval's. These were sold by some as "Semi-APOs" but seem to be Achromatic Petzvals and apparently these do show significant false color.
2. You are using a prism diagonal. Not really sure on this one but it is my understanding that is a fast scope and F/6 is fast, a prism diagonal can cause false color. There are no stars out to night so I cannot give it a good test but looking at a distant street lamp with at 81x, I beleive I see some hints of purple with a prism diagonal that I don't see with the mirror diagonal. Slightly out of focus it is more apparent.
3. Optics have some problem...
Thats my take...
Jon
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With any normal eyepiece, you should not be seeing a purple haze with any of the various FPL-51 based 66mm F/5.9 refractors. I have one of these and have used it with Plossls, orthos as well as various widefield designs on targets like Rigel, Vega and Venus with no apparent false color at and beyond the 50/inch limit. In comparison, I can see a significant amount of purple haze around Rigel and Venus in a well made 80mm F/11 refractor.
To me, a couple of possibilities occur:
1. The scope is one of the 66mm Petzval's. These were sold by some as "Semi-APOs" but seem to be Achromatic Petzvals and apparently these do show significant false color.
2. You are using a prism diagonal. Not really sure on this one but it is my understanding that is a fast scope and F/6 is fast, a prism diagonal can cause false color. There are no stars out to night so I cannot give it a good test but looking at a distant street lamp with at 81x, I beleive I see some hints of purple with a prism diagonal that I don't see with the mirror diagonal. Slightly out of focus it is more apparent.
3. Optics have some problem...
Thats my take...
Jon
Posted 02/19/2007 04:49PM
#4
The term ED might imply that it is fully corrected for color error at all visible wavelengths. However ED does not mean anything about correction at all. ED glass CAN be used to correct color error, but does not necessarily do so on its own.
Full color correction comes from a combination of ED glass and crown glass, either in doublet or triplet configuration. If ED glass is combined with Flint glass or Crown-Flint or other similar types, you will have more or less color error. Only crown glass mates can achieve true apo performance over a wide wavelength range.
Why do manufaturers use ED glass with flint or crown-flint and not the ideal crown glasses? After all, a crown glass is actually purer, clearer, less prone to striae and less costly than any crown-flint or flint glass. They do this to achieve fast focal ratios without messy aspherization, or to keep the cost of fabrication down because flint glass makes for easy manufacturing and looser tolerances. The tolerance level will need to be as much as 10 times tighter when you use a crown mate with ED glass, but the color correction achieveable this way can be as much as 10 times better.
Rolando
Full color correction comes from a combination of ED glass and crown glass, either in doublet or triplet configuration. If ED glass is combined with Flint glass or Crown-Flint or other similar types, you will have more or less color error. Only crown glass mates can achieve true apo performance over a wide wavelength range.
Why do manufaturers use ED glass with flint or crown-flint and not the ideal crown glasses? After all, a crown glass is actually purer, clearer, less prone to striae and less costly than any crown-flint or flint glass. They do this to achieve fast focal ratios without messy aspherization, or to keep the cost of fabrication down because flint glass makes for easy manufacturing and looser tolerances. The tolerance level will need to be as much as 10 times tighter when you use a crown mate with ED glass, but the color correction achieveable this way can be as much as 10 times better.
Rolando
