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ES 102 ED APO CF false color?

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#1 exmedia

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Posted 26 August 2013 - 09:27 AM

I've had this refractor for about 8 months and the last (near) full moon was the only time I've viewed the moon through it. It's the only refractor I've owned, and because of the apo reputation expected none of the reported achromat characteristics. However, with 32mm and 25mm plossls the moon had a distinct, bright green line running fully around its circumference. It looked like someone had taken a fine green sharpie and carefully drawn the line. Is this normal? Is it perhaps the eyepieces (they're not expensive)? It's the only time I've seen anything like this--nothing like it on planets or stars--and it was disconcerting.

Thanks,

Richard

#2 dennilfloss

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Posted 26 August 2013 - 10:40 AM

My 102 F/7 doublet also shows some very pale green around the lunar rim but at 223x and it's very faint and thin. I have to look for it to notice it.

http://www.cloudynig...6033410/Main...

And the type of glass used in your ES is supposed to be basically equivalent to the FPL-51 in my objective.

Maybe I'm less sensitive to it.

At 102mm, a FPL-51 doublet would need to be longer than F/7 to be colour-free. Even a FPL-53 doublet would need to be longer IIRC. You could achieve apochromacy at f/7 with a triplet but that has other disadvantages (heavier, more cool-down time needed, more expensive).

#3 SKYGZR

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Posted 26 August 2013 - 03:11 PM

Sounds like "green cheese" you were getting. A higher quality EP may help, yet a moon filter would be best when viewing the moon near full.

P.S. a moon filter also works good on Jupiter<<

#4 Joe Aguiar

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Posted 26 August 2013 - 08:21 PM

the moon is very very bright compared to the planets so iam guessing that's why and you would need to have a triplet to be totally colour free on an object that bright.
joe

#5 PowellAstro

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Posted 26 August 2013 - 08:40 PM

My bet would be the eyepieces. In my APOs I see this with long FL eyepieces but not with the higher power Naglers.

#6 dennilfloss

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Posted 27 August 2013 - 03:23 AM

Just read the OP's sig. Looks like his 102 is indeed a triplet.

#7 Joe Aguiar

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Posted 27 August 2013 - 07:09 AM

ok on his sig yes i see it says triplet but on the title it just said Ed apo which i would normally take as a doublet,nor did i read anything saying it was a triplet thats y i assumed was doublet, i also dont read every scope on the sig all the time either,

so iam not so sure then was he says he was using low power ep not high power
joe

#8 crow

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Posted 27 August 2013 - 10:07 AM

Thats an interesting one. I have the original metal version. I don't experience any false colour at all on anything. I mentioned it a while ago on CN as to be honest I was surprised. I had it out on the moon recently and there was none. That includes using the Axiom eyepieces that came with my Edge as well as ES/naglers etc. I think I need to start looking after it more, it seems to be my grab and go scope and its had a hard life but optically its impressive.

#9 la200o

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Posted 28 August 2013 - 09:07 PM

May be the ep; does the green show when the limb of the moon is in the center of the FOV?

If not, probably the ep. In any event, I wouldn't worry too much about it.

Bill

#10 ken svp120

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Posted 29 August 2013 - 07:08 AM

Swap eyepieces - its either the scope or the eyepiece but one or both of them are giving you CA

#11 Jon Isaacs

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Posted 29 August 2013 - 07:29 AM

May be the ep; does the green show when the limb of the moon is in the center of the FOV?

If not, probably the ep. In any event, I wouldn't worry too much about it.

Bill


I suspect it is the eyepieces, in my experience the moon is not particularly demanding at low magnifications and even a fast achromat can provide quite nice views without much CA.

Point it at Venus shortly after sunset and crankup the magnification. If it is present, an apo or reflector will show the atmospheric dispersion with the purple one limb and orange on the other... if there are color correction issues with the objective, there will be a purple haze surrounding Venus.

jon






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