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TAK TSA102s Yellow Green Fringe around Moon

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#1 BullTerrier

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Posted 14 July 2013 - 12:08 PM

Last night, as I was observing the Moon (about 50+ degrees above horizon),with my new Tak TSA102s and Pentax XW20 EP, I noticed a yellow-green fringe around the moon. Is this normal for this instrument or is it possible that it's CA ?
One thing, if I moved my eye around the eyepiece, it disappeared. I've used this ep with my SCTs and did not see this. I also tried a 31mm Plossl and also saw the yellow-green fringe.

Is this common for this scope or do I have an issue that I need to take up with TNR.

Thanks
Martin

#2 T1R2

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Posted 14 July 2013 - 12:16 PM

On a trip-apo, its probably because its not reached thermal eq.,,, or its slightly out of alignment.

#3 ggalilei

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Posted 14 July 2013 - 12:41 PM

I have never seen that in my TSA 102. I could check my notes but I think I would remember seeing color while in focus on any object. I've noticed that some eyepieces - even quality eyepieces - may produce a bit of color fringe on the Moon when you move the Moon's edge near the edge of the field of view. Perhaps that's what was happening?

#4 Simoes Pedro

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Posted 14 July 2013 - 03:00 PM

I have never seen that in my TSA 102. I could check my notes but I think I would remember seeing color while in focus on any object. I've noticed that some eyepieces - even quality eyepieces - may produce a bit of color fringe on the Moon when you move the Moon's edge near the edge of the field of view. Perhaps that's what was happening?


+1

Does this happen at the center of the field of view?

#5 BullTerrier

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Posted 14 July 2013 - 03:11 PM

The limb of the moon was not in the center of the field.
I will try again tonite, 1st making sure everything is in thermal equilibrium, and the if I see anything, I'll move the image within the FOV to see if anything changes. The only curious thing was that the colors disappeared when I adjusted the angle of my eye to the eyepiece.
Guess should also try looking with the other eye to rule out my right eye from being an issue.

#6 StarDust1

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Posted 14 July 2013 - 03:52 PM

Martin, As you said, it's the eye position that causes the the CA.

#7 BullTerrier

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Posted 14 July 2013 - 03:56 PM

So does that rule out there being a problem with the optics ?

Thanks
Martin

#8 ggalilei

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Posted 14 July 2013 - 04:12 PM

I imagine that they design eyepieces to be used from near the optical axis. I guess strange things could happen at a very different angle.

#9 OJS

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Posted 14 July 2013 - 04:47 PM

Martin:
In my TAK FS 128, I notice the following:
I see such a fringe with my 13 mm nagler type 6, less so in my two Panoptic eyepieces, not there in my Tak LE's. So, maybe it's a function of design/no. of elements in an eyepice.
In all the ones it does show up in, I don't find it objectionable; just noticable. I have yet to try out my new Delos 4.5 mm on the Moon. Will report when I do. I've had this scope for 10 yrs. now and still marvel at the quality of images.

OJS

#10 art1942us

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Posted 15 July 2013 - 09:50 AM

Please contact us if we can help.

Art

#11 Niklo

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Posted 15 July 2013 - 10:47 AM

Hello Martin,
this seems to be an eyepiece problem otherwise the fringe would not disappear if you look from another angle into the eyepiece.
Cheers,
Roland

#12 StarDust1

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Posted 15 July 2013 - 12:53 PM

Yes, Martin, It's the position of the eyepiece.

Did you perform a star test? When performing a star test you need good seeing. And give the scope enough time to cool down.

As Art suggested, you can also call TNR. I'm sure they can help you out.

#13 Scott99

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Posted 15 July 2013 - 01:55 PM

Every wide-field eyepiece I've ever used has some lateral color error, sounds like what you're describing.

Try something like an UO HD ortho and you'll get the best view of the moon, less scatter, no lateral color, and more contrast than an 8-lens wide field in my experience. My XW's add a tiny bit of color error on bright objects as well.

#14 cw00

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Posted 15 July 2013 - 02:21 PM

I was using my Tak FS-152 and a few eyepieces to look at the moon last night. I can see a blue fringe around the moon with Edmund RKE 28mm and a yellow fringe around the moon with TMB 25mm aspherical when the moon is centered in the view. They will disappear once I move the edge of moon to the center of the view. But I do not see any color fringe at all with other low power eyepieces, including TV 25mm/40mm plossl and Brandon 48mm. So I think it is due to the characteristics of the eyepiece.

#15 BillP

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Posted 15 July 2013 - 03:11 PM

Most, if not all wide fields show lateral color in their off-axis, especially noticable with bright objects like the Moon. I suspect it is just lateral color you are seeing that is generated by the optics of the eyepiece. As already mentioned, for sure way to tell if it is this is to move the limb of the Moon across the FOV. If the color fringe goes away for the portion of the limb in the dead center of the FOV, then comes back as it reaches the other side of the FOV, then this is lateral color from the eyepiece. "Typically" the lateral color on the lunar limb also changes hue when on one side of the FOV of the eyepiece vs the other side of the FOV. or if you have a full Moon in the eyepiece then the hue of the color on the limb may be different for opposing limbs.

#16 GlennLeDrew

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Posted 15 July 2013 - 03:43 PM

Based on the description, this seems to be lateral chromatic aberration introduced by the eyepiece.

I hereby register my *extreme* exception against the notion that a lack of thermal equilibrium will introduce any perceptible amount of longitudinal chromatic aberration in the objective! Other aberrations will rear their heads *long* before color error.

#17 BullTerrier

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Posted 15 July 2013 - 03:47 PM

Thanks everyone for the replies and insight. One thing though, does the design of the OTA impact whether and eyepiece introduces color ? I ask because I don't see this same color fringing when I use the same eyepiece with my C8. I know we're talking Reflector vs Refractor, but is it incorrect to assume that if I saw this issue in the Refractor that the same should appear in the reflector (SCT).

Thanks
Martin

#18 BullTerrier

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Posted 15 July 2013 - 03:53 PM

One more thing... would mediocre "seeing" conditions have an impact of this ?

Martin

#19 Nippon

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Posted 15 July 2013 - 03:59 PM

I think the faster focal ratio of the refractor versus the C8 would be why you don't see it in the C8.

#20 Kunama

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Posted 15 July 2013 - 04:08 PM

My vote also goes for this being eyepiece induced, I sometimes get the same effect (albeit very faint ) when using a Vixen LVW in my TSA120 with the moon at the very edge of the field of view.

#21 BullTerrier

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Posted 16 July 2013 - 12:58 AM

Interesting test tonite. The only EP I have which showed absolutely no color fringing on the moon was a 1985 Celestron 26mm Plossl eyepiece which came with the C8 I bought back then. All my other eyepieces (mainly Celestron X-Cell and a new Pentax 20MM XW) show fringing. Seems like the more complex the EP, the more color. Still perplexed.

#22 BillP

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Posted 16 July 2013 - 09:31 AM

Not surpising. When the AFOV is under one radian (~57 degrees) it is easier to tame most of the off-axis aberrations. So not unusual to see no lateral color on eyepieces with smaller AFOVs like Plossls and Orthos.

Glenn - I agree with you...I don't see how thermal equillibrium could induce lateral color. Like atmospheric seeing, I can see how it might induce axial chromatic aberration, but not just lateral chromatic aberration.

#23 Rutilus

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Posted 16 July 2013 - 03:22 PM

Yes, eyepiece induced. I noticed it quite a lot when using
my TSA 102. My Vixen LV eyepieces showed a colour fringe.

#24 BullTerrier

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Posted 21 July 2013 - 10:52 AM

Just purchased and used the TAK 24mm LE eyepiece to see if I experienced the same issue. Fringe color around the moon was alot less. I did however, notice that when seeing was very steady, it disappeared completely. So, would turbulent seeing cause this phenomenon ?

Thanks
Martin






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