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Stray light and chromatic aberration

refractor
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9 replies to this topic

#1 ddegroot

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Posted 31 May 2020 - 04:55 AM

Hi everyone, 

Last night, I had little choice but to observe the Moon in a brightly illuminated environment. I brought out my Takahashi FC-100DC with a Takahashi focal extender. This should produce just about color-free views, as I experienced recently while observing Venus. But last night, I saw more chromatic aberration than I've ever seen while viewing the Moon. 

Could the culprit be stray light entering the OTA? Or maybe just atmospheric conditions? 

Thanks! 
D



#2 Tapio

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Posted 31 May 2020 - 05:11 AM

How high was Moon when you observed it ?

If it was low it can make such color faults.



#3 25585

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Posted 31 May 2020 - 06:06 AM

Was it your eyepiece instead?



#4 SeattleScott

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Posted 31 May 2020 - 08:35 AM

Stray light shouldn’t manifest itself as a purple halo. I have had issues with stray light causing internal reflections when using a focal reducer (0.63x SCT style) or coma corrector (reflector). But it was more a matter of an undesirable glare impacting part of the view, not a purple halo or purple shadows in craters.

Personally my Vixen Apo is extremely well baffled and I pretty much can’t induce stray light even shining a flashlight down the tube. I would expect nothing less from Tak.

Yes I would question how high the Moon was in the sky. It set around 1 pm last night?

Scott
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#5 ddegroot

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Posted 31 May 2020 - 09:06 PM

Thanks for the replies.

Moon was quite high, and I've never seen such false color before with the Tak . . . I expected less last night, since it was one of my first times using the extender. 

Strange! 


Edited by ddegroot, 31 May 2020 - 09:07 PM.


#6 Moondust

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Posted 01 June 2020 - 12:46 PM

Could be atmospheric dispersion. I've seen this effect in my 92mm Stowaway on several occasions even with moon/planet relatively high in the sky. Try again on a different night before cursing the scope, eyepiece, or diagonal. 



#7 Alan French

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Posted 01 June 2020 - 01:23 PM

A little hard to speculate when essentially no details were provided. 

 

 

Hi everyone, 

Last night, I had little choice but to observe the Moon in a brightly illuminated environment. I brought out my Takahashi FC-100DC with a Takahashi focal extender. This should produce just about color-free views, as I experienced recently while observing Venus. But last night, I saw more chromatic aberration than I've ever seen while viewing the Moon. 

Could the culprit be stray light entering the OTA? Or maybe just atmospheric conditions? 

Thanks! 
D

 

For meaningful answers, you really need to provide a detailed description of exactly what you saw. 

 

Clear skies, Alan


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#8 mikeDnight

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Posted 01 June 2020 - 02:07 PM

Hi everyone, 

Last night, I had little choice but to observe the Moon in a brightly illuminated environment. I brought out my Takahashi FC-100DC with a Takahashi focal extender. This should produce just about color-free views, as I experienced recently while observing Venus. But last night, I saw more chromatic aberration than I've ever seen while viewing the Moon. 

Could the culprit be stray light entering the OTA? Or maybe just atmospheric conditions? 

Thanks! 
D

If you've observed a CA free Venus, then It's definitely not your DC. I used a DC for five years and its a top class apo. Is the extender a 1.6X extender-Q that screws into your visual back, and if so is it new or second hand? The Q should make an already stunning scope even more stunning. You could try viewing at its native F7.4 and see if the CA disappears. If it does there may be a fault with the Q, but I've never heard of such a thing. What diagonal and eyepice were you using?



#9 daquad

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Posted 01 June 2020 - 07:00 PM

I don't own a Tak, but this seems weird to me.  I'll be interested to see how you Tak guys figure this one out.

 

Dom Q.



#10 Jon Isaacs

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Posted 02 June 2020 - 01:36 AM

You could try viewing at its native F7.4 and see if the CA disappears.

 

 

I like this idea. Let the eyepiece provide the magnification.  A decent eyepiece in a truly color free scope like a Newtonians will not show on-axis chromatic aberration...

 

If it's not seen at the same mag with the eyepiece but seen with the Qextender, question answered.

 

My guess is atmospheric dispersion.

 

Jon




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