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I thought reflectors weren't supposed to show chromatic aberrations?

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#1 Buqibu

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Posted 25 July 2021 - 12:24 PM

I was at a public viewing site and looked through a reflector. Can't say the exact model, but when I looked at Venus through it there was this crazy rainbow blue and red fringing around it. Thought it looked very odd, even more so than in my achromatic refractor. Any ideas why this could be? It was relatively low near the horizon, could've also been slightly out of focus, but still surprised me

#2 spazmore

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Posted 25 July 2021 - 12:31 PM

Could be:

- Atmospheric dispersion (most likely it was this since it was near the horizon)

- CA in the coma corrector (this was obvious in a GSO coma corrector that I had)

- CA from the eyepiece


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#3 macdonjh

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Posted 25 July 2021 - 12:31 PM

That sounds like atmospheric dispersion to me.  The closer an object gets to the horizon, the more of Earth's atmosphere it's light has to penetrate so you can see it.  The short blue wavelengths get scattered, and the long red wavelengths come through and that might be what caused the red halo you saw.  I think I have that right.

 

You may also have seen the effects of poor seeing: unsteady air.  Since you were looking at Venus when it was near the horizon (looking through lots of air as opposed to viewing when an object is near zenith) and fairly close to sunset (the atmosphere was still heated by the Sun), you had a recipe for turbulent air and distorted images  This winter, try looking at Sirius just before or just after a storm passes through your neighborhood (if you get the chance).  Atrocious seeing gives Sirius the opportunity to put on quite a light show.  I've seen it twinkle red, green and blue at the same time once or twice.


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#4 skybsd

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Posted 25 July 2021 - 12:42 PM

Yes reflectors do show false colour even more so I believe in faster systems that’s why thay are preferred for deep sky observing however you can puchase coma correcters, ...

What does coma have to do with false colour??? 

 

skybsd 


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#5 Buqibu

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Posted 25 July 2021 - 12:42 PM

Thanks for the replies, definetly could've been the atmosphere

#6 Starman1

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Posted 25 July 2021 - 12:46 PM

  • lack of cooling of the mirrors early in the evening
  • atmospheric dispersion when an object is low (one of many reasons I set my scope not to go below 10°)
  • poor seeing due to low altitude
  • possible mediocre collimation
  • large amount of blue light from the object, which is the color most effectively scattered by the atmosphere.
  • larger scopes are more affected by poor seeing than smaller scopes.

That's about it.


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#7 CBM1970

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Posted 25 July 2021 - 12:47 PM

It can be caused by certain eyepieces. I used to see it in my C90 and my C8 when using 2 element Huygenian and Ramsden eyepieces.

It affected Jupiter, Venus, and the rims of lunar craters!

It was pretty bad, but hey, I could go all the way up to 400x with those eyepieces! (My thought process at age 13...)🤣


Edited by CBM1970, 25 July 2021 - 07:49 PM.


#8 Sleep Deprived

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Posted 25 July 2021 - 01:11 PM

CA has to do with a differing index of refraction that varies with wavelength.  It is only caused by refractive elements in the system - correcting plates, eyepieces, coma correctors, or the atmosphere. Don't blame the mirror.  Blame the lens.


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#9 Cotts

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Posted 25 July 2021 - 01:50 PM

How to distinguish between Atmospheric (Refractive) Dispersion (AD) and Chromatic Aberration.

 

If one side of the planet - especially Venus, Jupiter and Saturn - , lunar crater or other object has a RED fringe on only  and the opposite side has a VIOLET fringe then it is AD.  This can occur on objects as high as 45º above the horizon...

 

If the coloured fringe is all around the object equally and changes from red to blue as you change focus then it is CA.

 

Fun story:  My first trip to the Texas Star Party the guy next to me had a new (to him, anyway) AP 175.  Awesome scope.  He was very upset about the "False Colour" he saw on Venus......  SEveral of us gathered around and tried to explain that his scope was fine, it was just AD,  but he remained skeptical and was 'gonna call those people at AP' as soon as he got home...  I often wonder what ever happened.  I also wondered about 'too much scope for too little experience'....

 

Venus is the absolute worst test object for 'false colour' that could possible be chosen.... 

 

Dave


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#10 Sky Muse

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Posted 25 July 2021 - 02:01 PM

A cheap, bad barlow will do that, too, like this one...

 

2x chromatic barlow2.jpg

 

...and when I inserted it into this Newtonian...

 

optical tube assembly2.jpg


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#11 CHASLX200

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Posted 25 July 2021 - 03:05 PM

Many times it is too low so you get a rainbow effect. I like seeing it way high up before dark.



#12 Dobs O Fun

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Posted 25 July 2021 - 03:46 PM

Last night I was star hopping, I'm new at this so at first I was not sure what it was. I had three stars in the field, one blinking red, looked like a stationary aircraft but it never moved. I forgot which one it was, I live in a high pollution area anyways.

At th hat home my scope has been out 45 mins. I thought wow....

#13 Dobs O Fun

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Posted 25 July 2021 - 03:47 PM

Last night I was star hopping, I'm new at this so at first I was not sure what it was. I had three stars in the field, one blinking red, looked like a stationary aircraft but it never moved. I forgot which one it was, I live in a high pollution area anyways.

At th hat home my scope has been out 45 mins. I thought wow....

#14 Starman1

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Posted 25 July 2021 - 04:32 PM

Last night I was star hopping, I'm new at this so at first I was not sure what it was. I had three stars in the field, one blinking red, looked like a stationary aircraft but it never moved. I forgot which one it was, I live in a high pollution area anyways.

At that home my scope has been out 45 mins. I thought wow....

Blinking is due to seeing conditions (atmospheric turbulence).

Some stars are red, but you may also have been looking low through the atmosphere.

The air is 1 atmosphere thick at the zenith, 2 atmospheres thick at 30° off the horizon, and about 10 atmospheres thick on the horizon.

The best views of everything occur when the object is high in the sky.

And, if it's in the south, where it never gets above 30°, then it's best when it crosses the imaginary N-S line called the meridian. 


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#15 sixela

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Posted 25 July 2021 - 05:01 PM

I was at a public viewing site and looked through a reflector. Can't say the exact model, but when I looked at Venus through it there was this crazy rainbow blue and red fringing around it. 

You need a reflector outside of the atmosphere to get rid of that. There's one almost at the end of its research life; perhaps you could buy it on the cheap? A bit difficult to do maintenance on, I've heard, though.


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#16 Echolight

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Posted 25 July 2021 - 05:34 PM

I don’t often look low in the sky, except with binoculars. And when I’m looking around 10 degrees above the horizon with my binoculars, all the stars look like variable stars.



#17 Gregrox

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Posted 25 July 2021 - 05:37 PM

What does coma have to do with false colour??? 

 

skybsd 

nothing, but lenses do, so coma correctors which use lenses can potentially add false color



#18 skybsd

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Posted 26 July 2021 - 01:14 AM

nothing, but lenses do, so coma correctors which use lenses can potentially add false color

Appears to me that you neglected to review the actual post and quote to which I was responding.., 

 

skybsd ., 



#19 Tom Stock

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Posted 26 July 2021 - 07:12 AM

I was at a public viewing site and looked through a reflector. Can't say the exact model, but when I looked at Venus through it there was this crazy rainbow blue and red fringing around it. Thought it looked very odd, even more so than in my achromatic refractor. Any ideas why this could be? It was relatively low near the horizon, could've also been slightly out of focus, but still surprised me

Cheap bird jones style reflector?

 

 


Edited by Tom Stock, 26 July 2021 - 07:12 AM.


#20 Jon Isaacs

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Posted 26 July 2021 - 07:15 AM

I would have to guess it was almost certainly Atmospheric Dispersion.  As Dave said:

 

"How to distinguish between Atmospheric (Refractive) Dispersion (AD) and Chromatic Aberration.

 

If one side of the planet - especially Venus, Jupiter and Saturn - , lunar crater or other object has a RED fringe on only  and the opposite side has a VIOLET fringe then it is AD.  This can occur on objects as high as 45º above the horizon...

 

If the coloured fringe is all around the object equally and changes from red to blue as you change focus then it is CA."

 

Here's some images of AD..

 

5937359-Venus Through the Eyepiece.jpg
 
Venus March 6, 2017.jpg
 
1767401-Venus Crescent July 29 2007 .jpg
 
4065510-Venus March 22, 2009.jpg

 

The top one is a Newtonian, the others, refractors. The last photo was taken when Venus was a narrow crescent, less than 2% illuminated but very large, nearly 1 degree and very bright, magnitude -4.1.  At sunset, Venus was only 8 degrees above the horizon so it was difficult to find and the image was taken though a lot of atmosphere. 
 
I think one sign of good color correction in a refractor is when it shows the AD on Venus rather than the CA.
 
Jon

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#21 SteveG

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Posted 26 July 2021 - 07:04 PM

Another example of AD

 

saturn with AD.jpg


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#22 ram812

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Posted 26 July 2021 - 07:31 PM

I actually like to look at those low hovering bright stars like Sirius... they can be beautiful flickering the color spectrum and a great teaching tool to our young, newly minted astronomers out there when it comes to CA and AD. Great post!

Clearer Skies, Ralph😁
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#23 Bean614

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Posted 26 July 2021 - 07:42 PM

By the way, for what it's worth, the Jetstream has been steering the Western wildfire smoke up over the top of the Country, and then right down onto a good chunk of New England.  The last 2 days/nights there's been a constant brown 'fog' around, causing spectacular sunsets, and some extremely interesting views of Saturn and Jupiter.   Almost unnatural,  actually. 

 It's our 3rd go-round this year with the smoke, and there were several each of the last few years.  Talk about "False Color"!!!


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#24 ShaulaB

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Posted 26 July 2021 - 07:55 PM

If Venus was too low on the horizon, the odd color can be explained away. At public star parties, due to partial cloudiness or lots of Moonlight, telescope operators sometimes pick objects when they are not in the best position. Often, an object is chosen because the public has heard of it. With Venus at a phase, the shapes can be explained.

This happened a few years back. A fellow with a beautiful Starmaster Dob, using an Ethos eyepiece, was dismayed that Venus had purple around it. But Venus was lower than 10 degrees above the horizon.
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#25 MitchAlsup

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Posted 26 July 2021 - 08:30 PM

For reasons that must remain a secret here, we call this atmospheric chromatic distortion the "Pechonis effect".




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