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Purple glow around Venus

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#1 Diomedes

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Posted 21 February 2020 - 04:27 PM

Hey all, I was observing Venus through a 70mm Meade refractor and I noticed bright purple coloring around the disk. Is this due to poor focusing or atmospheric turbulence ?


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#2 jeremiah2229

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Posted 21 February 2020 - 04:32 PM

Due to the optics in the scope. It is expected. Just keep looking up and enjoy, nothing to fret about.  waytogo.gif

 

 

Peace...


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#3 cst4

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Posted 21 February 2020 - 04:32 PM

Chromatic aberration.  If you recall Isaac Newton's old experiment of shining white light through a bent prism, the colors come out separated as a rainbow on the other end due to their varying frequencies.  This happens in refractor glass too due to the lens curvature.  The effect is that not all of the colors come to focus at the exact same spot and you can see some of the colors like purple green and yellow.  If you are slightly one side of focus it's usually purple and if you look slightly on the other side of focus you might see yellow or green.  The expensive apochromatic or "Apo" refractors have much more expensive glass that does not do this near as much or practically at all.


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#4 vtornado

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Posted 21 February 2020 - 04:36 PM

The most likely explanation is you have an achromatic refractor.

 

An achromat cannot focus all three primary colors to the same plane.

The purple you are seeing is a blend of  blue and red light that is not in focus.

 

What is the f ratio of your telescope?  The higher the f ratio for a given aperture, the less

purple you will see.

 

Here are some things you do to help alleviate it.

Make an aperture mask.  You are making a donut out of a piece of card board and placing it in your dew shield.

Try with the center hole 50mm.

 

You can also use a colored planetary filter.  A light green or light yellow filter will probably work the best.

 

There are more expensive filters call fringe killers/apo filters that will remove more of the purple, and render a more

color balanced image, but they are quite expensive compared to the cost of your telescope.


Edited by vtornado, 21 February 2020 - 04:52 PM.

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#5 TOMDEY

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Posted 21 February 2020 - 05:33 PM

Yeah, plain vanilla classic residual chromatism... fine to notice it ... and then just get used to ignoring it. I'm betting that you're not geriatric yet, unlike most of us CN guys. That out of focus coloration will diminish, over the decades, as your cataracts filter out more and more of the shorter wavelengths.   Tom


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#6 coopman

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Posted 21 February 2020 - 05:41 PM

Venus is the most brutal thing that you can point your scope at.  It takes no prisoners.


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#7 Dave Mitsky

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Posted 21 February 2020 - 06:56 PM

http://www.pas.roche...refracting.html

https://www.skyatnig...al-aberrations/


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#8 Diomedes

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Posted 22 February 2020 - 01:03 PM

thank you all, I had read about aberrations but I was not sure. This is article is really good  https://www.skyatnig...al-aberrations/ !



#9 ww321q

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Posted 22 February 2020 - 02:20 PM

Hey all, I was observing Venus through a 70mm Meade refractor and I noticed bright purple coloring around the disk. Is this due to poor focusing or atmospheric turbulence ?

Thats exactly what I saw a few nights ago. More to do with the telescope design than anything else I think, like vtornado said . On my Celestron 120mm f8 refractor I got the big purple haze but the sky was good enough to get a good focus and see a little white/gray detail on the planet. I was pretty happy about that. I was looking at 70 to 90mm refractors before I got my 120mm. Whats the focal length of your Meade 70mm ? 



#10 Diomedes

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Posted 22 February 2020 - 04:07 PM

I have been straining for a couple of weeks trying to see any details at all.  The focal length on the Meade is 700mm, I observe with a 8mm eye piece so that’s about 87.5x magnification. I tried using the Barlow that came with the telescope but I noticed the image got worst.
 

 

 

Thats exactly what I saw a few nights ago. More to do with the telescope design than anything else I think, like vtornado said . On my Celestron 120mm f8 refractor I got the big purple haze but the sky was good enough to get a good focus and see a little white/gray detail on the planet. I was pretty happy about that. I was looking at 70 to 90mm refractors before I got my 120mm. Whats the focal length of your Meade 70mm ? 


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#11 ww321q

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Posted 22 February 2020 - 04:40 PM

I have been straining for a couple of weeks trying to see any details at all.  The focal length on the Meade is 700mm, I observe with a 8mm eye piece so that’s about 87.5x magnification. I tried using the Barlow that came with the telescope but I noticed the image got worst.
 

The details were really faint . I wasn't sure if I was seeing things but when I went inside and searched some online pics I decided I wasn't. I was using a Celestron Omni XLT 120mm f8.33 and a Explore Scientific 6.5mm 52°eyepiece with a Svbony 2X screw on barlow.(that's really like 1.5X) Thats like 250X to 300X.


Edited by ww321q, 22 February 2020 - 04:41 PM.


#12 Diomedes

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Posted 22 February 2020 - 05:00 PM

Good to know, I'll try using the Barlow. I think the max amount of magnification I can do with my telescope is about 140x-150x, I'll go up to that and see what happens.

 

The details were really faint . I wasn't sure if I was seeing things but when I went inside and searched some online pics I decided I wasn't. I was using a Celestron Omni XLT 120mm f8.33 and a Explore Scientific 6.5mm 52°eyepiece with a Svbony 2X screw on barlow.(that's really like 1.5X) Thats like 250X to 300X.



#13 Rutilus

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Posted 23 February 2020 - 04:43 PM

The details were really faint . I wasn't sure if I was seeing things but when I went inside and searched some online pics I decided I wasn't. I was using a Celestron Omni XLT 120mm f8.33 and a Explore Scientific 6.5mm 52°eyepiece with a Svbony 2X screw on barlow.(that's really like 1.5X) Thats like 250X to 300X.

I have the same scope, and over the last 14 years I have had some pretty decent views of Venus with it.

Sure, there is a big purple glow, but faint markings are visible.  I usually do not go above 200x and the only

filter I tend to use is a yellow one.

Here are a few drawings made with the scope, and also a drawing of Jupiter to show how the C.A.appears to my

eyes.

Attached Thumbnails

  • v5-cn.jpg
  • Jupx.JPG

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#14 Jon Isaacs  Happy Birthday!

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Posted 23 February 2020 - 07:18 PM

A 70 mm F10 refractor can have quite decent color correction. The "chromatic ratio" (focal ratio/aperture in inches) is about 3.6, 3.0 is considered acceptable, 5.0 will be nearly color free.

 

You can also further reduce the chromatic aberration with an aperture mask. A heavy piece of paper cut to fit the dew shield with a 50 mm hole cut in the center will dramatically reduce the chromatic aberration. The chromatic ratio is 7 and it will be apo like.

 

Venus is generally relatively close to the horizon and subject to atmospheric dispersion, the atmosphere functions as lens and has its own chromatic aberration. When you see this, one side of Venus will be orange/red, the other side, violet/blue.

 

5001261-Venus Through the Eyepiece.jpg
 
That taken with an apo. The fact that the two sides are different rather than a uniform purple indicates it's the atmosphere and not the lens.
 
Jon

 


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#15 Diomedes

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Posted 24 February 2020 - 04:19 PM

I tried observing again over the last few days. The aberration decreased when I used the aperture mask but all I could see was a very bright disk. I took a picture for reference

 

https://imgur.com/a/fsTKxG7

 

 

Very cool sketches/pictures

I have the same scope, and over the last 14 years I have had some pretty decent views of Venus with it.

Sure, there is a big purple glow, but faint markings are visible.  I usually do not go above 200x and the only

filter I tend to use is a yellow one.

Here are a few drawings made with the scope, and also a drawing of Jupiter to show how the C.A.appears to my

eyes.

 

 

 

A 70 mm F10 refractor can have quite decent color correction. The "chromatic ratio" (focal ratio/aperture in inches) is about 3.6, 3.0 is considered acceptable, 5.0 will be nearly color free.

 

You can also further reduce the chromatic aberration with an aperture mask. A heavy piece of paper cut to fit the dew shield with a 50 mm hole cut in the center will dramatically reduce the chromatic aberration. The chromatic ratio is 7 and it will be apo like.

 

Venus is generally relatively close to the horizon and subject to atmospheric dispersion, the atmosphere functions as lens and has its own chromatic aberration. When you see this, one side of Venus will be orange/red, the other side, violet/blue.

 

 
 
That taken with an apo. The fact that the two sides are different rather than a uniform purple indicates it's the atmosphere and not the lens.
 
Jon

 



#16 Dave Mitsky

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Posted 24 February 2020 - 11:13 PM

I posted an afocal image of Venus that I took on February 17th at https://www.cloudyni...uary/?p=9991057



#17 project nightflight

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Posted 25 February 2020 - 05:47 AM

I tried observing again over the last few days. The aberration decreased when I used the aperture mask but all I could see was a very bright disk. I took a picture for reference

 

https://imgur.com/a/fsTKxG7

 

Diomedes, try experimenting with different exposure settings.

The image you are referring to is heavily overexposed.

 

I compiled a comparison sheet for you that shows the effects of over- and underexposure. The data is from a mirror lens test we did on February 8:

 
Venus Exposure Comparison

(click on image)

 

In this comparison, at 1/60 and 1/125 second the planet is overexposed and shows no phase. These first two shots are ruined. At 1/250 and 1/500 exposure is about right, Venus shows its gibbous phase and the planet's disk is not fully saturated. On the other end, 1/1000 second would have been too short an exposure.


Edited by project nightflight, 25 February 2020 - 06:11 AM.

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#18 Diomedes

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Posted 25 February 2020 - 12:08 PM

Thanks for the example, the moment I saw it, it made perfect sense. I realized the pictures I took with my DSLR were overexposed. I tried using a moon filter to observe the phase visually but it was still bright to the point where I could not recognize the phase it was on. I’m looking forward to trying to identify the phase using my DSLR.


Diomedes, try experimenting with different exposure settings.
The image you are referring to is heavily overexposed.

I compiled a comparison sheet for you that shows the effects of over- and underexposure. The data is from a mirror lens test we did on February 8:


(click on image)

In this comparison, at 1/60 and 1/125 second the planet is overexposed and shows no phase. These first two shots are ruined. At 1/250 and 1/500 exposure is about right, Venus shows its gibbous phase and the planet's disk is not fully saturated. On the other end, 1/1000 second would have been too short an exposure.



#19 project nightflight

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Posted 25 February 2020 - 04:13 PM

Thanks for the example, the moment I saw it, it made perfect sense. I realized the pictures I took with my DSLR were overexposed. I tried using a moon filter to observe the phase visually but it was still bright to the point where I could not recognize the phase it was on. I’m looking forward to trying to identify the phase using my DSLR.
 

When observing visually, an easy way to decrease the blinding brightness of Venus is to observe it as early in the evening as possible. To the unaided eye, Venus is visible shortly after sunset. If you point your scope at it during this time, the contrast between the still bright-blue evening sky and the planet is not so high as it becomes later in the evening. In the eyepiece, the phase of the planet should be obvious. Good luck!


Edited by project nightflight, 25 February 2020 - 04:23 PM.


#20 ww321q

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Posted 27 February 2020 - 12:27 AM

I made a mask with a 2.5" hole for my 6.25" reflector. It dims down the haze along with everything else. 




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