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Equipment Discussions >> Refractors

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FaronD
sage


Reged: 03/25/10

Loc: Richmond Hill, Ontario
What color is your refractor's lateral color?
      #5632300 - 01/19/13 10:04 PM

This question may sound like it belongs in the eyepiece forum but it doesn't. My eyepieces are Tele vue Naglers and Ethos...or is it Ethoi?? My first apo refractor was a Skywatcher 80ED. This was an early pre-order model and it was color free to my eyes back then. I did notice that there was a green ring on the rim of a FULL moon when using Naglers. I know this is lateral color and is a result of the wide field eyepiece design.

My second apo was a WO Flt-132. This was absolutely color free both in and out of focus. Again, when using wide field eyepieces, there was a color fringe on a full moon, this time it was a deep blue.

My current scope is a Tak TOA130. This scope is unequivocally an apo. It doesn't matter what the subject is or at what power, there is no color. Once again, there is a color ring around a full moon, this time it is red.

To reiterate, the Skywatcher 80ED had green lateral color, William Optics FLT132 blue, Tak TOA130 red.

I don't have near the optical knowledge as many of you do, so this is pure conjecture on my part. Seems to me if one were to look at a spot diagram for each scope, the color fringe exhibited by each scope and wide field eyepiece combination, will be larger on the spot diagram when compared to the other colors? I guess the real question should be why are the color fringes different.
Am I on the right track here or way off. I'm searching for the spot diagrams now to confirm this.

Faron


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: What color is your refractor's lateral color? new [Re: FaronD]
      #5632581 - 01/20/13 02:33 AM

I look for lateral color by looking at bright stars near the edge of the field of view. When I see it, it is always bluish.

If one wants to see the inherent lateral color in an eyepiece, a Newtonian is good tool...

Jon


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FaronD
sage


Reged: 03/25/10

Loc: Richmond Hill, Ontario
Re: What color is your refractor's lateral color? new [Re: Jon Isaacs]
      #5633727 - 01/20/13 06:23 PM

Hi Jon, thanks for taking the time to reply. From what you are saying, the different colors are in the eyepiece. From my experiences the actual color has more to do with the design of the refractor optics. Of course it is a result of using wide field eyepieces. I'm still trying to get my head around this. Could it be that the scopes aren't really designed to be used with such wide eyepieces?

I don't generally look at the moon in wide field eyepieces, they were purchased for deep sky. However, on occasion I'll throw one in for friends and family. Perhaps it's time for a plossil or two.

All of this isn't really an issue for me just an observation and a little food for thought. See what happens when there's three months of cloud

Faron


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: What color is your refractor's lateral color? new [Re: FaronD]
      #5634619 - 01/21/13 10:03 AM

Quote:

Hi Jon, thanks for taking the time to reply. From what you are saying, the different colors are in the eyepiece. From my experiences the actual color has more to do with the design of the refractor optics. Of course it is a result of using wide field eyepieces. I'm still trying to get my head around this. Could it be that the scopes aren't really designed to be used with such wide eyepieces?

I don't generally look at the moon in wide field eyepieces, they were purchased for deep sky. However, on occasion I'll throw one in for friends and family. Perhaps it's time for a plossil or two.

All of this isn't really an issue for me just an observation and a little food for thought. See what happens when there's three months of cloud

Faron




Faron:

I enjoy looking at the moon at low magnifications in a larger Newtonian, the 31mm Nagler in a faster Newtonian, the moon is hangs there looking like a large rock suspended in space. I don't notice any issues with lateral color, I will check it out next time I get a chance.

Sorry about the clouds.. We have had a streak of clear nights, not so unusual for San Diego. The flu has got me so I am only sneaking out for short looks.

Jon


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BillP
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Reged: 11/26/06

Loc: Vienna, VA
Re: What color is your refractor's lateral color? new [Re: FaronD]
      #5635296 - 01/21/13 04:22 PM

IME, most all wide field eyepieces (including Naglers) show some degree of lateral color...especially with the Moon since it is so very bright. Why would it be a different color with different scopes? Perhaps the eyepiece's performance alters to some degree with the focal ratio of the light cone -- this would be my guess. Doubt very strongly it is the scopes.

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FaronD
sage


Reged: 03/25/10

Loc: Richmond Hill, Ontario
Re: What color is your refractor's lateral color? new [Re: BillP]
      #5635881 - 01/21/13 10:47 PM Attachment (26 downloads)

Jon, I hope you feel better soon.

Bill, thanks for chiming in. I agree that some lateral color will be seen when using wide field eyepieces. However, the actual color of the fringe must be inherent in optical design (execution) of the refractor, since I've observed different colors in different scopes.

Attached is a spot diagram of the Tak TOA130 with the TOA130RD reducer/flattener. Notice the red color towards the edge of the lens. If I'm interpreting this correctly, this corresponds to the red color fringe seen on a full moon when using wide field eyepieces. Obviously I wouldn't be using the reducer during visual use but I couldn't find a spot diagram without it.

Faron

Edited by FaronD (01/21/13 10:48 PM)


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FaronD
sage


Reged: 03/25/10

Loc: Richmond Hill, Ontario
Re: What color is your refractor's lateral color? new [Re: FaronD]
      #5635907 - 01/21/13 11:05 PM Attachment (24 downloads)

Here's the WO FLT132 diagram it shows the violet not converging at the same point as the other colors. This seems to support the deep blue I observed with the FLT132 and wide eyepieces on a full moon. Then again I could be wrong.

Faron


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dan_h
Carpal Tunnel


Reged: 12/10/07

Re: What color is your refractor's lateral color? new [Re: FaronD]
      #5636003 - 01/21/13 11:53 PM

I have an ED 80mm and from my experience, the ring of unfocued light around the moon is very dependant on focus position. When you consider that there is very little difference in focus from red to green to blue, it is possible to see any color you choose at the edge of a bright image. At least that has been my experience with the one scope.

dan


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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: What color is your refractor's lateral color? new [Re: FaronD]
      #5636096 - 01/22/13 01:21 AM

You are misinterpreting the FLT graph. That one shows on-axis color correction, not lateral chromatic error. The variation in color correction is as you move from the center of the lens itself to the edge of the lens, not as you go from the center of the image field to the edge. Since there is more light coming from the outer edges of the scope than from the center of the lens, most designers shoot for their best correction between 70% and about 85% of the way to the edge.

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Mark9473
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Reged: 07/21/05

Loc: 51N 4E
Re: What color is your refractor's lateral color? new [Re: Jared]
      #5636143 - 01/22/13 02:21 AM

You know, Jared, that's the first time in all these years of seeing such graphs that somebody offers such a great simple explanation.

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BillP
Postmaster
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Reged: 11/26/06

Loc: Vienna, VA
Re: What color is your refractor's lateral color? new [Re: Jon Isaacs]
      #5637774 - 01/22/13 09:07 PM

Quote:

The flu has got me so I am only sneaking out for short looks.

Jon




Jon...you've got the flu! Stop going outside and stay put and get better!!! The stars can wait!!!!!


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FaronD
sage


Reged: 03/25/10

Loc: Richmond Hill, Ontario
Re: What color is your refractor's lateral color? new [Re: Jared]
      #5637927 - 01/22/13 10:44 PM

Jared, thanks for the explanation. Would it be safe to say that a plossl will only utilize 75-85% of the actual refractor lens and stay within that well corrected area? Whereas a Nagler will encompass the outer less corrected edge?

Jared you are an imager correct? If I may, I'd like to use an analogy. Let's compare a Canon crop DSLR to a full frame camera. It has been established that lenses with less than stellar edge correction can be used with a crop camera(plossl), the chip doesn't utilize that outer area. Use that same lens on a full frame cam(Nagler). The imperfections at the edge of the glass can be seen in the photo.

Is this why we see different color fringing in different refractors. The eyepiece designers may have pushed the envelope to the point where future lens designers will consider better correction in outer 15% of glass? I like using wide field design eyepieces. Given the generous lens size, it's hard to go back.

Faron


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Jeff B
Carpal Tunnel
*****

Reged: 12/30/06

Re: What color is your refractor's lateral color? new [Re: FaronD]
      #5640833 - 01/24/13 02:28 PM

Don't forget, your eye also has TONS (that's right, thousands of pounds) of lateral color. Simply move your eye around a bit and you will see radical changes in off-axis lateral color.

Jeff


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