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Noob question on chromatic aberration

refractor binoculars
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#26 Jon Isaacs

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Posted 18 April 2019 - 06:59 PM

Good to hear from a fellow observer who thinks his sensitivity to color is weak. Thanks. This was the gist of my question: Is an APO worth it for a person who looks up and seldom sees much color in the heavens?

 

My eyes are less sensitive to chromatic aberration than a younger observer. 

 

But I still see plenty of chromatic aberration at higher magnifications in scopes that are not free of chromatic aberration.  

 

Jon


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#27 gnowellsct

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Posted 18 April 2019 - 08:38 PM

Joe:

 

As I have said, I currently own a 120 mm F/8.3 achromat with the same optics as yours. I have owned two others. The views have been essentially identical.

 

I posted a bunch of analysis, I can post some more but the point is to show that the chromatic aberration does exist, it affects the view of the planets. 

 

This is not to say that a 120 mm F/8.3 achromat does not provide enjoyable views of the planets, it can for sure. But I think that if you and I were out under the night sky with  a 120 mm F/8.3 achromat and a 120 mm F/7.5 ED APO, you would see a definite difference and you would come to see the purple fringing.

 

This is not to say the 120 mm F/8.3 is a bad scope, it's a good scope and a good balance between field of view, aperture, manageability, color correction etc, unlike the 120 mm F/5, this one does a reasonable job on the planets and on deep sky.. When discussing a scope, I want people to have reasonable expectations. If they see some purple haze around Jupiter, at 200x, they should not be surprised, it should be expected.

 

Jon

Actually red-green color blindness--to various degrees, seldom in absolute form--is very common in the male population.  I suspect this variation among color sensitivity, which extends into the population that is considered average or "normal," is one of the reasons why there is such variation in people's reports about satisfaction with their instruments.

 

I have a good friend who is able to pick out red carbon stars but h-alpha reds, the fine feather reds of prominences, are extremely difficult for him.  To the extent that it's not fair to ask him out for solar viewing unless there's a white light filter handy.

 

So some achromat lovers may not, in fact, see the purple, or not see it as a very big deal.  In fact, variations in color sensitivity may be one of the factors, along with cost per square inch of aperture, that makes achros as popular as they are.

 

Greg N


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#28 Sky Muse

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Posted 18 April 2019 - 09:14 PM

A lot of good knowledge and great suggestions here. Thanks, all! I consider myself a little more educated.

Whenever you see a "Dob" or "Dobsonian" mentioned or advertised, it refers to a Newtonian mounted upon a Dobson-type alt-azimuth mount.  Newtonians are mounted upon traditional tripods, too.  Newtonians are 100% false-colour free, but they also require regular maintenance in the form of collimation, which is the alignment of the two mirrors within the tube in tandem with the focusser.  To give you an idea of what's involved...

 

http://www.schlatter...y/collimate.htm

 

Refractors require the least amount of collimation, if at all.  Many refractors cannot be collimated, as the design practically eliminates the need.  After Newtonians, Maksutov-Cassegrains are the next down in difficulty, but they rarely require it.  Schmidt-Cassegrains require collimation more frequently than a Maksutov, but are the easiest of those two catadioptrics to collimate, when needed.  Maksutovs however exhibit greater contrast and sharpness, and over a Schmidt.  

 

Those four designs of telescopes are what you will commonly see online, and from which to choose in the end.


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#29 Sky Muse

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Posted 18 April 2019 - 09:35 PM

Note to self: Stay away from short achromats. Thanks for the schooling.

Well, that is not exactly what I suggested.  I wouldn't get one as my only telescope, but as a low-power companion to an other more versatile telescope.  For example, you made mention of your binoculars.  With binoculars, the magnification is fixed, and at a low power.  Short achromats excel at and are primarily configured and intended for low-power wide-field views, like a pair of binoculars.  But in the case of a short achromat, and practically a low-power monocular, larger apertures are available relatively inexpensively, yet most important of all, it's capable of several differing magnifications, as it can accept separate eyepieces of varying focal-lengths, again, from 4mm to 40mm, in general. 

 

I have a few pairs of binoculars, but I rarely if ever use them for astronomical observations.  Not too terribly long ago, I brought a pair out one night and saw a flock of geese or other in formation, and silhouetted against a background of stars; a beautiful sight, make no mistake, but then I took the binoculars back inside and brought out one of my telescopes.


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#30 Jon Isaacs

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Posted 18 April 2019 - 09:59 PM

Actually red-green color blindness--to various degrees, seldom in absolute form--is very common in the male population.  I suspect this variation among color sensitivity, which extends into the population that is considered average or "normal," is one of the reasons why there is such variation in people's reports about satisfaction with their instruments.

 

I have a good friend who is able to pick out red carbon stars but h-alpha reds, the fine feather reds of prominences, are extremely difficult for him.  To the extent that it's not fair to ask him out for solar viewing unless there's a white light filter handy.

 

So some achromat lovers may not, in fact, see the purple, or not see it as a very big deal.  In fact, variations in color sensitivity may be one of the factors, along with cost per square inch of aperture, that makes achros as popular as they are.

 

Greg N

 

Greg:

 

The fact that someone is color blind means they cannot distinguish the colors, it does not necessarily mean they cannot see the light of that color, they just don't know what color it is.

 

  Out of focus light has the same basic effect whether it's red or gray scale.

 

Jon


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#31 gnowellsct

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Posted 18 April 2019 - 10:33 PM

Greg:

 

The fact that someone is color blind means they cannot distinguish the colors, it does not necessarily mean they cannot see the light of that color, they just don't know what color it is.

 

  Out of focus light has the same basic effect whether it's red or gray scale.

 

Jon

That's true, but some situations in astro observing are particularly difficult.  Red prominences are one (h-alpha).  Not just my friend I've had it come up with outreach to adults.   My 18 year old has RG color blindness and does better with h-alpha tha my friend and some other people I've met. 

 

But I have no idea about purple haze and how that would work.   I just think about 10 to 15% of the male population isn't going to respond the same way as the others.  Let's say you're right.  Jupiter throws off a lot of light.  So they "see" the purple as roughly the same as the other light.  Therefore, they're not bothered by color.   But again, I don't know how often blue/purple is affected.    GN


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#32 aeajr

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Posted 18 April 2019 - 10:38 PM

Note to self: Stay away from short achromats. Thanks for the schooling.

Well, if you end up with one you can send it to me.  I like short achromats.  I have two. 


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#33 Jon Isaacs

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Posted 18 April 2019 - 11:32 PM

But I have no idea about purple haze and how that would work.   I just think about 10 to 15% of the male population isn't going to respond the same way as the others.  Let's say you're right.  Jupiter throws off a lot of light.  So they "see" the purple as roughly the same as the other light.  Therefore, they're not bothered by color.   But again, I don't know how often blue/purple is affected.    GN

 

 

The thing about chromatic aberration is not the color, it is the fact that it represents defocused light which blurs the image.  Whether one sees the light as purple or white, it affects the contrast. In terms of how chromatic aberration affects the contrast of planet, the haze surrounding the planet is not the issue, it is a symptom of the issue.  It is the haze that is part of the actual image that affects the contrast. Purple is the combination of the out of focus light at the ends of the visual spectrum, red and blue/violet.   

 

What a color blind person actually sees is another part of the equation.  Apparently red-green color blindness generally means they are lacking sensitivity in green. 

 

https://www.color-bl...olor-blindness/

 

If someone sees the red and the blue but was missing part of the green, It could be that they were more sensitive to chromatic aberration since green is one color an achromat is corrected for.  And then the red-green is function of the cones and not the more sensitive rods so how it all adds up is not clear.  

 

I think this is probably beyond the scope of this thread.. 

 

Jon


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#34 Jon Isaacs

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Posted 18 April 2019 - 11:36 PM

Well, if you end up with one you can send it to me.  I like short achromats.  I have two. 

 

The thing about achromats is they are more specialized than ED/apos.   A 102mm F/5 or F/6 can be very good for low power, wide field views but will suffer at high magnifications. 

 

There are 102mm F/5.4 ED/apos that are essentially perfect at the lowest powers and the highest powers but they are very costly.

 

Jon


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#35 Frisky

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Posted 19 April 2019 - 12:14 AM

I went out and observed Castor tonight. I split it and looked closely and could see a faint purple haze around it. Very faint. I might not have noticed it if I hadn't been looking for it. I put the built-in, 2" aperture mask on the scope and nearly all the purple was gone. The faint haze was totally absent. That third star in the triple was a bit dimmer with the mask applied. Overall, I observed the Orion Nebula, Sigma Orionis, Struve 761, Struve 747 and 745, Caster/Pollux, Winter Albireo, Algieba, Mizar/Alcor, the Beehive and the moon. Never even noticed color except that faint haze around Caster that I was looking for and the yellow sliver on the lunar limb. I'm gonna have to call the 120mm achromat the best scope, overall. Better than the big and heavy triplets and the big and heavy 16" reflectors and pretty much all other scopes!

 

Joe


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#36 midwestastronomer

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Posted 19 April 2019 - 03:33 PM

The combination of a 5" f/8 or f/9 and a fringe killer should do well for the most part

 

I don't know who makes the best fringe killers, but at low focal ratios even fringe killers don't do the job. 

 

If you have a rich field achromat, it isn't designed for planets and bright double stars. Don't use it for that. It IS great for super wide field views of open star clusters and areas rich in bright deep sky objects. My 120 f/5 and my 2" 24mm ES 82 eyepiece will give me 3.28 degrees true field of view at 25x. That is what they're meant for. 

 

Explore Scientific offers a 5" f/9.4 refractor, that I'd love to get my hands on with a fringe killer. I can't afford an apo at this time, but Explore Scientific is making them more and more affordable. You can get a 5" Apo for just under $2000, and that is a far cry from what triplet Apos cost 10 years ago. Hell, you can get a used ES 5" apo for $1100-1200 on the classified section. 

 

I wouldn't invest that kind of cash into one until you're sure that astronomy is a lifetime hobby. 


Edited by midwestastronomer, 19 April 2019 - 03:38 PM.

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