frequency dispersion in binocs?

astropat · #1 26-10-2006, 12:55 PM

Hi friends,

Recently I got a 20*80 triplet binocs. When I look at bright stars (and only bright stars) with the binocs, there is a red+blue "corona" around the star. Red on one side and blue on the other. Is this frequency dispersion normal (i.e. binocs are not apochromatic)? Or is the optics of my new triplet binocs misaligned?

Thanks,
Patrick

[1ponders] (Paul) · #2 26-10-2006, 01:02 PM

Sounds like normal chromatic aberration to me. Strange that you would be getting it with triplets. Mind you the brighter the star the harder I imagine it would be to correct.

Are they well reviewed binos?

iceman (Mike) · #3 26-10-2006, 01:03 PM

Hi Patrick.

Does it happen on stars at all altitudes, or only when they're at low altitude?

astropat · #4 31-10-2006, 11:29 AM

May I please know how would the altitude lead to aberration? thanks

janoskiss (Steve H) · #5 31-10-2006, 11:38 AM

The atmosphere introduces visible chromatic aberration at low altitudes because there is a stronger refractive index gradient there and because the refractive index of the atmosphere is wavelength dependent.

iceman (Mike) · #6 31-10-2006, 11:47 AM

Hi Pat

At low altitudes, the atmosphere is thicker. Different wavelengths of light are affected by the atmosphere in different ways.

When you see the stars twinkling, that's due to the light being affected by the atmosphere. The light is dispered by the atmosphere and the channels will come to focus at different points.

Lee · #7 01-11-2006, 06:12 PM

I have a pair of the Andrews 20*80 triplets - they are fairly colourful - at their low focal length I didn't expect colour free for $200.... I wish!

johnno · #8 02-11-2006, 12:35 AM

Hi All,

I think Lee,
Hit the nail right on the head,they have a fairly small focal ratio,I think.

Although,the Triplet configuration,I would have thought, would correct for this.

Maybe,one cancells out the other,
I am far from an expert,
Perhaps John,(Ausastronomer),if he reads this thread will have the answer.

I have the 20x 80 LW's,and extremly little CA is seen,it's there allright,but minimal.

Just the Barrels on these are 280mm,or 11 inches long,so I would imagine the f/r is reasonable.

Regards.
John

janoskiss (Steve H) · #9 02-11-2006, 11:21 AM

What I've been told by the guys at AOE is that the triplet lens does two things: 1) allows the binos to be shorter while still maintaining good colour correction 2) produces a flatter field, compared with the LW.

So you get similar CA but less FC and in a more compact bino.

Lee · #10 02-11-2006, 06:14 PM

Isn't false colour (is that what you mean by FC) just chromatic abberation? I thought they were the same, or false colour is a product of chromatic abberation....
Hey, these cheap binos are great for what they cost!

janoskiss (Steve H) · #11 03-11-2006, 10:49 AM

FC = field curvature. Less FC means flatter field, or better sharpness across a greater portion of the FOV. Sorry all these abbreviations are a bit confusing. (Like CA = chromatic aberration, but also = clear aperture.)

74tuc · #12 03-11-2006, 08:43 PM

Sounds like differential refraction through the atmosphere- Red on top of the disc and blue on the bottom. At low altitudes the atmosphere looks like a wedge of atmonphere and as such will cause dispersion. BTW large telescopes have atmospheric dispersion compensators and for spectroscopy guide on a particular wavelength.

The differential refraction effect looks wonderful in an APO telescope you might see the whole spectrum not just one colour. Its easy to tell the difference between chromatic aberration(CA) and differential refraction. CA has a violet fringe of unfocused red and blue light and the atmospheric dispersion will be vertical (mostly).

An APO 'scope cannot correct for atmospheric dispersion as the light has been dispersed before it reaches the optics but it gives one a better view of this aberration!!!

Jerry.