Atmospheric Dispersion

[MarkS]

I've just discovered the existence of atmospheric dispersion (so I've started a new thread).

Discussing the Mars webcam image http://forum.orpington-astronomy.org.uk/index.php?topic=2909.0 I said:
"I also needed to align the RGB in Registax because the raw frames showed obvious colour fringing, but why, I'm not quite sure."

To which Ian replied:
"colour fringing is an atmospheric effect, because of the really high focal length you're using you'll see it much higher up than normal. It's not objectionable in your image, but it is still a little blue bottom left..."

Now, the amount of dispersion I had to correct for between red and blue was approximately 8 pixels which equates to around 0.8 arcsec.  Mars was quite high in the sky at the time so I was really surprised at this.

However, this site http://www.licha.de/astro_article_atmosphere.php gives some estimates for dispersion as a function of the angle away from zenith:

Angle(deg)  Dispersion(arcsec)
0               0.0
10              0.3
20              0.6
30              1.0
40              1.5
50              2.1
60              3.1
70              4.9
80              9.9

I've independently verified these figures using schoolboy optics and the refractive indices for air at various wavelengths found elsewhere on the internet.

So 0.8 arsec is pretty much what would be expected for Mars at that altitude.
 
And this finally explains why I've noticed colour fringing on all my high magnification moon and planet shots - even when the object is high in the sky.

So registration of RGB is an important step in any planetary work.

Thanks Ian!

[Ian]

Mark, you are quite scarily thorough...

[Mike]

Far too technical for me. I just like to take pretty pictures. 

[Rocket Pooch]

See see it's not just me who is sad, actually just to add to that the lower an object is you also have to adjust your RGB balance more because of the different amount of atmospheric dispersion on the different colour channels!!!  Blue is affected more that green and red, which also happens to be why the sky is blue on a clear day, the blue is dispersed more that the green and red, and when the sun goes down the red is caused because its the best wavelength at getting through the atmosphere.

Chris

[Tony G]

So where does the BLOODY GREY SKY come from then.

Tony G

[Rocket Pooch]

Ah yes well, that however is a little more difficult to explain, actually this must have been the wettest January on record.

[Mac]

i'm sorry, very very sorry.

Ever since i got my filter its been nothing but cloud and rain.

I'll post it back to america in the morning.

See if that makes a difference.

[MarkS]

Hey Chris, you're right - frequency dependent atmospheric absorption.  I hadn't though of that before. 

Oh no, I can see the need to spend yet another (cloudy) evening doing bizarre mathematics!

Mark

[Mac]

If anyone is interestd in the maths, i'm covering this in my degree

I've made this module available onlie

the info is from page 7 onwards.

have fun.

http://www.belvedereconcertband.co.uk/Macs/section5.pdf

ps. If anyone is interested the the previous two modules, Introductions to astronomy, and Cosmology.

I can let you have the modules.

Something to read whilst its raining.

[MarkS]

Thanks Mac, that's an interesting read.  Looks like you can join the sad club as well 

[Carole]

Mac,

I wouldn't mind the Introduction to Astronomy if it is a step further than what we did in the GCSE course (which I would expect it to be if it's a degree course).

I have copied the spectroscopy folder (some of which I have glanced through) to my PC for reading at a later date.

Thanks
Carole

[Mac]

If anyone does want the last three modules,
Drop me a PM and i'll send you a link.
Each one is about 10meg so you can download just the ones you want.