Veil + Bright Doughnut

[MarkS]

Mac,

I love those diagrams - thanks for taking the time and trouble!

I see one problem - because of refractive index, the raypath through 3mm of glass is equivalent to only 1mm of air so your multiple raypath suggestion is looking less likely as an explanation.

[Mac]

The refractive index of the glass will only alter the angle of internal reflection, the light path will still be ~3mm

Snell's Law.   http://en.wikipedia.org/wiki/Snells_law

The distance is still ~3mm for the air gap and ~3mm for the thickness of glass.

The difference in refractive index for the glass and air, will change the angles of reflections at the boundrys, The ray paths will still be the same. give or take the refractive indexes.

[MarkS]

Sorry, I got my figures the wrong way round - I think 3mm of glass is equivalent to either 4mm or 4.5mm of air. 

The way it works is like this.  If you focus your camera and then put a sheet of plane glass between the lens and the CCD then the rays of light will no longer focus on the CCD - they will focus behind it.  The reason is that glass has extended the effective raypath.  It's because the speed of light in glass is only 2/3 of that in air so the waves "bunch up" closer together in glass. 

I'm not sure of the exact mathematics and the exact ratio though ...

[MarkS]

I worked through the maths whilst cycling to work.

Putting 3mm of glass between a (focused) lens and the CCD (or film) pushes the focal point 1mm behind the CCD i.e. it adds 1mm to the effective raypath.  So 3mm of glass is equivalent to 4mm of air in terms of "effective raypath".

The same principle is true of astronomical filters.  If you focus the telescope onto your camera CCD and then add a filter into the raypath, it will throw the focus out.

[Ian]

which is why type one RGB (or CMY) filter sets should include a plain glass filter for the L exposure.

[JohnP]

I think we need to create a new forum sub-section... something along the lines of :

'Highly Technical Question Area - don't bother looking unless you have at least three hours to try & understand the thread......' 

'fraid you all lost me hours ago........!!!!!

Mac - have you just been swatting up on reflection theory or something for you Astro degree....?????

John..  (confused)   

[Mike]

John I agree !  I can feel my synapses popping.

[MarkS]

John, Mike,

True - the discussion became a bit technical but it does have some important practical consequences:

1) To avoid the need for re-focusing for every change of filter, filter sets should be made of glass of the same thickness and should include a clear filter (of the same thickness) for the luminance channel.

2) If you mod your camera with a filter of a different thickness you also need to re-position the CCD for autofocus to work properly.

O.K. so I lied about (2) - it's not an common or garden practical consequence!

Mark

[JohnP]

1) To avoid the need for re-focusing for every change of filter, filter sets should be made of glass of the same thickness and should include a clear filter (of the same thickness) for the luminance channel.

Mark - Exactly why I bought Astronomik L, RGB type II filters...

[Ian]

Except with Type II filters, the "clear" one is actually an IR block... but, the point remains.

[Rocket Pooch]

Its the secondary.

[mickw]

Mark, just curious.............

Has this problem not been brought up on any of the forums ?

Or did you use a different thickness glass - Could this be rectified by changing the glass ?
Should only take you 2.5 hrs as you have now got the knack 

And stop doing sums on your bike - you'll cause a nasty accident 

[MarkS]

More practical consequences:
(3) Swimming pools always look shallower then they really are
(4) Without this effect, glass lenses would not exist in the first place - light would pass straight through without deviating.

Mick, the reason I have this problem and the reason it's not on the forums (present forum excepted!) is (probably) because I modded my camera using cheap uncoated glass - I gave up waiting for Baader to re-supply the market with a bespoke replacement IR filter.  In fact, we're all still waiting ...

Chris, why do you think it's the secondary?

[Mac]

Mac - have you just been swatting up on reflection theory or something for you Astro degree....?????

John..  (confused)

Part of this years course included, light, reflection and refraction boundrys and ray paths throught lens's as well as spectroscopy.

So i was cheating in a sort of way as this info was fresh in my mind.

If you want more detail, explained with loads of pretty diagrams and maths have alook here.

http://www.belvedereconcertband.co.uk/Macs/Year3EMU.zip

unzip it and have a look at the physics section 5.

should give you loads of info.

Mac.

[JohnP]

Cheers Mac - That explains a lot - I'll take a look at the notes but I'm not promising to understand any of it...

John.

PS - How's that Pier coming along - Have you got it built yet?