RE: [APML] Sky Pollution Astrophotography - on film

["Lane, Jason R" <jason.lane@navy.mil>]

Hi John,

Just adding to your comment here...If the filters are the same thickness but have a different index of refraction and/or dispersion, then you would need to refocus.  This is true even with a reflective system.  This correction is a separate effect than the difference in focus due to chromatic aberrations.  It is also more applicable to faster lenses.  Filters generally use the same substrate glass (BK7 or related), which is why focus usually stays the same when swapping out filters of same thickness.  However, if you ever come across two filters of the same thickness that change the focus on a reflective system...this is why.  It is the exact same effect from adding a filter as opposed to not using one.  In that case, you are basically replacing a thickness of material (air) with the same thickness of material (glass) with different index and dispersion.  

An APO won't be off if you are focusing to yellow and red wavelenghts....they typically are designed to be parfocal at blue, red, and yellow (I forget the actual numbers that are typically used)...which as a side benefit constrains focus at other wavelengths to be extremely close.   However, the preceding effect still applies.

-Jason


-----Original Message-----
From: John C. Mirtle [mailto:spampit@shaw.ca]
Sent: Monday, December 01, 2003 7:10 PM
To: Discussion of Film Astrophotography
Subject: Re: [APML] Sky Pollution Astrophotography - on film


Bill,
    I'm not an optics whizz, but this is how I understand it...

> John could you please explain this last statement so that I can wrap my
> brain around it ? Why would there be a different focus shift from a
> filter based upon the optics (reflective vs refractive) ? IIRC a Schmidt
> Camera id primarily Reflective :^) and there is a definient shift with
> the filter as opposed to not using one. Lets assume for a moment though
> that we are discussing "normal" f-ratios of f/4 and above :^) I just
> figured that the glass in the filter has a refractive index that will
> produce a focus shift. Period. So if I am wrong I would like that
> cleared up.

You are correct with the Schmidt, it is reflective. Because of this, all
colours of light focus at the same point. If you stick something in the
optical path, you will need to refocus. The point with reflective optics is
that no matter what the colour, as long as the filters are the same
thickness, the amount you need to refocus is the same. With a non-apo
refractor, blue and red focus at different points. The H-alpha is a really,
really deep red - it will focus at a different point than say green
(visible) light. So, if you were to focus with a deep sky and swap in the
H-alpha, it will be out of focus, even thought the filters are the same
thickness. You will need to focus with the filter you plan to shoot with.
With an APO refractor, you *shouldn't* have this problem. As I don't happen
to have an APO refractor lying around, I can't confirm this. With my 180mm
telephoto I know for a fact that yellow and H-alpha focus at different
points. A 2-hole mask focus test will confirm this. Have a look at
http://members.shaw.ca/jmirtle/ha.htm . All my telephotos focus deep red at
a different setting than that of yellow light or just visible light. Brain
wrapped yet?

John Mirtle
Calgary, Ab. Canada

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