Re: [APML] Sky Pollution Astrophotography - on film

["John C. Mirtle" <spampit@shaw.ca>]

Jason,
    I didn't want to go into too much detail, but you are right. I was
assuming that the filters had the same index of refraction, perhaps I should
have mentioned that. As for APO's, the term seems to be used in advertising
perhaps more than it should. I have seen images taken with "APO" scopes that
clearly had blue halos and real APO images that don't. I do not own anything
this fancy, so all my refractive optics must be focus tested with a mask.
Especially when using an H-alpha filter.

John Mirtle
Calgary, Ab. Canada

----- Original Message ----- 
From: "Lane, Jason R" <jason.lane@navy.mil>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Tuesday, December 02, 2003 9:12 AM
Subject: RE: [APML] Sky Pollution Astrophotography - on film


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

_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo