RE: [APML] need suggestions on how to use a reducer with a Taurus TIII

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

Hi Wei-Hao

While reducers are designed to be placed at a fixed distance from the film, this doesn't mean it must be designed to go next to the camera.  In the particular case of Alan's Taurus Tracker (I own one also), the distance between camera and reducer is fixed since the reducer is placed in a lens cell within the guider and the camera is attached to the guider with no ability to adjust the distance between the two components.

The reason is that on a basic level, all the reducer/flattener needs to do is flatten the field and correct for the color that the reducer itself introduces.  The first job is performed by a specific variation of glass thickness from center to edge of the lens to cancel the effect of the curved field...a.k.a. a lens with some power on it (the side effect is a change in magnification..or effective system f/# if you will).  Glass type (ie the refractive index) also comes into play here.  The second job is performed by making the lens an achromat.  Distance to image plane doesn't really come into play, except it (slightly) changes the radius of curvatures on the reducer.  Because of this, placing the reducer at a different distance than it was designed for gives an increasing (but not a large) under- or over-correction of the curved field (note color is still ok).  Depending on the distance and the amount of field curvature in the original system, this may or may not show up in the final image.  I think the change in f/# is more noticeable.  So while as you say there is an ideal distance for best correction, there's quite a bit of latitude on where the lens can be placed before the effects show up in practice.  

Hope this helps,
Jason



-----Original Message-----
From: astro-photo-bounces@seds.org
[mailto:astro-photo-bounces@seds.org]On Behalf Of Wei-Hao Wang
Sent: Tuesday, February 15, 2005 4:35 PM
To: Discussion of Film Astrophotography
Subject: Re: [APML] need suggestions on how to use a reducer with a
Taurus TIII


Hi,

I don't know much about this guider stuff, but I wonder shouldn't the distance
between the camera and the reducer be fixed?  If you add something extra
between the reducer and the camera, can you still have the best aberration
correction and field flattening (even if you can focus)?
Just a question of mine.

Cheers,

Wei-Hao

-- 
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Wei-Hao Wang  :)

Institute for Astronomy at University of Hawaii

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