Re: [APML] Nikon DW-4 help

[Stephen Pitt <lthuedk@pe.net>]

Hi- The DW4's helical focuser allows me to get very sharp focus, not by
achieving smallest star artifacts, but by using the telescope's focuser to
focus on the defocused image made by the DW4.  It is a comparitively quick
method of high certainty, since the eye/brain compensatory mechanism is
bypassed.  It relies on achieving four simultaneous features indicative of
sharpest focus of a bright, defocused star: (1) minimum diameter, (2) sharp
diffraction rings, (3) central asterism with (4) radial rays.  Since the
defocused star is large, these elements of focus are quite apparent and their
nature is not ammendable by compensation.  The use of a precision focuser is
especially advantageous.

Stephen

Emmanuele Sordini wrote:

> Dear all,
> just back from vacation, I've got a question to pose to the whole list
> about correct focusing with the Nikon DW-4 magnifier.
> Some weeks ago I tried to use it for prime focus photography with a Nikon
> F3 (with red-dot, D-type focusing screen) and a 8" SCT: when framing, the
> field seemed correctly in-focus, but when I got my slides developed stars
> did have very cool donut-like shapes. So, here is how I proceeded.
>
> Since one can focus both by turning the DW-4 pipe and through the scope's
> focus knob, I thought the most correct way was to block the DW-4 and use
> only the scope's knob. The DW-4 body has a vertical white line on its back,
> and its turning part has a small white dot which, I suppose, should be
> brought to perfect match in order for the focus on the film plane to be OK.
> Here's what I noticed:
>
> 1) When the dot and the line were aligned by turning the magnifier's pipe
> clockwise, then by no means could I get the stars in the magnifier to look
> properly focused by turning the scope's focus knob.
> 2) When I turned the DW-4's pipe clockwise _all the way_ down, the star
> field looked as crisp as I could ever want, but the actual shots did not.
>
> What am I missing here? Could it be my fault (most likely) or perhaps a
> flaw with the DW-4? Any suggestions/links will be highly valued.
> Please note that the DW-4 (used) was bought some months ago in San
> Francisco by a friend of mine, so I guess I can't do much about it if it
> happens to be the DW-4. It is in overall good condition, except for
> slightly twisted hooks, which make attaching it to the camera not so smooth
> an operation: however, once mounted, it stays firmly in its place.
>
> Thanks
> Emmanuele Sordini
>
> --  APML Archives at <http://astro.umsystem.edu/apml/>  ---
>              Unsubscribe at <majordomo@seds.org>


--  APML Archives at <http://astro.umsystem.edu/apml/>  ---
             Unsubscribe at <majordomo@seds.org>