Re: ATM RE Mirror testing

[lindberg <lindberg@olywa.net>]

Krajci Tom Maj Det 5 wrote:

> With the pin stick you see the whole mirror, and adjust your tester to put
> the "crest of the donut" at the pin/marker.  Seeing the whole mirror makes
> it easier (in my opinion) too see where this crest lies in your shadow
> patterns.
> 
> Also, you can easily evaluate overall smoothness of your figure, and the
> presence or absence of turned edge.

Tom: 
        Allow me to add a few comments, especially for those unfamiliar with
the Everest pin-stick test (ATM 2, first chapter). The position of the
pins needs to be known accurately and the pinstick positioned precisely
to the same spot in front of the mirror for each testing session. This
is important because the zone radius marked by each pin, r, is squared
in the equation:

                r(squared)/2R = knife-edge position (slitless tester)

so a small error in pin position is magnified mathematically. This is
true of the windows of Couder masks, too.  Here is how I do it.
        My pinsticks are of balsa approx. 3/8" square. The pins are positioned
with the aid of a 10x magnifier and a machinist's rule that can be
directly read to .01".  I can estimate to half of that, easily. I
measure from pin center to center. The nice thing about balsa is that
the pins can be pushed laterally into exact position (and out again,
too, so guard your pinstick against bumping). Pins for each zone are cut
to different heights for easy identification. The tallest is 3/8" high,
the shortest a little less than 1/8".  The pinheads are removed.
        The stick rests in front of the mirror on coat hanger wire attached to
the testing stand.  The stiff wire can be adjusted up and down for exact
position by bending, and relied upon not to move unless bumped. The
mirror itself sits on separate rests. The stick is restrained laterally
by straight pins projecting out the back of the stick, pinheads
attached. These grip the side of the mirror and slide on with light
friction. If the positioning pins are pushed almost all the way through
the balsa, I have found that the pin's springiness centers up the
pinstick with good repeatability, judging by occasional checks with a
machinist's rule. There are doubtless better ways, but I have found this
method easy and accurate enough. 
        I have found the Everest pinstick method better than Couder style
testing masks for the reasons you mention, Tom, especially on faster
mirrors. I have not tried marking the front of the mirror with a felt
tip pen, but I worry that this method would degrade the repeatability of
the test, although it might be OK for testing during early figuring.
        Best regards, Steve.