From: "Wm. D. Hanagan, Jr., Ph.D." 
To: J Killea 
CC: atm{at}shore.net
Reply-To: "Wm. D. Hanagan, Jr., Ph.D." 


J Killea wrote:
>
> Rank beginner:
>
> 1.  I overdid it and put a 1/4" bevel on my 8" blank (I misread some
instructions).  Will this
> have a material impact on my resolution and contrast?  Simple math tells me I
could achieve 98% of
> diffraction-limited resolution of a similar mirror having a 1/8"
bevel.  But
what is the effect on
> contrast, and is any of this really going to be noticeable?
>

You didn't overdo it.  A 1/4" bevel on the mirror side is not a
mistake, particularly since you are nowhere near finished hogging yet.  As
you deepen your sagitta, the bevel will shrink.  I wouldn't let it get to
less than 1/8". Search the archives for the term "chip" if
you want any more reassurance that your bevel is OK.  Legions of mirror
makers before you have put chips on their mirrors and wished they had
started with as wide a bevel as you have.

> 2.  I begun grinding a sagitta into this side of the blank and it stands at
about .034" max depth
> (my goal is an f/6 mirror).  Is this a shallow enough curve that I can stop
grinding that side,
> use it as the mirror's back side, and instead grind the other side on which I
only put a 1/8"
> bevel?
>

I personally don't like to leave coarse ground glass anywhere on the blank,
because grit retention and subsequent release of both grit and glass
particles after you move to finer grits can cause scratches.  Also, it's
generally better if the back of your mirror is flat so that the glass is
uniformly supported while grinding and polishing.  For both of these
reasons and because you don't need to do it, I would NOT switch to the
other side of your blank to make your mirror.


> 3.  TDE is everyone's nightmare apparently, but isn't a bevel an extreme
example of TDE - so is
> it necessary to grind the curve out to the extreme edge?  If not, how much of
the bevel can I
> reasonably expect to use up by the time I'm done (8" Pyrex blank,  f/6)?
>


A bevel and a turned down edge (TDE) are totally different!  Unfortunately, it's
all too easy to see how a beginner could confuse the two.  The problem is
that the mirror defect that we refer to as TDE was misnamed.  With a
Newtonian primary, the glass itself does not in fact turn down in the
region of a TDE. The glass in the region of the TDE is still decidedly
concave and it still brings light to a focus--albeit a longer focus than
desired.  The defect would have been better named "Long Focus
Edge" (LFE).  A high percentage of the light reflecting from a long
focus edge, whatever name we give it, still finds its way
into the image and ends up in the wrong place, severely damaging the image.

By comparison, nearly all of the light striking a bevel is reflected far
away from the mirror's axis and never even gets close to the image and
hence does little harm.  Some minute fraction of the light striking the
bevel may be scattered into the image, but the impact of that is minimal
compared to the misplaced light reflected from a long focus edge.
Unfortunately, we'll probably never be rid of the term "turned down
edge", regardless of how poor a name it is.  The use of this term is
too pervasive in ATM history, folklore, and writings for it to be gotten
rid of now.  Just remember, the point at which the glass actually does turn
down is no longer part
of your mirror surface, it's part of your bevel.

Clear Skies,

Bill Hanagan

> TIA!!
>
> __________________________________________________
> Do you Yahoo!?
> Yahoo! Shopping - Send Flowers for Valentine's Day
> http://shopping.yahoo.com

--- BBBS/NT v4.00 MP