ATM Re: Pegasus response to mirror questions and comments

[Bratislav Curcic <epabcc@epa.ericsson.se>]

John,

what a thoroughly written response ! 

I really have nothing to add or comment on further on your
excellent analysis of Foucault testing.

What I will comment on is apparent equality between 'diffraction
limited' and 'relative transverse aberration' (or D-C criteria #1)
that is repeated throughout your dissertation.

As I said, Dick Suiter apparently revised what he thought (and said in
TM#32) recently in this forum. Check his posting in ATM archive
(http://www.system.missouri.edu/ics/staff/andy/ATM/ARCHIVES/FEB97/0824.html)
And for a good reason, too. Ray traced based evaluation (as it is how
we can look upon Danjon-Couder criteria #1, or RTA) doesn't describe
optics too well for a simple reason that light behaves much more like
wave once we get close to the 'diffraction limited' ( i.e. when we make
mirror capable of directing majority of its light into a spot whose
dimensions are comparable with Airy disc).

Let's have this hypothetical mirror example : have two PERFECT
paraboloids that have just slightly different foci and join them at 0.7
radius. In other words, inner 0.7 is perfect parabola of focus F, from
0.7 onwards is another perfect parabola of focus F + delta_F. We can
pick delta_F so that at a minimum transverse aberration point ("best
focus") this blur is still smaller than 2 * rho. So, formally we have
met D-C #1 100%. All 'rays' will be directed within Airy disc. But such
mirror would perform very poorly, as LOT of its light will get
diffracted into rings, once we use it outside in the real world. The 
Strehl ratio of this mirror is appreciably less than required 0.8 !
That is why professionals 'invented' other means of describing optics
(a-ka RMS, Strehl etc.). And even they don't always work too well as
we can again get examples of mirror with Strehl of 0.8 that don't
work too well in the field (ubiquitous example of 0.7 wave TDE 
starting at 0.95). That's why apparently no self respecting
optician will ever let himself dragged into 'wave waving' or 'number
stamping' of optics. "1/4 wave mirror" is similar to going to a grocery
shop and asking for "a kilo of". Kilo of WHAT ? Quarter wave of WHAT ?
Astigmatism ? Coma ? High order spherical ? Some aberrations be quite 
benign at quarter wave. Quarter wave of edge roll taking outer 20% of 
the mirror is a disaster.
Just see what Bob Kestner had to say as introduction to 'Thin mirror
making' series, and why Bob Goff has such problem in getting any kind
of uniform standard at all for amateur optics. And this is repeated
among experienced ATMs as well. Many of them here simply stated that
optics should be categorized into junk, poor, good, very good and 'so
close to perfect that it hurts' (sorry Mel, but I liked that one very
much!).

> Note that Texereau did not say that it's easier to get large mirrors
> diffraction limited than to make them 1/4 wave.  Quite the contrary.  What
> Texereau said, in English translation, was:   "For small mirrors,
> generally speaking, the first condition [RTA less than 1.0] is the more
> difficult to satisfy." (How to Make a Telescope, p. 8,
> Willmann-Bell).   Notice he says nothing here about "big" mirrors. 

OK I admit I have let myself to interpretation of something not explicitly
said. Unfortunately I don't have any relevant experience in making large 
mirrors, so I just assumed that he pointed this out to emphasize that this
condition is EASIER to satisfy for larger mirrors. Obviously I was wrong!
( note again that Texerau doesn't use term 'diffraction limited' but
  "first condition" [of Danjon-Couder] )

> Many planetary observers would disagree that a 1/4 wave pvwf mirror will
> do as well as a 1/10th wave pvwf mirror.

Note that I said "20 inch". There is quite few VERY experienced (should
I say legendary) planetary observers that commented on seeing and
telescope size in 'golden oldie' (Sidgwick). Again, I have to admit 
that I'm yet to see a 20 incher capable of 1/10 wavefront, so I can't
talk first hand; but my limited experience with smaller optics points
to the conclusion that I expressed before. It is very difficult to pick
between 'essentially perfect' mirror and 'good enough' one (0.8 Strehl,
1/14 RMS, 1/4 pvwf etc.) once we pass about 8 to 10". Planets, doubles, 
whatever. (disclaimer : I have never lived in Florida, Pic du Midi or
atop Mauna Kea)

> Incidentally, 1/4 wave of error is not difficult to detect at the
> wavefront for an experienced star-tester, either in terms of reduced
> contrast on planetary objects, or in the presentation of the  defocused
> star image under good seeing.

Again, I meant "in focus" images. It is too easy to detect errors using
defocused star (in fact so easy that in 20+ years I'm yet to see scope 
that will pass this test; Questar & Zeiss included)

In the end I can just express my sorrow that companies like Pegasus
with their honesty and modest approach to self advertizing cannot enjoy
market popularity (and share) as unscrupulous other ones do. Wave
inflation of 70s took ages to contain, and we can see it again coming
in the ads, this time much more elaborate. I guess it would take another 
crusade (like TM did in its heyday), but do we REALLY have to do 
that again ?

I hope that everyone benefited from this discussion. I certainly did.
If I ever look into buying a mirror, I surely know where to ask !

Best regards,
Bratislav