diagonals

[MooEy]

found a interesting article by roland christen on the ap-ug.


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From: chris1011@a...
Date: Sun Oct 17, 2004 2:24 am
Subject: Star Diagonal questions


Hi everybody,

This subject has come up time and again, so I feel it is important to take a
stand again. I recently received the following letter from a concerned amateur
about the specifications of our Maxbright. I know exactly how this is being
exploited by our infamous competitors, so here is my answer to him and to the
rest of you who may have been wondering. Also, I want to remind you that this
has come up before and I thought that I had put it to rest by posting the
interferograms in the Files section, so you might want to check them out.


> Regarding your 2" diagonal.
> What is the mirror made of? (pyrex?, quartz?)
> How flat is the mirror? (what fraction of a wavelength)
>
> Thankyou,
>

Dear Sir,

Thank you for the inquiry of our Maxbright diagonals. The diagonal elements
are made and coated in the US by our coating subcontractor using the most
advanced multi-layer all-dielectric design. The elements are oversized and thick

enough so that the end result is a highly accurate reflective surface over the
clear aperture that is actually used with any kind of telescope.

I personally assemble and test every diagonal under controlled conditions to
assure that there is no detectable error of a perfect star image at very high
powers. Measurements that I have made using interferometer techniques show
that over the clear aperture of an F4.8 telescope, the flatness runs typically
1/40 wave, and I will guarantee at least 1/20 wave. This is not a flatness
measure of the entire surface, just that part of the surface that is acually
used
to form an image of any particular star anywhere in the field of view.

I believe that you are asking these questions because of certain ongoing
marketing efforts by competitors that may claim some kind of performance
advantage
by using different wavefront numbers or coatings or substrates. I've been in
this business long enough to know what is important for each optical element
in the optical train, and how these should be designed, specified and
constructed to perform at their peak. I will not sell any accessory that would
diminish
the performance of our basic telescopes.

In the case of a star diagonal, what is most important is that the surface
polish quality be very high (very deep polish with no remnant pits or subsurface

damage to cause scatter), and very high quality low scatter coatings. These
will insure highest image contrast with lowest scattering. Surface wavefront is
also important, but the only effect of that is to introduce astigmatism into
the image. A diagonal mirror cannot cause any other image defect. Astigmatism
is very easy to detect, and I do test each and every mirror at very high power
to insure that the star image seen through each Maxbright will be perfectly
round. There is nothing more that you can expect from a star diagonal.

Arguments have been made that certain low expansion materials are required
for best performance in star diagonals, but this has no basis in fact. For an
optical element close to focus, having a zero expansion material has no effect
whatsoever on final image quality. In fact, if you examine any eyepiece or
Barlow lens, you will find all kinds of different glass materials in them that
have very high thermal expansion co-efficients, with nary an effect on image
quality. If thermal expansion were an issue, then all eyepieces and Barlows
would
need to be made of quartz or Zerodur, but in fact none ever has been. Thermal
expansion also has no effect on cooldown. The two effects are unrelated.

The proprietary material we use is is low expansion, but not zero expansion
type. It is not cheap common glass. It was chosen for its hard surface
qualities and specifically to allow the manufacturer to achieve the excellent
polish
that we require. The material is being used in countless large diameter
professional astronomical mirrors. It will not deform or produce unwanted image
defects under any kind of temperature extremes.

Roland Christen

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~MooEy~

[MooEy]

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> In particular the 12.5" RCOS delivers star images that are "searing"
> in quality...very tiny/tight points of light with not much shimmering
> due to seeing....very similar to the AP scopes...but something the
> albeit smaller SCT never does...which I have to say is very well
> corrected, spherically at least.
>

I doubt that the commercial SCTs leave pits in their surfaces. They are
usually fully polished out. It could be the difference between a smooth pitch
polish versus a more rippled high speed polish using plastic or poly-urethane
polishers. Pitch polishing takes 10 to 20 times longer but results in very
smooth
surfaces.

Roland Christen

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[MooEy]

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>
> Here's a post from Carl Zambuto and Markus regarding SCTs, polish and
> contrast that I believe is on topic
>

The main reason why SCT optics are cheap is that the vast majority of
amateurs cannot see the difference between a smooth optical surface and a rough
one.

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