Below is a series of videos on mirror testing and mirror quality that
are either used in the mirror workshop directly or are intended to be a suppliment to
it. They are derived from a series of presentations I gave at the Stellafane telescope
makers convention over the period of a few years. They are freely available for you to
view. For those of you who have attended the workshop before, you might recognize them,
but they have been completely improved and remade. I can guarantee you'll enjoy
watching them again and you'll be treated with sections you haven't seen before. All in
all, I hope that you will find them useful as you tackle your mirror making projects.
The first video lays out several mirror test concepts and methodologies used by
amateurs during the figuring process. The second examines two commonly accepted
criteria for determining acceptable mirror quality and discusses the analysis methods
that will help you determine if your mirror meets those criteria. Finally, taking the
tests and analysis techniques from the first two videos, the third discusses mirror
quality terms that you may encounter as you research or shop for mirrors other than
Downloading These Videos
These videos are pretty large and can take a long time to download depending on your
internet connection speed. To save download time on your side and bandwidth on mine, I
kindly request that you download and save your own copy. This will allow you to view it
as often as you'd like without the need to download them each time. There are webpages,
however, with the videos embedded if you find yourself in a situation where you are
unable to save a local copy. Please remember that, while they're free for download, I
have placed some copyright and redistribution restrictions that are detailed at the bottom
of this page.
The Video Format
These videos are in the Windows Media format. (WMV) If you are running windows, you
should already have everything you need to view these videos: including Windows Media
Player and the Media Player plugin for your internet browser. If not, you will need
to get a video player capable of viewing the WMV format.
Telescope Optics Testing
This video covers the several different methods that can be used measure the shape
of your telescope mirrors. It describes the underlying principles behind the null
test along with the use of the Ronchi screen. It explains the conditions under
which various mirror shapes will produce a null return. It defines spherical
aberration and then studies how you can use it and a knife edge to reveal
different characteristics about your mirror.
It explores four tests that use a knife edge, or a variation of it, to measure
the spherical aberration including: the Focault knife edge test, the Ronchi test,
the wire test, and the caustic test.
On the other hand, null tests provide the advantage of returning a single,
immediate result. The video covers five null tests that are at the mirror makers
disposal. It explains the Mobsby screen, the artificial star test, the Dall and
Ross tests, the autocollimation test and finally the star test. By the end of
this video, you will have a good understanding of the tests as they are being
applied in the workshop.
Duration: 44 min
Validating Your Mirror Quality (Part I)
This video covers the two most commonly established standards for a good mirror:
the Rayleigh 1/4 wave criteria and the Danjon & Couder (D&C) criteria. While here,
it takes a practical look at mirror wavelength ratings and shows you where you can
get the most performance out of your mirror for your effort.
Using data that can be collected via the test techniques discussed in Telescope
Optics Testing, it explains three data analysis techniques used to determine
if a mirror meets those criteria. Those techniques include: the Classic Method,
Texereau's method, and the Milles-Lacroix method.
Duration: 51 min
Validating Your Mirror Quality (Part II)
This video starts with a summary recap of the criteria and analysis methods covered
in Validating Your Mirror (Part I). It then uses them as a basis for
discussion of other mirror quality terms that you may encounter as you read
articles about mirror quality or mirror specification data sheets. If you
would like to skip the recap. and go straight to the new material, you may
fast-forward to minute 14:00.
The new material talks about four different mirror specification measures. It
highlights the differences between surface error vs. wavefront error. Often
times they are interchanged without your knowledge. Along the way, it takes
a few minutes to talk about some considerations when evaluating secondary mirrors
as part of your telescope's optical system. Additionally, it studies
the Peak-to-Valley (P-V) measure and juxtaposes it with the Root Mean Square (RMS)
measure. Additionally, with RMS, it talks about its intended application, it
reveals its actual application, then highlights the difference between the two.
It finishes with the Strehl Ratio measure. Once again, it talks about the
difference between its intended application and its practical application.
Duration: 54 min
This video dives farther into the autocollimation test. It is a superior test used by several
amateur mirror making workshops including this one and has a long history of being used by
professionals and amateurs alike. This video covers a brief history of the autocollimation
test, discusses the minimum requirements of a flat, describes suggested features of a test bench,
and reviews some of the avoidable pitfalls one can encounter in their attempts to use the test.
Finally, it shows how the test can be used to test entire optical systems including optics already
installed in a telescope.
Duration: 45 min
Copyright & Distribution
Permission is herein granted to download these videos for your own personal purposes.
You may not modify these videos in any way without the express written permission of
the author. You may not redistribute these videos without the express written consent
of the author. You may not use these videos for any commercial purpose or financial
gain. You may not include these videos in any composition, either in whole or in part,
for commercial purposes or financial gain.
© 2009 Richard S. Parker, Thomas L. Parker
WinZip and the WinZip logo are registered trademarks of WinZip International LLC.