Free mirror testing

Or strip it and star test uncoated it on a bright star. Newtonian
paraboloids are straight forward enough with the star test.; I
routinely figure my mirrors with an uncoated glass star test.

My sons and I have never done this, but have occasionally talked about it.
Seems to me that the drawbacks are, at least, 1) getting skies that are
clear enough, 2) waiting for the mirror to reach equlibrium (much longer
than indoors), 3) getting good enough seeing (from a lot of the posts to
this group, you would think this happens only twice per year and accounts
for all optical problems), 4) having a mount ready, and 5) having to
collimate each time. Care to share your experiences?

Dennis

Dennis Woos wrote in message ...
Or strip it and star test uncoated it on a bright star. Newtonian
paraboloids are straight forward enough with the star test.; I
routinely figure my mirrors with an uncoated glass star test.

My sons and I have never done this, but have occasionally talked about it.
Seems to me that the drawbacks are, at least, 1) getting skies that are
clear enough, 2) waiting for the mirror to reach equlibrium (much longer
than indoors), 3) getting good enough seeing (from a lot of the posts to
this group, you would think this happens only twice per year and accounts
for all optical problems), 4) having a mount ready, and 5) having to
collimate each time. Care to share your experiences?

Dennis



don't need any of the above. Use an artificial star.

best regards,
matt tudor

don't need any of the above. Use an artificial star.

best regards,
matt tudor



This, too, is something we have noodled around, but haven't seriously tried
to implement. Seems that whenever the topic comes up here there is quite a
bit of discussion/controversy about the different techniques. Care to
relate what you do?

Dennis

There are a large number of tests, really, that can tell of the quality of a
telescope.
For quick and dirty, I use a Ronchi grating that I put in a eyepiece blank
to look at a star and see the quality of the surface. The image will be of
a bunch of lines on a disk and a perfect scope will have perfectly straight
lines.
Next is to use a knife edge to see how the star (again a big disk) dims as
the image of the star is occulted at the focal plane. The perfect scope
will dim evenly over the whole disk. Both of the above tests don't use an
eyepiece but rather use the testing equipment to do the modifications to the
light to show the shape of the surfaces of the scope. Both of these tests
do have problems with seeing but, if you know what you are looking at, you
can see through the seeing and see what the accuracy of the scope is.
Next comes a Ronchi test which is done at the ROC of the primary and as
such, you don't need to remove the secondary to do the test to a fair
degree. The test is done with a Ronchi grating of about 100-200 lpi and the
mirror is illuminated by a corner of the grating having a light shown
through it and the returned beam looked at through another part of the
grating. Very simple test to do and setup. The problem with this test is
that the lines returned will be curved and thus the test becomes a test more
for the smoothness of the surface rather than exactly how accurate the
surface is to a parabola.
Next is the Foucault test which is also done at the ROC of the mirror
although it is better to remove the spider and secondary for best results.
This test will show the accuracy of the surface to what a parabola is. It
is best done with a Couder mask and then reduce the data (there are a number
of programs available over the web for free) to find out what the accuracy
of the primary is.
After this come varous other tests that take some additonal optics or other
stuff to do the test, such as Waineo, Dall and other such tests or the use
of an interefrometer to read the surface.
--
Why isn't there an Ozone Hole at the NORTH Pole?

In article , Orion
wrote:

Don't know who out there has seen this:
http://www.wodenoptics.com/freetest.html


I have occasinally tested mirrors for my own amusement. If someone
want a commercial Newtonian mirror tested I woudl consider it. Here is
my result from a cheap E-bay mirror:

http://tlepage.home.mindspring.com/6F5_mirror.html

Scott

I would not want to send any of my optics to such an address without
having substantially more information about their financial status,
reputation and liability insurance.

But can anyone provide a rough guestimate as to what would be involved
in obtaining such services from a reputable optics company? Are there
recommendations as to firms or individuals who would evaluate, regrind
if necessary, and recoat a primary mirror (16 inches, f4.5 in my case),
and if so, what's a ball-park figure for such services? I'm assuming
that they would first evaluate and test the mirror and then provide a
cost estimate for recoating and for corrections to a given degree of
precision (e.g., 1/4 wave-length, 1/12 wave-length, etc.) if such
corrections were recommended.

As a practical matter, I'm trying to obtain arrange a Foucault
evaluation of my mirror locally and would do so before considering any
such work, but I would be interested in knowing what's involved in
getting a mirror reground and recoated.

Jim




Orion wrote:

Don't know who out there has seen this:
http://www.wodenoptics.com/freetest.html
Anyone sending in their mirror? My Meade 8" mirror is now about 10 yrs old,
and the Aluminum coating is flaking off from the edges badly, Would this
adversely effect an optical test? I would not have time to get this recoated
and sent to Woden by the Nov cut off time.
Any comments?
Clearskies!
Orion

In article , Jim Cate
wrote:


Jim,

A professional opticial would not touch it for any price you would be
willing to pay. There are a few specialists opticians who make mirrors
for amateurs and one of them would do it for you. Steve Swayze has a
good reputation and he advertises such a service.

http://www.europa.com/~swayze/refigure/refigure.html

For a 16" mirror he charges $45 for testing and $450 for refiguring.
It would need to be recoated after that.

I have no financial interest, blah blah blah.

Scott



I would not want to send any of my optics to such an address without
having substantially more information about their financial status,
reputation and liability insurance.

But can anyone provide a rough guestimate as to what would be involved
in obtaining such services from a reputable optics company? Are there
recommendations as to firms or individuals who would evaluate, regrind
if necessary, and recoat a primary mirror (16 inches, f4.5 in my case),
and if so, what's a ball-park figure for such services? I'm assuming
that they would first evaluate and test the mirror and then provide a
cost estimate for recoating and for corrections to a given degree of
precision (e.g., 1/4 wave-length, 1/12 wave-length, etc.) if such
corrections were recommended.

As a practical matter, I'm trying to obtain arrange a Foucault
evaluation of my mirror locally and would do so before considering any
such work, but I would be interested in knowing what's involved in
getting a mirror reground and recoated.

Jim




Orion wrote:

Don't know who out there has seen this:
http://www.wodenoptics.com/freetest.html
Anyone sending in their mirror? My Meade 8" mirror is now about 10 yrs old,
and the Aluminum coating is flaking off from the edges badly, Would this
adversely effect an optical test? I would not have time to get this recoated
and sent to Woden by the Nov cut off time.
Any comments?
Clearskies!
Orion

Hi Folks,

Just an FYI. This Woden Optics geek is my husband. He is President of
the SVAS and he runs the Full Moon Telescope Workshop, a free service
to club members to teach them how to make/build/repair their own
telescopes. If you are in the region, stop by for our open workshop on
Saturday, November 27th and you can meet us in person and learn about
what we do. Here is a link to the site:
http://fullmoon.telescopebuilding.com/

Actually, I do some of the work myself, although I am currently
focusing on running the big mirrors. We have had 10 mirrors come in on
this offer so far and I promise to treat them with tender-loving care.
;-)

As far as the more technical questions are concerned, I will drag him
to the keyboard and make him post some answers.

Dawn Baird-Chleborad
www.astronerds.com



(Dan Chaffee) wrote in message ...
On Wed, 17 Nov 2004 18:53:14 -0500, "Dennis Woos"
wrote:

It is better to test the mirror uncoated anyway, so you could strip it and
then send it in. Depending on the results, you could then refigure it
before having it coated.

Or strip it and star test uncoated it on a bright star. Newtonian
paraboloids are straight forward enough with the star test.; I
routinely figure my mirrors with an uncoated glass star test.
If at one time, the mirror gave great high power images, no real
need to test it.

Dan Chaffee

On Thu, 18 Nov 2004 10:09:42 -0500, "Dennis Woos"
wrote:

Dennis,

2) waiting for the mirror to reach equlibrium (much longer
than indoors),

You also have to do that for observing; I don't see that as a
significant drawback. As for this being my practice in making optics,
I'm in no hurry--I accept that high quality takes lots of time.

3) getting good enough seeing (from a lot of the posts to
this group, you would think this happens only twice per year and accounts
for all optical problems),

I live where the typical seeing has never been raved about and have
found at least 30 nights /year for the steadyness to be sufficient to
reveal all but the least significant errors. Certainly not all night,
but for long enough bouts to make an evalutaion. You do NOT need
near perfect seeing for a revealing star test, especially detecting
3rd order correction. In P-3 conditions I can EASILY see differences
of outer and inner focus that are under .25 wavefront global under or
overcorrection in an 8" mirror. Certainly P7 or better is necessary
for ultra critical detection of smoothness, small amounts of
astigmatism and higher order errors that a primo paraboloid should be
free of, but a good performer could be verified with average seeing.

I typically use Vega or Arcturus for uncoated mirrors. WHile these
stars are bright enough for the traditional star test, they are barely
bright enough for a crtical evaluation with a ronchi at focus of
uncoated mirrors.

Dan

Hi!

Any chance some of these results being posted soon.
Just curious.

Craig