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[ATM] Re: ATM Digest, Vol 19, Issue 8
My lack of experience will no doubt show thru with this question,but I've been told by that "well known astronomer guy" mentioned not to long ago on these pages, that it's difficult to tell the difference between a 1/8 wave mirror and say a 1/16 wave or 1/20 wave(assuming all other factors in the optical path are essentially perfect, a fantasty situation I'm sure.) What say the masses? Let the salvo begin! Thanks--Jeff Newsom
-----Original Message-----
From: atm-request@atmlist.net
Sent: Jul 5, 2005 9:00 PM
To: atm@atmlist.net
Subject: ATM Digest, Vol 19, Issue 8
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Today's Topics:
1. Apooligies (David Harbour)
2. Re: Re[2]: [ATM] Question about "Null-testing"
(mbartels@bbastrodesigns.com)
3. Re: Question about "Null-testing" (Mike Lockwood)
4. Re: Question about "Null-testing" (hermit)
5. Re: Question about "Null-testing" (mbartels@bbastrodesigns.com)
6. Re: Question about "Null-testing" (Scott Ewart)
7. Ultra Thin blanks, Making the mold (vorblesnak@peak.org)
8. Re: Re[2]: [ATM] Question about "Null-testing" (Jeff Baldwin)
9. Re: Question about "Null-testing" (George Anderson)
----------------------------------------------------------------------
Message: 1
Date: Tue, 5 Jul 2005 13:50:22 -0500
From: "David Harbour" <scarab2@cox.net>
Subject: [ATM] Apooligies
To: "ATM List" <atm@atmlist.net>
Message-ID: <001101c58192$67789050$37d0e544@david6t8539rbu>
Content-Type: text/plain; charset="iso-8859-1"
Hey, guys-
I'm going to move on, like all of us do in life. I can feel myself dying
prematurely (like my beloved wife, Theresa). I hope we can go together in
the same minute. I PRAY FOR IT. If I still had my Fairchilid I could
arrange it. If I still had my old beloved Fairchild I could arrange it; it
would be so easy. I'm tired of lifes' experiencesl. I broke my back as a
little boy, my children both died right after they were born; Theresa is
periwshing of muscular dystrophy. Really, I leave because I can barely keep
house now, trying to take care of her. I don'tg really care how much the
King's man undervalues razor blades; call it a man's excuse to quit
something he can't keep up with. Remember the Hooker 60", though: KNIFE
EDGE.
>Move on David - knife edges are for shaving with :))
(Anonymous: "R-101" man)
What do I know- knife edges- nothing at all.
I am not really hurt by this; I do not let anyone hurt me any more. They
have to have my permission nowadays. Believe it or not, I like the guy who
told me this. But I'm out of here, because I haven't kept up, and my wife is
dying right in front of my eyes, and the other people who love me are
repudiating me, and my health is disintegrating. We're both dying. Don't
feel sorry for me for it- EVERY LAST ONE OF YOU WILL GET TO EXPERIENCE IT.
COUNT ON IT; DON'T BE SURPRISED ABOUT IT LIKE MARCUS AURELIUS SAID YOU WOULD
BE.
Goodbye, and you all have my email address. I have nothing more to
contribute than what injector blades are (two questions from several
members).
Dave,
Flyer and mirror maker
http://www.atmsite.org/contrib/Harbour/Foucault.html
http://www.enidcemetery.com/stoneart2med/ROLL4.JPG-5.JPG
http://www.enidcemetery.com/stoneart2med/ROLL4.JPG-6.JPG
------------------------------
Message: 2
Date: Tue, 05 Jul 2005 12:34:08 -0700 (PDT)
From: mbartels@bbastrodesigns.com
Subject: Re: Re[2]: [ATM] Question about "Null-testing"
To: ATM List <atm@atmlist.net>, INVALID_ADDRESS@.SYNTAX-ERROR
Message-ID: <1120592048.42cae0b096190@www.epud.net>
Content-Type: text/plain; charset=ISO-8859-1
> Why do atms resist this wonderful test, which is so easy to do (tape a
> razor
> blade across the opening of the eyepiece draw tube?). Does anyone have
'Wonderful' test??? Not for me - I tried it more than a couple of times. The
problem is that seeing and the EVERY SO SLIGHTEST jitter make it impossible to
use. At most one gets an impression of the surface. Now, if you are willing
to take several seconds digital image, then that's different. I think there's
great room here for practical innovation. BTW, the razor knife edge so close
to your eye is a safety issue.
Mel Bartels
------------------------------
Message: 3
Date: Tue, 05 Jul 2005 16:19:51 -0500
From: Mike Lockwood <melockwo@uiuc.edu>
Subject: Re: [ATM] Question about "Null-testing"
To: ATM List <atm@atmlist.net>
Message-ID: <42CAF977.2010107@uiuc.edu>
Content-Type: text/plain; charset=us-ascii; format=flowed
Hi,
mbartels@bbastrodesigns.com wrote:
> 'Wonderful' test??? Not for me - I tried it more than a couple of times. The
> problem is that seeing and the EVERY SO SLIGHTEST jitter make it impossible to
> use. At most one gets an impression of the surface.
I have never tried a razor blade. I got rid of the razor blade on my
Foucault tester and replaced it with a piece of brass that had been
prepared as per Texereau's instructions. It works much better than a
razor blade - seemed to be less diffraction artifacts as my eye moved
around.
As I was shown it, null testing on a star is not that hard utilizing a
Ronchi grating and Polaris. The scope must equilibrate and the sky
must cooperate and be fairly steady, but useful results are
obtainable. The Ronchi grating is mounted in a hole in the top of a
film canister, which is placed in the focuser.
On one night at Astrofest last year, with some assistance and quite a
bit of instruction from a friend, we used an 85-line-per-inch (I
think) Ronchi grating at the focus of several telescopes. Using
Polaris as a source, its small motion will, with a little waiting, put
it just at the right place for one of the lines to serve as a "knife
edge". (Turn off your tracking platform!)
A slight pressure, in the form of fingertip pressure on the secondary
cage, is applied to bring the line into the source. The pressure is
NOT enough to move the scope - just enough to flex it every so
slightly. (Wooden Dobs fare well with this.) Foucault-like shadows
are then seen, but the clarity of the shadows will be a function of
the seeing conditions. In the slight motion allows the shadows to be
seen while Polaris is correctly positioned. Some practice is required.
My 16" F/4, which tested consistently very good with my slitless
tester, produced a clean null. Defocusing and observing the Ronchi
grating lines at a focal position that showed a few lines, a clean
diffraction line was seen between each grating line. I am told that
the "cleanness" of this diffraction line indicates smooth optics. The
grating lines were very straight.
On our 30" F/3.8, we saw a high zone in the shadows at about the 80%
zone. This agreed with the form of the mirror as indicated by
slitless Foucault testing. The test did NOT obviously indicate the
astigmatism that was present due to our diagonal mirror being 2 waves
concave. (The flat has since been fixed - see my web site under "A
tale of four flats" for a description of that very fun operation!)
If we had rotated the grating and repeated the test, we might have
seen some astigmatism. I will have to try this again some time and
see if I can detect it.
An 8" SCT nearby revealed almost straight bands, but the diffraction
lines were less obvious. This indicated roughness, on the corrector I
suspect. Overall, not bad.
In short, I found the "Ronchi eyepiece" test to be a useful
confirmation and sanity check of my Foucault testing results for two
large, fast mirrors that I have figured.
Null tests without extra components are a wonderful thing. (Yes, an
autocollimation flat is in my future.)
Mike Lockwood
Champaign, IL
http://bi-staff.beckman.uiuc.edu/~melockwo/index.html
------------------------------
Message: 4
Date: Tue, 05 Jul 2005 17:39:13 -0400
From: hermit <hermit@outofoptions.org>
Subject: Re: [ATM] Question about "Null-testing"
To: Mike Lockwood <melockwo@uiuc.edu>
Cc: ATM List <atm@atmlist.net>
Message-ID: <42CAFE01.4030904@outofoptions.org>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Mike Lockwood wrote:
> Hi,
>
> mbartels@bbastrodesigns.com wrote:
>
>> 'Wonderful' test??? Not for me - I tried it more than a couple of
>> times. The problem is that seeing and the EVERY SO SLIGHTEST jitter
>> make it impossible to use. At most one gets an impression of the
>> surface.
>
>
> I have never tried a razor blade. I got rid of the razor blade on my
> Foucault tester and replaced it with a piece of brass that had been
> prepared as per Texereau's instructions. It works much better than a
> razor blade - seemed to be less diffraction artifacts as my eye moved
> around.
>
Ronchi grating down to one line ain't bad either. ;-)
Ken
------------------------------
Message: 5
Date: Tue, 05 Jul 2005 16:17:28 -0700 (PDT)
From: mbartels@bbastrodesigns.com
Subject: Re: [ATM] Question about "Null-testing"
To: ATM List <atm@atmlist.net>
Message-ID: <1120605448.42cb150889eca@www.epud.net>
Content-Type: text/plain; charset=ISO-8859-1
> it just at the right place for one of the lines to serve as a "knife
> edge". (Turn off your tracking platform!)
I use the Ronchi with a barlow in place of the eyepiece all the time, though
it's pretty insensitive. A mirror has to be pretty bad to show any deviation
from straight bands. I was responding specifically to using a knife-edge at
the focal plane.
Mel Bartels
------------------------------
Message: 6
Date: Tue, 5 Jul 2005 20:44:25 -0400
From: "Scott Ewart" <scott.e@quicksurfer.com>
Subject: Re: [ATM] Question about "Null-testing"
To: "ATM List" <atm@atmlist.net>
Message-ID: <02cc01c581c3$de9500c0$3e2aef04@DELL>
Content-Type: text/plain; charset="iso-8859-1"
Mel,
Effectively, that's what he's doing. If you use a Ronchi in the focuser
AT (or very near) focus, you can easily use the edge of ONE band as a knife
edge. It's very simple to position since you can use any band out of dozens
near the center of the field. As one drifts by, wait for the next one.
I was able to see a bend in the shadows that was hardly detectable with
the usual star test.
Scott Ewart
----- Original Message -----
From: <mbartels@bbastrodesigns.com>
To: "ATM List" <atm@atmlist.net>
Sent: Tuesday, July 05, 2005 7:17 PM
Subject: Re: [ATM] Question about "Null-testing"
|> it just at the right place for one of the lines to serve as a "knife
| > edge". (Turn off your tracking platform!)
|
| I use the Ronchi with a barlow in place of the eyepiece all the time,
though
| it's pretty insensitive. A mirror has to be pretty bad to show any
deviation
| from straight bands. I was responding specifically to using a knife-edge
at
| the focal plane.
|
| Mel Bartels
| _______________________________________________
| ATM mailing list http://www.atmlist.net/
------------------------------
Message: 7
Date: Tue, 5 Jul 2005 17:45:26 -0700 (PDT)
From: vorblesnak@peak.org
Subject: [ATM] Ultra Thin blanks, Making the mold
To: atm@atmlist.net
Message-ID: <1944.64.28.62.212.1120610726.squirrel@webmail.peak.org>
Content-Type: text/plain;charset=iso-8859-1
I had made a couple of slumping molds out of plaster and was prepared to
send those off to various kind souls with kilns who had offered to slump.
Alas! plaster will not take the heat, as I was warned, and as I proved to
myself. So it was back to square one. I opted to make the molds out of
ceramic. I used a cheap casting slip for the slumping mold, but I am
ahead of myself.
To begin I made two more disks of plaster to create casting molds. These
were made by pouring plaster into a plastic mold. The mold was a 'planter
diaper', bought at the local variety store. They are $1.99 each and are
18 inches in diameter. I allowed these to dry for a week, then then
turned them out and baked them at 300 degrees F for four hours and allowed
them to cool. Tapped with a hard object they ring.
Now basic r squared over two R. I wanted a sixteen inch diameter F5
mirror blank. This would require an 84 inch focal length. Sagitta is
about three sixteenths of an inch. I used the plaster disks just as I
would a glass pair and ground out a plaster blank. If the mirror is three
sixteenths inch deep, the tool is three sixteenths humped. Using an old
hole saw I hogged out the middle of the mirror blank and hogged off the
edge of the tool blank. Now it was just a matter of rubbing the two
together and smoothing the curve. Once this was done I checked the curve
using a straight edge across the diameter and a caliper. I was within a
sixtyfourth and considered that good enough.
Now I took the mirror blank disk and wrapped a strip of paper around it
and poured in the slip. This has taken a month to fully dry, though I
took it out of the mold after a week. Next I used spray adhesive to stick
sixty grit sandpaper to the plaster mold and smoothed the curve of the
cast ceramic slumping mold.
I have made two of these now in case one breaks while firing. I had an
old mold from Mel Bartels and that one I reduced the curve and slumped the
eighteen inch blank. So I have one ultra thin blank to grind.
The last word right now is glass. The last piece of borosilicate, (Pyrex)
I bought cost me $150.00 and I broke it. OUCH! However, I found old
microwave oven turn tables at Goodwill for $1.99 each and they are in the
twelve to fifteen inch diameter range. These are Pyrex so thin blanks
might be cheap.
I have three waiting for the molds to fire, so hopefully I will get some
more glass slumped soon. Grinding is next.
David Davis
Toledo, OR 97391
------------------------------
Message: 8
Date: Tue, 5 Jul 2005 18:16:46 -0700
From: "Jeff Baldwin" <bald@olypen.com>
Subject: Re: Re[2]: [ATM] Question about "Null-testing"
To: "ATM List" <atm@atmlist.net>
Message-ID: <003d01c581c8$621039c0$3144433f@D9Z1Y521>
Content-Type: text/plain; charset="iso-8859-1"
This is hard. I've put the razor at an acute angle with the drift of the
star to allow it to cross through the razor slowly, and it still is very
tough to capture visually. Adjusting the razor to the right focus is also a
trial and error eternal game. A controlled autocollimation test in the shop
is not only easier to handle, but allows double reflection for increasing
error detection
JBald
> > Why do atms resist this wonderful test, which is so easy to do (tape a
> > razor
> > blade across the opening of the eyepiece draw tube?). Does anyone have
>
> 'Wonderful' test??? Not for me - I tried it more than a couple of times.
The
> problem is that seeing and the EVERY SO SLIGHTEST jitter make it
impossible to
> use. At most one gets an impression of the surface. Now, if you are
willing
> to take several seconds digital image, then that's different. I think
there's
> great room here for practical innovation. BTW, the razor knife edge so
close
> to your eye is a safety issue.
>
> Mel Bartels
------------------------------
Message: 9
Date: Tue, 05 Jul 2005 22:32:05 -0400
From: George Anderson <tillerman1@videotron.ca>
Subject: Re: [ATM] Question about "Null-testing"
To: Richard <cnc@cncservo.co.uk>
Cc: David Harbour <atm@atmlist.net>
Message-ID: <42CB42A5.A5AA013B@videotron.ca>
Content-Type: text/plain; charset=us-ascii
Richard wrote:
SNIP
> BTW, what is an *injector* razor blade?
>
> --
>
> Best regards,
> Richard in the UK
Watch the Alfred Hitchcock movie "Topaz" for details on the Schick Blade
Injector.
George Anderson
Montreal Canada
Clear skies and good health
------------------------------
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End of ATM Digest, Vol 19, Issue 8
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