Stefan Rostyne
scholastic sledgehammer
Reged: 10/19/04
Posts: 940
Loc: Assenede, Belgium
|
|
Last week we started polishing an 18" mirror, ground and smoothed to f/4. The mirror is 33mm thick Schott borosilicate. Grinding, smoothing and polishing is done with a homemade Draper style machine. Turntable speed is currently set at 6RPM with a pulley, overarm stroke speed is fully adjustable 0-50 strokes/minute by a Siemens frequency controller. We use two 3-phase electromotors at 1200W each. Grinding & smoothing went at about 40 strokes, for polishing we note that such a high rate was near impossible to maintain for the machine. We work now at about 25 strokes /minute for polishing out, and intend to go much slower for figuring. Overhang is set at 1/6 mirror diameter, and we use a 12" subdiameter lap with gugolz pitch #73.
It is my intention to post a few foucault images after every polishing session. Right now, the mirror has had 3 hours of polishing time. I know that it is *very* premature to start testing in this early stage, yet putting the glass on the test stand may -or may not- give insights that may help others that are in a similar process, or who are thinking about making a lrge thin mirror. we are a local group of amateurs, and we gather only on monday evenings for mirrormaking. I will try to post news each week.
I an earlier attempt to start polishing this mirror, a couple of months ago, we managed to polish into astigmatic due to unefficient support. We are currently supporting the mirror by floatation on a sheet of some latex covered mesh, normally used in home decoration. Thanks to Mark Cowan for that tip.
Other tips and techniques that are followed or that have inspired us, are found in a most interesting discussion on a spin polishing howto, (or better just not?), by Mark Cowan and others.
Further reading is found in the thread about 'astigmatism : causes and misconceptions', by Mark Harry. I would like to thank these gentleman for their insights.
OK back to our own. We know that it is far to early for actually testing. Nevertheless, putting the mirror in the test stand and shooting a picture of the surface, you can't do wrong, we think.
In this first image, what immediately is seen, is the still rough surface. Image taken at CoC(2F) with a classic foucault tester,with slit and a blue power LED. We know that you can test with a slitless LED, but it seems like we loose resolution in our image due overlighting. The image shown comes just right off the camera and is only cropped to size, no further processing.
At current, a zone of about 2" stays matte at the edge. We just have to continue polishing. What is also seen is a slight central hill, perhaps t is seens as because the edge zone still lays flat. The concentric rings you see near the center are actually markings on the back of the glass.
Any considerings, tips, etc... will be greatly respected!
|
kfrederick
sage
Reged: 02/01/08
Posts: 223
|
|
hi are you using pads or pitch ? kevin
|
Stefan Rostyne
scholastic sledgehammer
Reged: 10/19/04
Posts: 940
Loc: Assenede, Belgium
|
|
We are polishing with Gugolz pith. I know, it's the old way. Pads go faster, but we 've plenty of time.
--------------------
Stefan Van de Rostijne
4.5" F4.5 newt 5°widefield/finderscope
8" f/5.6 travel dob
old 12.5" F5 dob (used to look better...)
30 cm f/30 Classic Cassegrain (polishing primary)
23" f/4 dob project
|
rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 6024
Loc: Bremerton Washington
|
|
Quote:
We are polishing with Gugolz pith. I know, it's the old way. Pads go faster, but we 've plenty of time.
Stefan...Good beginning. How is the polishing going as far as evenness? Are you polishing out the center first....?
My 18's (now in the trash)first few shadowgrams looked just like this...with a slight bump in the middle. You may want to clean up those ring marks on the back of the mirror...they may lend to difficult reading in later tests.
Good luck.
Rob
--------------------
www.goldmtobservingcenter.com
Providing a great place for amateur astronomers, and ATM's to come and enjoy their hobby.
|
Glig
sage
Reged: 10/02/05
Posts: 270
Loc: Baltimore, MD
|
|
You still have astigmatism right? The shadowgram seems to be lopsided.
--------------------
Richard Caldwell
|
Stefan Rostyne
scholastic sledgehammer
Reged: 10/19/04
Posts: 940
Loc: Assenede, Belgium
|
|
I don't know if it is astigmatism.. It can be from the mirror not exactly on a line to the camera.
--------------------
Stefan Van de Rostijne
4.5" F4.5 newt 5°widefield/finderscope
8" f/5.6 travel dob
old 12.5" F5 dob (used to look better...)
30 cm f/30 Classic Cassegrain (polishing primary)
23" f/4 dob project
|
Glig
sage
Reged: 10/02/05
Posts: 270
Loc: Baltimore, MD
|
|
What have you done since the astigmatism was detected 2 months ago? Reground the mirror? Or just resumed polishing?
What size tools?
--------------------
Richard Caldwell
|
Mark Harry
Carpal Tunnel
Reged: 09/05/05
Posts: 2492
Loc: Northeast
|
|
Just my 2 millicents worth, it still looks astigmatic. The central area astigmatism looks like it's oriented far off of what the astigmatism is at the edge.(meaning, there are 2 orientations to different radii-???)
****
One other note, I have never seen Gugolz pitch come out as rough as this, unless there is something grossly amiss.
My opinion on parameters of polishing, your overhang isn't far enough or fast enough -IF- you want to have stroke dependent polishing. If you are spin-polishing, I think MC just lets the polisher sit in one position,and he cranks up the RPMs, though I think everyone knows I don't really favor that type of polishing for reasons such as revealed in your picture. But not knowing all of the aspects, I can't say for sure what's causing the rough surface/astigmatism. I do know that a nice wide sweeping stroke will tend to concentrate more action on the lagging edge.
*********
I can't ever remember seeing a condition quite like this, unless pad polishing was used with resonance issues of stroke/RPM setting.
Perhaps we should wait until you have 10-12 hours of polishing completed?
Regards, Mark
One other note, I have a 3/4 horse DC motor pulling the eccentric on my machine, and it can do 120 very wide strokes/minute, with an 11" polisher, and the motor just about gets lukewarm to the touch. In my way of guesstimating wattage used, it should be about 500-600 watts max under load. Yours should equate to about 1.5-1.75 horsepower easily. Why won't the machine pull a more "active" polishing regimen?
--------------------
Scopes in the works-
Too many for putting down here! Favorites- 8" F/6, 8" F/4.72, 4.5" F/5.4, 14" F/4.455, all completed.
|
Mark Harry
Carpal Tunnel
Reged: 09/05/05
Posts: 2492
Loc: Northeast
|
|
I think I see 2 issues with the settings on your machine. You have 25 strokes/min, with 6 RPMs- darn close to 1:4, which could explain the astigmatism to a degree.
Also, you'll find frequency controlled motors lack a lot of low speed torque- the possible reason for their not being capable of handling the load at slow speeds. Do you have the speed turned down from -normal operation- at 50 hertz?
Mark
--------------------
Scopes in the works-
Too many for putting down here! Favorites- 8" F/6, 8" F/4.72, 4.5" F/5.4, 14" F/4.455, all completed.
|
mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1370
Loc: salem, OR
|
|
Quote:
If you are spin-polishing, I think MC just lets the polisher sit in one position,and he cranks up the RPMs, though I think everyone knows I don't really favor that type of polishing for reasons such as revealed in your picture.
Not my mirror. And he's not using my technique, as quite clearly noted.
Best,
Mark
|
mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1370
Loc: salem, OR
|
|
Hi Stefan,
Could just be test stand sag, that's common enough. You might want to use a longer stroke with a 12" polishing lap than 3" I'm thinking. Also, is the lap turning evenly or is it tending to go in fits? Hard to know that much after 3 hours.  What it looks like fully polished out will be more telling. 
Best,
Mark
|
Mark Harry
Carpal Tunnel
Reged: 09/05/05
Posts: 2492
Loc: Northeast
|
|
"Could just be test stand sag, that's common enough."
*****
Not if he supports it correctly. And for information, I've never seen it here. (in fact, about as common here as finding hen's teeth!) Also, there are what appear to be 2 separate surfaces present, not exactly due to test stand technique.
M.
--------------------
Scopes in the works-
Too many for putting down here! Favorites- 8" F/6, 8" F/4.72, 4.5" F/5.4, 14" F/4.455, all completed.
|
Stefan Rostyne
scholastic sledgehammer
Reged: 10/19/04
Posts: 940
Loc: Assenede, Belgium
|
|
@ MH :
Quote:
You have 25 strokes/min, with 6 RPMs- darn close to 1:4, which could explain the astigmatism to a degree.
I know. Our intention is to work with 40 strokes at 6RPM, but at current the machine just can't get that. The lap sticks to glass, and the drive belt slips on the pulley. It has nothing to do with the motor not being strong enough, only poor design.
Keep also in mind that the mirror has only seen 3H of polishing time, so the image is very premature. Lap movement currently is raw, irregular and at times very bumpy. The lap shows an incredible tendency to stick.
@ MC : We over here are also thinking that the astigmatism-like thing we see is induced by the test stand. I am allmost sure that the mirror is too thin for the crude and elementary stand (it rests on 2 rubber pads at 60° on the bottom and is hold on the upper edge by a 3rd rubber that presses on the edge. We are currently making a better stand. Can someone provide a pic of how your stands for large thin mirrors look like? That would be great.
>> OK that was yesterday and I didn't get a chance to get online after that<<
We made an improvement on the lap drive system, which is stronger now (more tension on the belt), no more stickin' lap. Overarm can easily take 50 strokes /minute now, with evenly rotation counterwise the mirror rotation. However, what we see now, is that the turntable (6RPM) goes in small fits at the overarm speed of 40 strokes/min. Still not favourable for the figure, I guess, so we'll do similar adjustment to the turntable drive belt, one of the upcoming days.
About stroke lenght, we intend to keep using the 3" overhang until fully polished out. We 'll see what the figure looks by then; no shadowgram in the meantime.
We 'll focus now on a better mirror stand.
Thanks for your toughts as these were very instructive.
Regards!
--------------------
Stefan Van de Rostijne
4.5" F4.5 newt 5°widefield/finderscope
8" f/5.6 travel dob
old 12.5" F5 dob (used to look better...)
30 cm f/30 Classic Cassegrain (polishing primary)
23" f/4 dob project
|
Stefan Rostyne
scholastic sledgehammer
Reged: 10/19/04
Posts: 940
Loc: Assenede, Belgium
|
|
Quote:
What have you done since the astigmatism was detected 2 months ago? Reground the mirror? Or just resumed polishing?
We reground it. Then suffered for a while with unbelievable scratches (see one of my previous posts). Made new tool with coins. Scratches gone. Started polishing. Go astigmatic again. Asking help at the CN fellows. Adjust machinery. Start again. And again. And again. Oh well...
It is... Learning! 
--------------------
Stefan Van de Rostijne
4.5" F4.5 newt 5°widefield/finderscope
8" f/5.6 travel dob
old 12.5" F5 dob (used to look better...)
30 cm f/30 Classic Cassegrain (polishing primary)
23" f/4 dob project
|
Stefan Rostyne
scholastic sledgehammer
Reged: 10/19/04
Posts: 940
Loc: Assenede, Belgium
|
|
Again about that astigm : there may also be a geometry problem with our testing setup. Investigating that.
--------------------
Stefan Van de Rostijne
4.5" F4.5 newt 5°widefield/finderscope
8" f/5.6 travel dob
old 12.5" F5 dob (used to look better...)
30 cm f/30 Classic Cassegrain (polishing primary)
23" f/4 dob project
|
kfrederick
sage
Reged: 02/01/08
Posts: 223
|
|
hi Stefan one thing i did on my last big thin mirror i ground the back of my mirror on the turn table so they were the same surfaces then used carpet under the mirror . i used tape to hold them at the edge /they were the same diameter /if you have a ronchi grating see if the single band tilts or has a s shape / kevin
|
mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1370
Loc: salem, OR
|
|
Quote:
Also, there are what appear to be 2 separate surfaces present, not exactly due to test stand technique.
Quote:
I am allmost sure that the mirror is too thin for the crude and elementary stand (it rests on 2 rubber pads at 60° on the bottom and is hold on the upper edge by a 3rd rubber that presses on the edge.
Well, there's your problem! Could be potato chipping (primary) and trefoil.
Best,
Mark
|
mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1370
Loc: salem, OR
|
|
Quote:
Can someone provide a pic of how your stands for large thin mirrors look like?
Here's a picture of what I use, with the optical bench for the new interferometer (under construction) behind it.
The front surface is 1/2" Corian (countertop material). It's very flat. The posts are adjustable 1/2" rod with rubber pads that engage about the outer 1/3 thickness of the glass. Making sure the mirror's back is also flat, the mirror is pressed up against the flat support and then slid down under some pressure to contact the pads. This prevents the support points from moving (forward) and the center of the mirror from moving (back) which is enough to overcome the small force of gravity that wants to fold the top forward.
It works better if tilted back a few degrees though, so this stand actually needs to be shortened and the tester angled down along the optical axis. The supports produce a very local stress in the surface that is easy to see when used as described, but if tilted back there's no need to apply any force in setting it up and the pads can support the entire depth of the glass.
The interferometer will run from ground level to 4 or 5 feet elevation over its 16 foot bench span, with the same kind of mirror support as shown, although more adjustable, probably with some motorization, and it might get a coat of paint as well...
Best,
Mark
|
Stefan Rostyne
scholastic sledgehammer
Reged: 10/19/04
Posts: 940
Loc: Assenede, Belgium
|
|
Found it!
It seems to come down on geometry problems in the setup. A few minutes ago I did some work on te test setup alignment.
Also did some basic work on the mirror stand. Made sure there is no more tension on the edges. The mirror now only rests on the stand without anuy stress. What also showed up is a little play in our tester. That last problem will son be gone anyway as we are building a new tester from an old and heavy brass microscope that we were donated.
shadows are still little lopsized, but the magnitude is far less than in the first shadowgram.
image only cropped to size
--------------------
Stefan Van de Rostijne
4.5" F4.5 newt 5°widefield/finderscope
8" f/5.6 travel dob
old 12.5" F5 dob (used to look better...)
30 cm f/30 Classic Cassegrain (polishing primary)
23" f/4 dob project
|
rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 6024
Loc: Bremerton Washington
|
|
Quote:
Quote:
Can someone provide a pic of how your stands for large thin mirrors look like?
Here's a picture of what I use, with the optical bench for the new interferometer (under construction) behind it.
The front surface is 1/2" Corian (countertop material). It's very flat. The posts are adjustable 1/2" rod with rubber pads that engage about the outer 1/3 thickness of the glass. Making sure the mirror's back is also flat, the mirror is pressed up against the flat support and then slid down under some pressure to contact the pads. This prevents the support points from moving (forward) and the center of the mirror from moving (back) which is enough to overcome the small force of gravity that wants to fold the top forward.
It works better if tilted back a few degrees though, so this stand actually needs to be shortened and the tester angled down along the optical axis. The supports produce a very local stress in the surface that is easy to see when used as described, but if tilted back there's no need to apply any force in setting it up and the pads can support the entire depth of the glass.
The interferometer will run from ground level to 4 or 5 feet elevation over its 16 foot bench span, with the same kind of mirror support as shown, although more adjustable, probably with some motorization, and it might get a coat of paint as well...
Best,
Mark
That critter looks like my stand...cept mine is made from metal, and mine can tip.
Rob
--------------------
www.goldmtobservingcenter.com
Providing a great place for amateur astronomers, and ATM's to come and enjoy their hobby.
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
[Re: 