Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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This is a thread for my 20" mirror grinding project. I hope to get lots of great advice and/or constructive criticism.
Here is the background...
The mirror is 20.25" dia, 1.22 thick quartz. It is being ground/polished/figured on a machine.
I am using is 15" diameter hydrostone porcelain tile tool that is about 2" thick (yeah, I went a little crazy).
The f ratio I am shooting for is somewhere between 3.5 and 4. The mirror has a curve generated on it (by our very own Mark Cowan).
I've started grinding with 80 grit and have been at it for about 2 hours now. The table is turning around 19 RPM. I have also placed a 25# weight on top of the tool while grinding for a total weight of about 50#.
It has taken a bit longer than I expected at this grit but I had some curve generation anomalies that needed to be removed.
Currently I am still grinding at 80 trying to get a constant RoC over the surface of the mirror.
I have a 4" diameter spherometer that measures to .0001 accuracy.
So my current question I have is how consistent from the edge to the center should my spherometer measure before I move on to 120 grit? i.e. should I have it exactly right across the surface?
As of now there is a .001 measurement difference between the edge and the center. Right now I am proceeding to try and get a consistent measurement before moving on.
Also, I suspect that the grind is taking longer because the grit is breaking down quicker than I think it is. I've never worked with quartz before, could anyone give some insight as to how long a given "wet" should last? To me it seems after 5 minutes the grit is history (even though it is still making a fair amount of noise).
Thanks...
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Brian....I've never used glass tiles for my tools..I use pennies...and coupled with the grit it takes me around a couple of hours on my machine to get to fine grinding. But I don't grind quartz either. I use my spherometer and make sure the measurement is nearly consistant all across the mirror before I go to fine grinding.
OH..and I do that during curve generation too.
Good luck with that big mirror. 
Rob
--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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Their actually tiles, not glass....
I moved the tool in a bit and added grit every 5 minutes and it quickly brought the curve into shape. The sound of the mud running on the tool/glass was fooling me. Indeed the grit is done in about 5 minutes.... The quartz makes quick work of it!
Everywhere on the surface now is within .0001. On to the next grit..
Thanks...
Quote:
Brian....I've never used glass tiles for my tools..I use pennies...and coupled with the grit it takes me around a couple of hours on my machine to get to fine grinding. But I don't grind quartz either. I use my spherometer and make sure the measurement is nearly consistant all across the mirror before I go to fine grinding.
OH..and I do that during curve generation too.
Good luck with that big mirror.
Rob
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MitchAlsup
super member
Reged: 08/31/09
Posts: 185
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At each stage in the grinding process (beyond hogging out and sphericalizing), you are going to go through about 3X as much glass as the grit you are at.
So add up all the grit thicknesses you are going to go through and your tollerance is better than 3X that number ratioed by the amount of surface you are mesuring.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Brian,
A "wet" at 80 will last 30 seconds to 1 minute under good pressure. Pushing it beyond where it's noisy doesn't accomplish anything. With a water drip on the machine I add fresh grit every 30 seconds to 1 minute, depending on contact and how the mud behaves.
Best,
Mark
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 668
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one test to do a fiew wets with something finer say 220 then see if it is fine grinding all over evenly
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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And Sharpie test if you want to know - recall how the generated blank looked when it arrived... 
Best,
Mark
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Mert
Carpal Tunnel
Reged: 08/31/05
Posts: 1803
Loc: Spain, Pamplona
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Any pics you can share with us?????
Soo nice to see these big projects coming along!
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Mert
42º49"N 1º38"W
3" Polarex refractor
6" F12 SW Maksutov,CS2-S
EQ6 + EQMOD 
SPC900NC/DFK21AU04.AS
My web-page don't laugh, I should make it better! 
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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So I've progressed thru 220 and my next grit is 320 SiC. The f ratio is coming in around f/3.9. I did have a chance to make it 3.5 during 80 grit but decided against it. So I backed the arm out a bit and have it at the "neutral point" for f/3.9.
My grit progression is going to be this...
80 SiC
120
220
320
500
25M AOX
15M AOX
9M AOX
Maybe 5M AOX, depending...
Before someone points it out, yes I know 320 SiC is about the same size as 25M AOX and that I will be "back tracking" thru the grits. I am doing this because it is quartz and on the recommendation of Mark (whose done many more mirrors than I ever will).
I've also rigged up a dripper to drip water on the mirror as it is grinding. I used a standard IV drip bag, poured the saline out and refilled with water and some dawn. I know this is not revolutionary but it is cheap and works REALLY well.
BTW, just wondering if anyone uses dawn mixed with water during grinding? It make such a huge difference in the action of the tool.
Sharpie and spherometer tests confirm the mirror is being ground evenly.
I am still amazed how quickly the SiC is consumed. The last mirror I did (27" over 10 years ago) was basically plate glass and I got longer times before having to put grit on. With this quartz blank I find myself putting grit on every 1-2 minutes (if not more frequently).
I've included 3 pictures below. The tile tool, the generated blank with tiles on it with my "bib" with some #12 copper wire holding it up so it doesn't touch the back of the center tiles and the mirror on the turntable ground thru 220.
Quote:
And Sharpie test if you want to know - recall how the generated blank looked when it arrived...
Best,
Mark
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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Tile Tool
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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Mirror ground thru 220
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Quote:
I am still amazed how quickly the SiC is consumed.
Looking good. I'd recommend skipping the 5 micron, though.
Just think how much fun it'd be hogging out the 5 lbs or so of quartz dust from the initial generation with SiC instead of diamond tooling!
Best,
Mark
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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Yeah, I am thinking of skipping the 5 micron. Since I have a machine I can let it polish all day. The only reason I would/have used 5 micron is to shorten polishing time.
BTW If you had to guess, how many pounds of 6O grit SiC would it take to hog out a quartz blank from a flat? 10 pounds? more?
Quote:
Quote:
I am still amazed how quickly the SiC is consumed.
Looking good. I'd recommend skipping the 5 micron, though.
Just think how much fun it'd be hogging out the 5 lbs or so of quartz dust from the initial generation with SiC instead of diamond tooling!
Best,
Mark
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 668
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how flat is the back i checked my 14.7 quartz blank it is NOT flat .
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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I don't really know. I know it was blanchard ground and my spherometer measures pretty consistantly across the back.
Geez... I hope it is flat. I have been using it as a reference to zero out my spherometer indicator.
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JohnH
professor emeritus
Reged: 10/04/05
Posts: 734
Loc: vancouver near the wilds of B...
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Quote:
I don't really know. I know it was blanchard ground and my spherometer measures pretty consistantly across the back.
Geez... I hope it is flat. I have been using it as a reference to zero out my spherometer indicator.
You need to use a proper reference surface to zero a spherometer. Most recommend using a granite plate made for just that purpose
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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So I am grinding at 500 SiC right now and having some trouble with the action of the tool. The tool seems to be mostly floating and not doing much work. Every so often I will get good action (from the sound) but mostly it seems like it is floating.
Any advice? Do I need more weight? The tool plus the weight I put on top of it is around 50 pounds.
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 668
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i fine grind with NO weight .
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Quote:
Yeah, I am thinking of skipping the 5 micron. Since I have a machine I can let it polish all day. The only reason I would/have used 5 micron is to shorten polishing time.
Any changes in polishing time are negligible, compared to the potential for sleeks from the tile tool at this stage. Your very thick tool base might keep that at bay, but I quit using anything below 12 micron years ago.
Quote:
BTW If you had to guess, how many pounds of 6O grit SiC would it take to hog out a quartz blank from a flat? 10 pounds? more?
Something like that. The removal rate is not very high even with SiC on quartz. I know that manually hogging Pyrex with SiC used a lot of grit...
Quote:
So I am grinding at 500 SiC right now and having some trouble with the action of the tool. The tool seems to be mostly floating and not doing much work. Every so often I will get good action (from the sound) but mostly it seems like it is floating.
Well, it could be, but you don't get much sound from 500 anyway. If the tool is Hydrostone don't let it dry out, if it does dry out soak it in water for a couple hours. It will change curve on you if it dries out.
If you let it run a while with a drip and then add some 500 you should hear and see it pick up action immediately. You might want to try the Sharpie test and see how it's working.
Best,
Mark
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3127
Loc: Northeast
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I'm pretty good at economising the grits. When I ground a 20, I used about 7+ pounds to get an F/5.1 curve on it.
M.
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So many projects, so little time!
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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I've ground thru 500 and I am making my way thru the AoX grits.....
I was just wondering mark, how long (approx) should it take @ 25M and 15M. I spent about 30 minutes and 1/2 a pound @ 25M and now I am on to 15M and plan to do the same.
The surface seems to be ground evenly (judging by pits from the center to the edge).
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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A half pound?! I mix all grits from 220 on with water, and 25 micron makes a dilute slurry at about 10:1 or 15:1, of which I'd use maybe half a liter at most, so that's like 50g perhaps. The 25 and 12 last almost forever due to how far they spread.
Working time is about right though, the finer grits require progressively less time. I'll give 30 minutes to 25, 15-20 to 12.
Best,
Mark
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3127
Loc: Northeast
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"A half pound?! "
****************
Same here, half a pound!!! Hate to see your floor! 
I think my progression, I used 220, then went to 25 micron, think I used about half what you did, over 3x the interval. Reason for doing it that way, I could see the difference quite a bit better when thoroughly done. Then switching to 12 as final grit, about an hour and a half for good measure. (maybe 4 oz total) I went 60, 120, 220, 25m, and 12m.
The finer you go, the more particles for any given volume. Just a little squirt of 12 goes a long way. If most is just squashing out, then it's wasted. At least that's the way I look at it.
M.
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So many projects, so little time!
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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It is roughly half a pound... Probably 1/3 if I really weighed it.
What I've been doing is running a water drip sprinkling on the grit with a shaker as I go.
I've finished thru 15M,for 9M I will mix it into a slurry to avoid any chance of sleaking.
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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I've finished fine grinding.... Only managed to get 2 very small scratches near the edge (which I mirror clip will completely cover) and one sleak about an inch long near the edge. It may possibly polish out but I think I am just going to live with it.
I've cast the 15" polishing tool. This time I did something a little different....
I added acryl 60 (1 Quart) to the hydrostone mix to add strength and water resistance to the hydrostone.
I need some advice.....
My environment (a garage) is going to be about 60 degrees (a little less at times), despite my efforts otherwise.
I have gugolz 64 pitch and was wondering if anyone has any recommendations on whether I should soften it up with some mineral spirits? I would rather have it a bit too hard than too soft (TDE).
Thanks...
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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My recommendation is Aculap standard grade. Polishing out the quartz is fairly brutal and the microenvironment on the surface will be / should be warm. The only problem I have with that combo is in very hot weather when I get too much pitch flow.
How thick is the polisher tool? Hydrostone is a bit squirrely for making polishers and I don't use it for that, as it changes shape too easily.
Best,
Mark
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3127
Loc: Northeast
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If you're working with 64, I would definitely -NOT- thin that pitch, and particularly not with mineral spirits! It should be adequate as is. I polished a 6" F/13, and a 6" F/4.5 both with relatively hard, used #73 in a 60 degree cellar this last week, and they came out fine. Very smooth, no edge, and I'll say adequately (!) accurate. The polisher was cut more like one used for a flat; few, and very shallow grooves. I was suprised how well it worked with the fast mirror!(new technique)
M.
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So many projects, so little time!
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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My polisher tool is 2" thick and weighs more than the mirror  . With the acryl 60, I would think the flexural strength should be plenty good - also the acryl 60 make the plaster more water resistant. I also plan to seal it up with polyurethane as well.
As it is I guess I won't mess with the pitch. I've been out of the game too long to make a subjective determination if the pitch is tempered correctly. I will go with the straight gugolz 64 and accept my destiny .
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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The polisher should be plenty thick enough then.
I've used G64 a lot in polishing out by machine, I like the Acculap better because (a) it lasts better (b) recycles endlessly (c) it doesn't suddenly go south on the machine like I've seen the current formula of G64 do.
Mark mentioned faceting and I use large facets with narrow channels near the end, just enough to hold some thin slurry.
Best,
Mark
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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I did consider acculap but I've used G64 in the past and liked it alot (light years better than "burgondy" pitch).
Large facets, how large? I was going to do 1" square facets. Not sure if I can visualize what you mean by narrow channels near the end.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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See pic (that's a 12" polisher) - around 2" facets. The faceting pattern is deliberately non-periodic. The facets can be smaller if you like.
If you start out with wide channels and polish a long time they close up to become narrow, and/or close completely. As you're completing the polishing out it's better to have narrow channels as the smoothness benefits.
Best,
Mark
Edited by mark cowan (11/05/09 12:12 AM)
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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Poured and channelled the 15" pitch lap and I am having a little bit of trouble.
Basically the lap is not contacting over the surface of the mirror. I figured I would solve the problem "hot" pressing the lap to the mirror. I have tried 2 hot pressings both about 15 minutes long with about 75 pounds of weight on top.
The lap is slowing coming into shape but the better the contact is, the harder it is to get off! My last pressing took *ALL* of my strength (and weight) to free it. I am using about 5-1 mixure of cerium/water with a few drops of dawn for the "release" agent.
I am afraid to press longer as the tool may become permanetly stuck.
The other odd thing is the surface of the lap is pretty "tacky". i.e. it feels like a giant piece of scotch tape (same feel of tackyness). Again, it has been awhile for me making mirrors but I don't remember my laps ever being that tacky. I am sure this tacky/sticky ness is a big part of the reason it is do damn hard to take the lap off.
So any suggestions for a better release agent? Pressing tips?
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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Try cutting a clean common white polyethylene trash bag down one side, open up the trash bag, wet the lap, and just lay the trash bag across the lap or mirror before pressing. The lap won't stick to the polyethylene, but it will come into perfect contact through it. Saran wrap also works but it's not usually wide enough for your 20". Keep the trash bag clean between pressings.
Also, before trash bag pressing, take a razor blade and make 3 or 4 scratches in each of the big facets to break them up into smaller subfacets. That will let the lap flow into contact better, and reduce the drag and tendency to chatter or zone.
Mike
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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Duh, I should of thought of that  ... This is exactly what I did when I cast my tile tool.
That seems to be the trick, after several hot/cold pressings and rechannelling I am nearly getting full contact with the lap. I guess I need to find a better technique pouring the lap to get good contact from the start.
Quote:
Try cutting a clean common white polyethylene trash bag down one side, open up the trash bag, wet the lap, and just lay the trash bag across the lap or mirror before pressing. The lap won't stick to the polyethylene, but it will come into perfect contact through it. Saran wrap also works but it's not usually wide enough for your 20". Keep the trash bag clean between pressings.
Also, before trash bag pressing, take a razor blade and make 3 or 4 scratches in each of the big facets to break them up into smaller subfacets. That will let the lap flow into contact better, and reduce the drag and tendency to chatter or zone.
Mike
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3127
Loc: Northeast
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Sounds like you're having trouble with contact. What kind of base are you using for the pitch- flat, or curved to match the mirror?
Mark
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So many projects, so little time!
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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It is curved. I cast hydrostone directly on the surface of the mirror.
I am slowing getting there.... After many hot pressings and channelings the lap is mostly in contact. I just have one more cycle to go thru (hopefully).
Man... this sucks, I must of did something wrong in pouring the pitch, inital pressing or something. 
Quote:
Sounds like you're having trouble with contact. What kind of base are you using for the pitch- flat, or curved to match the mirror?
Mark
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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Finally I got the lap in good contact with the mirror and polishing has begun. I have about 2 hours in now and I already have a "flash" polish on the glass.
One thing I noted was after about 2 hours the lap just seemed to be skating on the surface. And indeed I was wasn't getting much action. So I took the tool off and brushed the surface of the pitch with a brass brush to "deglaze" it and wow what a difference that made in the lap action.
Not that this is a new idea or anything (many people do it) I was rather amazed what a difference it made.
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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Wondering if I can get some expert advice here...
I am little concerned that my pitch is too hard. During my 1st polishing session I was getting great contact. So I stopped for a few days (lap was stored on its "back") and went to resume.
I cold pressed the lap for 30 minutes on some wedding tool (micro faceting) with 50 pounds of weight on it (the tool weighs around 25 so 75 total). After all of that I was not getting good contact at all. The wedding tool barely left a mark on the pitch, you could see it but it was not very pronounced.
The room enviroment is about 57 degrees and I did add a few drops (3 or 4) of olive oil to the pitch when I was melting it. I heated the pitch very slow and it never came anywhere near boiling.
So I am going to press it for another 30 minutes to see if the contact improves.
Any thoughts?
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Do some hot (or warm) pressing. Hot pressing on quartz is almost a gimme, as you can do it very quickly (30 secs or so with the lap treated with hot tap water and a wire brush) and you get good contact immediately - with no penalty for deformation on the glass. It's what I do day to day and it works great.
Note this is only for polishing out!
Best,
Mark
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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After another 30 minutes the contact is better and after polishing for 3 hours it is decent.
My gauge for good contact is that the lap is spinning somewhat fast and constant(no stops and starts). I would expect that as the surface gets smoother the friction gets less, therefor the lap will spin slower and slower?
I have about 7 hours total polishing time and still needs a lot more. I am going to put it up on the test bench to see what kind of mess I have....
Quote:
Do some hot (or warm) pressing. Hot pressing on quartz is almost a gimme, as you can do it very quickly (30 secs or so with the lap treated with hot tap water and a wire brush) and you get good contact immediately - with no penalty for deformation on the glass. It's what I do day to day and it works great.
Note this is only for polishing out!
Best,
Mark
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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I managed to get up on my foucault tester.
I couldn't find the most important thing I need right now - a ronchi screen but I was able to get a real good look at the surface.
Right now it looks like a record with grooves. Kind of like Rob's shadowgrams but instead of a pinwheel pattern I have a circular pattern. Although I will say that they don't appear to be as bad/deep.
I also have a really nice hill in the middle. Interesting that I can see the *exact* center of the optical axis because there is a point right in the middle of the mirror.
I hope the hill isn't ground in. I am going to move the arm in a bit to get more action in the center.
I am going to have to figure out how to get better contact.
I am also going to have to redesign my test stand support as I can see the vee block deforming the mirror at the bottom where I am supporting it.
The good news is the edge looks fine.
Thanks....
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Quote:
I would expect that as the surface gets smoother the friction gets less, therefor the lap will spin slower and slower?
Well, no, the polisher works due to embedded cerium and the friction will increase over time with perfect contact. It will generally spin a little slower than the turntable, but if different areas are making the best contact that can change - if, say, only the center is in good contact, the polisher will spin extremely fast...
Best,
Mark
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3127
Loc: Northeast
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In all machine work that I have occasioned, any polisher that's smaller than the mirror (TOT) will spin somewhat faster than the spindle, if in good contact. (Elgin) If it spins slower, and can be spun easily by touching it, the polisher's center-acting, and will drill a hole. If it's center acting and it spins very easily, (fast as Mark says)the conformity is almost non-existent. If it starts to spin, and stops rotating in mid stroke, I find that it's not fully conformed, generally. Or microfacetting is needed.
Sounds like you're "spin-polishing". On your own there!
Mark
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So many projects, so little time!
Edited by Mark Harry (11/13/09 07:16 AM)
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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So I put down a piece of wedding tool on the mirror and placed the lap on top of it overnight (10 hours or so) with no weight on it.
I think that resolved my issue but I was kind of expecting the channels to be closed up after sitting that long. Still concerned about the pitch hardness maybe I will now get the pinwheel pattern .
I youtubed a clip of the lap spinning on the turntable. I apologize for the quality for some reason the camera made it really green...
http://www.youtube.com/watch?v=Pe9FkF-IIpY
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Quote:
I managed to get up on my foucault tester.
I couldn't find the most important thing I need right now - a ronchi screen but I was able to get a real good look at the surface.
Right now it looks like a record with grooves. Kind of like Rob's shadowgrams but instead of a pinwheel pattern I have a circular pattern. Although I will say that they don't appear to be as bad/deep.
I also have a really nice hill in the middle. Interesting that I can see the *exact* center of the optical axis because there is a point right in the middle of the mirror.
I hope the hill isn't ground in. I am going to move the arm in a bit to get more action in the center.
I am going to have to figure out how to get better contact.
I am also going to have to redesign my test stand support as I can see the vee block deforming the mirror at the bottom where I am supporting it.
The good news is the edge looks fine.
Thanks....
Fun stuff huh?
Those circular marks are definately a result of poor tool contact. I've found that stroking and spin polishing leave difinitively different marks if the tool is improperly contacting.
My latest shadowgram shows a nice even smooth surface.
Contact..contact...contact
I'm done working on the 13"f/3.8 so no more imges will be posted...however I will be following up with Droid's mirror images.
Glad you can begin to see where you have to go.....
Rob
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www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Quote:
So I put down a piece of wedding tool on the mirror and placed the lap on top of it overnight (10 hours or so) with no weight on it.
I think that resolved my issue but I was kind of expecting the channels to be closed up after sitting that long. Still concerned about the pitch hardness maybe I will now get the pinwheel pattern .
I youtubed a clip of the lap spinning on the turntable. I apologize for the quality for some reason the camera made it really green...
http://www.youtube.com/watch?v=Pe9FkF-IIpY
Brian.....I've found that subdi tools that are not perfectly conformed and that are not overhanging the center and the edge will definately drill a hole in the center.
On my machine I try to create a slight amount of lateral stroke(it doesn't appear you can) along with the spin polishing...it prevents any singular artifacts from being imbeded in the figure.
Good luck and keep us posted.
Rob
--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
|
rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Quote:
If you're working with 64, I would definitely -NOT- thin that pitch, and particularly not with mineral spirits! It should be adequate as is. I polished a 6" F/13, and a 6" F/4.5 both with relatively hard, used #73 in a 60 degree cellar this last week, and they came out fine. Very smooth, no edge, and I'll say adequately (!) accurate. The polisher was cut more like one used for a flat; few, and very shallow grooves. I was suprised how well it worked with the fast mirror!(new technique)
M.
I've experimented with shallow grooves with #64 and it works well in a cool shop.
Rob
--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
|
rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Quote:
Try cutting a clean common white polyethylene trash bag down one side, open up the trash bag, wet the lap, and just lay the trash bag across the lap or mirror before pressing. The lap won't stick to the polyethylene, but it will come into perfect contact through it. Saran wrap also works but it's not usually wide enough for your 20". Keep the trash bag clean between pressings.
Also, before trash bag pressing, take a razor blade and make 3 or 4 scratches in each of the big facets to break them up into smaller subfacets. That will let the lap flow into contact better, and reduce the drag and tendency to chatter or zone.
Mike
Exactly my process.....Mikester 
Rob
--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
|
Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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|
So far I have a nice big hill in the middle. I've had the arm offset from the radial to avoid drilling a hole and it looks like it worked too well in that I now have a hill.
I have put the arm on the radial and moved the tool in just a touch to try and knock it down. Since I am still polishing out I am not trying to do too much but I was to make corrections to gross errors as I get thru the polishing.
I am getting pretty good contact now hopefully that will smooth things out.
Quote:
Quote:
So I put down a piece of wedding tool on the mirror and placed the lap on top of it overnight (10 hours or so) with no weight on it.
I think that resolved my issue but I was kind of expecting the channels to be closed up after sitting that long. Still concerned about the pitch hardness maybe I will now get the pinwheel pattern .
I youtubed a clip of the lap spinning on the turntable. I apologize for the quality for some reason the camera made it really green...
http://www.youtube.com/watch?v=Pe9FkF-IIpY
Brian.....I've found that subdi tools that are not perfectly conformed and that are not overhanging the center and the edge will definately drill a hole in the center.
On my machine I try to create a slight amount of lateral stroke(it doesn't appear you can) along with the spin polishing...it prevents any singular artifacts from being imbeded in the figure.
Good luck and keep us posted.
Rob
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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|
Quote:
It will generally spin a little slower than the turntable, but if different areas are making the best contact that can change - if, say, only the center is in good contact, the polisher will spin extremely fast...
Clarification - in spin polishing to finish the edge for figuring, a freshly prepped polisher around 75% diameter at a 15% overhang, the polisher if making proper contact and driving the mirror toward a good sphere spins just a bit slower than the table does. This is what I look for to verify contact and action - the polisher will have a great deal of drag under these conditions.
Change the size, the surface, the offset, and the above will not be true.
I don't have an analysis of what causes the described situation, it's all empirical based on the results I get.
Best,
Mark
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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|
That is pretty much what I am seeing now. The lap is spinning about 1/2 the speed of the turntable.
Just curious, what ratio do you dilute your CeO with water? I've been adding 4 "heaping" teaspoons of CeO to 1 liter of water. I get a diluted milk looking mixture.
Quote:
Quote:
It will generally spin a little slower than the turntable, but if different areas are making the best contact that can change - if, say, only the center is in good contact, the polisher will spin extremely fast...
Clarification - in spin polishing to finish the edge for figuring, a freshly prepped polisher around 75% diameter at a 15% overhang, the polisher if making proper contact and driving the mirror toward a good sphere spins just a bit slower than the table does. This is what I look for to verify contact and action - the polisher will have a great deal of drag under these conditions.
Change the size, the surface, the offset, and the above will not be true.
I don't have an analysis of what causes the described situation, it's all empirical based on the results I get.
Best,
Mark
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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|
A hill..would seem to indicate you are not permitting the tool to slightly cross the center. The polishing action removes glass around the center from roughly the outter edge of the inner zone to the edge of the mirror.
I had this problem when I used tools under 75%.
Your tool on a 20" mirror...may give you fits. Been there.....  How big is your tool again.....?
Quote:
So far I have a nice big hill in the middle. I've had the arm offset from the radial to avoid drilling a hole and it looks like it worked too well in that I now have a hill.
I have put the arm on the radial and moved the tool in just a touch to try and knock it down. Since I am still polishing out I am not trying to do too much but I was to make corrections to gross errors as I get thru the polishing.
I am getting pretty good contact now hopefully that will smooth things out.
Quote:
Quote:
So I put down a piece of wedding tool on the mirror and placed the lap on top of it overnight (10 hours or so) with no weight on it.
I think that resolved my issue but I was kind of expecting the channels to be closed up after sitting that long. Still concerned about the pitch hardness maybe I will now get the pinwheel pattern .
I youtubed a clip of the lap spinning on the turntable. I apologize for the quality for some reason the camera made it really green...
http://www.youtube.com/watch?v=Pe9FkF-IIpY
Brian.....I've found that subdi tools that are not perfectly conformed and that are not overhanging the center and the edge will definately drill a hole in the center.
On my machine I try to create a slight amount of lateral stroke(it doesn't appear you can) along with the spin polishing...it prevents any singular artifacts from being imbeded in the figure.
Good luck and keep us posted.
Rob
--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
|
Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
|
|
Quote:
Your tool on a 20" mirror...may give you fits. Been there..... How big is your tool again.....?
It is roughly 15 inches, so about 75%
I am starting to think I got a bad batch of pitch. I actually tempered the stuff with about 4-5 drops of olive oil, knowing the working tempertures I have. The stuff is pretty hard in the container. Easily fails the "thumbnail test".
Guess I shouldn't be so lazy and make a pitch tester so I can stop guessing....
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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|
Brian,
The video looks OK only the offset is too high. The polisher always needs to be well over the center for effective work; for a 15" polisher the offset (overhang) should be about 2-1/4" and the distance it goes past the center thus is around 2.6"...
Best,
Mark
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
|
|
Quote:
Just curious, what ratio do you dilute your CeO with water? I've been adding 4 "heaping" teaspoons of CeO to 1 liter of water. I get a diluted milk looking mixture.
I do two (or three dilutions) depending - in the stock it's maybe 10:1 or 15:1 water by volume (I cook, what can I say), and the stock is used for charging laps. For figuring I dilute that another 8:1 or so; for the cerium drip I shoot a healthy amount of diluted cerium into a gallon of water, which stays in suspension and produces a drip that has just a tinge of white to it.
Charge the lap heavily before spin polishing is my advice. What gets embedded is what does the work.
Best,
Mark
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
|
|
Quote:
Quote:
Your tool on a 20" mirror...may give you fits. Been there..... How big is your tool again.....?
It is roughly 15 inches, so about 75%
I am starting to think I got a bad batch of pitch. I actually tempered the stuff with about 4-5 drops of olive oil, knowing the working tempertures I have. The stuff is pretty hard in the container. Easily fails the "thumbnail test".
Guess I shouldn't be so lazy and make a pitch tester so I can stop guessing....
I don't have a pitch tester....I have a thumb nail and a good guesser... 
I used lots of turpentine in the older #76 and it made it much softer....However..I'm using the new #55 and #65 in my cold shop. Friction will cause the pitch to deform over time anyway. From my experiences a good conformed tool with irregular fasceting and shallow grooves works very well.
Letting the machine do its work for many hours is also part of the equasion in my view. However...I've adapter a regular system of checkin in on the polishing via tester. If a problem is arrising..you can correct it before a trainwreck happens...(been there) 
I never handle the mirror with bare hands..transfering heat.
I use insulating gloves when transfering the mirror from the machine to the wash station to the tester.
Good luck and I'm glad someone else here is tackling a mirror via machine.
Rob
--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
|
rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
|
|
Quote:
Brian,
The video looks OK only the offset is too high. The polisher always needs to be well over the center for effective work; for a 15" polisher the offset (overhang) should be about 2-1/4" and the distance it goes past the center thus is around 2.6"...
Best,
Mark
Very good...that is my very experiences.....Sweet advice Mark.
Rob
--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
|
rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
|
|
Quote:
Quote:
Just curious, what ratio do you dilute your CeO with water? I've been adding 4 "heaping" teaspoons of CeO to 1 liter of water. I get a diluted milk looking mixture.
I do two (or three dilutions) depending - in the stock it's maybe 10:1 or 15:1 water by volume (I cook, what can I say), and the stock is used for charging laps. For figuring I dilute that another 8:1 or so; for the cerium drip I shoot a healthy amount of diluted cerium into a gallon of water, which stays in suspension and produces a drip that has just a tinge of white to it.
Charge the lap heavily before spin polishing is my advice. What gets embedded is what does the work.
Best,
Mark
Good advice Mark....I charge by initially pressing with a strong delution of cerium on the mirror...this way the initial contact while the pitch is its softest will capture lots of cerium. I polish in the same fashion.
Rob
--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
|
Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
|
|
By offset do you mean how far the tool is in on the radial?
If so, that would explain the hill I have. I have moved the tool in a bit. I just hope I can polish it out and it is not "ground" in.
Quote:
Brian,
The video looks OK only the offset is too high. The polisher always needs to be well over the center for effective work; for a 15" polisher the offset (overhang) should be about 2-1/4" and the distance it goes past the center thus is around 2.6"...
Best,
Mark
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
|
|
I actually do the same.... I have a "rich" mixture I use for pressing and initially charging up the tool and a weaker mixture for the drip. A little dawn in the water keeps the CeO suspended nicely. I also vibrate the bag to prevent clumping and keeping everything homogenus.
Quote:
Quote:
Just curious, what ratio do you dilute your CeO with water? I've been adding 4 "heaping" teaspoons of CeO to 1 liter of water. I get a diluted milk looking mixture.
I do two (or three dilutions) depending - in the stock it's maybe 10:1 or 15:1 water by volume (I cook, what can I say), and the stock is used for charging laps. For figuring I dilute that another 8:1 or so; for the cerium drip I shoot a healthy amount of diluted cerium into a gallon of water, which stays in suspension and produces a drip that has just a tinge of white to it.
Charge the lap heavily before spin polishing is my advice. What gets embedded is what does the work.
Best,
Mark
|
rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
|
|
Quote:
By offset do you mean how far the tool is in on the radial?
If so, that would explain the hill I have. I have moved the tool in a bit. I just hope I can polish it out and it is not "ground" in.
Quote:
Brian,
The video looks OK only the offset is too high. The polisher always needs to be well over the center for effective work; for a 15" polisher the offset (overhang) should be about 2-1/4" and the distance it goes past the center thus is around 2.6"...
Best,
Mark
Brian...yes. If the tool is not crossing the center it will produce a hill. Imagine the radius being eclipped by the tool and the edge roughly the same distaance...like MARK said.
Believe me I learned the hard way with subdiameter laps.
Rob
--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
|
rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
|
|
Quote:
I actually do the same.... I have a "rich" mixture I use for pressing and initially charging up the tool and a weaker mixture for the drip. A little dawn in the water keeps the CeO suspended nicely. I also vibrate the bag to prevent clumping and keeping everything homogenus.
Quote:
Quote:
Just curious, what ratio do you dilute your CeO with water? I've been adding 4 "heaping" teaspoons of CeO to 1 liter of water. I get a diluted milk looking mixture.
I do two (or three dilutions) depending - in the stock it's maybe 10:1 or 15:1 water by volume (I cook, what can I say), and the stock is used for charging laps. For figuring I dilute that another 8:1 or so; for the cerium drip I shoot a healthy amount of diluted cerium into a gallon of water, which stays in suspension and produces a drip that has just a tinge of white to it.
Charge the lap heavily before spin polishing is my advice. What gets embedded is what does the work.
Best,
Mark
Brian....I think the best method is what I devised. Simply use a gallon jug. filled with water and your several Tbl spoons of cerium. Then use a very small line and use capillary action to drip the cerium on the mirror.....suspend it with a simple(cheep) fish tank bubbler. Run both tubes in the jug....down toward the bottom and let the bubbles keep things suspended.
I use this method for all my polishing/fine grinding slurries. slick..slick..slick. You don't have to use gravity....just capillary action.
Rob(but if your system works for you...)
--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
|
Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
|
|
I am starting to see a pattern here....
After pressing and starting polishing contact/action is good. As the polishing session goes on the contact gets worse (meaning the friction is less). By the end of a 4 hour session I can spin the lap with my fingers.
I don't understand what is going on here. Should I have weight on the lap?
Also I've used the lap for about 8 hours now and the channels have not closed up on the lap. In fact the are not even near closing up.
I am thinking the pitch is bad.
Any thoughts?
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
|
|
Quote:
I am starting to see a pattern here....
After pressing and starting polishing contact/action is good. As the polishing session goes on the contact gets worse (meaning the friction is less). By the end of a 4 hour session I can spin the lap with my fingers.
I don't understand what is going on here. Should I have weight on the lap?
Also I've used the lap for about 8 hours now and the channels have not closed up on the lap. In fact the are not even near closing up.
I am thinking the pitch is bad.
Any thoughts?
Test the mirror...see what is being created by the lack of cantact.
Rob
--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
|
rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
|
|
Also the tool shoulkd have a kinda apaque sheen to the surface..where the cerium has worked the glass.
Wight might be good.
I use 15lbs.
Rob
--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
|
mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
|
|
Here's a short video of a 14.7" in final spin polishing right now. I compressed the hell out of it to make the upload tolerable, but it shows what it should look like...
spinning.mov
Best,
Mark
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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|
I get that kind of action at the start of a session (after pressing) but as the session goes on the contact gets worse. I would think the action would get better as the session goes on. Maybe brushing the lap more often would help.
I also noticed you have a weight on top, 25 pounds?
I would think my pitch channels should have closed up by now with 7-8 hours of work plus leaving the tool sit on the mirror all night (10 hours or so). Also the "micro facets" left on the pitch from the overnight press (using wedding tool) are still on the pitch facets after 4 hours.
Again, it has been awhile since I've done this but the pitch seems too hard to me. I really don't want it to be as I dread the idea of remaking it! 
Ironically enough 2 days after I went thru the pain and suffering of many hot presses to get initial good contact, Carl Zambuto posted a message on his yahoo list on the way he does laps..... Man, 2 days eariler and that would have saved me so much suffering  .
I am going to throw the mirror up on the test stand to see where I am at. If it is not getting better I am going to be forced to do something about the lap.
Quote:
Here's a short video of a 14.7" in final spin polishing right now. I compressed the hell out of it to make the upload tolerable, but it shows what it should look like...
spinning.mov
Best,
Mark
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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|
Yes, the weight is important. I want about 1/3 lb per square inch on the pitch for polishing out, the tool is about 12" diameter and weighs about 10 lbs.
The tape thing is interesting in pouring laps, when I've tried it the tape won't come off - but I didn't try just leaving it there and trimming it off later...
When I pour a pitch lap, I immediately form it to the mirror and press channels, a la John Dobson, using a thin aluminum bar. The key is having the pitch the right softness just before you pour, and the tool the right temperature, but if you don't do it often the result can be a horrid mess.
Your pitch could be too hard, especially if the channels don't change at all when polishing with some weight - note that with good contact the mirror should at least get warm, it may get hot. This is just a sign of proper working going on.
Just throw the polisher in the freezer for an hour or two and chip the pitch off it if you need to remake it. I recommend tempering it with pure gum turpentine, it works really well with Gugolz.
Best,
Mark
BTW that mirror came off the polishing session ready to figure about a half-hour later. If I remember tomorrow I'll take a pic of the result on the stand before I start working it.
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
|
|
I put it up on the tester and the good news is the hill is gone and the surface is smooth.
I am seeing some weirdness with the knife edge not cutting "cleanly" (not sure how else to describe it) in the mirror that could be astigmastism. Given that my test stand is supporting it with 2 points 3-4 inches apart on the bottom, this may explain it (given the mirror is 1.22" thick).
Would a sling on my test stand eliminate any stand induced stig? What do you guys use?
Thanks....
Brian (now where did I put that ronchi screen)....
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
|
|
Just an update.... I discovered the mirror on the test stand was "leaning" against the top clip (the thing that keeps the mirror from tipping over). Once I leveled the stand such that all the weight was down vertically on the support points the "weirdness" went away. However I can still see the bottm support points deforming the mirror. Kind of scary...
I've tested quite a few mirrors in my day but I've never done one this thin - it is amazing how easily deformed it is....
Everything still looks really smooth and my hill might be turning into a hole so I am going to back the tool off of the radial a bit (offset).
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
|
|
I printed up a ronchi screen and tested my mirror with it.
The lines are pretty straight. I see some bending around the center where I had my hill. The best news of all is that it appears no stig is present which was my greatest fear.
How do you guys make your ronchi screens or do you buy glass ones? Printing on acetate doesn't work all that well for me, mainly issues with "clarity" thru the lines.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
|
|
Quote:
I am seeing some weirdness with the knife edge not cutting "cleanly" (not sure how else to describe it) in the mirror that could be astigmastism. Given that my test stand is supporting it with 2 points 3-4 inches apart on the bottom, this may explain it (given the mirror is 1.22" thick).
Would a sling on my test stand eliminate any stand induced stig? What do you guys use?
What you want are two support points 90 degrees apart a little outboard of the COG of the mirror, and a very flat back for it to sit against. Slings, flexible backings, all cause the mirror to potato chip when supported vertically. I've tested this, and detected test stand sag in a 12.5x2.125" Pyrex mirror when supported on a sling and/or against bubble-wrap.
Edmunds sells quality ronchi gratings on glass - you only need a 1" square, around 133 LPI or so for bench testing.
Best,
Mark
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
|
|
Mark...your video is nice.
One question..no there'll be 2..
When making your polishing tools do you first pour a slight mound on the center of the tool with pitch....then fill the rest in to the damn..so you have a slight bump in the center to make better initial contact with the mirros curve?
If not...what is your trick to get great central contact initially?
Second if use a camera to take lots of your shadowgrams/ronchigrams..you need a lot larger grading then 1" square. You have to pass it in front of the light source and the KE...and beable to permit a camera lens to view the center of ROC. In my set-up anyway.
Thanks for your wonderful advice and help.
Great 20" project Brian....
Rob
--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
|
mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
|
|
Hey, at 6 seconds total run time it's hard to go wrong...  Thanks!
Well, first I've not had success with using a dam, so I just start pouring in the center and let it spread outwards, adjusting as needed. Then take the warmed mirror and spread some cerium on it, press from the center until it spreads out to the edge. Slide the mirror off and start channeling with a 1/8x1/2" aluminum bar. If you do this right the pitch just comes up to the edge and not much beyond, but that's where the dam would help I think.
I also use the flat side of the bar to even up the facets (bending the bar along the curve) with the idea being the mirror should make good contact with all the facets across their faces before it's finished. In other words, using the 1/8" bar to make the channels and roll it back and forth a bit, which raises the edges of the facets, then using the 1/2" side to smooth the facets out, leaving the channels intact, and so on. Do this a few times, reapplying the mirror in between, until you get good contact overall and a nice facet pattern. That's pretty much the way Dobson did it in his class, though with a stick as I recall, and he could talk about the laps while he was making them, which I thought was great - the process is very fast if you've got the hang of it.
Gratings bigger than 1" square? Why's that? I've used a 50 micron pinhole as the source for the fixed-source tester and the camera looking through the grating, and I've also used the 1" grating on the moving-source (slitless) tester where the grating covers both the LED and the return beam - again, the camera looks through the grating just like it would past a KE. Could you post a picture of the setup you have - it might be the tester design or it might be the camera itself. If you can put your eye in position to view the image from the tester you should be able to put the lens of the camera in the same place.
Best,
Mark
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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Just wondering Mark, how fast is that turntable spinning? I have mine running at 20 RPM.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Just over 40 RPM. 4 revolutions in 6 seconds.
Best,
Mark
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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In an effort to get better contact I channeled the lap and added some new channels to the bigger facets, which had grown to like 2" with all of the pressings.
Now the tool is getting damn good contact and spinning like crazy.
If I can get thru the polishing with something close to a sphere, no stig, smooth surface and no TDE I will consider myself lucky :-)....
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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Maybe someone can offer some insight on this...
I am spin polishing my mirror and when I start I press the lap then start up the machine. The lap makes great contact and spins consistanly with lots of friction for about 2-3 hours. But slowly over a 2-3 hour session the contact/action gets worse and worse such that the tool almost stops spinning and you can spin the tool around with your finger.
Why is does the contact get worse during the session? This makes no sense to me. I would expect with a 25 pound weight on top of the tool the contact would be getting better!
So every 2-3 hours I have to stop, brush the lap (brass brush) and press it again for 15 minutes or so. Then I get good lap action again.
Thoughts?
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Mert
Carpal Tunnel
Reged: 08/31/05
Posts: 1803
Loc: Spain, Pamplona
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I'm not an expert in any way, but this sounds
like a mecanical problem maybe!
Imagine the tool not freely moving in the Z-axis
but slightly being hindered somehow, when it begins
to wear down a bit, it could loose contact.
Just my 2 cents
--------------------
------------------
Mert
42º49"N 1º38"W
3" Polarex refractor
6" F12 SW Maksutov,CS2-S
EQ6 + EQMOD
SPC900NC/DFK21AU04.AS
My web-page don't laugh, I should make it better!
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 668
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That is the way it works. Dress the polisher when it stops spining correct ..scratch it all up .and have at it
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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Quote:
That is the way it works. Dress the polisher when it stops spining correct ..scratch it all up .and have at it
I've learned that . The slowing rotation is not a result of bad contact (but can be) but the pitch facets developing a glaze on them that reduce polishing action.
I've observed this both visually (lap stops spinning) and by sound - if I put my ear next to the tool it becomes much quieter as time goes on.
If I simply take the tool off, brush the glaze off and put it back on, action is great again for another 2 hours or so.
On my 20 RPM table,50 pounds of total weight, with pitch on the hard side; it takes about 2.5 hours to build up a glaze.
I've read lots of explainations on how polishing works and I don't think I buy all of them completely :-). Still magical to me .
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 668
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It may be a litle like a bald tire hydroplaining on water .NOT SURE
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3127
Loc: Northeast
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"Why is does the contact get worse during the session? This makes no sense to me. I would expect with a 25 pound weight on top of the tool the contact would be getting better!"
*************
It is! Unfortunately, the pitch squares are/have gotten too large, and the microfacetting is filled in. The squares as a result, suck right to the glass. This is where I believe "dog bisquit" comes from. Every so often, a square gets stuck down, and through rotation or strokes, rotates and creates divots in the glass surface.... Short of it, when it just starts to do this, stop and microfacet the polisher. You should see an immediate improvement in the smoothness of the polishing action afterward.
Mark
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So many projects, so little time!
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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So I managed to get a decent 100 lpi ronchi screen together and had a look at my mirror.
I still have a slight center issue that I expect to come in line with further polishing but it also appears I have the dreaded TDE. The outer 1" or so of the mirror is "hooking" on the ronchi test.
The outer 1" or so of the mirror still needs alot more polishing as well (I would guess around 10 hours left).
Question is should I do anything now to try to deal with it or just keep polishing? My opinion is that the edge will come into line as the polish progresses but I can't say I 100% sure of this.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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That's typical (the outer edge) and it's what I fix with the final polishing prep (the 20-30 minute sessions in spin polishing at the neutral spot). It pushes the spherical center out to the extreme edge, eventually. Keep polishing until you're done polishing out, then think about what to do next, is my advice.
Best,
Mark
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