Click here if you are having trouble logging into the forums
Privacy Policy |
Please read our Terms
of Service | Signup and
Troubleshooting FAQ | Problems? PM a Red or a Green Gu.... uh, User
molniyabeer
Confused and Asleep
Reged: 01/08/05
Posts: 2556
Loc: Central Coast, California
|
|
I am polishing my first mirror, an f/8 8", and after about 5.5 hrs of polishing have a nice clean center out to about the last 1" on the rim. The rim has a superficial polish but under grazing light shows the faintest frosting. Polishing stroke is a W with about 1/3 stroke in length and about 1" side to side overhang. I'm using a pretty thin cerium oxide mix (more like milky water than creamy). Polishing sessions are about 30 minutes long with around 20 minutes cold pressing prior to each spell.
I'm not worried about the figure at this point (it shows a classic oblate spheroid on the Foucault tester, by the way...) but am looking for affirmation that I'm more or less in the "normal" ballpark for polishing out.
I'd appreciate any feedback. Thanks.
--------------------
Steve
16" Meade LightBridge (Beowulf)
10" Hardin DSH, StarMax 127mm Mak, PST H-a
Oberwerk 11 x 70 binocs, Tasco 10 x 50 binocs
Santa Maria Clear Sky Clock
Figueroa Mt Clear Sky Clock
|
MitchAlsup
super member
Reged: 08/31/09
Posts: 184
|
|
Texereaux recommends the straightforward 1/3 CoC stroke. He goes on to indicate that the polishing action becomes more efficient near the end of a wet when the friction is almost to the point of squeeking. The subtle side stroke is preventing the edge of the mirror from having as much polishing time on the lap as the center--thus, it lags behind.
Polishing an 8" with rouge should take ~5.5 hours; polishing with cerium, you should have had the whole surface done by now.
But the glass pushers crede: "Grind more--worry less" applies equally well with polishing as with grinding.
Good luck
|
DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
|
|
It takes on the average 1 hour per inch of aperture to achieve a true "black" optical polishes using pitch. To get the edge to polish out, put the mirror on the bottom and alittle edge pressure on the lap.
You also do not want to polish with a thick polishing solution. It is the action of the polishing compound stuck in the pitch that is doing the polish. All a thick solution does is roll around on the surface and stops the wiping action of the pitch and just slows down the polishing.
- Dave
--------------------
Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
|
ed_turco
member
Reged: 08/29/09
Posts: 27
|
|
You could use the same stroke with the tool on top for a few hours and this will clear up nicely.
|
molniyabeer
Confused and Asleep
Reged: 01/08/05
Posts: 2556
Loc: Central Coast, California
|
|
Thanks, all, for the tips. I've done 30 minutes with the lap on top, short strokes with a little bit "W" stroke and it does seem like the edge is coming a bit more clean. I'll the next round as plain CoC. And I'll make sure to keep the cerium thin.
Given that it took me far longer than the "standard" time to grind out the mirror, I'm not really surprised that the polishing is taking a bit longer, too.
Clear skies.
--------------------
Steve
16" Meade LightBridge (Beowulf)
10" Hardin DSH, StarMax 127mm Mak, PST H-a
Oberwerk 11 x 70 binocs, Tasco 10 x 50 binocs
Santa Maria Clear Sky Clock
Figueroa Mt Clear Sky Clock
|
GaryS
super member
Reged: 10/30/06
Posts: 191
|
|
Quote:
I am polishing my first mirror, an f/8 8", and after about 5.5 hrs of polishing have a nice clean center out to about the last 1" on the rim. The rim has a superficial polish but under grazing light shows the faintest frosting. Polishing stroke is a W with about 1/3 stroke in length and about 1" side to side overhang. I'm using a pretty thin cerium oxide mix (more like milky water than creamy). Polishing sessions are about 30 minutes long with around 20 minutes cold pressing prior to each spell.
I'm not worried about the figure at this point (it shows a classic oblate spheroid on the Foucault tester, by the way...) but am looking for affirmation that I'm more or less in the "normal" ballpark for polishing out.
I'd appreciate any feedback. Thanks.
Hi Steve:
A couple of points. First, ignore all rules of thumb. There are many variables and when it's your first mirror,andyou probably don't have them under control well enough for generalities to apply. Okay, here's one rule of thumb that seems to hold true: the first time you think you're done, you're probably only half way done.
Second point: Don't get too hung up on keeping your strokes exactly the same. Some degree of variation is good. You only want to follow the 1/3rd stroke length as an *average*. Plus or minus a little will help you from straying too far from a sphere.
Third, an oblate spheroid is common at the end of polishing. You're right not to worry about this yet though. One job at a time.
Finally, the edge always seems to take forever to polish out. Be patient -- it'll get there. Building a scope from scratch (especially when you make your own mirror) takes, many, many hours. Don't try to save a couple here by stopping short of full polish. The two or three hours you save here will mean next to nothing by the end of the project.
Hope that helps.
Gary
--------------------
Sky & Telescope Contributing Editor
SkyNews Columnist & Blogger
www.GarySeronik.com
A place for stargazing enthusiasts.
|
Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3124
Loc: Northeast
|
|
On an F/8, with same size tool, if your stroke is somewhat 'timid' as a first timer, you'll have quite a bit different shape than an oblate. I can guarantee the edge will look drastically turned with what you're doing.
For polishing out TOT I'd prefer a tool about 7" diameter to maintain the same ROC, and lengthen stroke and increase the speed of the strokes with a med-light pressure. Of course this all goes in the can if the polisher isn't in good contact. I'd also replace the "W" with an "oval". Save the full size tool for parabolizing later. Compound no thicker than maybe slightly watered down milk, as Dave suggests.
Good luck, and have fun.
Mark
--------------------
So many projects, so little time!
|
molniyabeer
Confused and Asleep
Reged: 01/08/05
Posts: 2556
Loc: Central Coast, California
|
|
Thanks again, guys. I've put a couple more hours on it, with some tool on top, and it seems to be coming in. Still the slightest bit of frosting under glacing light but much less than it was. With luck, I may get it fully polished out tomorrow.
Just for giggles, I took a foucogram (sp?) this evening. Here is the result. I could be wrong but I see this as an oblate spheroid with a bit of turned edge. Or am I whacked? The turbulence is from the oven near by. Yeah, I know that you should avoid sources of turbulence for a serious measurement but this was just a quicky shot.
Clear skies.
--------------------
Steve
16" Meade LightBridge (Beowulf)
10" Hardin DSH, StarMax 127mm Mak, PST H-a
Oberwerk 11 x 70 binocs, Tasco 10 x 50 binocs
Santa Maria Clear Sky Clock
Figueroa Mt Clear Sky Clock
|
molniyabeer
Confused and Asleep
Reged: 01/08/05
Posts: 2556
Loc: Central Coast, California
|
|
Update. Ok, after 16 hours of polishing, I am satisfied with the edge and have started correction to bring it back spherical. The extra time, I think, may reflect less than perfect contact from grinding. The figure at the moment is an oblate spheroid. The shadows in the Foucault test look pretty much spot on with several references, so at least I have a good idea where to go from here. Thanks again for the advice.
Clear skies.
--------------------
Steve
16" Meade LightBridge (Beowulf)
10" Hardin DSH, StarMax 127mm Mak, PST H-a
Oberwerk 11 x 70 binocs, Tasco 10 x 50 binocs
Santa Maria Clear Sky Clock
Figueroa Mt Clear Sky Clock
|
|
14 registered and 6 anonymous users are browsing this forum.
Moderator: Don W, Mike I. Jones
Print Thread
|
Forum Permissions
You cannot start new topics
You cannot reply to topics
HTML is disabled
UBBCode is enabled
|
Thread views: 190
|
|
|
|
|
|
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
