40 replies to this topic

[bdemario3]

Hello everyone! Unfortunately, I am getting nowhere in polishing a 14 inch mirror. I have made a pitch lap and managed to polish away all of the visible pits and haze from fine grinding. However, the mirror can not pass the laser pointer test, and has had the same figure (slightly oblate) for as long as I can remember. I have been polishing for around 15 hours of total time, and I have seen no difference in polish quality for probably 8 hours worth of polishing time. I was wondering if there are any other tests apart from the laser pointer test that I can use to determine if I actually am making any progress or not? How long should polishing take for a mirror of this size? I am worried that my recent efforts are not accomplishing anything, and I occasionally add a small scratch to the mirror just from the action of the pitch lap. Thanks.

[Stephen Kennedy]

First of all, what color laser are you using for the test. If it is a green laser, your mirror will not pass the laser test no matter how long you polish it. You have to use a red laser to determine if you have completed polishing. The red laser dot will disappear first in the center of the mirror and last at the edges.

I just finished making an 8" mirror and I think that I spent about 12 hours polishing until I was satisfied with it. Your mirror has (14/8)^2 = 3.06 times as much surface area as mine did so it is probably just a matter of spending more time polishing. This is the longest, most physically demanding and tedious part of making a mirror.

[bdemario3]

Thanks, Stephen. I'm using a red laser, so I shouldn't be worrying about that. At this point, the front surface reflection looks pretty uniform as I go from center to edge. Should I expect the front surface reflection to get dimmer as the polishing gets closer to done? Because, it's seemed about the same for my mirror for quite a while now.

 

I also have most of the rest of the telescope done, so I can quickly load the mirror into its cell for a star test.

Edited by bdemario3, 24 August 2014 - 10:29 PM.

[mark cowan]

 

 

 

Never mind.

Edited by mark cowan, 25 August 2014 - 09:07 PM.

[GlennLeDrew]

I find an inexpensive, plastic 30X illuminated pocket microscope to be the cat's meow for checking on polishing status. It's all I've used.

[Stephen Kennedy]

Thanks, Stephen. I'm using a red laser, so I shouldn't be worrying about that. At this point, the front surface reflection looks pretty uniform as I go from center to edge. Should I expect the front surface reflection to get dimmer as the polishing gets closer to done? Because, it's seemed about the same for my mirror for quite a while now.
 
I also have most of the rest of the telescope done, so I can quickly load the mirror into its cell for a star test.

The reflection from the red laser should disappear from the center of the mirror surface first and then move towards the edges.

[bdemario3]

Glenn, where did you get such a microscope? That sounds perfect.

[kfrederick]

Might wont to try pads you can put them on your grinding tool or on the pitch lap . Working by hand takes time to get a black polish

[Pinbout]

Glenn, where did you get such a microscope? That sounds perfect.

 

 

Radio shack.

 

Polishing pads help speed up polishing more than half.

 

A 10" polishes is 4hours.

 

 

[474747]

Glenn, where did you get such a microscope? That sounds perfect.

They're available from places like Amazon, depending on the quality you want.

I got mine from American Science Surplus: http://www.sciplus.com/

[bdemario3]

I'm doing my best to continue polishing. I have found that I can typically take about 100 strokes back/forth (5 minutes) before I have to pause for a bit. I think the laser reflection gets dimmer, but I'm not sure of anything more than that. I tried to take a picture of the Ronchi grating about 0.2 inches outside of focus. I notice that there are three things: the edge is turned down, most of the mirror surface is flatter than a sphere, except for the center which is steeper than a sphere. What figuring strokes can/should I be doing now to get the mirror as close as possible to Spherical when the mirror has completed polishing? Thanks.

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Edited by bdemario3, 31 August 2014 - 08:29 PM.

[planet earth]

How long are your strokes.

What stroke are you using COC or W stroke or both.

What polishing compound are you using.

Polishing TOT or MOT or both.

Did you micro facet the lap.

Are you cold pressing often.

Does the lap looked glazed.

Have you tried the Focault test with mask to see how far the reading are out. 

This will give you a idea of the general shape of the mirror and also indicate if you need to go back to fine grinding if reading errors are drastic.

A 15 inch should be well polished out with CO in roughly 20 to 25 hours, although it can vary.

My last two mirrors were 8 inch, fine ground with 5 micron and I polished  them for 15 hours each with CO.

Sam

[Pinbout]

 

. I was wondering if there are any other tests apart from the laser pointer test that I can use to determine if I actually am making any progress or not?

 

Swayze showed me to hold the mirror up toward the sun at a grazing angle and see if there is a white haze where its not fully poslished out.

 

Edited by Pinbout, 01 September 2014 - 01:53 PM.

[Tom and Beth]

Like Pinbout I use the Sun.

 

No more haze.

 

@bdemario3

  That Ronchi image is showing a fairly rough surface

What does your lap look like? Does it have a Glaze buildup?

[Stephen Kennedy]

I have only made one 8" mirror but from rough grinding throughout fine grinding I always made sure the mirror passed the "sharpie" test before moving on to the next abbrasive and never had any problems achieving and maintaining a spherical surface through the end of polishing. 

 

One mistake I did make was not testing the sagitta during fine grinding with mostly MOT and ended up with a F6.5 mirror instead of the intended F7.5 at the  end of polishing.  I tried polishing with TOT for several hours to lengthen the focal length but was unsuccessful.  It appears that polishing just did remove enough glass to change things like focal length.  I decided that instead of going back to grinding I would live with the 6.5 focal ratio.  If your mirror surface is still mostly flat when you are polishing, you may find that it will be impossible to polish it into a sphere and will have to go back to grinding to achieve a sphere which is required for figuring.  I realize that is a step you would not want to take and I do not reccomend it until some of the more experienced ATMs on this forum think it is your only alternative.

 

Steve 

[bdemario3]

Here's a picture of the pitch lap. I've been doing some polishing with a slight overhang (left/right), which caused the inside-focus Ronchi lines in the center of the mirror to bow slightly inward, indicating overcorrection. I've tried to do the Foucault test, and it seems that the zones differ by only around 0.1 inch.

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[Pinbout]

after trimming the polisher, i would just work on the edge for 10mins at a time. really fast strokes, MOT only 1/2" overhang, to turn the edge back.

 

cold press a star of wax paper in the middle of your polisher where your hole is. by the time your edge is fixed the middle hole will be fixed and the polisher will be worked back to full contact.

[Tom and Beth]

Nothing in your lap looks bad.

 

Just a suggestion, but this is a great time to introduce micro-facets to your lap. An onion bag from the grocery bag (as an example) pressed between the lap and mirror.

Doing this by hand, you'll have popeye forearms in no time ;D

[planet earth]

I suspect the lap may be too hard, or not enough cold pressing making little contact in the centre or the stokes were too short causing the oblateness for such a long period.

10 to 15 minutes of polishing (5 minutes spells each with cold pressing) with a  longer stroke MOT should noticeably deepen the centre checking after with the Focault test.

Forget about turning the edge, you need to deepen the centre to see whats going on.

The only other way I can think of is to use a smaller polisher to deepen the centre.

Then there's the question of, did you use the sharpy test when fine grinding?

Sam

[bdemario3]

So, I'm a bit confused by what is meant by 'surface roughness'. I understand that I am not done with polishing, so will the issue that you see in the Ronchi test go away by the time the mirror has been fully polished?

 

I came across this which seems to offer advice to my specific condition: http://www.loptics.c...ate/oblate.html I am going to try and polish mirror on top with some left/right offset, to hopefully reduce the oblate sphere to a sphere.

 

I doubt my pitch is too hard, since the pitch is gugolz 64, and I polish in a house which is kept at 76 degrees for now.

 

Will post more updates

[Stephen Kennedy]

I think that you have to keep in mind what I found out in making my mirror is that the polishing process, even if continued for hours, does not remove very much glass.  If you completed fine grinding with a very smooth surface than the remaining roughness should polish out.  However, if the surface was insufficiently smooth when you went from fine grinding to polishing, the roughness will consist of more glass than can be removed by polishing. 

 

I do not see how an oblate spheroid could have passed the sharpie test during fine grinding.  If you started polishing an oblate spheroid that was present at the end of fine grinding that is likely what you will end up with no matter how long it is polished.

 

I have the greatest respect for Mike Lockwood but I am not sure I can agree with the following statement in his essay.

 

"The surface area of the edge of the mirror is FAR less than the area of the center."

 

We can take as an example a mirror with a diameter of 300 mm and consider a 10 mm ring around the outermost part of the mirror as the edge and a disc 100 mm in diameter at the middle of the mirror as the center.  To calculate the precise areas of the center and the edge of this mirror would require the use of integral calculus but a rough estimate of the relative areas can be obtained as follows:

 

Area of Center of mirror  = pi*r^2 = 3.14 * 50mm^2 = 7,850 mm^2

 

Area of edge of mirror = 2 * pi * r * d  =  2 * pi * 150 mm * 10 mm = 9,576 mm^2

 

There seems to be much more surface area in the edges of the mirror than the area around the center of the mirror.  This may be why a TDE is so damaging to the images the telescope produces but a hole in the center of the mirror such as in a Cassegrain does not cause noticeable problems.

[Pinbout]

 

So, I'm a bit confused by what is meant by 'surface roughness'.

 

after polishing and even during figuring. you can see the surface roughness with a knife edge.

 

several things wrong with this mirror but I posted it 'cause of the roughness.

 

 

 

it can be from  too hard pitch, short fast strokes or poor contact between the polisher and mirror.

 

it's polished in, not ground in. good thing is since its polished in it can be polished out.

 

re-press the the polisher, slower longer strokes will easily get rid of the roughness. but I fixed the edge first as well as the overcorrection, then smoothed the surface.

 

here's the pic of the polishing pads I want to post earlier.

 

Edited by Pinbout, 07 September 2014 - 08:48 AM.

[soldevilla]

That mirror does not look bad. Work with the polish very smoothly, without any pressure, as if skating on ice, and the surface will become smoothed magically.

I have polished some hundred mirrors. There are a few basic rules that must be followed scrupulously:

You can never switch to a finer abrasive until the thickest has done its job. A fine abrasive never solves the error of a coarse abrasive.

Wet the mirror allows you to check the focus. But final focus is achieved with thicker abrasive; thin abrasive only soften the engraving produced by the coarse abrasive.

You can never change abrasive until it has disappeared all traces of the "engraved" produced by the coarse abrasive. An eyepiece is a good magnifying glass to check that the edge and center of the mirror look the same matte.

Polishing can not begin until the form is exactly spherical. Two minutes polishing is enough to produce an image on Foucault to verify this.

Polishing varies only very slightly the form. Moving from spherical to parabolic. Polishing never solved a previous problem.

The surface quality depends of the times that the polisher passes over the mirror. A too much pressure work may faster the polishing, but the softness of the form suffers greatly.

There is no more wonderful magic thing that to behold as some work with polisher doing ever going to change the shape of the mirror. Get him!

[bdemario3]

I did some chordal strokes with the mirror on top to deepen the center. Then I switched to some longer normal strokes to try and spread correction to the edge and get the mirror spherical. I took Foucault knife edge readings:

 

effective radius of zone, zone curvature relative to center

 

1.0", 0

4.0", -0.016"

4.9", -0.081"

5.8", -0.125"

6.6", -0.158"

 

Am I in trouble, or what? I put the data into figureXP and it seems that I am at a conic constant of 1.348 which means that getting to a parabola would require moving about twice as much glass as getting to a sphere. Because I am not yet done with polishing I would like to get to a sphere first, this time without turn down edge or oblate condition. If I keep using the normal stroke, is it likely I will return to a sphere?

[Pinbout]

Post a new ronchi gram to compare to your 1st image.