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Should I polish this?

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#1 Giorgos

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Posted 24 May 2019 - 12:06 PM

Hi all!

 

Should I polish this fine ground mirror? There is a 3mm pit on it because of an accident but I guess I can live with it. I just want to know whether this pit will do any harm during polishing (scratches, ripple on surface etc). Please do not suggest to regrind with #80 carborundum! I'd rather make it a paperweight grin.gif  It is full thickness (2.5cm thick) 150mm diameter plate glass (6") approx. f/7.5. I was aiming to f/8 but I overshot it a little.

Thank you in advance for your kind replies!

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Edited by Giorgos, 24 May 2019 - 12:07 PM.

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#2 Stephen Kennedy

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Posted 24 May 2019 - 01:00 PM

If it is at all deep, further grinding would require the removal of so much more glass that it really would not be worth the effort.  I do not think it will have a noticeable impact on performance of the mirror since the area of the pit is so small compared to the entire surface area of the mirror.


Edited by Stephen Kennedy, 24 May 2019 - 01:01 PM.

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#3 Richard O'Neill

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Posted 24 May 2019 - 01:36 PM

 If the hole's edges are sharp they could break off during polishing, get embedded n the pitch and scratch the mirror face. If so I think it best to clean it up by twirling a small abrasive covered pencil eraser head around the edges before proceeding.  


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#4 TOMDEY

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Posted 24 May 2019 - 03:28 PM

 If the hole's edges are sharp they could break off during polishing, get embedded n the pitch and scratch the mirror face. If so I think it best to clean it up by twirling a small abrasive covered pencil eraser head around the edges before proceeding.  

Yep, that little divot is just a minor cosmetic. At work, we indeed did similar to what you recommend there, the shop guys called it "graying out the oysters" Our PMs ranged from one to four meters. I don't think I ever saw a mirror with no scratches, digs or chips. We ~mapped~ their locations, and that went with the delivery report. The only people who demand perfect cosmetics are --- amateurs! Conversely, the professional customers demand meticulously-certified great wavefront... of the entire telescope, all fields, orientations and use configurations!   Tom


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#5 jdupton

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Posted 24 May 2019 - 05:02 PM

Giorgos,

 

   As others have already said very well here, it is perfectly OK to move forward. Slightly abrading the edges as Richard O'Neill says is a good remedy against tiny chips causing scratches later in fine grinding.

 

   The only additional caution I would suggest is to be wary of the pit when you get to polishing and figuring. It will be best to always make sure the divot in positioned under a channel in your polishing lap when pressing (cold, warm, or hot). You only want to avoid having the chipped area impressed into the lap as a raised nib. That will tend to cause havoc when figuring. So long as it is under a channel when pressing, you'll be fine in all stages of finishing the mirror.

 

 

John


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#6 Lognic04

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Posted 24 May 2019 - 06:13 PM

A pit is a pit, who cares! Same with scratches really. I cringe at the idea of people ruining their 20 hour polish and going back to fine grinding because they got a scratch.

Grind/polish more, worry less :)


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#7 TOMDEY

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Posted 24 May 2019 - 10:24 PM

Ummm... I also ~did a study~ to image and quantify cosmetic flaws of components, sub-assemblies, and entire optical systems. When someone asks, "Well, how much scatter is there?" --- that's a loaded question! That leads to things like TIS (Total Integrated Scatter), BRDF, (Bidirectional Reflectance Distribution Function)... a whole Pandora's Box of theory and metrologies. Here are a couple of figures from one of those studies that I did... as "independent research" It was very quantitative!

 

The take-away is that a mirror can appear to be filthy and still function wonderfully.

 

Exception: Coronagraphs (as in planet finders)... must be meticulously clean and (nearly) blem-free, one part in a billion is desirable. Just talk when looking at it --- and you've ruined it!    Tom

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Edited by TOMDEY, 24 May 2019 - 10:27 PM.

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#8 ed_turco

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Posted 25 May 2019 - 01:10 PM

Worry not one second about that pit; it doesn't affect your figuring the glass and or enjoying the view.


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#9 ckh

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Posted 25 May 2019 - 04:53 PM

What about the pitch that conforms to the pit? I suppose it just breaks off and finds its way to a channel or gets smushed because its tiny? Or maybe it gets stuck in the pit which seems OK since it will break when you start polishing.  I believe you Ed, just wondering what happens.



#10 dogbiscuit

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Posted 25 May 2019 - 07:02 PM

I've never worried about a pit or bubble pressing a raised spot in the lap. No trouble from that.

But those holes can develop turned edge and other strange anomalies at their edges, some visible directly by eyeball.  These things can cause a 3 mm wide pit to be a 10 mm wide defect on the surface.  Polished into a bubble once and had those things.  After some hours of polishing, the bubble filled with swarf and the anomalies disappeared.  If I get another one of those things I will fill it with pitch and continue work.  Maybe letting the lap press into the hole would allow it to shave off at first stroke and soon be a level surface across the hole.



#11 TOMDEY

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Posted 25 May 2019 - 08:18 PM

Sometimes when I'm mowing the lawn, I notice a too-big rock and stop the tractor and toss it into the woods (the rock, not the tractor). I mow about five acres and need/use an actual real tractor to efficiently do that. Around here, a too big rock is about the size of  coffee cup or saucer... but tougher and more abrasive. For divots, big 4WD tractors have tires that can drive right over chuck and rabbit holes without even hiccoughing. I mowed one field today, just before the thunderstorm... perfect!

 

I don't know what our shop guys do about keeping pitch outa divots. Believe they press the giant laps on ~break-in bodies~ while other laps are polishing. Big, industrial shops can afford to be efficient that way. It's all about getting stuff in one door and out the other as efficiently as possible. The giant mirrors still take quite some time --- but are worth the effort.    Tom



#12 ed_turco

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Posted 26 May 2019 - 03:32 PM

What about the pitch that conforms to the pit? I suppose it just breaks off and finds its way to a channel or gets smushed because its tiny? Or maybe it gets stuck in the pit which seems OK since it will break when you start polishing.  I believe you Ed, just wondering what happens.

All those things can and will happen but none have affected the final finish of the many pieces I have worked on.



#13 ccaissie

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Posted 27 May 2019 - 07:23 AM

When putting a tool or lap to a mirror,  good to move it around gently at first with as little pressure as possible...even lifting it up as you make a couple careful strokes...

 

This gives you a chance to lift it off if you hear a noise, and with a pitch lap it takes care of maybe a bit of pitch that is protruding from a pit, or a raised sliver from the edge of a facet when pressing....

 

You're good to go.




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