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10" grinding

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#1 danjones

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Posted 10 January 2013 - 11:29 PM

i'm grinding a 10" mirror. most of the hogging/roughing is done i think. just a bit more to do. as you can see in the pic, the meter reading at center of mirror is 0.044. if this miirror were divided into zones going toward the edge, the zone closest to the center part reads 0.043, then 0.042 and lastly 0.041 at the edge. my goal is 0.046. I'm using a full sized plaster/tile tool. the sharpie test indicated great contact, the marks being worn away almost immediately with one revolution of grinding around the mirror. i'm not sure how to proceed for correction. the tool obviously has the same curvature, so my thoughts with continuing COC strokes, TOT, will produce the same results but shallowing the fl. i know my goal is to make the sag 0.002 deeper then match the rest of the curve to that. I'm thinking MOT with a different stroke might resolve this to correction. Lend me your feedback.

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#2 danjones

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Posted 10 January 2013 - 11:33 PM

here is the tile tool. all the marks were worn away consistently after just one grinding revolution around the mirror.

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#3 Arjan

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Posted 11 January 2013 - 04:18 AM

How much is the focal length off, have you tried this? Just wet the mirror and project the sun or a flashlight. In the second case you have to divide by two...
Even with 360 grit you will change the focal length up to a couple inches.

#4 ed_turco

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Posted 11 January 2013 - 11:43 AM

I'd go with MOT and 1/3 stroke to even out the curve, and while it might lengthen a bit, it will surely deepen over time. I am a believer of nailing the fl on #80 and getting a decent sphere. The mirror then can be used MOT and TOM alternatively within a finer grade, and it always comes out right for me. Remember, the edge needs as much care in grinding; this TOM you are forced to do will get that edge nice and clean.

I've seen too many mirrors by amateurs that are not fully polished at the edge. Dam shame with the price of aluminizing what it is.

#5 John Carruthers

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Posted 12 January 2013 - 03:25 AM

My Duran 10" F6 lost about an inch fl with each grit until the polishing stage.

#6 KerryR

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Posted 12 January 2013 - 11:53 AM

...I am a believer of nailing the fl on #80 and getting a decent sphere...

...I've seen too many mirrors by amateurs that are not fully polished at the edge...


I added a bit to the f.l. when grinding my 6" f5, following the advice in texts that stated the f.l. would shorten during polishing. I haven't found this to be the case-- it hasn't budged a bit, despite all polishing MOT. So, I'd suggest nailing the f.l. at 80, and maintaining it with MOT and TOT to the end, too. I wish now that I had done mine that way.

re: edge polishing-- I think it may be helpful to frequently re-distribute the grinding medium between new abrasive charges. It seems the edges tend to push the grains towards the middle, an effect compounded by a slightly deeper center. Though I ground each grade for an hour or more each, I'm finding the edge VERY slow to polish out. It's distinctly possible that I need better distribution much earlier. At this point, I'm not 100% certain my edge is going to polish out, and I may have to backtrack to fine grinding. So, watch that edge!

#7 Gary Fuchs

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Posted 12 January 2013 - 12:32 PM

The "grazing" light test described here might help to see if the grinding is even.

Gary

#8 ccaissie

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Posted 12 January 2013 - 12:57 PM

try marking the tiles and then work the tool and mirror dry. Harsh, but it will clearly show the contact.

The readings say it is hyperboloidal, so as you get close to R, shorten strokes/alt. You can do a lot of glass removal at your next grade once you know where you are and where you're going.

C

#9 Arjan

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Posted 12 January 2013 - 04:10 PM

That test described by Bob May i salso called red-out test. It also tells you when you're ready for polishing. See picture below.

By the way, I don't think that #80 will ever get you a sphere, because the grit is too large.

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#10 danjones

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Posted 14 January 2013 - 11:25 PM

I was having an issue getting a sphere, the meter readings differing towards the outer 2/3 of the mirror. Tonight, I did what Ed said, put MOT doing 1/3 strokes (I hate losing all the good grit in the tiles), and the spherometer now reads the same everywhere on the mirror. Finally! But, the FL is just a hair long. I need the spherometer to read 0.046 and I have 0.043. I'm using 80 grit. I'm guessing the next step is to grind a bit more with MOT with short strokes til reach target, then move down the grit scale alternating MOT and TOT.

#11 Arjan

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Posted 15 January 2013 - 03:13 PM

Have you measured focal length yet? I still think you will change it with finer grits too.
How deep do you think the pits are? For #80 this surely is more than 0.001...

#12 danjones

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Posted 19 January 2013 - 01:09 AM

thanks arjan. i will be using the long stick focal lenght test this wknd and looking at the pits. magnifying glass good enough for looking for pits?

#13 StarDusty

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Posted 19 January 2013 - 05:02 PM

Once I reach my target sag and have a nice uniform surface appearance I move on to the next finer grit. I continue to monitor the sag through each grit but really don't see much movement in the sag once beyond 220 grit.

Since I generally build the telescope after making the mirror I don't worry too much if I am a little long or short especially with a Newtonian telescope. I just adjust my OTA dimensions as needed.

#14 Arjan

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Posted 20 January 2013 - 02:47 AM

You can also use a 10mm eyepiece in reverse as magnifying glass.

I usually move to #180 when sag is approximately OK and only then focus on getting a nice sphere. Once sufficiently sperical I wet the mirror and use a flashlight to measure F. Use longisch MOT or TOT to shorten resp lengthen.

#15 ccaissie

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Posted 20 January 2013 - 08:29 PM

A segmented tool like yours will help distribute the grains more evenly, and you'll get a truer sphere, especially towards the end. Spend the extra time at each grit to make sure the pits are gone...time well spent. Redoing it or regretting spending an extra half hour is sad.
Last grits, really overdo the time spent and you'll have an easy time polishing.

lookin good






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