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6" F3 Mirror and Scope Build

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#51 Aaron_tragle

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Posted 17 December 2019 - 05:21 PM

My target is a sagitta of .135 which yields a F ratio of F2.8, I was advised by Zane to go for a bit past the target for margin of error during parabolization. So maybe stop with the 60 grit when I set .1 sagitta and then start the finer grits.


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#52 dogbiscuit

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Posted 17 December 2019 - 05:22 PM

6 hours with 80 grit to get to 500" ROC.  Wonder why it's going so slow?

Maybe it's not that slow.   I haven't seen a description of the speherometer, so maybe the measured sagitta is across a smaller diameter circle than the 6" mirror.  For example, if the spherometer circle is 3" diameter, a measured sagitta of 0.009"would mean the ROC is 125".

Let's hope it's that simple.

 

 

Other considerations:

Grinding on that machine, or by hand?

What method of stroke and/or overhang?

 

 

Don't like the idea of 40 or 60 grit for a 6" mirror.  Use that to hog central area to sagitta.  Expect edge chipping if it is worked too close to the edge.


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#53 DAVIDG

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Posted 17 December 2019 - 05:33 PM

 If  you make a simple Sine table which  is nothing more then two pieces of wood and a  hinge  which is much easier then a grinding machine and buy a 3" diamond edge coring bit from THK diamond tools on Ebay for $11 you can cut the f/3 curve on both the tool and the mirror in under an hour. I wrote an article in Sky and Tel a couple of years ago on how it is done. Here is link to one of  the threads here on Cloudy Nights how it is done https://www.cloudyni...ration-part-ii/

 Here is another thread. https://www.cloudyni...rve-generation/

 

  Here is link to THK store on Ebay and 3" diamond coring bit. https://www.ebay.com...t4AAMXQATlRaE3N

 

 

                    - Dave 


Edited by DAVIDG, 17 December 2019 - 05:36 PM.

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#54 Aaron_tragle

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Posted 17 December 2019 - 06:04 PM

6 hours with 80 grit to get to 500" ROC.  Wonder why it's going so slow?

Maybe it's not that slow.   I haven't seen a description of the speherometer, so maybe the measured sagitta is across a smaller diameter circle than the 6" mirror.  For example, if the spherometer circle is 3" diameter, a measured sagitta of 0.009"would mean the ROC is 125".

Let's hope it's that simple.

 

 

Other considerations:

Grinding on that machine, or by hand?

What method of stroke and/or overhang?

 

 

Don't like the idea of 40 or 60 grit for a 6" mirror.  Use that to hog central area to sagitta.  Expect edge chipping if it is worked too close to the edge.

My spherometer has a 3.56in diameter, is there a way to convert my sagitta measurement from my spherometer to what it would be with a 6in spherometer?


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#55 davidc135

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Posted 17 December 2019 - 07:00 PM

Roughly, square the ratio of diameters times your reading. So; x2.84.  David


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#56 dogbiscuit

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Posted 17 December 2019 - 07:26 PM


My spherometer has a 3.56in diameter, is there a way to convert my sagitta measurement from my spherometer to what it would be with a 6in spherometer?

To reasonable accuracy   r^2 / measured sagitta / 2 = ROC

 

Radius r of your 3.56" diameter spherometer = 1.78"

r^2 = 3.1684


 

So 3.1684 / measured sagitta / 2 =  ROC

to take one step out of that, lets divide r^2 by 2 and then for your spherometer it's just

1.5842 / measured sagitta = ROC

 

 

For your target ROC of 36" sagitta is 1.5842" / 36" = .044"  (1.12mm)

 

 

 

 

How 'bout that.  Grinding sped up significantly.  lol.gif

 


Edited by dogbiscuit, 17 December 2019 - 07:33 PM.

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#57 Aaron_tragle

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Posted 17 December 2019 - 07:59 PM

Well I've got some bad news, I somehow flattened my mirror. It started with a sagitta of around 45/1000th of an inch and now has a sagitta of about 17.75/1000th inch. I am going to try doing this by hand for a bit just so I can get an idea of what I am doing.


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#58 Aaron_tragle

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Posted 17 December 2019 - 08:28 PM

 

To reasonable accuracy   r^2 / measured sagitta / 2 = ROC

 

Radius r of your 3.56" diameter spherometer = 1.78"

r^2 = 3.1684


 

So 3.1684 / measured sagitta / 2 =  ROC

to take one step out of that, lets divide r^2 by 2 and then for your spherometer it's just

1.5842 / measured sagitta = ROC

 

 

For your target ROC of 36" sagitta is 1.5842" / 36" = .044"  (1.12mm)

 

 

 

 

How 'bout that.  Grinding sped up significantly.  lol.gif

 

 

That math makes sense to me, but I am confused as to what ROC is. ROC x 2 = FL, got it. So I am aiming for F2.8 which means my ROC needs to be ~33.1 at the end of 5um. 


Edited by Aaron_tragle, 17 December 2019 - 08:50 PM.


#59 MGAR

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Posted 17 December 2019 - 08:30 PM

Well I've got some bad news, I somehow flattened my mirror. It started with a sagitta of around 45/1000th of an inch and now has a sagitta of about 17.75/1000th inch. I am going to try doing this by hand for a bit just so I can get an idea of what I am doing.

Sorry to hear that but I think your on the right track trying by hand. You'll get a good feel of the grinding strokes.

 

I'm so old school I used a sagitta curve traced on piece of paper and used that to measure my progress. As I got close I put it up on the knife tester and measured how close I was to my F target.

 

Gary


Edited by MGAR, 17 December 2019 - 08:31 PM.

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#60 dogbiscuit

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Posted 17 December 2019 - 08:48 PM

That math makes sense to me, but I am confused as to what ROC is.

Radius Of Curvature.  The distance from the mirror's surface to the center of the curve.

That is twice the focal length.   For 6" f:3 mirror the focal length would be 18" so ROC = 36".

 

 

Well I've got some bad news, I somehow flattened my mirror. It started with a sagitta of around 45/1000th of an inch and now has a sagitta of about 17.75/1000th inch. I am going to try doing this by hand for a bit just so I can get an idea of what I am doing.

Those sags measured with the spherometer?   That would mean you started at the target.

 

Are you zeroing the spherometer on a flat surface?
 



#61 Aaron_tragle

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Posted 17 December 2019 - 08:52 PM

Radius Of Curvature.  The distance from the mirror's surface to the center of the curve.

That is twice the focal length.   For 6" f:3 mirror the focal length would be 18" so ROC = 36".

 

 

Those sags measured with the spherometer?   That would mean you started at the target.

 

Are you zeroing the spherometer on a flat surface?
 

I am trying to figure out exactly what is going on here, head is spinning. Doing the math to figure out exactly what I did, but I think that initial 45/1000th of an inch was based on the fact it was a F8 mirror and the sagitta measured by my spherometer when you consider the 2.84 multiplication factor does line up with a ~F8 mirror.



#62 Aaron_tragle

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Posted 17 December 2019 - 09:13 PM

Ok, I cracked the maths! My current ROC is 89.1254 which means the focal length is nearly 4527.57mm which makes the mirror currently F30.


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#63 dogbiscuit

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Posted 17 December 2019 - 09:56 PM

I am trying to figure out exactly what is going on here, head is spinning. Doing the math to figure out exactly what I did, but I think that initial 45/1000th of an inch was based on the fact it was a F8 mirror and the sagitta measured by my spherometer when you consider the 2.84 multiplication factor does line up with a ~F8 mirror.

Ok, I cracked the maths! My current ROC is 89.1254 which means the focal length is nearly 4527.57mm which makes the mirror currently F30.

No. 

Mixing inches and mm is confusing, but I figured  out what you meant.  89.1254" ROC and that would be 2264mm.

then half of that would be the focal length, 44.6" or 1132mm.  About f:7.4.

I guess all that is based on a spherometer reading of about 0.0178".

 

the ratio of mirror diameter to spherometer  diameter that davidc135 mentions as a factor of 2.84 is to convert sagitta measured on the spherometer to sagitta of the 6" mirror.  Actual sagitta of the 6" mirror is 2.84 times what is measured by the 3.56" diameter spherometer.

 

 

Whether the focal length becomes longer or shorter in polishing and figuring depends on methods, and how you go about parabolizing and correcting errors.  It is not certain it will go longer, and if most efficient parabolizing, it will go shorter.  If you spend a long time correcting errors expect more change and who knows which way.
 


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#64 DAVIDG

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Posted 18 December 2019 - 09:27 AM

  At this stage of grinding a spherometer is over kill and can lead to problems. The simplest way to measure the sagitta is to just use a straight edge and  gauge of known thickness that is close to the sagitta you want. You place the straight edge across the full diameter of the mirror and see if the gauge will just fit under it   So you want a sagitta of 0.135", a #29 drill bit has a diameter of 0.136" close enough and will allow for a both quick and accurate way to measure your sagitta without the need to zero your spherometer and doing  the math to determine the sagitta. 

   At this  stage of  just hogging the mirror out, the center of the mirror should always be over the edge of the tool. The goal is to  just hollow out the center so it is  close to the correct sagitta. You will also maximize the effectiveness for your grit and not waste it.  Once you reach that depth you can smooth the curve out with a longer stroke. 

 

                 - Dave 


Edited by DAVIDG, 18 December 2019 - 10:32 AM.

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#65 Pinbout

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Posted 18 December 2019 - 09:41 AM

 

At this  stage of  just hogging the mirror out, the center of the mirror should always be over the edge of the tool.

 

very similar to how Im taking down a hill while figuring.

 

https://www.youtube....h?v=Vr5c3NL9yR4

 

sometimes I think sharpies were invented for ATM'ers  lol.gif


Edited by Pinbout, 18 December 2019 - 09:42 AM.

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#66 MGAR

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Posted 18 December 2019 - 10:11 AM

 You place the straight edge across the full diameter of the mirror and see if the gauge will just fit under it   So you want a sagitta of 0.135", a #29 drill bit has a diameter of 0.136" close enough and will allow for a both quick and accurate way to measure your sagitta without the need to zero your spherometer and doing  the math to determine the sagitta. 

bow.gif 

Great post Dave! Simple is the way to go and good ole Jean Texerau recommends this way as well.  

 

Gary 



#67 Augustus

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Posted 18 December 2019 - 10:28 AM

At this stage of grinding a spherometer is over kill and can lead to problems. The simplest way to measure the sagitta is to just use a straight edge and gauge of known thickness that is close to the sagitta you want. You place the straight edge across the full diameter of the mirror and see if the gauge will just fit under it So you want a sagitta of 0.135", a #29 drill bit has a diameter of 0.136" close enough and will allow for a both quick and accurate way to measure your sagitta without the need to zero your spherometer and doing the math to determine the sagitta.
At this stage of just hogging the mirror out, the center of the mirror should always be over the edge of the tool. The goal is to just hollow out the center so it close to the correct sagitta. You will also maximize the effectiveness for your grit and not waste it. Once you reach that depth you can smooth the curve out with a longer stroke.

- Dave


This is what I’ve been telling him.

#68 Aaron_tragle

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Posted 18 December 2019 - 10:59 PM

Took a day break from the mirror, I plan on using the rest of my 80 grit tomorrow by hand. The rest of the grits arrive Friday or Saturday, I made a tool using a penny and a quarter that is .12" thick as DAVIDG mentioned.



#69 DAVIDG

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Posted 19 December 2019 - 10:45 AM

Took a day break from the mirror, I plan on using the rest of my 80 grit tomorrow by hand. The rest of the grits arrive Friday or Saturday, I made a tool using a penny and a quarter that is .12" thick as DAVIDG mentioned.

 Just remember that if  you used  the full diameter of the penny or the Quarter then your sagitta will be deeper then  what you measured.. This is because the coins will touch at  the edges and there will be a gap under them.. A rod is a  better thickness gauge  like a piece of wire or  a drill bit since it will make contact in only one point with the glass so it will give you a more accurate measurement.  I piece of standard #10 solid wire with the jacked stripped off  so it is bare metal is  9/64" ( 0.135") in diameter so right where you want to be. 

     By the way you said you have been using a spherometer, how are zeroing it and have you checked it against any know radii to make sure that it reads accurately ? 

      Once you get the sagitta hogged out and  your into fine grinding and if you want to get the  focal length right on the money to F2.8 there are simple and accurate ways to it. Simply wetting the mirror with a soapy solution and finding were the Sun comes to focus will get you within  1" or so. When you get down to 600 grit or finer you can rub some clear shoe polish on the surface or flash polish it on some polishing pads for a few minutes. That will make the surface reflective enough to do a Foucault test  to measure the radius of curvature ie 2x the focal length. You can measure the radius of curvature to within 1/4" using that method so the focal length will be within 1/8". So there are simple ways were the Laws of Physics and Optics are on your side to give you accurate results without the need  for complex methods.

 

                  - Dave  


Edited by DAVIDG, 19 December 2019 - 03:58 PM.

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#70 Aaron_tragle

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Posted 19 December 2019 - 08:04 PM

 Just remember that if  you used  the full diameter of the penny or the Quarter then your sagitta will be deeper then  what you measured.. This is because the coins will touch at  the edges and there will be a gap under them.. A rod is a  better thickness gauge  like a piece of wire or  a drill bit since it will make contact in only one point with the glass so it will give you a more accurate measurement.  I piece of standard #10 solid wire with the jacked stripped off  so it is bare metal is  9/64" ( 0.135") in diameter so right where you want to be. 

     By the way you said you have been using a spherometer, how are zeroing it and have you checked it against any know radii to make sure that it reads accurately ? 

      Once you get the sagitta hogged out and  your into fine grinding and if you want to get the  focal length right on the money to F2.8 there are simple and accurate ways to it. Simply wetting the mirror with a soapy solution and finding were the Sun comes to focus will get you within  1" or so. When you get down to 600 grit or finer you can rub some clear shoe polish on the surface or flash polish it on some polishing pads for a few minutes. That will make the surface reflective enough to do a Foucault test  to measure the radius of curvature ie 2x the focal length. You can measure the radius of curvature to within 1/4" using that method so the focal length will be within 1/8". So there are simple ways were the Laws of Physics and Optics are on your side to give you accurate results without the need  for complex methods.

 

                  - Dave  

The penny/quarter tool is just for using a straight edge to get an idea of the sagitta while I am outside grinding. I zeroed my spherometer with the second glass blank that I bought to use as a tool and I'ver verified it using a few windows and a glass plate my neighbor had for some reason. Thanks for the tips about simple and accurate methods for getting my focal length accurate, I'd like this to be close to F2.8 as possible and the end. I just received a foucalt/ronchi tester from Augustus and will definitely be trying the method you described. As always, thank you for the help all.



#71 Aaron_tragle

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Posted 21 December 2019 - 07:45 PM

I switched to hand grinding today with the turn table, I made sure to have the tool stay in the mirror and not overhang but the mirror is still being flattened. The mirror is ~4 thou from flat, I have absolutely no idea what I am doing wrong bawling.gif



#72 MGAR

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Posted 22 December 2019 - 12:58 AM

I switched to hand grinding today with the turn table, I made sure to have the tool stay in the mirror and not overhang but the mirror is still being flattened. The mirror is ~4 thou from flat, I have absolutely no idea what I am doing wrong bawling.gif

Aaron,

 

You should have overhang when roughing out. Preferably mirror on top with 40% overhanging the tool using a chordal stroke. I used a 6" pyrex blank as my tool when hogging and used this stroke with good success.

 

The video on the linked page shows a good example of this stroke. 

 

Gary 


Edited by MGAR, 22 December 2019 - 01:00 AM.


#73 Dale Eason

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Posted 22 December 2019 - 01:03 PM

I switched to hand grinding today with the turn table, I made sure to have the tool stay in the mirror and not overhang but the mirror is still being flattened. The mirror is ~4 thou from flat, I have absolutely no idea what I am doing wrong bawling.gif

Hard to say because I think nobody knows what you are actually doing.  Mirror on top? Tool on Top?  What is your tool?  What is your stroke length and width?


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#74 Aaron_tragle

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Posted 22 December 2019 - 02:16 PM

Hard to say because I think nobody knows what you are actually doing. Mirror on top? Tool on Top? What is your tool? What is your stroke length and width?


I am doing what is in the first part of this video. Tool on top and the tool is a 1 kilo weight with a 3.5” diameter pipe flange. Stroke length is around 3 inches and I do 30 back and forth strokes per minute.

#75 dogbiscuit

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Posted 22 December 2019 - 02:57 PM

TOT with for a 3.5" pipe flange, should be stroking center over center (COC) .

With a small tool TOT stroke center over center. Not much overhang on the ends of the stroke, maybe edge of 3.5" tool goes past edge of mirror about 1/2" at ends of the stroke. That would be a stroke of about 3".  Plenty of weight.  You want to remove glass in the center of the mirror.

 

I thought you had two glass disks.  I think you would find it easier to use a glass disk same size a the mirror  as a grinding tool for your first mirror.

For that use a chordal stroke MOT.

https://stellafane.o... Chordal Stroke

There is a video there to show the stroke.


Edited by dogbiscuit, 22 December 2019 - 03:16 PM.

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