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Looking for assistance in figuring 6.25in f2.88

ATM Mirror Making
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#1 oscaroboto

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Posted 28 February 2021 - 03:42 AM

Hello, I've been working on figuring a 6.25in f2.88 mirror for years. I've gone back started over at a sphere 6 or 7 times now, actually I've lost count, it may be more than that. Anyhow, I always seem to end up either way over corrected or with various kinks that I can't seem to get out.

 

Currently I have a kink at the 85% zone, that according to Mel Bartels Ronchi overlap program is showing -0.7 undercorrection at the 85 zone and overall under correction of -0.5.

 

Any help is appreciated.

 

Here are some pictures

I'm using 133 lines per inch grating

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#2 Pinbout

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Posted 28 February 2021 - 03:51 PM

with a full size tool, MoT, use the edge of the tool to scrub over those areas you need to dig. 

 

kind of how I'm taking down this hill, but more localized on the kink... and then a little more in your middle zones

 

but get rid of the kink first

 

https://www.youtube....jPcDoB&index=11


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#3 oscaroboto

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Posted 01 March 2021 - 01:18 AM

Thanks ISS!

 

I got a sudden idea this morning of doing really tiny W strokes about 0.5in side swing, and its looking better. I'll post a photo when I can.



#4 ccaissie

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Posted 01 March 2021 - 07:24 AM

Edge looks good.  Whatever you try, concentrate action inside edge.  Sub laps can do just that....work a zone and not drag the lap over the edge.  The only way, in my hands at least, to get below f/4.



#5 DAVIDG

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Posted 01 March 2021 - 02:17 PM

 Your fighting the fact the you are inferring the figure by looking at a  Ronchi pattern. The problem is that method for such a fast mirror has sources of error to understand what the exact figure is and the fact that a F2.8 mirror has an extremely small error tolerance to be just a 1/4 wave.

    I recommend you switch to testing  with double pass autocollimation. This will  turn figuring the parabola into what you do to figure a sphere.  There is nothing to measure and the test SHOWS you exactly were the figure departs from the parabola your trying to figure. The errors read just like they do when figuring a spherical mirror with a Ronchi screen or knife edge. So when you achieve three dead straight Ronchi bands you now have the parabola you want. A hill, hole ,turned edge  or under or over correction  in the parabola,  tests  just like for that of sphere but those errors are in reference to the parabola you want. So you treat them  the same way and want to achieve a figure that tests like a sphere but again when achieve what looks like a perfect sphere it will be the parabola that you want.  Also light is reflected off the mirror 2 time so any errors are doubled so they are easier to see. 

 

                      - Dave 


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#6 ccaissie

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Posted 02 March 2021 - 10:21 AM

 Your fighting the fact the you are inferring the figure by looking at a  Ronchi pattern. The problem is that method for such a fast mirror has sources of error to understand what the exact figure is and the fact that a F2.8 mirror has an extremely small error tolerance to be just a 1/4 wave.

    I recommend you switch to testing  with double pass autocollimation. This will  turn figuring the parabola into what you do to figure a sphere.  There is nothing to measure and the test SHOWS you exactly were the figure departs from the parabola your trying to figure. The errors read just like they do when figuring a spherical mirror with a Ronchi screen or knife edge. So when you achieve three dead straight Ronchi bands you now have the parabola you want. A hill, hole ,turned edge  or under or over correction  in the parabola,  tests  just like for that of sphere but those errors are in reference to the parabola you want. So you treat them  the same way and want to achieve a figure that tests like a sphere but again when achieve what looks like a perfect sphere it will be the parabola that you want.  Also light is reflected off the mirror 2 time so any errors are doubled so they are easier to see. 

 

                      - Dave 

If you have a 7"+ flat available.  Also, the contact of a lap at f/3 asphere is different than a sphere, so some consideration is needed there...I go for smaller laps to handle that contact issue.

 

But yes...do DPAC if possible.



#7 DAVIDG

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Posted 02 March 2021 - 11:21 AM

If you have a 7"+ flat available.  Also, the contact of a lap at f/3 asphere is different than a sphere, so some consideration is needed there...I go for smaller laps to handle that contact issue.

 

But yes...do DPAC if possible.

 With the focal length being under 20" the mirror could be mounted in a OTA  and mounted vertically and then one could use an oil flat and ultra bright LED as light source to do double pass testing. How  I have figured these very fast mirrors is first get them too a clean sphere. then with the mirror on the bottom I place the center of the lap over the 70% of a do a W stroke taking the center the lap to about the 30% zone  and out to the 90% zone. Using double I can monitor the amount of correction and if I need more in outer area for example I move the center of the W stroke outwards and go inward less. 

    The beauty of  double pass is that  it allows me to SEE exactly were the figure departs from  parabola I want just like one does when using a standard Foucault test to see how the figure departs from a sphere, no measuring, just look. It also allows me to quickly determine if the stroke I'm using is making a positive difference or not.

   Having taught mirror making classes for many years I have seen ATM's come to the classes with mirrors  they have tried to figure for years and I do mean years using other test methods and with in a day finish  their mirrors to show a nice null using double pass. The "magic" is that you can see were the figure departs vs inferring via other test methods that also have sources of errors that are not accounted for. 

 

                     - Dave 




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