Mirror+making+and+optical+testing

**__Interferometry__**
Our test methods since the early 90's have been Focult, Couder, Ronchi and star testing. These methods have served us well, and we will still use them (especially star testing for final confirmation) to get the mirrors figured. We have now entered the world of Interferometry. This method of testing has now helped us make better large and fast mirrors in the final testing stage. Again, star testing is always used to confirm if the mirrors are good, or not! We do use the interferometer for production. This is contrary to normal production techniques in the amateur world, but we have developed a way that is efficient and accurate to polishing, figuring and testing the whole surface using the interferometer. See the section on Test Equipment below. We still use the knife edge test for ROC testing and keeping an eye on roughness. This cannot be avoided.

=Test Equipment= Some information on the latest test equipment used for making the mirrors is given in the link above.

=Foucault Tester= We used a Foucault test for many years as the prime instrument for making mirrors. We still use it, but mainly for qualative inspetion and radius of curvature testing at the vertex for OpenFringe configuration.

=__Testing Optical Flats__= []

__Old mirror making web site__
http://members.aol.com/DATSCOPE/mirrormake1.html

__Mirror making, things to remember__
(xls spreadsheet procedure download here!)

Useful web links. Need to look at this in detail yet:- http://www.atmsite.org/contrib/May/grind/ http://www.atmsite.org/contrib/May/grind/polish.htm

Things to remember when making a large thin mirror working face up using plate galss. From experience in making mirrors. It is easy to forget!


 * Keep a log, notes and images, of what you are doing at each stage. Make notes of how long you have spent grinding and polishing etc. Use a tick list (yet to be attached to this wiki) to check each stage.
 * Get the blank water cut out. This saves time and money. The cost of water cutting as relatively cheap. £40 for a 20 inch.
 * Bevel the edge. This can be done during water cutting at most places. But does not take long by hand with a stone and water. Ensure you use a fine stone to complete the bevel. A rough stone will give a rough edge were small chips of glass can come off during fine grinding and polishing and cause scratches.
 * Ensure the table to work on is flat. No central hole. Thin glass will flex andrebound in the hole. We have seen this.
 * For ALL hogging, fine grinding, polishing and figuring rotate the mirror often to prevent astigmatism. **This is the single biggest thing you can do to prevent astigmatism**.
 * Flatten both sides of the blank to prevent print through. Plate galss is not flat enough. Use at least a 50% tool to flat. 240 grit is enough.
 * Hogging out can be done with very cheap aluminium oxide from places like Machine Mart. This is used in sand blasting guns that are used to produce a smooth surface on metal.
 * Use a 50% (max 55%) tool to hog out. A bar bell weight works well, but ensure the central boss does not touch or ware when the outer edge wears down. We had a problem when we used the same tool to make three 20 inch mirrors.
 * Use the Sun (with care) to check the focal length. Wet the mirror slightly.
 * Use a straight edge to check the sagitta. Take a number of samples and average (discard readings that are way out) to get the sagitta.
 * Stop short with the bar beel weight of the required sagitta. Use the glass tile tool to get to the required sagitta. This way you will end up with a better figure without going too deep.
 * Use complient carpet that is quite thick. Bathroom carpet with no pattern works well.
 * Working temperature. Stay as close to ambient as you can. Water out ofthe tap is colder than the glass. You will get a turned edge. Use bottled water stored in the smae room as you are working (see below).
 * Use cheap bottled water (15 pence/bottle) from supermarkets. It is filtered. We have seen less sratches.
 * Stay clean. Keep all work surfaces clean. USe plenty of cheap kitchen roll to clean and mop up. Clean you nails and wash your clothes between grits. Remove watches etc.
 * Keep grits in separate bottles/bags etc. Keep grinding and polsihing agents in separate cupboards from each other.
 * We have been using metal washer tools for many years but have had some recent seriouse problems using them. The main problem has been caused by using old washer tools that have the approximate curve on the mirror after hogging out. We think the tools have been oversized and that they may have astigmatism or other defects that take too long to conform. We have more recently be using glass tile tools. These are easy to make and conform quickly. They allow fine grinding to be taken much further to at least BM303 (15 micron). We have sen no scratches.
 * Ensure the WHOLE surface is fine ground with no pits left over from previouse grade. We have made this mistake and it has caused a lot of grief. Pay particular attention to the edge. Pits can easily be missed. We have had a few mirrors were the outer 2 inches or so were not completed properly. This caused a lot of time to materials to be wasted.
 * Grinding and polishing strokes. If 50% tool then need to work the edge enough or else it will fall behind rapidly. Mix the stroke, but do work with the tool overhanging the edge by 1 inch for 50% of the time.
 * Sherometers can not be trusted!! They will give you a good feel for depth of curve but that is it. Small base lines will sample small sections of the mirror. Large baselines cannot easily test the edge of the mirror. They can lead you into a false sense of security. The middle may say that it's curve is too long, leading you to belive that the middle is HIGH. It might not be. It may be a low zone with a longer radius of curvature. We have seen this and other examples.
 * Us the pencil test a lot in fine grinding. This is the most useful test for fine grinding before polishing to ensure the mirror has a regular sphere of revolution across the WHOLE surface. Use either a pencil or an indellible marker. BUT, care is even required here. If the tool is small (50% say) then it can be difficult to ensure that you are moving the tool evenly across the surface to bring out the surface irregularities.
 * Look at the mirror at a shallow angle with a bright light source. This can show irregularities in the later grinding and early polishing stages.
 * Using a laser pen to test for pits. Thus is an excellent tool. Ignor dust particles. The laser will not be seen easily on a good polished surface.
 * Look at the mirror face on and look at the reflected surface. This can show edge defects etc.
 * If after 1 hour (not any more) of polishing there are zones that are not polishing at all, then it is probably a fine grinding problem. Go back to fine grinding and check you technique.
 * Pre polsihing laps using BM303??
 * Polsihing laps
 * Star laps
 * Size of polsihing laps
 * Making pitch laps
 * Making pitch sqaures
 * Preforming the tool to the curve (filler etc)
 * Hot water bath for bedding in laps
 * Pitch hardness/type
 * Cutting facets with knife
 * Making micro facets with a net
 * Pressing polishing laps
 * Testing methods (focult, couder mask, ronchi, star testing)
 * Test equipment. Knife edge, light sources, set up, taking images of the tested mirror
 * Test stands and angle of the mirror during testing. Straps, bubble pack, wiffle trees
 * Data reduction. Milles, Star test
 * Mirror support
 * Secondary flat
 * Collimation

__Mirror making seminar report__
Report on the mirror making seminar at the AAC back in January 2003. The purpose of the meeting was to discuss the way forward for the 42 inch BVC blank that they had purchased.



__Corrector lens web page__
http://scopes.wikispaces.com/Coma+Correction
 * Ian Bakers designs:-**

http://www.telescope-optics.net/catadioptric.htm
 * Other:-**

__Table of grit sizes__

 * **Mesh** || **Decimicron** || **M** || **Size** || **Elutriative** || **W** ||
 * ||  ||   || **(microns)** || **time** || **(Garnett?)** ||
 * **60** ||  || **M60** || **293** ||   ||   ||
 * **80** ||  ||   || **185** ||   ||   ||
 * **100** ||  || **M100** || **148** ||   ||   ||
 * **120** ||  ||   || **123** ||   ||   ||
 * **180** ||  || **M180** || **86** || **1 min** ||   ||
 * **220** ||  ||   || **73** ||   ||   ||
 * **280** ||  ||   || **39** ||   ||   ||
 * **320** ||  ||   || **37** ||   ||   ||
 * **400** ||  ||   || **34** ||   ||   ||
 * **600** ||  ||   || **25** ||   || **W2** ||
 * **700** ||  || **M302** || **22** || **5 min** ||   ||
 * ||  ||   || **20** ||   || **W3** ||
 * **850** ||  || **M302 1/2** || **17** || **10 min** ||   ||
 * ||  ||   || **16** ||   || **W4** ||
 * **1000** ||  || **M303** || **15.5** || **20 min** ||   ||
 * ||  ||   || **14** ||   || **W5** ||
 * ||  ||   || **12** ||   || **W6** ||
 * **1300** ||  || **M303 1/2** || **11** || **40 min** ||   ||
 * ||  ||   || **10** ||   || **W7** ||
 * **1800** ||  || **M304** || **8** || **60 min** || **W8** ||
 * ||  ||   || **6** ||   || **W9** ||
 * **3000** ||  || **M305** || **5** ||   || **W10** ||
 * ||  ||   || **4** ||   || **W11** ||