30+inch+F3+Secondary+manufacture

The secondary mirror used for initial testing of the 30 inch F3 primary mirror was water jet cut from a large precision flat. This was done to accelerate the project and ensure that the telescope was ready for demonstration at the 2015 Kelling Heath star party.

The minimum size of secondary that appeared to be useful was just under 7 inches. Finding secondary mirrors of around 7 inch minor axis off the shelf is quite rare. Not many manufacturers go beyond this size as they do not make mirrors large or fast enough to use secondary's much above 6 inch. Orion optics UK advertise a 6.6 inch for £805 +VAT. Quotes for larger than this are significantly more in cost. Ideally a 7 inch minimum was what I was looking for. A little extra on size would not be an issue and would allow for some lateral displacement without loosing light from the primary.

As the size of telescope mirrors in the community starts to rise again and this time with much shorter F ratio's, the secondary is going to become an issue. Making large flat mirrors is not easy! A great starting place for secondary sizing is to use Mel Bartels site link below.

[]

A minor axis size of 200mm was finally selected to enable binocular views to be used. This is approximately 26% obstruction which is still a lot less that most SCT telescopes. It would allow for a small roll off at the edge too (if that happened) that could be blacked out. This is a big flat!



When the moon is full and reflected onto the secondary from the primary it nearly fills the 8 inch flat!

Sadly the surface of the secondary mirror that was water jet cut sprung more than I would have hoped. I have had secondary's water jet cut before (4 to 6 inch minor axis from large precision flats) had their surface has not been effected. The surface of this flat had power, astigmatism and other issues. It worked OK at low power but with a total PV error of 2 waves was no good beyond that. A new flat was required before this one would be re-figured. Other wise the telescope would be out of action for quite a while.

Using the 4D PhaseCam 6000 in work I measure the reference flat and 3 areas of the water jet cut flat. The worst end was the one pointing towards the primary. This was confired from the star tests to be the area that was causing the worst effects.

Below is the reference flat taken using the beam expander. I use this reference flat a lot and is nominally 15nm rms surface.

Below is a shop screen of the middle section of the water jet cut flat.

Below is a shop screen of the primary end section of the water jet cut flat. This is the worst section. A large area is falling away to the right. Below is a shop screen of the top section (away from the primary) of the water jet cut flat. The left side is raised up compared to the other end which is falling away.

Testing of the flats would be done initially using a test plate and then when within a wave or so a reference sphere would be used to tweak the surface to a good figure. The flat would be at 45 degrees to the reference sphere. Initial tests used a commercial 200mm sphere of good known quality. This test confirmed the results of the beam expander and the 4D PhaseCam 6000.

The interferometer would be used to analyse the surface and provide an error map to correct the surface. The aim being to figure the flat to bring the sphere back to being good again. I.e. surface errors of the flat would impact on the surface of the sphere. Tests showed that this would work but a larger sphere would be needed. Ideally 10 inches would be better. Luckily Geoff had a 12 inch F3.22 mirror just lying around! We tested it and it was half way between a sphere and a parabola.

You can see the error in the first water jet cut flat in the image below on the laptop screen. Controlling power of the flat is critical to ensure that the flat remains flat and no significant curve is generated when re-manufactured. The test sphere will be rotated and data averaged to remove the majority of test errors. Final figuring tests will be confirmed with the star test.



The radius of curvature for the 298mm diameter test sphere was 1932mm and work correcting the mirror only took a dozen sessions with the error map from the interferometer.



When the secondary was water jet cut I has 3 other blanks water jet cut as well. The outer waste material from this was kept to allow the blanks to be blocked into plate on the machine. Image below shows the waste material (28 inch diameter 1 inch thick blank) with the 3 water just cut holes of the 8 inch minor axis flats. One flat is in place. This has been bevelled on all edges now ready for the flats to be waxed into place.

The waste material and secondary blanks were mounted on a flat plate with a thin piece of paper underneath. The gaps from the water jet cutting were made even around the blanks using cardboard. Beeswax was melted (65 deg C) and injected into the gaps.

Holding the syringe in the hot wax released wax that had solidified. A pad floating in hot water is best to melt the wax so that It does not overheat. A 24 inch diameter 1 inch thick plate glass blank was used to grind both sides of the blanks in the waste material. The tool and the blanks were flipped often and rotated to prevent curves being formed and astigmatism. The machine was used for most of the work as quite a lot of grinding was required.

The solid plate tool has proved to be too big. It has been sticking to the waste material and blank disc. Therefore the tool has been covered in glass tiles similar to the way the primary is fine ground. Fibre glass resin holds the tiles in place. This will stop the tool sticking to the part.

Unfortunately a week later the wax failed holding the secondary blanks in place. The blanks were removed and each was then ground by hand on the large flat tool.

The pencil test (indelible marker really) was used to monitor wear rate.



One side of the first flat showed that the edge was high. Grinding continued until the pencil marks were removed evenly.



T pitch lap was made using a rubber mat. This turned out not to be as easy to make as first thought. The tool is over sized at approximately 14 inch in diameter. After half an hour a polish was across 95% of the flat. Testing will being to determine the surface form.



The test rig for testing the flats required a larger spher than the 200mm diameter reference sphere I had, so we made a new 300mm diameter one as can be seen below. This was made in a couple of weeks over a few nights and is quite a good sphere. The sphere will be rotated during testing of the flat to average out any residuals. Using only 200mm of it improves the surface form also (as the flat is only 200mm in diameter at 45 degrees). Ignor the old war ship of mine that is in the background. I had this as a kid back in the early 70's.....it still floats. It's quite old. Probably mid1900's when it was first made.



With teh new sphere now completed, we started to test the new 200mm minor axis flat with the interferometer and correct teh surface similar to the way we make the parabolas. The error map is projected down onto the part in the corret orientation. The fist test showed over 10 microns of astigmatism!! I thought we may never polish this out. But, with dosed polishing laps and some hard work, it was removed ina few hours. A small amount of washing up liquid helps keep the cerium on teh lap and part and also seems to help improve contact and removal rate for some reason? A small 3 inch star lap is used for most of the corrcetion. A 6 inch eliptical tool is used to smooth teh surface every now and again. Constant testing is required.



Image below is of teh flat now with less than 1 micron of total surface error. It is getting close and now has less error than the orginal flat we had water jet cut out.



The flat above had too much power and another flat was reworked. A spherometer was used to check flatness before the flat was polished on the large polishing lap as above. The test equipment was also modified to make testing easier and remove the difficulty of supporting the flat on its edge. Both the sphere and the interferometer are mounted at 45 degrees with the flat under test mounted flat on the bench.





One difficult error to control is power (shows up as defocus in OpenFringe). The amount of power is very small with the ROC of the flat being in the order of 10 miles. Power is controlled by placing a reference flat in place of the one to be tested (thickness has to be the same or the surface height altered to match). The reference flat fringes are then nulled out. The mirror to be tested is then put in its place and the fringes aligned but no movement in axial motion is allowed. The reference flats I use have been independently tested and have essentially no residual power and are very good flats (12nm RMS surface). Power can be controlled using a large tool (6 inch as below) that is moved over the flat following the shape of the flat with little overhang. The tool must be moved full length of the major and minor axis as the flat rotates below. The motion 'removes/reduces' power by deepening the middle very slightly. The opposite would be required to raise the middle if the power was the opposite phase. E.g. If the interferometer has to move closer, then the flat is concave.



The final error map is shown below. The shows the surface to be 16.2 nm RMS which is good for a plate mirror. I decided to stop at this due to the fact that the purpose of this was to develop the manufacturing and testing techniques. A new mirror will be made from borosilicate glass soon. Plate takes a long time to stabilise between polishing sessions. But this mirror is significantly better than the one in use at the moment.



To get the error map projected down onto the mirror, the CONTOUR view is selected and then the image saved. It then needs 'stretching' to match the oval shape of the secondary.



Below shows the error map modified in MS Paint (140% major axis) for projection down onto the mrror.