30+inch+F3+Figuring+and+Testing


 * Figuring and Testing**

Image below shows some of the smaller sub diamter lap used in figuring. 3 inch, 4 inch and 6 inch. The 3 and 4 inch are star lapped. The 6 inch is to be star lapped soon. A smaller 2 inch lap is not show but is needed for very narrow zones at the edge. Star laps reduce the effects of sharp zone interfaces. The laps are micro faceted and pressed for each figuring session.



Polishing was completed on the 30th May 2015. So figuring would now be started. The mirror is mounted in the test rig for initial testing with the interferometer.

The amount of correction needed is shown below in a FigureXP screen shot. Quite a lot of correction is required if starting from a sphere!



30th May 2015. The first test of the mirror with the Interferometer gave the following results. Surface PV was approximately 4 microns. Therefore the edge and centre work was pushing the figure away from a sphere towards the desired figure but still a long way to go.





4th June 2015. The large lap has now been star lapped to reduce zoning and the smaller 10, 6 and 3 inch star laps have been re-formed to the F3 curve follwoing completion of the 30 inch F4.6.

Smoothing continued for an hour with the large lap which improved smoothness and removed some fine zones. The mirror was tested again and was seen to be 3.75 microns PV.

Figuring continued with the 10, 6 and 3 inch star laps. The 10 and 6 inch laps used in the middle high area and the 3 inch star lap on the outer edge.

After 32 rotations (each rotation of the table is approx 1.5 minutes) of the 3 inch lap and 16 rotations of the 10 and 6 inch lap the mirror was tested to be 3 microns PV. Surface was still smooth.



8th June 2015. After another 40 rotations with the 4 inch lap at the edge and the 6 inch lap in the middle the PV is now 1.9 micons. The curve is a little rough as only a few interferograms were used to obtain it. Lots will be used as the figure begins to converge (sub 1 micron PV). In addition, the mirror will be marked and rotated through 72 degree positions with many iinterferograms taken at each posiion. The wavefronts will then be de-roated and averaged.



9th June 2015. After another few hours with the 4 inch lap on the edge and the 6 inch lap in the middle, the figure is starting to converg. The PV is now around 1 micron surface. and other surface details are starting to appear. But the edge is very narrow and is going to be difficukt to sort out.......Images below show surface and profile.





Mirror has now been marked on the edge with marks every 72 degree from the 0 mark on the edge. These are used to precisely rotate the mirror in the cell for taking interferograms at each position. These are the de-rotated in OpenFringe and averaged to make one error map for figuring. This method is common practise in metrology laboratories and I have done this myself on commercial optics for Glyndwr Innovations.

My original methods were to 'print out' error maps and place these under the optic. This was OK but seeing through the cerium was not easy to work on local zones. So I decided to 'project' the error map down onto the optic with a PC projector. The mirror to 'bend' the light has to be quite flat. I have bonded the mirror to 2 layers of MDF that was checked for flatness. The mirror must not be twisted or else the error map will be twisted too. I get the error map projected to within +/- 2mm out of round and diameter. Image below is a little more than that but the optic was still miles from being finished so no need to be that precise at that time. There are 2 methods for figuring. Either have the table rotating for symmetrical errors (circular zones), or leave the table fixed for non-symmetrical errors (astigmatism/lumps bumps etc.)

The orientation of the optic and error map is critical to ensure that the correct parts of the optic are being worked. That is why the optic is marked on the edge and the orientation of the error map be check carefully. It takes LOTS of interferograms at each position to get a quality error map! I do not post interferograms here as this would not be appropriate (and would take a LOT of space). Only error maps and profiles are uploaded.

We have been using this method reliably for a few years now (since winter 2012) and have made numerous mirrors from 16 inch and up with F ratios as fast as F3. Proof is on sky star testing which is also used to verify the results. The Bath Interferometer laser is checked precisely for wavelength with a spectrometer on a regular basis too. All these issue need to be considered and checked to ensure that the surface errors are correct before any figuring is undertaken. I have undertaken a lot of verification work of the interferometer on a number of optics and compared the results to expensive commercial interferometers such as the 4D PhaseCam 6000 (which I use a lot in work). The results show that with care, the interferometer is comparable. Vibrations and air currents are an issue so your environment has to be controlled, but the results I have been getting are very similar. . Max did this at Glyndwr Innovations as part of his masters. Some additional information is contained in the short presentations here also.









The radius of curvature is checked with the Foucault tester at the vertex with a laser ranger that is checked for calibration on a regular basis. This gets the ROC to within +/-2mm or better. The ranger is inserted into the tester repeatable. The tester has an offset of 11.3mm from the moving knife edge and source. This is added to the reading taken. The staging for the tester is par-focal with the Bath Interferometer used for surface measurements. This makes testing easy and quicker to set up.



13th June 2015. Another few hours of figuring. This time using a 2 inch lap on the edge as well as a 3 inch and 4 inch. 6 inch only in the inner zones. PV is now around 700nm (0.7 microns) on the surface with the edge significantly reduced. Mirror rotations now being undertaken. Fringes difficult to get orientated and the appropriate amount to get reliable data but just about OK. Higher resolution web cam is on order to help out. It will not take too much more figuring before the Strehl starts to jump up. Still 0 at the moment. Ooppss...noticed recently that the file name is wrong in teh images below! Will update in teh future at some point....





At the end of June the polishing machine went through an upgrade to change the table motor. The original motor system used a 240V motor and belt system that was quite noisy and slipped. In order to keep the neighbours happy and my ears too, the motor was changed. This took over a week to complete and put the project back a little. The new motor works great and you can hardly hear it. In addition the supply is variable and the speed of the table can now be changed too. The image below is without the plastic water guard for clarity. The motor is from an old stair lift. 24V dc supply maximum but works well for this application even at 9V dc.



The servo system has now also arrived and the secondary has been water jet cut too. So it looks like the project will be still be on target for the end of August 2015.

The mirror is now very close to being finished. We have now modified the test rig for star testing that removes the secondary to ensure that only the primary is used fro imaging Polaris. This is done to compare the error map from the interferometer to the star test results. The arrangement looks a bit rough and ready but it works fine. The 3 end points of the wooden spider vanes have adjustment to allow for collimation in the usual way with a laser collimator. The most important thing to do is to ensure that final collimation of the primary is done on Polaris when it is centred on the CCD chip.



The project has slipped a few weeks and it is touch and go if the telescope will be completed for Kelling Heath star party in September 2015. But we will keep working away.

17th August 2015. The figure now has a Strehl of 0.331. Some astigmatism is now present as we converge to the final figure but will be easily removed using the error map and projection method we have developed.







The above image shows each position of the mirror in the test stand. 0, 72, 144, 216 and 288 degrees. The astigmatism goes with the mirror so it is really in glass. This level of astigmatism only really pops out when you get closer to convergance. We do tetsing in the early morning when we can as the temperature has stabalised in the workshop and there is less vibrations. Only the 288 degree image is slightly different to the others with 'less' astigmatism seen. There always some variance between data sets, but when averaged data sets of different days (without working the mirror) are very similar.

Since the above data was taken LOTS more data runs and final tweaks have been done......this takes AGES! We have not pput all the data up as it would take Giga Bytes of space. Litteraly thousands of interferograms over weeks have been taken to get to this point. Best time for measuring is early moring when the temperaure inside and out are the same and vibrations are minimised.

The image below is a bit of a cheat....we do this now and again to look at zones. Astigmatism is removed from OpenFringe temporarily. This helps reduce zones caused by small laps. Mel Bartels is correct that small laps do cause zones, but I did not fancy using an oversized lap like he does with this size mirror!! We always work face up. It is too risky turning the mirror. This mirror has more local zones and irregularity at the moment than I would like, but we will try and reduced or remove these in the final figure. In the end if the stars look good and the planets look good then it must be a good mirror....we always star test! Without astigmatism, the Strehl is 0.861....so removing this astigmatism is the priority.



The secondary has now been cut from a precision flat. This is a 200mm minor axis flat. A slightly smaller one could have been made, but we wanted to have the option of using binocular viewers. The flat when tested has moved a little from the original figure so is only usable really at the lower powers. The views using the 21mm Ethos and Type 2 Paracorr are great. Small stars with a 100 degree field. Epsilon Lyrae has plently of black sky between each of teh doubles so we know we are getting close. The goal is to make this as good as we can.

Since the above data was taken a even more tweaks have been done, data has taken with the interferometer and on sky testing completed. Too much to tabulate here. Maybe I will put it in one day.

The images below are of the surface as of the 23rd august 2015. The surface is probably better than this as turbulance could stil be seen in individual interferograms. Strehl is now 0.826 which is not bad. The surface has more zones, lumps and bumps than I would like, but not bad for a first attempt at a 30 inch F3! The next one will be better (another blank has just arrived) Asigmatism has essentially gone as can be seen by the Zenike terms. Surface RMS is 19 nm, and probably better, as mild turbulence did change each result a small amount. Data improved with more data sets. The OpenFringe final error map is shown at 550nm no matter what laser wavelength is used hence ((1/14.3 * 550) / 2) =19

This has been a difficult project but we have learnt a lot from it. Figuring an optic like this is not easy! It was tempting to carry on but the star testing siad it was good, so we have stopped. Key issues are ensuring that the diameter is measured EXACTLY to the millimetre!! This is critical, more so than laser and ROC. Laser has to be +/- 1nm tollerance really and ROC has to be +/- 2mm tolerance. Carefully placing the ellipse in OpenFringe is critical also as well as the interferometer set up. Smaller slower mirrors are more forgiving.

Final star testing (23rd August 2015) confirmed that the mirror was good enough to be coated. This will be done at GalvOptics ([]) Epsilon Lyrae (double double) had plenty of black sky between each double and stars were near pin point and snapped to focus. I even managed to use the 8mm Ethos with a 4x Barlow at 1145x and the image was good. Most of the time the 21mm Ethos and Paracorr type 2 will be used to give 100 degree fully corrected (around 3/4 degree) field.