TRO+-+Polishing

=__Polishing the TRO 30inch mirror__=

The polishing lap used was 16 inch diameter and originally used to poish the old 16 inch telescopes we had made some years ago.

This was made of layers of 3/4 inch plywoord stuck together with fibre glass resin and painted.

The front surface was profiled to the curve of the mirror with car body filler, sealed with resin and then coated with pitch.

Pitch squares were then bonded (gently with a blow torch) to the surface. The tool was then dipped in warm water to soften the squares and pressed into the curve with cerium oxide/water slurry. The tool was then left to cool on the mirror to set. Over night the tool had a sheet of thin netting material to prevent the tool from sticking to the mirror and also provide thousands of small facets to the pitch squares.

The machine was used for 1 hour initially with no overarm, only the table was driven. The over arm was then used for another hour or so (2 hours in total) before initial testing.

[|TRO_30inch_polishing_centre.avi] [|TRO_30inch_polishing_offcenter.avi]
 * Videos of polishing the mirror:-**

Table of work completed for polishing
 * Log of polishing work:-**



Image above shows the new machine poishing the 30 inch TRO mirror (TRO_30inch_machine_1.jpg)



Above is a close up of the mirror on the turn table (TRO_30inch_polishing_1.jpg). The image below is of the initial makeshift test stand the worked quite well. This was made from a 3/4 sheet of plywood (the old TRO mirror cell) covered in carpet and then a layer of large bubble pack.

The mirror was tested after just over two hours to determine if there was any astigmatism and if the mirror figure was within acceptable limits to continue polishing.

TRO_30inch_teststand_1.jpg

Image below is of the 16 inch pitch lap. Notice the fine micro facets put in by the fine netting when left on the mirror over night.



First tests, at around 12 degrees altitude using the focult tester, showed not perceptable astigmatism or altitude induced flexure! But the center was very high (throwing the light long). The outer part of the mirror looked good though.

The tester was then used with the old 30 inch TROK Coude mask to determine what the knife edge movements were. This showed that the outer zones were fine and mostly going greay at the same time at the same knife position, but the centre was around minus 46mm away from teh outer zones. It should have been 0mm for sphere or around +24mm for a porabola.

We would either have to correct the central 15 inch diameter zones with an 8 inch star lap and BM303 emery, or go back to fine grinding, otherwise it would take forever to posih this plateau back down to the outer zones.

Above image is after 135 minutes of polishing (TRO_30inch_polishing_1_135mins.jpg)

A 12 inch polishing lap was converted into a star lap to try and reduce the high center of the mirror. This was used with Cerium and BM303 mixed to try and speed up the process. Based on some approximate calculations, we expected to reduce the negative knife eduge movement of minus 46mm down to 0mm (approx 30 microns) in about an hour.

The image below shows the star lap on the mirror. Work was concentrated around teh center 16inches of the mirror. The tool was moved off center only an inch or two and the stroke varied by an inch or two as well to blend teh strokes.

TRO_30inch_polishing_3_starlap.jpg

The Ronchi lines improved quickly. In total 1.5hours work was done using the star lap. This reduced the knife edge movement from edge to center from -46mm to -12mm. This was a big jump and we decided to re-press teh large lap and continue polishing. Image below is after 2 hours with the main lap and 1.5hours with the star lap.

Above image is after 225 minutes of polishing and after correcting the high center with the star lap (TRO_30inch_polishing_2_225mins_b.jpg)

Above shows the mirror after more work with the main lap after finishing with the star lap (TRO_30inch_polishing_3_440mins.jpg)

The polishing lap was positioned 5 inches off center and had an overhang or approximately 3 inches to try and polish the edge and prevent it being turned down by not being polished. This was exepcted to raise the center again, but after inepection it did not appear to in the time (TRO_30inch_polishing_4_offcenter.jpg)



Above image shows the mirror after working 5 inches off center. The edge is more polished and does not appear to be any longer. The mirror has a much higher refelctivity as can be seen by the better contrast in these Ronchi images (TRO_30inch_polishing_4_502mins_b.jpg). We will continue with this stroke to try and polish the edge more.



Above image is TRO_30inch_polishing_5_622mins_b. The below image shows the center over center position of the 16 inch lap.



One test that we have been is a to use a laser to test for the amount of polish the mirror has been getting. The mirror is cleaned and the laser pointed at the mirror surface at around 40 degrees. If poished, the laser will not show any signs on the front surface of teh mirror, but will show on the back side of the mirror as thsi was finsihed to 120 grit.

The edge of the mirror has been taking longer to polish than the front (normal) The image below shows the edge of teh mirror after 15 hours. The center of teh mirror is completely poished and shows no pits using the laser method.





Above image is taken after 1262 minutes of polishing. The polish is now much greater and the edge is beginning to get a good polish too. Little zoning of the polish in now evident across most of the mirror. The image is now much brighter. There was some turbulance in the garage at the time the image was taken, and this can be seen in the image. Also this was not long after coming off the lap. The Ronchi lines imrpoved over the next 30 minutes. Image reference is TRO_30inch_polishing_6_1262mins_b.jpg This is after another 3 hours work. Image reference is TRO_30inch_polishing_7_1442mins_a.jpg

The edge was being still quite slow to polish out. We decisded to move the tool even further over the edge of the mirror to try and speed things up, and yet hopefully not turn the edge down or bring up the center too much. The image below shows how the lap was placed.





The above image is after 1637 minutes (over 27 hours) of polishing. Image reference: TRO_30inch_polishing_8_1637mins_a.jpg

Above image is after 1817 minutes (over 30 hours) of polishing. The edge is still slightly hazy. TRO_30inch_polishing_9_1817mins_a.jpg

TRO_30inch_polishing_9_1817mins_b.jpg

TRO_30inch_polishing_9_1817mins_d.jpg

Above image is after over 34 hours of polishing. A cord stroke was used extensively to try and polish the edge more. This did not appear to make the edge worse. The edge has now much more polish. Image reference: TRO_30inch_polishing_10_2047mins_a.jpg

TRO_30inch_polishing_10_2047mins_b.jpg

Above images is after 39 hours of polishing. The edge is now almost complete. The temperature is now rising as the summer appraoches. We are starting to get thermals when taking these images. We may have to delay figuring and testing until later in the year. The polishing tool has now worn too thin and needs re-building too. Image reference: TRO_30inch_polishing_11_2347mins_b.jpg

While polishing the mirror, we had to re-build the pitch lap as it had become worn out. The page below has images showing this

Re-building the 16 inch TRO pitch lap

TRO_30inch_laser_test_5.jpg

TRO_30inch_laser_test_4.jpg

Above image is after 37.8 hours of polishing. Image reference is: TRO_30inch_polishing_12_2567mins_a.jpg

Polishing is now complete enough for figuring to be started. See the figuring page.