Polishing and testing

Polishing

The polishing tool was made from four disks of 20inch diameter exterior plywood and one piece of 18 inch diameter 45mm thick worktop glued and screwed together. A strip of wood 2x1was also added in the middle of the top piece and overhanging the edges a few inches helped lifting the tool off the mirror face. The edges where sealed with PVA glue and painted all over with white exterior paint. The edges where then sealed again, after the pitch lap was complete, with fibreglass resin to seal again and also prevent flaking of paint. The tool has to be thick to prevent flexture and inducing astigmatism into the mirror during polishing.

2 1lbs of cerium oxide and 3 1lbs of optical pitch were purchased for the polishing process. Geoff and I made the pitch lap on the 6/6/97. The pitch was broken up into small pieces before adding it to the pan. It was during this that Geoff noticed something shiny in the pitch. On inspection it was found to be aluminium swarf. More pieces where found and we had to scan all pieces before they where added to the pan. The pitch was heated in an old pan using a petrol gas camping stove on a low heat so as not to boil the pitch. The lap was pre-formed with car body filler the same as the previous tool and wiped with turps to give a clean surface (sheen, but not wet). The lap was duct tapped to 1/2 an inch and the pitch (now a runny honey consistency) was poured in a spiral from the outer edge to the middle. A dollop was made in the middle to form a higher area, When the pitch began to set the dam was removed. This proved a problem as the dam had stuck to the pitch and began to damage the pitch lap. We waited another minute for it to go harder and used a Stanley knife dipped in water to cut the dam away. This waiting proved to be a problem as the lap had set too hard and when we pressed the lap into the mirror, after wetting and soaping (washing up liquid) both surfaces well, the lap did not fully take up the shape of the mirror. The outer facets where touching but the middle was not. We heated the lap 6 or 7 times with a portable heater and blowlamp and repressed each time. Eventually most of the facets where touching. This was not easy work and took some time to complete. During each press we cut the facets out with a saw so that to pitch had somewhere to goto. Some damaged or stubborn facets where repaired by adding a small amount of pitch off a spoon or by putting a small amount of pitch dust on the surface and flashing the blow lamp across to melt it in.



Above shows Geoff Regan pouring the melted pitch onto the pre-formed tool.

Above image shows the completed lap ready for use.

We cleaned up all surfaces, hoovered and changed the plastic sheets. The floor was dampened and all doors shut. Cerium oxide was added to a clean sauce bottle. 6-heaped teaspoons to a full bottle of water. A spiral was added to the mirror and tool. The surfaces where misted with the spray bottle and we began polishing. We began with normal back and forth strokes with a 2 inch overhang, each stroke lasting 4-5 seconds. The tool was sticking in places and difficult to move at some points. The tool quickly came to equilibrium and was easier to move. Slight W strokes where then used of 3-4 seconds having a slow consistent drag (this is when most effective polishing is done). We did 30-40 minute sessions at a time
and checked the tool each time. The facets at the outer edges became tinted with the colour of the cerium oxide and began to touch more and more towards the middle as polishing went on. Polish on the mirror was seen after only 40 minutes work and in was noted that it was more polished in the middle zone than at the edge or middle (annulus shaped). This gradually widened and became more polished and the middle began to have more polish also. After 2 hours 40 minutes we stopped work and decided to do a focult test. The mirror was removed and mounted in the test rig. The mirror appeared to show slight astigmatism with a central depression. This later proved not enough polish was done on the edge to test for a turned edge.

On the 14/06/97 we began work by cleaning the work area, washing the mirror and the damming the tool and pouring on hot water ready to press. After scolding Geoffs hands we removed the dam after only 30 seconds, soaped the mirror surface and pressed the mirror down until cool. The tool had been covered over the week with cling film. This had imprinted shapes and wrinkles into the pitch and did not get fully removed by the cold press. We began work with normal strokes for 1/2 hour and then went onto W strokes for the rest of the session. We completed work after 2 hours and 20 minuets. By now the mirror showed much more polish with only the outer edge and a small central region requiring more work. The tool was well covered in cerium oxide and water then covered with thicker plastic sheet instead of cling film. The mirror was placed in the test rig and a focult test done at 1800 hours. This showed uneven shadows at the centre but the same if not slightly improved knife-edge inside of focus. The mirror was retested at 2300 hours and showed a much clearer figure. This was attributed to the mirror cooling down evidence of this cooling was noticed by Geoff during the earlier test at 1800 when the shadows appeared to be changing slightly.

What was most striking was that it had appeared that we had mis-interpreted the results of the test the previous week. This time it was very obvious at 2300 hours that the mirror had a raised ring near the edge and a large raised hill running from the base of this ring to the centre of the mirror and not a depression. But see below!!

Caution !! When checking other readings on mirror figuring one showed focultgrams through the stages of sphere to hyperaboliod. The oblate spheroid looked remarkably similar to the foucult test done on the 14th. Shadows change side from left to right going from oblate spheroid to paraboloid.

One interesting test was placing a thumb on the mirror while doing the focult test. Big hill, very bright on the right and dark on the left. Thermals off hand very prominent.

Another was placing a bag of peas in the centre too cool the middle. This had the effect of deepening the centre after only a few seconds!!

One solution to storing the lap between polishing sessions was to let the lap dry out and then cover with one sheet of newspaper. This did not stick to the surface or make any imprints on it. This has now been done many times successfully.

Figuring the mirror

1) Making an 8" tool to bring down the centre

The centre of the mirror was way too high and we decided to lower it to a better shape before we did any more polishing. This would be done with a sub diameter lap of 8". On the 6/9/97 we made the 8" lap. Lessons learnt by Geoff when doing the 6 inch finder scope mirror where not to form a dam at the edge of the tool as this will build up the pitch at the edge and not form the correct shape. It is also easier and quicker to make. Also a large blob is first put in the centre of the tool and left to cool for a while so as to provide depth to the centre to give shape during pressing. The rest of the pitch is then added and allowed to flow evenly over the surface up to and even over the edge. Do not worry about this as it can easily be chipped off later when cool. The tool is left for only 30sec before splashing with water and quickly placed and pressed into the mirror. If the lap has not made full contact the put under hot running water, dispersing the flow, to soften and then repeat the pressing.

after 2 sessions of 25mins each the centre of the mirror was seen to be getting lower. After 2 hours the centre was getting well down but had low and raised zones within it. Reworking with the larger lap when repressed would smooth these out. Rochi lines where now straighter and less bent towards the middle.

On Friday the 12 September Geoff and I did further work on the centre and retested. The centre now had an 8" diameter hole but did not seem to be too deep. We then decided to reform the main lap and evenout the work done by the small lap. We heated the pitch that was left over from making the 8"lap and cleaned the main lap. Pitch was painted on the low areas. After heating the surface with a paint stripper and blowlamp we poured on hot water to give a final overall heating of the lap. The lap was then well pressed in and removed. The high spots where well grooved and the lap faceted all over. It was then repressed two more times before use. We did an hours work doing mainly backward and forward strokes and finishing with 10 minutes of W stroke. The focult test and ronchi test showed a much-improved surface. The ronchi lines where much straighter near the middle of the mirror and not curving in. The focult test showed that the central hill had been planned down with some of the central hole still remaining. Further work will be required to gradually deepen the centre with care not to dig a too deep a hole therefore using sparing bouts of the 8" tool together with the main lap to even out any irregularities caused by the 8".

Work level was increased over the second week in October, as it was obvious that the edge was going to take a long time to get done. Evening sessions during the week and extra work on Sundays began to take effect on the edge. Only the last 5mm had any significant pits on Sunday the 12th of October after 6 hours polishing on Saturday and Sunday. I had my doubts that the outer 5mm would ever polish due the pits in this zone being much larger than the rest of the surface. This could be due to not carrying through fine grinding enough on the edge. It may be best to stop work and figure and mask off this very outer edge when aluminised. It would still be a 29 to 29.5 inch mirror, which is still large!! The extra work required to remove these larger pits would not be acceptable and be very costly in cerium oxide. (£100 to date to polish the mirror!)



Building the testers for the mirror

1)The focult tester

I had already completed a focult tester some weeks before to get some experience in testing other mirrors. I had always held back from these optical tests because the texts in books had always been obscure in the construction and operation of the device. One book did give a more basic outline and I decided to have a go. All you need is a 7watt-pigmy bulb, baton holder, piece of flex and plug, short piece of drainpipe, few pieces of wood and a razor blade. All items from B&Q (except the razor blade) for a few pounds. It took only an hour to make it and I was testing my first mirror (my original 8.75 inch mirror) soon after. The mystery of optical testing soon began to subside. The main objective left would be to understand how to take measurements of these variations to get an accurate surface profile.

2) The ronchi tester

The Focult tester was adapted so that the ronchi tester could be used. This was done using fine gauge transformer wire and two fine thread screws. The screws where screwed sideways into a piece of wood that would be fixed to the moving platform of the focult tester and positioned either side of the same position that the knife edge would have been. Fine wire was then wrapped around the threads and superglued into position. When dry one side of the wire was cut away to leave a 48 lines per inch ronchi grating. This tool was later to be used a lot during polishing to ascertain the general figure of the mirror. The ronchi lines are very easy to see (more so inside focus) and you get a feel for the overall shape of the mirror as the entire surface is visible. The book Star Testing Astronomical Telescopes says that the ronchi is no good at determining the fine figuring requirements of a mirror by looking at the shape of the lines alone. The shape of the lines vary very little between an undercorrected 1/2wave mirror and a perfect paraboloid. Actual measurements must be taken to determine the true shape and figure.

3) The Zonal Mask tester

This would turn out to be one of the main tests used to figure the mirror. Only by measurement can the slope of the mirror be actually measured to say if it is parabolic or not. Phil Layton, patron of the L.A.S., gave us the first insight into how to use a zonal mask.

More details of the mask construction and use will be detailed here at a later date.

Above image shows Dave Owen checking the edge of the mirror for pits. He is using a modified 25mm Orthoscopic eyepiece to determine how many are visible over a certain area.

Above iamge shows myself with one of the many graphs produced by the zonal mask measurements. This graph was one produced early in the figuring process.

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