New projects throws their long shadows ahead. - This page is a documentation of the grinding process of two new telescope mirrors:

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24" - 610 mm f/4 "Johann"

The latest status of the work on the big blank inclusive new images can checked out here.

During many spechtling years with the telescopes Bismarck and Christina I always had the wish to have more aperture. Still before the construction of Christina I gave mirror making a try and I've begun the rough grinding of an 8" mirror. Now this work will be resumed. The mirror making course at the Volkssternwarte Munich encouraged me to resume the work with this forgotten project and also start the work on the 24" monster. In November 2003 the project was started with the ordering of a thin pyrex blank from Newport Glass. At March 5th 2004 the time had come! The pyrex blank reached airport Munich and after succesfull customŽs check it was greeted from the hole family:

Our cat Picard was engaged as receiving inspection but his interests were more concentrated on the shipping box in which the glass was packaged. The milled courve was checked with a spherometer and declared ok. According to the spherometer the blank should have focal length of 2.3 metres. The blank was also checked for distress. This was done with a flat computer screen. The blank was positioned before the screen and viewed through a polarisation filter. Because a flatscreen emits polarised light, distress in the glass would be visible instantly. This check was also positive. This row of images shows the receiving inspection:

Between March 8th and March 15th 2004 the construction of a tiled plaster tool took place with following method: Cover of the mirror and making of a ring which was used as mold for the concrete from which the tool was casted. After hardening and drying the border tiles were preparated. Their thickness is 1 cm. They were glued with tile plaster and all surfaces were covered with plaster in order that nothing from the grainy concrete can ripple out off the tool during grinding. The back side was also covered with a tile. Through the molding the courve of the mirror was copied exactly into the tool. The tiled tool has a diameter of 47 cm. With the same technique a 35 cm diameter polishing tool was made simultaneously. It has on the grinding side no tiles. After grinding with carbo 180 both tools were covered with epoxy in order that really nothing can ripple out of the tools. After this, Picard had to inspect one of the finished tools. In this image you can also see tool and blank of the 8" .

At March 18th grinding started. After three strokes I remembered the bevel. It was not yet finish-worked. This was sentenced with a few small shell cracks. The grinding was processed mainly with W 1/3 strokes but also with spiral strokes. Because mirror and tool have a combined weight of ca 50 kg, I have to work around the grinding table, to perform the movement of the mirror during grinding. After one hour carbo 180 about 60 to 70 percent of the tool was ground. During the first two hours I changed between TOT and MOT, than I remained at TOT. Every grinding session started with a pattern of permanent marker strokes drawed at the surface of the mirror to perform a spherical check. After three hours the 180 carbo was finished at March 25th 2004. To check if the grain is done, I observed the fading of a milling mark, based from the work at Newport, with a 10x magnifier. The wear pattern of the tiled tool showed now a ground of 90 percent. It is always advisable to mark a large pit or a micro shell crack close to the edge and observe it during grinding for considering the grain is done.

After the common cleaning procedure during grain change there was the opportunity to fullfill an old dream: Since ever I wished to serve a pizza from a large telescope mirror. Sadly our oven is not capable of making an adequate pizza, the size of a griddle is ridiculous compared to the 61 cm pyrex blank!

Now the 230 carbo is almost done. The further progress will be off course documented.

At April 11th the 230 carbo was done. A bevel was ground into the tool. At April 18th I proceeded with the 320. After two sessions at every 70 minutes at May 2nd the grain was done (prooved with magnifier). A few pits were founded close to the edge. June 14th: seventy minutes 400 carbo, strokes as usual. The inspection with the magnifier shows: the edge is better done than the middle! During the next session two days later I ground more over the center. The blank is still nice spherical. With the magnifier I found just two small pits. The 400 is done. After grain changing procedure the grinding process was stopped for a while. This room was then in use for carpenter jobs, I've also encountered health problems and the making of the lightweight focusers for Christina had to be completed until the ITT. At Oktober 12th the work resumed diligent now with the 15my Microgrit. The following row of exposures shows impressions of this session. At the wear of the tool one can see that the complete area is now ground. The view from below at the tool shows with its reflection how polished the surface already is.

At October 12th I proceeded with the 15my. Now a sprayer was used for humidification. With this device the amount of water at the blank can metered exactly. The sphere was again ok. With usual strokes (W 1/3 - 1/4 and spiral strokes) the blank was ground 75 minutes. During the last removal of the tool from the surface of the blank the tool got stuck and it was impossible to move it! How to solve this problem? I attached a piece of timber at the grinding table and clamped a block of wood between blank and timber. Now it was possible to loop ratched belts around the tool and the timber. With two belts it was possible to drag the tool with a force of more than 1000 N until the tool went loose. This is a good method to remove a stucked tool! Later the inspection with the magnifier showed at first that the grain was almost done. Furthermore I found a tiny scratch ca 10 mm long which should ground out during the next session. This happened at October 17th and lasted more than one hour wherein the scratch and a few sleeks almost disappeared. But 10 new sleeks appeared at the edge. Assumption: The tool is blamable for this, perhaps because of its weight of ca 25 kgs. As corrective all edges of all tiles got new bevels.

At October 20th it continued with common strokes pattern. The session lasted two hours and during recharge with new Microgrit the tool remained at the blank. Effect: All sleeks of the recent grinding actions disappeared! But I found three little new ones at the edge and an already partly ground scratch which must be arosed at the beginning of this session.

November 2004: Continuation of the usual process. Infeeding of the blank with Microgrit was done with overlying tool. At the end of the two hours lasting session the ground scratch disappeared, but there are still sleeks. Dezember 20th 2004 I changed to 9 my Microgrit. I continued my stroke pattern (W 1/3 and spiral strokes). The tool got stuck again at the beginning of this session. This problem could also be solved easily with the ratched belt method. The tool remained at the blank and I continued grinding promptly but infeeded more often and used more water. At the end there were some sleeks close to the edge but the middle was in good condition, the 9 my is almost done. Until now I worked altogether 18 hours and 15 minutes with Johann.

February 03rd/04th 2005: Continuation of the grinding process with 9 my Microgrit. Spherical probe with permanent marker shows a good sphere. During grinding the tool stucked again but it was harmless and the belt has to be tightened only softly to proceed. The rest of the time grinding moved on without problems and the established stroke pattern was choosed. Often infeeding with new Microgrit. The tool remained during the whole 1 1/2 hours at the blank. The final inspection shows 2 longer sleeks at the edge and a few short ones. The center looks very good the whole surface is tidy ground and in better shape than after the last session. Because of the heavy tool and the expected problems I decided to skip the 5 my. Thus fine grinding is declared as finished! Until now a total working time of 19 hours and 45 minutes at the large blank has been completed. The edge has to be rebeveled again because of the measurable glass removal during the fine stages of grinding. The following images are showing the condition of the tool but also the reflecting capability of the fine ground mirror:

This project started at January 11th 1991. During 10 hours of grinding I hollowed the duran blank out to a focal length of 1.1 metres. Than this project was postponed because the construction of Christina became higher priority. This blank was considered primarily for the ULT, but it got a cheap 10" mirror. At Oktober 25th 2003 the work on the 8" blank was started again, motivated through the grinding course. Now the eagerness pushed me until Dezember 9th to f/3.77. I changed busy between TOT and MOT, made mostly 1/2 or 1/3 strokes. The bevel on the tool had to be worked out. At February 2nd 2004 the 180 carbo was done and I started with 320. At February 11th I skipped more and more performing O and spiral strokes, because the marker pattern probe showed that the mirror was not exactly spherical. I remained longer at the 320 to be sure that the blank will be really spherical. For this I spent all designated 320 carbo. The aperture ratio got longer to tame f/3.9. The complete working time until this point was 24 hours and 15 minutes. From February 17th until March 8th 2004 I worked 135 minutes long with the 15 my Microgrit. After the cleaning procedure the room was occupied for the making of Johann's tiled plaster tools.

February 15th 2005: After finishing fine grinding the 24 inch blank working with the tiny mirror is almost like playing. Kalliope was treated 75 minutes with 9 my Microgrit. spherical probe was ok. From now on humidification was done with the sprayer and no washing in between. Strokes like usual W, O and spiral, always change between MOT and TOT. At the end of this action the 9 my is almost done.

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Last editing: February 27th 2005