New projects throws their long shadows ahead. - This page is a
documentation of the grinding process of two new telescope mirrors:
All Images of this gallery are also visible
using this way if you like it.
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 customīs 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
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
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
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:
After succesful finishing of fine grinding a foucoult testing device
will be built soon and polishing will start!
8" - 207 mm f/3.9 "Kalliope"
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
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.
Last editing: February 27th 2005