this site I write about my astronomy projects. In the summer 2006 I
finished construction of my observatory. And in the summer 2007 I will
build a telescope.
21. november 2007: The 10" telescope
I have finished my telescope. The prime mirror is mounted in the
telescope, the secondary mirror and the ocular is mounted too. But I
got two problems:
1. I ordered a new 40 mm. secondary mirror,
but my suplier have not got it and send it to me yet. So I have used an
old 30 mm. secondary mirror from a 6" newtonian. It is acceptable, but
not good enough. I still waiting for the 40 mm. secondary mirror.
2. The next problem is more difficult. When the
mirror returned from aluminizing I discovered a kind of ring near the
edge of the mirror. I think it is a problem from the polishing phase. See the Youtube-video from the link below.
spite of theese problems I have tested the telescope with a 25 mm.
Kelner ocular. The stars was sharp, I could see many double stars and
other details. I just need to align my viewfinder, before it
will be posibly to determine which star and their magnitude I can see.
At Youtube.com I published a video with my telescope, see this link: My first telescope
22.september 2007: Mounting - Dobson
miror is send for aluminising and while I wait for the mirror to be
returned the mounting is constructed. A Dobson-mounting have I made as
you can see at the pictures below:
september 2007: Starparty
weekend an astro-club in Jylland arranged a starparty. 58 proffesionel
and amateur astronomers went outside a little town named Ejstrupholm.
Work shops, introductions, discusions, activities and much other was
arranged for the two days.
Telescopes and equipment ready for use. Friday evening and night the weather was fine for observing.
Radio astronomy was also represented at Ejstrupholm.
Jens Jakobsen show how to grind a mirror.
Thanks to Midtjysk Astronomiforening for organizing a succesful starparty.
have polished the mirror and now it's time for testing the surface. The
foucaulttest consist of a lightsource also called the
artificial star placed in radius of cuvature (two times focuslength).
By refracting the light inside and outside focus it is
possibly to analyzing the surface. The light source is in radius of
curvature 3 cm. left from center axis, digitalcamera in focus right to
center axis and the knife-edge moves from right to left.
my little digitalcamera the mirror makes these images in foucaulttest:
Knife-edge cut light outside focus
Knife-edge cut light inside focus
testing the focal plane the lightsource is placed in radius of
curvature, a white screen is placed in focus and by moving the light, I
testing the focuslength is equal on different points on the
The test is repeated 5, 10, 15 and 20 cm. from optic axis and the
is also turned arround five times, approximately 72 degrees each time.
The test showed that focuslength is uniform and therefore the
image is formed on a curved focal surface.
was also tested for
astigmatism. If there is astigmatism the image out of focus
will be eliptical and
not round. And the eliptical images (inside and outside focus) will be
right angles to each other. With the white screen inside and outside
the focal plane I could
not registrate any astigmatism.
an eyepiece in focus the mirror is tested for represent the
artificial star sharp and distinct. The test was excellent - the
artificial star was represented very fine.
surface was controlled by visuel test and the mirror was accepted.
last test is test of polishing. Light from the sun concentrated by a
magnifying glass controle the surface. If the surface make different
kind of reflections then the mirror is not polished enough. I decided
to polish 30 minutes more and then it got a smooth and
well-polished surface. At the image below you can see how I check up
the surface, here there is a little scratch:
Vacuum coatings, maybe they shall aluminize my mirror. In the
meantime I will construct the tubus and
juli 2007: Mirror
polished and first test - Foucault
mirror is now polished and ready for being tested first time. When I
saw the artificial star first time I was very fascinated, alle that
work and the little faint star was reflected quite sharp on a
white screen at the focal fix point. The radius of curvature was
measured to 2370 mm, the focal length is then 1185 mm and the F-number
is f:4.7. Pictures of the polishing lap, testbench and the testresults
scratch up in left corner and the dark area too means I
probably have to return to grinding with
carbo320. There is a dark area in the center and also just above the
center. Near the edge below there is a yellow zone. And the surface
have faint schratches. I hope some ATMs with experience will
comment these test-pictures.
Here is a picture
showing how the mirror is mounted under the test.
artificial star is placed 291 cm. (twice the estimated focal length)
from the surface of the mirror. And moved 5 cm. right to the center of
the curvature of radius. I also tried the slitless-method, but it was a
little complicated to make the slit sharp enough. When my holidays is
over (two weeks yet) I try to make a slitless-tester at work.
knife-edge-device is a prototype which isn't finished yet. It
is moveable along the center and acros the center of the
the background is the artificial star.
The knife-edge one
The polish lap is
made by pitch, wich is melted and formed in a mould. The lap is used
with cerium oxide and was very effective. The pitch have to fill out
the whole lap, and have to be more uniform and regular.
july 2007: Grinding
to much and making a new the tool
grinding have been a little complicated. I
was recomended to
weight and used a heavy stone up on the mirror durring the rough
grinding. And then it went fast and
easy - a little to much fast and easy. I hollowed out 4
mm. and it
is 1.1 mm. to much. The order of tool and mirror was changed,
the mirror down and the tool up. And the sagitta went 3.3 mm. which is
A new tool is
constructed by stone cast plaster and tiles. The tool is a little
konvex as shown in the picture:
the konvex shape was not very smart. I couldn't control the
grinding and the mirror went hollowed out again. A new and
tool was constructed and then I was able to control the grinding
the mirror have the right shape and a sagitta at 3.2 mm. which is 0.3
mm to much, but it is acceptable and grinding is
continued with next kind of
The purpose of the grinding proces is not
to hollowing out anymore.
The shape is approximately as it have to be and I'm ussing silicon
silicon carbide 120, silicon carbide 220 and silicon
carbide 220. For every shift of abrasive, the tool, mirror and
workbench is cleaned carefully.
The grinding strokes
used now is a W-stroke , it is a to-and-from-movement like a
W. See this movie-clip (the strokes is to long, I make a new
movie next time I grind a mirror):Movie finegrinding
grinding proces is splitted up in trhee steps:Rough grinding
with iron-tool, the mirror just have to be hollowed out. Using
strokes-on-the-edge and carbo60 and coabo80. Sagitta have to
10%. Time spending on grinding: 6 hours.Fine grinding
with a tool constructed by glass-tiles or ceramic-tiles. The aim is to
make small corrections at the shape of the mirror otherwise the surface
have to be smooth. Using W-strokes and carbo120,
carbo220, carbo320, emery 125E and emery 50E.
Fine grinding finished when foucaulttest is OK. Spending 6 hours on
with pitch-tool, which will be described in details later. Using
w-strokes like in fine grinding and cerium oxide. Spending
The abrasive is shifted when the surface is
uniform from edge to edge.
juni 2007: Observatory
Observatory is just painted. I like the image in blue, white and green
rough grinding I have experimented and is satisfied with the
tool where I grinding on an iron-plate. Next time I grinding a miror I
will get a piece of iron bar. I have linked 75 seconds movie from the
rough grinding-proces. I'm using the method described by
Texereau and Howard, the file take up 6 mb.Movie rough
from this link
for showing the file)
is some pictures from the rough grinding, the sagitta (the hollowing)of
the mirror is controlled by the slide caliper. The curvature is
controled by a template. Adding water and grinding powder to
mirror. The grinding realized by chordal stroke.
with the tool
have started grinding of the mirror, but the tool
have problems. The concrete dissolve under the
grindingprocess, the consume of grinding powder went to heavy
the mirror-grinding don't succeed. I will make a new tool, in iron I
have made a 2 mm sheet which will be mounted upon the concrete.
mirror is grinding on a block of concrete, other ATM's use stone, piece
of iron bar, plate glass or something else wich is more hard than the
hours grinding sagitta (the hollowing) is 0.9 mm, it shall
be 2.9 mm.
17. may 2007:
proces, methodes and the supplies
day I write some documentation on what telescope I want to
make, the supplies which have to be used and how the mirror
mirror (precisely is the diameter 254 mm), f:5,6 focuslength:
mm. The mirror have to be mounted in a Newtonian with dobson-mounting.
pyrex (260 x 36 mm)
60 grade 2 lbs
220 grade 1/2lb
320 grade 1/2lb
Method or progress of
Testing blank for error
tool for rough grinding
Rough grinding with carbide
60, 80, 120 og 220
Control sagitta and curvature
tool for fine grinding
Fine grinding with Carbide
320, Smergel 125E og 50E
Control sagitta and
Make tool for polishing
Cerium Oxide do also control by Foucaulttester
to make test and control:
Template made in
plastick, curvature marked by a calipers/dividers, radius of curvature
= 2 x focuslength.
Sagitta(S) is the curvature depth
in the center of the mirror.
S = R - squareroot(R^2 - r^2)
R = radius of curvature = 2 x fokuslength
r = radius of mirror.
S = 2912 -
squareroot(2912^2 - 130^2)
S = 2912 - 2909,0967
is constructed about a lightsource (a virtuel star) and a screening
which do it posibly to analyze the surface of the mirror.
15. may 2007: Mirror has arrived and
constructing the Foucaulttester have started
I have recieved miror and supplies from Wacuum Coatings in England. And
I have started to make a Foucaulttester, it is assembled by
sledges adjusted by a screw.
mirror is a 10" Pyrex-glas, but there is a little bubble. I
it is 1,5 mm long and is 5 mm beneath the surface. I
hope it will make troubles. So do an ATM from Denmark say.
have ordered supplies from Wacuum
Coatings i England. I was recommended Suprax-glas from a danish ATM,
but that kind of glass was sold out, so I got a pyrex-glass. When I
waiting for the supplies I make a working table and a grinding tool.
The tool is made in concrete, have a diameter at 25 cm. and is 8 cm.
high. At the picture you can see the tool and behind the tool I have
some materials to a Foucault-tester.
of the Observatory
started to build my observatory in the fall 2005 and in the summer 2006
I finished. I will upload a plan for the building once in the future.
Here is some pictures from the building site, when the foundation/base
was build it looked like chaos.
My oldest son Rasmus
helping with the foundation.
The rib for the dome is marked
by a caliper/divider.
My two youngest sons Jakob and
Emil helping to glue the rib.
Now are the rib assembled and
waiting for the veneer to be mounted.
My father-in-law helped with the
roof. And it did also suceed to move the doome around.
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