Testing of the 10" RC Cassegrain optical system

The Secondary and the System

The secondary of the Cassegrain system is convex and can be tested in several different ways. The ones that I am going to use are interference test with a concave test plate and with autocollimation with the entire system.

Interference test with a test plate

A test plate is easy to create. I used an identical glass disk as grinding tool. This tool became a concave mirror and after polishing can be tested with Ronchi, Foucault or wire test.

Here is the setup of the test plate in the Ronchi tester and a sample Ronchi image at a time when it was spherical (by chance).

The convex secondary is also polished and when placed in contact with the test plate under monochromatic light interference fringes form. A gas discharge lamp can be used as a cheap light source and its light is sufficient to show the fringes.

Here is the setup of the lamp and the interference pattern between the two pieces of glass. The circular fringes show that the two surfaces are not matching. The distance between two fringes is 1/2 wave length. In this example we have 6 fringes or 3 waves of deviation. Moving and pushing the disk around one can determine if the spacing is convex (more spacing in the center) or concave (less spacing in the center). In this case the spacing is convex and one would polish on the outer zones of the glass to make them fit.

For figuring the correct shape is polished into the test plate. In the case of the Cassegrain project this will be a hyperbolic curve. At an aspheric constant of -6.74 and an F-ratio of 4.0 this will be a pretty steep curve. The correction of the secondary will be measured against the finished test plate. If things are perfect there should be no fringes, or when pushed to the side the fringes should be straight.

Autocollimation test with a flat mirror

This test requires a perfect flat mirror. Normally this is difficult to get by but a friend from a telescope making workshop was so kind to allow me use his one for the project.

In the test the finished telescope is pointed against a flat. The Ronchi or Foucault light source is placed at the focus point. The light is then exiting the telescope perfectly parallel hits the flat mirror and is reflected back into itself. The telescope focuses the light and again at the focal point we can test it with the Ronchi grating or the Foucault knife edge. This sounds easier than it is because in a Cassegrain a total of 5 reflections are happening. All surfaces have to be collimated and aligned. During polishing the secondary is not aluminized and reflects only 5%. This happens twice! So at best only 5% of 5% of the light make it back. So to make the test I had to have the primary and the flat aluminized. So there is no going back to fixing the primary now!

Here is a picture of the setup of the scope against the flat and a sample of how the Ronchi test pattern looks like.

In the Ronchi pattern we see the heavily under corrected shape of the instrument. When this image shows straight fringes then the scope is finished!

Auto collimation T2 Ronchi T2 - M2 Interference Test T2 - M2 Edge Interference Test Comment

First light with the autocollimation test setup

2004-05-09
30min TOT 30% overhang circles, 5-6 fringes off

2004-05-09

2004-05-10

2004-05-11

2004-05-14
An interesting idea.When offsetting the disks in the interference test one can differentiate between the edge of the above plate (left) and the below plate (right). A good check for turnde edge testing.

2004-05-15
20min M2 MOT.
10% length
10% W
TDE on M2+T2

2004-05-15
20min T2 MOT.
10% len
5% W
TDE on T2 gets better.

2004-05-15
20min. T2 TOT.
30% len + circles
TDE worse. TOT bad for edge!

2004-05-15

2004-05-15
20min M2 TOT.
25% len
25% W
TDE on T2 (bottom disk) better.
M2:3mm 1/4F
T2:6mm 1/2F

2004-05-15
20min M2 TOT.
20% len
20% W

2004-05-16
20min. T2 MOT.
30% len
10% W
TDE on T2 same?
T2:5mm 1/2F
M2:4mm 1/4F
Zone in center.

2004-05-16
20min. T2 MOT.
30% len
10% W
3min center on edge.
TDE:
T2:4mm 1/2F
M2:3mm 1/4F
What to do about ring zone at R=2cm?

2004-05-16
Star press 4cm diam.
25min T2 MOT.
25% len
0% W
Center comes up.
TDE:
T2:2mm 1/4F
M2:3mm 1/4F
Not bad!

2004-05-17
Star press 6cm diam.
10min T2 MOT.
20% len
0% W
3cm hump in center.

2004-05-17
10min T2 MOT.
10% len
0% W + 2min center on edge.
Center hump better!

2004-05-17
16min T2 MOT.
10% len
0% W
Seems good fit. But !!
T2 edge high
M2 edge low
!!! Bad fit !!!

2004-05-18
20min M2 MOT.
20% len
10% W
TDE
M2:6mm 1/2F
T2:13mm high

2004-05-18
20min M2 MOT.
20% len
10% W
TDE
M2:8mm 2/3F
T2:13mm high

2004-05-19
30min T2 MOT.
30% len
30% W
Check in Foucault. No pictures. T2 is getting very spherical.

2004-05-19
30min M2 TOT.
25% len
10% W. No pressure.
TDE:
M2:7mm 3/4F
T2:15mm up

2004-05-19
20min M2 TOT.
25% len
10% W. No pressure.
TDE:
M2:6mm 3/4F
T2:15mm up
TOT good for shape but bad for TDE. Try harder pitch!

2004-05-20
18min T2 MOT.
30% len
30% W + center on edge
Check in Foucault. No pictures. T2 is VERY spherical. Now go and match M2 to it!

2004-05-22
New pitch lap 73deg Gugolz pitch.
30min M2 TOT.
Bumpy motion.
25% len
20% W
Center good. TDE bad!
M2: 8mm 1F

2004-05-22
20min M2 TOT.
25% len
20% W
Center good. TDE worse!!
M2:15mm, 1.5F.
TOT on hard pitch is VERY bad! Back to MOT!

2004-05-22
20min M2 MOT.
10% len
5% W
Center rises! TDE better.
M2: 6mm 1F

2004-05-22
60min M2 MOT.
10% len
30% W
Center up.
TDE little better.
M2:3mm 1/3F

2004-05-23
40min M2 MOT.
10min M2 TOT.
Center flat but zones.
TDE:
4mm 1/2F

2004-05-23
10min M2 MOT +
20min M2 TOT.
10% len
0% W
TDE worse.
No more hard pitch and TOT!
Bad ring zones!

2004-05-24
back to 64deg pitch!!!!
Star press 3-8cm.
10min M2 MOT
10% len
10% W
Shape spherical but 1F off.
TDE not better

2004-05-24
15min. M2 MOT.
Center better. High ring bulge moving out. How to pus\h it over the edge? More extrem star press!

2004-05-25
Star press 7cm-9cm.
5min M2 MOT.
Only a small ring of pitch is at work!
Push out ring furhter! Center good!

2004-05-25
6min M2 MOT.
Push out ring even more furhter! Center good! How to kill this last little ring??
Two spheres in almost perfect fit!!!
Edges great!

A later test of the test plate shows a nice sphere.

Now we start testing and figuring the secondary to the hyperbolic shape.


		The start
		2004-06-11 Picture shows motor turntable and petal lap. Motor turns at about 60RPM. 40min. 4" petal lap tool on turntable. Center: 7 fringes
		2004-06-11 20min 4" petal lap tool on turntable. Center : 11 fringes. FringeXP is wrong!! We need to do radius and CC fit on surface.
		2004-06-13 10min 2" TOT by hand on center. Turned down edge!! Need to fix later.
		2004-06-13 15min. 2" TOT by hand on center. Lots of artifacts from turntable polish. Need to smooth a lot!!
		2004-06-19 Center 5 fringes Edge 8 fringes. Last fringe wider -> TDE!
		2004-06-19 From the image above a profile was measured using AstroArt3 (right image). A surface error was calculated (left image) using this Excel spread sheet. Best fit for R and SC was done by hand.
		2004-06-22 20min 2" TOT on turntable 50%-100% Zone + 30min trimmed 4" petal lap MOT. Very small circles. Try to smoothen things.
		2004-06-23 40min 4" petal lap MOT. Seems we are loosing correction.
		2004-06-26 30min 1" TOT. 70% zone. Foucault shows ring zones. But this is double pass!
		2004-06-28 20min 1" TOT 80% zone. Getting more correction again. Good! Ronchi looks good after new collimation. If edge is taken away we only have 0.15wv hump at 80% R=793.24 SC=-6.70 Wow! Close!
		2004-06-28 Redo the photos of last session. Right Ronchtest in focus with one stripe filling the entire mirror. I can see the hump at 80% in the Ronchi lines!
		2004-07-14 10min 1" very close to edge point pressure. 10min 1" on 70% zone. Left/Right: inside/outside of focus. Still tiny undercorrection. Edge zone better.
		2004-07-17 10min 1" R=50%-80%. Left/Right: inside/outside of focus. Correction pretty good. Remaining error could be due to collimation effects. Next Startest.
		2004-07-18 Put it all together. Scope mounted. Baffle on secondary still missing. Lots of stray light. Right, first image ever! Later Startest shows good results at 400x. Stars very faint as M2 not yet coated. Maybe I am done!

mail : drgert1@yahoo.com

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Auto collimation	T2 Ronchi	T2 - M2 Interference Test	T2 - M2 Edge Interference Test	Comment
				First light with the autocollimation test setup
				2004-05-09 30min TOT 30% overhang circles, 5-6 fringes off
				2004-05-09
				2004-05-10
				2004-05-11
				2004-05-14 An interesting idea.When offsetting the disks in the interference test one can differentiate between the edge of the above plate (left) and the below plate (right). A good check for turnde edge testing.
				2004-05-15 20min M2 MOT. 10% length 10% W TDE on M2+T2
				2004-05-15 20min T2 MOT. 10% len 5% W TDE on T2 gets better.
				2004-05-15 20min. T2 TOT. 30% len + circles TDE worse. TOT bad for edge!
				2004-05-15
				2004-05-15 20min M2 TOT. 25% len 25% W TDE on T2 (bottom disk) better. M2:3mm 1/4F T2:6mm 1/2F
				2004-05-15 20min M2 TOT. 20% len 20% W
				2004-05-16 20min. T2 MOT. 30% len 10% W TDE on T2 same? T2:5mm 1/2F M2:4mm 1/4F Zone in center.
				2004-05-16 20min. T2 MOT. 30% len 10% W 3min center on edge. TDE: T2:4mm 1/2F M2:3mm 1/4F What to do about ring zone at R=2cm?
				2004-05-16 Star press 4cm diam. 25min T2 MOT. 25% len 0% W Center comes up. TDE: T2:2mm 1/4F M2:3mm 1/4F Not bad!
				2004-05-17 Star press 6cm diam. 10min T2 MOT. 20% len 0% W 3cm hump in center.
				2004-05-17 10min T2 MOT. 10% len 0% W + 2min center on edge. Center hump better!
				2004-05-17 16min T2 MOT. 10% len 0% W Seems good fit. But !! T2 edge high M2 edge low !!! Bad fit !!!
				2004-05-18 20min M2 MOT. 20% len 10% W TDE M2:6mm 1/2F T2:13mm high
				2004-05-18 20min M2 MOT. 20% len 10% W TDE M2:8mm 2/3F T2:13mm high
				2004-05-19 30min T2 MOT. 30% len 30% W Check in Foucault. No pictures. T2 is getting very spherical.
				2004-05-19 30min M2 TOT. 25% len 10% W. No pressure. TDE: M2:7mm 3/4F T2:15mm up
				2004-05-19 20min M2 TOT. 25% len 10% W. No pressure. TDE: M2:6mm 3/4F T2:15mm up TOT good for shape but bad for TDE. Try harder pitch!
				2004-05-20 18min T2 MOT. 30% len 30% W + center on edge Check in Foucault. No pictures. T2 is VERY spherical. Now go and match M2 to it!
				2004-05-22 New pitch lap 73deg Gugolz pitch. 30min M2 TOT. Bumpy motion. 25% len 20% W Center good. TDE bad! M2: 8mm 1F
				2004-05-22 20min M2 TOT. 25% len 20% W Center good. TDE worse!! M2:15mm, 1.5F. TOT on hard pitch is VERY bad! Back to MOT!
				2004-05-22 20min M2 MOT. 10% len 5% W Center rises! TDE better. M2: 6mm 1F
				2004-05-22 60min M2 MOT. 10% len 30% W Center up. TDE little better. M2:3mm 1/3F
				2004-05-23 40min M2 MOT. 10min M2 TOT. Center flat but zones. TDE: 4mm 1/2F
				2004-05-23 10min M2 MOT + 20min M2 TOT. 10% len 0% W TDE worse. No more hard pitch and TOT! Bad ring zones!
				2004-05-24 back to 64deg pitch!!!! Star press 3-8cm. 10min M2 MOT 10% len 10% W Shape spherical but 1F off. TDE not better
				2004-05-24 15min. M2 MOT. Center better. High ring bulge moving out. How to pus\h it over the edge? More extrem star press!
				2004-05-25 Star press 7cm-9cm. 5min M2 MOT. Only a small ring of pitch is at work! Push out ring furhter! Center good!
				2004-05-25 6min M2 MOT. Push out ring even more furhter! Center good! How to kill this last little ring?? Two spheres in almost perfect fit!!! Edges great!