Testing of the 10" RC Cassegrain optical system

The primary mirror with its f2.6 focal ration requires a special test methodology. The lateral wire test has shown to give the best accuracy here. For the test a test setup was built. For the measurements a transparent ruler is placed in front of the mirror and the position of the wire is measures. From the measurements the shape of the mirror can be calculated. To get an over view of the shape a ronchi test is also performed after each polishing session.
 

Polishing turn table

I made a simple turn table to rotate the glass and have more control over the zones that the tool works on.
 

Polishing progress

 
Ronchigram Plot matched to best conic fit Plot matched to target Comment and polishing remarks
    30min. 4" TOT center stroke 20% offset full length. 50% point pressure.
    30min. 10" MOT parabolizing W-stroke.
    30min. 10" MOT parabolizing W-stroke.
    20min. 4" TOT center stroke 10% offset full length. 80% point pressure.
    20min. 4" TOT center stroke 10% offset full length. 80% point pressure.
    30min. 10" MOT parabolizing W-stroke.
    30min. 10" MOT parabolizing W-stroke.
    20min. 4" TOT center stroke 10-20% offset full length.
    20min. 10" MOT parabolizing W-stroke
    20min. 4" TOT center stroke full length.
    30min. 4" TOT center stroke full length.
    20min. 4" TOT center stroke full length. 50% point pressure.
    20min. 4" TOT starlap center stroke full length.
  20min. 4" TOT starlap center stroke full length.
  30min. 4" TOT starlap center stroke full length 
  30min. 4" TOT starlap center stroke 33% full length 33% 2/3 length, 33% 1/3 length.
  30min. 4" TOT starlap center stroke 33% full length 33% 2/3 length, 33% 1/3 length.
    30min. 4" TOT starlap center stroke 33% full length 33% 2/3 length, 33% 1/3 length.
  40min. 4" TOT starlap center stroke 60% full length 40% 1/2 length.
  20min. 4" TOT starlap center stroke 60% full length 40% 1/2 length.
    5min. 4" TOT starlap circles on edge.
    10min. 4" TOT starlap circles on edge.
    20min. 4" TOT starlap center stroke 60% full length 40% 1/2 length.
    10min. 4" TOT starlap circles with point pressure on edge.
  10min. 4" TOT starlap circles with point pressure on edge.
30min. 4" TOT starlap long central stroke 40% point pressure.
3min. 4" TOT starlap circles R=60mm
6min. 4" TOT starlap circles R=70mm. 
4min. point pressure circles on edge.
    10min. 4" TOT long strokes 20mm offset from center. 50% point pressure. 
5min. circles on outer zone.

10min. 4" TOT long strokes 20mm offset from center. 50% point pressure
20min. 4" TOT starlap circles on outer zone.
5min. 2" TOT circles on R=50mm.
  10min 2" TOT circles on R=40-70mm.
12min. 2" TOT circles on R=50-70mm.
30min. 2" TOT on turn table. Close to edge. Repress lap.
30min. 2" TOT on turn table. Close to edge. Bad last measurement data point.
10min. 4" TOT starlap. Long center strokes 50% normal, 50% point pressure. Too much.
    45min. 2" TOT turntable edge on edge +-5mm. Point pressure. Groove 5mm from edge.
    15min. 2" TOT turntable center on edge. No pressure. Groove softer. Needs more.
    20min. 2" TOT turntable center on edge. No pressure. Groove softer. Needs more.
    30min. 2" TOT turntable center on center. No pressure. Needs 4" to soften.
20min. 4" TOT starlap  turntable center on edge. No pressure. Outer zone softer. Center on edge is best. More!
40min. 4" TOT starlap turntable center on edge. No pressure. Edge better.
30min. 4" TOT starlap turntable center on edge. Edge down? Bad measurement.
10min. 4" TOT starlap circles R=80mm.
5min, 4" TOT starlap circles R=50-80mm.
20min. 2" TOT turntable. Center on edge.
20min. 2" TOT turntable. Center on edge.
20min. 4" TOT starlap turntable. Center on edge.
20min. 2" TOT turntable. Center on edge 
+ 6min. R=40-60mm
10min. 1" TOT  turntable R=115mm 
+ 6min. 2" TOT R=40-90mm.
20min. 2" TOT turntable . Center on edge.
5min. 2" TOT turntable R=70mm. 
+ 5min. center on edge.
5min. 2" TOT turntable R=40-70mm. 
+ 5min. center on edge.
5min. 1" TOT turntable R=60-80mm. + 5min. R=110-120mm.
8min. 2" TOT turntable R=50-90mm.

From now on test with 100mm diam. central obstruction

 
Repeat measurement 2003/12/24b.
5min. 2" TOT turntable R=60-110mm.
3min. 2" TOT turntable R=70mm. 
Try 1" TOT on 2 zones. 2min each.

At this point the center hole in the primary was cored out. Thanks to Bill Marriott and his optical shop for the great work!

 

Only a very few scratches showed up around the hole. But that area is covered by the secondary anyway!

From now on testing could not be performed with the reference pont in the center of the mirror. An averaging technique was used to find a virtual center mark. Again 100mm obstruction is assumed.
 
The hump at R=75mm has gotten a bit higher. But that can be a measurement artifact. I'll go 3min 2" TOT at R=75mm
  4min.2" TOT turntable. Zone at R=75mm better. Probably measurement artefact at R=120mm? Remeasure!
  No polishing! Remaesured. Try 3min. 1" TOT R=100mm.
  4min. 2" TOT turntable. R=70-110mm.
Too much!
  2min. 1" R=105-120mm + 2min. 1" R=65-85mm.
Very close!
  1min. 1" R=75mm. 
1min. 1"
R=105mm.
Measurements at 0deg and 240deg agree. 120deg seems to have a problem. 
110mm zone still visible. 

One last try.
2min. 1" R=110mm + 2min. 1" R=68mm.

After re-measuring the ruler used in the wire test the tick marks on the ruler were made more accurate. Some were off by perhaps 0.3mm. With the updated ruler the measurements were repeated and it seems that the shape of the mirror has been slightly misjudged. From now on all measurements are with the updated ruler and 80mm obstruction to give a better overview of the mirror over a larger area.
 
  Remeasurement shows high zone at R=55-85mm and low zone at R=85-100mm.
Try 3min. 1" TOT on R=55-85mm.
  1" 3min. TOT R=55-85mm.
  2" 3min. TOT R=75mm.
  1" 5min. TOT R=50-80mm
  1" 6min. TOT R=72mm
  1" 6min. TOT R=67mm.
1min. Edge zone.
  1" 5min. TOT R=72mm.
2min. R=115mm
  1" 6min. TOT R=55-75mm.
3min. R=115mm.
  1" 5min. TOT r=105-110mm.
  1" 10min. TOT R=100-125mm.
  1" 5min. TOT R=95-120 
+ 5min. R=65-90mm.
  1" 5min. TOT R=110mm 
+ 8min. R=70-90mm
  1" 12min. TOT R=80mm.
  New shaving and pressing of 1" lap.
1" TOT 6min. R=80mm.
  1" TOT 15min. R=75-80mm..
  2" TOT 6min. R=75=80mm.
  2" TOT 12min.  R=75-80mm.
  2" TOT 8min. R=85mm 
+ 4min. R=105mm.
  2" TOT 8min. R=65-82mm 
+ 4min. R=105mm.
  2" TOT 10min. R=65-90mm.
   2" TOT 6min. R=55-80mm.
  2" TOT 
3min. R=55mm,
3min. R=85mm,
3min. R=55-85mm.
  2" TOT 
5min. R=65mm,
3min. R=65-85mm.
 
  2" TOT
4min. R=65mm.
    Replot with 100mm diam obstruction + only 9zones. 
96%Strehl.
DONE!
       
     

 
 

Email : drgert1@yahoo.com

Back to my astronomy home page
Zurueck zur Astronomie Startseite