Test of a dry objective with correction collar

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Test of a dry objective with correction collar
G. Sirri for the Bologna group
Dry objectives having an high numerical aperture
give aberration when imaging through coverslips
that deviate from design thickness and refractive
index. To prevent artifacts, many objectives
are equipped with correction collars that help
compensate for coverslip thickness variations.
By means of the Cover Glass Correction slider the
position of a movable lens group is adjusted
inside the objective. In this way, it is
possible to observe the specimen through variable
glass thickness from 0 to 2 mm. This can be
useful in emulsion scanning because the thickness
of the intermediate medium between the front lens
and the object plane ranges from 0 (when we scan
the top) to about 0.3 mm (bottom). Note that
when the collar is adjusted, focus tends to shift
and the magnification can slightly change.
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Characteristics of the objective
This type of objective can be found normally in
the market (from Leica, Nikon, Olympus or Zeiss)
and are largely used by biologists. We have used
for this test the ZEISS LD Achroplan (code
440864) magnification 40x numerical aperture
0.60 working distance 1.8 mm correction range
0-2 mm We installed this objective in our
prototypal microscope equipped with Zeiss tube
lens and our custom CMOS camera. The Field of
View is about 395x310 µm² The micron/pixel
ratio is 0.310 µm (top) and 0.305 (bottom).
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Test conditions
In our case the usage of this objective requires
a continuum collar rotation synchronously with
the movement of the vertical stage. However, we
used only 2 correction positions d0 for the top
and d0.25 for the bottom. 1.6 cm² has been
scanned in the plates 34,35,36,37,38,39 of the
reference brick (CERN June 2004, Brick 5 without
lead) and one-by-one compared with the same
measurements with the standard oil objective in
the standard microscope. 20 levels (4 inactive)
for view were grabbed. The minimum area of the
clusters was 3 pixels. Since we do not have any
automatic rotating system, we scanned all the
fragments of the top and then of the bottom.
After the merging the two raw data files into one
as in an ordinary acquisition, we analyze the
data with FEDRA.
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Grains analysis
z
Level-to-level distance 0.5 µm Vertical
clusters chains obtained using the libEGA of
FEDRA
cluster
eZ
z0
grains
g
OIL
DRY
Cluster area (pixels)
Cluster area (pixels)
cluster-to-grain_center vertical distance (µm)
cluster-to-grain_center vertical distance (µm)
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Base Tracks (pl35, 1.6 cm²)
QUALITY CUT
BACKGROUND
SIGNAL
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Base Tracks (pl34, 1.6 cm², chi_puls cut)
Slope Y
After the quality cut DRY 110
tracks/mm² OIL 92 tracks/mm²
Slope X
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Base Tracks microtracks base tracks agreement
DRY
Plate 35(0.0 , 0.0) s 6 mrad
Plate 35 (0.4 , 0.0) s 23 mrad
1.6 cm²
1.6 cm²
OIL
Plate 35(0.0 , 0.0) s 9 mrad
Plate 35 (0.4 , 0.0) s 21 mrad
4.8 cm²
4.8 cm²
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Volume Tracks (6 plates no holes 1.6 cm²)
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Volume tracks angular residuals
DRY
OIL
Linear fits
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Volume tracks position residuals
DRY
OIL
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Volume Tracks efficiencies
DRY
OIL
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Conclusions

• These are preliminary results because the on-line
  parameters were not well tuned.
• However, the use of this technique seems to give
  results comparable to our oil results on the same
  conditions.