The Giant Magellan Telescope (GMT) consists of seven 8.365 m segments, with
gaps of 0.345 m between adjacent segments. A unique challenge for GMT lies
in phasing the segments and, in particular, how to measure segment piston
optically while the telescope is in science operation. In this paper, we present
a dispersed fringe sensor (DFS) to make these measurements using a novel
algorithm. We show that using four off-axis DFSs operating at J-band with
10 ms exposures, we are able to measure segment piston to the required
50 nm accuracy every 30 s with over 90% sky coverage.
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The photometric zero point is for the whole telescope on-axis.
The background at K-short depends on the off-axis distance, as
explained in the text.
Table 4.
Limiting Magnitude as a Function of Sky Coverage at the South Galactic
Pole
Sky coverage (%)
50
80
90
95
99
I-band
12.89
13.67
14.03
14.27
14.81
J-band
12.31
12.98
13.34
13.58
14.01
H-band
11.86
12.44
12.82
13.11
13.50
K-short
11.72
12.32
12.66
13.01
13.36
Table 5.
RMS Segment Piston Error in nm as a Function of Off-Axis and Guide Star
Magnitude Using a J-Band DFS
J-Band Magnitude
Off-Axis Distance (′)
10
11
12
12.5
13
13.5
14
6
30
29
38
49
68
73
126
7
30
32
37
44
52
81
127
8
34
34
43
53
58
86
133
9
31
34
42
41
53
88
146
10
30
34
43
44
64
87
139
Table 6.
Off-Axis Aberrations for the GMT in Narrow-Field Mode as a Function of
Guide Star Location in the Directiona,b
Off-Axis Distance (′)
(nm)
(nm)
(nm)
6
2042
−7
10
8
3632
−47
−18
10
5675
−149
−30
The values are the coefficients of the Zernike polynomials [23] that have significant
power.
Tip-tilt and focus errors are ignored as they are easily corrected by
recentering the probes.
Table 7.
Performance of J-Band DFS Piston Sensor with Segment Tip-Tilt under
Different Wind Shake Scenarios
The photometric zero point is for the whole telescope on-axis.
The background at K-short depends on the off-axis distance, as
explained in the text.
Table 4.
Limiting Magnitude as a Function of Sky Coverage at the South Galactic
Pole
Sky coverage (%)
50
80
90
95
99
I-band
12.89
13.67
14.03
14.27
14.81
J-band
12.31
12.98
13.34
13.58
14.01
H-band
11.86
12.44
12.82
13.11
13.50
K-short
11.72
12.32
12.66
13.01
13.36
Table 5.
RMS Segment Piston Error in nm as a Function of Off-Axis and Guide Star
Magnitude Using a J-Band DFS
J-Band Magnitude
Off-Axis Distance (′)
10
11
12
12.5
13
13.5
14
6
30
29
38
49
68
73
126
7
30
32
37
44
52
81
127
8
34
34
43
53
58
86
133
9
31
34
42
41
53
88
146
10
30
34
43
44
64
87
139
Table 6.
Off-Axis Aberrations for the GMT in Narrow-Field Mode as a Function of
Guide Star Location in the Directiona,b
Off-Axis Distance (′)
(nm)
(nm)
(nm)
6
2042
−7
10
8
3632
−47
−18
10
5675
−149
−30
The values are the coefficients of the Zernike polynomials [23] that have significant
power.
Tip-tilt and focus errors are ignored as they are easily corrected by
recentering the probes.
Table 7.
Performance of J-Band DFS Piston Sensor with Segment Tip-Tilt under
Different Wind Shake Scenarios
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