Michelle Repeats
Estimating the number of repeats required in a
Michelle observing sequence
How to use the tables
Imaging:
Each filter uses a default exposure times, which, under normal
circumstances, should not be changed by the observer. This is listed in
column 2. The
default observation time in the OT is 20 seconds, which means 20
seconds
of observing time in each chop beam. The Michelle software
automatically
calculates how many exposures, or coadds, it can fit into 20 seconds
and
this is listed in column 3. Bear in mind this includes overheads the
observer
may not be aware of, such as array idling, readout and reset times.
Column 4 gives the total exposure time in one nod position,
which is exposure time x coadds x 2 chop beams.
Column 5 lists the total exposure time for one ABBA pattern.
To find out how many repeats you should specify in the OT for an
observing
sequence, you should divide your total required integration time by the
time given in column 5.
Column 6 lists examples of how many repeats are required to give a
total on-source integration time of 30 minutes.
Notes:
- The table assumes you have selected an observation time of 20
seconds in the OT instrument component. Halve the numbers if you use 10
seconds, or double if you select 40 seconds etc.
- The table assumes you are nodding and chopping on the array.
If your source is extended and you have to nod off the array, you
should double the number of ABBAs.
Spectroscopy:
The Michelle and OT software will calculate the required exposure time
for any combination of grating, wavelength and slit width. This is
listed in column 2 of the spectroscopy tables. NB. The exposures
times listed are for the 2-pixel wide slit. They will double if the
slit width is halved, and be halved if the slit width is doubled. As in
imaging, column 3 lists the number of exposures that are calculated to
fit into 20 seconds of observation time in each chop beam and columns
4, 5 & 6 provide the same information as imaging.
Notes:
- As above, the tables assume you have selected an observation
time of 20 seconds. Adjust appropriately if you have changed this
setting.
- The tables assume 1x1 sampling. If you use 1x2 for example,
then the total time in one nod is doubled, and if you select 2x2, the
time
is quadrupled. This obviously affects the total exposure time in an
ABBA
the same way. If you oversample, it is recommended that you
decrease the observation time by an appropriate amount, e.g., use 10
seconds if
you use 1x2 sampling.
- When the required exposure time is equal to or greater than
4 seconds, Michelle will not chop and instead use simple nodded
observations. In this case, the total time in one nod (or ABBA) is
halved, as chop beam B is not used.
- The tables assume you are nodding and chopping on the array.
If your source is extended and you have to nod off the array, you
should
double the number of ABBAs.
Imaging
Filter (microns, passband)
|
Typical single exposure time
(ms)
|
Coadds in 20 sec chop beam
|
Total exposure time in one nod
(sec)
|
Total exposure time in one
ABBA (sec)
|
No. of ABBAs for 30 minutes
integration
|
7.9, 10%
|
80
|
155
|
24.8
|
99.20
|
19
|
8.8, 10%
|
100
|
134
|
26.80
|
107.20
|
17
|
9.7, 10%
|
70
|
168
|
23.52
|
94.08
|
20
|
10.3, 10%
|
70
|
168
|
23.52
|
94.08
|
20
|
11.6, 9%
|
90
|
144
|
25.92
|
103.68
|
18
|
12.5, 9%
|
70
|
168
|
23.52
|
94.08
|
20
|
18.5, 9%
|
30
|
338
|
20.28
|
81.12
|
23
|
10.5, 53%
|
10
|
621
|
12.42
|
49.68
|
37
|
11.2, 21%
|
40
|
225
|
18.00
|
72.00
|
25
|
20.5, 43%
|
10
|
621
|
12.42
|
49.68
|
37
|
19.7, 27%
|
10
|
621
|
12.42
|
49.68
|
37
|
Spectroscopy
LowN Grating
Wavelength (microns)
|
Typical single exposure time
(ms)
|
Coadds in 20 sec chop beam
|
Total exposure time in one nod
(sec)
|
Total exposure time in one
ABBA (sec)
|
No. of ABBAs for 30 minutes
integration
|
10
|
200
|
78
|
31.20
|
124.80
|
15
|
10.5
|
210
|
72
|
30.24
|
120.96
|
15
|
21
|
430
|
42
|
36.12
|
144.48
|
13
|
LowQ Grating
Wavelength (microns)
|
Typical single exposure time
(ms)
|
Coadds in 20 sec chop beam
|
Total exposure time in one nod
(sec)
|
Total exposure time in one
ABBA (sec)
|
No. of ABBAs for 30 minutes
integration
|
21
|
360
|
48
|
34.56
|
138.24
|
14
|
MedN1 Grating
Wavelength (microns)
|
Typical single exposure time
(sec)
|
Coadds in 20 sec chop beam
|
Total exposure time in one nod
(sec)
|
Total exposure time in one
ABBA (sec)
|
No. of ABBAs for 30 minutes
integration
|
7
|
0.73
|
25
|
36.50
|
146.00
|
13
|
8
|
0.84
|
22
|
36.96
|
147.84
|
13
|
9
|
0.94
|
20
|
37.60
|
150.40
|
12
|
10
|
1.05
|
20
|
42.00
|
168.00
|
11
|
11
|
1.16
|
16
|
37.12
|
148.48
|
13
|
12
|
1.26
|
15
|
37.80
|
151.20
|
12
|
13
|
1.37
|
15
|
41.10
|
164.40
|
11
|
17
|
1.79
|
10
|
35.80
|
143.20
|
13
|
18
|
1.90
|
10
|
38.00
|
152.00
|
12
|
19
|
2.01
|
10
|
40.20
|
160.80
|
12
|
20
|
2.11
|
10
|
42.20
|
168.80
|
11
|
21
|
2.22
|
8
|
35.52
|
142.08
|
13
|
22
|
2.33
|
8
|
37.28
|
149.12
|
13
|
23
|
2.44
|
8
|
39.04
|
156.08
|
12
|
24
|
2.54
|
8
|
40.64
|
162.56
|
12
|
MedN2 Grating
Wavelength (microns)
|
Typical single exposure time
(ms)
|
Coadds in 20 sec chop beam
|
Total exposure time in one
nod (sec)
|
Total exposure time in one
ABBA (sec)
|
No. of ABBAs for 30 minutes
integration
|
7
|
2.22
|
8
|
35.52
|
142.08
|
13
|
8
|
2.54
|
8
|
40.64
|
162.56
|
12
|
9
|
2.86
|
7
|
40.04
|
160.16
|
12
|
10
|
3.19
|
6
|
38.28
|
153.12
|
12
|
11
|
3.52
|
6
|
42.24
|
168.96
|
11
|
12
|
3.85
|
5
|
38.50
|
154.00
|
12
|
13
|
4.18
|
4
|
16.72
|
66.88
|
27
|
17
|
5.54
|
3
|
16.62
|
66.48
|
28
|
18
|
5.89
|
3
|
17.76
|
70.68
|
26
|
>18.3
|
6.00
|
3
|
18.00
|
72.00
|
25
|
Echelle
Wavelength (microns)
|
Typical single exposure time
(sec)
|
Coadds in 20 sec chop beam
|
Total exposure time in one nod
(sec)
|
Total exposure time in one
ABBA (sec)
|
No. of ABBAs for 30 minutes
integration
|
All wavelengths
|
6.00
|
3
|
18.00
|
72.00
|
25
|
Imaging Polarimetry
Filter (microns, passband)
|
Typical single exposure time
(ms)
|
Coadds in 20 sec chop beam
|
Total exposure time in one nod
(sec)
|
Total exposure time in one
ABBA (sec)
|
No. of ABBAs for 30 minutes
integration
|
7.9, 10%
|
160
|
88
|
28.16
|
112.64
|
16
|
8.8, 10%
|
200
|
75
|
30.00
|
120.00
|
15
|
9.7, 10%
|
140
|
96
|
26.88
|
107.52
|
17
|
10.3, 10%
|
140
|
96
|
26.88
|
107.52
|
17
|
11.6, 9%
|
190
|
81
|
30.78
|
123.12
|
15
|
12.5, 9%
|
140
|
96
|
26.88
|
107.52
|
17
|
18.5, 9%
|
60
|
184
|
22.08
|
88.32
|
21
|
10.5, 53%
|
10
|
621
|
12.42
|
49.68
|
37
|
11.2, 21%
|
90
|
144
|
25.92
|
103.68
|
18
|
20.5, 43%
|
10
|
621
|
12.42
|
49.68
|
37
|
19.7, 27%
|
10
|
621
|
12.42
|
49.68
|
37
|
|
