Estimating the number of repeats required in a Michelle observing sequence

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 1×1 sampling. If you use 1×2 for example, then the total time in one nod is doubled, and if you select 2×2, 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 1×2 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