OSIRIS Hieroglyph The OSIRIS User's Manual
[Contents]

Imaging Observations

This section explains how to set up imaging mode, techniques for data acquisition, and observing strategies. The topics covered in this section are:

Setting up Imaging Mode

If OSIRIS is not in imaging mode already, type the IMMODE command. This command switches the instrument into imaging mode by retracting the diffraction grating used in spectroscopic mode and flipping the prefilter wheel and pupil mask into the optical path. The next step is to select the camera and filter. Table 1 below lists the plate scale for each camera at the CTIO and SOAR telescopes and the current best camera focus positions.
Table 1: OSIRIS Imaging Scales
Camera
Telescope
Blanco 4m f/14.5 SOAR 4.1m f/16
Scale FOVa Scale FOVa
f/2.8
0.403"/pixel
233"
0.331"/pixel
191"
f/7
0.161"/pixel
93"
0.139"/pixel
80"

Table Notes:

a) Assumes that the imaging masks restrict the field to 577x577 pixels.
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Photometric Performance

Estimates for signal to noise calculations can be obtained from the Exposure Calculator. There is only one cold stop in OSIRIS, chosen to match the 4m at f/14, hence the 1.5m is stopped down approximately 15% in aperture relative to CIRIM. SOAR performance will be added here.
 
 

ADU/s
(15th mag)


e-/s
sky
(e-/s/pixel)
ADU/s
(15th mag)
e-/s
sky
(e-/s/pixel)
sky
(mag/sq arcsec)
 ADU/s
(15th mag)
 e-/s
sky
(e-/s/pixel)
sky
(mag/sq arcsec)
 
SOAR
 
4m
      
 1.5m
    
J    f/7


2037
6110
100
15.50
 200
 600
 49
 16.04
H    f/7


2007
6020
600
13.53
 200
 600
 240
 14.31
K    f/7


1113
3340
900
12.46
 125
 375
 840
 12.44
Ks   f/7


1000
3000
225
13.85
 134
 450
 571
 13.06
J    f/3


2037
6110
600
 15.54
 333
 1000
 300
 16.61
H    f/3


 2007
 6020
 3750
 13.53
 333
 1000
 2050
 14.52
K    f/3


 1113
 3340
  5625
 12.46
 186
 557
 5625
 12.79
Ks   f/3


1000
3000
1410
13.85
167
500
2934
13.38

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Observing Strategies: Point Sources

Once you have set up imaging mode, you then need to either select a filter from the filter wheel or the prefilter wheel. The current population of the filter wheels can be found here.

To select a filter from the filter wheel, type the command:

FILTER X
Where X is the position number of the filter in the wheel. To select a filter from the prefilter wheel, type the command:
PREFILTER X
For observing point sources, we recommend using a dithering pattern with offsets in arcseconds that are at least 5 times the size of the full-width-half-maximum (FWHM) of the point spread function (PSF). Telescope offsets can be done directly within Prospero with the command:
OFFSET RA=XX DEC=XX
where the values of RA and DEC should be specified in arcseconds and only one of the two keywords is required. You can also use the following shortcuts to OFFSET:
NORTH XX
SOUTH XX
EAST XX
WEST XX
where again the size of the offset is in arcseconds. See the Prospero Home Page for more information about Prospero commands and writing scripts.

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Observing Strategies: Extended Sources

Observing extended sources is similar to observing the Point Sources described above. The one additional complication is that you must also sky chop in order obtain sky frames. The OFFSET command or NORTH, etc. may be used to chop the telescope to obtain sky frames. However, this process is much more efficient if the sky chopping is done only north-south as the DEC drive is more efficient with offseting than the RA drive (the DEC drive does not have to track in addition to performing the offset).

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Photometric Calibration

To calibrate your data, we recommend using infrared standards on the CIT system as defined by Elias et al. [1982 AJ, 87, 1029]. However, these stars are too bright to observe with OSIRIS (due to current lower limit on the length of an integration). Instead, you can use the fainter standard stars of Persson et al. [1998 AJ, 116, 2475]. Here are links to the standard star lists and finding charts from this paper: Further photometric information is in Appendix B. It is important to observe standards that bracket your program objects in airmass in order to correct for atmospheric absorption. An estimate of the infrared, atmospheric extinction at CTIO is given by Frogel [1998 PASP, 110, 200]. If your observing program includes red objects (e.g. J-K > 0.5), you should also observe several stars from the Red Standards list in addition to stars from the Faint Standards list. Once color terms have been calculated for the current filter set, we will post that information here.
 

Color Transformations

The OSIRIS system is expected to have similar color transformations to the CIT/CTIO system as for CIRIM. The limited data we have obtained in photometric conditions on engineering nights is consistent with this expectation, though sufficient data are not in hand to compute accurate transformations. In particular, we expect the K band magnitudes to be the same for OSIRIS and CIT/CTIO, H-K to have little or no dependence on color, and J-K to have similar dependence (i.e. a significant non-zero color dependence) as for CIRIM (about 4%).

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Flatfielding

We will probably recommend using dome flat to calibrate the data once we have performed some tests during the engineering run. The best way to flatfield the data and how to take flats will be posted here once the information is available.

Typically, one should take dome flat sequences of lights on and lights off images. Each set is combined and then the two are used to construct a flat (on - off). Here is an image of a typical dome flat. The brightness enhancement near the center and vignetting in the corners are normal. This flat was obtained using the f/7 camera on the CTIO 4m. Images taken on other telescopes will look a bit different. If your flat looks much different than this, check with your support astronomer,and/or ask telops to check the dome lamps.

One can also make flats with sky or data images suitably combined (median and bright object filtering). The key here is to make sure you obtain a sequence of darks with the same frame integration time and number of coadds. We recommend taking such a series of darks several times each night. The basic illumination pattern should (obviously) look like the image above for the dome flats.

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Updated: 2005 June 29 [rdb]