wiki:Overview

Overview

RATIR is a multi-channel imager for the OAN/SPM 1.5-meter Johnson robotic telescope. This page summarizes RATIR and the telescope for the benefit of potential users.

Observing Process

  • Once you have been awarded observing time, you should submit Phase 2 visit description files. We will use these to schedule and execute your observations.
  • You can follow the progress of your observations by consulting the nightly summary logs and the global exposure log in the archive.
  • You can download your data from the archive.

Architecture

The architecture of RATIR is shown in Figure 1. Three dichroics are used to image the same field with four channels. The dichroics reflect shortwards of 0.69, 0.83, and 1.03 µm and transmit longwards of these wavelengths.

The architecture of RATIR.

Figure 1. The architecture of RATIR, showing how three dichroics divide light between four channels.

Table 1. Channels

ChannelDetectorField SizePixel SizeFilters
(arcmin)(arcsec)
C0CCD5.40.32SDSS ugr and seven others
C1CCD5.40.32Fixed SDSS i
C2H2RG100.3Fixed WFCAM Z and Y
C3H2RG100.3Fixed MKO J and H

The four channels are summarized in Table 1. In more detail:

  • C0. This uses a Fairchild 3041 CCD with a UV coating. The detector format is 2048 × 2048 pixels each 15 µm square, but the CCD will normally be binned 2 × 2 to give 0.32 arcsec pixels. The total field of view will be 5.4 arcmin square. This CCD will be equipped with a filter wheel containing up to ten 50 mm diameter filters.
  • C1. This uses another Fairchild 3041 CCD, but this time with a broad-band coating rather than a UV-optimized coating. The scale and field are close to that of the C0 channel. This CCD is equipped with a fixed SDSS i filter.
  • C2. This uses a HAWAII-2RG detector with a 1.7 µm cut-off. The detector format is 2048 × 2048 pixels each 18 µm square. Powered optics give a pixel scale of about 0.30 arcsec and a field of about 10 arcmin. A fixed filter is installed close to the focal plane. The filter is split along a N-S axis, with the eastern half of the detector being imaged in a WFCAM Z filter and the western half in WFCAM Y filter. The 45-arcsec wide strip centered on the join is not imaged cleanly.
  • C3. This uses another HAWAII-2RG detector with a 2.5 µm cut-off behind the same powered optics as the C2 channel. Again, a split filter close to the focal plane images the eastern half of the detector in MKO J and the western half in MKO H.

Fields

Figure 2 shows the instantaneous fields of the detectors. The effective field of view will depend on the dithering strategy:

  • If one wishes to obtain images in riZYJH, one will need to dither between the two regions common to riZJ and riYH. If this is done, the effective field of view will normally be slightly smaller than 2.7 × 5.4 arcmin.
  • If one wishes to obtain images in ZYJH, ignoring the CCDs, one will need to dither between the two regions common to ZJ and YH. If this is done, the effective field of view will normally be slightly smaller than 5 × 10 arcmin.
  • If one wishes to obtain images only with the CCDs, the effective field of view will be 5.4 × 5.4 arcmin.

Of course, it will be possible to map larger areas by mosaicing multiple fields.

The fields seen by each channel.

Figure 2. The approximate fields over the RATIR detectors. Any of the filters in the filter wheel can substitute for the r filter.

Filters

The C0 channel has a 10-position filter wheel for 50-mm round filters. We intend that the SDSS ugr filters be always installed in the filter wheel. The CATT will decide which additional filters will be installed in the other seven slots in the filter wheel, from possibilities that include:

  • Bessell UBV
  • Strömgren-Crawford uvbyNW
  • Nebular filters such as H-alpha.

The C1, C2, C3, and C4 channels have fixed i, ZY, JH, and z filters.

For more details, see the wiki page on filters.

Telescope

The telescope is located at

longitude L = -115° 28' 00'',
latitude φ = +31° 02' 43'',

and

height above mean sea level = 2790 meters.

The telescope can point between +57° and -27.5° in declination, ±5h20m in hour angle, and up to 85° in zenith distance.

The standard pointing precision of the telescope is about 15 arcsec RMS. In fields with a sufficient density of moderately bright stars (TBD), a special pointing mode is expected to achieve a precision of 1 arcsec (TBC).

The offset precision is about 2 arcsec RMS. Offsets of up to a few arcmin take about 10 seconds.

The median image quality of the telescope in riZYJH is about 1.0 arcsec.

Operation

The instrument and telescope will be operated robotically.

Observations will be organized into “visits”, consisting of a single slew followed by a number of exposures and offsets. The scheduler will allow the execution of a visit to be constrained by the sky brightness, the airmass, the UTC, and also the likelihood of photometric conditions.

The instrument is expected to be used in two main modes:

  • “Infrared Mode”. In this mode, all four instrument detectors are used for science exposures with the same exposure time of up to 60 seconds. The finders are used for guiding.
  • “Optical Mode”. In this mode, one of the instrument CCDs is used for a science exposure of up to 1800 seconds and the other is used for guiding.

Sensitivity

The sensitivity of RATIR is given in limiting AB magnitudes in ugri and limiting Vega-based magnitudes for UBV. To convert AB magnitudes to approximate Vega-based magnitudes, see Table 7 of Hewett et al. (2006, MNRAS, 367, 454).

The estimated point-source 10-sigma limiting magnitudes in 60 seconds are shown in Table 3. These limits do not include any penalty for sky subtraction and assume image quality of 1.0 arcsec FWHM. We envisage that most infrared observations will be carried out with 60 second exposures, as this exposure time is needed to reach the background limit in iZYJH.

Table 3: The estimated point-source 10-sigma limiting magnitudes in dark and bright time in 60 seconds.

FilterDarkBrightMagnitude Type
u18.918.8AB
g21.520.9AB
r21.020.7AB
i20.820.3AB
Z20.119.9AB
Y19.719.7AB
J19.919.9AB
H19.119.1AB
U18.518.4Vega
B20.920.7Vega
V20.920.7Vega

The estimated point-source 10-sigma limiting magnitudes in 960 seconds of infrared mode observations are shown in Table 4 and Figure 3. The 960 seconds of total exposure consists of 16 exposures each of 60 seconds, with 8 exposures on the object in riZJ and 8 exposures on the object in riYH. These limits include the penalty for sky subtraction assuming that 7 sky exposures contribute to the mean sky image and assume image quality of 1.0 arcsec FWHM.

Table 4. The estimated point-source 10-sigma limiting magnitudes in dark and bright time in 960 seconds of simultaneous observations in riZYJH in infrared mode.

FilterDarkBrightMagnitude Type
r22.522.2AB
i22.321.8AB
Z21.220.9AB
Y20.820.8AB
J20.920.9AB
H20.120.1AB

The estimated 10-sigma limiting AB magnitudes in infrared mode.

Figure 3. The estimated point-source 10-sigma limiting AB magnitudes in dark and bright time in 960 seconds for simultaneous observations in riZYJH (“infrared mode”).

The estimated point-source 10-sigma limiting magnitudes in 1800 seconds of optical mode observations are shown in Table 5 and Figure 4. These limits do not include any penalty for sky subtraction and assume image quality of 1.0 arcsec FWHM. Since only one CCD will be used for science, only one filter can be observed per exposure, unlike infrared mode observations.

Table 5: The estimated point-source 10-sigma limiting magnitudes in dark and bright time in 1800 seconds of observations in optical mode.

FilterDarkBrightMagnitude Type
u21.221.1AB
g23.722.8AB
r23.222.7AB
i22.822.3AB
U21.020.9Vega
B23.222.8Vega
V23.122.6Vega

The estimated 10-sigma limiting AB magnitudes in optical mode.

Figure 4. The estimated point-source 10-sigma limiting AB magnitudes in dark and bright time in 1800 seconds for observations in ugri using one CCD (“optical mode”).

To give more detail of the expected sensitivity of the CCDs as a function of magnitude and exposure time, Figure 5. shows the signal-to-noise ratio as a function of magnitude for 10, 60, and 1800 second exposures for dark and bright time.

Limiting magnitude in ugri and UBV for 10, 60, and 1800 second exposurs.

Figure 5. The estimated signal-to-noise ratio in dark and bright time. The magnitudes are AB for ugri and Vega-based for UBV.

Issue: The efficiency in u and U is unexpectedly low and the dark current in C0 is unexpectedly high. In combination, this leads to a low sensitivity in u and U.

Ghosts

There are weak ghosts in C0 and C1.

Last modified 3 years ago Last modified on May 23, 2015 2:04:30 AM

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