Note: RATIR was removed from the telescope in June 2022.
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.
Figure 1. The architecture of RATIR, showing how three dichroics divide light between four channels.
The four channels are:
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 is always binned 2 × 2 to give 0.32 arcsec pixels. The total field of view is 5.4 arcmin square. This CCD is 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 needs 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 needs 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 is possible to map larger areas by mosaicing multiple fields.
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 Hα.
Note that the D1 dichroic transmits to the red of 690 nm, so redder filters are not useful in the C0 filter wheel. This means that Bessell RI are not available with RATIR.
The filters that are currently installed are listed in the status page.
The C1, C2, C3, and C4 channels have fixed i, ZY, and JH filters.
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 1. These limits do not include any penalty for sky subtraction and assume image quality of 1.5 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 1: 10-sigma limiting magnitudes in dark and bright time in 60 seconds.
| Filter | Dark | Bright | Magnitude Type |
|---|---|---|---|
| u | 18.5 | 18.3 | AB |
| g | 21.0 | 20.4 | AB |
| r | 20.5 | 20.2 | AB |
| i | 20.3 | 19.8 | AB |
| Z | 19.6 | 19.4 | AB |
| Y | 19.2 | 19.2 | AB |
| J | 19.4 | 19.4 | AB |
| H | 18.6 | 18.6 | AB |
| U | 18.0 | 17.9 | Vega |
| B | 20.4 | 20.2 | Vega |
| V | 20.4 | 20.2 | Vega |