The software for the MOS mask design requires an image ("preimage") which is distortion-free with an accurately-measured pixel scale. A mosaicked image by MOIRCS is usually used. The preimaging data must be properly distortion-corrected by the software provided by the SS (MCSRED or QMCS package).
Usually your preimaging data will be taken 2 to 3 months prior to your actual MOS observation, and it will be executed by asking to give us a bit of the observing time of the other observers (*1). The number of the preimaging fields should be equal (or less than) the number of the MOS mask approved by TAC: namely we can not accept any back-up preimaging fields. This is because we usually have to take your preimaging data by depriving the time of other unrelated observers for most cases, so as a policy the preimaging fields should be minimal. The other reason is that currently we cannot accept the back-up MOS masks due to the limited number of the available MOS slots.
(*1) Therefore it is highly recommended that you may also give small amount of your observing time for the preimaging of others instead. This is the observatory's policy for the preimaging data acquisition. Usually the preimaging observation takes about 30 minutes, and will be executed during the twilight time before sunrise.
As we should minimize the loss of the other open-use observers' time for your preimaging data acquisition, we want to keep the exposure for preimaging to minimum. By default, the net exposure time will be 5 times 30 seconds with some dither. It may not go so deep as to be able to detect objects as faint as Ks~20 mag. What we suppose you to do on that preimage is to register your deep images on that preimaging data and choose your targets from it, not from the preimage itself (*2). Please note that the world coordinate system (wcs) on the MOIRCS fits header is just the nominal one and does not have enough accuracy to use it for the mask design. If you want to use it, you have to put the correct wcs on the image header by yourself.
You will receive the raw data with the appropriate flat frames soon after we take the data. We ask you to do the data reduction by yourself. To do so, we will supply a simple IRAF scripts for quick data reduction. That software includes the distortion correction and mosaicking program.
The preimage for MOS design is made as follows. Firstly the raw data is flatfielded, sky-subtracted, and distortion corrected. After pasting the channel 1 and 2 data at the "shadow" region by the beam-splitting roof mirror to make a mosaic image, it will be rotated to counterclockwise by 90 degrees. Namely, the x-y axis of the preimage has been rotated from those of the raw image, with the channel-2 data at the top and the channel-1 data at the bottom.
Please note that the mosaicking rule we use in the mosaicking software is the canonical one which is based on the set of images we took near the zenith. The actual mosaicking rule for your preimaging data may differ a bit from the database we use due to the gravitational flexure. Therefore we do not recommend to use the data taken at elevation lower than 45 degrees. We will try to take your preimage at the elevation higher than 60 degrees to avoid the effect of the flexure. It is also recommended to avoid targets that does not go up higher than 50 degrees for MOS observation.
(*2) If you are the IRAF user, this will be done by the tasks called geomap and geotran. For the task geomap you can use the parameters below: fitgeometry="general", function = "polynomial" with the order of the fitting function at least higher than 3.
- The detector for channel 1 (chip 1) has more dead pixels and patchy high-dark regions compared to the one for channel 2 (chip 2). Also, there exists one-pixel-width non-data rows/columns between each quadrant. The unreadable regions are [1:1024,1025:1025], [1025:2048,1024:1024], [1024:1024,1:1024], and [1025:1025,1025:2048]. Examples of raw images are here (CHIP 1 [gif, fits] / CHIP 2 [gif ,fits]).
- As is shown in Figure 9 of the MOIRCS MOS website, the area available for putting slitlets is limited to within the 6'-diameter circular field of view, not the whole 4' by 7' MOIRCS fov.
- Each channel has its own grism. This mean that the spectra beyond each detector FOV is not acquired. You should keep the whole spectral range on single detector, unless you only need a particular part of the spectra.
- The MOIRCS pointing position lies around the center of the boundary between channel 1 and 2 (i.e. the image center after mosaicking). Please see the figures on the MOIRCS web page. The pointing accuracy of the telescope is usally within a few arcseconds.
- Position angle (PA) is defined as counterclockwise of the MOIRCS f.o.v. on the sky after mosaic, with PA=0 when the channel-2 side is to the North. See Figure 1 in the OPE file manual.
- The spectra from a slitlet extends differently with the different grism settings. Below is a rough position of the spectra for each setting. In the preimage coordinate, longer-wavelength direction is -Y for channel 1 and +Y for channel 2.
- If the preimage is properly distortion-corrected, you can shift the center of the preimage slightly from the original one for mask design. Small amount of the rotation should also be okay. But if you try the shift or rotation, please do it by your own risk, because we still does not test it extensively. Currently no trouble caused by the slight move of the center during the design has been reported so far.
- Bright stars are used for the alignment of the MOS mask and the target. These "alignment stars" should be more than three on each channel, i.e. at least >6 stars. Please select stars with 15 mag < Ks < 18 mag. Stars brighter than Ks=15 mag may saturate on the detectors, which makes the accurate measurement difficult. Faint stars are also not recommended because of the inaccurate positional measurement. For more information, please refer the section 3.3 of the MOIRCS MDP manual.
Basically the use of images other than MOIRCS data for preimage is not recommended with some exceptions. The FOCAS imaging data ("bigimage" i.e. the reduced and mosaicked data by FOCASRED software) can also be used as the preimage. If your target field is close to the center of the FOV, red band image by Suprime-cam may also be useable. The preimaging data by these instruments must be properly distortion-corrected by the software provided by each instrument team (FOCASRED for FOCAS, and NekoSoft for Suprime-cam). For the data reduced and distortion-corrected other than the above software we cannot guarantee the result, simply because we do not know how you reduced the data. If you are confident about your data reduction including the distortion correction, and if you know very accurate pixel scale of your data with the accuracy of 0.05% or better, you can use those data under your own risk. In principle it should work (*3).
(*3) The accrate pixel scale of MOIRCS preimage data reduced by QMCS or MCSRED package still has a slight uncertainty (0.1168--0.1169 arcsec/pixel: the value will be updated soon). Therefore, we still recommend to take all/part of your fields even if you are planning to use your own data.
If you have any question, please send emails to Ichi Tanaka.