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 we will take your preimaging data 2 to 4 months prior to your actual MOS observation. It will be executed during the morning 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 programs that are unrelated to you 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) As an observatory policy, we will ask you to complete your observation about 1 hour before sunrise for taking preimage. Instead we will try to minimize the time we would ask you to provide. The actual plan for preimage can be changed case by case, so you may want discuss about the preimaging plan with the SA considering your observing plan. For example, we may skip the preimaging observation if we do not have much pre-imaging fields. If you have a bit of non-target time earlier than the morning time, we may execute the preimage acquisition instead of the morning time if it is possible. Please ask the SA for the actual plan when your observation become near.
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 to 7 times 30 seconds with some dither (default is 25") in Ks band. 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 so poorly set that it is useless for the mask design. If you want to use it, you have to put the correct wcs on the image header by yourself by your own responsibility.
After we take your preimaging data, we will do a quick reduction and send them to you as soon as possible. The data reduction of the preimages is executed as follows. Firstly the raw data is flatfielded (by self-flats), sky-subtracted, and distortion corrected. After pasting each channel 1 and 2 data at the "shadow" region by the beam-splitting roof mirror to make a set of mosaic images, they are registered and median-combined. Then the resulting combined image is rotated to counterclockwise by 90 degrees. This is the image you will get as the preimage(3). Note that 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. If you try to do MOS observation to the targets that does not go up higher than 50 degrees, please be aware that the alignment accuracy may be worse than usual (though the previous experiences all worked fine).
(*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 equal or higher than 3.
(*3) Usually we do not provide the raw data of your preimage. If you want to receive the raw data, you can ask for it to the SA. You will be able to get them through STARS system.
- Our Hawaii-2 detector has several bad regions. The sensitivity variation is much larger than CCD. You can take them into account for the mask design. See the Detector Information page for detail.
- Currently the area available for putting slitlets is limited to within the 6'-diameter circular region in the FOV, not the whole 4' by 7' MOIRCS fov, as shown in Figure 9 . It may be changed in the future.
- You do not need to determine the coordinates with too much accuracy, because the pointing accuracy is sometimes limited to several arcseconds. The pointing position is the center of the 4' x 7' FOV after mosaicking.
- Targets close to the preimage center (assuming you do not shift the center fov during the design) cannot put the slit: i.e. avoid putting targets several arcsec around the boundary of chip 1 and 2 mosaic. Also, we recommend to avoid putting targets ~10 arcsec close to the edge of the preimaging fov.
- As is shown in Figure 2b and Figure 9 on the MOIRCS web page, the spectra of each channel is independently taken in each detector. Even though the spectra in one channel apparently goes on the other channel in the mask design software display, the actual spectra will be terminated at the end of the detector.
- The 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 2a and its note (4) for explanation.
- The spectra from a slitlet extends differently with the different grism settings. Be careful not to drop the wavelengths of your interest off the detector. 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.
- You can shift or rotate the preimage provided by the MOIRCS SA if it is taken at high elevation. But please note that we do not guarantee the result by the post-processing by users, though we have never had any serious problem before by such operation. Anyway it is always safer if you can use the data as it is.
- 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 as a recommendation. Please select stars with 15 mag ‹ Ks ‹ 18 mag. If all alignment stars has similar magnitude, selecting stars brighter than Ks=15 mag may still work fine by reducing the exposure time. If the star saturates, we can just ignore during the alignment. If the stars has enough good S/N in your preimage, this may mean you can use these stars for alignment. 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.
- If you want to simulate the mask design by actually running the design software, you can download it from the wmdp_moircs website. You can change your pixel scale to 0.117 arcsec/pixel and cut the image size to 2048 x 3569 to mimic the preimage.
- Typical slit length is around 10 arcseconds. Shorter slit length may gain multiplicity of slitlets. But at the same time it may lead a poor result during the residual night line (OH etc) subtraction if it is too short.
- If your target fields are in dark clouds or high latitude region with few stars, finding the suitable star for AutoGuider without vignetting the MOIRCS FOV may be difficult. It is highly recommended to check the availability of AG star before sending preimaging coordinates. See below for more detail.
We highly recommend to check the availability of the Autoguider stars before you finalize the preimaging coordinates. You can find the description of the AG issue in the Spectroscopy Information Page. Please refer the following link below.
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Go to MOIRCS Spectroscopy Mode: Information -- AutoGuider Issue.
Basically, the use of images taken NOT BY MOIRCS for preimage is not recommended but 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(*4)(*5).
(*4) 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 at least a part of your fields even if you are planning to use your own data. Namely, the use of any images that are well distortion-corrected should be okay if the magnification factor of the each pixel scale is well determined by the actual MOIRCS data.
(*5) Sometimes the FOCAS or Supreme-Cam data has advantage on astrometry, because there is no uncertainty by mosaicking. As MOIRCS is has two independent optics for each channel, the mosaicking rule is unfortunately affected by the gravitational flexure (change is still a level of ‹ 1 pixel). If the preimaging data is taken below 40degrees in elevation due to bad weather etc, you may check the mosaicking accuracy by your such data if it is available.
Please send the coordinate information of your preimaging fields using the following format to the SA by email. The SA will send the confirmation email within a couple of days. The submission should be BEFORE your targets become available (EL › 45 degrees) in the morning sky (1hr before sunrise). The delay of the submission may result in the failure of taking preimage. The early preparation and submission is always welcomed.
The format of the preimage is as follows.
TANAKA1=OBJECT=TANAKA_TARGET1 RA=123456.78 DEC=+123456.7 EQUINOX=2000.0 PA=0.0- Change the name (TANAKA_TARGET1) and its abbreviation (TANAKA1) as you like.
- The format of RA and DEC is in hhmmss.ss and ddmmss.s, respectively. For above case, it is RA=12h34m56.78s, DEC=12d34m56.7s
Put the RA, DEC, and PA very carefully because we do not check the fields individually. Sending correct coordinate is the responsivility of users. Usually we do not re-acquire your preimages even if you found it is worng, as long as it is your fault. You do not need to submit the finding charts for preimages because we usually have no time/ways to check on site. If you want the preimage other than Ks bands, you can explicitly request it when you submit the coordinates.
If you need the exposure much deeper than the default for your preimage, it mus be already approved by the TAC during the scheduling: i.e. it must be explicitly requested for approval in the proposal. The time for your deep preimages must be scheduled at the begginging of the semester. If the scheduled date is cloudy, however, the standard preimage will be done.
If you have any question, please send emails to Ichi Tanaka.