MOIRCS Multi-Object Spectroscopy Mode

The Layout of the Slit Mask

Layout of a long slit mask and a multi-object slit mask projected onto the detectors of MOIRCS is shown in the following diagram. Also, please refer to the FOV page.

Figure 9: A layout of the slit masks.

Preimaging for mask design: Guideline

Usually we ask all MOS observers to do the mask design based on the shallow MOIRCS images (the "preimage") of your fields. We take your preimages when your MOS targets are available at > 50 degree in elevation during the morning time, which will occur roughly 2 to 4 months before your observation. The PIs granted MOS time should start the preparation of the preimaging coordinates immediately after you are announced awarding the observation time. For more detail, please read the following Pre-imaging Information Page carefully.

Pre-imaging Information Page

As an observatory policy, we will ask ALL MOIRCS MOS observers to surrender a part of your time for pre-imaging for other observers. Pooling pre-imaging times this way is necessary due to the limited amount of engineering time: your pre-images are also prepared by using observation times of other observer. All spectroscopy observation is requested to be completed one (1) hour before sunrise^(*1). Please make sure that your observation program can be completed by the time including the time for the mask store.

Despite the above policy, the actual use of the preimaging time can be revised by the situation. You and the SA may need to discuss about the actual plan when your observation become near. Especially, during the season when weather is generally bad, we may have to ask to increase the pre-imaging pool time a bit longer than 1 hr. We will greatly appreciate it if you can understand the situation.

Please be aware that there is always a risk of failing the preimaging data acquisition due to bad weather, instrument problems, or any other unforeseeable causes, even though we make our best to obtain your pre-image in time for the mask preparation. Therefore, all MOS observers are recommended to prepare for situations where pre-images by MOIRCS are not available.

(*1) This guideline has been considered as the burden of all MOS observers by the recommendation of the 14th SAC Meeting since S08B.

Preimaging data: observation and the mask design software

The preimages will usually be the shallow (a few minutes exposure) data. This may not be deep enough for your targets. What we suppose is that you superpose your own deep science images to the preimaging data by yourself, and choose targets from the superposed science image, not from the pre-image itself. Therefore, all applicants for MOS observation should already have your own deep imaging data of the target fields in FITS format before the mask design.

If you need much longer exposures for preimaging data than described in the Pre-imaging Information Page, or if you need special setting for preimage that may take long time, you must explicitly request the time for imaging observaton with the description about the detail (including the reason for special requirement) on your proposal (technical detail section). We execute it only when it is approved by the TAC. Any small change (e.g. change of the filter etc) may be okay if you send the SA about the change well before your preimaging date.

For the mask design you can use the dedicated software provided by us. The software, wmdp_moircs^(Note), is the IDL-based program (you can run it with a free virtual machine). The mask design file has the same format as those for FOCAS: i.e., it is so called .mdp file. You can download the most recent version of the wmdp_moircs.sav from the website below.

Mask Design Software (wmdp_moircs) Distribution Page

The Users Manual for MOIRCS MDP has also been prepared.

(Note) The software wmdp-moircs.sav is based on the FOCAS MDP software (wmdp4).

MOS Acquisition

The MOS acquisition sequence is summarized as follows (see Figure 10).

Pointing
Imaging the target field
MOS mask install (at zenith position)
Imaging the MOS mask
Measure the position of the alignment stars and holes
Offset calculation, and moving Telescope
Re-imaging with MOS mask.
Alignment check
Final adjustment of telescope pointing
Final imaging for confirmation
MOS exposure

Typical magnitudes of alignment stars are roughly 14>K_s>17. If alignment stars are too bright, it will cause saturation on the image and may introduce the position measurement error (though we can reject the star during the alignment). If the stars are too faint (or if you choose a galaxy instead of a star), again the measured position may become less accurate. Selecting more than three alignment stars for each channel is strongly recommended. It takes about 25 minutes for MOS acquisition (see the Guide to Exposure Time section).

The alignment accuracy is better than ~0".1 RMS, enough small compared to the typical slit width for a MOS observation of 0".8.

Figure 10: The MOS acquisition sequence (Tokoku 2006, Ph.D. Thesis)

Please note that all data on these pages are subject to changes as the evaluation of the performance of MOIRCS progresses.

Updated 2009-06-15