Information for Observers

Further information

Questions regarding this page should be directed to Kentaro Aoki ( ).


Planning observation

  1. Prepare a source list
  2. Observers first need to make a list of coordinates for science targets, guide stars, coordinate calibration stars, telluric absorption correction stars and flux calibration stars (see below for details except for science targets). All the candidates can be included in the list, so-called "fld" file for the spine allocation software, at this stage, since target selection for an optimal spine allocation will be performed by a software (see the next section). To ensure the accurate spine allocation and subsequently minimize flux loss from the fibers, observers need to be careful about the following things:

    • Astrometry

      Relative astrometry of your science targets compared with guide stars and coordinate calibration stars is critical because:

      • The fibers are positioned to science targets by using guide star positions as fiducials.
      • Coordinate calibration stars are used to "align" the telescope and instrument to the target field by applying lateral offset to telescope pointing and rotational offset.

      The error of spine allocation in a purely mechanical sense is ~10 μm (i.e. ~0.1 arcsec) and the error of relative astrometry should not be much worse than this. In order to check the relative astrometry of science targets, guide stars, and coordinate calibration stars, observers need to cross-check the coordinates of stars on their catalogs with those in a good astrometric catalog (e.g. UCAC4, 2MASS point source catalog). UCAC4 would be particularly recommeneded because stars are catalogued down to R ∼ 16 mag from the entire sky, which is suitable for selection of guide stars and coordinate calibration stars. - Please be aware that the cameras to observe guide stars & coordinate calibration stars are CCD cameras for visible wavelengths, while FMOS is a NIR instrument.

      Note:

      The guideline of the brightness of guide star and coordinate calibration star is give below in R band magnitude, since the sky camera and guide camera are used with a red-pass filter applied. Meanwhile, as explained in this page, in Section 3 (Properties of the catalog and important notes for the user), Item d (Magnitudes), the "R-band" magnitude in the UCAC3 catalog is not that in the standard R band but in a 579-642 nm bandpass which is between V and R. Observers need to take this into account in selecting stars from the UCAC3 catalog based on the "R-band" magnitude.
    • Guide stars (GS)(F stars in the spine allocation software)

      Auto guiding (AG) is performed by taking images of the guide fiber bundles (GFB) and calculating the centroids of GS. In total there are 14 GFB located at the edge of the field of view (FOV) as shown in Fig. 1a (NOTE: Two GFB are currently unavailable due to their less good positioning cababilities). The AG camera is a CCD camera with a long-pass filter (OG590, transparent at > 0.6 μm) in front of it. Observers therefore need to have GS bright in red optical bands and located near the edge of FOV. A guideline for GS brightness is R = 12 - 16 mag (this of course depends on seeing condition and sky transparency). Observers are highly recommneded to choose field center and PA to maximize the number of available guide stars: The more guide stars are available, the more stable AG operation is expected. Especially, the operation should be rubust against unexpected contamination of stars with large proper motions, double stars, those with large errors in astrometry, magnitude, etc. In addition, if all GS are bright, the exposure time for AG could be shortened. Then telescope pointing correction could be applied at a shorter interval and subsequently guding error could be smaller.

    • Fig. 1a - A sketch of the guide fiber bundles. Note that two guide fiber bundles are unavailable due to their less good positioning cababilities. Fig. 1b - A collection of the images of 14 guide fiber bundles during an actual auto guiding. 9 guide stars are clearly visible around the center of each bundle.

    • Coodinate calibration stars (CCS)(K stars in the spine allocation software)

      These stars are used to align the telescope and instrument to the target field and acquire GS on GFB, by applying corrections to the telescope pointing and/or instrument rotator angle. This alignment (or field acquisition) process is necessary because:

      • The typical pointing error of the telescope (≈ 4-5 arcsec) is larger than the FOV of GFB (≈ 2.5 arcsec in diameter), GS are not necessarily acquired by GFB after telescope pointing.
      • If a rotational offset remains, it cannot be fully corrected by auto guiding, resulting in a significant amount of flux loss of science targets.

      To do this alignment, the sky camera (a CCD camera in the Echidna Focal Plane Imager with an FOV of ~ 1 arcmin, with a red-pass filter OG590) moves to the positions of CCS in the FMOS FOV and takes images of CCS to measure the displacements of their positions from where they should be. A guideline for CCS brightness is R = 12 - 15 mag. To work out not only lateral offset but also rotational offset simultaneously, observers need to select one CCS near the field center and a few stars surrounding it in the outer field.

      NOTE(1): The CCS near the center of FOV is also used for focusing operation: The same magnitude limit as other CCS (R = 12 - 15 mag) is applied. If there are CCS only at distances larger than 10 arcmin from the center of the FOV and one of those stars has to be used for focusing operation, the focus value derived could be less accurate due to the effect of curvature of focal plane.

      NOTE(2): Observers may leave many CCS available in S2O files (see below for details about S2O file): Although the actual field acquisition is performed by using ∼ 5 CCS, if more CCS are available in an S2O file then the instrument control system picks up several from them considering the positions in the FoV.

    • Telluric absorption correction and flux calibration stars (Z and C stars, respectively, in the spine allocation software)
    • What has been usually done so far is to assign a few fibers to observe faint stars simultaneously with science targets. A guideline of the brightness of stars for this method is 15-18 mag (AB) in JH. The brighter limit of this range is set to minimize the effect of ghost features after the typical exposure time of an individual frame (i.e. 15 min), which tends to appear at a three-orders-of-magnitude (i.e. 7.5 mag) fainter level than the original brightness. The fainter limit is set to keep S/N of these stellar spectra high enough for calibration. The spectral types preferred are F, G, and K early dwarfs (A stars can be handled by the reduction package but are not recommeneded). Broad-band colors are expected to be useful to select them in advance, while it is also possible to estimate spectral type from observed spectrum, given the instrument throughput.

      In theory, such correction/calibration is possible if one star is observed per spectrograph, but it is strongly recommended to observe a few to several stars possibley of different types so that the result of the correction/calibration can be cross-checked.

      An alternative method is to observe a standard star in a different field before and/or after observing a science target field. Observers would need to prepare an S2O file for a standard star observation separately from those for science target field observations, where field center, CCS, & GS are necessary in the same way as science fields. This method would be recommended e.g. when a target field is at a low Galactic latitude and standard stars available there as well as science targets are highly affected by Galactic extinction.

  3. Run the spine allocation software
  4. !! Important notes for S14A observers !!

    • We request S13A observers to download the latest version (20130311 version) from the link below and prepare s2o files using it. We beieve the known issues on the previous version have been solved.

    Observers will need to prepare and provide the "s2o" file which is an output from this S2O software. The input file of the S2O software is called "fld" file. The details and formats of those files are described in the documents in the software package. You need to prepare the information below:

    • Field center
    • Position angle
    • Observing wavelength

      Due to the effects of atmospheric dispersion and chromatic aberration, the position of an object on the focal plane is slightly different as a function of wavelength. Observers therefore need to rightly input "observing wavelength" in the S2O software. Below listed is the guideline of observing wavelength for each observation mode:

      Operational mode Observing wavelength
      Low resolution 1.3-1.4 μm
      High resolution J-short 1.0 μm
      J-long 1.20-1.25 μm
      H-short 1.50-1.55 μm
      H-short prime 1.55-1.60 μm
      H-long 1.65-1.70 μm

    • Observation date
    • Maximal hour angle acceptable for observation
    • Source list (priorities can be given to the science targets)
    • RA & DEC offsets for beam switching (unless the observing method is Point & Stare - see below)
    • Observing method
      • Normal beam switching --- In this mode, the telescope is offset between "ON" and "OFF" positions, where the fibers look at objects (sky) at the "ON" ("OFF") position, respectively. Half of the observation time will be spent for sky exposure.
      • Cross beam switching --- In this mode, two fibers are allocated to one object, and the telescope is offset between two positions so that either of the two fibers observes the object and the other observes sky. The advantages of this method are: (1) 100% of the time can be spent observing objects, and (2) at least in principle, sky subtraction is not affected by time variation of sky brightness. The disadvantage is that the maximal number of spines allocated to objects is 200. Since the geometrical constraint to spine allocation is strong, the actual number of allocated spines could be even smaller in reality.
      • Point & stare --- There is no telescope offset in this mode. Instead, some fibers need to be placed on blank sky region and the average of the sky spectra is applied to other fibers for sky subtraction. For example, when most of the objects are relatively bright and the accuracy of sky subtraction is not extremely critical, this mode can be used.
    The software then derives an optimal spine allocation and position angle of the instrument considering a number of matters such as:

    • Maximizing the number of high priority targets with spines allocated
    • Maximizing the number of guide stars located on the guide fiber bundles
    • Minimizing spine tilts
    • Avoiding spines to cross each other

    Fig. 2 - A snapshot of the spine allocation software. The arrows show the spines allocated to science targets or guide stars.

  5. Save the output file(s) (*.s2o) and send it to the support astronomer.
  6. The spine allocation software produces an output file (*.s2o). According to the information in this S2O file, the Echidna Instrument Control Software (ICS) will configure the positions of the science fibers and guide fiber bundles for actual observations.

    To finalize the spine allocation, please refer to the items in this check list and check if everything is considered properly.

    Observers should send the final version of S2O files to support astronomer(s) for FMOS open-use observations (Kentaro Aoki ). If you observe more than one spine allocations/target fields, please send all the corresponding S2O files.

    For clarity and maintenance purpose, please name your S2O files following the format "PROPOSALID_PILASTNAME_ANYCOMMENT_OBS.s2o". In the "ANYCOMMENT" part, observers can choose a useful phrase to classify the file (e.g. name of target field). "OBS" should be replaced with one of "nbs", "cbs", or "pas" depending on the selected observing method: Normal Beam Switching, Cross Beam Switching, or Point & Stare, respectively (see above in this section for the details of the observing methods). For example, if your proposal ID is S10A-000 by PI: Tamura and you observe a target field named "somewhere" by cross beam swiching, then please name the S2O file as "s10a_000_tamura_somewhere_cbs.s2o".

    NOTE: S2O file name needs to be CASE-INSENSITIVE. Although it is OK to write an S2O file name using either all upper cases or all lower cases, please DO NOT mix them. For example, tamura.s2o and TAMURA.S2O are recognized as the same file by the Echidna ICS.

  7. Prepare an OPE file

    • What is OPE file?
    • Observation Procedure Execution (OPE) file contains a list of abstract commands to operate the telescope and instrument in actual observations. Like the other instuments on Subaru, observers using FMOS will also need to prepare an OPE file according to their plans of data acquisition during a night. Target information, observing method, exposure time, and so on will also need to be specified in this file.

    • How is it used?
    • In an actual observation, the content of an OPE file is displayed on the main telescope system control GUI. A support astronomer or telescope operator then selects the line for a certain command in it (with parameters in many cases) to execute when it is necessary. Observations proceed usually by repeating this for different commands.

      Although an OPE file can be edited on the GUI during a night, to minimize overhead, it is important for observers to provide all the target information and prepare all the commands in OPE files that are used in their observations. On the other hand, since a command is manually picked up from an OPE file and is executed one by one, it is not extremely important to organize the commands exactly along the time sequence in an OPE file.

    • Template OPE file
    • A template of OPE file for FMOS observation is available here:

      Observers will need to download the text version of template and edit it to make one for your observation. Please refer to the PDF version as a brief manual. Then it needs to be sent to the support astronomer for FMOS open-use observations (Kentaro Aoki ).

      For clarity, please name your OPE file following the format "PROPOSALID_PILASTNAME.ope". For example, if your proposal ID is S10A-000 by PI: Tamura, then the name of an OPE file would be "s10a_000_tamura.ope". Again the OPE file name needs to be CASE-INSENSITIVE: Except for the extension (".ope", which has to be lower case), either upper-case or lower-case characters can be used for the file name but please do not mix them.

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Observation checklist

  • [Operational mode] In S11B, LR will be available on both IRS1 and IRS2, while HR will be in operation only on IRS1. From S12A, HR as well as LR will be available on both IRS1 and IRS2.
  • [S2O file submission] Have you sent S2O file(s) to the support astronomer (Kentaro Aoki )? If you observe more than one spine allocations/target fields, please send all the corresponding S2O files. If you saw any error messages or warnings in making the S2O file(s), please consult well before your observation.
  • [S2O file name convention] Did you name the S2O files following the format "PROPOSALID_PILASTNAME_ANYCOMMENT_OBS.s2o"? In the "ANYCOMMENT" part, observers can choose a useful phrase to classify the file (e.g. name of target field). "OBS" should be replaced with one of "nbs", "cbs", or "pas" depending on the selected observing method: Normal Beam Switching, Cross Beam Switching, or Point & Stare, respectively (see this page for the details of the observing methods). For example, if your proposal ID is S10A-000 by PI: Tamura and you observe a target field named "somewhere" by cross beam swiching, then please name the S2O file as "s10a_000_tamura_somewhere_cbs.s2o".

    NOTE: The S2O file names should be CASE-INSENSITIVE, i.e., you should NOT mix lower-case and upper-case characters in the file name.

  • [OPE file submission] Have you sent an OPE file to the support astronomer (Kentaro Aoki )? Are the target information and the names of S2O files correct? Did you name the OPE file following the format "PROPOSALID_PILASTNAME.ope"? For example, if your proposal ID is S10A-000 by PI: Tamura, then the name of an OPE file would be "S10A_000_TAMURA.ope".
    [See also this section. for some details of OPE file.]
  • [Guide stars] Did you manage to have two or more guide stars on the guide fiber bundles (NOTE: Two bundles are currently unavailable due to their less good positioning cababilities - see this page)? Are they bright (R = 12 - 16 mag) and on the same astrometric system as science targets? Observers firstly should try to choose field center and position angle (PA) to maximize the number of available guide stars, with one or more guide stars available on each row (but, there is one technical restriction in choice of PA - see the next item). The more guide stars are available, the more stable AG operation is expected. Especially, the operation should be rubust against unexpected contamination of stars with large proper motions, double stars, those with large errors in astrometry, magnitude, etc. In addition, if many GS are bright, the exposure time for AG could be shortened. Then telescope pointing correction could be applied at a shorter interval and subsequently guiding error could be smaller.
  • [Position angle] In the spine allocation, observers need to choose a PA. For a long interation across the transit in the southern (northern) sky, it is better to set PA somewhere around 0 deg (180 deg), respectively. (In fact, 0±90 deg (180±90 deg) is acceptable). This is due to the specification of the rotator in the Prime Focus unit for IR (PIR, used by FMOS), which can go only from -180 deg to +180 deg. Hence, if you set PA to 180 deg to observe southern sky, for example, the rotator comes to 180 deg at the transit and stops there due to the limit. For another example, if you set PA to a value large in negative (e.g. -150 deg) and try to observe southern sky right after it rises, the rotator may hit on the negative limit. If this happens in the middle of an exposure, this data frame is unlikely to be very useful. Also, an additional overhead occurs to get the rotator back to -180 deg (or the other way) and restart the tracking.
  • [Coordinate calibration stars] How many coordinate calibration stars are available in your S2O file? Are they bright (R = 12 - 15 mag) and on the same astrometric system as science targets? Observers are recommneded to prepare at least one CCS near the field center and a few stars surrounding it in the outer field to calculate not only lateral offset to telescope pointing but also rotational offset to instrument rotator. Observers may leave many CCS available in S2O files. Although the actual field acquisition is performed by using ∼ 5 CCS, if more CCS are available in an S2O file then the instrument control system picks up several from them considering the positions in the FoV.

    The CCS near the center of FoV is also used for focusing operation. If a star at a distance larger than 10 arcmin from the center of the FoV is used for focusing operation, the focus value derived could be less accurate due to the effect of curvature of focal plane.

  • [Brightness of guide star and coordinate calibration star] Please make sure, GS and CCS are bright enough in the R band. Please be aware that the "R" band in the UCAC3 catalog is somewhat different from the standard R band (see this section). Also note that FMOS is a near-infrared spectrograph, but the guide camera and SKYcamera are CCD cameras for visible wavelengths.
  • [Several fibers showing throughput variation] The throughput of 11 fibers fed to IRS1 and 3 fibers to IRS2 seems to vary with time by a few percent and the amount of this variation is different as a function of wavelength. Observers should confirm that these fibers are not allocated to stars for telluric absorption correction and flux calibration (at least spectra from these fibers should not be used for such purposes).
    [See this page for more details.]

  • [Ghost feature] Are there any spines allocated on very bright objects (e.g. 13 mag or brighter), while much fainter objects will be observed simultaneously? Brighter objects would make brigher ghost features on the detector, which could affect the spectra of fainter objects. Empirically, a ghost feature appears at a three-orders-of-magnitude (i.e. 7.5 mag) fainter level than the original brightness. For example, if you observe a star of 13 mag, a ghost feature would appear on the detector with a brightness of 20.5 mag, which may be unacceptable for the spectra of objects of ∼ 18 mag or fainter in the same exposure, depending on the goal of the observation.
  • Please refer also to this page. Ghost feature tends to appear just below the spectrum of a bright object.

  • [Overhead estimation] Please remember to take overhead into account appropriately. Typically ~20 minutes is necessary for target acquisition, including telescope pointing, dome rotation, focusing, and Echidna spine configuration. In addition, for long integration, Echidna spine re-configuration which takes ~10 minutes is necessary every ~30 minutes. Resultantly, the typical observation efficiency in long integration is ~60% (i.e. the overhead is ~40%).
  • [High Resolution mode] In the operation of HR, a few important restrictions will be applied. Applicants should take the following notes into account in making proposals:

    • The FMOS spectral coverage (0.9-1.8 um) is divided into four bands in HR: J-short (0.92-1.12 μm), J-long (1.11-1.35 μm), H-short (1.40-1.60 μm), H-short prime(1.45-1.67 μm) & H-long (1.60-1.80 μm), and one of them is covered by a single exposure. HR observation is carried out by using these pre-defined bands only.
    • To minimize the risk of increasing additional overhead, observers need to keep the same setting of IRS1 from beginning to end of a night - i.e. no change is strongly recomeemnd in observation mode (e.g. LR → HR, HR → HR but a difference band) during a night.
    • It would be acceptable to use IRS1 and IRS2 in different observation modes (e.g. LR in IRS1 and J-long in IRS2), while it is strongly recommended to use IRS1 and IRS2 in the same mode to accurately position Echidna spines/fibers: Due to the effects of atmospheric dispersion and chromatic aberration, the position of an object on the focal plane is slightly different as a function of wavelength, and the spine-to-object (s2o) allocation software cannot allocate spines according to different requests (e.g. observation wavelength) to spines for IRS1 and those for IRS2. This means, if one tried observation in J-long with IRS1 and in H-long with IRS2 and set observation wavelength for spine allocation to 1.3 μm, less flux would fall into the fiber for the H-long observation.
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Preparation of an OPE file

  • What is OPE file?
  • Observation Procedure Execution (OPE) file contains a list of abstract commands to operate the telescope and instrument in actual observations. Like the other instuments on Subaru, observers using FMOS will also need to prepare an OPE file according to their plans of data acquisition during a night. Target information, observing method, exposure time, and so on will also need to be specified in this file.

  • How is it used?
  • In an actual observation, the content of an OPE file is displayed on the main telescope system control GUI. A support astronomer or telescope operator then selects the line of a certain command in it (with parameters in many cases) to execute when necessary. Observations proceed usually by repeating this for different commands.

    Although an OPE file can be edited on the GUI during a night, to minimize overhead, it is important for observers to provide all the target information and prepare all the commands in OPE files that are used in their observations. On the other hand, since a command is manually picked up from an OPE file and is executed one by one, it is not extremely important to organize the commands exactly along the time sequence in an OPE file.

  • Template OPE file
  • A template of OPE file for FMOS observation is available here:

    Observers will need to download the text version of template and edit it to make one for your observation. Please refer to the PDF version as a brief manual. Then it needs to be sent to support astronomer(s) for FMOS open-use observations (Kentaro Aoki for S11B).

    For clarity, please name your OPE file following the format "PROPOSALID_PILASTNAME.ope". For example, if your proposal ID is S10A-000 by PI: Tamura, then the name of an OPE file should be "S10A_000_TAMURA.ope".

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List of OH airglow masks

Based on the list of OH-airglow lines by Rousselot et al.(2000, A&A, 354, 1134), those to be masked were determined. The list of the masked lines can be retrieved from here. The width of each mask line is 8Å. Note that there is no mask between ~1.35μm and ~1.40μm because no spectral information is available at these wavelengths due to the presence of the fiber slit.

The figures below show the locations of OH-airglow lines and masks adopted in the FMOS spectrograph. The top left panel shows the entire spectral coverage of FMOS (0.9μm-1.8μm), while the others indicate the details by dividing it into 9 pieces (i.e. 0.1μm sampling). The following information is color-coded in these panels:

  • Blue - These OH lines are masked.
  • Cyan - Same as blue, but weaker lines are indicated.
  • Green (barely visible in this figure around the zero level) - These OH lines are NOT masked.
  • Red - Locations of the OH masks adopted on FMOS.
  • Magenta - The solid line along the horizontal axis indicates the threshold to the brightness of OH-airglow lines. The OH masks on FMOS were designed to block those brighter than this threshold.


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Echidna spine ID and spectrum ID match-up

Below is a list of match-up between Echidna spine IDs on the focal plane (you can see them on the spine allocation software) and spectrum IDs on the detector. To the spectra, the numbers of 1-200 are assigend in the ascending order from bottom to top of the detector (i.e. from 1 to 2048 in y-axis).

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FIBRE-pac: FMOS data reduction package

In the series of engineering observations, we have been developing a data reduction & calibration package ( "FIBRE-pac: FMOS Image-Based REduction Package", Iwamuro et al. 2011, PASJ in press, arXiv.1111.6746). This consists of IRAF tasks and C programs using the CFITSIO library that are excuted from a command line on a Linux or Mac OS X. This software package has been used to reduce and calibrate the data from recent observations including those of open use. After a long period for optimization, bug-fix, and verification, it has almost reached to a stable version. Please go to the web site below to download the package. A manual and training data set are also available.

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Last updated: March 11, 2013



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