Information for Proposal Applicants

• Observation procedure
• Data acquisition for flat fielding and calibration
• Operation of high resolution mode
• Proposal checklist

Note: Please refer to this page for details of the Echidna system, which will be helpful to plan observations, especially Echidna spine/fiber configuration.

Further information

Observation procedure

1. Pointing the telescope, rotating the instrument, checking the focus, & configuring spines
2. Target field acquisition by making corrections to telescope pointing and instrument rotator angle

The typical overhead to complete 1. and 2. and get ready to start exposures is ~20 minutes, which is usually dominated by the spine configuration time. Spine configuration can start with slewing the telescope and rotating the instrument. The overhead will be longer when the telescope pointing is involved with a large amount of dome rotation and/or when the next target field is still at EL<30 degree: Since the spine configuration should be executed at a telescope elevation NO LOWER THAN 30 degree, the telescope elevation is temporarily raised to 40 degree to do a spine configuration.

3. Start auto guiding, and take exposures

Notes for operation:

• There are three observing methods available, Normal beam switching (NBS), Cross beam switching (CBS), and Point & stare.
• 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.

• Operations of IRS1 and IRS2 are independent. For example, observers can set up IRS1 for LR and IRS2 for HR if wanted. Also, as long as the telescope stays at a certain position and the fiber configuration stays the same, exposures by IRS1 and IRS2 do not need to be synchronized: Duration of individual exposure and number of exposures can be chosen independently.
• As indicated in the "basic instrument parameters" page, currently the readout noise of the detector in the spectrographs is not very low. In a Correlated Double Sampling (CDS) readout, this noise will have to be fully included to every output frame. However, the noise level can be significantly reduced by exploiting non-destructive readouts (i.e. ramp sampling): Given an observer wants to take an 1800 sec exposure, the pixel count can be measured N=(1800/(minimum exposure time)) times in the process of exposure and the final frame can be obtained by a linear fit to the N data points. This is expected to reduce the readout noise by a factor of sqrt(N). Observers are therefore recommended to use this ramp sampling unless they need to repeat very short exposure.

!! Important !! : Corrections to the spine positions during long integration

Unfortunately, even if the instrument is rotated as necessary, the positions of objects on the focal plane gradually change as time goes by for several reasons as below:

• Since the rotator axis is slightly misaligned with the optical axis of the telescope main mirror, the pattern of field distortion on the focal plane rotates as the instrument rotates.
• Strength of differential atmospheric dispersion effect changes as the telescope elevation changes.
• Plate scale changes, e.g., when the truss temperature changes, and subsequently the distance between the telescope primary mirror and wide-field corrector in the PIR changes.

Consequently, before flux loss from fibers starts being significant due to a large displacement between fiber and object positions, the spine positions need to be re-configured at a regular interval to keep observing the same objects with the same spine configuration. In the recent engineering observations, it has been confirmed that re-configuring the spine positions every 30 minutes enables the object flux to be reasonably constant. Each re-configuration process takes about 10 minutes. The typical observation efficiency in long integration is ~60% (i.e. the overhead is ~40%), while the efficiency gets lower for shorter integration because the contribution of initial configuration time (20 minutes) becomes more significant.

As of Feb 2011, we are still in the process of characterization and optimization and this frequency of re-configurations may be reduced in future as the instrument characteristics are better understood. However, the observers are strongly recommeneded to follow this sequence (i.e. executing the re-configuration process every 30 minutes) for long integration.

Data acquisition for flat fielding and calibration

1. Flat fielding

Dome-flat frames are taken in the evening and/or morning by closing the dome and using the dome-flat lamp. In doing this, the Echidna spines are configured to the same as for a scientific exposure. If observers have more than one target field/spine configuration to observe, then they would need to take the same number of sets of dome-flat frames with the spine configuration changed as appropriately.

2. Wavelength calibration

A Th/Ar lamp is available in the buffle structure of the tertiary mirror pointing to the focal plane at the prime focus. Using this lamp, CAL frames are taken in the evening and/or morning (spine configuration at the time of this data acquisition should not matter for the accuracy of the calibration). A line list will be provided on this web site in the near future.

3. Telluric absorption correction and flux calibration

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.

Operation of high resolution mode

There will be a few restrictions to the HR operation as follows, and applicants/observers should keep them in mind about the use of HR:

1. 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-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.
2. It takes about one hour to chnage one mode to another one (e.g. LR → HR, HR → HR but a difference band). To minimize the additional overhead, we recommend to keep the same setting of IRS1 & IRS2 from beginning to end of a night.
3. 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, more light would tend to be lost at the fiber entrance for the H-long observation.

Proposal checklist

• There are three observing methods available and the main differences are summarized in a table below. Have you chosen one for your observation with them considered?

Observing method	Fraction of fibers for science targets	Fraction of on-source integration time (1)	Fraction of on-source integration time (2)
Normal beam switching	~100%	50%	~30%
Cross beam switching	~50%	100%	~60%
Point & stare	~80-90%	100%	~60%

(1): This percentage represents 100 x ([On-source Integration Time]) / [Total Integration Time]).
(2): This percentage represents 100 x ([On-source Integration Time]) / [Total Observing Time (including overhead)]).
• [Operational mode] From S12A, the High Resolution mode (HR) as well as the Low Resolution mode (LR) are available on both IRS1 and IRS2.
• [Number of fibers] There are 12 fibers that are not available for observation of science target, mainly because the spines move not well enough (so most of them are still useful to take sky spectra). Therefore the total number of fibers available to observe science target is 388.
• [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-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.
  • It takes about one hour to chnage one mode to another one (e.g. LR → HR, HR → HR but a difference band). To minimize the additional overhead, we recommend to keep the same setting of IRS1 & IRS2 from beginning to end of 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 plan 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.

• [Overhead estimation] Have you calculated the total amount of time to request with taking overhead into account appropriately? Typically, ~20 minutes is necessary for telescope pointing, dome rotation, Echidna spine configuration, target field acquisition, and focusing. 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%).
  [See this page for more details.]
• [Magnitude zeropoint] If you justify on-source integration time based on a certain magnitude in the proposal, have you clarified which zeropoint, Vega or AB, should be applied to the magnitude? In NIR, the difference between Vega mag and AB mag against a given flux density is 1-2 mag, which is significant.
• [Guide stars and coordinate calibration stars] In the target fields, are there a number of bright stars suitable for guide stars (R = 12-16 mag) & coordinate calibration stars (R = 12-15 mag) with accurate relative astrometry to science targets?
  [See this page for more details.]
• [OH mask] Is the spectroscopic feature you need to observe expected to be robust enough against the effect of OH masks? You can check the list of OH masks here. Also, FMOS spectrum simulator may be helpful to see the effect.

Last updated: July 23, 2012