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Like all Gemini facility instruments, NICI is operated through the Gemini Observing Tool (OT). This page guides you through the main steps and considerations for constructing NICI observations in the OT.
The Observing Tool is available in two versions, one intended for stand-alone use on a computer outside of Gemini (referred to as “OT” in this document), and one which connects with the Observing Database from a computer within the Gemini firewall (“OTR”). In this document the OT is assumed unless otherwise noted. This version of the document supports the OT version for the currently offered semester.
- OT Basics - Basic information about the NICI OT.
- - Baseline configuration in the OT (weather constraints, target etc.)
- Observing Conditions Component - Setting the weather constraints for NICI
- Target Component - Selecting the science target, AOWFS target, PWFS2 guide star(tip/tilt)
- NICI Component - NICI instrument configuration in the OT (filters, exposure times etc.)
- Sequence - Configuring the observing sequence (offsets, repetitions, etc)
- Observe - Configuring the observing sequence type(Dark, Flat or Science)
For NICI, the OT is used primarily to specify the configuration of the Science Camera mechanisms and detectors. Most AO system parameters (such as loop gain) are set manually during observing by the instrument operators. One exception is that the NDFW is set from the OIWFS magnitude in the Target component. AO acquisitions are also executed manually, so the details of these acquisitions are not required in the OT.
The first step after opening a new program is to define one or more observations by selecting the Observation menu, then NICI Observation. The observation may be placed in a group,either by selecting Group:Organizational Folder first, or by moving it into an existing group at a later time.
NICI observations consist of four components:
- Observing Conditions
- NICI Component
The NICI-specific aspects of each component are discussed below.
The Observing Conditions component contains no special NICI features, but note the following practices for NICI.
- For 2011A the IQ must be 70 or better, and CC must be 70 or better.
- CC70 programs must have modest sensitivity requirements and previous approval from the TAC. Observations of faint coronagraphic targets normally require CC50.
- Use the airmass constraint to specify the airmass limits which may be desired in Cass Rotator Fixed mode to limit the rotation rate on the sky.
The Target component is standard for all instruments, but for NICI there are some special requirements. The recommended procedure for defining targets is as follows.
Verify that the target type is Base (the default), then enter a target name into the Name field. A standard name such as an HD number is preferred so the name will be resolved properly. If you are starting from a Phase II skeleton, the target name and the Phase I coordinates should already be present but the proper motions will be zero. Therefore, the following procedure is still recommended.
- Press Enter in the target Name box or Click the magnifying glass icon (Resolve) and wait until the target data are retrieved from the catalog server. (We’re assuming sidereal targets here...) The RA, Dec, and proper motion fields should be filled automatically. Proper motions are important due to NICI’s small field of view and stringent acquisition requirements. Note that the Equinox is not retrieved, so if the catalog is not in the default J2000 you must change the equinox by hand! The Magnitude table of the Base target may also be filled in values and bandpasses/units. For NICI V and R magnitudes are the most relevant.
- If the target was not found by the Resolve, enter the coordinates and proper motions manually. These data must be entered properly before proceeding to the next step.
In the top window select the Base target just created, then below click on the rightmost Duplicate button. Change the type of the new target from Base to OIWFS. This creates the OIWFS target used by the OCS to control NICI’s Tip-Tilt Steering Mirror (TTSM). For on-axis targets, the Base and OI coordinates and proper motions must match exactly, so we recommend using the Duplicate button to avoid errors.
This is an important NICI-specific step. Enter a V or R magnitude in the NICI OIWFS Magnitude entry list (see figure above). This magnitude will be transmitted to the OCS in order to select the NICI AOWFS Neutral Density Filter, so it must be set properly. In most cases it can be a low-precision (~ 0.1 – 0.2 mag) V or R magnitude; the transmissivity step between filters is a factor of 10 so high precision is not required. The range of neutral density filters is ND5 to Open, so the OIWFS magnitude can be from –1.6 to > 13. Fainter than 13, the counts on the APD counters are nearly zero and the Strehl ratio declines to very low levels. We have little experience in this regime, so if your program contains very faint targets please contact us to discuss your requirements.
Add PWFS2 guide star. Click Find Guide Stars... at the bottom, and select a PWFS2 guide star using the normal procedure. NICI requires a PWFS2 guide star for slow tip-tilt corrections via M2. Because the P2 guiding is very slow with NICI the guide stars can be as faint as 14.
The main tab is visible in all OT versions and contains the following elements. See the NICI web pages for more information, especially on available masks and filters.
- Cass Rotator – Choose one of the two available modes.
- Follow: The CR rotates continuously to track the sky. In this mode the value entered in Position Angle will be the on-sky position angle oriented toward the top of the Red channel detector – eg. Enter 0 for north to be up. The Blue channel will also be approximately at this angle except for a small relative rotation between the detectors.
- Fixed: The CR is fixed in position, allowing the sky to rotate on the detector as the telescope tracks. In this mode the text entry label changes to CR Angle and the CR is set directly to the entered angle in degrees. With NICI on either the side or the bottom port, we prefer CR Angle "180.0" to set the instrument to an orientation which appears to minimize flexure as elevation changes.
- Position Angle / CR Angle – The label and meaning of this text entry field changes according to the Cass Rotator mode; see above.
Note about Spider Mask Rotator: In both CR modes, the SMR is automatically set before the start of each integration to align the mask spider vanes with the telescope’s. This can be overridden in the Engineering tab (in the on-site OTR), but the default should be satisfactory for most science observations.
Focal Plane Mask – Choose one of the focal plane masks for coronagraphic observations, or Clear for normal imaging. The names indicate the mask half-power radii in arcsec. Note that the F0.32 mask is most commonly used and provides satisfactory performance for most stars. The F0.22 mask may be desireable for fainter stars, and one of the larger masks for brighter stars and/or long integrations to avoid halo saturation.
Pupil Mask – The 95% mask is the only mask available for the current semester.
- Dichroic Wheel – Mechanism which splits light going to the red and blue detector channels.
H 50/50 Beamsplitter: The default, and the only element that provides light simultaneously to both channels. Note that it is truly a 50/50 beamsplitter, not a dichroic. The transmissivity / reflectivity is close to 50/50 throughout the H and K bands, but the reflectivity is only about 0.16 at J so the Mirror is recommended for that wavelength.
- Mirror: Send light to the blue channel only.
- Open: Send light to the red channel only.
- Block: This setting effectively blocks light to both channels for taking dark images.
Filter Red Channel – Choose the desired filter for the red channel.
Filter Blue Channel – Choose the desired filter for the blue channel.
- A Note on Filters:: For dual-channel imaging, eg. with the CH4-4% S and L filters, we usually set the Red channel to the L filter and Blue to S. Standardizing on this order simplifies taking flat fields and other tasks. However, note that both the S and L filters are available in both channels, and swapping them reverses the non-common-path optical aberrations present at each wavelength, which may help to detect and/or verify extremely faint targets detected in the PSF speckle pattern. Swapping filters does increase the complexities of observing, doubles the number of flat fields, and complicates data reduction, so it may or may not be desirable for your program.
Imaging Mode – Contains six modes which set the Dichroic Wheel, Red and Blue filters, and the Pupil Imager (in the Engineering tab) together. Also contains a Manual setting so that these mechanisms can be set individually.
- Detector Tab
- Exposure Time – The time per frame (individual readout of detector) in seconds. Although any value can be entered in the OT, the actual time generated by the instrument will be in steps of 0.38 sec with a minimum of 0.38 sec and a maximum of 150 s (due to limitations in the sample-up-ramp firmware).
- No. Coadds – The number of frames coadded per saved exposure, which corresponds to one MEF file written to the DHS. The effective integration time per saved observation is therefore Exposure Time * No. Coadds.
- Well Depth
- Shallow (200 mV): Provides slightly better performance (fewer hot pixels) for faint targets.
- Normal (300 mV, default): Provides a good balance between well depth and number of hot pixels, and is preferred for most targets.
- Deep (400 mv): May be necessary for bright targets when greater well depth is desired at the expense of a larger number of bad pixels.
- Set Well Depth to Normal. If you wish to use a different well depth please contact the instrument team to discuss your program’s requirements.
Set Exposure Time according to recommendations on the NICI web pages. A useful rule-of-thumb is the “rule of fours” that we have used for the H 50-50 beamsplitter, CH4-4% filters and F0.32 coronagraph spot combination: a star of H = 4.0 requires exposure time = 4.0 sec in order to expose the core and halo properly without saturating. The integration time can be scaled from this reference point to any H > 3.
For H < 3, the exposure time required to prevent saturation will be < 2 sec and the frame-readout overhead of 0.38 sec will be > 20%. A choice must be made either to accept the higher overhead, or choose a larger mask to avoid saturating the halo. For the 1% filters, exposure times can be scaled by a factor of 4 from the “rule of fours.” For broadband and other filters, see the NICI web pages for further recommendations.
Finally, the core and halo signal levels are usually checked during target acquisition, and real-time adjustments can be made for proper exposure. If your requirements are critical, please describe them in an Observing Note, or better yet discuss them with an NGO or instrument team representative.
Set No. Coadds to give the desired exposure time per saved file. In Cass Rotator Fixed mode, this is set to limit the sky rotation in the duration of the saved image.
ISS Port Tab – Select between the ISS up-looking and side-looking ports, which affects the coordinate transformations in the NICI Offset component. In 2011A NICI will be on the up-looking port.
Two types of NICI sequences are available in the Iterator Menu for normal observations and for offsets. Either type of sequence may be placed within a Repeat component in order to execute it multiple times.
The NICI Sequence, as with other instruments, allows most of the NICI Component parameters to be changed during a sequence of observations. Please avoid entering items which do not change during the sequence – this will simplify checking programs and correcting errors for yourself, your NGO, and your Gemini Contact Scientist.
Please consult the NICI OT Science library for observing sequence templates which can be used as starting points for your observations.
The NICI Offset component is used for executing dither patterns within an observation sequence. This has special features for NICI which allow control of the Focal Plane Mask Wheel. There are two modes:
Fixed: The FPMW is not moved during dithers. The user enters p,q offsets directly as with other Gemini instruments.
Follow: A special NICI mode The FPMW is moved in synchronization with the telescope to keep the target centered under the occulting spot. Because the masks are mounted in a wheel, their motion is restricted along a certain arc across the detector. In follow mode, therefore, a value d is entered which represents the distance along the arc, in arcsec, from the default spot position near the center of the field; this may be positive or negative. The OT contains the approximate conversion from d to a p,q offset, according to the ISS port selected in the main NICI component, and displays the resultant p,q in the Offset component and in the visual representation.
When a Follow-mode offset sequence is executed, the OT transmits the parameter d to the OCS, which coordinates the moves of the FPMW, AO steering mirror, and telescope using more precisely calibrated transformations than are present in the OT. The precision of the OCS transformations is good to a few tenths of an arcsec over the science detector field, which is insufficient to keep stars precisely centered under the occulting spots. Therefore, manual re-centering is required after each dither for best centering. Although coronagraph-mode dithers are possible, they have relatively low efficiency and should be used sparingly. Dithering every 15-20 minutes, for example, is acceptable but dithering every few minutes significantly reduces efficiency and places an unacceptable burden on observers.
The standard observe used for most observations.
Used to take NICI flat fields using GCAL. In this mode GCALand the NICI detectors are configured from the parameters in the Flat component, overriding detector parameters which may be set elsewhere in the observing sequence. We recommend consulting the NICI OT Calibration Library for example flat field sequences.
NICI flat fields are difficult to take properly because of the Focal Plane mask. The masks have surface imperfections in addition to the coronagraph spot, and it is not possible to have a mask with the same pattern of imperfections minus the spot! There is no ideal solution;the user must choose between deep flats made at a different time and shorter, shallower flats made at the time of the observations.
Narrow-filter twilight flats require very long exposure times,possibly with the sun above the horizon for the K-bandfilters, and thus were very difficult to acquire during commissioning. GCAL flats produce essentially the same results much more quickly, so we have come to rely on them exclusively.
As we have relatively little experience taking flats in all but a few instrument modes, we recommend discussing your flat field requirements and strategies with the instrument team early in the Phase II preparation of your program.
Used for NICI darks. In this mode the NICI Dichroic Wheel (DW) is automatically set to Block and the detector parameters are set from the Dark component. As with Flats, please consult with the instrument team about Dark requirements for your program.