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Preparing Observations

This section describes how to prepare and check GHOST observations at the Phase II stage. Key to successful observation preparation is starting from the GHOST OT Library. Preparing observing sequences involves ensuring correct coordinates of the target, the desired instrumental parameters, and overall sequence components. In addition, observations of telluric, flux, radial velocity standards, or non-standard calibrations may be needed depending on your science goals.

Required OT observations and components

The table below lists an overview of the basic requirements from the PI. A Phase II will not be accepted without this type of observation or component being defined. As needed means that the user can add this type of observation or component as needed by the science goals of the program. From Phase I means that the same values defined during the Phase I stage must be used, or else a change request should be initiated. Use the GHOST OT library as a source of template and example observations.

Calibrations taken during the night, or any special standards are charged towards the program's allocated time. Baseline standard stars, flats, and arcs are not charged (see the calibrations webpages for more details). The time for acquisition is already included in the science observation overhead, and no additional time is charged. However, for long observations (>1 hr) extra time for recentering, or reacquisition is charged.

Table 1: Phase II requirements
Component	Sub-component	Requirements
Observing conditions		From Phase I
Timing windows		As needed; from Phase I
GHOST	Position Angle	As needed
GHOST	Resolution and Target mode	From Phase I
GHOST	Exposure time and count	Required from PI
GHOST	Binning and read mode	Required from PI
Target component	Science coordinates and proper motions	Required from PI; from Phase I
Target component	Science magnitudes (V magnitude essential)	Required from PI
Target component	Guide star	Required
GHOST sequence component		Required
Observe component		Required

For any changes in targets, observing conditions, or substantial change in instrument mode please send a request detailing such to the Head of Science Operations at Gemini South. Further details on the change request process maybe found here.

Overheads

For calculation of specific system overheads where these are critical to your observing program (for example time-resolved observations), the detailed information on this page may be used. The detailed overhead information is also useful for the Phase II planning of your observing program. The various overheads can be broken into the following categories described in the table below. The subsections give a detailed description of each overhead.

Table 2: Overheads overview
-	Time (s)	-
Setup and acquisition	480	Every target
Reacqustion	480	Every two hours of observations
Recentering	300	Every one hour of observation
Readout	98 (maximum)	Every write of either red/blue detector
DHS	7	Write of every file

Telescope acquisition, and instrumental setup

8 minutes of program time is charged for setup, slewing to a new target, starting guiding, and accurate centering. One reacquisition is required for every two hours of observing time (including any overheads and calibrations). One recentering is required for every hour of observation (5 mins). Long observations may be split to accommodate them in the queue. For splits of observations shorter than 1 hour, the reacquisition is not charged to the program.

Readout times

Readout times are a function of the read mode chosen, and the binning defined in the instrument section of the OT. Below is a list of overheads for the various combinations of both. Since the blue and red detectors have different read out times, one can in principle use different exposure times in both cameras so as to not leave any camera idle. In addition, the data handling system adds 7 seconds of overhead per file to be saved.

Given the readout time, and detector noise levels, we strongly recommend that users use readout modes of BLUE/SLOW and RED/MEDIUM, respectively. This is because the overheads and noise levels of the camera in those read modes match best. 

The readout times for both detectors are given below in each binning and read mode.

Table 3: Readout overheads
-	Spectral binning	Spatial binning	Red read time (s)	Blue read time (s)
SLOW	1	1	97.4	45.6
SLOW	1	2	49.6	24.8
SLOW	1	4	25.7	14.4
SLOW	1	8	13.8	9.1
SLOW	2	2	27.5	15.4
SLOW	2	4	14.7	9.8
SLOW	2	8	8.4	7.0
SLOW	4	4	9.5	7.9
MEDIUM	1	1	50.1	24.6
MEDIUM	1	2	26.1	14.3
MEDIUM	1	4	13.9	9.1
MEDIUM	1	8	7.9	6.5
MEDIUM	2	2	15.7	10.1
MEDIUM	2	4	8.8	7.2
MEDIUM	2	8	5.4	5.6
MEDIUM	4	4	6.5	6.5
FAST	1	1	21.7	12.0
FAST	1	2	11.7	7.9
FAST	1	4	6.8	5.9
FAST	1	8	4.3	4.9
FAST	2	2	8.6	6.9
FAST	2	4	5.2	5.6
FAST	2	8	3.6	4.9
FAST	4	4	4.7	5.8

Observing strategies

The following section contains advice on different observing strategies for various components depending on your science goals.

Two target observations

GHOST offers the ability to observe two targets in the standard resolution mode. The principal requirements are the separation of the targets on the sky. They must be at least 102 arcsec apart, and fall within a field of view of 7.34 arcmins. The position angle of the instrument may be rotated to accommodate them, or reach a suitable guide star.

The final spectrum of both targets is read out combined, separated by the sky spectra for both the red and blue detectors. The reader is referred to the instrument capabilities for the spectrum format description. It is therefore strongly recommended that the two science targets are of comparable magnitudes. For large magnitude differences (>3-5 mag), users must ensure that there is enough signal in the faint target, and that the brighter target is not saturated using the exposure time calculator, for the observing conditions requested. Since the spectrum of both targets falls on the same detectors, it is not possible to have different exposure times, readout, or binning modes for the two different targets. Also, heavily binned spectra in the spatial direction tend to overlap between neighbouring fibers, making extraction difficult. For this reason, binning in more than 4 in the spatial direction is not recommended in two object mode.

An important consideration is also to account for the proper motion of the targets during the approximate date of the observations, since this will change the relative orientation of the two IFUs. A description on how to do so is found below. Additionally, it is strongly recommended that the user places the two targets equidistant from the centre of the focal plane. This is because GHOST introduces a focus correction for objects away from the centre of the focal plane to the secondary mirror. This is calculated based on the average distance of the two IFUs from the centre, and having the two targets non-equidistant places them both slightly off focus. This is only important for science targets, which require a well focused object and not any sky only targets.

Sky observations

GHOST offers the possibility to obtain simultaneous sky spectra along with science, for all modes. In standard resolution mode, IFU1 has its own dedicated sky IFU (whose position cannot be changed), where as IFU2 does not. When using IFU2 for standard resolution science target, if sky is needed, IFU1 must be used. Using IFU1/IFU2 in target+sky mode allows for 10 sky fibers, and a better sky subtraction. For target+sky modes, ensure link to base is not selected to not be off center. For high resoluiton mode in IFU1, there is a dedicated sky IFU in IFU2, whose relative position can be changed. Since the relative position of the SRIFU1 sky IFU cannot be changed, one needs to change the PA to avoid any targets in this case. More details on the IFU configuration can be found in the components webpages.

There is no automatic check to ensure that the sky IFU is on blank sky. It is the users responsibility to ensure that during Phase II, the IFUs are pointed at blank sky. The position of the sky IFU, and the relative IFU positions (for target+sky observations) can be seen in the OT imager. While it is only possible to overlay a DSS image at optical wavelengths currently, there may exist fainter deep objects not visible in DSS images. If users believe this to be the case, one can use the coordinates of the IFU in the static components to determine using other catalogues (for e.g. Gaia) whether there is a faint target present. This is of particular importance for faint science targets in crowded fields.

Timing windows

Some observations, such as exoplanetary transits require strict timing windows. In such scenarios, it is essential that the PI enter the timing window in the timing window component of the OT, shown below. In addition, the PI should fill in the standardized note available in the GHOST library on how to approach changing conditions. The PI must indicate if the conditions are worse than requested after the observations have begun, whether the observer should continue with the observations (for how long, and in how much worser conditions). Without this information, once the conditions alter the observations will be aborted if started.

Faint targets

Since GHOST uses the science target also as a guide star, faint targets can be difficult to acquire and guide. In this situation, where targets are fainter than V=19 mag, it is suggested that either a blind offset acquisition, or companion guiding by employed.

Extended targets

GHOST's IFU have a limited FoV (~1 arcsec). GHOST utilizes guide microlenses centered around the science IFU to acquire and guide on targets. Therefore, extended targets come with the disclaimer that they cannot be correctly centered, since the centering is done by the highest point of flux in a PSF. So if the source is extended with uniform brightness, acquisition and guiding is not precise.

For extended targets, desiring offsets around the object, or for slightly extended objects one might need to turn off the centering during acquisition, and observe blindly (during observations) once acquired. For significantly extended objects, the centering again is impeded and one might need to observe blindly and remove centering once acquired. Please contact your CS in these cases.

Additional calibrations

All programs include, as part of the baseline calibration plan arcs, flats, and biases necessary to reduce science data using the DRAGONS pipeline. Any additional calibrations mixed in with science (i.e. during the night) are charged to the program as part of the nighttime program calibrations. Any additional daytime calibrations necessary will be charged as daytime calibrations, and must be iterated during the Phase II process.

In addition, some programs may require a flux standard taken sequentially with science, or radial velocity or telluric standards. These are not part of the standard calibration program, and must be defined by the PI during the Phase II process. All additional calibrations taken during the night are charged to the program, and must be defined, and ideally requested during the Phase I proposal.

Counts versus Observe

Based on the current DRAGONS data reduction version which delivers spectra without stacking, we strongly recommend that users employ only observe to increase the number of spectra for a single object. 

The total exposure time for GHOST can be defined as: each individual observe × individual exposure times × exposure count, and each observation is defined as: individual exposure times × exposure count. The individual exposure time × counts are bundled as a single file and each count is taken sequentially (i.e. cannot be split).

Therefore long observations (i.e. only individual exposure times × observation counts) must be splittable into more reasonable chunks (around 1hr long) for queue scheduling. In such cases, the counts should be split up into reasonable individual sequences. This should be followed unless science necessitates (for e.g. planetary transits, orbital phases) longer observation times (counts × individual exposure times).

GHOST OT details

We strongly recommend that users read this section and also that they start from the automatically-generated OT templates from the OT library when preparing observations. The library contains detailed instructions for various modes, as well as standardized notes for the observer. This web page is intended to explain the various OT components in depth, which are:

• GHOST static component
• GHOST target component
• Sequence component

After creating your GHOST observations, please go through the Checklist. If you still have questions, email your contact scientist or ghost_science @ noirlab.edu for support.

GHOST static component

When adding a GHOST component to an existing observation, or adding a "GHOST Observation" a GHOST static component is created. The component is used to define the basic instrument configuration and looks like this.

The various subsections of the static component are described below.

• Position angle gives the instrument position angle on the sky, from north to east. We recommended that the position angle remains at 0. However, one can edit the position angle to reach a better guide star, or to better access dual targets.

• Instrument resolution gives the choice of standard, high, or precision radial velocity resolution modes. The details of the final resolution achieved is given in the GHOST instrument capabilities section, for each binning mode. Note that the resolution mode chosen should match the mode requested in Phase I. Note that PRV mode is not offered currently.

• Target mode offers only in standard resolution mode the options of single target, dual target, of target+sky modes. The mode chosen should match the mode requested in Phase I. For target and sky observations, please select the option best suited for the sky IFU.

• Red/blue exposure times and counts give the exposure times, and total number of exposures written to one file for the red and blue detector respectively. These can be edited separately for the different detectors, but it is recommended that one edit the total time for one observation to be more of less equal to avoid overheads. It is also suggested that one edit the exposures times to match including the readout overheads (given above). For e.g., in 1x2 binning red/med overhead is 30s, and blue/slow is 24s. If both have a science exposure time of say 60s, one can increase the blue/slow readout to 66s without any overhead increase. See also example below.

• Red/blue binning sets the spectral and spatial binning in the red and blue detectors respectively. Users are suggested to use the same binning in both detectors, unless necessary for science. High spatial binning (1×8, 2×8, 4×4 options) in the dual target mode may lead to cross contaminations between the two science fibers, since they can overlap on the final image along with the sky spectrum.

• Red/blue read modes sets the read modes in the red and blue detectors respectively. Users are suggested to use the blue/slow, and red/medium read mode since they have usually equal readout times at the same binning, and offer the similar readout properties. Other readout modes are suggested such as fast for bright (V<6 mag), or slow for faint targets. Further details on binning and read modes can be found in the components webpages.

• Fiber agitator radio button turns on or off the fibers agitator for GHOST. In general, they should be turned off unless science case requests it. For SNR<500, the fiber agitators adds no benefits, and for higher SNR the benefit is minimal.

• ISS port sets whether the instrument is using an up-looking, or side-looking port. This also translates into the ITC in the OT. By default, GHOST always uses a up-looking and it is recommended that the users do not edit this. Although depending on the schedule the instrument schedule, GHOST may be put on the side-looking port.

GHOST target component

When creating an observation using the the "GHOST observation", or adding a target to an existing component a GHOST target component is created. This is used to define the basic target parameters, and examples in the three different target modes, single target standard resolution, dual target standard resolution, and high resolution are given below. Each subsection is also described.

• SRIFU1 gives the coordinates, space motions and the magnitudes of the standard resolution IFU1 target. Users can use the magnifying glass to search automatic databases for their target. However, it is their responsibility to ensure the correct target coordinates, motions, and magnitudes are entered. For standard resolution IFU, the relative sky position stays the same, and can only be changed by either changing the position angle or the centre of the focal plane.

• SRIFU2 gives the coordinates, space motions and the magnitudes of the standard resolution IFU2 target, only in dual or target+sky modes. It is the users responsibility to ensure this is on blank sky.

• HRIFU gives the coordinates, space motions and the magnitudes of the high resolution IFU1 target. It is the users responsibility to ensure the are on the chosen science target.

• HRIFU Sky gives the coordinates of the high resolution sky in IFU2. It is the users responsibility to ensure this is on blank sky.

• Link base to target radio button allows users to automatically ensure that the target is on the centre of the focal plane. For dual target mode, it sets the centre position (the base) to be equidistant from both targets. For target+sky mode (including HR mode) it sets the base to be the target position. To set the coordinates off-axis, this radio button must be unchecked.

GHOST sequence component

The GHOST sequence component represents the final step in completing a single observation, and decides the total number of frames to be written during the observation. Below is an example. Each component is also explained.

• Counts are the number of individual exposures in one file, per detector. These cannot be split, and total exposure time×counts are one observation.
• Observes are the total number of files created by a GHOST sequence.
• Sequences. They serve the same purpose as observes for GHOST, but one can modify the parameters more cleanly between steps.

If the observation sequence includes a mix of long and short expousres, the slit exposure may be missed in the short expousre. To avoid this issue, consider splitting the seuqnece into separate groups of long and short expousres. 

Checklist

• Have you selected the appropriate template from the GHOST OT library? Have you gone through the checklist in the Top-level Program Overview note and included relevant standardized notes? Add notes with information about the program that will make it easier for the observer. Try to use the standardized notes provided in the OT Library.
• In the GHOST component, check that the correct target and instrument mode is chosen.
• Are the exposure times reasonable? Sequence longer than ~2 hours to execute will likely not be executed all on one night, and you must allow adequate time for re-acquisitions on subsequent nights when filling the allocated time.
• Have you checked baseline calibrations to see what is offered/required, or if additional calibrations are required for your science. Additional calibrations such as telluric, or RV standards are charged to the program but must be setup during the Phase II process.
• Have you included the correct target coordinates, including proper motions where applicable, and magnitudes?
• Is the guide star selected vignetting your science target?