Observation Preparation 

This section outlines how to prepare and verify GHOST observations during the Phase II stage. A successful setup begins by using the GHOST OT Library as a starting point. Preparing observing sequences requires specifying accurate target coordinates, selecting appropriate instrument configurations, and including all necessary sequence components. Depending on your science goals, you may also need to include observations of telluric standards, flux standards, radial velocity standards, or other non-standard calibrations.

Video Tutorial

Need help getting started? Watch the Phase II preparation video tutorial below.
Can't see well?  Get to a full size video in a separate window.  

Required Phase II components

The following table summarizes the essential components that must be defined in the OT at Phase II. Proposals lacking these components will not be accepted. Use the GHOST OT Library as a reference for templates and example observations. The table below lists an overview of the basic requirements from the PI.

• Required from PI: Must be specified in Phase II.
• As needed: Optional, depending on your science goals.
• From Phase I: Must match Phase I proposal. Changes require a formal change request.

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 mag essential)	Required from PI
Target component	Guide star	Required
GHOST sequence component		Required
Observe component		Required

Calibration and Time Accounting

• Baseline calibrations (biases, flats, arcs, and standard stars) are provided and not charged to your program. See the calibrations page for details.
• Nighttime calibrations or special standards (e.g., telluric, flux, or RV standards) are charged to the program. These must be defined by the PI during Phase II.
• Acquisition time is already included in the overheads for science observations.
• No additional time is charged for initial target acquisition. For long observations (>1 hr), extra time for recentering or reacquisition is charged.

Change Request

For any changes in target(s), observing conditions, or substantial changes in instrument mode, you must submit a request to the Head of Science Operations at Gemini South. See the Change Request page for instructions and approval requirements. 

Overheads

Accurately accounting for overheads is essential when preparing Phase II observations, especially for time-resolved programs or long integrations. The following table summarizes GHOST overheads.

Table 2: Overheads overview
Component	Time (s)	Applied
Setup and acquisition	480	On
Reacqustion	480	Every two hours of observations
Recentering	300	Every one hour of observation
Readout	Up to 98	Every write of either red/blue detector
DHS (Data Handiling System)	7	Every file wrriten

Set up and Target acquisition

• Each new target acquisition requires 8 minutes, which includes slewing, guiding initiation, and centering.
• For long (>1hr) observations, additional overheads are applied:
  • Recenterings are required every 1 hour (+ 5 min)
  • Reacquisitions are required every 2 hours (+ 8 min)
• Long observations may be split across nights to accommodate queue scheduling. If an observation block is shorter than 1 hour, the reacquisition is not charged.

Readout times

GHOST uses independent red and blue detectors, each with its own read mode and binning options. Because these detectors operate independently and have different readout times, users can set different exposure times for each to minimize idle time and improve efficiency.

• Readout times are per file write and must be added to the exposure time when calculating total time.
• An additional 7-second DHS overhead is applied per file.
• Recommendation: We strongly recommend using BLUE/SLOW and RED/MEDIUM read modes. These settings offer the best balance between readout speed, overhead, and detector noise performance.

Table 3: Readout overheads
Readmode	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 enables simultaneous observation of two targets in Standard Resolution mode. To use this capability:

• Target seperation: Targets must be at least 102 arcsec apart and within a field of view of 7.34 arcmins. 
• Instrument position angle can be rotated to accommodate target seperation and reach a suitable guide star. 
• Final spectrum: Both targets are read out on the same detectors, with spectra sparated by sky fibers. This requires: 
  • Similar magnitudes (within ~3-5mag) 
  • Shared expousre time, binning, and readout time. 
• Exposure planning: Use the exposure time calculator to avoid saturation of bright targets and esure S/N for faint ones. 
• Spatial binning: Avoid binning more than 4 in the spatial direction to prevent fiber overlap. 
• Proper motion: Check proper motion shifts near the observation date, as they affect the relative orientation of the two IFUs. 
• Place both targets equidistant from the field center to ensure optimal focus. 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 allows simultaneous sky subtraction in both resolution modes. More details on the IFU configuration can be found in the components webpages.

• Standard Resolution:
  • IFU1 has its own dedicated sky IFU (whose position cannot be changed).
  • IFU2 lacks a dedicated sky IFU; if sky subtraction is needed, use IFU1.
  • For target+sky mode: IFU1 (science) + IFU2 (sky) enables 10 sky fibers. Ensure 'link to base' is not selected in OT to avoid off-centering.
• High Resolution
  • IFU1 (science) use IFU2 (sky) as a dedicated sky, whose relative position can be changed.
  • Change PA if necessary to place the SRIFU1 sky IFU on blank sky.
• User responsibility: There is no automatic check to ensure that the sky IFU is on blank sky. During Phase II, confirm the sky fibers fall on 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

Program requiring specific timing windows (e.g., transits) must: 

• Define the timing window using the Timing Window component of the OT.
• Include a the standardized note available in the GHOST library.  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

GHOST requires the science target for guiding. For faint targets V> 19mag, use blind offset acquisition. The template is available in GHOST OT library. These options must be defined clearly during Phase II. 

Extended targets

The GHOST IFUs have a limited FoV (~1 arcsec). Guiding is done via microlenses around the science IFU: 

• Extended or uniform-brightness targets may not center correctly. 
• Consider disabling automatic centering and using blind offset mode. 
• Contact your CS if your program includes extended sources. 

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. All additional calibrations taken during the night are charged to the program, and must be defined, and ideally requested during the Phase I proposal.

• Any additional calibrations mixed in with science (i.e. during the night) as part of the nighttime program calibrations. 
• Any additional daytime calibrations requested beyond the baseline 
• A flux, telluric, or RV stadnards must defined at Phase II. 

Counts vs. Observe

In the OT, user define expousres using two parameters: 

• Exposure time and Counts
  • Number of exposures taken within a single 'Observe' step, saved as a single MEF file.
  • Example: 3 counts = 3 exposures written into one MEF file
• Observes
  • Number of times that full 'Observe' step is repeated, each producing a separate file.
  • Example: 3 Observes = 3 seperate files
• For scheduling flixibitlity, long observations should be split across multiple Observe steps (ideally ~1hr or less each). Only if your scinece demands uninterrupted expousre (e.g., transits or orbital phase moitoring) should you bundle many counts into a single Observe step. 
• NOTE: The current DRAGONS data reduction does not stack multple counts within a single Observe. We strongly recommend using multiple Observe to create independent exposures for each target. ​

GHOST OT details

We strongly recommend starting from the automatically-generated OT templates from the OT library when preparing GHOST observations. The library includes detailed instructions for each mode, as well as standardized notes for observers. This web page explains each OT component in depth, which are:

• GHOST static component
• GHOST target component
• Sequence component

After building your GHOST observations, please review the Checklist. For any questions, contact your CS or ghost_science @ noirlab.edu for support.

GHOST static component

When adding a GHOST component to an existing observation, or creating a "GHOST Observation", a GHOST static component is automatically included. This component defines the basic instrument setup:

• Position Angle (PA): Specifies the orientation of the instrument on the sky (North through East). Default is 0°. You may adjust it to reach a suitable guide star or align dual targets.

• Instrument Resolution: Choose from Standard, High, or Precision Radial Velocity (PRV) mode. Must match what was requested in Phase I. PRV is currently not offered.

• Target mode (Standard Resolution only): Options include Single Target, Dual Target, and Target+Sky. Must match the Phase I setup. 

• Red/Blue Exposure Time and Counts: Set exposure time and number of exposures per file for each detector. To avoid unnecessary overheads, keep red and blue total times roughly equal. You may slightly adjust blue exposure times to match red+overhead without adding delay. 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.

• Red/Blue Binning: Defines spectral and spatial binning. Use the same binning in both detectors unless scientifically required. Avoid high spatial binning (>4) in dual-target mode to prevent fiber overlap.

• Red/Blue Read Mode: Recommended setting is Blue/SLOW and Red/MEDIUM for balance of speed and noise. Other readout modes are suggested such as FAST for bright targets (V<6 mag), or SLOW for faint targets. Further details on binning and read modes can be found in the components webpages.

• Fiber Agitator: Should be OFF for most programs unless sicence case requests it. For SNR<500, the fiber agitators adds no benefits, and for higher SNR the benefit is minimal.

• ISS Port:  Default is Up-looking. Users should not modify unless instructed; internal telescope configuration handles port changes.

• Engineering tab: The Slit Veiweing Camera (SVC) Exposure time is automatically calculated based on the target V magnitude. However, for faint targets, users may manually adjust the SVC exposure under the Engineering tab. 

GHOST target component

When creating a new observation with "GHOST Observation", or adding a target to an existing component, the GHOST Target Component is created. This defines 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: Coordinates, proper motion, and 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: Coordinates, proper motions, and magnitudes of the Standard Resolution IFU2 target. Used only in Dual or Target+Sky modes. Ensure target is blank sky if for sky subtraction.

• HRIFU: Coordinates, proper motion, and magnitudes of the High Resolution IFU1 target.

• HRIFU Sky: Sky IFU coordinates for High Resolution observations. Must be placed on blank sky by user.

• Link Base to Target: Automatically centers the focal plane base. For Dual Target Mode, places base equidistant between targets. For Target+Sky and High-Resolution mode, links base to the target position. To set the coordinates off-axis, this radio button must be unchecked.

GHOST sequence component

The GHOST sequence component defines the exposure structure for a complete observation. 

• Counts: Number of individual exposures in one file, per detector. These cannot be split. 
• Observes : Number of complete Observe steps/files. 
• Sequences. They serve the same purpose as observes for GHOST, but one can modify the parameters more cleanly between steps.

NOTE: 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?

• Did you review the Top-level Program Overview note and include all relevant standardized notes?

• Have you added any extra information that could hlep the observer? 

• Did you select the correct target mode and instrument resolution, matching your Phase I setup?

• Are the target coordinates, proper motions, and magnitudes correctly entered. V magnitude is required, as it is used to calculate the slit viewing camera expousre. 

• Are the exposure times reasonable for your science gaols? Sequence longer than ~2 hours may span multiple nights and you must allow adequate time for re-acquisitions on subsequent nights when filling the allocated time.

• For bright star, have you ensured they won't saturate under better conditions? If there's a risk, leave a note in the OT. 

• Have you reviewed which baseline calibrations are provided? If additinoal calibrations (e.g., telluric, flux, RV standards) are needed, are they deifned in Phase II? 

• Does the selected guide star avoid vignetting your science target?