How QSO works at CFHT

Updated August 2022

Abstract -- This document explains how Principal Investigators (PIs) can submit QSO mode proposals and observations, and what services can be expected from CFHT. This document is not instrument-specific and applies to MegaPrime, WIRCam, ESPaDOnS, SITELLE, and SPIRou.

Keywords -- QSO, Regular/TOO/Snapshot programs, Deadlines, NorthStar, Exposure Time Calculators, Technical Evaluations, TAC, Ranking, QSO grade, C programs, Snapshot programs, Phase 2, PH2/K2, Observations, Sky conditions, Calibrations, Night Reports, Data reduction and distribution, Proprietary period, QSO rules, Contact information.


The main concept behind the queue observation scheme is to execute programs only during the sky conditions or time constraints required to meet their science goals, as defined by the investigators.

The Queued Service Observing mode used at CFHT offers many advantages over Classical Observing: higher efficiency on the sky (because Remote Observers are highly trained and observe frequently), high priority given to the most important projects (i.e., those highly ranked), observations carried under constraints specified by PIs (e.g. Image Quality requirement), very short observations possible (e.g. 2 hrs), unexpected and short notice events can be observed (e.g. Gamma Ray burst). QSO started in 2001 with CFH12k, and is now used with MegaPrime (2003), WIRCam (2005), ESPaDOnS (2008), SITELLE (2015), and SPIRou (2019).

Regular, TOO, and Snapshot programs

Three types of programs can be carried under QSO:

Deadlines and Phase 1 using the K1 tool

PIs interested in submitting proposals should consult the CFHT website about a month prior to the Spring and Fall equinox. When a semester opens for proposals, announcements are made on the web site and via the agencies. The deadline for submitting proposals is usually around March 21st for the "B" semester (Aug 1st to Jan 31st), and around September 21st for the "A" semester (February 1st to July 31st). Director's Discretionary Time proposals can be submitted at any time.

Proposals are submitted using the kealahou tool K1, a Phase 1 tool created at CFHT.

Exposure time calculators

Each instrument has an Exposure Time Calculator that must be used to correctly calculate required exposure times. Proposals must include the exposure times and the readout time for the specific instrument, and other overheads as instructed in the instrument-specific documentation.

Technical evaluations, TAC

After the deadline, all proposals are reviewed by CFHT astronomers to make sure there are no technical issues present in the proposals. Each agency has a Time Allocation Committee (TAC), and reviews its proposals. Proposals are ranked by TACs.

QSO grade, C and Snapshot programs

The accepted proposals are split into A and B programs, with a 30/70 or 40/60 ratio of hours. The QSO Team chooses appropriate C programs to overfill the queue. The chosen C programs are usually close to the cut-off line and are suitable as overfill programs (for example, they use an Image Quality which is not requested much, or they have targets in a range of RA not used by accepted A and B programs). Snapshot programs, which must request IQ>1.2" and have to accept extinction, are usually also accepted. The table below presents the priority given to the QSO grades and the approximate fraction of the telescope time given to A and B programs. Snapshot programs are not accounted for and are not charged to the agencies.

Grade Grade Name Global Priority % of Telescope Time
A "Must-Do" Highest 30-40
B "Prioritized" Good 70-60
C "Best Effort" Medium N/A
S "Snapshot" Lowest N/A

PH2 and kealahou K2

Each accepted program must be entered in CFHT's database using the Phase 2 Tool (PH2) or kealahou K2. These web-based tools allow PIs to list their targets, configure their observations (instrument, filter(s), mode(s), exposure times, etc.), specify acceptable constraints (Image Quality, airmass, background), and schedule observations for specific times or with a monitoring period, if desired.


Each day during a QSO run, a CFHT QSO Team member prepares various queues suitable for the coming night, based on PI requests for Image Quality and sky conditions, time constraints, and ranking of the programs. "A" programs are given priority, followed by "B", "C", and Snapshot programs. Queues are also made as much as possible to give each agency its share of the time; if an agency has 30% of the allocated time on a given instrument, it will be found on average in about 30% of the queues. At night, depending on the sky conditions, the CFHT Remote Observer selects and executes one or more queues. Each exposure receives a grade indicating the quality of the data (grades 1 and 2 are good for the science proposed), and most exposures also receive comments about sky conditions, technical issues, etc. The next morning, a CFHT QSO Team member reviews those grades and comments, and validates exposures which are good enough for the science goals proposed; only validated exposures are taken out of a PI's allocated time. Exposures which are not validated will be tried again if possible.

Sky conditions

The following table presents average seeing conditions in the R band (taken with FOCAM at CFHT between 1993-1995).
Image Quality (IQ) Frequency (%)
IQ ≤ 0.55" 5
0.55" < IQ ≤ 0.65" 25
0.65" < IQ ≤ 0.80" 30
0.80" < IQ ≤ 1.0" 25
1.0" < IQ ≤ 1.2" 15
IQ > 1.2" 5

The table below gives the average weather statistics for Mauna Kea. In general, the amount of time lost is higher during "A" semesters than during "B" semesters, in particular during the first few months of the winter when it is not unusual to see more than half the time lost to bad weather.

Sky Conditions Frequency
Usable Nights ~70%
Lost to Weather ~20-30%
Usable Photometric Nights ~50%


For all instruments, appropriate instrumental calibrations are obtained by the QSO Team only (biases, darks, twilight flats, lamp exposures, fringe frames). Broad-band photometric standard stars and astrometric standard fields are also taken for the imagers; no spectrophotometric standard are taken with ESPaDOnS or SPIRou. Narrow-band calibrations (for the imagers) must be included by PIs in their proposal and PH2.

Night Reports

The QSO schedule is put on the QSO web page and updated whenever there is a change. During a QSO run, Night Reports are available on the web, and provide weather information and a log of validated exposures for each QSO night. PIs are responsible for following the progress of a QSO run and the progress of their program in particular. CFHT does not necessarily contact PIs each time data are obtained.

Data reduction and distribution

Data are detrended or reduced, either after each night, or after each run by a team of astronomers and software engineers. PIs who request immediate (or quick) access to their data can receive raw data or detrended data as soon as available. When data are ready, PIs receive an email from CFHT with the location of their data; the data (detrended, and raw for some instruments), and sometimes documentation, instructions, and metadata are downloadable from a private URL with a unique key sent to PIs.

Proprietary period

The proprietary period of QSO data extends to 1 year + 1 month starting at the end of the QSO semester. For instance, data taken for the 2009B semester (Aug 1 2009 - Jan 31 2010) will have a default release date set to 02/28/2011. If an extension is requested in NorthStar and approved by TAC, a new date will be set for this program through the QSO system. The release date for the data is indicated in the fits headers by the keyword REL_DATE. For snapshot programs, the proprietary time is 3 months following the end of the semester.

QSO rules

Preparation and execution of queues follow three steps:
  1. Selection: observations stored in the database are selected according to instrumental constraints, sky constraints, actual sky conditions, completeness level, and the position of the targets.
  2. Ordering: selected observations are put in a prioritized list to be sequentially executed according to the TAC grade, rank, target positions, time constraints, and user's priorities.
  3. Human filtering: the QSO observer may modify the order in a queue according to special constraints like focus sequences, filter change, calibration plan, etc.
Here are a few general rules that the QSO Team follows as much as possible:

Contact information

PIs may contact the QSO Team at any time by email, by using qsoteam -=at=-