How QSO works at CFHT
Updated November 2021
Abstract -- This document explains how Principal
Investigators (PIs) can submit QSO mode proposals and observations, and
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,
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:
- Regular programs: default type of program requesting good sky
conditions and with known targets.
- Target-of-Opportunity programs (TOO): programs for which the targets
are not known in advance.
- Programs submitted at the regular time and as a Regular QSO
program, before the deadline, but
for which targets will be identified during the semester.
- Programs submitted during a semester to observe unpredictable
objects (e.g. supernova). The programs are submitted as Director's
Discretionary Time proposals.
- Snapshot programs: programs requesting the worst sky
conditions (Image Quality over 1.2", and preferably non-photometric
conditions). Snapshot programs have to be simple (e.g. using broad-band
filters only, and without time constraints), preferably have short
observations, and have to be scientifically useful even if only a small
fraction of observations are obtained. Snapshot programs are not
charged to the agencies.
Deadlines and NorthStar
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). Proposals are submitted using NorthStar, a Phase 1 tool
that resides 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.
||% of Telescope Time
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.
The following table presents average seeing
conditions in the R band (taken with FOCAM at CFHT between
|Image Quality (IQ)
|IQ ≤ 0.55"
|0.55" < IQ ≤ 0.65"
|0.65" < IQ ≤ 0.80"
|0.80" < IQ ≤ 1.0"
|1.0" < IQ ≤ 1.2"
|IQ > 1.2"
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.
|Lost to Weather
|Usable Photometric Nights
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
PIs in their proposal and PH2.
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
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.
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
Preparation and execution of queues follow three
Here are a few general rules that the QSO Team follows as much as
- 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.
- 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.
- Human filtering: the QSO observer may modify the order in a queue
according to special constraints like focus sequences, filter change,
calibration plan, etc.
- Images will not be obtained in worse IQ (or sky brightness)
conditions than requested. The IQ (sky brightness) measured should not
exceed the upper limit of the IQ band (sky brightness) requested by more
- Images can be obtained in conditions better than requested, if no
other observations actually requesting these conditions are
- If the IQ>1.2" or the sky is non-photometric, the snapshot programs
requesting bad conditions will be executed, unless other possibilities
exist among the regular programs.
- The priority of the programs started is automatically increased
compared to programs not started.
- When possible, the observations will be tentatively
done with airmass smaller than 1.5.
- During selection and ordering of the queues, the priority goes
from grade A to B to C, followed by the snapshots. Inside these grades,
priority is given according to the TAC rank as much as possible.
- Balancing Agency time has a very high priority and might exceed
the other selection and ranking criteria.
- The observations are executed according to the priority index (high,
medium, low) given by the PIs during the Phase 2 period.
- A QSO run should never be completed without getting all the
necessary calibrations for all the programs fully or partially executed
during the run.
- All fields requesting photometry and done during non-photometric
conditions will be calibrated with short exposures during photometric
- No programs will be recycled for completion during the next
- When started, a monitoring program receives a higher priority so
that the observations to be repeated can be carried out within the
specified time frame period.
- The QSO Team will always try to obtain the requested number of
observations for a given monitoring program. In case of other
constraints, the minimum number of observations specified during Phase
2 is the minimum acceptable.
PIs may contact the QSO Team at any
time by email, by using qsoteam -=at=- cfht.hawaii.edu