Information for Proposers

Instrument Description
Proposal Guidelines and Restrictions
Mailing List
Raw Sensitivity
Observing Modes and Scan Strategy
Pointing and Calibration
Observing Efficiency
Data Archiving
Analysis Software
Revision History

Instrument Description

Bolocam, a large-format bolometric camera for observations at 1.1 and 2.1 mm, is open to proposals. The camera will in general be available for only one of these wavelengths in any given semester. The camera has 115 working pixels with 8 arcmin FOV (approximately circular). The beam FWHM is 30 arcsec at 1.1 mm and 60 arcsec at 2.1 mm. At all wavelengths, the pixel spacing (nearest neighbors of hexagonal close-packed array) is 38 arcsec.

wavelength unless otherwise indicated: 1.1 mm

Proposal Guidelines and Restrictions

Proposals that overlap the instrument team's key projects (blank-field surveys at 1.1 mm for dusty extragalactic point sources, blank-field surveys at 2.1 mm for clusters using the Sunyaev-Zeldovich effect, and pointed observations of galaxy clusters for dusty extragalactic point sources and the Sunyaev-Zeldovich effect) will only be considered if in collaboration with or approved by the instrument team.

All other science topics are open to all proposers, though collaboration with the instrument team is strongly encouraged. While analysis software will be made available publicly via this web site, the lack of funding for support limits the instrument team's availability for providing technical support on the software to those projects with instrument team members involved as collaborators.

Prospective proposers are encouraged to contact members of the instrument team with questions or to investigate possibilities for collaboration:

Jamie Bock (jjb@astro.caltech.edu)
Jason Glenn (jglenn@origins.colorado.edu)
Sunil Golwala (golwala@astro.caltech.edu)
Phil Mauskopf (philip.mauskopf@astro.cf.ac.uk)

Prospective proposers should endeavor to make such contacts or requests well in advance (> 1 month) of proposal deadlines; the instrument team cannot guarantee immediate response to such requests.

Mailing List

As of March, 2004, we have started a mailing list for Bolocam proposers/users who would like to be notified of major web page updates, especially with regard to sensitivity calculations and analysis software. Please contact href="BolocamSupport.html">the Bolocam support person if you would like to be added to this list.

Raw Sensitivity

Due to the short history of observations with Bolocam, exhaustive sensitivity numbers are not available. We give below those numbers that are available.

At 1.1 mm:

Observations taken during the early 2003 El Nino give the following per-pixel instantaneous sensitivity percentiles at 1.1 mm:

unchopped mode

25%: 68 mJy √sec

50%: 78 mJy √sec

75%: 92 mJy √sec

chopped mode:

25%: 102 mJy √sec

50%: 112 mJy √sec
75%: 167 mJy √sec

Cumulative distributions are shown here (NOTE: the horizontal axis must be scaled by 3.4!): pdf

The chopped observations took place partially after the end of the El Nino and give more representative sensitivities. For the purpose of proposals, we suggest the use of the following 1.1 mm sensitivities:

unchopped mode: 100 mJy √sec

chopped mode: 143 mJy √sec

For survey mode, one can calculate a mapping speed from

MS = N_pix Ω_beam / sensitivity²

N_pix = 115
Ω_beam = 0.32 arcmin²

giving (approximately)

unchopped mode: 13 arcmin² hr^-1 mJy^-2

chopped mode: 6 arcmin² hr^-1 mJy^-2

The above results are based on an aggressive sky subtraction. For "blank-field" data, this aggressive subtraction seems to have a noticeable but small effect on source fluxes (~20% currently). For bright and/or extended sources, such an aggressive sky removal cannot be applied if accurate recovery of flux from bright and/or extended sources is desired. A much less aggressive sky subtraction preserves such structures but also results in poorer sensitivity. Therefore, we ask that proposers use the following mapping speeds depending of their type of observation and observing mode:

unchopped, blank-field: 10 arcmin² hr^-1 mJy^-2
unchopped, bright and/or extended objects: 1 arcmin² hr^-1 mJy^-2
chopped: 5 arcmin² hr^-1 mJy^-2
We quote the same number for all kinds of fields when chopping because we assume the proposer is uninterested in extended structures because chopping will remove them.

Observations not taken in survey mode should begin with the above instantaneous sensitivity numbers and then correct for the number of pixels on-source at any given time as discussed below.

Regardless of observing mode, proposers should take account of various observing efficiency factors listed below in estimating the amount of time needed for a given observation.

Observing Modes and Scan Strategy

Observations with bolometric cameras require some sort of temporal modulation of the astronomical signal in order to separate such signal from sky noise and instrumental 1/f. This need is reflected in the available scanning modes.

simple raster scanning, unchopped: If chopping is not employed, the telescope must be scanned sufficiently quickly that sky noise is mitigated to the point where it can be subtracted using the array format. As noted above, good sensitivities are achieved at 1.1 mm when a scan speed of 60 arcsec/sec is used. We do not recommend changing the scan speed until the sensitivity as a function of scan speed has been characterized. While scanning slower will result in better observing efficiency, the degradation in sensitivity more than cancels any such gains. Faster scanning has not been explored in detail; the pointing stability will likely degrade at faster scan speeds and the degradation has not yet been characterized. See the XRASTER_SCAN command in the CSO manual for available coordinate systems and options.
raster scanning + chopping: When the secondary chopper is used, much slower scan speeds are possible because chopping removes the bulk of the sky noise. However, the system remains unstable at DC and so the telescope must be scanned. Good sensitivity has been demonstrated at a scan speed of 5 arcsec/sec. Higher scan speeds in chopped mode are not advised because the individual bolometer sampling of the target field becomes poorer. Slower scan speeds are susceptible to slow drift in the chopper-synchronous offset. We recommend that drastically different scan speeds not be used until the sensitivity at different scan speeds has been characterized. Only azimuthal chopping combined with azimuthal raster scanning has been used. Observers may use other chop directions at their own risk. See the XRASTER_SCAN and SECONDARY commands for further possibilities.
jiggle mapping, a.k.a. chopping + nodding: Not currently available, may be implemented.

more complicated scan modes: The LISSAJOUS and BOX_SCAN modes used by SHARC-II are available to Bolocam but have not been characterized. The analysis software also cannot deal with these modes yet. Observers may use them at their own risk and with the understanding they may have to modify the pipeline (substantially). Detailed information about these scan modes can be found on the SHARC II web page, available via the CSO main page, http://www.cso.caltech.edu.

Important additional notes on scan strategy:

Sky subtraction: To first order, sky subtraction removes the average signal seen across the array. It thus attenuates or removes all astronomical structure comparable in size to the FOV and larger. We expect that more advanced analysis methods may allow recovery of some of this information, but no guarantees can be made at this point.
Striping: For large fields, observers should be aware that striping due to residual 1/f can be a problem. As noted above, sensitivities have been calculated for point-source searches; larger-scale structure will likely be observed at poorer sensitivity. To mitigate the effects of striping, observers should use cross-linked scan strategies. These can consist of alt-az scans coupled with field rotation, or equatorial-mode raster scans at two different angles. The instrument team is developing optimal destriping methods, but these can only work when the maps are cross-linked in the above manner.
Point-source photometry: The most efficient observing mode for photometry of single sources small compared to the FOV is a jiggle-map mode. Such a mode has NOT YET been implemented for Bolocam, though we expect it will at some point. Instead of jiggle-mapping, observers will have to employ either a fast, unchopped raster scan or a slow, chopped scan.

Proposers must mention their intended observing mode(s) so the CSO TAC may judge whether the desired observations are consistent with the constraints imposed by a bolometric camera.

Pointing and Calibration

Pointing observations must be done frequently with Bolocam. It is strongly suggested that observers use multiple pointing sources near the field to be observed (within 10 to 20 deg), checking pointing once every 2 hours or so, more frequently if near zenith and the field's local coordinates are changing quickly. The pointing sources should be spread out so that the pointing on the field of interest can be interpolated, not extrapolated, from the grid. Pointing sources with fluxes of 2 Jy and higher are easiest to use, though lower-flux sources can be used if necessary. Pointing sources effectively become relative flux calibration observations by dint of doing them frequently, but they must be tied to primary or secondary flux calibrators at some point during your observing.

Absolute flux calibration on a primary or high-quality secondary calibrator should be done once or twice per night, more frequently if observing conditions are highly variable. Bolocam has continuous monitors of sky loading (the bolometer resistances) that can be used to do a running relative flux calibration correction, but sufficient data must be accumulated to measure check that this relationship maintains its standard form. In general, your pointing observations will serve this purpose.

A beam map/flux calibration of the full array should be done once every few nights using a primary calibrator such as Mars (Jupiter and Saturn are too bright) or a bright secondary calibrator (Uranus, Neptune, or bright quasars or galactic sources). Even for short (1-2 night) runs, at least one beam map should be done.

Relative gains of the array elements can be monitored using sky noise.

More information on doing calibration observations is provided on the PreparingForObserving page.

Observing Efficiency

In calculating time requirements, five important observing efficiency factors should be taken into account:

Turnaround Efficiency

At high scan speeds and for small fields, a significant amount of observing time may be spent in turnarounds. We have not optimized the turnaround time, but we have found that the standard turnaround time of 10 sec (5 to slow down, 5 to speed up) provides the sensitivities given above. Shorter turnaround times may be possible. To be conservative, observers should assume 10 sec turnaround time and all turnaround time is lost. This contributes an efficiency factor

eta_turn = (l/v_scan) / [ (l/v_scan) + t_turn ]

where l is the scan length, v_scan is the scan speed, and t_turn is the turnaround time.

When in slow raster + chopping mode, the efficiency is greatly increased due to the slow scan speed.
"Sampling" Efficiency

The Bolocam array does not instantaneously provide full sampling of the sky. The array has a hexagonal close-packed format, with pixels spaced by 38 arcsec. The beam FWHM is 30 arcsec at 1.1 mm and 60 arcsec at 2.1 mm, so the effective pixel spacing is 1.3 f lambda and 0.6 f lambda, respectively. Full sampling requires a rectangular array with 0.5 f lambda pixel spacing, so clearly instantaneous full sampling is not achieved.

However, the camera can be rotated relative to the scan direction such that full sampling is achieved after one full pass of the array across a line perpendicular to the scan direction. While the sky is fully sampled in this mode, it is not necessarily uniformly sampled due to the distribution of bad pixels in the array. When the sampling is not uniform, the noise will vary across the map. Observers must therefore realize that one pass across the source will not suffice to provide a uniform map! For most observers, who will make many scans across a field, the scan strategy can be arranged to provide fairly uniform (to better than a few percent) sampling after many scans. But observers proposing to observe a field smaller than or comparable to the field of view for only a short amount of time should consult the PreparingForObserving page to determine whether they will need to observe a source for longer than expected in order to build up a uniformly sampled map.

"Mapping" Efficiency

The coverage in any scan mode will decrease at the edges of the map because the entire array does not sweep over the edges of the field defined by the scan strategy. The field edges defined by the scan strategy correspond to the motion of the center of the array. If the center of the array scans to a certain point, only half the array has scanned past that point, so the coverage is reduced to approximately 50% of its value at the center of the field. The coverage will grade down linearly from 100% to 50% over the last FOV/2 = 4' at each edge of the map. To conservatively estimate this inefficiency, observers should assume that an edge of width FOV/2 = 4' is lost around the edges of the map.

For example, for a square map of size 30' x 30' (dimensions indicate the path that the telescope boresight follows), the full coverage region will be only 22' x 22', corresponding to a mapping efficiency of only 0.54. In general, the mapping efficiency is

eta_map = (l_x - FOV) (l_y - FOV) / l_x l_y

where l_x and l_y are the dimensions in the x and y directions of the scan. Alt-az rasters over long periods (where field rotation has an effect) will yield uniform coverage only over a circle whose diameter is the smaller of l_x - FOV and l_y - FOV, corresponding to lower mapping efficiency. These formulae become inaccurate when l_x and/or l_y become comparable to the FOV or smaller.
Point-Source Photometry Mode

Observations of a single object small compared to the FOV, as opposed to operation in survey mode, can be quite inefficient because only a few pixels are on-source at any time. For photometry of point sources, effectively only 1 pixel is on-source at any given time and so one should assume the per-pixel instantaneous sensitivities given above rather than the survey-mode mapping speeds. For distributed sources small compared to the field of view, one must similarly correct, calculating a mapping speed based only on the number of on-source pixels.
Pointing and Calibration

The time spent on pointing and calibration should be included in calculation of observing time. Typical pointing and flux calibration observations only require 5 min + slewing time because they only require the center bolometers of the array cross over the source. Full-array beam maps and flux calibration scans require large amounts of time (30 min in unchopped mode, 60 min to 2 hrs in chopped mode!), but need only be done once every few nights.

Data Archiving

See the DataArchiving page.

Analysis Software

An archive of the Bolocam analysis pipeline (written in IDL) with some example execution scripts will be made available via the AnalysisSoftware page. Directions for operating the pipeline are provided there also. A more detailed manual will be available in the future. The pipeline is installed in the bolocam account on kilauea.submm.caltech.edu, one of the summit computers, for use during your observing run.

We emphasize that this pipeline is neither foolproof nor fully automated. While we expect the pipeline to improve with time (and hopefully user contributions!), it is important that proposers realize that bolometer camera data is, unlike any other kind of astronomical data, a nontrivial exercise in timestream signal processing and noise correlation analysis. Proposers should judge whether the time investment is worth the expected science return. The instrument team will not be available for on-demand technical support except in collaborative relationships.

Revision History

2003/10/20 SG
First Web version
2003/10/22 SG
Correct MAJOR error in mapping speeds! Sensitivities were 3.4 times too good, mapping speeds were 10 times too good!
2004/01/31 SG
Add link back to main page, add links to SCUBA calibrator pages, correct error in mapping efficiency formula (have FOV/2 instead of FOV in the numerator).
2004/03/18 SG
Post reduced mapping speed for bright/extended source observations, add section on mailing list.
2004/04/07 SG
Move some of the Sampling Efficiency discussion to the PreparingForObserving page, clarify details on Mapping Efficiency, reduce turnaround time to 10 sec, other minor changes.
2004/05/09 SG
Link to DataArchiving page, modify AnalysisSoftware section.

Questions or comments? Contact the Bolocam support person.