JCMT Observing (4)
Flexible observing practice involves determining what the next MSB queued to observe should be by assessing the currently known weather conditions, the source position and the scientific priority of the project. This can be updated on a constant basis and aims to ensure the best use of the available weather conditions and maximise the scientific aims of a project.
When we invite observers to the telescope we always aim to do so at the best time for observing their targets (according to their RA distribution). So we hope that you will get to observe your project when you are at the JCMT. However, if the weather on the night(s) you are at the summit is not in your allocated weather band, then the Queuy Tool will select something else.
Yes! Because this ensures that we match as best we can the current conditions to those for which you calculated your observing time and subsequent sensitivities. It also means there is a good possibility we might have already gathered data for your project before you have arrived at the facility. So when you get here, you may already have freshly collected data to start making great science with.
No. The observatory takes care of all calibration observations (pointing, focus and standard source observations) and this time is not charged to your project. We suggest you look at the calibration pages on specific JCMT instruments to guide you as to our quoted calibration uncertainties. If you have a special requirement, for example with SCUBA-2: a faint source observation (like cosmology) where pointing is critical, you may request in the MSB notes that, if possible, additional pointings are taken bracketing the observations to provide additional precision. Likewise, with a spectral observation, if intensity is of critical importance, you may wish to request a standard source observation be taken closely in time to your science observation. We will do our best to accommodate such requests. If uncertain, ask your Friend of Project for further guidance.
JCMT Operations (2)
JCMT operates a 12-hour shift from approximately sunset to just after sunrise (though submillimeter observations are not strictly limited to nighttime hours). We have an attending Telescope System Specialist (TSS) who operates the telescope from the summit, and who is accompanied by a visiting observer. The visiting observer assists by helping to guide selection of MSBs with the Query Tool (according to weather and priority) and providing scientific feedback on the quality of the observations being collected. If weather conditions are favourable, we might also complete evening observing and then handover to a remote operator (Extended Observer) in Hilo, to continue observing for a few hours in the morning whilst conditions are still good.
Yes it can. Submillimeter telescopes are not limited to night time hours. Telescopes like the JCMT are mostly restricted by the amount of water vapour present in the atmosphere. This tends to be lower at night, and in the early morning, but can often (especially on sites like Mauna Kea) be low enough throughout the day to allow us to observe at any time. Scientific observations of planets and comets are the most common targets for daytime observing.
However usually the seeing becomes bad at submillimeter wavelengths during the day and pointing and focus observations become difficult after about 10am HST.
JCMT Data (4)
The reduced data at CADC are mainly for inspection of the results, and also have not always been reduced with the latest version of the software.
They are reduced with the default DR recipe in the MSB, and for SCUBA-2 the default FCF was applied, which may not be the best for your observation. Also observations of different nights are not co-added.
You probably want to rereduce SCUBA-2 data using the correct FCF, the pixelsize of your choice, and possibly using external masks and other refinements.
For heterodyne observations the resulting files are often acceptable, but in some cases you may e.g. want to blank parts of the raw data or modify the velocity range with noisy pixels at the edges of the bands and rereduce.
Dealing with data from different telescopes is a common activity for astronomers. Here is a rough method for convolving a beam (i.e. PACS, Herschel) to another beam (i.e. SCUBA-2 at 850um).
A very rough and ready method is to assume that both beams are Gaussian. Let’s say PACS has an FWHM of “A” arc-seconds and SCUBA-2 has an FWHM of “B” arc-seconds. If “A” is larger than “B”, then you need to smooth the SCUBA-2 map using a Gaussian kernel of FWHM equal to sqrt(A*A – B*B) arc-seconds. If the SCUBA-2 map has a pixel size of Pa arc-seconds, then first convert the above size into pixels by calculating:
width = sqrt( A*A – B*B )/Pa
and then smooth the SCUBA-2 map using the “gausmooth” command in the starlink kappa package:
gausmooth in=<your scuba-2 map> out=<smoothed map> fwhm=<your "width" value>
The smoothed map is put into the file specified by the “out” parameter. Alternatively, if “B” is larger than “A” smooth the PACS map inthe same way, using a width of:
width = sqrt( B*B – A*A )/Pb
where Pb is the pixel size in the PACS map.
You will need to know what the A and B values are (at 850 um “B” is about 13.5 arc-seconds). If your maps have point sources in them, then you could determine A and B by measuring the widths of the point sources in your maps. For instance, the “psf” command (“Point Spread Function”) in the starlink kappa package allows you to determine a mean beam shape from one or more point sources in an image. It does this by fitting a generalized Gaussian function to the mean radial profile of the indicated point sources.
The above assumes that both beam shapes are Gaussian. The SCUBA-2 beam shape is not quite Gaussian and so the above method can be improved, but it involves a lot more time and effort. You need first to get accurate models for the two beams (either as analytical functions or as 2D images), then you smooth the SCUBA-2 map using the PACS beam, and then smooth the PACS map using the SCUBA-2 beam. This approach requires no deconvolution, but results in maps that have lower resolution than either the PACS or SCUBA-2 maps. The kappa package includes the “convolve” command that will smooth a map using a beam shape specified as a 2D image (the kappa “maths” command can be used to generate a 2D image if your beam shape is expressed as an analytical expression). The details of this method depend on the form in which you obtain the beam shape information.
A common question we receive here at the JCMT is regarding observations with SCUBA-2 and the conversion from mJy/beam to mJy. Before we being it should be noted that for a real point source, a peak brightness value reported in units of mJy is the same as a peak brightness value reported in mJy/beam.
Now what happens if we have a map in mJy/beam and we want to obtain an integrated intensity value, a total flux value. We first sum up a number of pixels and now we want to get our units correct from mJy/beam to mJy…
Total Flux = flux summed over a number of pixels/(number of pixels in a beam)
Then your units are:
[mJy*pixels/beam] / [pixels/beam] = [mJy].
Beam Area = 2 × π × σ² [arcsec]
where the σ of the Gaussian beam can be calculated from the JCMT FWHM values at 850 and 450 microns (reminder the beam components are provided in Dempsey’s 2013 paper).
FWHM = 2 σ √(2 ln 2) [arcsec]
So we can use the FWHM to obtain σ to calculate the Beam Area and report the beam area in terms of pixels:
number of pixels in a beam = Beam Area [arcsec] / (pixel length)²
JCMT OT (10)
Normally a repeat counter is not necessary. A single MSB should be between 30-70minutes in length. To build up the total time require on a source you should increase the “Observe Counter”. A repeat counter may be used when performing a Heterodyne stare observation. In this case it may be ideal to have each stare last ~15-20minutes and use the “repeat” counter to produce a sensible MSB length.
Observers usually base a reference/off position on knowledge about a region/source from previous data or information from the literature.
If you are unsure of your reference position you might want to add a quick 5 minute stare observation of your reference position in the same set up as your science observation. You can then add to the notes that the science observation must only be taken if the reference position is clear.
Use what you need. i.e. only request wideband mode if your source requires it. Only request multi subband observations if you need it and not to get “free lines” – typically the more complex an ACSIS set up the higher the chance for baseline issues due to less stable power supplies. Choosing two 250 MHz bands with lines in the center of each band is better than choosing a 400 MHz band centered on one line.
A basket weave is an observation in which one scan ha a “Scan PA” component set to “Along Width” and a second scan has the “Scan PA” set to “Along Height”. If you are mapping a small region you can simply do this within the same MSB. To do this copy the scan component and paste it below the original scan component and adjusting the “Scan PA” accordingly. You might also need to update the Sample Time to ensure your MSB remains a sensible length.
For larger scan areas (>0.5 degrees in size) you might have to have a separate MSB for the “Width” and “Height” due to the total length. If this is the case it is recommended you request in the notes that two observations are taken back to back to ensure an even coverage in terms of sky transmission (in terms of elevation and weather).
Do not edit an older copy you have on disk and upload.
In JCMTOT click on Fetch Program to retrieve the MSB’s from the database. Then do any necessary modifications and Store to the Online Database. This will prevent repeating observations which already have been done, and keeps modifications done by your FoP.
Heterodyne: The aspect of the MSB (Minimum Schedulable Block) you most want to think about is its length. If you have been allocated e.g. eight hours for a single source, you could submit a single 8-hour MSB if you wanted. But that would be a very silly thing to do. If your source is not up for more than eight hours, it would never be scheduled as our software would detect that it would set before the MSB could be completed. But even if your source were observable for 8 hours solid, the observer won’t choose to undertake it except near the very beginning of the night, which vastly decreases your scheduling opportunities.
On the other hand, if many short observations are all scheduled one after the other they would have to be sent to the queue one by one, thus driving the TSS and observer insane.
If your sources are well separated across the sky a pointing will be required between them (even if they are only separated in time by ten minutes). However if they are close by each other they can be done consecutively. If you are including multiple sources in an MSB try to pair or group them so they are all are all close together. If this is not the case then include an note highlighting this to the operator.
The golden mean? A 30-to-90 minute MSB. If you have any questions about the best way to distribute your sources please consult your Friend of Project for ideas.
SCUBA-2: We have a 40 minute limit on SCUBA-2 observing blocks. This upper limit usually concerns large pong maps while small daisies fall at the other end of the scale being potentially only a few minutes each. Having short MSBs for SCUBA-2 does not incur the same overheads as heterodyne. Like heterodyne observation consecutive sources that include a large slew will likely require a pointing between them, however SCUBA-2 has the additional factor or requiring the arrays to be setup again after a move between sources.
There isn’t one. However if you don’t submit MSBs for your sources by the time they are observable you could be throwing away opportunities of having your sources observed. Nevertheless, note that there is no requirement that you submit your entire project at once. Therefore you can submit some MSBs, wait for them to get done so you can analyze the results and then submit other MSBs to follow up on those results. Your MSBs will keep on being scheduled until you have exceeded the total time allocated to your project.
You certainly can. However I suggest you let us know for two reasons. First, for your benefit, we’ll be able to look your MSBs over to ensure that they will actually do what you intend. Secondly, for our benefit, we would like the chance to create a library MSB that matches your observing mode in case that will be useful to other users.
We would rather you did not. Your Friend Of Project will not enable your program for observing until they are sure the construction is good. Uploading to the database is the best way to work with us, and get help. Upload it and then notify your F.O.P. so they can review it and suggest or make changes. Also there is a risk with xml sent via email that the lines are not wrapped – this will produce illegal xml.