|Revision 1.5||2017 June 1||jbs|
|Added information about how to use the rapid response mode|
|Revision 1.4||2016 May 9||jbs|
|Updated information about the new high-frequency 1934-638 flux density model|
|Revision 1.3||2015 Oct 21||jbs|
|Updated information about remote observing procedures|
|Revision 1.2||2014 Oct 15||jbs|
|Fix a couple of typos regarding 1934-638 flux density models|
|Revision 1.1||2014 May 28||jbs|
|Rewrite chapter in FAQ format|
|Revision 1.0||2014 Mar 20||jbs|
|Initial Docbook revision|
This chapter deals with how to prepare for observations on the ATCA after having been granted observing time. It is presented as an FAQ, so that new observers can get all the information they need, and experienced observers can quickly find the information they need.
How much integration time is required on source to reach the necessary sensitivity?
What calibration is required to ensure the observations succeed, and how much observing time will it require?
How can observational overheads be minimised?
How will weather and RFI affect the observations?
After reading this chapter you should be able to optimally plan an observation, and know how to make the schedule that carries it out.
The ATCA antennas have a hard elevation limit of 12°, and for observations in the millimetre bands it is recommended that you stay above an elevation of 30° in order to avoid the thickest parts of the atmosphere.
A quick estimate of how long a particular source will be observable on a single day with the ATCA can be made using Table 2.1. It shows, for a source at a particular declination, the LST range before/past transit it is above elevations of 12° and 30°.
Example: The source 0823-500 (coordinates RA=08:25:26.869, Dec=-50:10:38.4) will rise above the 12° elevation limit at an LST of 01:05 (take 07:20 from 08:25), and it will fall below 30° at an LST of 13:42 (add 05:17 to 08:25). It will be above the 12° elevation limit for approximately 14 h 40 m each day.
Table 2.1. For sources at particular declinations, this table displays the amount of time the source would take to get from rise to transit, or from transit to set at the ATCA. The LST ranges are shown for both the hard 12° and the recommended millimetre 30° elevation limits.
|-80°||always above horizon||06:09|
|+40°||02:31||never above horizon|
To calculate precisely how long your source is observable each day with the ATCA you can use the coord program, available from the Parkes website. When using coord from the website you must remember to change both the site to “Australia Telescope Compact Array”, and the elevation limit to the value you require.
CABB makes available two 2048 MHz wide bands, and the centre frequency of one IF must be within 6 GHz of the centre frequency of the other. Generally speaking however, the closer the two centre frequencies are to each other, the more sensitive the observations will be.
This is due to the design of the CABB system. All frequency bands observable by the ATCA are first mixed to fit into the primary CABB frequency window of 4 to 12 GHz (except for the 4cm band, which doesn't require any mixing). By default, closely-spaced IF pairs are mixed so that they lie at the top of this frequency window because the lower frequency end can suffer from poor image rejection. If two widely-spaced IFs are selected however, the observational sensitivity for one of the IFs may suffer because of this.
It is important to note that the ATCA sensitivity calculator does not account for this effect.
To counter this problem, you may wish instead to use frequency switching techniques; you can read about how to do this in Section 2.3.6.
For normal continuum observations we recommend that you use our recommended frequencies, shown in Table 2.2.
Table 2.2. Recommended CABB continuum centre frequencies.
|Band||IF1 (MHz)||IF2 (MHz)|
If you are using the 3mm system, the choice of simultaneous frequencies can affect the observational sensitivity in another way, due to restrictions on the LO frequencies:
If the highest central frequency is > 100.6 GHz and the lowest central frequency is > 97.8 GHz, then the band will be USB, and sensitivity will be optimised.
If the lowest central frequency is < 97.8 GHz and the highest central frequency is > 100.6 GHz, then the band will be LSB, but the LO will be driven past its rated limit, reducing the sensitivity.
If the lowest central frequency is < 97.8 GHz, and the highest central frequency is < 100.6 GHz, then the band will be LSB, and sensitivity will be optimised.
If the highest central frequency is < 100.6 GHz, and the lowest central frequency is > 97.8 GHz, then the band may either be LSB or USB, and will be automatically configured by caobs to optimise sideband rejection (but this may still result in less than optimal sensitivity).
RFI at the ATCA falls into two groups: external, and self-generated. External RFI is worst at lower frequencies, and can significantly reduce the bandwith usable in the centimetre bands. Self-generated RFI is less destructive, but also mostly unavoidable, and can be present regardless of which band you're observing in.
The observatory keeps track (as much as possible) of the licensed transmitters in the surrounding area. A searchable database of these transmitters is made available for users of the ATCA (who already have an ATNF UNIX account). You can search for transmitters based on their frequencies and distance from their observatory, and learn about where they are, their transmission power, and pointing direction.
The ATCA sensitivity calculator also has information about frequency ranges that are commonly flagged due to RFI, and so can take into account the bandwidth losses when doing its calculations. This functionality is recommended for all continuum observers.
Continuum observers should expect to lose approximately 270 MHz of bandwidth in the 16cm band (13% of the 2048 MHz total bandwidth) and approximately 200 MHz of bandwidth in the 4cm band (5% of the 4096 MHz total bandwidth) due to RFI that is almost always present. There are other sources of RFI that will be present for only some of the time that will cost even more bandwidth, but most of these have fairly short durations and will not generally significantly affect the continuum sensitivity.
In the 16cm band there is a transmitter that produces what is commonly referred to as “mid-week” RFI, since it usually appears on weekdays during business hours (it can however appear at other times). This transmitter is so powerful that it can send the 16cm receivers into saturation, or at the very least cause intermodulation products to contaminate the entire 2 GHz band; during either of those scenarios, no sensible astronomical data is produced by the system. The observatory will sometimes receive advanced warning about this RFI, and observers will be notified before they start in that case, but this is not guaranteed to happen. If warning is given sufficiently in advance, the operations staff may be able to assist you in organising a swap with a project that can observe in a band other than 16cm; again, no guarantee is made that this will occur.