# Chapter 2. Preparing for Observations

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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.

Broadly speaking you will need to address at least the following considerations when planning an observation with the ATCA:

• 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.

## 2.1. Observational Sensitivity

### 2.1.1. How long is my source up per day?

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.

DeclinationLST range
12°30°
-80°always above horizon06:09
-70°10:3505:47
-60°08:1405:32
-50°07:2005:17
-40°06:4405:02
-30°06:1704:46
-20°05:5304:28
-10°05:3104:07
+0°05:0703:42
+10°04:4203:09
+20°04:1202:20
+30°03:3200:21
+40°02:31never 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.

### 2.1.2. Is it possible to tune my observing frequencies to obtain optimal sensitivity?

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.

BandIF1 (MHz)IF2 (MHz)
16cm21002100
4cm55009000
15mm1700019000
7mm (LSB)3300035000
7mm (USB)4300045000
3mm9300095000

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).

### 2.1.3. How can I avoid RFI?

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.

#### 2.1.3.1. What external transmitters are located near the observatory?

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.

#### 2.1.3.2. How much continuum sensitivity will I lose due to RFI?

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.

#### 2.1.3.3. What is “mid-week” RFI?

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.