Status Report for Common User Observations, December 2011- May 2012

*LastUpdate: 2012-02-09

H28/32 is not available due to a problem on the mixer.
New spectrometer SAM45 is available (shared risk release). Please check here for details.
Integration time calculator for position switch

ANTENNA

Beam Size and Efficiency



	Latest measurements : [Antenna Efficiency (2011-2012)]
	Older  measurements : [Antenna Efficiency (2010-2011)]
                      [Antenna Efficiency (2009-2010)]
                      [Antenna Efficiency (2008-2009)]
                      [Antenna Efficiency (2007-2008)]
                      [Antenna Efficiency (2006-2007)]
                      [Antenna Efficiency (2004-2005)]
                      [Antenna Efficiency (2002-2004)]
                      [Antenna Efficiency (2001-2002)]
                      [Antenna Efficiency (before June 2001)]

Measurement of eta_fss of the 45 m radio telescope with the Moon: [

_fss]

Surface Adjustment

In the autumn of 1998, we did holographic measurements to improve the surface accuracy of the telescope.

Pointing Observations

The tracking precision of the telescope is typically 3" (rms.). We measured the pointing accuracy of the telescope during a night with no wind by observing strong SiO maser (43 GHz) sources. The derived rms. residuals from the pointing model were 2" both in azimuth and in elevation. This level of pointing accuracy can be achieved only under good conditions (night time, clear and no wind).

Pointing offsets are variable from day to day, possibly due to thermal effects on the structure and the master collimator. In practice, measurements for pointing corrections on a nearby source are needed every 1-1.5 hour. The major limitation in the pointing of the telescope is now tracking errors due to the wind loading effects on the main reflector structure. Below a wind speed of 4 m s^-1, the pointing accuracy seems to be satisfactory, i.e., less than 3 arcseconds rms.

The pointing observations are usually done by observing SiO maser lines from nearby evolved stars with either the H40 receiver or the S40 receiver. The list of evolved stars with the SiO maser lines, its positions, and its intensity are easily obtained from the program making the observation table. Continuum sources also can be used for the pointing observations.

It has been noticed that for observations of objects within 35 deg. from the Sun, the pointing can change very much. Please, take this into account when applying for observing time or when planning your schedule of observations.

RECEIVERS

There are three receiver groups: old receiver port (a low frequency group comprising H22, H28/32 and H40), new receiver port (a high frequency group consisting of S40, S80/S100 and T100V/H), and multi-beam port (BEARS). Mirrors and polarization splitters are used for receiver selection. It takes about one minute to change receivers by changing the mirrors and polarization splitters. Receiver retuning takes 20 - 40 minutes per receiver.

The combinations of S80/S100, S40/S100, S100/T100H, S40/T100V and T100V/T100H are available for simultaneous observations.
The available LO frequency ranges for the receivers are listed on the Table 1. Please confirm whether your LO frequency is in the range when you make your observing tables.

Table 1
Receiver	LO frequency range (GHz)
H20	12.75-13.25, 14.75-15.25, 16.75-17.25, 18.8-19.3
H40	36-38.5
S40	35-48.5(*)
S80	71-77, 79-115(**)
S100	75-115(***)
T100H/V	86-112
BEARS	85-115

(*) Please don't use 38.0GHz and 47.5GHz.
(**) For the LO range of 112-114.5GHz, there are some points where the local oscillator doesn’t work well. Especially, please don’t use 113.7GHz.
(***) For the LO range of 112-114.5GHz, there are some points where the local oscillator doesn’t work well. Especially, please don’t use 113.5-113.6GHz.

Table 2 Receivers Available
Receiver	Freq. Range (GHz)	IF Range (GHz)	IF Range (GHz) for SAM45	Tsys^(*1) (K)	Remarks
H22	20.0 - 25.0	5-7	5-7	100	(*3)
H40	42 - 44	5-7	5-7	250	(*5)
S40	35 - 50	1.075-1.675	not available	150 - 300	(*5), Fig.1
S80	72 - 116	1.075-1.675	not available	250 - 900	(*6), Fig.2
S100	77 - 116	1.075-1.675	not available	250 - 500	(*6), Fig.3
T100V	78-120	5-7	4-8(2SB)	115 - 290	(*7), Fig.5
T100H	78-120	5-7	4-8(2SB)	140 - 370	(*7), Fig.5
BEARS	82 - 116	2.0-2.6	not available	400 - 800^(*2)	(*8), Fig.4

H: HEMT; S: SIS Mixer; BEARS = SIS 25-BEam Array Receiver System.

*1) The system temepratures shown in the following graphs.
*2) Typical T_sys of BEARS is 200 - 400 K in DSB (double side band) antenna temperature. After applying the scaling factors in the reduction process, it corresponds to 400 - 800 K in SSB (single side band) antenna temperature. You have to use T_sys in SSB for estimation of the observing time.

*3) H22 has 2 channels (ch1 and ch2: circular polarization). These two channels can be used simultaneously.
*5) H40 is usually used for pointing calibration, by observing SiO maser sources.
*6) S80 and S100 share a common dewar. A polarization splitter in the feed enables them to be used simultaneously. The two beams are aligned to 2". The mm receivers (i.e., higher than 40 GHz) are kept as closely aligned as possible, to about 1".
*7) The IF center frequency is 6 GHz. USB and LSB can be used simultaneously.
*8) In order to plan your observation, please read "detail information" carefully.

[Fig. System temperature of S40]
Fig. 1: System temperature of S40.
Note that these Tsys were measured at EL = 70 deg under good condition in winter.

S80
Fig. 2: System temperature of S80.

[Fig. System temperature of S100]
Fig. 3: System temperature of S100.
Note that these Tsys were measured at EL = 70 deg under good condition in winter.

Fig. 4: Tsys(BEARS) vs. Elevation angle.
Note that the shown values are in DSB: scaling factors (typically 2) should be multiplied to be converted into SSB scale.

T100

Fig. 5: System temperature of T100 V/H.

SPECTRAL LINE OBSERVATIONS

Spectral line observations can be made in a position switching mode, frequency switching mode or OTF(On-The-Fly) mode. A real time monitoring system ("Quick Look") for spectral data is available. Quick Look can also display the integrated spectra.

[Note for OTF observation]

        Read OTF manual carefully!

AOS can NOT be used for OTF mode.
Only T100 can be used for OTF mode with SAM45 (other receivers cannot be used).
Please bring your own HDD to save your data. The capacity of our disk array is very limited. If you use the OTF mode by BEARS, the disk space needs 1.1GB/hr x (observing time [hr]) (maximum). In order to secure the disk space, all your data except for raw data need to be deleted after your observation finishes. Transferring the data files back to one's home institution by ftp or scp is strictly prohibited, because this will severely affect our network system. Thus, the observers should bring their own HDD.

Table 3. Back-End
Type	No. of array	No. of channels in an array	Freq. coverage fo an array	Frequency resolution	Noise bandwidth
AOS-W (Wide band)^(*0)	8	2048	250 MHz	250 kHz^(*1)	494 kHz^(*2)
AOS-H (High resolution)^(*0)	8	2048	40 MHz	37 kHz^(*1)	103 kHz^(*2)
AC^(*3)	25	1024	Wide mode 512 MHz High mode 32 MHz 16 MHz 8 MHz 4 MHz	see below^(*4)
SAM45^(*5)	16^(*6)	4096	16MHz 31MHz 63MHz 125MHz 250MHz 500MHz 1000MHz 2000MHz	(^*5)

*0) Please check here for AOS status (2011-03 --)
*1) Frequency resolutions are defined from the FWHM of the standard signal inputted to AOS. These resolutions correspond to about 2 channels for AOS-W and AOS-H.
*2) The noise bandwidth is a measure of noise characteristics of AOS. When estimating rms noise level obtained after integration, it is appropriate to use this noise bandwidth instead of frequency resolution (The noise bandwidth is the same as "fluctuation bandwidth" or "reception bandwidth" in literature, for example, Klumb et al. Proceedings of SPIE Vol. 2268, 305 (1994).).
*3) All the receivers can be used with AC. However, for the wide band observations (512 MHz) with the H22 and H28/30, noise level may not be uniform within the 512 MHz bandwidth due to the band characteristic of these receivers. The ratio between max and min of noise within the band is typically less than 1.5. Only AC can be used for BEARS.
*4) Frequency resolution of AC depends on both the band width and the window function you selected. You can select from three window functions- hanning(x2.00), hamming(x1.82), and brackman(x2.30). Frequency resolution is calculated by the next equation;

Frequency resolution = Bandwidth / 1024 x factor(window function)

For example, in the case of the 32 MHz bandwidth and the window function “none”, resolution is 37.8 kHz

*5) Please check here for details.

DATA REDUCTION

Facilities for data analysis are available at the observatory.

[Position switching mode]: Data reduction is performed using NewStar on Linux PCs. We provide the export version of NewStar for convenience. Please see the manual page in detail.

[OTF mode]: By using the reduction software for OTF observations, you can make FITS file of the cube data. Please see the manual page for details.

OBSERVING PLAN

In making your observing plan, please note that:

1. Elevation: The observing elevation range of the telescope is 12-80 degrees. Since the telescope is located at 138° 28' 21.2" (E), 35° 56' 40.9" (N), objects (ON or OFF positions) with dec. 26 deg. < dec. < 46 deg. will reach elevations higher than 80 degrees (upper limit). (SeeFig. 3)
2. Avoid the sun: In clear days, when observing at less than 35 degrees from the Sun, the pointing of the telescope worsens due to the heating of the antenna. Please take that into account when writing your proposal and during your observations. Table of Sun Position
3. Receiver tuning: Depending on the setup used by the previous observer and the status of the receivers, the time needed for receiver setting varies greatly. However, from our experience, an average of 20 min per receiver is required. In the case of BEARS, it takes about 30 min for the setting of 25 beams. Note, however, that if you use the H40 receiver for pointing, almost no time is required for its setting since it is usually kept tuned at 43 GHz.
4. Expected noise level: The expected one sigma noise level dT(K) for the position switching observations is
dT = sqrt(2) * T_sys / sqrt(df * t)
where T_sys is the system temperature (including atmosphere, see table 1), df (Hz) is the frequency resolution and t (sec) is the integration time ON source. In the case of AC, the factor (1/0.87) is nessesary to correct for the quantization noise. How to estimate the expected noise level for OTF mode is here.
5. Total observing time: Apart from the ON source observations, the total observing time must includes the OFF source observations and the movement of the antenna between the ON and OFF positions. For position switching mode, the integration time of OFF position has been set to the same as the ON points. The fraction of time spent for telescope movement and software overheads is roughly 30%. How to estimate the total observing time for OTF mode is here. Integration time calculator for position switch is here.
6. Telescope movement:: The quickest speed at which the telescope can move is 18 degree/min, thus taking 10 min to rotate 180 degrees in azimuth.
7. Pointing: It is recommended to do pointing observations every 1-1.5 hours. The time required for the pointing check will, of course, vary depending on the brightness of the SiO maser source used and the weather conditions, but is usually of the order of 15-20 min. If an SiO maser source with a line strength of 5 K in antenna temperatrue and a velocity width of 3 km/s is observed as a pointing calibrator with our standard pointing receiver H40 (T_sys = 250 K), one can get an S/N ratio of 20 by spending a ON source time of 10 seconds, which should result in a measurement accuracy of 1 arcsecond rms (see a plot of Pointing Error vs. S/N ratio).
8. Standard sources [List]: The intensity calibration of the spectra is done by the chopper wheel method. It is recommended to observe a standard source at least once every day to establish the absolute line intensity scale and also to check the observing frequencies. When the standard source is quite separated from the objects being observed, a pointing check will be required previous to its observation.
9. Total observing time and LST limits: In calculating the total time needed in the observations, please take also into account the receiver tuning, and the pointing and standard source observations. The LST range within which a certain object can be observed is easily deduced from Fig.6. For a source at a certain declination, the LST range at which it can be observed above a certain elevation is the R.A. of the object +- the hour where the curve for its declination that elevation in Fig. 6. For example, an object of (R.A.,Dec.) = (12h,10d) can be observed above an elevation of 30 degrees for LST = 8h-16h.

Fig. 6: Elevation Angle vs Hour Angle (click to enlarge)

CONTINUUM OBSERVATIONS

Front End

Performance of receivers for continuum observations.
RX	Frequency range (GHz)	Band width (GHz)	No. of beam	Beam separation (arcsec)	Sensitivity^* (mJy)
H22	20-25	2	2 (ch)	---	40 / sqrt(sec)
S40	35-50	0.6	1	---	90 / sqrt(sec)
S80	72-115	0.6	1	---	160 / sqrt(sec)
S100	77-115	0.6	1	---	130 / sqrt(sec)
BEARS	82-112	1	25	41.1	300+-100 / sqrt(sec)

* The sensitivities were measured in a clear night in winter with wind speed less than 3m/s.

Back End

Output from the receivers is filtered and A-to-D converted to 16 bit data. Up to 48 channels can be sampled simultaneously, which enable the use of BEARS. Demodulation of the switched signal is made by a workstation.

Observing mode

On-The-Fly (OTF) mapping mode is used for mapping observations. Cross scan mode is used for pointing and observations of a point source. On-Off scan mode also can be used for a point source. For On-Off scan mode, the total observing time, including off-source time and antenna movement, is about 2.6 times longer than the on-source time. For OTF mode, a grid spacing of the final map is the same as the spacing of scan paths. The total observing time t_obs to get a map with Nyquist sampling, including the dead time for acceleration and return of the antenna, is roughly calculated by next equation,
t_obs (sec) = 1.3 * (S/F_rms)² * N,
where S is the sensitivity listed above, F_rms rms noise level reqired, N grid number in the map.

Switching

To remove atmospheric offsets, beam switching is used for the heterodyn receivers. The switching frequency is 3-20 Hz with beam separation of either 4-6' in azimuth direction or 8 deg in the same direction with 5 deg beam.

Quick Look and Data Reduction

Quick look can be used to check the data during observations. The noise level of the integrated data can be checked also. IDL-based reduction software is prepared to visualize the data. Mapping data can be converted to FITS format.

2012-02-24 nro45mrt @ NRO