ON APPLICATION OF MULTI-FREQUENCY SYNTHESIS FOR RADIOASTRON MISSION.
Astro Space Center of P.N. Lebedev Physical Institute, Moscow, Russia
Space radio interferometers as expected permits a dramatically increasing of angular resolution.
The main problems for it related to restrictions by sensitivity and poor coverage of UV-plane. The first problem for RadioAstron mission for K-band will improved with using a new LNA / probably identical to
NRAO K-band amplifier designed and build for the MAP satellite and will used for VLA /. The second problem resolved partly by operating on the ground VLBI network with single space radio telescope on the orbits with small apogee /less 30,000 Km / or if to use the Moon perturbed orbit with regular rotation of the orbital plane in the time of mapping of 3-6 months . The construction of multi-element array in space is extremely expansive and relate probably to the very distant future. The Multi-Frequency Synthesis /MFS/ permit to realize of imaging for the continuum spectrum radio sources in short time [2-5]. Switching of N values of frequency are equivalent at some assumptions to the switching of the space frequencies and therefore equivalent to the operating simultaneously of N space radio telescopes distributed along the same radius Earth - RadioAstron. If two-element interferometer operate simultaneously at two frequencies, it is possible to measure the amplitudes and difference phases of two space harmonics.
For RadioAstron mission it is proposed to operate one K-band circular polarized channel at fixed frequency 22.232 GHz. The second simultaneously operating channel with opposite circular polarization can switched in the band from 18.392 GHz to 25.112 GHz. Fmax/Fmin=1.37. It permit to construct one-dimensional image in time determined the integration time for each channel multiplied on the number switching frequencies N=8. Two-dimensional imaging is possible twice per orbit with maximum angular resolution and 1-(Fmin/Fmax)^2=46% of elliptic area on the UV plane. It is significant that this values are independent from the size and others parameters of the orbit.
Tunable channel permit also to observe the extragalactic water masers by 4 values of central frequency from 22.136 GHz to 22.232 GHz.
In general one K-band RadioAstron channel with fixed frequency will be unchanging and compatible with all on the ground radio telescopes. The tunable K-band RadioAstron channel will be compatible with the upgrade VLA K-band.
Advantages and operational modes.
1. The one-dimensional imaging of the sources in time shorter than one hour with extremely high angular resolution and at anyone part of the orbit.
2. The two-dimensional imaging in time 3-5 days near the apogee for Moon perturbed orbit and in time of 1/2-1 day near perigee.
3. The obtaining of the spectrum of different elements of the image.
4. A determination of the angular size - frequency dependence connected with scattering, or absorption, or other physical processes in the envelope of the sources.
5. A construction of the map of linear polarization and map of the rotation measure or map of the circular polarization, and also determination a degree of polarization changing with the frequency.
6. A determination of the differential coordinates and the proper motions with extremely high accuracy.
7. An investigations of the own variability structure of the sources and/or their scintillation variability in interstellar or own envelope plasma as function of the frequency.
8. A measurement of the systematically changing of coordinates of the sources with the frequency or polarization which connected probably with refraction or others physical processes.
9. An investigations of water megamasers with the radial velocity up to 1,600 Km/s.
All this new possibility are very bright prospect for RadioAstron and other Space VLBI missions as well it is very useful to expand of two frequency simultaneously observations for on the ground VLBI observations.
A new /additionally to existing / observation modes can be selected for realization the mentioned above targets. The frequencies of tunable channel for MFS are 18.392 /F-4/, 19.352 /F-3/, 20.312 F-2/, 21.272 /F-1/, 22.232 /F0/, 23.192 /F1/, 24.152 /F2/ and 25.112 GHz /F3/.
A. Both circular polarization channels are operated at the same frequency 22.232 GHz /F0/.
B. One circular polarization channel operates at frequency F0 and the opposite polarization channel operates at minimal frequency F-4.
C. The same but tunable channel switch the frequency consequently from F-4 up to F3 with time interval of T /see Fig. 1/ each of 8 values.
D. The same as mode B but switching of circular polarization between channels with time interval T.
E. The same as mode C but switching of circular polarization's for each frequency of the tunable channel / 16 measurements /.
F. For water masers one circular polarized channel operates at frequency F0 and opposite circular polarized channel operates at one from 4 values: 22.136, 22.168, 22.200 and 22.232 GHz.
G. For water maser polarization observation operate at switching of polarization with time interval of T.
All signal and LO frequencies and diagram of new K-band on-board RadioAstron receiver are shown on the Fig.1 and Fig.2.
All new blocks detach by bold lines. It contains a new wide-band horn feed /AF-1.35/ for both circular polarization's, new two-channel wide-band LNAs, new designed box MFS [ unit for frequency conversions, frequency switching and "fine" frequency switching /FCS/, a wide-band commutator /WBC/, narrow-band commutator /NBC/] and existing Receiver-1.35, feed AF-6, LNA-6, and Receiver-6.
It is used here a conversion down of K-band to fixed input frequency of L-band receiver /4832+-16 MHz/. This commutation can be used for each of two polarization's of K-band and LNA-6 can be switch off at this time.
LO frequencies can be formatted by the special synthesizer / Fig.3/, input reference frequencies of which are 8 and 160 MHz can be used from existing synthesizer.
LO frequencies for water masers observations are 17.304, 17.336, 17.368 and 17.400 GHz and its forming also demonstrated on Fig.3.
LO frequencies are installed by selection of M = 14, 15 16, 17, 19, 20 or 21 and N = 15 for MFS observations. LO frequencies for the masers observations are installed by selection of M = 18 and N = 15, 11, 7 or 3.
The requirements to power supply are 27 V and < 15 W.
Mass and dimension of additional box are < 3 Kg and 250x150x100 mm^3.
Modeling of MFS imaging.
Model of the source is shown on the Fig.4a and Fig.5a. It is three point details with fluxes are Fa = 0.5 Jy , Fb = 1 Jy, Fc = 0.5 Jy, and angular sizes are AB =12, AC =15 and BC = 9 mcas. Normal to the orbit is coincide with direction on the source, AC is parallel to big ax of the orbit and base-line projection equal to center of Earth - RadioAstron distance.
Parameters of the orbit are excentricitet 0.85, perigee radius 28,800 Km, apogee radius 355,191 Km, period 9.5 days.
Fig.4b demonstrate UV-coverage for short time observation near apogee with MFS switching.
On the Fig.4c is shown the one-dimensional "clean" map of such observations, constructed by using an algorithm "CLEAN". It is very good reflection of scanning by narrow "knife-like" diagram.
On the Fig.5b are shown UV-coverage by 6 similar short time observations near apogee. Time interval between neighboring observations is 22.9 hours and interval between the first and last observation about 5 days. On the Fig.5c is shown the two-dimensional "clean" " image of the source constructed by algorithm "CLEAN" .
1. Memorandum " A new science strategy of RadioAstron mission with a high apogee orbit ", ASC of P.N.Lebedev Phys. Inst., Moscow, February 1998.
2. Andreyanov V.V., Gurvits L.I., Kardashev N.S. et al., 1984, "Wide range space radio interferometer", in Proc. of workshop "Quasat a VLBI observatory in space" at Gross Encendorf, Austria,ESA SP-213, p.161.
3. Conway J.E., Cornwell T.J., Wilkinson P.N., 1990, MNRAS, v.246, p.490.
4. Sault R.J., Wieringa M.N., 1994, A&AS, v.108, p.585.
5. Sault R.J.,Conway J.E., 1998, in "Synthesis imaging in radio astronomy" / Taylor G.B., Carilli C.L., Perley R.A. eds./, II ASP Conf. Series, v.30, p.1.
Fig.1. Signal and LO frequencies.
Fig.2. Diagram for upgrade of on-board K-band channel. Bold lines mark a new or replaced equipment.
Fig.3. Simplified scheme of LO forming. VCO - voltage control oscillator, PD - phase detector, N - frequency divider, M - frequency multiplier, -- - mixer.
Fig.4: a - model of the source; b - UV-coverage for short time one-dimensional imaging near apogee;
c - one-dimensional "clean" map.
Fig.5: a - model of the source; b - UV-coverage for 6 short time observations in 5 days; c - two-dimensional "clean" map.