Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.naic.edu/alfa/memos/ealfa/memo041206.pdf Дата изменения: Mon Dec 6 22:32:57 2004 Дата индексирования: Sat Dec 22 05:20:58 2007 Кодировка: Поисковые слова: moon

The ALFA Fixed Azimuth Drift Observing Mode
M. Haynes, R. Giovanelli and the memb ers of the A1946 and A1963 observing teams 06 Decemb er 2004 Several E-ALFA observers have b een confused ab out the function of the Fixed Azimuth Drift Map mo de and asso ciated CIMA widgets, designed originally by Jeff Hagen in collab oration with the A1946 team, later adapted for A1963 purp oses by Lyle Hoffman and further refined by Mikael Lerner for the A1961 and A1963 teams. Here we attempt to explain several p oints ab out the mo de which may help to reduce future confusion. The basic idea of Fixed Azimuth Drift Mapping is to p osition the telescop e at a particular azimuth and zenith angle, with ALFA rotated to a particular angle, so that a source of given RA and Decl will transit exactly through Beam 0 at a predetermined time, yielding an ALFA b eam configuration giving b eams equally spaced in declination. Because of ALFA symmetry, there are several such configurations. All observations conducted by the A1946 and A1963 E-ALFA teams adopt a single configuration placing Beam 4 at the highest zenith angle, given by a rotation angle of +19 for a source on the meridian (Az=0 or 180 ). The use of the term "Right Ascension" as an entry in the "Fixed Azimuth Drift" widget is apparently confusing to those not familiar with the strategy, since it implies that the telescop e should p oint towards that p osition on the sky. Actually, the telescop e needs to p oint at the appropriate azimuth and zenith angle so that a source of RA,Dec in Beam 0 will pass exactly though that Az, ZA at a designated time in the future. The data-taking b egins at the instant that such source transits Beam 0. So when the telescop e slews to p oint, it do es **not** p oint at the entered RA, but rather a bit to the west, as given by the commanded azimuth, and then waits for the targetted RA to drift by. Observers need to understand this function in order to make sense of the sequence of events, p ositions and times, imp ortant b oth in the practical exp erience of observing and in the design of most efficient and effective observing schedules (such as those demanded by high precision programs like A1963; for practical examples, see http://www.naic.edu/ngc2903). Once the sp ecial widgets, IF/LO Control and Backend Control parameters are set in CIMA, and the "Fixed Azimuth Drift Map" is selected in the "Sp ectral Line Observing" widget, it is this latter one which is relevant to the Fixed Azimuth Drift Map observing strategy. In particular, after the b eginning of a night's run (when you may wish to start the telescop e moving if a large slew is needed from where the telescop e was last p ointed by the previous observer), the "Pointing Control" widget should **NOT** b e used to p oint the telescop e or to rotate ALFA. Those are incorp orated into the "Sp ectral Line Observing" widget when "Fixed Azimuth Drift Map" is selected, and the apparently similar commands in the "Pointing Control" window do not function identically. Once you start the telescop e moving towards the first source, use the "Pointing Control" window **ONLY** to read p ositions from a source catalog ­ or, b etter yet, not at all (to avoid confusion). Simply, for a given RA,Decl., enter the desired azimuth and ALFA rotation angle in the "Sp ectral Line Observing Control" widget and hit "Observe" to start the telescop e in motion. It will move to p osition and then wait for the prop er time to b egin integrating. For maximum efficiency, typ e in the new parameters b efore the end of the previous scan and hit "Observe" as so on as the trump et flourish sounds the end of the previous observation. Keep in mind that the azimuth you enter must allow adequate time for the telescop e to get to p osition and for ALFA to rotate, along with their asso ciated not-insignificant overheads. It should also b e noted that most observers' calculations are p erformed in on-sky ep o ch J2000, but most of the telescop e parameters that app ear on the control monitors and practical timing considerations are apparent. Pointing mo del and even precession corrections from apparent to J2000 are considerable. Likewise, slew and ALFA angle rotation times are large and dep end on Az, ZA and Angle, and overheads allowing for communication, acceleration, deceleration and settle times add up. Furthermore, while scramnet communications regarding p ositions o ccur every second, the rotation angle is sensed only every 10 seconds, so the dead time b etween command and action can vary by as much (dep ending on your luck). It is very imp ortant that observers understand the intention and function of the fixed azimuth drift mo de and its asso ciated widget. Since CIMA control of ALFA is still evolving, care should b e taken to make sure that the mo de functions in the same way that it do es at the time of this writing (04Dec06).

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