Introduction to Phase Blocking (equivalently Phase Filling)

Phase blocking uses a "phaseblock file" named for the objectname as given exactly in the Phase II for your program. For example "TauBoo1" would have a phase block file called "TauBoo1.block". When a target is considered for observation, or actually observed, the correspondingly named file is searched for. If found, the phase at the predicted current observation mid-time is compared with the constraints implied by the objectname.block file. Several Phase II entries can use the same phase block file by simply using the same objectname.

There are three currently available methods (and two others could be provided) to block and unblock targets according to a phase variable:

To remove the annoyance of having to become an expert in all manner of phaseblocking notation, before being able to select the method most applicable to your target control, the remainder of this section attempts a rough sketch of the choices and conventions. For the full descriptions please go on to the later sections.

The phaseblock file kept on disk at the HET controls the phase-dependent temporary blocking or unblocking of visits to any target for which a phaseblock file exists. The need for different "styles" of phaseblocking arises from distinct science requirements for the control of visit phasing, between different celestial object classes.

The idea of "pulsator style" phase-blocking involves two concepts. The first is a "phase-filling-in" capability whereby a chosen phase range, or the complete range, is to be filled in to a desired phase filling density. The visit dates and phases already accumulated are inserted into the phaseblock file, and these dynamically determine the remaining "open phase window." There are no pre-determined phase bins or spacing buffers. The second concept concerns a pure and immutable phase block (or blocks), not dependent at all upon the happenstance of visits already obtained. This variation is realized simply by a convention of setting the period parameter NEGATIVE, and starting off the phaseblock file with one or more FICTITIOUS phases looking like phase points already obtained. In "pulsator style" format, the negative period prohibits the date and phase information from the actual visits from being added to the phaseblock file. That means that the phaseblocking effect is held precisely fixed, and while no visit will penetrate the blocked phase ranges, yet multiple visits are free to be arbitrarily distributed within the unblocked phase ranges. Lastly, if the goal is no visits in certain ranges, and more like quasi-evenly space visits in other ranges, that combination can be effected with a bit of manipulation involving both one or more fictitious suitably phased pseudo-points, in combination with the "phase-filling-in" capability described above (period parameter being POSITIVE).

The idea of "binary style" phase-blocking involves two concepts. The first is the added ability to enforce precisely specified phase bins that are intended to receive 0, 1, 2 or any positive integer number of visits. This more precise ability to control vis a vis pulsator style is paid for by having to accept phase near-coincidences and optionally "dead" phase buffering bins that sacrifice observing window. The second concept is the added ability to specify correlation between phase ranges in the sense that an observation within range A can, with provisos, cancel the need for an observation within range B (generally because the science can be provided by the one OR the other). Notice that the period parameter remains positive and the phaseblock file undergoes modification with EVERY successful visit.


Phase Filling "Pulsator Style"

Suppose you want your allocated observations to fill in a phase curve "smoothly with about N phase points." No two points should be too close together in phase. Blocking should be dynamic and depend on the phases already obtained, not on pre-set "dead phase" buffers to keep phase points separated. As the curve fills in and the trimester runs out, blocking should progressively retract to allow enough fractional open phase window to make productive use of the remaining hits.

The following illustrates a "pulsator style" phaseblock file. At the start of observing it looked like:

target01.block:
---------------------------------------------------------------

 8.637930
12
.064
         0.0000

period
npts
current block
current zero
phases and xjds
---------------------------------------------------------------

and some weeks into observing it looked like:

target01.block:
---------------------------------------------------------------

 8.637930
12
.064
         0.0000
.129   882.1871
.287   918.1028
.474   859.2478
.559   808.1573
.935   863.2348

period
npts
current block
current zero
phases and xjds
---------------------------------------------------------------

The first line is the period[d].

The second line is the hypothetical number of phase points aspired to (kind of the "independent variable").

The third line is the current phase-block radius (kind of the "dependent-variable") in terms of which an existing phase point blocks any future phase point within +- of that phase radius. The value can decrease automatically as many phase points are obtained, to permit sufficient remaining gaps for the remaining to-be-obtained visits.

The fourth line is an xjd (see notes below for the xjd, a modified-modified julian date) of any special "zero phase" relative to which all the collected phases should be expressed. Since the "phase zero xjd" is here set to zero, it means that the curve filling phase is not meant to be relative to maximum light or any other special phase, but is left arbitrary (relative to whatever phase happened to be at xjd=0).

The data lines added after that, one per accepted observation, record the phase and xjd at which the observation was obtained.

The last 6 lines are for observer convenience as a reminder of what the preceding lines contain. They are optional.


Phase Blocking "Pulsator Style"

Suppose you want high SNR data on a star and you don't care about phase-dependence in detail, but just want some handle for improving your SNR outcome. An example would be to get a few phase points "but avoid minimum light" or "but only near maximum light." In addition you don't care how close in phase are the phase-points which you DO obtain; hence subsequent phase-points must NOT be phase blocked by previously obtained ones.

The following file illustrates the Phase Blocking Pulsator Style:

target02.block:
---------------------------------------------------------------

 -7.507945
 2
.190
         0.8118
.650     5.6920

period
npts
current block
current zero
phases and xjds
---------------------------------------------------------------

The first line is the period[d] times -1. Note that here the period is made negative, which flags that phase points obtained will NOT be inserted into the phaseblock file (hence it stands unchanged the entire trimester).

The second line is again a number of points, but due to the negative period, and the non-addition of further blocking points, it means something slightly different. The npts here should equal the number of phase ranges to be blocked, plus one. In the current example you are blocking phase range within a phase radius of 0.19 either side of the phase 0.65. Hence you already "have" a fictitious point (at 0.65), and npts=2 signifies that you intend to obtain a phase point additional to that.

The third line is the phase-block radius in terms of which an existing phase point blocks any future phase point acquisition within +- that phase radius. You can supply more blocking phases than one, but only a single value for the phase-block radius can be used per file.

The fourth line is an xjd (see notes below for the xjd, a modified-modified julian date) of any special "zero phase" in relative to which all the collected phases should be expressed. The phase zero IS however essential now, and in the above case is the xjd of a maximum light phase.

This phase block file allows an htopx target to collect arbitrarily many new phase points (limited only by the htopx visit parameter, never by the phaseblock npts parameter), such as are NOT blocked by the chosen fictitious phase points and radius. The newly observed phases are NOT added to the blocking effect since they are NOT added to the phaseblock file.

The last 6 lines are again for observer convenience as a reminder of what the preceding lines contain.

SPECIAL NOTE. Combination of phase blocking and phase filling. In the event that phase filling in some range and phase blocking in another is wanted, you can try the phase filling approach above, but first adding some (=nblock) initial fictitious (hence blocking) phases. If you want filling of the unblocked phase range by nfill real visits, increase the npts parameter to nfill+nblock. Since the blocking radius is dynamically recalculated and can slightly decrease as the phase window closes, you might consider some advantages of the following, alternative, phaseblocking method called "Binary Style."


Phase Filling "Binary Style"

Binary style phaseblocking allows choosing observation phases in a way which is more suitable for science from orbital radial velocity curves, than the pulsator style phaseblocking software is able to facilitate. The three main new features are enabling 1) observing a pre-set number of points within pre-set phase ranges, 2) observing an either-or choice of two correlated phase ranges, meaning an observation in one range removes the need for a further observation in that range AND its correlated range, and finally 3) requiring a minimum number of observations to be obtained in a phase range, that observations in a correlated phase range cannot stand in for.

The following illustrates a "binary style" phaseblock file. At the start of observing it looked like:

target03.block:
---------------------------------------------------------------

 14.361036
 0
0.000
       1172.397
  120  180  320  380
  620  680  820  880
 -999    0    0    0
  320  380  120  180
  820  880  620  680
 -999    0    0    0
  200  240
  250  300
  700  750
  760  800
  760  800
---------------------------------------------------------------

The first line is the period[d].

The second and third lines are just dummy stand-ins for the number of points and the phase blocking radius, as these are not used except as a flag to tell the software to treat the phaseblock file as inputting binary (not pulsator) phaseblocking information.

The fourth line is an xjd of a significant phase of the orbital binary curve, probably the zero-crossing.

The remaining lines have a special "binary" not "pulsator" format. The first three subsequent lines indicate that a point in the phase range (.120,.180) is desired, but if obtained then the point requested in the phase range (.320,.380) becomes unrequested. Also a point in the phase range (.620,.680) is requested, but if obtained then the point requested in the phase range (.820,.880) becomes unrequested. The line beginning -999 (or other negative integer -- see below) puts an end to the first bunch of requested lines (dubbed "the Bunch"). The Bunch list can have any number of phase ranges (items 1 and 2) , with their correlated phase ranges (items 3 and 4 on the same line). Multiple observations requested for the same range information are effected by simply duplicating the same line.

The correlated phase ranges appear in the second three subsequent lines. The first of those lines indicates that a point in the phase range (.320,380) is desired , but if obtained then the point requested in the phase range (.120,.180) becomes unrequested. Also a point in the phase range (.820,.880) is requested, but if obtained then the point requested in the phase range (.620,.680) becomes unrequested. The line beginning -999 (or other negative integer -- see below) puts an end to the second bunch of phase ranges (dubbed "the Antibunch" since their execution is kind of anticorrelated with the execution of the phase ranges in the (first) Bunch of phase ranges.

After the second flagging line (starting with a negative integer), the rest of the lines are the "Loose Phases" that are requested but have no correlation relationship to any other phase ranges. In this case one phase point is requested in each of the phase ranges (.200,.240), (.250,.300), (.700,.750), and two are requested in the phase range (.760,.800).

After the execution of a new phased visit, the phaseblock file is updated by deleting the requested lines that were satisfied. E.g. an observation at phase .150 would eliminate the first Bunch line and the first Antibunch line, an observation at phase .850 would eliminate the second Bunch and Antibunch line, and an observation at phase .780 would eliminate one of the last two lines.

The following illustrates an additional useful feature with a "binary style" phaseblock file. At the start of observing it looked like:

target04.block:
---------------------------------------------------------------

  4.617140
 0
.000
          424.5
  105  195  605  695
  205  295  705  795
  305  395  805  895
  405  495  905  995
  505  595  005  095
   -2    0    0    0
  605  695  105  195
  705  795  205  295
  805  895  305  395
  905  995  405  495
  005  095  505  595
   -2    0    0    0
---------------------------------------------------------------

A point is requested in five either-or phase ranges. If obtained in a phase-range in the Bunch, the corresponding Antibunch request is also removed. If obtained in a phase- range in the Antibunch, the corresponding Bunch phase range is removed. However it is also required to have at least 2 of the points in each of the Bunch choices (no more than 3 in the Antibunch choices), and correspondingly at least two in the Antibunch choices (no more than 3 in the Bunch choices). This last requirement is carried out by the fact that the (negative) flagging integer sets out the minimum number of points required to come from that bunch of phases. [as -0 does not function as a negative number, in the earlier examples which had NO pre-set required number, -999 is used to mean that there is no pre-set required number].

Therefore, here is the next phaseblock file after a successful observation at phase .150:

target04.block:
---------------------------------------------------------------

  4.617140
 0
.000
          424.5
  205  295  705  795
  305  395  805  895
  405  495  905  995
  505  595  005  095
   -1    0    0    0
  705  795  205  295
  805  895  305  395
  905  995  405  495
  005  095  505  595
   -2    0    0    0
---------------------------------------------------------------

and then a subsequent observation at phase .250:

target04.block:
---------------------------------------------------------------

  4.617140
 0
.000
          424.5
  305  395  805  895
  405  495  905  995
  505  595  005  095
 -999    0    0    0
  805  895  305  395
  905  995  405  495
  005  095  505  595
   -2    0    0    0
---------------------------------------------------------------

At this stage at least 2 of the upcoming accepted phases must come from the Antibunch. The logic allows more phases from the Bunch only as consistent with the requirements of the Antibunch.


Phase Filling "Period Finding Style" and "Short Period Variable Style"

HET spectroscopy of survey candidates of uncertain periodicity would benefit from a cadence suitable to best constrain the period. This cadence can be achieved by software requiring any new jd interval to be distinct (by parameter1, say factor 1.2) from the jd intervals between all so far successfully obtained visits. Parameter2 for visit censoring would be the minimum jd gap of any possible interest. By careful two-parameter selection, excellent periodicity estimation would be enabled. Consult with the RAs to inquire about the current availability of this particular automated track selection mode.

The phaseblocking methods described above are designed to work best, i.e. highly automatedly, with periods of 2d or more. For periods much less than that, each track corresponds to a very large range of phase, and an alternative method is thereby required. Consult with the RAs to inquire about the current availability of this (only semi-automated) short-period option.


Notes

1. There is transparency to phase < 0.000 and > 0.999 in the calculations. Pulsator style phases are expressed as .000-.999 and binary style phases are expressed as 000 - 999 notation.

2. There is no correction attempted to heliocentric, and thus phasing must be insensitive to errors of order 8/1440 = 0.006d

3. xjds are calculated from the modified julian date minus 54000. The modified julian date is the julian date minus 2400000.5 Since HET phaseblocking was initiated shortly after modified JD 54000, working with xjds reduces the required number of accurate digits. Therefore xjd = jd - 2454000.5

4. The logic of the pulsator phase filling is as follows:
a) Start the early filling with the requested phase block radius.
b) As filling proceeds, with each new phase point obtained, order the remaining phase gaps by size, and adjust the phase blocking radius such that the remaining desired hits will fit into JUST that number of the biggest remaining gaps
c) As phase filling becomes significant (hits looking for small remaining phase gaps with shrinking chances), further shrink the phaseblock radius such that a minimum net open phase window (currently about 20%) is preserved. In that way the program accepts a slightly more bunched phase outcome with good throughput, in preference to an extremely evenly phased outcome with bad chances of completion.

5. Choice of initial phaseblock radius. Use 0.000 if binary style. If pulsator style where you want to block n phases ranges without adding any newly obtained phases to the blocking effect, choose a phaseblock radius and blocked phases to produce the desired blocking zones effect (ALSO MAKE PERIOD ARTIFICIALLY NEGATIVE).

For simple phase filling, where the successful visits add their incremental phase blockage to the total blockage, then start the initial blocking radius at 1/npts/1.3 of the total available phase range. E.g. if you want complete curve coverage with 12 points, use a starting blocking radius of 0.064. If you are using fixed "fictitious" blockage of 0.2 phase at minimum light, and want the rest filled in with 8 phase points, use .077, etc.

6. Choice of initial phaseblock npts. Use 0 if binary style. Use n+1 if pulsator style where you merely want to block n phase ranges without adding any newly obtained phases to the blocking effect (ALSO MAKE PERIOD ARTIFICIALLY NEGATIVE). For simple phase filling, npts is the desired number of phases defining the ultimate curve.

If you employ the additional tactic of starting your phaseblock file with artificial phase-blocking pseudo points for the purpose of preventing real data being taken at certain phases, then use npts equal to the sum of your desired phase filling points AND pseudo blocking points. For example, if you want a 12-point phase curve that avoids two zero-crossings, create two fictitiously "already observed" phase points, one per zero crossing, and set npts = 14.

Note that the TSL keyword VISITS (the number of visits to the target, each with total exposure time of keyword EXP) and the npts parameter serve distinct roles and need not be identical.

7. For objects with both secular and periodic effects, the maximal control comes from combining all three means: the STATUS parameter (D for deferred, "" for ready), the synoptic date parameters, and a phaseblocking file. Batteries of targets on a single object can be kept on deferral until activated to ready. For activated targets, the software will night-by-night track-by-track re-parameterize the targets in the database so that they are blocked or unblocked according to phase. Optionally synoptic parameters can afford additional selection on top of that, but would be in most circumstances unnecessary.

8. The issue of very marginal blocking. The target listing database ("htopx2") checks on the phaseblocking of all current targets when making the observing selection and plans for the upcoming night. Currently if a target is not phaseblocked at at least one of the two times corresponding to 1/4 and 3/4 of its track, it will be listed as attemptable and will appear in the plan. However, for very short period targets, each track contains a nonnegligible range of phase, and so when the RA actually undertakes the visit, the estimated mid-exposure phase can very slightly miss the open phase window by a few 0.001 in phase. Currently, without other PI directives, the RA will proceed with such very marginally blocked visits. The binary-style phaseblock file will be manually modified as though the visit did satisfy the relevant phase range (pulsator-style is self-managing in this regard).



Last updated: Sun, 08 Jan 2012 03:54:58 -0600 caldwell



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