As one can expect only certain stars will meet the criteria required for FGS guiding. The selection of GSs is determined by the Guide Star Selection System (GSSS) Request Processor, which provides the Science Operations Ground System (SOGS) with information required to perform GS selection and target acquisition to support HST science operations. In the process of implementing HST proposals there are many instances were GSs are not available for the execution of programs. The following document consists of a guideline which will aid the Program Coordinator to troubleshoot and understand the common GS problems which are encountered while implementing HST proposals. In Part 2, a short introduction of the Guide Star Request Processor is presented. Part 3 then discusses the selection and evaluation criteria of candidate GSs imposed by the Guide Star Request Processor Algortihm. Part 4 describes the scenarios to explore when no GSs (pairs/singles) are available with no specified scheduling constraints (eg. ORIENTs, Timing Requirements, etc.) included in both fixed & moving target proposals. In part 5, these constraints are taken into account and alternatives are considered. Part 6 provides additional tips for moving target proposals. Finally, Part 7 describes the GSSS Status and Guide Star Reports which are available for troubleshooting GS problems.

Each GSSS GS request includes all the relevant data about the observation set (the proposal, reference RA & DEC, time ranges, selected constraints), target description (position & uncertainty information, magnitude, epoch, proper motion & parallax), target/aperture alignment data (position angles, angular separation, pointing position, orientation type, aperture description & viewing options), Chebyshev coefficients for moving targets and special parameters for running GSSS (such as pair prioritization functions, diagnostic request, min & max pair counts & ambiguous and bright object criteria). GSSS then uses all this data along with information from the Guide Star Catalog (GSC), to provide SPSS with characteristics of the potentially usable GS.

In order to understand why sometimes no GSs are available for particular observations, we need to review the major functions of GSRP: the selection, evaluation and prioritization of GSs or GS pairs. A star or pair of stars is selected as a candidate GS or GS pair if these fall within the FGSs field of view (FOV) for a particular Target/Scientific Instrument (SI) Aperture combination and within a certain time and V3/U3 angle constraints (the FGS's field of view corresponds to 69 square arc minutes and in the STScI's jargon is referred to as the "pickle"). The evaluation of a candidate GS includes the estimation of HST's ability to acquire and hold the GS pair (if they meet centerlineable and magnitude thresholds & spoilers criteria), whether the GSs allow the observation of parallel targets, and whether the candidates satisfy the FGS's bright object & boundary alert criteria.

Once the GSRP functions have been executed, GSSS generates a GS response to SPSS which includes information regarding the science target (target name, GS catalog plate used, RA & DEC epoch, proper motion, magnitude, color etc.), information about each star in a GS pair (star ID, boundary alert flag, high proper motion flag, magnitude and uncertainty, acquisition probability , ambigous object data etc.), information about the GS pair (acquisition & separation probabilities, pair separation, usable times, V3 position angle ranges, probability of acquiring an ambiguous object etc..) and bright object alert data (such as alert ID, apertures in which alert falls, V3 position range etc..).

a) The rectangular position uncertainty range specified by the target coordinates uncertainties in the data field of the request.

b) The rectangular position uncertainty range representing the parallax error due to the HST's orbital motion.

c) The rectangular real-time adjustment range specified in the TA data field of the request. It can have its center anywhere in the position uncertainty area or parallax error range.

Thus, in general for every GS in the FGS's FOV, the requested aperture reference point of the primary SI used (eg. center) must align in the CUR centered at the nominal pointing position, and with every point in a scan area having its origin anywhere in the CUR. This constitutes the primary condition that must be satisfied in order to meet the TA accessibility criteria. In addition, the TA criteria must be satisfied for all the different orientations possible. Hence, if the observations will be performed at nominal roll, all alignments (set of exposures performed at the same pointing, thus same set of GSs) specified must be satisfied at the nominal roll orientation on some date within a given date window. If the orientation type corresponds to absolute or real-time, then all alignments must be satisfied at the specified V3-axis position angle, or at every angle in the given date window. Finally, if the orientation type is range, then all alignments must be satisfied at some angle within the given range.

Failure to meet the TA criteria is responsible for the majority of GSs which are not selected as candidates during GS requests.

Now, after the "Primary" GS has been selected (the GS which is acquired first), uncertainties in the search of the "Secondary" GS are reduced and the search radius for the secondary is minimized to 15". Information provided by GSSS then takes into account the GS pair separation distance which defines a particular pair. Thus, given the position & magnitude information for the GS acquired, if the separation is not consistent with the GSC, memory of the wrong star pair (spoiler) is kept and the searching algorithm is performed again while ignoring the previously acquired spoiler.

The potential for acquiring spoilers is estimated by the Request Processor algorithm, where the probability for other stars included within the magnitude range of the desired GS is calculated for stars enclosed within the search radius. This probability is referred to as the "Estimated Spoiler Pair Probability" and it serves as a variable to prioritize and filter unacceptable GS pairs. Since significant values for this function (10%) are not tolerated, this aspect of the selection process is responsible for the reduced number of GS pairs returned in high GS density areas (eg. star fields of low galactic latitude).

Thus, GSs which are selected as candidates must satisfy the allowed FGS magnitude limit and have low spoiler pair probabilities. Therefore, in scenarios were the science targets are located in low density star fields (eg. targets with high galactic coordinates) the probability for successful GS pairs candidates clearly decreases due to the smaller number of GSs available within the FGS's acceptable magnitude range. In high density star fields, the number of GS available is also reduced, but in this case due to the added complexity of the large superposition of stars in the FOV and therefore, the higher probability for acquiring spoilers. However, we have to keep in mind that in some cases spoiler pair probabilties can attain large values at high galactic latitudes as well (eg. globular clusters).

Inspection of the FOV program will yield information as to which of above factors is playing a role in a GS request.

 

Task Parameter File Name	Usage/Scenario
taskpar_fine.dat	fixed target obset guided by one pair or two pairs of guide stars/BASE1 or BASE2 acquisition scenario
taskpar_fine_single.dat	fixed target obset guided by a single guide star/ONEB1 acquisition scenario
taskpar_fine_moving.dat	moving target obset guided by one pair or two pairs of guide stars/BASE1 or BASE2 acquisition scenario
taskpar_fine_single_moving.dat	moving target obset guided by a single guide star/ONEB1 acquisition scenario
taskpar_fine_snap.dat	SNAP obset. Prevents FGS2 usage in order to help prolong FGS2 lifetime

Occasionally, a GS request will return no GS pairs for any time during the year. In such cases, the alternatives are to either changed the default GS scenario to single GS guiding, or override the default GS magnitude selection criteria in the GSSR Task Parameter files and either search for higher magnitudes stars or modify the spoiler search radius. These overrides can be performed via an interactive GS request to Full-Look, or by creating and editing a GAQ record for the obset in the PMDB. In all cases trade-offs are to be taken in consideration. If none of this possibilities proved successful there is no alternative but to changed the target of observation.

When performing single GS guiding it is important to note that the pointing precision is degraded as compared with the GS pair scenario. Any potential problems with single GS guiding will be determined by the on-target time length and the aperture size used during the observation. The table below shows the worst predicted target motion on single guide star guiding.

                 
		Target position shift in arcseconds as a function of time 
      
                     seconds                               orbits
      1000    2000   5000   10000   15000   20000 |    1     2     3     4
     0.005   0.011  0.029   0.055   0.084   0.110 | 0.03  0.06  0.10  0.13

As stated before, these types of searches can be done via GAQ records too, which is easier than an interactive request, but which requires correct keyboard mapping of the Unix workstation for SmartStar pforms.

----------------------------------------------------------------------------------------------
Report Title:  FULL-LOOK REQUESTS FOR WHICH RESPONSES HAVE BEEN RECEIVED AND PROCESSED IN SPSS
----------------------------------------------------------------------------------------------

prop  ob vn   req num     req date   req typ |  resp num      response time    val spa da    num pairs
----- -- --  ----------   --------   ------- | ----------   -----------------   -   -   -    ---------
07289:02:01       39687   1998.114   Auto    |      74836   1998.114:18:42:10   Y   N   N           13

GSSS-STATUS Command Terminated

In addition, there are four GS reports available from the ASSIST database which display information generated by the GSSS: Guide Star Pairs, Guide Star Roll Ranges, Guide Star Spoilers and Guide Star Acq Data Sets. These reports might prove useful when troubleshooting GS problems. They are available from the ASSIST Reports tool (which can be accessed through the PC Toolbox under the Database Controls). These reports are also available via the line command on SOGS.

RESPONSE: 74836          FOR PROPOSAL ID: 07289

Guide Star Availability Report

OB                             SEL  MAGNITUDE
ID PAIR ID                     SEQ  #1    #2      
-- --------------------------- ---  --------------

02 --0268800307F20269200814F1  13   12.2  12.4
02 --0268800307F20269202944F3  8    12.2  12.9
02 --0269200814F10269202944F3  1    12.4  12.9
02 --0269200814F10269203521F3  10   12.4  12.7
02 --0269202909F10269200832F3  11   13.2  13.9
02 --0269203020F10269200832F3  3    13.9  13.9
02 --0269203241F10269200832F3  6    12.1  13.9
02 --0269203400F30268800061F2  12   11.3  12.9
02 --0269203400F30269200814F1  9    11.3  12.4
02 --0269203400F30269203241F1  2    11.3  12.1
02 --0269203521F30268800061F2  4    12.7  12.9
02 --0269203521F30269202909F1  5    12.7  13.2
02 --0269203521F30269203020F1  7    12.7  13.9

                         GUIDE STAR ROLL RANGE REPORT
                            FOR PROPOSAL ID: 07289

                                                  -- V3 PA RANGES --
OB                             SEL     TOTAL         ACQUISITION     A   CENTERLINE    C
ID         PAIR ID             SEQ MIN      MAX      MIN      MAX    S MIN      MAX    S
-- --------------------------- --- -------- -------- -------- -------- -------- --------

02 --0269203020F10269200832F3  3   320.63     8.10   331.35   345.76 B 320.63   349.93 B
02 --0269203521F30269203020F1  7   323.54     8.10   338.18   345.76 B 323.54   357.95 B
02 --0269202909F10269200832F3  11  325.54     9.35   336.55   349.94 B 325.54   354.65 B
02 --0269203521F30269202909F1  5   325.54     9.35   338.18   352.25 B 325.54     2.87 B
02 --0269203241F10269200832F3  6   338.30     9.98   346.53   349.94 B 338.30   358.56 B
02 --0269203400F30269203241F1  2   338.30    17.42   349.78   359.80 B 340.54    17.42 B
02 --0269203400F30268800061F2  12  346.70    23.80   353.91     8.12 B 346.70    20.51 B
02 --0269203521F30268800061F2  4   346.70    23.80   353.91   357.94 B 346.70    16.65 B
02 --0268800307F20269202944F3  8   348.47    14.56   357.64   357.89 B 355.87    14.56 B
02 --0269200814F10269202944F3  1   354.90    14.56     1.41     1.58 B 354.90    14.56 B
02 --0269200814F10269203521F3  10  354.90    27.23     0.00     0.00 G 354.90    23.06 B
02 --0268800307F20269200814F1  13  354.90    28.38     0.00     0.00 G   0.98    28.38 B
02 --0269203400F30269200814F1  9   354.90    28.38     1.41     8.12 B 354.90    28.38 B

Total # of pairs retrieved for this obset(s) =   13


AS = Acquisition Status
Key:    B = Both stars can be acquired
        P = Primary star only
        S = Secondary star only
        G = Acq range for both stars is outside GS 'Total' range
        N = Neither star can be acquired
CS = Centerline Status
Key:    B = Both stars can be centerlined
        P = Primary star only can be centerlined
        O = Both stars can be placed near but not on the centerline
        N = Neither star can be centerlined

SPOILERS REPORT

                                     PRI-      DOMINANT       SUB-DOMIN
OBSET ID         PAIR ID             MARY FGS SNGL  DBLE  FGS SNGL  DBLE
----------- ------------------------ ---- --- ----- ----- --- ----- -----
07289:02:01 0268800307F20269200814F1  S    2  81.19 120   3   11240 12080
07289:02:01 0268800307F20269202944F3  S    2  81.19 120   3   11240 12080
07289:02:01 0269200814F10269202944F3  S    1  84.19 86.80 3   11240 12080
07289:02:01 0269200814F10269203521F3  S    1  84.19 86.80 3   11240 12080
07289:02:01 0269202909F10269200832F3  S    1  30    120   3   11240 12080
07289:02:01 0269203020F10269200832F3  S    1  30    118.8 3   11240 12080
07289:02:01 0269203241F10269200832F3  S    1  30.60 58.20 3   11240 12080
07289:02:01 0269203400F30268800061F2  D    3  50.20 87.19 3   11240 12080
07289:02:01 0269203400F30269200814F1  S    3  50.20 87.19 3   11240 12080
07289:02:01 0269203400F30269203241F1  D    3  50.20 87.19 3   11240 12080
07289:02:01 0269203521F30268800061F2  D    3  95.40 120   3   11240 12080
07289:02:01 0269203521F30269202909F1  D    3  95.40 120   3   11240 12080
07289:02:01 0269203521F30269203020F1  D    3  95.40 120   3   11240 12080

GS ACQUISITION DATA SET REPORT
PROP/OB  VR SL                   DAT GS PROB     LOW V3     UP V3                                     ROLL   MOD1    MOD2  MIN     NOM
  ID     #  FL CREATE DATE       ID  #  SUCC     ANGLE      ANGLE      RA         DEC        ATT ERR  ERR    TIME    TIME  TIME    TIME
----- -- -- -- ----------------- --- -- -------- ---------- ---------- ---------- ---------- -------- ------ ------- ----- ------- -------

0728902  01 Y  1998.114:18:48:03   1 6  0.414000 323.539280 28.3774770 314.026028 31.9351105 3.757257 50.000 621.000 0.046 622.000 819.000

Acknowledgements: I thank B. Workman, B. Owens, G. Chapman, M. Reinhardt, D. Jones and J. Baum for many stimulating and useful discussions and D. Taylor for suggestions and a thorough revision of the document.

Last Update: June 5, 1998