GRO observing plan changes

[SHRADER@GROSSC.GSFC.NASA.GOV]

Dear Colleagues,

Please find attached information from Dave Bertsch and Dick Kroeger on 
proposed modifications to the GRO viewing plan for select periods between 
mid-February and mid-July.  The proposal maintains the basic pointings of 
the December 5, 1994 plan, but with some small shifts in the Z-axis, some 
changes in OSSE targets and some rearrangement of dates.  The actual plan is 
given at the end of the message.

Please let me know if you have any comments on these modifications 
(gehrels@lheavx.gsfc.nasa.gov).  I will move quickly to implement the plan 
since the first altered period (VP 411.1 in mid February) is fast 
approaching.

Sincerely,
Neil Gehrels
Compton Project Scientist


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Dave Bertsch (EGRET) has developed a proposed modification to the GRO cycle 
4 viewing program for the purpose of reducing background trigger rates in 
EGRET, and thereby extending the life of EGRET.  I have made a few 
adjustments in Dave's original proposal in order to improve the OSSE 
observations, and to ensure that all spacecraft constraints are met.  The 
proposed plan with the small adjustments, is contained in this message.

Richard Kroeger
OSSE


EGRET position:

EGRET will have a significant problem with the proximity of the orbit pole 
to the Z-axis in viewing periods 414, 415, and 418 in the plan as it now 
exists.  An earlier suggestion of swapping the Anticenter and LMC/MRK 421 
pointings met with considerable opposition due to planned coordinated 
campaigns.  I (Dave Bertsch) offer here another suggestion that would permit 
observations in 415 and 418, and perhaps for a portion of 414. 

By shifting the 415 and 418 one week later, the pole impact is not so great. 
The dark moon interval for 418 (for coordinated TeV observations) is 
actually improved, and the coordinated ASCA observation is closer to the 
center of the period.  Both periods still have the pole as close as 25 
degrees for short times, but we think this is acceptable given the 
circumstances. 

I think the least painful way to achieve this shift is to move one week of 
the Orion observation to follow 414.  If this is done, CAS A cannot be the
secondary target for that week.  Neil Gehrels felt that the loss of one week
for CAS A was very undesirable, but thought that the CAS A observation did 
not need to be contiguous in time.  One way to make up the CAS A time is to 
make that source the secondary target in the first half of 411 and observe 
NGC 7172 during the second week of 411 and for the one week of 412.  One 
week of NCG -2-58-22 is then displaced, but can be made up as the secondary 
for the one week Orion observation that moved to follow 414.  As far as I 
can tell, all of the constraints (except perhaps for a guide star in one 
period) are satisfied, and I think no coordinated campaigns will suffer. 

Even if this modification is adopted, EGRET may be turned off during 414, 
and we may find it necessary to restrict the field during some part of 415 
and 418. 

OSSE position:

We have discussed the proposed changes with the PIs of the OSSE Orion 
(Bloemen) and Cas A (The) observations. We have also discussed delaying the 
3C 109 (Bassani) observation until VP 425 and observing PKS 0528+134 in VP 
413 instead. 

1) The Cas A team seems to have no problem with the proposed change.
2) The Orion team has been concerned that an isolated week with a different
position angle would be of little value.  However, they decided that,
by moving the OSSE scan plane so that it passes mid-way between Orion A
and Orion B, that the exposure from the three observations could be used
together.  
3) The 3C 109 team seems would like to observe nearer the ASCA observation.
As it turns out, ASCA has scheduled 3C 109 in September, and therefore
delaying this observation can only help.  The delay until VP 425, however,
is only until late July.

We have several small adjustments to the EGRET proposal designed to improve
the OSSE observations.  Our suggested changes are incorporated into the plan 
presented below.

Proposed Viewing Program:

VP 411.1 - 14 Feb - 21 Feb
OSSE targets are: Mrk 3 and Cas A
There are no guide stars in the attitude proposed in Dave's plan.  A -2 
degree pitch is the smallest shift in the z-axis which seems to cure this 
problem.  I (Kroeger) propose the following attitude which finds a guide 
star in FHST 1.
z-axis   :  RA =  98.492, DEC =  69.754 J2000
x-axis   :  RA = 328.383, DEC =  13.367 J2000
z-axis   :  l2 = 145.096, b2  =  23.851
x-axis   :  l2 =  70.393, b2  = -30.826

VP 411.5 - 21 Feb - 28 Feb
OSSE targets are: Mrk 3 and NGC 7172
z-axis   :  RA =  93.909, DEC =  71.034 J2000
x-axis   :  RA = 344.064, DEC =   6.654 J2000
z-axis   :  l2 = 143.300, b2  =  22.720
x-axis   :  l2 =  79.196, b2  = -46.207
This week is unchanged in targets, or spacecraft attitude from original 
plan.

VP 412 - 28 Feb - 7 Mar
OSSE targets are: Crab and NGC 7172
The attitude for this period in Dave's plan places the two OSSE targets 
just off the scan plane.  I would suggest the following change:
z-axis   :  RA =  90.091, DEC =  24.675 J2000
x-axis   :  RA =   9.864, DEC = -20.278 J2000
z-axis   :  l2 = 185.342, b2  =   0.666
x-axis   :  l2 = 100.492, b2  = -82.622
This moves the z-axis 2.35 degrees toward the right (galactic projection)
from the original viewing plan.  This is the smallest motion of the z-axis
consistent with placing the Crab and NGC 7172 on the OSSE scan plane. We 
could consider pitching the spacecraft toward galactic North or South by up 
to 4.5 degrees if this position is not acceptable. 

VP 413 - 7 Mar - 21 Mar
OSSE targets are: PKS 0528+134 and MCG -2-58-22
The target, PKS 0528+134 is 0.69 degrees off.  We would like to move the
z-axis by about this amount in order to center the PKS source on the scan
plane.  The new attitude would be:
z-axis   :  RA =  95.886, DEC =  14.738 J2000
x-axis   :  RA =   6.846, DEC =  -3.641 J2000
z-axis   :  l2 = 196.674, b2  =   0.660
x-axis   :  l2 = 108.143, b2  = -65.820

VP 414 - 21 Mar - 4 Apr
OSSE targets: LMC X-1 and S. Gal Pole
z-axis   :  RA = 108.378, DEC = -69.636 J2000
x-axis   :  RA =   7.294, DEC =  -4.082 J2000
z-axis   :  l2 = 280.752, b2  = -23.434
x-axis   :  l2 = 108.978, b2  = -66.349
A 0.65 degree change in the z-axis position from original viewing plan is
suggested that places LMC X-1 on the OSSE scan plane.  This is a slight
shift toward the galactic left (higher longitude).

VP 419.1 - 4 Apr - 11 Apr
OSSE targets: Orion and MCG -2-58-22
The coordinates of the mid point between Orion A and Orion B are
RA = 84.621, DEC = -3.649, l2 = 207.752, b2 = -17.874
The attitude that allows OSSE to scan through this mid point is:
z-axis   :  RA =  83.395, DEC =  -3.839 J2000
x-axis   :  RA = 352.773, DEC =  -9.188 J2000
z-axis   :  l2 = 207.346, b2  = -19.053
x-axis   :  l2 =  72.549, b2  = -63.888
This moves the z-axis 1.61 degrees to the "right" on the galactic projection
from where Dave had proposed it.

VP 415 - 11 Apr - 25 Apr
OSSE targets: LMC X-3 and S. Gal Pole
z-axis   :  RA = 103.997, DEC = -65.304 J2000
x-axis   :  RA =  11.780, DEC =  -1.019 J2000
z-axis   :  l2 = 275.720, b2  = -24.010
x-axis   :  l2 = 120.480, b2  = -63.870
No change in targets, or spacecraft attitude from original plan.
Dates have slipped by 1 week.

VP 418 - 25 Apr - 9 May
OSSE targets: NGC 4151 and 4U 0352+30
z-axis   :  RA = 173.197, DEC =  45.552 J2000
x-axis   :  RA =  53.912, DEC =  25.633 J2000
z-axis   :  l2 = 158.080, b2  =  65.840
x-axis   :  l2 = 163.360, b2  = -24.069
No change in targets, or spacecraft attitude from original plan.
Dates have slipped by 1 week.

VP 419.5 - 9 May - 23 May
OSSE targets: Orion and Cas A
Again, we need the OSSE scan to pass through the same mid point between the
Orion clouds as in VP 419.1 
The attitude we would use is:
z-axis   :  RA =  86.674, DEC =  -6.997 J2000
x-axis   :  RA =   7.942, DEC =  57.868 J2000
z-axis   :  l2 = 211.865, b2  = -17.564
x-axis   :  l2 = 120.309, b2  =  -4.904
This moves the z-axis 1.44 degrees from where Dave had proposed to put it.

VP 420 - 23 May - 6 Jun
OSSE targets: Orion and Cas A
As in VPs 419.1 and 419.5, we need the scan plane through the mid point
between the Orion clouds.
The attitude we would use is:
z-axis   :  RA =  79.913, DEC =   4.126 J2000
x-axis   :  RA = 343.156, DEC =  58.485 J2000
z-axis   :  l2 = 198.209, b2  = -18.257
x-axis   :  l2 = 107.919, b2  =  -0.880
This attitude has the effect of moving the z-axis about 1.45 degrees in
the direction of the galactic north.

VP 425 - 25 Jul - 8 Aug
OSSE targets PKS 0202+149 and 3C 109
We need to replace PKS 0528+134 by 3C 109 that was bumped earlier.
The attitude in the unchanged viewing plan places 3C 109 4.38 degrees off 
the scan plane.  I have had some trouble coming up with a perfect attitude 
for this observation due to a lack of valid guide stars.  I found one 
solution that I would like to tentatively propose:
z-axis   :  RA =  22.902, DEC =  14.438 J2000
x-axis   :  RA = 113.288, DEC =   1.496 J2000
z-axis   :  l2 = 137.353, b2  = -47.305
x-axis   :  l2 = 216.382, b2  =   9.959
This moves the z-axis by only 0.07 degrees from its original position.
However, PKS 0202+149 is 0.73 degrees off the scan plane, and 3C 109
is 0.86 degrees off the scan plane.  I also note that the viewing
statistics for the pair PKS 0202+149 and 3C 109 are rather poor.

--------------------------------------------

A heads up on a future change that I am developing in Cycle 4.  Roland Diehl
has asked me to work on improving his OSSE observation of the Vela region in 
VP 428.  This will amount to a slight yaw of the spacecraft and should have 
no significant impact on the Z-axis targets. 

--------------------------------------------
The VSTAT file segment for these changes is:
Coordinates are Galactic

411.1 14-Feb-1995 04:00  21-Feb-1995 15:09   145.10  23.85  70.39 -30.83  
MRK 3         143.30  22.72   CAS A         111.74  -2.14
411.5 21-Feb-1995 04:00  28-Feb-1995 15:11   143.30  22.72  79.19 -46.21  
MRK 3         143.30  22.72   NGC 7172       15.13 -53.06
412   28-Feb-1995 04:00   7-Mar-1995 15:04   185.34   0.67 100.49 -82.62  
CRAB          184.50  -5.87   NGC 7172       15.13 -53.06
413    7-Mar-1995 04:00  21-Mar-1995 15:23   196.67   0.66 108.14 -65.82  
PKS 0528+134  191.37 -11.01   MCG -2-58-22   64.09 -58.76
414   21-Mar-1995 04:00   4-Apr-1995 15:09   280.75 -23.43 108.98 -66.35  
LMC X-1       280.21 -31.52   GAL 000-90     99.01 -90.00
419.1  4-Apr-1995 04:00  11-Apr-1995 15:08   207.35 -19.05  72.55 -63.89  
ORION MID     207.75 -17.87   MCG -2-58-22   64.09 -58.76
415   11-Apr-1995 04:00  25-Apr-1995 15:02   275.72 -24.01 120.48 -63.87  
LMC X-3       273.65 -32.95   GAL 000-90     99.01 -90.00
418  25-Apr-1995 04:00   9-May-1995 15:16   158.08  65.84 163.36 -24.06   
NGC 4151      155.08  75.06   4U 0352+30    163.09 -17.14
419.5  9-May-1995 04:00  23-May-1995 15:20   211.86 -17.56 120.31  -4.90  
ORION MID     207.75 -17.87   CAS A         111.74  -2.14
420   23-May-1995 04:00   6-Jun-1995 15:33   198.21 -18.26 107.92  -0.88  
ORION MID     207.75 -17.87   CAS A         111.74  -2.14
425   25-Jul-1995 04:00   8-Aug-1995 15:17   137.35 -47.31 216.38   9.96  
PKS 0202+149  147.93 -44.04   3C 109        181.83 -27.78
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