Spacecraft absolute pointing

After we get into orbit the SOHO spacecraft will be pointed at Sun Center. We need to define where that really is on the scale of arcmin. The question is: should we set it at EIT sun center, LASCO C2 center with the legs at 0,0, SUMER sun center with the mirror at 0,0, some compromise, etc.? It is obvious that we need to run an experiment after launch to determine where each instrument actually is with respect to each other. We will then decide as a group where we should set the spacecraft. In order to develop a procedure for running this experiment and developing some criteria I have asked each PI to tell me how much flexibility they have in their instrument. I have gotten all the replies that I expect to get at this point, so I will make an initial proposal. Below is a compilation of the list.


Instrument          FOV       Pointing       Range        note

EIT                 50'x50'   0,0+-8'        0' 0'

LASCO               30Ro      0,0+-8'        10' 10' maybe some more

UVCS                10Ro      0,0+-8'        7' 7'      57' 
                                                  available for science

CDS                 X"xY'     variable       ~40'x40'

SUMER               X"xY'     variable       64'x64'

MDI                 34'x34'   0,0+-8'        15'x 15'

VIRGO               X'xY'     0,0+-8'        7' 7'

GOLF                X'xY'     0,0+-8'        0' 0'

My assesment of the table:

EIT and GOLF: have no flexibility. They both look at full sun and have no further pointing capability. If spacecraft 0,0 and EIT 0,0 are significantly different, EIT will lose part of the sun or corona. GOLF will run an experiment early in the mission to see the effect of offset on their data.
VIRGO: Has some flexibility, not a lot.
MDI: They have some flexibility; they need to look at sun center.
LASCO: Has legs that can move by 10'x10'. They need to look at sun center.
UVCS: Has legs that can move by 7'x7'. They need to look at sun center
SUMER: Has a mirror for looking at different locations on the sun and in the corona with a range of 64'x64'
CDS: Has legs that can move by 40'x40'
The other instruments: Don't care to the accuracy we are talking about; a few arcmin.??

PROCEDURE

I propose we use the following procedure to determine where we set the spacecraft 0,0.

The spacecraft will begin at some electronic 0,0.
Each experiment will make a measurement of where their 0,0 is with respect to the spacecraft 0,0.
- EIT will make an image and see how well the sun is centered on their CCD.
- GOLF will see how much off center they are.??
- VIRGO ??
- MDI will make a full sun image with their legs at 0,0 to see how much off center they are.
- LASCO will see how much offset they need in their legs to center each coronagraph.
- UVCS will see how much offset they need in their offset mechanism to center the instrument.
- SUMER will make scans at each of the 4 limbs to see how much off center the mirror 0,0 is with respect to spacecraft 0,0.
- CDS will make 4 limb images to see how much off center their leg 0,0 is.
After looking at the data we will all negotiate where we want the spacecraft to be pointed.

This is a suggestion. The SOWG should plan on improving this at the next meeting.

Art Poland
Last modification: 29 November 1995