Hi Tim,

This is a little worrying, since the base position is the same (in the 
OT programme) for the observations from 20071123 and 20081008. We did 
change the OT programme for the observations on 20081008, putting in a 
1/4 array step in the raster, rather than a 1/2 array step, and an 
offset iterator. Both MSBs are still in the OT, with the original 1/2 
array step MSB set to 'REMOVED'. In the old (11003 Orion B South: 
NGC2024 12CO 10.8'x22.5' raster PA=-40) and new (11003Q+off(0,0) Orion B 
South: NGC2024 12CO 10.8'x22.5' raster PA=-40) the target components are 
identical, for project mjlsg11.

Regards,

Jane

tim.jenness wrote:
> On Jan 13, 2009, at 11:57 AM, Brad Cavanagh wrote:
>>
>>
>> Observation 54 from 20081007 ends up in its own group because it has 
>> a difference reference RA/Dec. You can see this by doing 'ndftrace 
>> a20081007_00050_01_0001 fullframe' and 'ndftrace 
>> a20081007_00054_01_0001 fullframe'. For observation 50 the reference 
>> Declination is -1:54:27, whereas for observation 54 it's -1:54:10. 
>> This change in the reference position means they get split up into 
>> separate groups -- the reference position has to be the same to 
>> within an arcsecond. I can adjust this "slop" though, if necessary.
>>
>
> Little bit of clarification here. The AST nomenclature is Sky 
> Reference but that is not to be confused with a heterodyne REFERENCE 
> position (the off position). Here Brad is saying that the BASE 
> position has changed between observations. Since oracdr does not want 
> to start doing astrometry itself it looks at the base positions and 
> uses those to decide when to group (the JSA will be cleverer). It 
> assumes that you will use offset iterators in the OT to move around 
> your field rather than changing the telescope base position.
>
> --Tim Jenness

-- 

Dr Jane V. Buckle 	
Astrophysics Group, Cavendish Lab, J J Thomson Avenue, Cambridge CB3 0HE
Tel: +44 (0)1223 337298