[Comm2011] rotator and ONECAL during calibrations

[Konrad Kuijken]

You want ABSROT if you want to know the rotation of the detectors wrt the telescope.
POSANG is indeed the position angle with respect to the sky.
PARANG is the parallactic angle, which think is independent of the actual rotator setting. I think it is the POSANG that would correspond to up-down alignment. We do not need it. But probably there is a relation like 
ABSROT = PARANG+POSANG (up to signs).

K

On Aug 9, 2011, at 23:33, Andrea Baruffolo wrote:

> FYI
> 
> -----Original Message-----
> From: John P. McFarland [mailto:mcfarland at astro.rug.nl] 
> Sent: Tuesday, August 09, 2011 11:15 PM
> To: Steffen Mieske; Muschielok Bernard; Andrea Baruffolo
> Cc: G. Sikkema; E.A. Valentijn; E.M. Helmich
> Subject: rotator and ONECAL during calibrations
> 
> Steffen, Bernard, and Andrea,
> 
> We were discussing the schedule for the remaining days of the commissioning 
> and some questions came up regarding the rotator and the mirror shape during
> 
> calibrations.
> 
> As for the rotator angles, We wanted to get clarification on the rotator 
> keywords and their values.
> 
> ADA.ABSROT.START / starting rotator angle w.r.t. telescope?
> ADA.ABSROT.END   / ending rotator angle w.r.t. telescope?
> ADA.POSANG       / angle between N and Y (sky and detectors)?
> TEL.PARANG.START / starting rotator angle between N and ABSROT?
> TEL.PARANG.END   / ending rotator angle between N and ABSROT?
> 
> The definitions have never been completely clear to us.  The primary thing 
> we need to determine here is the angle between the detector block and the 
> telescope.  We need to properly characterize any illumination differences 
> and if they are rotation angle dependent and we can only do this if we know 
> what the recorded angles really mean.
> 
> Also, what are these values when we take dome flats?  The adapter keywords 
> are not written to the header, so we can't tell.  My understanding is that 
> the rotator is parked during this time and always at the same absolute 
> position angle.  Is this indeed the case?  What angle is this and where is 
> it recorded?  And, when an angle is requested during an on-sky exposure 
> (i.e., twilight flat), what angle is this, and how does it relate to the 
> absolute position angle?
> 
> As for the mirror shape during calibrations, when is ONECAL used during dome
> 
> and twilight flats.  In comparing dome and twilight flats, we notice quite 
> often a trefoil shape to the residuals and wondered at it.  Also, is there 
> any logic to when ONECAL is and is not used for calibrations, and if so, 
> what is it?
> 
> Thanks in advance for the clarifications.
> 
> Cheers,
> 
> 
> -=John
> 
>