Background

The MOSFIRE observer must be aware not only of the position angle of the instrument, but also what that position angle refers to. There are several position angles pertinent to MOSFIRE, as described in the table below.

MOSFIRE Keyword Name Description Value for ROTDEST=0° Relation to ROTDEST

SKYPA0 Position angle of a vector normal to the CSU bars relative to celestial North. Note: Contrary to standards proposed in KSD 40 which stipulate that ROTPOSN is defined by the guider, the CSU normal vector is accepted as the fundamental definition of the position angle on MOSFIRE. MAGMA specifies this PA for slitmasks. 0.00° ROTDEST=SKYPA0

SKYPA1 Position angle of MOSFIRE guider detector columns relative to celestial North. +90.00° SKYPA1=ROTDEST+90.00°
ROTDEST=SKYPA1-90.00°

SKYPA2 Position angle of MOSFIRE science detector columns relative to celestial North. +0.22° SKYPA2=ROTDEST+0.22°
ROTDEST=SKYPA2-0.22°

SKYPA3 Position angle of MOSFIRE longslit relative to celestial North. +4.00° SKYPA3=ROTDEST+4.00°
ROTDEST=SKYPA3-4.00°

MOSFIRE Keyword Name	Description	Value for ROTDEST=0°	Relation to ROTDEST
SKYPA0	Position angle of a vector normal to the CSU bars relative to celestial North. Note: Contrary to standards proposed in KSD 40 which stipulate that ROTPOSN is defined by the guider, the CSU normal vector is accepted as the fundamental definition of the position angle on MOSFIRE. MAGMA specifies this PA for slitmasks.	0.00°	`ROTDEST=SKYPA0`
SKYPA1	Position angle of MOSFIRE guider detector columns relative to celestial North.	+90.00°	`SKYPA1=ROTDEST+90.00°` `ROTDEST=SKYPA1-90.00°`
SKYPA2	Position angle of MOSFIRE science detector columns relative to celestial North.	+0.22°	`SKYPA2=ROTDEST+0.22°` `ROTDEST=SKYPA2-0.22°`
SKYPA3	Position angle of MOSFIRE longslit relative to celestial North.	+4.00°	`SKYPA3=ROTDEST+4.00°` `ROTDEST=SKYPA3-4.00°`

Depiction

The image below (which is not to scale) depicts the relationships orientation of the various MOSFIRE components.

Notes on main plot

The tan outline represents the MOSFIRE detector.
The thin blue lines represent the MOSFIRE CSU bars. Note that the bars are tilted by 0.22° relative to the MOSFIRE detector rows.
The thick red line represents the MOSFIRE longslit (as well as any individual MOSFIRE slit). Note that the longslit is tilted by 4° relative to a vector normal to the CSU bars, and by 3.78° relative to the MOSFIRE detector columns.

Notes on legend

The vectors at right depict the correponding position angles for these components.

The black vector represents celestial North.
The angle labeled Θ₀ represents the position angle of the CSU relative to North (i.e., SKYPA0 or ROTPOSN).
The angle labeled Θ₂ represents the position angle of the MOSFIRE detector relative to North (i.e., SKYPA2).
The angle labeled Θ₃ represents the position angle of the MOSFIRE longslit relative to North (i.e., SKYPA3).

Applications

Slitmask position angle: To orient the CSU at a particular position angle relative to celestial north, simply set the MOSFIRE position angle (DCS keyword ROTDEST) to that value. Example: To put North "up" relative to the CSU bars, set ROTDEST to 0°.
Imaging position angle: To orient the MOSFIRE detector at a particular position angle relative to celestial north, set the MOSFIRE position angle (DCS keyword ROTDEST) to that value minus 0.22°. Example: To put North "up" on the image, set ROTDEST to -0.22°.
Longslit position angle: To orient the MOSFIRE longslit at a particular position angle relative to celestial north, set the MOSFIRE position angle (DCS keyword ROTDEST) to that value minus 4.00°. Example: To orient the longslit north/south, set ROTDEST to -4.00°.