Guiding and WFS (AGw)
Because of the sheer size of each MODS, it was necessary to integrate the autoguiding and wavefront sensing unit (AGw; little “w” since the wavefront sensor is a Shack-Hartmann WFS for active, not adaptive, optics) into the instrument, rather than mount the instrument onto it, as is the case for LUCI. In MODS, the AGw stage shares the same region above the focal plane as the calibration unit. Hardware and software interlocks operate to prevent collisions. [Add an image of the optical path: Figure 1: A schematic of the AGw and optical path layout for the guider and wavefront sensor is below.]
The AGw unit has a 4-position filter wheel in the guide channel (the WFS channel is unfiltered). The filter parameters are listed in the table below, with transmission curves shown linked in the table.
|Clear||Clear Fused Silica||6210||3670||Default acquisition & guiding
12 mag < R < 16.5 mag
|F525LP||Red long-pass||6930||3210||Moon suppression & red guiding
12 mag < R < 16.5 mag
|B_Bessel||Bessel B Filter||4310||686||Deep blue guiding
12 mag < B < 15.5 mag
|ND1.0||Neutral Density 1.0||6210||3670||Bright target acquisition|
Overall guiding accuracy with the MODS AGw unit and the current generation LBT GCS software is measured to be ±30mas RMS, except at very low elevations (<25°) where the guide-star images are smeared out due to differential atmospheric refraction and increased astigmatism coming from the adaptive secondary mirrors.
Most observations use the Clear filter when guiding. If using the Clear or LP525 filter, guide stars within the magnitude range: 12 mag < R < 16.5 mag should be selected, where the faint end is applicable only for good (<~0.8″) seeing and transparency. If using the B-Bessel filter to guide in blue light, the faint limit should be decreased by 1 magnitude due to account for the lower QE of the guide camera CCDs in the blue, or 12 mag < B < 15.5 mag. The bright magnitude limit is set by the need to avoid saturating the unfiltered wavefront sensor.
Patrol Field and Pickoff Mirror Shadow
The patrol field covers an 5′ x 5′ region that extends from the center of the science field of view (see Figure 2 to the left below). The observation planning tools, the OT and modsView, contain an overlay of the patrol field, which should be used when selecting guide stars.
The pickoff mirror shadow extends symmetrically above and below the guide star (by ~43 arcsec). [retrofit of MODS1] In both the MODS1 and MODS2 AGws, the beamsplitter which directs 50% of the light to the WFS camera and allows 50% to pass through to the guide camera covers the enter guide camera field of view. Because the guide probe is located above the f/15 focal plane, its shadow is larger than the size of the guide probe itself. The detailed shadow is fairly complex (shown below). A conservative zone of avoidance for PA=0° is such that you want no science targets within: 43″ North; 106″ East; and 44″ West of the location of guide/WFS star. In round numbers, if your guide star is located >243″ south of the science-field center at PA=0, the guide probe will not shadow the science field.
A schematic of the MODS AGw patrol field is shown above. The figure also shows the XY coordinate axes in the DETXY (rotator-invariant common focal plane coordinates) system. Offsets made in DETXY coordinates are used to move targets in acquisition images into the slit, and offsets along the Y axis of DETXY coordinates will dither the target along the slit.
MOS observers should take care not to choose guide stars too close to the science field of view, where the shadow of the pickoff mirror may fall on a slit. modsView and the OT both contain the overlay of the pickoff mirror shadow and are helpful when selecting guide stars.