Alignment procedures

During alignment and calibration of the TA, the High-speed Imaging Photometer for Occulta- tions (HIPO) is used as a reference instrument at the focal plane. It is one of the first-light science instruments for SOFIA and is involved in the telescope functional and performance testing and is therefore provided with specific design features [Dunham 2004a]. These include a very robust mechanical design ensuring only minor deflections due to gravity and a lowest resonance frequency at about 110 Hz, well beyond the structural frequencies that the pointing control system can address. HIPO has a removable Shack-Hartmann lenslet array, which pro- vides wavefront error measurement information and makes an effective tool for focusing and aligning. Furthermore, HIPO’s optical design includes a precisely located light source. It injects light from an internal LED at a known location relative to the SI mounting flange, through a pinhole at the focal plane into the telescope. A spherical button mirror mounted in the center hole of the secondary mirror reflects and re-images the light back into HIPO. This procedure permits measurements of the location and motion of the active secondary mirror. HIPO also has a pupil imaging mode. Beside its science application, HIPO will be used regularly for alignment and calibration during the operational lifetime of SOFIA. This hap- pens especially after demounting of the optical components for coating and cleaning when the realignment of the telescope optics has to be verified.

However, before alignment and calibration measurements of TARF and the sensor reference frames are performed, coarse mechanical alignments are performed on the telescope’s optical assembly which include:

• the collimation of the primary mirror to the secondary mirror,

• the alignment of the mechanical axes of the secondary mirror Focus and Centering Mechanism (FCM) to the primary mirror axis,

• the alignment of the headring imagers (WFI and FFI) optical axes to the primary mirror optical axis,

• the pupil matching of the nominal FPI pupil position to the secondary mirror representing the exit pupil of the telescope.

The optical assembly alignment and verification plan is described in [Erdmann 2001] and [Erhard 2004]. The optical alignment is afterwards measured using the HIPO Shack- Hartmann test capability [Haas 2005]. The optimum decenter and focus position of the secondary mirror is determined for optimum image quality. The position of the FCM is then kept constant over the course of the following sensor alignment measurements.

The alignment and calibration procedures are performed with HIPO using astrometric references on the sky. The alignment sequence is split into three consecutive parts:

The first part of the on sky alignment consists of determining the position of the TA boresight with HIPO relative to the TA imagers. A star is brought into the field of view of HIPO and

centered on the SI flange, i.e. at a calibrated pixel location in the HIPO detector. This pixel location identifies where the projected origin of TARF is located at the SI flange. Figure 3.3 shows the known location and orientation of the HIPO CCD with respect to the SI mounting flange for a typical operational telescope elevation of 40° [Dunham 2004b]. Pixel location (X0,Y0) on the HIPO CCD is measured to be the center of the flange with a known accuracy.

By comparing the centroid locations in the other telescope imagers, the origin of TARF is identified within these fields of view and recorded by pixel locations. The relations of the pixel locations on the HIPO CCD and on each of the imager CCDs are defined depending on image scale and the imager boresight positions relative to TARF. As the telescope structure flexes due to gravity, the relative alignment relationships for the imagers and the HIPO reference location also change for different elevation angles. For the FPI, the boresight posi- tion is additionally dependent on temperature variations. These dependencies are reflected in the camera alignment matrices with respect to TARF.

Figure 3.3. Defining TARF with HIPO at the SI mounting flange.

The second part consists of making the telescope rotation axes conjugate with the TARF axes. The orientation and position of HIPO on the telescope not only define the origin of TARF at the center of the SI flange but also the orientation of the TARF axes as projected on the focal plane. Laboratory measurements of the alignment of the rows and columns of the HIPO CCD detector – relative to the science instrument flange mount pins – enable HIPO to be used to define the telescope motion axes with a known accuracy. The telescope axes alignment measurements are done by commanding motions about the initially estimated TARF axes, while observing the motions of a bright star in the HIPO images. At initial setup of the TA, TARF is assumed to be conjugate with the gyroscope axes defining GYRF by their sensing

Center of SI mounting flange at pixel location (X0,Y0) VTARF WTARF HIPO CCD SI mounting flange ϕ Y X

TARF commanded rotations lead to a rotation about the gyroscope axes. Comparing centroid measurements and recorded gyroscope attitude data during these processes permits the rotational alignment relationship between TARF and GYRF to be quantified. The GYRF to TARF alignment ensures that an executed rotation about one of the TARF axes is reflected properly on the HIPO CCD, i.e. the focal plane. The GYRF axes are assumed to be orthogonal to each other, because the gyroscope box in which the gyroscope are mounted was measured to have non-orthogonality errors in the region of only 20-40 arcsec. On the left side of Figure 3.4, it is shown how the telescope rotations, i.e. the TARF axes, are defined on the HIPO CCD. After the gyroscope alignment, HIPO’s field of view moves along the designated axes due to rotations about EL, XEL and LOS in TARF. Image motion takes place in the opposite direction.

The third part consists of determining the axes orientation of the three imagers. TARF’s ori- gin in the imager was already defined in the first part and with it the misalignment of the imager boresight (the center of the CCD) to TARF. Here, the misalignment of the EL- and XEL-axes is assessed. If the gyroscope misalignment is already compensated, the commanded telescope rotations are performed about the TARF axes. The image centroid measurements before and after a move are again compared to the target position. Figure 3.4, on the right side, shows the field of view of an imager and the projected TARF axes as defined by the HIPO CCD. Maximizing alignment accuracies, the maneuvers are performed over the whole imager field of view.

Figure 3.4. Calibrating TARF with HIPO CCD. Column and Row Axes Orientation of HIPO and an imager

CCD and the telescope rotations EL, XEL and LOS on the sky as seen in these CCDs. The pixel reference frames are indicated for HIPO in the left lower corner and the imagers in the left upper corner.

Finally, the influence of the telescope structural deformation due to gravity is determined. The three parts of the alignment sequence are repeated at various elevations over the telescope

HIPO field of view

+EL ( Pixel X Pixel Y Center of SI Flange (X0,Y0) +LOS +XEL

Imager field of view

+EL Pixel X Pixel Y +LOS +XEL Center of SI Flange HIPO CCD Imager axes misalignment ϕ

elevation range. In this way, relative flexures between the focal plane, the imagers and the gyroscopes can be assessed.