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====== Focal Plane Unit ======

The focal plane unit consists of a heated [[#Iodine cell|iodine cell]] for wavelength calibration, an acqusition/guide camera for placing and keeping stars on the 50 micron spectrograph fiber, and a calibration channel for flat fielding and injection of ThAr calibration.


===== Guiding and acquisition =====

Initial plan is to use a Shelyak FIGU which has a 50 micron hole with circular fiber in a pickoff mirror with a guide camera imaging this focal plane, with a magnification of 1.5X (empirically could be 1.33X?).
The guide camera is an Atik 414ex 531. It is normally controlled by APOsong through ASCOM Remote, although the PWI4 software also contains an ASCOM driver, which is useful for pointing models, etc.

The Shelyak is mounted on a Zaber X-LSM050a motorized stage with 50mm travel. From the Windows desktop, this can be controlled with the Zaber Launcher software, but normal usage for SONG is through the remote interface in APOsong (although it would be nice to write a quick ASCOM Focuser server).

===== Iodine cell =====

The iodine cell has been graciously provided by Dr. Paul Butler. It is numbered as "AS17" and is 4 inches long and 2 inches in diameter.

The iodine cell is placed on a Thorlabs LTS150C motorized linear stage for moving it in and out of the optical path at the Nasmyth port. From the Windows desktop, this can be controlled with the Thorlabs Kinesis software, but normal usage for SONG is through the remote interface in APOsong.

Temperature control is achieved using a Thorlabs TC300 controller with two heating elements for each half of the iodine cell. From the Windows desktop, this can be controlled with the Thorlabs TC300 software, but normal use for SONG is through the remote interface in APOsong.

===== Mechanical platform mount =====

The platform is 1/2 inch aluminum and anodized. It was manufactured at CVE Machine and Welding in High Rolls, NM.

Here is a photo of the mount in the lab with an optical breadboard that holds the iodine cell, focusing and guiding unit, linear drives, and eventually calibration light input. 

{{ :fpu:mount_in_lab.jpg?direct&300|}}

Here is a photo of the PFU ,mounted on the 1m Nasmyth port for initial testing in Fall 2024.
 {{:fpu:fpu_1m_oct2024.jpg?300|}}

Documents
  * {{ :docs:fpu_mount_dims.pdf |Drawings and dimensions}} of mount
  * {{ :docs:fpu2.rtf |Sketchup design file}} (replace rtf with skp)


**FPU calibration relay stage**

The calibration relay path on the FPU allows fiber fed calibration light to be fed back onto the science fiber via the Shelyak FIGU. The FPU layout allows the cal light to pass with or without passing through the iodine cell. The cal relay consists of a fixed set consisting of a 75mm 300m focal length acromat and a 75mm mirror plus a movable set of the same two elements using a Thorlabs LTS150 linear stage (same type as used for the moveable iodine cell).

This a diagram of the FPU cal relay stage mechanical layout.

{{:fpu:fpu_layout_diagram_may27.pdf|}}