Background

In August of 2022 the HFTF started investigating the possibility of using traditional holography to calibrate the large scale deformations on the surface of the primary reflector of the GBT, potentially replacing out-of-focus holography. This was motivated by the need to commission a new holography receiver, which could also include a broadband option to perform the measurements on astronomical sources.

Analysis

Using "old" Holography maps to evaluate performance

We use the data acquired with the old holography receiver to estimate how a new receiver would perform.

Some notes related to the code:

• Mathematica saves files in binary format when using "Real32" as the extension. Python can do the same using:

     myArray = ...
     newFileByteArray = bytearray(myArray)
     with open("newfile", "wb") as log:
         log.write(newFileByteArray)
     

  This will produce files which are consistent with the input expected by the phase unwrapping code.
• mask.m (part of holo-analysis) has a boolean mask of the dish!
• holo-analysis grids the data using 0.0075 degree wide pixels (0.0075*1025~7.7 degrees map on the side).

• Cropping the complex valued beam map to 0.48x0.48 deg it is still possible to recover the same Zernike polynomials as when using the full complex valued beam map. For this to work, we need to mask the dish edges, and any large outliers in the aperture plane.

-- PedroSalas - 2022-09-23