Pointing at certain offset from target

[Borlaff]

Hi all,

We are trying to define a series of pointings with the Wide Field Instrument of Roman Space Telescope. In those pointings (RA_wfi, DEC_wfi), the center of WFI needs to be at a certain dX (RA) and dY (declination) from a selected target (RA_target, DEC_target).

We are currently using idl_to_tel to perform the estimations, but for some offsets, the target requires to be at a distance of ~5 - 10 degrees from the center of the FOV. From what I understand, Idl_to_tel performs a tangent plane approximation, which provides a distorted solution at such long angular distances.

My question is: Given a set of coordinates for a target (RA_target, Dec_target), and a delta X and delta Y in ecliptic coordinates, an a given position angle (PA) for a detector, what is the most optimal way to obtain the center of the focal plane using PySIAF? Is this currently supported at very long angular distances (~10 degrees or larger) or rather, PySIAF assumes tangent planar approximation for all delta-transformations?

Thanks,
Alex

[Skyhawk172]

Hi @Borlaff!

I'll have to think some more about your last question, but I can tell you now that idl_to_tel does support the full spherical formalism, which you can enable using the following method arguments:

aper.idl_to_tel(x_idl, y_idl, method="spherical", input_coordinates="polar", output_coordinates="polar")

where aper is a pysiaf aperture (e.g. WFI_CEN).

Admittedly, this has not been used or tested nearly as much as the planar approximation. We’ll be taking a closer look at its implementation.