RFE: add spectral order info.

[mcdittmar]

Presentations by Vandana Desai

• Nov. 2020 Interop: Implementation of the IVOA Spectral Data Model at IPAC” assessment of V1.1 Recommendation
• May 2021 Interop: "IVOA Spectral Models and Access in the Era of Big Data” enhancement request to support IPAC use cases

DM group meeting:

• DM Meeting 20220622: Notes
  • decision to add order / relorder to the SpectralAxis class

[mcdittmar]

Additional inputs to the 'order' RFE.

Interop DAL/DM Session - May 27, 2021

• PS: Concerning the orders - it would be useful for echelle spectra as well and there are in fact two order numbers. One is the relative order counting from 1 (in IRAF it is called aperture number) and then there is an ABSOLUTE echelle order number - it is the real n where the echelle optics is working (number of wavelengths whic will fit the distance between two grooves on echelle grating. This absolute is important for checking the wavelength calibration etc ...
• It sounds like those might even need distinct UCDs.
• BM: Very interested to see echelle spectra supported too. We have a lot of archival echelle spectra taken with the AAT (e.g. UCLES). Petr's note about the
  orders for a check on wavelength calibration are spot on. Reduction pipelines can sometimes misidentify orders, resulting in order-scale wavelength shifts
  that are quite bad, so it's useful to be able to pick up on those.

DAL/DM Running meeting - June 30, 2021

• PS: Order is more complex for Eschelle spectra - absolute order and relative order - useful for discovery and plotting so would like to see two utypes added.

IVOA - Spectrum Data Model change request - Oct. 15, 2021

• Group discussion
  • Petr gives some good text which I’ll use to seed the description in the document.

[lmichel]

You may try to find UCDs with the Vizier tool

http://cdsweb.u-strasbg.fr/UCD/cgi-bin/descr2ucd