M. Todd, C. Compton, S. Dwyer

Over the past few weeks I've been trying to understand what we can learn from some of the ETMY ring heater power changes that we've made over the past 9 months or so. By looking at the Higher Order Mode spacing from OMC data as well as substrate lensing from the HWS data, we hope to be able to make some models or estimates of the coupling factors of the different actuators to substrate and surface defocus.

In particular, we want to use the HOM spacing to estimate how much of the surface defocus comes from self heating.

This derivation is better illustrated in the attachment below, which has a neater write up of the following logic:

1) Known values : HOM spacing, FSR, Cold-State Surface Curvature, Ring Heater Power, Coupling Factor of Ring Heater Power to Surface Defocus, g-factor of the ITM and ETM

2) Desired values: Surface Defocus from Self-heating (coupling * absorbed power)

Through solving some of the equations relating these values, with the assumption that our coupling factors are correct, we estimate the self-heating to change the surface curvature by about 13%.