1. Comments on P. Hickson “TMT image quality at Mauna Kea and La Palma”
Document discusses effect of removing turbulence larger than certain length scales
It is partly correct in that it misses essential physics for quantifying ingress of turbulence into TMT enclosure
Result therefore is simplified worst-case scenario, but once other real world effects are added, we find:
Turbulence in the optical path is dominated by turbulence created by the enclosure, structures, instrumentation and telescope
Some input assumptions must also be modified
Full analysis cannot be done analytically; TMT results from detailed computational fluid dynamics (CFD) simulations (incl. validation work)
Specific comments
The vents are 2-dim filters, not 1-dim; the dividers between them are sufficiently wide for that
Vents and enclosure opening are not simply high-pass cut-on/off filters (in frequency)
Even if they were, relevant length scale is <~= half the dimension of the vents
In projection, only a few of the vents are full size, most of them are smaller reducing relevant length scale below that assumed in Hickson document
Edges add both mechanical turbulence and temperature gradients, erasing most of the incoming turbulence
The same is true for every structure (instruments, AO systems, Nasmyth structures, etc.) inside the enclosure
Once the air reaches the optical path, it has been strongly modified
Air from the lower vent layer assists the stability of the flow pattern, but for the most part of AZ/EL combinations it will never enter the optical path
Only a few vents get direct flow-through air, most of the air will go around the enclosure
Some fraction of the air is compressed and goes over the opening; this has, on average, higher steady gradients, but is less variable
We also disagree with two of the surrounding assumptions / statements
7-m seeing at ORM is not 0.94 arcsec; Racine formula cannot be used for this
Seeing is not universally more important for AO; this is only true under certain circumstances which do not apply in general to TMT