### Re: [arts-dev] Transit radius

```
Stefan,

Is there a smarter way?

Not inside ARTS itself.

And, for the way I outlined, what is the currently recommended way to get out
tangent altitude and opacity along the los?

Note that you don't need to make a Tb calculation, you can calculate the
transmission directly by iyTransmissionStandard. A bit quicker and you
can include particles.

The method TangentPointExtract extracts the tangent point data. First
element of the vector returned is the tangent altitude (or is it still

Bye,

Patrick
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### Re: [arts-dev] Transit radius

```Dear Stefan,

If you have a controlfile to compute the od and tangent altitude

it should be fairly easy to wrap this into a python function and

use scipy.optimize.root to find the zero of od - 1.0.

Cheers,

Simon

From: arts_dev.mi-boun...@mailman.rrz.uni-hamburg.de
on behalf of Stefan Buehler

Sent: Friday, March 9, 2018 3:17:42 PM
To: ARTS Development List
Subject: [arts-dev] Transit radius

Dear all,

here is an ARTS challenge, not strictly important right now, but for my own
curiosity. Probably mostly to Patrick, but perhaps interesting also for others.

For exoplanet research, one observation technique provides the planet's transit
radius, that is, the tangent radius where the optical depth of the limb path is
one.

Can we generate this with ARTS? The simple-minded way I can come up with is to
do a limb simulation and store opacity for each pencil-beam, along with its
tangent altitude. Then interpolate to find the tangent altitude for which the
opacity along the limb path is one.

Is there a smarter way?

And, for the way I outlined, what is the currently recommended way to get out
tangent altitude and opacity along the los?

If it is easy to generate, I feel tempted to include a “transit radius” view to
the nadir spectrum figure in the paper.

Best wishes,

Stefan

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```