Re: LIghtcone observer coordinates

From: Volker Springel <vspringel_at_MPA-Garching.MPG.DE>
Date: Wed, 13 Oct 2021 18:33:54 +0200

Hi Robin,

> On 11. Oct 2021, at 13:49, Robin Booth <> wrote:
> Hi Volker
> One of the things we would like to do in a future GADGET-4 run is to generate multiple lightcone realisations from the same simulation box by repositioning the observer in the box for each of the realisations, e.g. at each of the 8 vertices of the simulation box (q.v. DiRAC RAC 14, project 3.5). The GADGET-4 code paper (Springel et al 2020) implies that this feature exists in the code:
> Section 7.5 : "An arbitrary coordinate within the simulation box can be selected as an observer position.
> One or several lightcone geometries can then be defined through their starting redshift zstart,
> their ending redshift zend (which can be zero as in the sketch of Fig. 41,"
> but if that is the case, it does not seem to be documented and from a cursory examination of the code in, it does not appear that the code handles the reading of observer coordinates as a parameter option.
> I would be grateful if you could comment on whether this was a planned feature that has not yet been implemented, or perhaps I am missing something in the code?

You're right, at the moment the lightcone observer position is still hardcoded to the default (0,0,0). Changing this to another position is trivial, but allowing for multiple lightcones with different observer positions in a single run is slightly more involved as it requires to keep additional information for the particles stored in the lightcone buffer (namely to which observer position they belong). Especially if one in additions wants to be able to find halos on these different lightcones on the fly, a few small code changes are required. I wanted to originally implement this but had postponed it for simplicity. If there is a demand for this feature, I can however realize it.


> Regards
> Robin
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Received on 2021-10-13 18:33:55

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