[yt-dev] Photon fields

[Matthew Turk]

On Fri, Jun 15, 2012 at 12:11 PM, Eric Hallman <hallman13 at gmail.com> wrote:
> John and Matt,
>   so this sort of thing was on my to do list for a Chandra theory grant I'm
> currently on.  If something is already working or in the works, I'm happy to
> defer, and even help implement it.  I have a copy of PHOX, which I can run
> and do things with, but had planned to eventually incorporate into yt (with
> Matt's help).
>
> In the more general sense of generating photon fields, I totally support
> this, and would find it useful for all kinds of projects with synthetic
> observations.  I think your intuition is right on, that even though observed
> photons are along an particular direction, a "photon emissivity" in 3d in
> some band or set of wavelengths could function exactly as other emissivity
> type grid fields.
>
> Anyway, let me know what methodology you had in mind, and if you'd like an
> assistant ;)
> I have done a few of things with stand alone codes before, and they sort of
> worked.

As a first pass, would a 1:1 mapping between cells in the *volume* and
photons be a good approximation?  If so, we could implement this as a
pretty generalized yt solution and go from there.

>
> Cheers,
>
> Eric
>
> On Fri, Jun 15, 2012 at 11:59 AM, Matthew Turk <matthewturk at gmail.com>
> wrote:
>>
>> Hi John,
>>
>> Hopefully our resident X-ray observation syntheticist Eric Hallman can
>> also chime in, but I have a few comments.
>>
>> On Thu, Jun 14, 2012 at 3:33 PM, John ZuHone <jzuhone at gmail.com> wrote:
>> > Hi all,
>> >
>> > Just wanted to sort of poll all of you on an idea I've been kicking
>> > around. I don't know how useful it will be for non X-ray people, but I
>> > thought I'd give it a shot.
>> >
>> > There are a number of tools out there designed to create synthetic X-ray
>> > observations from simulations, both AMR and SPH. Of the tools adapted for
>> > AMR codes, they tend to be a little clunky and most of them use IDL to
>> > extract data. Almost always, gas grid data is regridded to a uniform grid
>> > before doing anything with it, making working on large volumes with small
>> > resolution prohibitive.
>> >
>> > PHOX is a tool written by Veronica Biffi and Klaus Dolag to generate
>> > X-ray observations from (primarily) Gadget simulations. You can find out
>> > about it here:
>> >
>> > http://www.mpa-garching.mpg.de/~kdolag/Phox/
>> >
>> > The pipeline for most virtual X-ray observatories goes like this:
>> >
>> > 1) Using the gas properties (density, temperature, metallicity,
>> > whatever), generate X-ray emissivity fields.
>> > 2) Project these fields along the chosen line of sight to give a 2D map
>> > of surface brightness.
>> > 3) Using this map as a DF for the photons, generate simulated photons
>> > which then may be processed later.
>> >
>> > PHOX's (in my estimation, better) approach is:
>> >
>> > 1) Using the gas properties (density, temperature, metallicity,
>> > whatever), generate X-ray emissivity fields.
>> > 2) Generate simulated photons from the emissivity field, assuming a
>> > fiducial instrument area and exposure time.
>> > 3) Project these photons along a given line of sight, which may be
>> > processed later.
>> >
>> > The nice thing about doing things this way is that you can save the
>> > photons to disk to be re-used for a variety of different projections and
>> > X-ray instruments.
>> >
>> > I am writing a tool that uses yt to get data from AMR simulations to
>> > generate photon files readable by PHOX. My intention is that this be a
>> > standalone tool that I will make publically available, but it got me
>> > thinking about the possible usefulness of "photon fields" in yt, akin to the
>> > 3D fields of photons that are written to PHOX photon files. Basically, these
>> > are akin to particle fields, with the (x,y,z) position of the photon, its
>> > (unshifted) energy, and the (vx,vy,vz) velocity of the emitting gas for
>> > Doppler shifting. I don't know if the more general concept of these photon
>> > fields would be a useful construct to include in yt at some level or not for
>> > constructing simulated observations.
>>
>> Are the photon files not 1:1 mapping with the individual cells?
>>
>> In general, I am 100% on board with this.  This speaks also to the
>> concept we've talked about in the past of 'sidecar' files in, say,
>> Grid Data Format.  You could specify a fallback file for IO, which can
>> also be written to.
>>
>> I think this is a very valuable concept, and we can work within the
>> PHOX goals to make something that could accept, say, contour
>> identification or expensive derivative-based fields (looking at you,
>> baroclinic terms.)
>>
>> >
>> > The main difference between these fields and others in yt is that these
>> > entities are slightly more abstract than physical: real photons are of
>> > course emitted in a particular direction and received by the observer. But I
>> > would guess that this subtle point would not be much of an issue since
>> > really the only thing you would be doing with them after generating them
>> > would be projecting them along a line of sight.
>>
>> No, I think it is fine, like you say.
>>
>> This could be very exciting -- and in general, I'm really very much in
>> favor of making it easier to interoperate with other analysis codes.
>> This mailing list would be a good place to sound off on design ideas
>> as well as figure out how to fit it in together.  yt gets used a lot
>> by cluster simulators, so I could see this also having broad appeal.
>>
>> -Matt
>>
>> >
>> > So I guess my idea would be to build in the framework for photon fields,
>> > along with capabilities for projecting them and doppler shifting them, while
>> > letting the details of the generation of their positions and energies up to
>> > the user. Would any of the rest of you or your colleagues find such a thing
>> > useful? Thoughts?
>> >
>> > Best,
>> >
>> > John Z
>> > _______________________________________________
>> > yt-dev mailing list
>> > yt-dev at lists.spacepope.org
>> > http://lists.spacepope.org/listinfo.cgi/yt-dev-spacepope.org
>> _______________________________________________
>> yt-dev mailing list
>> yt-dev at lists.spacepope.org
>> http://lists.spacepope.org/listinfo.cgi/yt-dev-spacepope.org
>
>
>
> _______________________________________________
> yt-dev mailing list
> yt-dev at lists.spacepope.org
> http://lists.spacepope.org/listinfo.cgi/yt-dev-spacepope.org
>