Hi Pengfei,

I'm not able to reproduce what you're seeing, but before I go into that, I can explain a few things about how the cosmology calculator works.  In yt, we are only able to define a comoving unit system in relation to a proper unit system at a specific redshift.  We are unable to define a comoving unit system that is the base unit system.  This is a problem for the cosmology calculator, which only calculates comoving quantities.  Therefore, in this case, we are forced to set the comoving and proper unit systems to be the same.  You can see this for yourself by running the following script:

from yt.utilities.cosmology import Cosmology

co = Cosmology()

print (co.comoving_radial_distance(1.92, 2).to("Mpc/h"))

print (co.comoving_radial_distance(1.92, 2).to("Mpccm/h"))

Both will be the same answer.

Now, back to the issue you're seeing.  I ran the following script:

http://paste.yt-project.org/show/6741/

to calculate the redshifts dumps needed for a 80 Mpc/h box to go from z = 2 to z = 1.92 and got the following result:

CosmologyOutputRedshift[0] = 2.000

CosmologyOutputRedshift[1] = 1.926

This is pretty close to what you expected.  Additionally, if I run the following script, 

http://paste.yt-project.org/show/6743/

with the above redshifts added to the parameter file, I get that dz_max (the change in redshift when moving 80 Mpccm/h at z = 0) of 0.0746521607364, which also seems to agree with what you are finding.

Can you share with me the simulation parameter file and script that you're using to make your light rays?  I can take a look at that and see if I still get what I expect.

Britton

On Mon, Aug 8, 2016 at 6:13 AM, Pengfei Chen <madcpf@gmail.com> wrote:

Hi all,

I was trying to generate light rays using yt 3.4-dev, but found that the light rays have wrong redshift intervals. For example, my simulation box is 80Mpccm/h per side, and I expect a light ray generated in a data dump at z=2 with length =1 simulation unit (i.e. 80Mpccm/h) would have a redshift interval from z=2 to z=1.92. However, the light ray generated by yt gives spans from z=2 to z=1.97.

As I look into this, I found that the function comoving_radial_distance in cosmology.py might return the wrong unit. I think the return value should be in comoving units, instead of physical units. To see this directly, I made the following change in cosmology_splice.py:

(root) ~/yt-conda/src/yt-hg/yt/analysis_modules/cosmological_observation $diff cosmology_splice.py cosmology_splice.py0

373d372

<             print target_distance, distance2, z2

And it shows:

80.0 Mpccm/h 4.65320708035e+26 cm 1.97387969592 dimensionless

80.0 Mpccm/h 1.19455177073e+26 cm 1.97343368065 dimensionless

80.0 Mpccm/h 1.21507519065e+26 cm 1.97342592996 dimensionless

Then I made the following change in cosmology.py:

(root) ~/yt-conda/src/yt-hg/yt/utilities $diff cosmology.py cosmology.py0

111,112c111,112

<         return self.quan((self.hubble_distance() *

<                 trapzint(self.inverse_expansion_factor, z_i, z_f)).value, 'cmcm')

---

>         return (self.hubble_distance() *

>                 trapzint(self.inverse_expansion_factor, z_i, z_f)).in_base(self.unit_system)

Then I get:

80.0 Mpccm/h 4.65320708035e+26 cmcm 1.92163908776 dimensionless

80.0 Mpccm/h 3.62771515661e+26 cmcm 1.92124702027 dimensionless

With the change of unit from cm to cmcm, the light rays have the right span.

Even though this solves my problem, I am not sure if similar problem still exists. For example, instead of making change in comoving_radial_distance, we might need to change hubble_distance into comoving units. Hopefully someone familiar with yt unit system could check this.

Thanks,
Pengfei

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