Circles of Redshift values

[selden]

People often don't realize just how nonlinear the redshift measurements are. Thia Addon provides circles showing the locations in the universe where redshift values go from a z of 0.01 out to z=1000, in both Equatorial and Galactic coordinate systems.

This addon goes well with the 2dF galactic redshift survey, providing a feel for the relationship between z and the age of the universe.

http://www.lns.cornell.edu/~seb/celestia/redshift.zip (8KB, 6Mar04)



(as usual, this links to a much larger image)

This somewhat cluttered picture shows the pink redshift circles in Galactic coordinates, accompanied by a few other Addons.

See http://www.lns.cornell.edu/~seb/celestia/graticules.html for additional information.

[Cham]

I can't wait to have a Mac OS X version of Celestia which can run those features. In 1.3.1, I just can't use your new addons.

[don]

Hi Selden,

Just tried this one on my system (ATI 9700 card) and it doesn't work due to the large radius values. Any chance of a version with scaled down radius values?

Thanks.

[selden]

Don,

The Radius values in the RedShift DSC files are "the real thing."
In other words, they are the apparent distances in LY of objects which have those z values. All of my other deep-space CMOD Addons include Radius values which correspond to their maximum apparent distances, too.
As a result, I'm very reluctant to rescale them to something non-physical.

That should not stop you from doing it, though. Just be sure that you
change all of the Radius values in the deep space DSC catalogs that use CMOD models by exactly the same factors (multiply them all by the same fraction). This includes the "Graticule" Addons. Otherwise the distances shown by the RedShift circles won't be meaningful.

In other words, in all of the Radius distance values, you could replace e10 by e5, e9 by e4, etc. This is equivalent to multiplying all the distances by 1/100,000.

Please post the values that you find you have to use, and remind us what graphics card you have. This may help to find appropriate scaling factors that Celestia should be using internally. It could be, for example, that Celestia isn't taking proper account of the depth of the Z buffer that's available in different models of graphics cards.

[don]

As a result, I'm very reluctant to rescale them to something non-physical.

I understand.

Just be sure that you change all of the Radius values in the deep space DSC catalogs that use CMOD models by exactly the same factors (multiply them all by the same fraction).

Right.

Please post the values that you find you have to use, and remind us what graphics card you have. This may help to find appropriate scaling factors that Celestia should be using internally. It could be, for example, that Celestia isn't taking proper account of the depth of the Z buffer that's available in different models of graphics cards.

As mentioned above, my card is the ATI 9700 (Pro). From what I've been able to find on the web, it's z-buffer depth is 24-bit, which means it's limit is 1.6777215e+7, right?

Older ATI cards use a 16-bit z-buffer depth (6.5535e+4 ?). I'm not sure about other mfg cards. Obviously, recent nVidia cards (and maybe other cards) use a 32-bit z-buffer depth, or nobody would be able to use these add-ons.