Celestia Forums

Hello all,

Although very beautiful (thank you Astroboy) the California Nebula is only a thousand years from earth. This means I see it clearly from the solar system which I confess, I don't like.

Is there any way I can alter the apparent size so that I can better simulate what exactly one can see from the earth (which is just a glimpse and even so only under good conditions)?

Mario

Mario,

If you change the Distance from 1500ly to 1000ly, you need to reduce the Radius in the same proportions, from 47 to 31.

The California Nebula is quite large in the sky: almost 3 degrees long, and slightly wider than the full moon. It is difficult to see because it is so dim. Astroboy's image is taken from a long exposure photograph, which makes it look much brighter. Unfortunately, Celestia does not (yet) provide brightness controls for Nebulae.

Also, for what it's worth, SEDS says that the nebula probably is illuminated by Xi Per. Celestia shows that star to have been measured by Hipparcos to have a distance of about 1772 ly. I think this suggests that the nebula is rather more than 1000 ly away, but closer to us than the star. (If it were further away than the star, we'd see it as a reflection nebula instead of as an emission nebula.) Many of the old distance guestimates have been thrown into confusion by Hipparcos.

Does this help?

Thank you Selden. It did help.

In fact, I had just been from SEDS description of this nebula when I wrote my email and followed it's "suggestion" of 1 kly without thinking. You are right, the nebula is at 1.5kly from earth in Celestia.

I decided to not change its radius even at 1,5000 ly to make up for that fact its is so visible under Celestia. Since the problem is due to brightness, I can live with it knowing that sooner or later that feature will be added into Celestia.

Thank you for your explanation, Selden

Mario

Though not within Celestia, brightness control is very simple. Just open the .png in Photoshop and adjust the transparancy of the picture: the lower the opacity, the darker the nebula.
This is just about the first thing I do with just about every billboard-texture I download. My M42 is at about 40% opacity, my california nebula much less. Imo it looks more realistic.

Hmm... good tip julesstoop. Thank you.

I don't have Photoshop. But I do have PainShop Pro and it should do the trick.

However this raises the following question:
Astroboy's nebulas are in 3ds format. He does provide the png/jpg files along with the download (which I unzipped to the mesh folder anyways). So what I should do is to change the dsc file so that instead of a mesh, I use a texture and point that texture to the png file. Right?

Mario

Mario,

The 3DS file used for a Nebula defines the shape of the object and specifies the texture image file(s) that should be applied to it. Celestia does not (yet) support specifying the image of a Nebula in any other way.

Astroboy's 3DS model of NGC1499 specifies the texture filename NGC1499.PNG. You can use either a 3D modelling program or a hex binary file editor to change that, or you can simply provide a replacement image that has the same name and type.

Under Windows the capitalization doesn't matter, but Linux would require the name of the image file to spelled with all caps to match the specification that's in the 3DS file.

If you use a hex binary file editor, you can change the file spec to be NGC1499.* and then Celestia will let you use a PNG, JPG or DDS texture image file. (This requires Celestia v1.3.0 or later.)

Thank you Selden.

I thought the image files Astroboy delivers within his zip files were just an extra. Now I know better.
I'll stop making gratuitous questions I know you provide on your website a very good tutorial on all this. Simply because I don't plan to be a Celestia artist, doesn't mean I shouldn't read it when I have doubts about something related.

Cheers,
Mario