Celestia Forums

I released the Helix nebula for celestia, but I still need some help.

1)How is this nebula oriented in the sky ?
2)Wath is his exact RA, Dec & Distance?
I use :
RA 22.4933333
Dec -20.8
Distance 45
but don't think they are very precise.

Heres some shots of it...

AstroBoy,

One way to get the orientation and position correct is to properly orient a photograph and then match your model to the picture.

These adjustments are needed because the accepted astronomical coordinates of an object do not necessarily match the centroid either of its photograph or of the model that you've designed.

Here's what I'd do:

First, place a photograph of the nebula, including the surrounding stars, within Celestia. My "Billboard" object can be used for this, or you can construct one of your own. See http://www.lepp.cornell.edu/~seb/celestia/billboard.html

Use a telescopic viewpoint from the vicinity of the Solar System to look at it. (Goto Earth, Track object, increase magnification with the "," command until the object fills your field of view appropriately.)

Set the Axis & Angle of the picture's DSC definition so it is face-on to the solar viewpoint and rotated into the correct position against the background stars. You can use Celestia's builtin positioning tool to determine these values. Chris described how to invoke this tool in the thread http://63.224.48.65/forum/viewtopic.php?p=11939#11939. I quoted the relevant paragraph at http://www.lepp.cornell.edu/~seb/celestia/celestia_notes.html#align.

Then edit the DSC file multiple times, adjusting RA, Dec and Radius values until the stars of the photograph precisely match those which Celestia draws in that region. You have to exit and restart Celestia each time you modify the DSC file. I usually use a Cel://URL to start Celestia with the appropriate viewpoint. In some cases I've had to install Pascal's 2million star database in order to have enough stars nearby. See http://perso.wanadoo.fr/celestia.stars/index.html

Once you've managed to get your photograph oriented correctly, add your model as the second object in that same DSC file. This ensures that the picture is drawn first and then your Nebula model is drawn in front of it. Celestia does not yet depth-sort Nebula objects.

Then use the same procedures as above to change the model's orientation and position until it precisely matches the photograph.

Finally, you can comment out the DSC entry for the photograph and your model will be shown at the right place.

I hope this helps.

p.s. I've now added this description of the procedure to the billboard page mentioned above.

Thanks for your help, selden.
I added the central star 13th magnitude white dwarf.
Here's the final result in Celestia compared to the real one...





Soon available for download.
Let me now what you think about my work...

Oups, I'm really sorry for the "double reply"... Just a newbie's misstake !!!
Never again, I promise !!

I'm not 100% certain (because you don't have enough stars visible in the sky) ... but it looks as if the nebula is upside down. Or rather flipped, around an axis of about 45 degrees through that picture.

Oops wasn't logged in.

Anonymous wrote:I'm not 100% certain (because you don't have enough stars visible in the sky) ... but it looks as if the nebula is upside down. Or rather flipped, around an axis of about 45 degrees through that picture.

I think it's rotated 180 degrees, or flipped both vertically and horizontally.

Andrea

I don't think so ... take a look at this pic, I've highlighted mirrored features:

Stupid BBS keeps logging me out

Astroboy... keep going with this..it looks fantastic. One thing...when it's finished, can you put in a star cluster? I just think it looks great when you can orbit a star (with a system) and see the animation.

Regards..bh.

Anonymous wrote:I'm not 100% certain (because you don't have enough stars visible in the sky) ... but it looks as if the nebula is upside down. Or rather flipped, around an axis of about 45 degrees through that picture.

You are right.
If you go to my page
http://www.ara-frasso-sabino.org/andrea_celestia.htm
you'll see the elaboration of the image, obtained with Photoshop.

By

Andrea

I'm not 100% certain (because you don't have enough stars visible in the sky) ... but it looks as if the nebula is upside down. Or rather flipped, around an axis of about 45 degrees through that picture.

In fact, the nebula was pointing backward the sun instead of towards !!
It has been corrected now.

Astroboy... keep going with this..it looks fantastic. One thing...when it's finished, can you put in a star cluster? I just think it looks great when you can orbit a star (with a system) and see the animation.

Unfortunately the nebula disappears when seen from closer than 0.5ly !! So when I go to the central star, it's invisible !!! Still don't now why...

Here's what append when approching:

I think its because you fly into the mesh.

But that's not exactly a bad thing ... I'm told that if you were inside a nebula, you wouldn't actually see the glowing gasses because you'd be too close to them, that the effect is only really noticable from a distance.

like fog or clouds??

Something like that. You see the effect of the nebula because of the relative density of the gasses in your field of view. If you're in the nebula itself, those gasses are all around you, not focused in one area, so the fact that they are glowing is less apparent. There may be areas that are brighter in the sky than others, like whispy strands of faintly glowing gas that you might be able to see, but the beauty of the nebula as a whole would be lost on you.

You can now download it on my Celestia Add-On page :

http://membres.lycos.fr/tompouce00/

Enjoy, and send me your comments !

Here is a quote from Leicester University Astronomy Society Website;

NGC7293, The Helix Nebula
The Helix Nebula is the largest and probably the closest planetary nebula. Although the Helix Nebula has a magnitude of 6.3, it is a massive 12x16 arcmins - that is half the apparent diameter of the Moon! The nebula is a spherical shell surrounding extremely hot dwarf star of magnitude 13. The true diameter of the Helix Nebula is about 1.75 light years, at a distance of 450 light years from us. The Helix Nebula reaches a maximum altitude of just over 16°, and is best observed at around 10pm on a clear night in October.
Map, RA=22h29.6m, Dec=-20°48'

Therefore going by the above figures would it not be correct for your STC file to read;



and your DSC file to read;



You will now find that your nebula does not disappear at around 0.5 LY and that you can see the nebula around you when you visit the central star!

Regards
TERRIER

You will now find that your nebula does not disappear at around 0.5 LY and that you can see the nebula around you when you visit the central star!

Yes, thank a lot !! The distance I used was wrong. This have now been corrected.

I've fixed the clipping problem for 1.3.1, so now small version at the wrong distances looks fine. Still, I'm glad to see it in the right location now.

--Chris

CHRIS
Great stuff! I thought it must have been a bug for viewing smaller sized objects.

ASTROBOY
The only prob now with these new settings is the nebula looks smaller if seen from earth. ie go to the space shuttle launch model (if you have it)and view the sky.
Your original figures made the nebula look the right size of about half the moons diameter.

Does this suggest the radius of the nebula should be increased?

I have checked out the NASA site and they suggest the nebula is SMALLER at 1.5 light years diameter (0.75 LY radius).

CHRIS
Can this mean that CELESTIA be used to prove it's actual radius!!!!???

Regards
TERRIER