Celestia Forums

Sorry to distrub you,but when I click in the link,the foloowing message appeared :
Object not found.The requested URL was not found in the server.The link on the referring page seems to be wrong or outdated.Error 404.
It?s strange because it?s the first time this happened to these links

danielj wrote:Sorry to distrub you,but when I click in the link,the foloowing message appeared :
Object not found.The requested URL was not found in the server.The link on the referring page seems to be wrong or outdated.Error 404.
It?s strange because it?s the first time this happened to these links

Thanks a lot danielj!

Indeed I had made a typo in writing the package filename: titan-clouds.zip instead of titan1k-clouds.zip.

Now it should work...

Bye Fridger

granthutchison wrote:

t00fri wrote:e.g. here in an official ESA-illustration of Titan's
anticipated atmosphere close to the south pole including
also a large , dark Methane cloud

Unfortunately, even the "official" illustrations generated by space agencies concentrate more on catching the eye than on accuracy. Saturn's rings are never so open when seen from Titan (its orbit is inclined at only a third of a degree to the ring plane), and the ring plane can never be so tilted relative to the horizon when viewed from "near the south pole" (since Titan rotates more or less in the plane of its orbit). Also, if this is a portrayal of the real Huygens mission, the season on Saturn (as shown by the ring shadows) is in error by quarter of a Saturn-year.
So I'm just guessing that they maybe didn't spend a hell of a lot of time trying to get the atmospheric transparency right ...

Grant

Grant,

sorry, in part, this was my mistake.

I had inferred (too quickly) that the image location was
close to the south pole region, since there the
occurrence of large methane clouds is most abundant.
But indeed, it must be close to the anticipated Huygens
landing site that is marked precisely on the latest Titan
texture from Cyclops. I went to the surface there within
Celestia to check: Saturn's orientation is quite OK, but it
is obvious (without checking, of course;-) that the ring
opening is far too wide...

The ESA-image has been specially prepared to illustrate the state of knowledge about Titan's /atmosphere/ to the media ...

I have not oriented my Titan texture in any way at such
media illustrations.

Bye Fridger

Fridger:

Thanks for the textures. They clearly took a lot of work and it is good to be able to represent them as "accurate" to my students. I presume at least one of them will be incorporated into the Celestia default. I opt for the almost opaque cloud texture with some hint of surface features.

Thanks again.

Frank

fsgregs wrote:Fridger:

Thanks for the textures. They clearly took a lot of
work and it is good to be able to represent them as
"accurate" to my students. I presume at least one of
them will be incorporated into the Celestia default. I opt
for the almost opaque cloud texture with some hint of
surface features.

Thanks again.

Frank

Frank,

well, I also use the "almost-opaque" texture myself for
the time being...

Provided we do not stumble over some hidden
copyrights, my Titan texture will indeed be part of 1.3.2.

If only I knew what surface colors are the most sensible.
We are now learning that the dark areas probably refer
to /water ice/ (the albedo of which may be quite low in
infrared light!) . Probably in a little while we shall know
much more on that crucial issue.

As Evil Dr. points out elsewhere (in agreement with
NASA statements), there is probably a fairly strong
similarity to Ganymede. Perhaps I'll do a Ganymede
clone, just for the fun of it...

Bye the way...

I have actually put 200 km for the atmosphere height in
my own ssc file, since that corresponds about to
the /realistic/ value of the haze thickness. It only looks a
little odd sometimes when viewed from certain angles,
but this is a Celestia artifact.


Bye Fridger

I'm baffled. We know you can't see through Titan's clouds in visible light - we have consistent theory and observations that prove it. And we work very hard at trying to make Celestia show as accurately as possible what you'd see with your own eyes.
So why the hell does everyone want semi-transparent clouds for Titan?

Grant

t00fri,

what's wrong with your Saturn texture ? On the picts you "pasted" in this topic, Saturn looks like all messed up.

granthutchison wrote:I'm baffled. We know you can't see through Titan's clouds in visible light - we have consistent theory and observations that prove it. And we work very hard at trying to make Celestia show as accurately as possible what you'd see with your own eyes.
So why the hell does everyone want semi-transparent clouds for Titan?

Grant

What's always annoyed me are those space-agency illustrations showing Saturn visible from the surface of Titan, when this clearly isn't true unless you have nonhuman eyes. They never seem to stop doing it.

granthutchison wrote:I'm baffled. We know you
can't see through Titan's clouds in visible light - we have
consistent theory and observations that prove it. And we
work very hard at trying to make Celestia show as
accurately as possible what you'd see with your own
eyes. So why the hell does everyone want
semi-transparent clouds for Titan?

Grant

Grant,

I knew you were going to protest. For that reason (and
others) I have added the "opaque" clouds texture in my
package.

Since quite a while, I /personally/ have a much wider
perspective as to the issue of which wavelength I am
going to use when watching Titan (and many other
objects in space!) from my little virtual "spaceship"...

If we did not cultivate this amazing amount of
"inertia" around here, it would long have been time to
contemplate a concrete "multi-wavelength" approach to
vision within Celestia!

For the time being I personally just carry these
"sun glasses" along on my "spaceflights" that feature
various filters...This is clearly a "primitive" compromise.

I find it close to ridiculous that I can just eliminate any
light pollution e.g. from street lights in my telescope with
a simple narrow band interference filter, and on the
other hand should not be allowed to see through Titan's
haze with an analogous device!

After all, Celestia is supposed to be a realistic space
simulation. Who of you would imagine to travel as far as
Saturn without taking filter devices along, just as Cassini
does!

As I wrote already earlier, I find it at least as
inconsequent to make Titan's haze entirely opaque and
then display its surface (after pushing "I") that has been
photographed in infrared light! Similarly with Venus...

If we had a sensible wavelength "switch" within Celestia
there would be no dispute whatsoever.

Bye Fridger

Cham wrote:what's wrong with your Saturn texture ? On the picts you "pasted" in this topic, Saturn looks like all messed up.

Cham, do you have ring shadows on your machine? I'm wondering if the "messed up" look is because you're not used to seeing ring shadows on Saturn - because Fridger's images look normal to me.

Grant

Cham wrote:t00fri,

what's wrong with your Saturn texture ? On the picts you "pasted" in this topic, Saturn looks like all messed up.

Cham,

I am confused. The illustration on the previous page does not involve my Staurn texture. I stole that image from the ESA site! They use it to illustrate the composition of Titan's atmosphere...

The ring opening of Saturn is clearly incorrect.

Bye Fridger

Matt McIrvin wrote:

granthutchison wrote:I'm
baffled. We know you can't see through Titan's
clouds in visible light - we have consistent theory and
observations that prove it. And we work very hard at
trying to make Celestia show as accurately as possible
what you'd see with your own eyes.
So why the hell does everyone want semi-transparent
clouds for Titan?

Grant

What's always annoyed me are those space-agency
illustrations showing Saturn visible from the surface of
Titan, when this clearly isn't true unless you have
nonhuman eyes. They never seem to stop doing it
.

I tend to agree with you largely, but not entirely.


All we know so far is that we cannot see to the surface of
Titan in visible light. However, the surface does not
contain any light sources that might well shine through
the haze!


So, who told you that I cannot make out at
least /something/ of the illuminated Saturn disc
when standing on Titan's surface??

Bye Fridger

t00fri,

look at the two last pictures on the first page of this topic. That's Saturn, from Celestia, I guess. There are all sorts of dark ovals and deformed circles on Saturn (not the rings). What's this ? Shadows ? This can't be ! It's all distorded. Just look at the two last pictures on the first page.

t00fri wrote:If we did not cultivate this amazing amount of "inertia" around here, it would long have been time to contemplate a concrete "multi-wavelength" approach to vision within Celestia!

I don't imagine you think I'm helping to cultivate the inertia, although it could be read that way.
1) I'm all for a multiwavelength approach, though I'm not sure how it could be implemented well, and I haven't really seen any concrete suggestions as to which wavebands would be selected and where we would obtain these data uniformly across all objects. The concept at present seems to involve too much handwaving for me to become actively engaged with it.
2) Given that we don't as yet have a multiwavelength Celestia, and we do at present try very hard to show people what they would really see with their own eyes, it seems to me that for the sake of consistency we shouldn't have transparent Titan clouds. If we're viewing Titan in IR, why is Saturn in the background in visible wavelengths?
3) We have a clear philosophical problem with the surface of Venus and now Titan, because we can't know what people would see with their own eyes, given that we can't strip off the atmosphere and take a look at visible wavelengths. In the past I've considered making Venus a featureless ball by default (perhaps with a suitable normal map, if one can be found), and offering "Radar reflectance" as an alternate texture. That would make me happy, but I do doubt if it would be well-received generally.

Some feedback on your texture bundle:
1) I very much like the Titan cloud and atmosphere colours. The current Celestia texture appears to be way too red, and this seems to be because it's effectively false colour - prepared from Voyager orange, blue and violet filters. The blueness of the high haze seen in Voyager views is probably really present, but again we're seeing a colour emphasis that isn't confirmed by the recent "true colour" Cassini image.
2) There are some dark bluish patches on the right side of your cloud texture that seem a little odd to me - certainly those at the top and bottom edge are creating wedge-shaped artefacts at the poles.
3) I know the atmosphere height of 200km looks a little odd, but I think this is just because we're not used to looking at a small low-gravity body with a dense atmosphere - I'd be keen to use that setting in Celestia.
4) The Voyager data put Titan's tropopause at around 40km, and the temperature minimum in that region suggests that's where the densest clouds would form. I've currently used 40 as the CloudHeight in my own setup - but do you have other information that would suggest a different altitude?

Grant

Cham wrote:t00fri,

look at the two last pictures on the first page of this topic. That's Saturn, from Celestia, I guess. There are all sorts of dark ovals and deformed circles on Saturn (not the rings). What's this ? Shadows ? This can't be ! It's all distorded. Just look at the two last pictures on the first page.

Cham,

what you are referring to are the shadows of Saturn's rings. These are perfectly correct. The great accuracy of Celestia's prediction of the ring shadows has now been confirmed on the first images of the Titan animation that I suggest interested people look at.
http://ciclops.lpl.arizona.edu/ir_index.php?id=6%20http://ciclops.lpl.arizona.edu/ir_index.php?id=6

What is a little off, is the color of the middle shadow ring. It is definitely bluish in reality...

Bye Fridger

t00fri wrote:If only I knew what surface colors are the most sensible.
We are now learning that the dark areas probably refer
to /water ice/ (the albedo of which may be quite low in
infrared light!) . Probably in a little while we shall know
much more on that crucial issue.

For my own, much cruder texture, I took a cue from Evil Dr. Ganymede and made the colors vaguely Ganymedean, maybe somewhat redder. But that's a pretty arbitrary guess.

I tend to mistrust any highly saturated colors in spacecraft photos (e.g. your Titan atmosphere color seems much more plausible than Celestia's texture, as Grant said).

t00fri wrote:

Cham wrote:t00fri,

look at the two last pictures on the first page of this topic. That's Saturn, from Celestia, I guess. There are all sorts of dark ovals and deformed circles on Saturn (not the rings). What's this ? Shadows ? This can't be ! It's all distorded. Just look at the two last pictures on the first page.

Cham,

what you are referring to are the shadows of Saturn's rings. These are perfectly correct. The great accuracy of Celestia's prediction of the ring shadows has now been confirmed on the first images of the Titan animation that I suggest interested people look at.
http://ciclops.lpl.arizona.edu/ir_index.php?id=6%20http://ciclops.lpl.arizona.edu/ir_index.php?id=6

What is a little off, is the color of the middle shadow ring. It is definitely bluish in reality...

Bye Fridger

Well, on the two last pictures of the first page, it doesn't make any sense. I see many distorded dark ovals on Saturn. Rings can't project shadows like these, unless the field of view is really that distorded. Something appears very wrong on those pictures.

Cham wrote:Well, on the two last pictures of the first page, it doesn't make any sense. I see many distorded dark ovals on Saturn. Rings can't project shadows like these, unless the field of view is really that distorded. Something appears very wrong on those pictures.

What may be disturbing you is that the shadows are being projected straight through the planet and appearing even on the night side, where we can see them because Celestia's artificial "ambient light" is turned up in the screenshots.

This is unrealistic; in a realistic view we would only see half of that oval, the half that is on the day side. The night side of Saturn really does get pretty significant ambient-like lighting from the rings, but the only ring shadow that appears from that light is along Saturn's equator, where the rings shadow themselves.

This is a known issue in Celestia. I could be wrong here, but I think that the unrealistic night-side shadows are already on somebody's to-do list for future versions...

granthutchison wrote:

t00fri wrote:If we did not
cultivate this amazing amount of "inertia" around
here, it would long have been time to contemplate a
concrete "multi-wavelength" approach to vision within
Celestia!

I don't imagine you think I'm helping to cultivate the
inertia, although it could be read that way.

Definitely NOT, Grant. I can perfectly see your point!

Actually, I think in the past the two of us (along with
Chris, of course) have been perfectly unanimous
in vigorously defending Celestia's basic design
philosophy of trying to model things as realistically as
possible!


Let me just point out a little subtlety here: I am --like
you-- clearly in favor of marking LOK boundaries on
textures, for example. In these cases all our
knowledge just ENDS at the boundary! But as to the
wavelength issue, things are slightly different. You seem
to adhere to the credo that we should refer to visible
light all the time. For me this is a rather artificial
limitation, since there are so many highly
valuable/instructive data in other wavelength bands. So
no LOK situation whatsoever!

Grant wrote:
1) I'm all for a multiwavelength approach, though I'm not
sure how it could be implemented well, and I
haven't really seen any concrete suggestions as to which
wavebands would be selected and where we would
obtain these data uniformly across all objects. The
concept at present seems to involve too much
handwaving for me to become actively engaged with it.

As I pointed out repeatedly, here is the point where we
have to make up our mind. In order to achieve real
progress with such a more challenging task, we
inevitably have to merge all our diverse know how and
DISCUSS actively. One 2 line post in one or two weeks
in the developer's list (as happens since quite some
time), will certainly be insufficient here...

Grant wrote:2) Given that we don't as yet have a
multiwavelength Celestia, and we do at present try
very hard to show people what they would really see
with their own eyes, it seems to me that for the sake of
consistency we shouldn't have transparent Titan clouds.
If we're viewing Titan in IR, why is Saturn in the
background in visible wavelengths?

OK, we obviously are not ready yet to display a fully
consistent picture.


But we do have beautiful pictures of Saturn in IR light
and there are many more to come. So a simple IR switch
could display BOTH Saturn and Titan in IR. Phantastic! All
we got to solve really, is what is to happen, if there are
no respective data available for an object....

Grant wrote:Some feedback on your texture bundle:

1) I very much like the Titan cloud and atmosphere
colours. The current Celestia texture appears to be
way too red, and this seems to be because it's
effectively false colour - prepared from Voyager orange,
blue and violet filters. The blueness of the high haze
seen in Voyager views is probably really present, but
again we're seeing a colour emphasis that isn't confirmed
by the recent "true colour" Cassini image.

2) There are some dark bluish patches on the right side
of your cloud texture that seem a little odd to me -
certainly those at the top and bottom edge are creating
wedge-shaped artefacts at the poles.

For the same reason I left out the blue upper
atmosphere color as suggested from Voyager and
apparently not visible recently.


Did you really download my most recent clouds
addendum?? The dark patches on the rhs of the haze
texture are gone, there is an opaque clouds texture
specially for you and the haze color is virtually
in /perfect/ agreement with the true-color Cassini photo.

Grant wrote:3) I know the atmosphere height of 200km looks a little
odd, but I think this is just because we're not used to
looking at a small low-gravity body with a dense
atmosphere - I'd be keen to use that setting in Celestia.

4) The Voyager data put Titan's tropopause at around
40km, and the temperature minimum in that region
suggests that's where the densest clouds would form.
I've currently used 40 as the CloudHeight in my own
setup - but do you have other information that would
suggest a different altitude?

I have put 70 km in my most recent 'titan-clouds.ssc' in
the clouds-addendum package for the simple reason that
200 km looks a little strange under some angles. But this
is due to a Celestia artifact. I have emphasized however
a few posts higher up that one should really take 200
km! So we agree here.


I put tentatively 3 km as the CloudHeight which I took
from a recent ESA caption. But no, there is no solid info,
I think.

Bye Fridger

Cham,

If it helps, here's a picture similar to Fridger's but with ambient light turned off.



For a Cel://URL to take you to this viewpoint, please visit http://www.lns.cornell.edu/~seb/celestia/gallery-001c.html#5a