Pan out of Encke

[abramson]

Dear Celestians,

Today I observed that Saturn's moon Pan is not in the Encke division (as it should) but a little towards the inside of A ring. I do not know what happened, I distinctly remember that Pan was correctly located in previous versions. I installed 1.3.1 final recently, and wonder if there is a bug either in the rings or in Pan's orbit (which is correctly defined in minormoons.ssc as 133583km).

Anybody else has seen this? Where do you see Pan, with 1.3.1 final?

Regards,

Guillermo

[JackHiggins]

Well, pan looked fine for me when I installed 1.3.1final first... Have you changed your ring texture to a different one, without updating solarsys.ssc...?

I'm using:

Code: Select all

   Rings {
   Inner   74000
   Outer  141000
      Texture "saturn-rings.png"

      # Bjorn Jonsson <http://www.mmedia.is/~bjj> recommends this
      # color for rendering Saturn's rings.  No idea if it's right,
      # but it looks nice
      Color  [ 1.0 0.88 0.82 ]
   }

With Jens' rings, from http://home.arcor.de/jimpage/saturn.html (Was 8k, scaled down to 2k)

[selden]

Guillermo,

It looks fine to me.

Saturn's rings and their locations have been improved in Celestia v1.3.1 final. If you're using a modified solarsys.ssc or ring image, that might explain what you're seeing.

Here's what I see with a version of Celestia that only has had all of the Locations files added:

[granthutchison]

Yes, there has been a change to the ring textures in 1.3.1 final. Uranus needed fixed because it was wrongly proportioned, and this led me to wonder why our ring textures were defined with bands of blank space at the edges - it just means that to position the rings precisely, the Rings definition has to be set up using a calculation to allow for the number of clear pixels at the inner and outer edges of the texture, rather than by simply entering the inner and outer margins of the rings themselves. So I trimmed off the space and tightened the Rings definitions from their previously rather imprecise settings. The current solarsys.ssc Rings definition for Saturn is set up like this:

Code: Select all

      Inner   74660
      Outer  140220

Jens' rings texture (which includes blank space at its edges) comes with the instruction:

Code: Select all

2. Edit with notepad 'solarsys.ssc' and change following lines in Saturn (ring) section.

   Inner   74000
   Outer  141000

so you'll find his rings are misaligned if you use them with the default Celestia solarsys.ssc.

Grant

[jim]

Hi Grant,

I think it's time to clarify some things about my Saturn rings. I'd started with the aim to build this rings as most precise as possiblbe by useing for all available information and pictures on the net. I'd uploaded 34 pictures and used only the best to extract texture parts for the rings. I'd tried also to pay attention to the different lighting on the pictures and compensate this as good as possible. Futher it was necessary to correct some optical distortions of the pictures. The next step was to position the single rings as precise as possible. I'd used following data to calculate
the possitions with an excel table.



You can see the reason why Pan is not in the middle of Encke gap. But what shall I do? These are the most excact values I found. Now the problem is that the possistion of rings and gaps is slightly influenced by saturn's moons. Further I'm not sure how accurate is Pan's orbit.

Now some words to the blank space at the edges of my ring texture. While I'd prepared the pictures I notice that there are still some visible pixels outsite the "official" borders of f- and c-ring. I was not sure how many space I would need therefore I expand my rings to the next round values. After the texture was fineshed I decide not to cut off this small blank space (less than 2% of the hole texture) to aviod additional resize distortions.

Why did I make an 8k ring texture? I'd known that Celestia can handle maximum a 4k ring texture (depend on the grafik card) but some raw material (less than 20% of the texture) allows to build 8k rings. Maybe a future version of Celestia can benefit from this.

So I trimmed off the space and tightened the Rings definitions from their previously rather imprecise settings.

I hope now you can unterstand that I was not very happy to read on developer mailing list about your modifications. Apart from the additional expenditure to install the highres texture my Rings definitions should be for away from being "rather imprecise".

Bye Jens

[granthutchison]

I hope now you can unterstand that I was not very happy to read on developer mailing list about your modifications. Apart from the additional expenditure to install the highres texture my Rings definitions should be for away from being "rather imprecise".

Jens:
You've misunderstood me. Of course I know your rings are very accurate - you'll maybe remember that I enthused about your ring texture in a message to Chris which I copied to you.
But the empty space you've left at the margins of your texture means that you have to provide Inner and Outer values that are not the precise edges of Saturn's rings - they're just the margins of your texture. To a user who hasn't inspected the texture, but who cares about accuracy, this is confusing, and the temptation is to "correct" your values to the real-world numbers, which then throws your texture into misalignment. Also, if we put your figures into solarsys.ssc, this then pushes other add-on creators towards abiding by your choice of ring margins. (Some, who understand things less well than you, might even fill up the whole space with ring texture.) Surely if we're going to impose such figures on other people they should be Saturn's own choice, rather than the end result of your personal approach to texture design?

So I trimmed off the spaces and entered the appropriate numbers for the real-world ring margins, so that I could have the best of both worlds - your texture and a solarsys.ssc definition that gave the correct values. (Since I was resizing from 8k down to 1k for the Celestia distribution texture, I had no fears that I was going to introduce any additional degradation of your texture by slightly trimming its edges before resizing!) Then I carefully checked, ring by ring, that I had not disturbed your texture and rendered it inaccurate - I did this by sending test objects into orbit along the ring margins and comparing their positions between my modification and your original set-up. Only then did I commit the new texture and the associated solarsys.ssc modifications.
I'm very sorry (but a little surprised) to hear that you were unhappy about this - it seems like a completely neutral change from your point of view, and a marginally beneficial one for the readability of solarsys.ssc.
Why didn't you just e-mail me at the time?

As to your comments about Pan, I'm not sure what you mean. Guillermo's problem has probably arisen (I think, but we need to hear back from him to be sure) because he has mixed your original ring texture with my Inner and Outer definitions from solarsys.ssc, which of course causes problems. There are certainly no problems with Pan, though, if your ring texture is used the way you describe in your readme, or if the Celestia installation is used with its default textures and definitions.

Grant

[jim]

Hi Grant,

But the empty space you've left at the margins of your texture means that you have to provide Inner and Outer values that are not the precise edges of Saturn's rings - they're just the margins of your texture.

That's correct.

To a user who hasn't inspected the texture, but who cares about accuracy, this is confusing, and the temptation is to "correct" your values to the real-world numbers, which then throws your texture into misalignment. Also, if we put your figures into solarsys.ssc, this then pushes other add-on creators towards abiding by your choice of ring margins. (Some, who understand things less well than you, might even fill up the whole space with ring texture.) Surely if we're going to impose such figures on other people they should be Saturn's own choice, rather than the end result of your personal approach to texture design?

Ahh, I unterstand.
But what are the "real-world numbers" Only the range with the most visible rings from C to F or all rings from D to E.

Since I was resizing from 8k down to 1k for the Celestia distribution texture, I had no fears that I was going to introduce any additional degradation of your texture by slightly trimming its edges before resizing!

Sure, an noticable additional degradation would only concern the 8k version.

Why didn't you just e-mail me at the time?

It was my failure. I'm sometimes a bit write lame.

As to your comments about Pan, I'm not sure what you mean.

I think something must inacurate with the current known data in this case.
Here are the values:

Distance from Saturn centre
inner Encke gap: 133570km
Pan: 133583km
outer Encke gap: 133895km

The Encke gap is 325km width but Pan is placed only 13km from inner Encke. I think Pan should be in the centre of Encke. Maybe the current Saturn mission will bring better data.

Bye Jens

[granthutchison]

But what are the "real-world numbers"? Only the range with the most visible rings from C to F or all rings from D to E.

Since your texture shows only the rings that are visible to the naked eye by reflected light (C to F), then the real-world numbers we use must be the margins of the C and F rings.

I think something must inacurate with the current known data in this case.

The Pan semimajor axis seems pretty tight - although it wasn't resolved in the Voyager discovery images, its mean motion was calculated, from which a semimajor axis of 133582.8 +/- 0.8km was derived. It's also assumed to be coorbital with the Encke ringlet, which is central in the Gap: so I'd guess that your data for the Gap edges are out of date, though I haven't seen anything better myself.

Grant

[jim]

so I'd guess that your data for the Gap edges are out of date

That's what I too think. But is only the inner value wrong or the complete Encke ringlet wrong placed ?
Maybe you can find better data... I can't.

Bye Jens