Saturn's rings wobble, as seen from Pan

[alex-w]

Original report by Pierluigi Panunzi: https://bugs.launchpad.net/stellarium/+bug/1559657

Hi!
I'm working with Saturn and it's famous rings...
I have created a data packet for its satellite PAN, whose orbit lies inside Enke's Division.
Collecting data from wikipedia, I organized the relevant data for Pan :

[pan]
albedo=0.5
color=1., 0.9, 0.75
coord_func=ell_orbit
halo=true
lighting=true
name=Pan
orbit_AscendingNode=263.0
orbit_Eccentricity=0.000035
orbit_Epoch=2453177.5
orbit_Inclination=0.0
orbit_LongOfPericenter=209.0
orbit_MeanLongitude=106.645
orbit_Period=0.57505
orbit_SemiMajorAxis=133584.0
orbit_visualization_period=0.57505
parent=Saturn
radius=15
rot_equator_ascending_node=213.7
rot_obliquity=0.0
rot_periode=13.8012
rot_rotation_offset=287
tex_halo=star16x16.png
tex_map=lune.png
type=moon

practically organizing data as other satellites with "ell_orbit" coord_func value: I started from Jupiter's Adrastea and modified values accordingly.
Please note the null values for "orbit_Inclination" and "rot_obliquity".
Now insert these data in ssystem.ini file and follow my instructions! You can witness a really awesome sight!

• choose Pan as Location (F6) and set coordinates to +40°00'00", +100°00'00", in order to see a magnificent view of Saturn

• with the mouse move horizontally a little in order to see rings edge-on

• choose a FOV of about 80°, just to see a smaller Saturn...
  Awesome view!!!

• press 3 times the L key, in order to increase the speed of time: you can see the magnificent show of Saturn crossed by the rings' shadows, but you can see also that there's something strange and wrong with the rings, that "flash" cyclically at that rate...

If you return to the normal speed of time (K) then you can see that rings are not always edge-on as you can expect: please remember that both inclinations are put to streight 0.0

What is wrong with my Pan's setup? Or is there any kind of issue in the "ell_orbit" algorithm?

If you try the same, starting now from Enceladus (for which we have "coord_func=enceladus_special") you can see that rings remain always edge-on without any wobbling.
then if you further zoom towards Saturn (I suggest you, in full screen view!) you can see also Pan wandering along the edge-on rings, as you would expect. Mimas instead has an orbital inclination of only 1.57°, but that's enough to see it detached from the rings' line. (as in the attached screenshot)

Thanks for your attention!
Pierluigi Panunzi

My log.txt file is this.
BTW what about the series of messages "libpng warning: iCCP: known incorrect sRGB profile"? I have never seen them...

2016-03-20T10:09:37
Windows 10
Compiled using MSVC++ 12.0 (Visual Studio 2013)
Qt runtime version: 5.5.1
Qt compilation version: 5.5.1
Addressing mode: 64-bit
Total physical memory: 8174 MB
Available physical memory: 7017 MB
Physical memory in use: 14%
Processor speed: 2794 MHz
Processor name: Intel(R) Core(TM) i5-2300 CPU @ 2.80GHz
Processor speed: 2794 MHz
Processor name: Intel(R) Core(TM) i5-2300 CPU @ 2.80GHz
Processor speed: 2794 MHz
Processor name: Intel(R) Core(TM) i5-2300 CPU @ 2.80GHz
Processor speed: 2794 MHz
Processor name: Intel(R) Core(TM) i5-2300 CPU @ 2.80GHz
C:\Program Files\Stellarium\stellarium.exe
 -------------------------------------------------------
[ This is Stellarium 0.14.2 - http://www.stellarium.org ]
[ Copyright (C) 2000-2016 Fabien Chereau et al. ]
 -------------------------------------------------------
Writing log file to: "C:\\Users\\pierlu\\AppData\\Roaming\\Stellarium\\log.txt"
File search paths:
  0 . "C:\\Users\\pierlu\\AppData\\Roaming\\Stellarium"
  1 . "."
Config file is: "C:\\Users\\pierlu\\AppData\\Roaming\\Stellarium\\config.ini"
Detected: OpenGL "3.3"
Driver version string: "3.3.0"
GL vendor is "NVIDIA Corporation"
GL renderer is "GeForce 405/PCIe/SSE2"
GL Shading Language version is "3.30 NVIDIA via Cg compiler"
GLSL Version Number detected: 3.3
GLSL version is fine, we should not see a graphics problem.
Cache directory is: "C:\\Users\\pierlu\\AppData\\Local\\stellarium\\stellarium\\cache"
Sky language is "en"
Application language is "en"
Loading Solar System data ...
Loading star data ...
"Loading \".\\stars\\default\\stars_0_0v0_5.cat\": 0_0v0_2; 4963"
"Loading \".\\stars\\default\\stars_1_0v0_5.cat\": 1_0v0_2; 21598"
"Loading \".\\stars\\default\\stars_2_0v0_5.cat\": 2_0v0_2; 150090"
"Loading \".\\stars\\default\\stars_3_1v0_3.cat\": 3_1v0_3; 428466"
Finished loading star catalogue data, max_geodesic_level: 3
navigation/preset_sky_time is a double - treating as jday: "2451514.25001"
Reloading DSO data...
Loaded 10756 DSO records
Loading DSO name data ...
Loaded 221 / 297 DSO name records successfully
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
Loading star names from ".\\skycultures\\western\\star_names.fab"
Loaded 340 / 340 common star names
Loading star names from ".\\stars\\default\\name.fab"
Loaded 4506 / 4506 scientific star names
Loading variable stars from ".\\stars\\default\\gcvs_hip_part.dat"
Loaded 6916 / 6916 variable stars
Loading cross-index data from ".\\stars\\default\\cross-index.dat"
Loaded 108279 / 108279 cross-index data records
Loaded 88 / 88 constellation records successfully for culture "western"
Loaded 85 / 85 constellation art records successfully for culture "western"
Loaded 88 / 88 constellation names
Loading constellation boundary data ...
Loaded 782 constellation boundary segments
Initializing basic GL shaders...
Creating GUI ...
libpng warning: iCCP: known incorrect sRGB profile
Loaded plugin "AngleMeasure"
Loaded plugin "CompassMarks"
Loaded plugin "FOV"
Loaded plugin "MeteorShowers"
libpng warning: iCCP: known incorrect sRGB profile
MeteorShowersMgr: Loading catalog file: "C:\\Users\\pierlu\\AppData\\Roaming\\Stellarium\\modules\\MeteorShowers\\showers.json"
Loaded plugin "SolarSystemEditor"
Using the ssystem.ini file that already exists in the user directory...
Unable to find module called "TimeZoneConfiguration"
Loaded plugin "TimeZoneConfiguration"
Dubious result: Landscape "Guereins" not calibrated. Opacity test represents mathematical horizon only.
Initializing planets GL shaders...
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
INFO Saving screenshot in file: "C:\\Users\\pierlu\\Desktop\\stellarium-000.png"
Unloaded plugin "TimeZoneConfiguration"
Unloaded plugin "SolarSystemEditor"
Unloaded plugin "MeteorShowers"
Unloaded plugin "FOV"
Unloaded plugin "CompassMarks"
Unloaded plugin "AngleMeasure"
Downloaded 0 files (0 kbytes) in a session of 1363.59 sec (average of 0 kB/s + 0 files from cache (0 kB)).

[gzotti]

Our built-in Pan has an orbital inclination of almost 1/2 degree, probably more than the correct value, but I don't know where the data are coming from.

The graphics issue seems to be a problem with projections. Using Perspective Projection it looks good (at least after adding a mimimum z_near in Planet::draw3dModel()), with others near triangles break apart. It does not help to tessellate the rings into a finer mesh.