Re: [Celestia-developers] RA/Dec display

[<vin...@fr...>]

Fridger,

I recently suggested on the shatters forum to display the RA/Dec coordinate
of the cursor position, as you just suggested. However, nobody seemed to be
interested in that option. On the other hand, many users have asked for
adding the Selection RA/Dec coordinates to the Selection info text, as it
is currently displayed by the Lua Tools. That's why I've opted for that
option.

I'm quite busy too at the moment, but if we come to a consensus about your
suggestion, I'll be happy to work on it as soon as possible.


@+
Vincent


Selon Fridger Schrempp <fri...@de...>:

> Vincent,
>
> when trying out your RA/DEC readout, I was a bit surprised...Years ago
> we implemented such a readout into XEphem in a quite different manner
> that I find way more practical.
>
> Correspondingly, my proposal would be as follows: The RA/DEC should be
> read out by the  actual CURSOR (+) position. The observer and the cursor
> define a vector in space (with origin at the observer) the RA/DEC
> coordinates of which are uniquely defined and much more useful. You can
> e.g. just move the cursor to a star and get the RA/DEC readout displayed
> wrto the observer position etc.
>
> Sorry I am very busy these days before Christmas and thus had little
> chance to participate in the relevant discussion before you committed
> that readout.
>
> Bye Fridger
>
>
> incent wrote:
> > Selden, Chris
> >
> > I made the change to use a coordinate system centered on the observer's
> > position:
> > http://vincent.gian.club.fr/celestia/celestiacore.cpp
> >
> > The RA/Dec displayed values seems to correctly match the position of
> objects
> > against the grid, even for low-orbit satellites.
> >
> > Please let me know if it works fine for you.
> >
> >
> > @+
> > Vincent
> >
> >
> >
> > Selon vincent <vin...@fr...>:
> >
> >> Selden,
> >>
> >> Thanks for your detailed example. I understand now that the current RA/Dec
> >> calculation can be problematic for object that are close to Earth. I fear
> >> fixing this issue is not as simple as you suggested, though. We can't
> >> simply replace the center of Earth with the observer's position, since the
> >> celestial grid is centered on Earth's center, not on the observer. As an
> >> example, doing what you suggested in the Lua Tools won't return the
> correct
> >> values for RA/Dec. You can try commenting out line 159 in celutil.lua:
> >> local rot = obs:getorientation() * base_rot
> >>
> >> and commenting in line 160:
> >> --local rot = pkg.EARTH:getposition():orientationto(sel:getposition(),
> >> pkg.LOOK)
> >> * base_rot
> >>
> >> Then, when at the surface of Earth, if you select any target and move the
> >> view with a left-drag, you'll see the RA/Dec values changing whereas the
> >> target stay at the same position against the grid!
> >>
> >> Here's a simple diagram that illustrate the problem:
> >> http://vincent.gian.club.fr/celestia/radec_diag.jpg
> >>
> >> In this diagram, simply replacing the center of Earth with the position of
> >> observer will return the V1 vector, whereas what we need to get is V2.
> >>
> >> If we can't find a quick fix for this, I'd suggest to restrict the display
> >> of the RA/Dec values to stars and DSOs only...
> >>
> >>
> >> @+
> >> Vincent
> >>
> >>
> >> Selon Selden E Ball Jr <se...@le...>:
> >>
> >>> Vincent,
> >>>
> >>> One obvious example is the one I mentioned:
> >>> what are the RA and Dec coordinates of the ISS?
> >>>
> >>> At any given time, the RA and Dec of the ISS are different for
> >>> different observers located at different locations on the
> >>> Earth's surface. For example, when it's directly over Paris,
> >>> the RA and Dec of the ISS will be different for someone observing
> >>> it from Le Havre than for someone observing it from Marseilles.
> >>>
> >>> This can be seen in Celestia: turn on the "Celestial grid".
> >>> Goto Paris and look at the position of the ISS against that grid.
> >>> Then go to some other city and look at its position at that same
> >>> simulated time. You'll see that the ISS' coordinates are different
> >>> with respect to the Equatorial grid.
> >>>
> >>> The Equatorial coordinate system (RA and Dec) is defined in
> >>> terms of apparent directions toward objects which are at "infinite
> >>> distance" (quasars, for example), not with respect to the center of the
> >>> Earth.
> >>>
> >>> For what it's worth, the Lua functions used to calculate RA and Dec in
> >>> the Edu Tools provide this coordinate transform, although it isn't used
> >>> by the Tools. I do make use of that functionality in the Lua
> ScriptedOrbit
> >>> functions used by my yet-to-be-released Hale Telescope addon.
> >>>
> >>> s.
> >>>
> >>>> Selden,
> >>>> RA/Dec commonly represent the coordinates of an object on the celestial
> >>> sphere
> >>>> - which is centered on Earth - using the equatorial coordinate system.
> >>>> I've never seen RA/Dec expressed using another coordinate system. Do you
> >>> have
> >>>> a more detailed example of the usefulness of defining RA/Dec in a
> >>> coordinate
> >>>> system centered on the observer?
> >>>
> >>>> @+
> >>>> Vincent
> >>>
> >>>
> >>>> Selon Selden E Ball Jr <se...@le...>:
> >>>>> Vincent,
> >>>>>
> >>>>> Your code seems to display the RA and Dec of the selected object
> >>>>> relative to some fixed origin, apparently the center of the Earth.
> >>>>> I think it needs to display the RA and Dec as seen from the user's
> >>> viewpoint.
> >>>>> Using the viewpoint as the origin is particularly important when
> >>>>> observing objects that are in low Earth orbit, like the ISS.
> >>>>>
> >>>>> s.
> >>>>>
> >>>>>> Here's a patch that adds the RA/Dec display to the Selection Info
> >> text:
> >>>>>> http://vincent.gian.club.fr/celestia/radec.patch
> >>>>>> http://vincent.gian.club.fr/celestia/radec.zip
> >>>>>> RA/Dec are displayed using a format that perfectly matches the
> >> display
> >>> of
> >>>>>> the celestial grid labels, i.e., RA in hour/min/sec and Dec in
> >>> deg/min/sec:
> >>>>>> http://vincent.gian.club.fr/celestia/radec.jpg
> >>>>>> Chris, please let me know if this is OK to be commited.
> >>>>>> @+
> >>>>>> Vincent
> >>> -------------------------------------------------------------------------
> >>>>>> SF.Net email is sponsored by:
> >>>>>> Check out the new SourceForge.net Marketplace.
> >>>>>> It's the best place to buy or sell services
> >>>>>> for just about anything Open Source.
> >>
> >> Selon Selden E Ball Jr <se...@le...>:
> >>
> >>> Vincent,
> >>>
> >>> One obvious example is the one I mentioned:
> >>> what are the RA and Dec coordinates of the ISS?
> >>>
> >>> At any given time, the RA and Dec of the ISS are different for
> >>> different observers located at different locations on the
> >>> Earth's surface. For example, when it's directly over Paris,
> >>> the RA and Dec of the ISS will be different for someone observing
> >>> it from Le Havre than for someone observing it from Marseilles.
> >>>
> >>> This can be seen in Celestia: turn on the "Celestial grid".
> >>> Goto Paris and look at the position of the ISS against that grid.
> >>> Then go to some other city and look at its position at that same
> >>> simulated time. You'll see that the ISS' coordinates are different
> >>> with respect to the Equatorial grid.
> >>>
> >>> The Equatorial coordinate system (RA and Dec) is defined in
> >>> terms of apparent directions toward objects which are at "infinite
> >>> distance" (quasars, for example), not with respect to the center of the
> >>> Earth.
> >>>
> >>> For what it's worth, the Lua functions used to calculate RA and Dec in
> >>> the Edu Tools provide this coordinate transform, although it isn't used
> >>> by the Tools. I do make use of that functionality in the Lua
> ScriptedOrbit
> >>> functions used by my yet-to-be-released Hale Telescope addon.
> >>>
> >>> s.
> >>>
> >>>> Selden,
> >>>> RA/Dec commonly represent the coordinates of an object on the celestial
> >>> sphere
> >>>> - which is centered on Earth - using the equatorial coordinate system.
> >>>> I've never seen RA/Dec expressed using another coordinate system. Do you
> >>> have
> >>>> a more detailed example of the usefulness of defining RA/Dec in a
> >>> coordinate
> >>>> system centered on the observer?
> >>>
> >>>> @+
> >>>> Vincent
> >>>
> >>>
> >>>> Selon Selden E Ball Jr <se...@le...>:
> >>>>> Vincent,
> >>>>>
> >>>>> Your code seems to display the RA and Dec of the selected object
> >>>>> relative to some fixed origin, apparently the center of the Earth.
> >>>>> I think it needs to display the RA and Dec as seen from the user's
> >>> viewpoint.
> >>>>> Using the viewpoint as the origin is particularly important when
> >>>>> observing objects that are in low Earth orbit, like the ISS.
> >>>>>
> >>>>> s.
> >>>>>
> >>>>>> Here's a patch that adds the RA/Dec display to the Selection Info
> >> text:
> >>>>>> http://vincent.gian.club.fr/celestia/radec.patch
> >>>>>> http://vincent.gian.club.fr/celestia/radec.zip
> >>>>>> RA/Dec are displayed using a format that perfectly matches the
> >> display
> >>> of
> >>>>>> the celestial grid labels, i.e., RA in hour/min/sec and Dec in
> >>> deg/min/sec:
> >>>>>> http://vincent.gian.club.fr/celestia/radec.jpg
> >>>>>> Chris, please let me know if this is OK to be commited.
> >>>>>> @+
> >>>>>> Vincent
> >
> > -------------------------------------------------------------------------
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