About GPO :
it shouldn't be with several "levels".
For Sure, it would be the best idea. However, can we have as much information in a little window as we have in a larger one? Do you think that a lot star names (using for reference when you are in intersellar locations) would be readable enough in a thumb? Do that mean thumb is not a good solution? Thumb is a quick stuff and this is why it's interesting element. Thumb and your GPO could be the same stuff, only differents scale views : thumbs for a quick view, with few reference elements to locate (like only planet, only planet orbits and names for solar system, or only galaxy, not more), and a larger view few with more elements, grids... Could Celestia show sizeable space windows?
-A difficult thing about first uses : why can't we select different objects and then change common properties like orbits viewed or not with mouse?
In what kind of use are you thinking of?
Everyone that uses a computer today, has already managed its own files in explorer. These are objects and everyone enjoys selecting easily severals with mouse (while pressing special keyboard button) and changing in "one click" each propriety (commons in selected objects). Surely, users discover Celestia understand that planets and moons are similar such objects. They decide to focus on only two moons of Jupiter's large moon system, they want to compare quickly they orbits and their trajectories for instance. They want only to show orbits of these two elements, and why not their own frames too? And they don't want learning and make script for only that!
That's means a "Show Home" feature?
Why not? All problem is knowing what is exactly our home ! Today, it's seems (using 'H') that Sun is our home and not Earth. However, Earth appears to me, more obvious home than hot Sun! But it's true that Sun is a better reference when you're out of solar system. Even in this case, it depends what user want to do : select home and no more, or goto home?
If you select home only, The good object between sun and earth depends where you are : either in solar system where Earth would be the best home, or far out of sun's neighboorhood where probably sun would become a better home reference. In the other case, where you select destination, this is obvious, you want as home only earth!
So, I don't really what is the best : the current solution, H for Sun, another one, where H is using for Sun and something like SHIFT+H for Earth, or a last one, automatic one, according where you are, an algorithm selects for you your home !? I don't know, then it would not be a user 's choice?
When you're out of solar system, you want to see home, or Jupiter (or anything else in solar system except sun itself), or you prepare yourself to go these destinations, then you press enter and write these names and nothing! Do they exist? Does Celestia recognize them? Ah, you've just remembered (because you've think a lot in the last time) that you have to enter sun instead. Then you restart again... But How to point Neptune, this is my next destination? The only solution is going first to sun and then in Neptune... Hooo! long stuffs for quick adventure! Would not be great and easy to enter just Jupiter whereas you're watching Acturus star, and goto home (or in other solar systems) (even if program is going to sun before and automatically to the planet finally)?
What about a general pop up menu (clicking on third mouse button for example) for universe and observer objects where you can quickly change own common properties? (for example Telescope view or not, magnitude limit with a local slider...)
I'm not sure to follow you on this one; the wheel (mouse) gives and optimal solution to travel; perhaps a key could change this function to FOV?
The mouse is a great tool to move in celestia, just only to move? I have posted an image that i've quickly done to illustrate how mouse could become very useful in my mind. It would be also a way to reduce the need of keyboard shortcuts for basics users (or even for the others?)
I draw quickly the image for the pop-up menu that, in my mind, appears after clicking on the wheel like a third mouse button. This is a menu where some functions, linked more or less to the observer, are listed and could be chosen on fly easily and quickly.
I have, at least, several reasons for this observer's menu (whose title is on blue background to distinguish it clearly from a classical pop-up menu from others objects like those of planets) :
-Users have in mind that for Celestia, they are an object call 'observer',
-Some basic functions get a quick access with mouse with out searching them in menu,
-Users have in mind that they have global proprieties, and changing them, change how they see universe and how universe is simulated by celestia,
-Mind is kept in space exploration using pop-up menus, more than going in menu which is more program stuff
-Easy to manipulate functions : For instance, when you want to change magnitude, you need a slider (and a text), no more. I think that you can't put slider in menu. That's why, i draw a new window, consider it as a tool windows with only minimum control and only those linked to magnitude ( for instance). I choose a blue transparent ballon window whereas I put a quick help/description of the current function and a link to an online help. Additionals features surely. It could be hidden or not... It sures this kind of stuff would not replace more advanced and complex observer panel available from top menu.
-Finally all would be below your hand!
(In this image, I use some functions that exist like field of view, and some others they probably don't