Celestia Forums

NASA/JPL have released a new image of Saturn as seen from the approaching Cassini spacecraft, which will go into orbit around the ringed planet next summer.



It's similar to the same view as simulated by Celestia (including the positions of five moons), although the ring's shadow on the planet appears darker than in Celestia, and Celestia is missing the planet's shadow on the rings (at least on my system).

Here's a story about the new image.

- Hank

The moons are way too bright in Celestia, compared to this pic (In mine anyway..)

Really nice pic though- not yet desktop background standard, but it won't be long now...!!

JackHiggins wrote:The moons are way too bright in Celestia, compared to this pic (In mine anyway..)

Rather, Saturn's too dim in Celestia - your monitor doesn't have the dynamic range necessary to depict all bodies at correct brightness. The photograph exposes correctly for Saturn, so the moons appear dim.

Grant

granthutchison wrote:

JackHiggins wrote:The moons are way too bright in Celestia, compared to this pic (In mine anyway..)

Rather, Saturn's too dim in Celestia - your monitor doesn't have the dynamic range necessary to depict all bodies at correct brightness. The photograph exposes correctly for Saturn, so the moons appear dim.

I think there is really a bug in Celestia. All moons appears to bright in Celestia. Set the magnitude limit to 0,8 and you can still see a lot of Saturn's satellites from Earth while all Stars are vanished. I noticed this while simulating the view with a telescope.

Bye Jens

Rather, Saturn's too dim in Celestia - your monitor doesn't have the dynamic range necessary to depict all bodies at correct brightness. The photograph exposes correctly for Saturn, so the moons appear dim.

Okay... But... To correct that why not just do the reverse and make the moons dimmer?

I'd have to agree with jim though about simulating in real life- last night I was out looking at saturn, and according to celestia- i should have been able to see about 5 of Saturn's bigger moons, when Saturn was that size in the scope.
With a 1.5" refractor- there's no WAY I could ever see titan!

They are actually far too bright, IMO.


Feature request:
Celestia should give a readout of the magnitude of objects too, not just stars!

JackHiggins wrote:Okay... But... To correct that why not just do the reverse and make the moons dimmer?

Because if the primary is very bright in reality, you'd need to step down the moons to the point of invisibility to achieve the correct relative "exposures" in Celestia. It's like those photos of planets and moons that you often see, where they have to be made up of two different exposures (one shows the planet properly exposed, and the other shows the satellites with a hefty flare from the planet) - the difference in brightness is just too great for a single image. So today, fine, you want to see Saturn and dim down the moons; but tomorrow you might want to see the moons properly and accept that Saturn is going to be rendered too dim because Celestia can't make it bright enough. There just isn't a way to portray the full range of brightness of the real world on a computer screen, all at once. So whenever Celestia shows you a planet as an extended object and its satellites as points, you're going to feel that the satellites are too bright when compared to a photograph that shows the planet correctly exposed.

But I'm much more interested in problems with the apparent magnitude of planetary satellites when seen from Earth, because that's something we can test against known magnitudes - and they certainly do seem to be misbehaving from what Jens says!

Grant

JackHiggins wrote:

Rather, Saturn's too dim in Celestia - your monitor doesn't have the dynamic range necessary to depict all bodies at correct brightness. The photograph exposes correctly for Saturn, so the moons appear dim.

Okay... But... To correct that why not just do the reverse and make the moons dimmer?

I'd have to agree with jim though about simulating in real life- last night I was out looking at saturn, and according to celestia- i should have been able to see about 5 of Saturn's bigger moons, when Saturn was that size in the scope.
With a 1.5" refractor- there's no WAY I could ever see titan!

They are actually far too bright, IMO.


Feature request:
Celestia should give a readout of the magnitude of objects too, not just stars!

Jack,

in view of these deficiencies explained clearly br Grant, how about getting an 8 inch telescope for Christmas?;-)

When I go out watching Saturn with my Celestron 8, I can see easily 5 moons, and still, Saturn does not look too bright. Altogether, things look quite a lot more similar to what I can see with Celestia. We all know the different amplification behaviour of "pointlike" versus extended light sources, when the diameter of a telescope increases...

Bye Fridger

grant wrote:There just isn't a way to portray the full range of brightness of the real world on a computer screen, all at once.

Anyone interested in designing a computer screen capable of blinding?

Nah seriously- that's ok... You've explained this before, but I must have forgotten about it!

Fridger wrote:in view of these deficiencies explained clearly br Grant, how about getting an 8 inch telescope for Christmas?

Oh, but I am! Hopefully it'll be this one, which is an 8" Newtonian. No motor drive or GoTo, but for €700 my choice in Newtonians, is a tad limited here in Ireland...! Should be getting it early in the new year, probably not in time for christmas...

JackHiggins wrote:

grant wrote:There just isn't a way to portray the full range of brightness of the real world on a computer screen, all at once.

Anyone interested in designing a computer screen capable of blinding?

Nah seriously- that's ok... You've explained this before, but I must have forgotten about it!

Fridger wrote:in view of these deficiencies explained clearly br Grant, how about getting an 8 inch telescope for Christmas?

Oh, but I am! Hopefully it'll be this one, which is an 8" Newtonian. No motor drive or GoTo, but for €700 my choice in Newtonians, is a tad limited here in Ireland...! Should be getting it early in the new year, probably not in time for christmas...

Jack,

I had a look at the specs in the url. Could be ~OK, if you are lucky with the quality of the optics (somewhat too cheap for an 8 inch). Ploessl eyepieces are good. Have a 50deg virtual field and are very well corrected. For my taste, the focus is too short. F/5 has too much Coma (nasty fan-type distortion), if you look /somewhat/ off-axis. Badly visible. The reason why they offer F/5 (or even shorter) is portability on the one hand (customers want it) and for being able to get along with a cheaper mounting (sells better). Moreover, at F/5 the converging light cone angle near focus is quite big, whence you need a relatively /big/ secondary mirror which deteriorates the contrast massively.

So I would /personally/ always go for an F/7 (= 140cm length) instrument. Coma is almost invisible and the amount of parabolization is very small, hence less chances for misfiguring! Exquisite definition due to small secondary mirrors. If you are a "stationary" observer (backyard) then a stable mounting can easily be made by youself with very little money (either Dobsonian type = 1 weekend of work or exploiting junkyards ...).

If you find a commercial F/6 (instead of F/5) this would be already much preferable.

Bye Fridger

Jack,

here comes an important addendum to my previous post:

You always strike me as a person who wants to know things well;-). So have a look at the site of an old internet friend of mine, Jim Klein, who has developed over more than a decade a most powerful optics simulation program. He is a professional optical lens designer himself:

http://www.westcoastengineering.com/

You can just easily simulate and display a bunch of rays entering your F/5 Newtonian telescope (under a certain angle of incidence) and compare it with an F/7 design. This exercise is almost too trivial for Jim's software;-).
You will easily be able to see the large amount of Coma a F/5 design has in the focal plane. Certainly, you could also check the quality of color correction in Chris' 4" Takahashi F102, for example;-).

You can download a "light" version for windows for free which is fully functional! The only restrictions are in the number of optical surfaces, the number of macros, zooms etc.

In practice, the light version is by far enough for telescope design. The program is highly professional including standard glass catalogs, sophisticated Monte Carlo parameter optimizations of lens/mirror parameters etc.

It is not super easy to use, since it is mainly command driven (which is a prerequisite for its power, in fact!). But there is an excellent manual with lots of examples in a tutorial. It also has a neat graphical display of the results. I have not looked at the recent versions, but they have for sure significantly progressed as I know Jim;-)

I have done many optics simulations with Jim's package in the past, found numerous bugs over the years and was working quite a bit to port the code to Linux. I have exchanged far above 1000 emails with Jim in the course of time;-)

I am sure you will enjoy looking "behind the scene" with this program...

Bye Fridger

PS: The role model for Jims software concept is a "Sportscar": powerful engine, but not luxurious, as opposed to a white Lincoln limousine with girls, TV, Whiskey included;-)

granthutchison wrote:So today, fine, you want to see Saturn and dim down the moons; but tomorrow you might want to see the moons properly and accept that Saturn is going to be rendered too dim because Celestia can't make it bright enough. There just isn't a way to portray the full range of brightness of the real world on a computer screen, all at once.

Now it would be nice to have a "true brightness" mode, similar to the effect of over- or underexposing a photo. The user should be able to easily choose wanted brightness-range (as if you could choose shutter speed) - so if you set to the equivalent of "long exposure", far away planets or asteroids will be shown as they are now, but the sun etc. would be completely overexposed (i.e. completely white), while at "short exposure" you hardly can see those dim bodies, and the sun would be correctly exposed (i.e. somewhat like SOHO images).

Of course this would require calculating true brightness of objects, adjusting textures-brightness on the fly and so on, and I wouldn't expect people to use this very often

What a beautiful image.
I hope we get a great surface texture from Casini when it gets into orbit.

Fridger

somewhat too cheap for an 8 inch

Everything looks cheap when it's in sterling- damn you with your great exchange rate!!!

Thanks for that info (every little bit helps!)- although I still can't go over €700, and almost all the 8" i've seen are F/5... Since dobsonians are cheaper, would I be better off getting an 8" F/7 or F/6 dob this year, and getting an EQ mount next christmas? Seems like a good compromise...!

Thanks for that link too- don't have the time now, but i'll download it tomorrow afternoon & check it out!

You always strike me as a person who wants to know things well;-).

Indeed...

I don't suppose you could suggest a few good european sites stocking this type of scope, could you? Over here, our prices are a bit, um.. *cough* RIP-OFF!! *cough* and so far very few of the overseas companies want to ship here (high cost and risk of damage, apparently.)

I would also very much apprieciate some way to easily swap between different exposure lengths for a more realistic image.

Additionally, Grant, wouldn't moving to support the higher bit depth modes capable by newer graphics cards address many of the range issues you refer to?

I understand that in anycase our output devices, monitors and LCD's, are greatly reduced in range from real life no matter what.

JackHiggins wrote:Fridger

somewhat too cheap for an 8 inch

Everything looks cheap when it's in sterling- damn you with your great exchange rate!!!

Thanks for that info (every little bit helps!)- although I still can't go over ?700, and almost all the 8" i've seen are F/5... Since dobsonians are cheaper, would I be better off getting an 8" F/7 or F/6 dob this year, and getting an EQ mount next christmas? Seems like a good compromise...!

Thanks for that link too- don't have the time now, but i'll download it tomorrow afternoon & check it out!

You always strike me as a person who wants to know things well;-).

Indeed...

I don't suppose you could suggest a few good european sites stocking this type of scope, could you? Over here, our prices are a bit, um.. *cough* RIP-OFF!! *cough* and so far very few of the overseas companies want to ship here (high cost and risk of damage, apparently.)

Jack,

have you checked APM telescopes?
http://www.apm-telescopes.de/englisch/index.htm

which is a German dealer selling to Europe and the site is kind of interesting. They notably have some freeware software for demonstrating the effects of aberations and the size of the secondary.

They specialize among US and Chinese brands etc. on the rather famous Russian INTES (Maksutov) telescopes that have a reputation of excellent optics for the money.

But somewhat above your budget perhaps.

For good F/7 Newtonian (Dobsonians) I would try the well known 'Discovery' telescopes (US made)
http://www.discovery-telescopes.com

They are using Pyrex mirrors, individually tested to good specifications.

or Orion

http://telescope.com

There are more firms in the American market (S&T !) that specialize on /high quality/ 8" F/7 Dobs.

Also, under all circumstances, you might want to check out the amateur astronomy usegroups in the net. There, people discuss their experiences with various telescope brands and dealers. You also find recommendations for the "toprunners"...

The mounting is often the dominant price for cheap telescopes, yet it is often flimsy i.e. unstable and unsuited for serious use. Always good-looking on photos!;-)

I would never buy complete telescopes by mail-order without touching the mounting myself or even hitting it;-) to check out how shaky it is.

With Dobsonians the situation is different. The optics determines much of the price. You can always add a more expensive mounting later, if you like the telescope optics!

The optics is clearly the main gamble when buying a telescope!

Since most customers (buying entire telescopes) have no idea how good optics should perform, the market is full of trash.

Try to buy optics with explicitly specified optical accuracy, so you can argue later in a well-defined manner if the optics is faulty. Attention: there are different ways of specifying the accuracy. A spec of "1/10 wave" is highly ambiguous, for example. Get yourself "educated" in that respect. It's crucial to know the differences...


Bye Fridger