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Question about comparing magnitudes

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#1 leafyseadragon

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Posted 22 November 2020 - 12:15 AM

Last night I was looking at the planets using my Celestron 8SE.

 

I saw Saturn just fine, but I wasn't able to spot Titan except by using my averted vision.  Titan's magnitude (according to Stellarium) is 8.83.

 

Then I used the Go-To feature to slew the telescope to Neptune.  I saw something which looked bluish towards the center of the field.  Or maybe I was just hoping it was bluish.  So I drew a diagram of Neptune and the stars nearby on a piece of paper, and compared my drawing with Stellarium.  Success!  I saw Neptune for the first time!  smile.gif

 

But here is my question: all the stars around Neptune in the FOV had magnitudes around 8 or 9.  I was able to see them plainly, without using my averted vision.  Why can I seem them plainly but not Titan, which has about the same magnitude?  Is it a reflectance issue?

 


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#2 Redbetter

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Posted 22 November 2020 - 03:27 AM

I don't know specifically where or when you are observing, but for most of us in the northern hemisphere Saturn is quite low in the sky by the time it is dark, and that likely applies to your situation as well.  That is a big factor on relative visibility--particularly if the sky is less than pristine or has any haze/transparency issues.  Seeing also tends to be muck low in the sky.  Neptune is substantially higher in the sky by comparison.

 

An even bigger factor is that Saturn is ~0.6 magnitude right now and this provides considerable glare around Titan, even though it has reasonable separation.   Imagine Saturn in the same field of view as the stars near Neptune, and at the same separation that Titan is, and you would find the same stars considerably more difficult to see. 

 

The night you observed, the Moon was also about twice as close to Saturn as Neptune, and Saturn would also have been well within the brightening of the zodiacal light.    


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#3 sg6

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Posted 22 November 2020 - 05:28 AM

Magnitudes are a bit of a pain, works OK on Stars, kind of falls down a bit on "extended" objects. Neptune being a planet and so having a size it sounds like it falls into the extended object hiccup.

 

If you had eyes (or an instrument) that only detected objects of say 1 unit magnitude and brighter but a nice big planet of say 8 units square but with a surface brightness of 0.5 units then by the way we define it the object has an magnitude of 4. 8 lots of 0.5 coming off it.

 

But as none is brighter then 0.5 it would not be seen/detected as no piece of it exceeds the 1 unit threshold.

 

And that is the problem. M31 is bright, in magnitude terms, simply because it is big, but every little bit of it is dim, very dim. So it is hard to see but "bright".

 

This where surface brightness comes into it all, and that can really add confusion.

 

Since blue and a blue/green is unusual in objects there is a good chance that you did see Neptune, especially if the object were a bit disk like rather then point like. To get a "blue" star the star has to be very bright, and "blue" is more a case of the eye response rather then the actual color - it's "color" would really be way up in the UV. The sun peaks in Green, but it isn't Green.

 

So if Green/Blue likely Neptune, if Blue/Green likely Uranus.



#4 Tony Flanders

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Posted 22 November 2020 - 07:13 AM

Last night I was looking at the planets using my Celestron 8SE.
 
I saw Saturn just fine, but I wasn't able to spot Titan except by using my averted vision.  Titan's magnitude (according to Stellarium) is 8.83.


That's weird! Normally an 8th-magnitude star appears very bright in an 8-inch scope. I routinely see not only Titan but also Rhea and Dione through my 80-mm refractor.

 

My guess is that you were observing Saturn after supper, by which time it was very low in the sky, and you also suffer from fairly severe light pollution, which is particularly problematic near the horizon. Hazy skies could be another problem.

 

If you want a good view of Saturn right now, observe it just as soon as it's visible to the unaided eye, roughly 40 minutes after sunset.


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#5 MellonLake

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Posted 22 November 2020 - 11:10 AM

Was your corrector plate or eyepiece fogged?  Had you let the telescope cool?  

 

Titan is usually really apparent and does not require averted vision.   In your 8" when Saturn is high (July-Sept) next year) you should be able to pick up 4 or 5 of Saturn's moons (and maybe even 6 on an ideal night). 

 

Neptune and Uranus are bright enough to be easily viewed in an 8" (as slightly large coloured stars at 240X), higher marginations will show these as discs and really help differentiate them from stars.  



#6 chrysalis

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Posted 22 November 2020 - 04:33 PM

Tony has some good points. Titan IS easily visible in smallish scopes, smaller than 8" for sure.

 

Is it possible also that maybe you were misidentifying a dim star for Titan? See below, this might give a feel for the minimum - maximum angular separation of Titan from Saturn: 11 arc-seconds to 181 arc-seconds.

 

https://www1.gly.bri.../titan_june.txt


Edited by chrysalis, 22 November 2020 - 04:33 PM.


#7 f74265a

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Posted 22 November 2020 - 05:39 PM

Was it out of your field if view? I can easily see it in an 85mm refractor in heavy light pollution
It is a little bit away from the planet

#8 leafyseadragon

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Posted 22 November 2020 - 09:33 PM

I have a lot of light pollution here.  Yes, I'm pretty sure I'm looking at Titan and not a dim star.  It's just a little away from Saturn.

 

I use Sky & Telescope's Saturn's Moons page so I know where to look.



#9 therealdmt

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Posted 24 November 2020 - 10:01 AM

I have a lot of light pollution here.  Yes, I'm pretty sure I'm looking at Titan and not a dim star.  It's just a little away from Saturn.

 

I use Sky & Telescope's Saturn's Moons page so I know where to look.

You probably already know this, but make sure you have the page set to display the correct view to match your scope. There are three choices, and among them ‘Mirrored reversed View’ applies for me with my refractor+diagonal, and I think that’s the same view you need as well (your scope is an SCT, right?).

 

The other two potentially tricky parts I’ve noticed with that S&T page are:

1) ‘T’ = Tethys, not Titan! Cornfused me a few times, I’ll admit (Titan is actually denoted by ‘Ti’)

2) Make sure to refresh the page for the current view if you have it opened from much earlier

 

If none of those apply, like the other responders, I’d also guess that Saturn was just down too low into the haze & light near the horizon for a good view of the moons. It’ll be back up higher in summer 2021 though, so just you hang on in there smile.gif

 

For now, try again on different nights, viewing it as early as possible, and hopefully you can still get some good views, Titan included


Edited by therealdmt, 24 November 2020 - 10:30 AM.



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