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Seeing Saturn's moons other than Titan...

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#1 Phaenomena

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Posted 22 September 2021 - 01:09 AM

I swear I saw Rhea this evening when I was looking at Saturn in an 8" f/6 dob. What other moons of Saturn (other than Titan) are possible in an 8" scope?
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#2 ButterFly

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Posted 22 September 2021 - 02:31 AM

Tethys, dione, and iapetus should be easy if you know where to look and the cobditions allow. Enceladus in good seeing. Mimas is unlikely.

Look when Saturn is at its highest for the night. It's a little past opposition, but not too much.

It helps to have something at the scope showing where they are right now.
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#3 Redbetter

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Posted 22 September 2021 - 02:36 AM

The big four are easy with an 8" in rural sky:  Titan, Rhea, Tethys & Dione.  Iapetus isn't hard either, and Enceladus can be seen on nights of better seeing.  Hyperion is far away from the planet most of the time that it can be seen in rural sky, if you know where to look.  Mimas is tough for an 8" because it is so close to the planet and rings...I managed to catch it once with the 8" SCT during elongation some months after a ring plane crossing.  That was in a location with excellent seeing as well.


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#4 Stellar1

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Posted 22 September 2021 - 06:21 AM

Perfectly doable in your dob as I have seen the big four in my 4” on a night of great seeing.


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#5 maroubra_boy

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Posted 22 September 2021 - 07:02 AM

I routinely see four through my 127mm Mak.  Six with my 9" Mak, and the last two, Enceladus and Mimas, make me work for them.  This is from my home in Sydney too - won't gain anything from trying for these from a dark site as Saturn is so bright.  I also have no problem pulling four moons with my own 8" dob, five if conditions are really good too (as Redbetter mentions).

 

Good seeing conditions are important for a steady image.  The higher the planet is in the sky the better too (less atmosphere for the light to travel through).  Some magnification is important too, from 150X to 250X.  Patience and confidence are important as well.

 

The quality of the action of your dob is important too.  If its action is stiff, has significant backlash or you have to fight the mount just to get fine, small movements, all of these impede your ability to concentrate.  If you are fighting the scope you may want to look at cleaning the bearings or doing whatever you can to improve its action.  And of course your scope needs to be well collimated so all the light is going where it should be going.  Not knowing your experience with collimating Newts (so please forgive me if you know this already) the secondary needs to be sorted first and then the primary, and a laser will NOT sort out the secondary mirror.

 

Alex.


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#6 Phaenomena

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Posted 22 September 2021 - 10:20 AM

Good to know that I can get four moons form my yard! Definitely saw Rhea then. I'll look for Tethys and Dione next time I'm out.

Scope was collimated and had been outside for several hours, though I really need to get some fans for the mirror - it's something like 1.4" thick.

EDIT: Looking at Sky Safari for when I was looking at Saturn last night, I'm pretty certain I saw Tethys and Dione too. After a year at this, I've gotten pretty good (I think) at spotting tiny flecks of light using averted vision.

Edited by Phaenomena, 22 September 2021 - 10:24 AM.

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#7 Redbetter

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Posted 22 September 2021 - 04:35 PM

Dark vs. urban/suburban sky still matters for these.  This is particularly true for Hyperion, but also for the others.  While glare from Saturn is the largest factor in many cases (requiring better seeing), several magnitudes of light pollution makes Tethys and Rhea harder to detect, because light pollution is additive to the glare.   With my AT60ED I can catch Dione on the better nights in town (~19 MPSAS) but not Tethys.  Mimas is best seen in dark and steady sky with the 20"--I can rarely detect it in town.

 

Iapetus is interesting because it is so much brighter on the west side of the planet than on the east (tidally locked, with a much darker surface on the side we see from the east.)   When it is on the bright side I have managed to pick it out with the AT60ED in town.

 

Recently we have had so much smoke overhead that there have been nights I couldn't see Titan with the 20"...it was tough even finding Saturn with the 80mm finder, Saturn wasn't visible naked eye.


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#8 Voyager 3

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Posted 23 September 2021 - 09:12 AM

Do you even observe planets in such bad conditions ? Cool! And here I'm sitting inside lazily and both Jupiter and Saturn are visible . I hope the conditions will change soon so that I don't feel guilty ;) .
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#9 ButterFly

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Posted 23 September 2021 - 12:42 PM

Depending on the scope, planets' moons remain point sources for quite a while of higher magnification.  Point sources do not dim with increasing magnification unless and until their extent becomes noticable.  That can be because the aperture is large enough to resolve the point into a disk, or because the seeing is so terrible that the point-like disk is already a blob.  The sky background, on the other hand, as well as the planet itself, do indeed dim with increasing magnification.  That makes it easier to see the moon against the background, and when the planet is still in the field of view.

 

I have seen all of the above moons (Edit: other than Mimas) from Northern Manhattan in NYC with a 120mm refractor.  Magnification helps, so long as you are looking in the right place, and seeing helps as well, so as to not dim the unresolved disk of the moon.


Edited by ButterFly, 23 September 2021 - 12:44 PM.


#10 Phaenomena

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Posted 24 September 2021 - 01:35 AM

Observing tonight, I definitely could see Tethys, Dione, and Rhea in addition to Titan. Now I don't think I'll be able to NOT see them!
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#11 Redbetter

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Posted 24 September 2021 - 03:18 AM

Depending on the scope, planets' moons remain point sources for quite a while of higher magnification.  Point sources do not dim with increasing magnification unless and until their extent becomes noticable.  That can be because the aperture is large enough to resolve the point into a disk, or because the seeing is so terrible that the point-like disk is already a blob.  The sky background, on the other hand, as well as the planet itself, do indeed dim with increasing magnification.  That makes it easier to see the moon against the background, and when the planet is still in the field of view.

 

I have seen all of the above moons (Edit: other than Mimas) from Northern Manhattan in NYC with a 120mm refractor.  Magnification helps, so long as you are looking in the right place, and seeing helps as well, so as to not dim the unresolved disk of the moon.

I assume you also have not seen Hyperion from Manhattan with that 120.  It would seem optimistic to reach 14's there with a 120, and particularly with Saturn so far south in recent years.  If so, Pluto would likely be visible as well.  

 

While maximizing magnification for the seeing and aperture helps contrast in both bright and dark sky, diffraction limitations kick in.  The problem with heavy amounts of light pollution is that you can only go so far with exit pupil and dimming of the background before point sources become extended sources.  With a 120mm refractor the airy disk diameter is about 2.3 arc seconds and the FWHM of the point spread function is about 40% of that, so ~0.9 arc seconds for the bright central intensity.  At 0.5mm exit pupil the magnification would be 240x, and the apparent size about 216 arc seconds, or 3.6 arc minutes. 

 

That's near the size where no further contrast enhancement could be expected of a threshold source.  0.5mm exit pupil equates to about 5.7 magnitude of surface brightness dimming.  Unfortunately, if the sky is still 17 MPSAS then that only gets down to about 22.7 MPSAS.  That might sound dark, but it is only slightly darker than non-light polluted night sky, and is still about 5-6 magnitude brighter than the darkest the eye can detect if fully adapted (about 28 to 29 MPSAS.)  There is substantial capacity for the eye to go dimmer.

 

If one started in rural sky instead, say 21 MPSAS, then the same exit pupil would dim the sky to 26.7 MPSAS, which isn't far from the dimmest one can hope to adapt at the eyepiece.  And it is even more favorable to be in dark or semi-dark sky, since surface brightness between stars is dimmer than what 22 MPSAS pristine average for the sky (which includes stars) would suggest.  The space between can be 24, 25 or darker as the starting point...while the space between stars in light polluted regions will be about the same as the nominal light pollution value.    



#12 ButterFly

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Posted 24 September 2021 - 02:38 PM

So one cannot split doubles closer than one minute of arc with a 120mm refractor at 240x?

#13 Redbetter

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Posted 24 September 2021 - 06:20 PM

So one cannot split doubles closer than one minute of arc with a 120mm refractor at 240x?

I don't understand your question.  1 minute of arc (60 arc seconds) at 240x will have an apparent size of 240 minutes of arc (4 degrees.) 

 

For a relatable reference point, the pairs of the Double Double are about ~3.5 arc minutes apart an can be seen to be double naked eye if one has sufficient visual acuity.  While this is difficult naked eye without distance focus or astigmatism correction, a few X magnification renders them clearly as a pair.   At 240x their apparent separation is ~14 degrees, and of course the pairs themselves are resolved.

 

But my comment was about how eventually the apparent spurious disc size of a threshold star expands until it begins to have apparent size to the eye.  From there on, further magnification will reduce the apparent surface brightness of the star, just as it is doing to the background sky.  It isn't a sharply defined thing, the spurious disc that can be seen at threshold is perhaps the light contained in the FWHM circle of the distribution or something similar.  That is about 0.4x the airy disk diameter as an approximation. 

 

The exit pupil at which this occurs might be smaller, as in 0.3 or 0.2x, but as the circle shrinks so does the percent of light from the point source, so at some point there is no longer sufficient flux to detect even if the background is continuing to dim.  If 0.4x diameter matched the threshold for 0.5mm exit pupil, then 0.3x might correspond to ~0.4mm or a little less.  I have found my limiting magnitude in very good seeing with a small refractor in 21.5 MPSAS sky is somewhere in this exit pupil range, but this is at the very margin of detection where some of the same magnitude are seen and others unseen.  0.5mm is a more relatable exit pupil to high magnification planetary viewing.

Some might find their limits higher or lower even if the seeing is perfect.   When the seeing is degraded, and depending on how much it is degraded, the limits of an object appearing stellar are lower in terms of magnification (larger exit pupil and smaller apparent size.)



#14 ButterFly

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Posted 25 September 2021 - 12:59 AM

And I don't understand your concern with using higher powers to see fainter moons.



#15 Redbetter

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Posted 25 September 2021 - 01:39 AM

And I don't understand your concern with using higher powers to see fainter moons.

My concern is that your statements are confusing to the audience and giving only a portion of the factors important to seeing these things.  You still haven't clarified whether or not you have seen Hyperion from Manhattan.  I don't think it is what you intended to say, but perhaps I am mistaken.

 

I tried to address the following:

Depending on the scope, planets' moons remain point sources for quite a while of higher magnification.  Point sources do not dim with increasing magnification unless and until their extent becomes noticeable.  That can be because the aperture is large enough to resolve the point into a disk, or because the seeing is so terrible that the point-like disk is already a blob.

 

You left out the other major limitation, and that is diffraction and its impact on stellar appearing objects.  Diffraction limits how much point sources can be magnified to gain contrast. When the magnification with a given aperture is sufficient (exit pupil small enough) an object has sufficient apparent size and will have its surface brightness reduced just as much as the background sky.  There is no contrast gain left to be had. 

 

Otherwise one could continue to crank magnification (shrink exit pupil) indefinitely through small enough aperture to see ever dimmer stars.  Yet, when a person tests this, they will find they begin having more trouble seeing dim stars at some point.  I can see the effect with a 110, an 80, and a 60...or with a 7mm hole in a mask over a short focal length refractor.  



#16 ButterFly

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Posted 25 September 2021 - 06:57 PM

I laud your efforts at clarification.



#17 E_Look

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Posted 27 September 2021 - 12:49 AM

On the original question, I use a 8" Newtonian and have been able to catch Titan, Rhea, Tethys, Dione, Iapetus easily almost every time except for the last few years; recently, Tethys has become sometimes difficult and Dione is now ordinarily hard.  My scope has aged, but I suspect it's light pollution (it's New York, after all).  I have also in the past been able to catch Enceladus every so often, most likely due to good seeing, though recently, I haven't seen it.  Mimas?  Over the years, once or twice (three times??).

Take tonight for instance; Iapetus was there, but I only caught a glance as I was zooming in on Saturn, as it was too far out for me to put in the field of view and still have decent magnification.  But, more to the point, Only Titan and Rhea was easy to see.  Tethys would appear every so many minutes as conditions permitted, and when it did, it wasn't too hard to see.  But Dione was difficult.  I could see it only at magnifications ≥ 287×, and even then, it blinked in and out, mostly out.  It was a nice sight those very few and very fleeting moments when both Tethys and Dione showed at the same moment, the two close together forming a line at an angle to the plane of the rings.  I could see Tethys and Dione best at 383×, even if Saturn wasn't too focusable at that power.  I actually did also catch Enceladus at this magnification for a very fleeting moment, too, but only once that I could remember tonight.  And no, no Mimas.

 

I also suspect that if I took an 8-incher to darker skies, I'd probably be able to catch the first five fairly easily and Enceladus.  Mimas would probably still be touch-and-go because it's sits square in Saturn's glare field.



#18 Tom Masterson

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Posted 01 October 2021 - 08:55 PM

With a reasonably dark sky, I see 5 in my 6" refractor.



#19 payner

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Posted 01 October 2021 - 11:30 PM

I easily saw four tonight through my 130 mm apo.  Yes, five are easy, but Enceladus was obscured behind the planet during the evening (21-9-1 EDT).


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#20 David Knisely

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Posted 02 October 2021 - 11:53 PM

I swear I saw Rhea this evening when I was looking at Saturn in an 8" f/6 dob. What other moons of Saturn (other than Titan) are possible in an 8" scope?

In my 8 inch f/7, I can see Titan, Rhea, Tethys, Dione, and Iapetus.  In my 9.25 inch SCT, I have also seen Enceladus when seeing is really stable under a dark sky and when it is well off from the ring ansae at its maximum angular distance from the planet.  I might also see Mimas when the rings are nearly edge-on, but with wider ring tilt, it usually takes 10 inches or more even to get a glimpse of it.  Also, I can occasionally catch Hyperion in my 10 inch under dark sky conditions when it is far from Saturn, but it is pretty faint at nearly 15th magnitude.  Clear skies to you. 


Edited by David Knisely, 02 October 2021 - 11:54 PM.

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#21 Dave Mitsky

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Posted 03 October 2021 - 01:08 AM

That's where our interactive observing tool comes in! For any date and time between 1900 and 2100, it shows the positions of Titan and four other bright moons: 10th-magnitude Rhea, Tethys, and Dione, and 12th-magnitude Enceladus.

 

https://skyandtelesc...script-utility/



#22 Dave Mitsky

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Posted 03 October 2021 - 01:16 AM

There's an article on observing the Saturnian satellites at https://www.skyatnig...e-saturn-moons/


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#23 E_Look

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Posted 03 October 2021 - 02:15 PM

Nice!  It sheds some light (no puns intended) on why each moon has the level of detectability it has.

I mean, I, of the 8" mirror, have seen Enceladus and Mimas, but very few times.  I don't seem to have nights as dark and transparent anymore, even if the seeing is good.  In fact, the best, most easy sightings of Enceladus was back when the rings were just about edge on.  Mimas was still hard for my scope back then.  I have seen Mimas, as I said, very few times, but strangely, I think it was during times other than those days when the rings were edge on; it must have been the conditions that night.



#24 chuckles

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Posted 03 October 2021 - 07:45 PM

I’ve been able to pick up 5 with my 92mm from just outside of downtown Atlanta on a good night, but it was not super easy. I’m fairly confident I caught Enceladus in my 160mm last week from the same location, but that’s the first time I’ve seen it. Admittedly I haven’t really tried much in the past. It was only visible here and there, and I couldn't hold it for any extended time. I’ve not tried for Hyperion, but if Saturn is still well placed I’ll try next time I’m at a dark site. It’s fun to try and detect the dimmer ones, but I do wish for more aperture to make them easier to see

#25 johnpeter2

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Posted 03 October 2021 - 11:51 PM

That's where our interactive observing tool comes in! For any date and time between 1900 and 2100, it shows the positions of Titan and four other bright moons: 10th-magnitude Rhea, Tethys, and Dione, and 12th-magnitude Enceladus.

 

https://skyandtelesc...script-utility/

I use this tool whenever I look for Saturn's moon.  Very useful.  But why is Iapetus not included?

 

John




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