6 replies to this topic

[Voyager 3]

These type of questions are utterly ridiculous without knowing the seeing etc etc but still I'm going to ask ...

What's the lowest magnification you took to see the moons of Mars and Uranus - I'm asking about Phobos , Deimos and the brighter 3 or 4 moons Uranus . Just murder your experience on me. 

 

[ismosi]

This thread should help you regarding Mars and its moons ... I won't spoil 'the ending'

https://www.cloudyni...-martian-moons/

 

For Uranus:

https://www.cloudyni...5-uranus-moons/

 

What aperture scope will you be using?

Edited by ismosi, 24 November 2020 - 11:56 AM.

[sg6]

First you need to be able to collect enough light to actually "see" them, and they will be a dot. Unless you collect sufficent light no amount of magnification will help.

 

Need to get out of the idea that "magnification" is the answer to everything.

Then comes having reasonable quality optics.

[Redbetter]

My recollection is that for the moons of Uranus and Mars I have begun detecting them at around 250x or greater, usually greater as in ~300 or 350x and sometimes considerably more, particularly for the two inner moons of Uranus.  

 

Magnification alone won't do the job, unless the seeing is adequate and one has the aperture to provide the needed resolution (smaller spurious disk).  Using 350x on a 200 or 250x night isn't particularly helpful.  

 

 

First you need to be able to collect enough light to actually "see" them, and they will be a dot. Unless you collect sufficent light no amount of magnification will help.

 

Need to get out of the idea that "magnification" is the answer to everything.

Then comes having reasonable quality optics.

It is less about sufficient light (aperture) than it is about contrast.  For point sources, increasing magnification increases actual contrast, up to some limits.  Seeing is an important part of that contrast, and tends to put a cap on effective magnification for these type of observations.  When the seeing is stable, magnification dims the background surface brightness while the central intensity of dim objects is effectively unchanged.  This increases contrast...until magnification is increased to the point that the seeing or diffraction pattern of the spurious disk begin to result in apparent size to the star/moon.   When the object begins to have apparent size, further increases in magnification do not improve contrast vs. the background.  

 

Mars' moons would be visible in nearly any telescope aperture in rural sky if this were merely about collecting sufficient light.  Instead, the moons pose considerable difficulty even for large scopes because of the presence of Mars which makes its surroundings very bright.  As the moons move further from the planet they are more readily seen, if conditions are sufficient, primarily seeing.  

 

Uranus' moons are closer to the telescopic limiting magnitudes of small and medium scopes, so aperture is more of a factor.  Fortunately, Uranus is not nearly as intense as Mars with respect to brightening the background sky.  Even so the inner moons are close enough to be impacted and generally require more aperture and/or better seeing.  Fortunately Titania and Oberon are far enough out to be observed with something like a 6" refractor or an 8" SCT.  

[maroubra_boy]

From my home under the light polluted skies of Sydney, I saw Phobos for the first time using a 9" f/13 Mak with a 12mm $100 orthoscopic eyepiece. I also kept Mars JUST OUTSIDE the field of view. And even then Phobos was a difficult target. Yet a couple of my "high end" $500 eyepieces couldn't do the job with the same scope.

Deimos is just too faint for 9". 10" would also be difficult, but a better chance. A dark site really won't help here much because of the glare that Mars puts out surrounding it.

Uranus' brightest Moon, Titania, at least 8", possibly 9". The advantage here being Uranus itself isn't as brilliant as Mars. The other moons possibly 10" minimum.

As has been mentioned, you need to start with sufficient aperture, good quality optics in scope and eyepieces (a good plossl or ortho will do the job), and then good seeing conditions. Poor seeing will scatter the feeble light from the moons.

I tried for Deimos with my 17.5" two weekends ago, but seeing conditions were poor, so no chance. But it wouldstill have been difficult because of Mars brilliance.

I hope this helps.

Alex.

[Redbetter]

Alex, did you mean the above to apply to heavily light polluted conditions rather than general?

 

For the record, Deimos is not too faint for a 9" in a general sense.  Allan has seen it with a 130mm Tak.  I have seen Phobos and Deimos with a 25 year old 8" SCT in the suburbs through smoke haze, 18.2 MPSAS conditions, but good seeing.  Deimos is easier to see than Phobos when both are at elongation, including in the latter case with the SCT.  In the 20" when the seeing settles I can sometimes see both simultaneously in averted vision while looking at Mars itself.

 

When it comes to Titania and Oberon, these are well within reach of an 8" in dark skies if the seeing is very good.  I have not had the 6" mask with me when I have observed them plus Ariel and Umbriel through the 8" mask on the 20"; but I know there is some more room there, particularly for Titania.  I have used the 6" mask for Pluto, which is low in the sky here and considerably dimmer, and I have also seen Pluto with a 110mm refractor. 

[luxo II]

At Alex’ place the sky is about Bortle 8. Where I am maybe Bortle 7 on a good night, 8 on an ordinary night.

Not so sure about Pluto being possible in these conditions, mag 14.4 could be rather optimistic. For galaxies mag 10, maybe 10.5 is the limit in the 9” and 10”.

Edited by luxo II, 26 November 2020 - 04:23 AM.