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Martian Moons

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#26 Allan Wade

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Posted 06 October 2020 - 05:37 PM

Awesome stuff Red. Cutting out the diffraction spikes is certainly going to make it possible to see the moons in smaller apetures as you’ve well proved. But I can’t say many people would have seen them in an 8”. My TOA130 is about as perfect as optics can get, and I hope to give it a go next week. But 5” might be a stretch too far.


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#27 Redbetter

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Posted 06 October 2020 - 06:51 PM

Awesome stuff Red. Cutting out the diffraction spikes is certainly going to make it possible to see the moons in smaller apetures as you’ve well proved. But I can’t say many people would have seen them in an 8”. My TOA130 is about as perfect as optics can get, and I hope to give it a go next week. But 5” might be a stretch too far.

Allan it probably isn't just the spikes, although it is much easier to figure out what-is-what without the distracting broad rays--actual real objects are more readily identified.  Some other factors probably play a part.  Even though the 20" has a small obstruction (~16% by diameter), this is throwing some additional light out around the planet.  Using the off axis mask also eliminates any mirror edge effects...and my edge has been roughed up from so much travel.  But the other thing it does is to better match the seeing.  The scope was handling the magnification decently as an 8" that night, but at 20" with the same eyepieces the image was too unstable at the same magnification. 

 

Instability makes it difficult to keep a tight focus on stellar objects and this is the critical aspect of limiting magnitude against any background.  I place the off-axis hole in the bottom quadrant where I assume the optical path is virtually free of any warm rising air.

 

My mirror and coatings are 17 years old and they are far from pristine at this point.  That can't be helping.  The clear aperture still suffers from transmission difference compared to a refractor, as well as some unknown level of mirror scatter that likely reduces contrast.  I figure that the transmission my old mirror with of that mask would be equivalent to ~7" refractor.  

 

I suspect that a 130 apo will be pushing it, but having viewed through one, that is the instrument I would try in that aperture range.   If the seeing is very good and transparency is good, then you will have the advantage of pristine dark sky, some sort of occulting device, and a tracking drive. I wouldn't bet against you...   I think a 6" apo would make this doable for me if the seeing was good and the sky was dark, along with using a drive and an occulting device of some type.   

 

p.s.  To give an idea of how much seeing matters, last night had somewhat better seeing.  Transparency was far better as well, a combination I have not had recently.  357x was marginal in the 20" at full aperture for Mars, but I was detecting Phobos at times.  An occulting bar at 338x was not really helping and I could not see Deimos.  I dropped down to 278x (no occulting bar) and Phobos was more readily seen and could be held at will.  I searched for a time and eventually found Deimos--again no occulting device.  I could hold Deimos at times when the seeing would improve.  Both were about 19 to 20 arc seconds from the planet limb at the time, the closest I have ever detected Deimos.  


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#28 happylimpet

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Posted 15 October 2020 - 03:08 AM

Gentlemen!

 

Thank you so much for reminding me of this challenge. Last night I managed Deimos and Phobos in my 16" Newtonian, (EDIT: my first ever sighting!) Not a major challenge you might think but it was still quite tough.

 

I used a 20mm erfle as it has an easily accessible focal plane for pushing a rectangle of aluminium foil into place as an occultation bar. Had to keep everything close to the centre of the FOV to minimise aberations. I was using my planetary camera setup with a flip-mirror, so one more reflection (and maybe not the highest quality one) than necessary. The barlow in this provides 4x, so i was at effectively x(1600*4/20)=320x. Mars showed a lot of detail at times.

 

I got Deimos first at around 2130 when it was close to western elongation. Helpfully there was a mag 11 star twice as far out which enabled me to know exactly where to look and the kind of brightness to expect. It took a while but eventually i was able to hold it for a few seconds at a time in a diffraction spike. After I got it convincingly for a while I moved Mars back into the FOV and noted the seeing had improved....so important for sure.

 

Then at around 2345 UT (after I got our 11 month old to sleep) I rushed out to catch phobos at greatest eastern alongation. Still very close eh? Took a lot longer than deimos but eventually i got it for a few seconds at a time. When i did it seemed so bright i tried it without the occulting bar, but no luck.

 

Incredibly satisfying, thanks all, particularly Allan!


Edited by happylimpet, 15 October 2020 - 04:24 AM.

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#29 Allan Wade

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Posted 15 October 2020 - 04:10 AM

Well done on sighting Deimos and Phobos. Was that your first time seeing the moons?


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#30 happylimpet

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Posted 15 October 2020 - 04:24 AM

Well done on sighting Deimos and Phobos. Was that your first time seeing the moons?

Yes it was! I should have made that clearer!! As a fairly old timer (well, 46, had a scope since i was ~10) this is a big deal!!! Thanks again Allan.


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#31 Allan Wade

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Posted 15 October 2020 - 04:44 AM

That’s a very big deal. Not many people would have seen Deimos or Phobos with their own eyes. You’re in the club.


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#32 happylimpet

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Posted 15 October 2020 - 04:59 AM

That’s a very big deal. Not many people would have seen Deimos or Phobos with their own eyes. You’re in the club.

Am honoured to be here. Not sure my girlfriend appreciated it fully when i told her at 2am. In fairness she did though to some extent!

 

I think Ive concentrated too much on imaging the last few years. I need to get back to the eyepiece a bit more. This is just the kick I needed.

 

Quick image of Deimos and field star taken just after I saw it. Flipped vertically to match my view.

 

2020-10-14-2159_4-NJH-RED-Mars_AS_P100_lapl6_ap2 fliplvls.jpg


Edited by happylimpet, 15 October 2020 - 05:10 AM.

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#33 Voyager 3

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Posted 15 October 2020 - 11:20 AM

Am honoured to be here. Not sure my girlfriend appreciated it fully when i told her at 2am. In fairness she did though to some extent!

I think Ive concentrated too much on imaging the last few years. I need to get back to the eyepiece a bit more. This is just the kick I needed.

Quick image of Deimos and field star taken just after I saw it. Flipped vertically to match my view.

2020-10-14-2159_4-NJH-RED-Mars_AS_P100_lapl6_ap2 fliplvls.jpg

Is that a smartphone pic ?

#34 happylimpet

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Posted 15 October 2020 - 02:16 PM

Is that a smartphone pic ?

No, a stack of 30 1 second exposures through a Baader red filter with an ASI290MM camera at f16.


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#35 Voyager 3

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Posted 16 October 2020 - 09:37 AM

No, a stack of 30 1 second exposures through a Baader red filter with an ASI290MM camera at f16.

Great 🙂 . I've not seen it visually and probably not see in the near future ... Thx a lot .
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#36 CrazyPanda

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Posted 16 October 2020 - 08:58 PM

I replied in another thread, but Phobos has been "easy" this season in my 15". Have seen it a few times without even looking for it (and confirmed the observations in Sky Safari after spotting it).

 

Deimos has been invisible though, even when I'm deliberately looking for it.

 

This was the configuration in my scope the first time I spotted Phobos unprompted:

 

scope-binoviewer.jpg


Edited by CrazyPanda, 16 October 2020 - 08:59 PM.

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#37 Jakke

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Posted 17 October 2020 - 12:28 AM

Last week I travelled 650 miles with only one thing on my mind... observing Mars !

I only had 4 days (nights) available, and the first few nights out were really disappointing.

Despite very clear skies, Mars was not more than a bright boiling pinky dot in the sky. 
Only the last night, against all expectations, there she was in all her glory. A clear and steady image showed the polar cap clearly visible, and even some minor darker surface detail.

After spending some time observing, I noticed a faint white dot at the edge of the field, and suddenly remembered this thread ! Moving the field of view a little bit a second faint dot appeared. Checked with Sky Safari and Yesssssss..... I couldn't have been happier.

I was using the 300mm(12”) F4 hexapod I build last year. No filters.

 

Thanks for starting this thread, the moons would probably have gone unnoticed without !

 

Jan.


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#38 kb58

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Posted 17 October 2020 - 08:18 AM

I have relatively little viewing time, so me finishing the 16" f/4.3 was a bit like a 16-yr old being handed the keys to a Ferrari, minus the danger. Anyway, I didn't even know that Mars had moons(!) so never thought to look for them. After reading this thread, I tried last night, and was a bit shocked and surprised to see Phobos on the first try (well, after consulting a star map and remembering to swap left and right. It appeared during moments of stable air enough times to conclude I wasn't imagining it, and it was in exactly the right spot, about 0.75" Mars-diameter to the "right" of its disc. Deimos on the other hand was a complete failure. I figured after Mars had risen higher, maybe then, but the seeing had degraded, with even features on Mars being washed out compared to earlier. Nevertheless, I'm very happy to take away that memory; it's amazing to see something that small and so far away.

 

[Edit: Phobos was best seen at 292x. More and it was blurry, less, too small. No filters were used]

 

[Edit again] Forgot to add that I couldn't figure out what you guys were talking about regarding diffraction spikes. I thought to myself that the spikes are too narrow to be of any issue... Yeah well now I understand, that looking at something as big and bright as a planet, it does indeed generate enormously wide diffraction spikes that are as wide as the planet itself. I get it now!


Edited by kb58, 17 October 2020 - 01:19 PM.

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#39 ANM

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Posted 17 October 2020 - 09:14 AM

Using my C8 I have also been looking for Phobos and Deimos the last few weeks but so far no luck, though last night provided me with the best views of Mars so far — from my backyard in El Paso yet. The best combination was the C8 (a classic orange tube) with a Baader 6mm Classic Ortho for ~340x. I was able to see some of the dark maria and a hint of one of the polar caps. It was a very satisfying evening.


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#40 Pcbessa

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Posted 17 October 2020 - 10:04 AM

As I posted in another thread I saw Deimos in my 10", twice in the past few weeks, when the seeing was above average. It wasn't that difficult at 350x but it required some attentive observation (I could hold it with averted vision at times so I was 100% sure it was Deimos).

The good seeing was the no1 critical factor. Mars was fantastic those nights. I also my very dark skies are a contributing factor. And high power too. And you need to use Sky Safari and know where to look.

In one of those two great nights, I glimpsed what was probably Phobos in two instances. Phobos is definitively a bit harder because it's so close to Mars (never more than a disk diameter away)

Interestingly in early September I couldn't see Deimos. The magnitude of the moons decreases quickly after opposition. So I think timing is also key.
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#41 CrazyPanda

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Posted 17 October 2020 - 11:52 PM

Saw Phobos again tonight very easily.

 

I was doing DSO observing of Stephen's Quintet in my 8 Ethos when I decided to see if seeing was good enough for some Mars observations, since it was at max altitude. When I swung over to it, the seeing was reasonably good (not great, but good), but a moon of sorts stuck out like a sore thumb. Right in between vane diffractions. It was approximately 80% of a Mars diameter away from the planet (so maybe 17" separation?). When I opened up Sky Safari, I saw that both Phobos and Deimos were fairly close to one another, with Phobos being the closer of the two to the planet. Based on distance, the moon I saw had to have been Phobos.

 

I *think* there was a very fleeting moment I saw Deimos sitting right at the edge of a vane diffraction. Tried a variety of eyepieces, but the 8 Ethos was giving me the steadiest, most consistent view of Phobos and the only one that I think I maybe saw a hint of Deimos in.

 

When I tried switching to a 4.5 Delos, I realized that the usual trick of using more mag to make faint stars easier to see *does not work* with Mars. The extra size really overwhelms things and makes the moons harder to see.

 

The 8 Ethos and 9 DeLite showed Phobos best, but it was still visible in the 11 DeLite. The optimum focal length seemed to be 8mm, though I would have liked to have tried a 7mm (wish I still had my 7 ortho).

 

Switching to the binoviewer still showed Phobos, but it was definitely more challenging to see.

 

What puzzles me is how I appear to be the only one who thinks Phobos is downright easy even when it's in close proximity to Mars (12 arc seconds or so), but cannot for the life of me see Deimos. I guess I have the misfortune of it always being in the diffraction spike of a vane when I go to observe it.


Edited by CrazyPanda, 18 October 2020 - 12:05 AM.

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#42 Allan Wade

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Posted 18 October 2020 - 12:23 AM

Saw Phobos again tonight very easily.

 

I was doing DSO observing of Stephen's Quintet in my 8 Ethos when I decided to see if seeing was good enough for some Mars observations, since it was at max altitude. When I swung over to it, the seeing was reasonably good (not great, but good), but a moon of sorts stuck out like a sore thumb. Right in between vane diffractions. It was approximately 80% of a Mars diameter away from the planet (so maybe 17" separation?). When I opened up Sky Safari, I saw that both Phobos and Deimos were fairly close to one another, with Phobos being the closer of the two to the planet. Based on distance, the moon I saw had to have been Phobos.

 

I *think* there was a very fleeting moment I saw Deimos sitting right at the edge of a vane diffraction. Tried a variety of eyepieces, but the 8 Ethos was giving me the steadiest, most consistent view of Phobos and the only one that I think I maybe saw a hint of Deimos in.

 

When I tried switching to a 4.5 Delos, I realized that the usual trick of using more mag to make faint stars easier to see *does not work* with Mars. The extra size really overwhelms things and makes the moons harder to see.

 

The 8 Ethos and 9 DeLite showed Phobos best, but it was still visible in the 11 DeLite. The optimum focal length seemed to be 8mm, though I would have liked to have tried a 7mm (wish I still had my 7 ortho).

 

Switching to the binoviewer still showed Phobos, but it was definitely more challenging to see.

 

What puzzles me is how I appear to be the only one who thinks Phobos is downright easy even when it's in close proximity to Mars (12 arc seconds or so), but cannot for the life of me see Deimos. I guess I have the misfortune of it always being in the diffraction spike of a vane when I go to observe it.

I definitely think it’s a timing thing for you. Granted, if Phobos and Deimos are the same distance from Mars, then Phobos is the easier moon to see. But then Deimos keeps moving out, and when it gets to three Mars discs away from the planet, it is much, much easier to see than Phobos. I saw Deimos in my TOA130, so you should be able to do it with your eyes closed in the 15”.


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#43 Redbetter

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Posted 18 October 2020 - 02:09 AM

 

What puzzles me is how I appear to be the only one who thinks Phobos is downright easy even when it's in close proximity to Mars (12 arc seconds or so), but cannot for the life of me see Deimos. I guess I have the misfortune of it always being in the diffraction spike of a vane when I go to observe it.

 

You must have had rather good seeing when you were observing Phobos, otherwise you wouldn't have a prayer of seeing Phobos when it was in close.  It is just so easy for Mars to blot it out.  Phobos comes and goes with the seeing, even when it is easier to see.  There are times when it jumps out easily, but in my experience those have always coincided with the best seeing.  In those situations Deimos is super easy assuming that it is well removed from the planet.  

 

It seems unlikely that Deimos would be on a spider vane ray from the planet each time you try for it, particularly since the orientation changes both as it orbits and in the altaz view of a Dob as Mars orbits.  Wait an hour or two and the relative positions can be substantially different.  Even if it were on the vane, this is not necessarily enough to block it from view.  Have you verified where the position should have been when you looked for it to confirm it would have been on a diffraction ray?

 

I don't know what the effective focal length of your 15" is (focal ratio and whether or not it is using a Paracorr providing ~1.15x factor), but the jump in magnification from an 8mm to a 4.5 is huge.  I'm guessing the latter puts you in the 400+ range.  Generally, I find it hard to get nights where point sources are steady at such magnification.  More magnification helps with limiting magnitude type observations (such as one of these moons on a very bright background glare), up until the point where it begins to blur and actually lose contrast with the sky.  Once one reaches large aperture, seeing usually dictates the optimum.  

 

Some folks have complained of various software, including Sky Safari, not showing Mars' moons properly, although I don't know if that is true or not.   Others have indicated Sky Safari's positions matched.  I don't know if some version difference or set up could give problematic results for Deimos or not.  It might be worth checking against WinJupos, Stellarium, etc.

 

It would be interesting to compare views in the same instrument when both moons were visible.  That is about the only way to understand if it is just a difference in the way one person sees things, if they are somehow looking in the wrong spot, etc.  That sort of multi-observer evaluation generally works when the instrument being used is one owned by/familiar too the person having difficulty.  If another person comes along, sees Deimos, then explains how to find it, then perhaps it was a matter of just having trouble locating it rather than actually seeing it.  If they can't see it, yet someone less familiar with the scope can, then it could be some physiological difference.  Dim objects next to bright ones seem to produce the widest range of experiences.

 

Deimos was discovered first by Asaph Hall, and then several nights later he spotted Phobos, having previously confirmed Deimos on the other side of its orbit the night before.  So he saw Deimos at least twice before picking up Phobos.  He was using the largest refractor in the world at the time, and during a very favorable opposition.     


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#44 CrazyPanda

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Posted 18 October 2020 - 02:37 AM

In those situations Deimos is super easy assuming that it is well removed from the planet.

 

But Deimos is a full magnitude dimmer. I would need *even steadier* seeing to see Deimos as easily as I see Phobos.



#45 Redbetter

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Posted 18 October 2020 - 05:14 AM

But Deimos is a full magnitude dimmer. I would need *even steadier* seeing to see Deimos as easily as I see Phobos.

The problem is not the magnitude of Deimos or Phobos, it is the glare of the planet which is several hundred thousand times brighter than either.  Neither moon would be difficult to see at these magnitudes in the absence of Mars, even in relatively bright night sky.  (I have observed Titania and Oberon in the red zone suburbs recently with an 8" SCT, and they are roughly 2 magnitude dimmer than Deimos.)  With Mars there is a brightness gradient that swamps things in close. 

 

For high delta magnitude pairs, having a multiple of separation (actual arc seconds) is far more valuable than being a magnitude brighter (as long as one isn't getting too close to the actual magnitude limit of the gear.)  Deimos reaches ~3x greater separation. 


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#46 Allan Wade

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Posted 18 October 2020 - 06:25 AM

But Deimos is a full magnitude dimmer. I would need *even steadier* seeing to see Deimos as easily as I see Phobos.

Yes this is the mistake lots of people make when they just look at the brightness of close orbiting moons. There are a lot of factors at play that contribute to a moons observability.

 

I found a website that described Amalthea as an easy moon to see in a vast number of amateur telescopes owing to its relatively bright 14th magnitude. The fact that perhaps as few as 20 people have seen Amalthea in almost 130 years was obviously completely lost on who ever wrote that description.


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#47 CrazyPanda

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Posted 18 October 2020 - 09:41 AM

Even if it were on the vane, this is not necessarily enough to block it from view.  Have you verified where the position should have been when you looked for it to confirm it would have been on a diffraction ray?

 

I've seen Phobos 3 times this observing seasons and Deimos 0. Two of those times, Phobos and Deimos were in radically different locations with Deimos being either much farther out, or on the opposite side of the planet, and Phobos being in close to the planet (1 Mars diameter away or less). There was *zero* question it was Phobos that I was seeing. 

 

Deimos, despite my efforts to see it after determining where it should be in relation to the view, was invisible.

 

Last night was the first night that both Phobos and Deimos were in relatively the same location.

 

IMG_2403.PNG

 

Phobos was right in the elbow of two diffraction spikes. Deimos SHOULD have been outside of the diffraction spikes as well, but I was unable to see it. I assumed maybe it was *just* on the edge of a diffraction ray (which is indeed where I think I saw that point of light flicker into view)

 

Last night the moon I saw was simply downright easy. I saw it immediately after I swung the scope over to it. Magnification was 246x with 8 Ethos and Paracorr. It was the same apparent brightness as the other two nights I'd seen it (maybe a bit brighter since I was monoviewing it tonight), so I assumed it was Phobos. Looking it up in Sky Safari showed it was just about right where Phobos should have been expected to see it. In fact, Sky Safari put it a bit farther away than it actually was. Sky Safari looked like it was almost exactly 1 Mars diameter, but what I spotted was closer to 80% of a Mars diameter.

 

Unless Sky Safari was *very* wrong and I did in fact see Deimos, and Phobos was only a few arc seconds away from Mars. But in my experience, Sky Safari is usually very accurate, and again, the moon I saw tracked with what I've seen previously when there was no question it was Phobos.

 

While not a huge separation for Deimos, I guess I can't follow the logic of "if Phobos was easy, Deimos should have been easier". I would have needed even steadier seeing to see it. I had maybe Pickering 6 seeing at the time of the observation.

 

And given I've seen Phobos at 12" separation without too much trouble, it seems the combination of magnitude and glare from Mars makes Deimos substantially more challenging despite its often greater separation from the planet.

 

Maybe for my eyes and scope, once Mars is in the field, Mag 12 objects are just outside of visibility for me? The Mag 11 of Phobos appears to be immune to Martian glare as long as it's not so close to the planet it's actually sitting in light scatter at the eyepiece.


Edited by CrazyPanda, 18 October 2020 - 09:54 AM.

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#48 Redbetter

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Posted 18 October 2020 - 06:43 PM

CrazyPanda,

 

You seem to be contending against an argument that isn't being made.

  • Deimos is much more difficult when it is in close, near Phobos separation.  I have seen it like this recently, but it was a real challenge.  I am not sure why you would try to make a non-sighting of Deimos in close as an example?  It isn't what any of us are saying...this is morphing into a strawman argument.
  • Deimos is easier to catch at elongation.  It is out there for long stretches, while Phobos darts around close to the planet.  That is why Deimos was discovered first and why most of us see it first.  Other than a few weeks at close oppositions, and when the seeing also happens to be good, Phobos gets lost in the glare that Deimos does not suffer from as badly at elongation.
  • I cannot answer why you cannot see Deimos at elongation--although from the discussion above, it isn't clear if you have tried for it in such conditions.  In good seeing it is easier than Phobos. It doesn't take better seeing for Deimos, quite the opposite.  Phobos is the one that needs better seeing because the rapidly fluctuating glare from the planet swamps it first.  Pickering 6 in a 15" is very good seeing, sub-arc second.   

I can't answer why an 11th mag object would be easy for you to see in the planet's glare, while a 12th mag object would go missing 2 to 3x as far from the planet.  The more you say about how easy it is for you to see Phobos, the more obvious it is that you have been doing this in very good seeing.  

 

Deimos is perhaps 1.1 mag dimmer, making Phobos about 2.75x brighter.  That might sound like a lot until you consider that the glow around the planet fades as one moves away from it.  I am not sure what the exact relationship is mathematically, but I suspect it is an inverse square law with the glow being 4x as diffuse at 2x the angular separation. 

 

Phobos reaches about 20 arc seconds of separation form the planet surface presently, while Deimos reaches about 60 arc seconds.  That is a 3x distance difference, which would make the glare about 3*3 = 9 times less at Deimos separation.  I'll take that boost in contrast over a less than 3x difference in brightness.  

 

From your magnification/eyepiece/Paracorr value, it appears you have a 15" f/4.5.  This explains why jumping right to ~437x with a 4.5mm was a problem in your earlier description.  My experience it that there is a big hole between 246 and 437x you have tried.   Unfortunately, this hole is right in the area where the optimum often is on the better nights--and by optimum I mean where limiting magnitude stars, galaxies, etc. still hold together, and beyond which visibility begins to suffer because the seeing is not quite still enough.  With my 8" SCT this very good but not quite excellent level was for a combo that provided 310+ (and this was about where it topped out on the dead still seeing nights as well.)   with the 10" and 20" f/5 it has been in the 357x range.  On the best nights I can push to ~400x with the 10" or 500x with the 20", but the 300+ range is more representative of the very good but not quite excellent nights.


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#49 CrazyPanda

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Posted 18 October 2020 - 08:30 PM

CrazyPanda,

 

You seem to be contending against an argument that isn't being made.

  • Deimos is much more difficult when it is in close, near Phobos separation.  I have seen it like this recently, but it was a real challenge.  I am not sure why you would try to make a non-sighting of Deimos in close as an example?  It isn't what any of us are saying...this is morphing into a strawman argument.
  • Deimos is easier to catch at elongation.  It is out there for long stretches, while Phobos darts around close to the planet.  That is why Deimos was discovered first and why most of us see it first.  Other than a few weeks at close oppositions, and when the seeing also happens to be good, Phobos gets lost in the glare that Deimos does not suffer from as badly at elongation.
  • I cannot answer why you cannot see Deimos at elongation--although from the discussion above, it isn't clear if you have tried for it in such conditions.  In good seeing it is easier than Phobos. It doesn't take better seeing for Deimos, quite the opposite.  Phobos is the one that needs better seeing because the rapidly fluctuating glare from the planet swamps it first.  Pickering 6 in a 15" is very good seeing, sub-arc second.   

I can't answer why an 11th mag object would be easy for you to see in the planet's glare, while a 12th mag object would go missing 2 to 3x as far from the planet.  The more you say about how easy it is for you to see Phobos, the more obvious it is that you have been doing this in very good seeing.  

 

Deimos is perhaps 1.1 mag dimmer, making Phobos about 2.75x brighter.  That might sound like a lot until you consider that the glow around the planet fades as one moves away from it.  I am not sure what the exact relationship is mathematically, but I suspect it is an inverse square law with the glow being 4x as diffuse at 2x the angular separation. 

 

Phobos reaches about 20 arc seconds of separation form the planet surface presently, while Deimos reaches about 60 arc seconds.  That is a 3x distance difference, which would make the glare about 3*3 = 9 times less at Deimos separation.  I'll take that boost in contrast over a less than 3x difference in brightness.  

 

From your magnification/eyepiece/Paracorr value, it appears you have a 15" f/4.5.  This explains why jumping right to ~437x with a 4.5mm was a problem in your earlier description.  My experience it that there is a big hole between 246 and 437x you have tried.   Unfortunately, this hole is right in the area where the optimum often is on the better nights--and by optimum I mean where limiting magnitude stars, galaxies, etc. still hold together, and beyond which visibility begins to suffer because the seeing is not quite still enough.  With my 8" SCT this very good but not quite excellent level was for a combo that provided 310+ (and this was about where it topped out on the dead still seeing nights as well.)   with the 10" and 20" f/5 it has been in the 357x range.  On the best nights I can push to ~400x with the 10" or 500x with the 20", but the 300+ range is more representative of the very good but not quite excellent nights.

I guess my point is that glow around Mars doesn't seem to be a factor for me (I'm talking about the actual glow from scatter and diffraction and atmosphere etc). There's very little of it much past about 10 arc seconds from the planet that I can see. It's not "etched", but it's quite well controlled.

 

But that's different from the actual *glare* of the planet and how that extra light impacts your vision. That glare is present whenever Mars is in the field, thus to me, it renders Deimos' separation an irrelevant factor. It could be 120" away from Mars and I still don't think that would help me see it short of moving Mars out of the field while keeping Deimos in it and letting my eyes dark adapt further.

 

Regarding seeing, I'm just replying to your early statement that if seeing is good enough for Phobos, then Deimos should be super easy (paraphrased). That has never been my experience with any faint star observing. The fainter the star, the better the seeing has to be to see it. The central star in M57 only comes out to play when seeing is close to perfect. I am only able to see stars dimmer than mag 6.1 with the naked eye when seeing is close to perfect. The ultra tiny stars dotting the Orion Nebula are wonderful, but again, only when seeing is close to perfect. They are invisible most of the time. I have been able to faintly glimpse a mag 16 glob in Andromeda in very good seeing. Most nights, mag ~15 stars are the limit in my scope.

 

So it could very well be that my seeing was good enough for Phobos, but not good enough for Deimos.

 

And yes, I grant that when Phobos is in very close to the planet you won't catch it. Every time I've observed it, it's been 12" or greater separation (by chance). The night it was 12" that I saw it, it was moving closer to the planet, and within an hour I could no longer see it. Deimos was still much farther away if I recall, but no luck seeing it.


Edited by CrazyPanda, 18 October 2020 - 08:33 PM.


#50 happylimpet

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Posted 19 October 2020 - 04:18 AM

Regarding seeing, I'm just replying to your early statement that if seeing is good enough for Phobos, then Deimos should be super easy (paraphrased). That has never been my experience with any faint star observing. The fainter the star, the better the seeing has to be to see it. The central star in M57 only comes out to play when seeing is close to perfect. I am only able to see stars dimmer than mag 6.1 with the naked eye when seeing is close to perfect. The ultra tiny stars dotting the Orion Nebula are wonderful, but again, only when seeing is close to perfect. They are invisible most of the time. I have been able to faintly glimpse a mag 16 glob in Andromeda in very good seeing. Most nights, mag ~15 stars are the limit in my scope.

 

You have seeing so bad that it affects your naked eye observing? That would be of the order of 60-200" to affect the naked eye view. Thats a poor observing site!

 

I dont think ive ever seen worse than 15" and that was noteworthy.




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