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Some thoughts on a Transit of Io

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#101 JasonBurry

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Posted 10 December 2012 - 12:56 PM

Boring disc? What's that? Never seen one without its own charm and character.... :)

I think that between you and Eddgie, me and historical accounts from EE Barnard, it's a hard door to shut now.

There are hundreds of amateurs out there with scopes far more capable than the 12" Lick refractor thru which Barnard made his initial observations.

Here's how I see it. In a Victorian refractor, Barnard saw well enough to be CERTAIN of the non-plain-ness of Io, enough to write about it quite extensively. Enough to be willing to wait 3 years to follow it up in the great 36" scope.

That's seeing something unknown.

We, today, have an advantage. We KNOW Io's disk isn't plain, even if it is on the edge of the capabilities of a 6, 8, 12, 14" scope. Knowing to look for a feature is a great advantage. I noticed it, in part, because I'd been puzzled by the football aspect of Io in astrophotos, combined with a fortuitous night of spectacular seeing.

This thread creeps up on 100 replies now. It's been top-of-list on this forum for 2 weeks. Others now know of this. They may not make it their mission to observe non-circular Io transits, but it'll be stuck in the back of their minds. One day, the seeing will work, the "dash" will appear, the memory tweak...

Others will observe. We'll see reports. Eventually. It took Barnard 3 years to confirm his observation.

J

#102 Eddgie

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Posted 10 December 2012 - 05:48 PM

My wish is that it would generate more general interest into how contast transfer (which is very much affected by diffraction caused by the size of the aperture and the obstruction (if present).

Everything on this thread to me was a 100% validation of the theory of MTF.

I just don't understand why more people aren't interested in this topic. It is like the ultimate question from Hitchhiker's guide or The Truth is Out There from X Files. It explains it all. To understand MTF is to understand how telescopes work, and what a given instrument can and cannot do.

A wonderful thread though.. I was excited when somone else saw my lovely Io Pearl. I felt like I shared a special little treasure with friends.

And maybe more people will look for this little jewel now.

#103 azure1961p

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Posted 10 December 2012 - 06:55 PM

I'm having Thursday purposely off to facilitate some astronomy. It's been a stretch of *BLEEP* weather since thanksgiving. It's gonna rain all the way till wednesday. Started yesterday rain all day today and prior just clouds clouds and clouds and patchy sun.

Pete

#104 Asbytec

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Posted 10 December 2012 - 11:12 PM

Agreed with everything said above (cuz lunch is ready.) :)

I'm pulling for you, Pete. You gotta see this if the weather holds. You can repeat Barnard and Pickering! To me that's exciting.

#105 azure1961p

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Posted 11 December 2012 - 07:01 PM

My wish is that it would generate more general interest into how contast transfer (which is very much affected by diffraction caused by the size of the aperture and the obstruction (if present).

Everything on this thread to me was a 100% validation of the theory of MTF.

I just don't understand why more people aren't interested in this topic. It is like the ultimate question from Hitchhiker's guide or The Truth is Out There from X Files. It explains it all. To understand MTF is to understand how telescopes work, and what a given instrument can and cannot do.

A wonderful thread though.. I was excited when somone else saw my lovely Io Pearl. I felt like I shared a special little treasure with friends.

And maybe more people will look for this little jewel now.


Well who's dissenting?

Pete

#106 fred1871

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Posted 11 December 2012 - 07:44 PM

Norme, I'm late to this thread - a very interesting one, and I'm impressed that the "Io effect" can be seen with a 6-inch - but I'm wondering about your suggestion of an observer who saw a 0.2" double elongated with a 6-inch telescope - who was that? SW Burnham?

I'd believe it with a 26-inch telescope, no problem; but a 6-inch? The best figure for SW Burnham, listed in Lewis's discussion of double star resolution in 1914 - yeah, that paper again - is Burnham managing 0.4" with a 6-inch (and 0.35" with a 9.4-inch).

Sidgwick seems to be the source for the 0.2" claim, but I haven't been able to find the source for this claim. And was it just one example? - so we'd need to know the separation of the pair at the time, and as a "one-off" whether it was a lucky guess, and why it didn't happen again.

The work by Christopher Taylor with a 12.5-inch scope on extremely close doubles, where he's recorded detectable elongation down to the 0.17-0.20" range, suggests that somewhere closer to 0.4" might be a better fit for Burnham as well.

I've seen detectable elongation at 0.50"-.53" with 14cm aperture (several pairs) - visible at 400x, confirmed at 570x. I'd suspect that under excellent seeing 0.45" might be possible with 14cm. An analog of Taylor's achievement would be 0.40" with 14cm - I did try one pair at that separation without success, and plan to try again if I can get a steady night and a suitable pair. Taylor may have excellent "out-of-round" eyesight, which is needed with such images (even at the 825x he uses). Extending Taylor to a 6-inch would suggest 0.37" as a limit.

So I have difficulty believing 0.2", even for Burnham. He was a remarkable observer but I don't believe he had to avoid kryptonite. :grin:

#107 Asbytec

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Posted 11 December 2012 - 09:11 PM

Fred, from the S&T article (posted above, but removed from the link), it was Burnham. He, "once correctly identified a binary star separated by only 0.2 arcsecond using a 6" refractor."

Anyway, the article mentioned he was so good at noticing elongation, yet he could not observe any on Io. Apparently he was looking at the full disc trying to elongate a circular disc. Io is not elongated in this way. The article did not say specifically what he was looking at, but I think that has to be the case. It also mentioned they could see the bright equator that would explain Io's apparent elongated effect seen by Pickering.

Io is still tiny small disc in my 150mm, but it has an aspect (best I can tell) near or a tad less than elongation seen in 72 Pegasi at .57" arc using 380x in excellent seeing.

#108 azure1961p

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Posted 11 December 2012 - 09:23 PM

I've not yet tried for a double closer than 0.5 with an 0.576 res scope but from the way 0.5 looked on several doubles I'd wager I could probably see an overlapped diffraction pattern or elongation of equal brightness somewhere between 0.4 and 0.3 arc seconds. It's projection as I haven't done it but 0.5 wasn't that hard in 7/10 or better seeing even when magnitudes were asymmetric.

Pete

#109 Asbytec

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Posted 12 December 2012 - 12:50 AM

Makes sense to me, Pete.

#110 fred1871

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Posted 12 December 2012 - 04:44 AM

Norme, what issue of S&T was the article in? - I can't find it in my DVD set.

#111 Asbytec

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Posted 12 December 2012 - 05:42 AM

I'm sorry Fred, I should have included that information.

"The Story of Jupiter's Egg Moons," by Thomas Dobbins and William Sheehan, Jan 2004.

If you cannot find a copy, I can email it for personal use without violating copyright.

#112 fred1871

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Posted 12 December 2012 - 07:48 AM

Thanks for the information - that'll be on the DVD set of S&T I have. I was looking in the 1990s issues because of the earlier reference to an item on Io back then.

#113 azure1961p

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Posted 12 December 2012 - 02:25 PM

It's a nice article. If it was pulled off the Internet by Sky as it used to be up, Id say it was a bad marketing move. Here's a great example of a single article playing into the needs of the amateur and broadcasted on the biggest amateur astronomy website on the Internet. For all that exposure and the paltry sim it cost anyone it was great PR underscoring Sky's "essential" need to the observer.

But perhaps they didn't pull it and its gone for other reasons. Regardless it was great promotional marketing for the mag and for free.

Oh well.

Pete

#114 Asbytec

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Posted 12 December 2012 - 05:45 PM

Yea, silly board rooms.

Actually, I'd be interested to hear Fred's reaction if he repeats the observation. I got a slice of humble pie riding on it and my fingers crossed. :)

Observed Io last night and place the result in the "pretty sure" category.

#115 JasonBurry

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Posted 13 December 2012 - 07:38 AM

I've been directly under the jet stream the last week or so, and so it will continue. Bugger. One day!

J

#116 azure1961p

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Posted 13 December 2012 - 11:32 AM

ok clear skys tonight, 39F - finally and on a night off [that i created] . Ill do my best with Io and drum up a jupiter sketch to boot. Particularly im going to see what i can do to kill the dew on my oculars when observing. Im going to fabricate something today.

Pete

#117 Asbytec

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Posted 13 December 2012 - 12:27 PM

Pete, try keeping them in your pocket. Good luck with Io, I look forward to your report.

#118 JasonBurry

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Posted 13 December 2012 - 12:32 PM

That's what I do... Eyepieces in pocket till time to use'em. It has worked for me down to -20C.

J

#119 Asbytec

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Posted 13 December 2012 - 02:33 PM

Reading up one the difference between point sources and extended objects, came across this interesting piece. Apparently, there is a limit to how small an extended object can be and still produce a PSF very similar to that of a point source. Io is a good example, and so are the individual chunks that make up Saturn's rings as seen edge on, as Pete mentioned in another thread. I'd say those chunks are well below the 1/4 Airy disc diameter in modest apertures and qualify as point sources.

It appears that angular size is given by 125,000 Lambda/Dmm. So, for D = 150mm and Lambda = 0.00055, anything smaller than 0.46" arc (~1/4 the Airy disc at 1.82" arc) is essentially a point source with no appreciable widening of it's PSF.

Io is well beyond that limit at 1.2" arc. It's image radius changes from 1.22 Lambda F for a point source to a little larger than 2 Lambda F as an extended object (with a apparent angular radius of a little more than half the Airy diameter.) Ganymede is nearly the same angular diameter of a 150mm Airy disc (~1.8" arc) and shows a disc a bit more than twice the Airy disc size at 3 Lambda F (~/> 2 * 1.22 Lambda F radius for a point source.)

http://www.telescope..._resolution.htm

I think this is just further proof one can see elongation in about 6" or more aperture, since Io's apparent diameter is well beyond that of a simple point source PSF or an extended object 0.46" arc. It would be very much like elongating 72 Pegasi at 0.57" arc separation. Although, Io is indeed more challenging.

#120 fred1871

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Posted 13 December 2012 - 07:33 PM

I've now read the article on "Jupiter's Egg Moons" - very good, and a nice portrait of Pickering as an eccentric observer. No surprise that he teamed up with Percival Lowell. :grin:

I'll make an attempt on the "Io effect" though Jupiter isn't well placed for high resolution stuff from where I am - it's at nearly its highest declination north, and I'm in the below-30 latitude region, so it doesn't rise as high as I'd like. I did have a look at Jupiter the night before last, and although seeing was very good in some parts of the sky, it wasn't good enough around Jupiter to see Io as more than a small disk off the planet. I could use 285x, but 400x was wobbly. Elsewhere in the sky I used 400x on some double stars with good effect. The moons were of obviously different sizes. So I'll try again, preferably with Io in transit. Best chance for the elliptical look.

On the claim about Burnham and a 0.2" double, no new information there. I'm now working my way through his General Catalog of Double Stars to see if I can identify the claim. With a number of these he remarks that he suspected a star double with the 6-inch refractor, confirmed later with larger (18.5 or 36 inch typically) scopes. Also a few where he suspected a double but couldn't confirm even with the Lick 36-inch. Once I have the list of possibles, I'll check out the measures list. A couple of these appear to have good orbits established, so a pretty accurate figure for when he first suspected them to be double can be obtained. Along the way I've found a few that fit Lewis's data collection - around 0.4". Very impressive for 6-inch. In a couple of places Burnham remarks on pairs around the 0.2"-0.3" level as being a challenge with larger telescopes.

I'll do a note on this when I've completed it, probably in the Double Star forum. Perhaps with a reference note to it in this thread.

Now, if the clouds (back again) will clear sometime soon, despite the weather forecast of days of more clouds ....

#121 Asbytec

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Posted 13 December 2012 - 09:33 PM

Fred, have been quietly following the rule of thumb thread, if that's the one you mean.

From the math above, I am wondering if somewhere near 0.4" arc (about 1/4th Airy disc) is at or near a limit for the 6". Below that separation seems to be the realm of point sources. According to the math, anyway. Curious...but seems consistent with some observations you mentioned.

I am at 16 degrees North and Jupiter passes pretty much through the Zenith. In our tropical climate upper air flow from the Pacific Ocean is pretty much laminar, the 6" really hits a sweet spot here. The first diffraction ring is nearly always visible and very steady much of that time. It bounces around from time to time, but generally it's very nicely presented.

#122 azure1961p

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Posted 13 December 2012 - 10:31 PM

The first diffraction ring in ct is cause for a gazzoo.

Pete

#123 fred1871

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Posted 14 December 2012 - 12:01 AM

Sounds as if you've got near-ideal conditions for high-res visual images, Norme. I don't get the laminar airflow, being somewhat inland from the wrong (east) coast. And Jupiter's rising only a bit over 30 degrees from my horizon.

Yes, I'd think 0.4" might/should be possible with 6-inch scope, as Burnham and a few others have seen elongation on even pairs at that separation. It also fits Christopher Taylor's experience, scaled from his 12.5-inch.

I'll make attempts on Io as weather allows. Should be interesting. I'm inclined to be doubting of Pickering's claim re 4-5-inch telescopes, but as I have a 5.5-inch (140mm) and it's a refractor I'll see what's possible. I've been surprised before by seeing things I'd thought needed a bigger scope.

Re Pete's last note - ummm, not sure what the slanguage means. :question:

#124 azure1961p

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Posted 14 December 2012 - 12:56 AM

As the other post mentioned, I can confirm with you folks on Ios egg shape. Europa near it made the comparison clear - on color too. Nice pale peach on Io and a nice pale yellow for Ganymede. Europa just appeared off white. At anyrate Europs tighter and symmetrical circle-dot made for good contrast to Io s Egg look.

Never never would e seen had I not specifically looked for it. Confirmed at 364x.

Pete

#125 Asbytec

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Posted 14 December 2012 - 02:59 AM

Fred, I have been literally stunned at what good seeing can allow. Six inches of aperture does not disappoint in the least. I would be pleased if you got stunned by Io, too. We can all add that one to our growing list of stunning sights, even if Pickering was, well, wrong about other things. :)

Pete, exactly. If Europa is in the same FOV, the comparison get's easier. Europa is the definition of round and there is no question, no doubt in my mind it's round. Io ("effect") is reasonably questionable in comparison. Sometimes it just hits you as elongated during the best moments. Over the cloud tops should be much easier.

So glad you caught that, Pete. You're part of a growing alumni. And you're right, you never would have noticed it at 30x per inch observing Jupiter.

Still trying to understand Io's extended nature and why it's difficult to see elongation visually a 1.2" arc - more so that 72 Pegasi at 0.56" arc, for example.






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