Return to the Cloudy Nights Telescope Reviews home page


Observing >> Solar System Observing

Pages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | (show all)
JasonBurry
sage


Reged: 04/27/12

Loc: Cape Spencer, NB, Canada
Re: Some thoughts on a Transit of Io new [Re: Eddgie]
      #5558965 - 12/07/12 10:11 AM

Eddgie, feel free to use that image, if in the future you find yourself trying to explain this again.

J


Post Extras: Print Post   Remind Me!   Notify Moderator  
Eddgie
Postmaster
*****

Reged: 02/01/06

Re: Some thoughts on a Transit of Io new [Re: Eddgie]
      #5558991 - 12/07/12 10:24 AM

And I think I might add, that this is a great peice of detective work.

This can easily explain why these early observers saw what they saw, but back then, diffraction theory and MTF were not really well known, and optical quality was not I think as well understood. The answer was simply to build bigger and bigger scopes!

The artice that stated that tube currents could be the cause is unlikely to me because these people were very serious observers and I think would be famaliar with these issues.

What a fun tread this has been for me. It is such a powerful testimony to the beauty of MTF that perhaps it will encourage more people to want to learn about it.

I am sure a lot of people got really tired of my MTF plots, but they explain all of this with great precision.

And obstructed scopes, near the limit of their performance, actually have better contrast than unobstructed scopes!

In fact, the observer using a smaller refractor may have not seen this effect so easily and may have been more likely to see Io as just a point, where a reflector user using the same size instrument would have been able to detect (sorry Norme) that the disk of Io was not evenly illuminated simply because of the fact that it appeared elongated and a quick look at the shadow of a transit would show it to be distincly round. The only inescapable conclusion would be that the disk is not evenly illuminated!

But again, the peer reviewers back then would not have had this theory to help them determine why the observations yeilded the results that were reported.

But this peice of modern detective work I think is a clear and concise explination that is anchored in the concrete physics of diffraction and MTF.

I think this is a minor historical mystery that has been completely solved! There is 100% correlation between theory and the observations reported and seperated by many, many decades of time.

My humble opinion, but one of the best threads ever on this entire group of forums. I feel like we have contributed to history by solving this minor mystery.

Edited by Eddgie (12/07/12 10:26 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: Some thoughts on a Transit of Io new [Re: Eddgie]
      #5559136 - 12/07/12 11:40 AM

Every scope transfers contrast, that's essentially what scopes do. The act of gathering light (or lack of light) and putting it on the focal plane involves the transfer of contrast (simply diffraction) from the target to the focal plane. That involves a good figure, etc., all the stuff we debate about performance. But, at it's core, it's all about taking an object and transferring it to the focal plane.

So, there is a level of performance that can be described by theory. Then there is actual performance that is not theory. In either case, the scope is performing to it's level of contrast transfer...it's MTF. Every time, all day, all night, from now until the end of time.

The MTF describes everything (or can, theory is all about concept rather than practice) that affects performance from diffraction, to CO, to aberrations, to seeing. The actual curve might not look like the perfect "textbook" curve on any given night, but it is always performing at a level of contrast transfer none-the-less. Always. That's what scopes do.

However, when all those variables (aberration, seeing, cooling, collimation, and the list goes on) are minimized, the scope's performance will be very closely matched to it's textbook MTF performance. And when that happens, look out! Superb views...

Anyway, I am a believer in the science of it.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: Some thoughts on a Transit of Io new [Re: JasonBurry]
      #5559173 - 12/07/12 12:00 PM

Quote:

On Sept. 8, .189, with the I2-inch this satellite presented a remarkable aspect while in dark transit. I noticed that it appeared elongated in a direction nearly perpendicular to the belts of Jupiter.




Ya! So, he saw it, too. And was not sure why, if only for a while. Fascinating, Jason, just fascinating. I think this exceeds reasonable doubt, and for the reasons Eddgie (and Pete, too, by the way) have always said. Such difficult observations are not impossible, they are within the laws of physics even if they (seem to) violate the old textbook standards of resolution.


Post Extras: Print Post   Remind Me!   Notify Moderator  
JasonBurry
sage


Reged: 04/27/12

Loc: Cape Spencer, NB, Canada
Re: Some thoughts on a Transit of Io new [Re: Asbytec]
      #5559229 - 12/07/12 12:36 PM

Yes, Barnard was seeing Io transit a light cloud band, and thus it was the dark poles he saw, making the relatively dark shape of the moon stand out, looking to be extended in the polar axis, as the equatorial region blended with the background clouds.

Further observations of similar transits against a light cloud band background lead him to begin to discern the 2 polar caps as individual objects, separated by the effectively invisible bright equatorial region.

Essentially, Barnard is describing a second possible aspect in which the Jovian background can affect the apparent shape of Io. On a light background, Io may appear only as 2 poles. On a darker cloud background, the poles blend and only the equatorial region stands out.

Rayleigh, Dawes, and Sparrow ("textbook" resolution limits) all concern separating point sources. The extended nature of Io, a gathering of infinite point sources, requires a examining the matter in a slightly different, though ultimately related, frame of mind. The diffraction effects for any one of those points remains the same, but they blend together to make the final image. The image is the sum of its parts.

We aren't breaking any rules of resolution, only refining our understanding of them.

J


Post Extras: Print Post   Remind Me!   Notify Moderator  
JasonBurry
sage


Reged: 04/27/12

Loc: Cape Spencer, NB, Canada
Re: Some thoughts on a Transit of Io new [Re: JasonBurry]
      #5559410 - 12/07/12 02:30 PM

EE Barnard's follow up of the his Jan 8, 1890 observation is available here:

Barnard's report

I ran the PDF thru a quick OCR and reproduce the content here:

*********************************************************
On the Dark Poles and Bright Equatorial Belt of the First
Satellite of Jupiter. By E. E. Ba.rnard, M.A.

It will, perhaps, be remembered that on 1890 September 8, while observing the transit of the first satellite of Jupiter with the 12-inch equatorial, the satellite appeared as a dusky elongated spot projected on a bright region of Jupiter. With a high power it appeared distinctly double, on a line nearly vertical to the belts of J~6piter. I called Mr. Burnham's attention to it, and we both saw it thus for upwards of half an hour. Subsequently it was seen several times when crossing a dark portion of the planet as a bright, very elongated spot, but the elongation in these cases was nearly parallel to the belts of h6piter. Yet when closely examined on the sky it always appeared perfectly round.

An account of these observations will be found in Monthly Notices, vol. Ii. , NO. 9. To explain these singular peculiarities I at that time offered two theories: the first of these was that the satellite was itself possibly double. This idea has long since been abandoned because of its improbability. The second theory supposed the satellite to be surrounded with a white equatorial belt, and that its poles were dark or dusky, and that it rotated on an axis nearly pflrpendicular to its orbit. If such were the case, when the satellite crossed a bright portion of the planet the white belt would cut it apparently in two, as it would be equal in brightness to the surface of Jupiter, and thus leave the two dark polar caps as two separate spots nearly perpendicular to the belts of Jupiter, and would thus give the observed appearance of duplicity.

If, however, the satellite should happen to be projected on a dark belt, then the dark poles would merge into the surface of Jupiter, and the white equatorial belt alone would be visible as an elongated white spot nearly parallel to the belts of Jupiter.

It was not until this year (1893) that an opportunity occurred to settle this question with the 36-inch.

On September 2S last the transit of this satellite was watched with the great telescope. It first appeared as an elongated white spot, east and west, when over the dusky region near the planet's limb, and later it appeared as two dusky spots in a line north and south, exactly as it appeared in the lz-inch 1890 September 8. But during moments of good definition it was seen distinctly as a small round disc, dusky at the poles, and with a white belt between them.

On November 11 it was again observed under nearly similar conditions, and presented the same phenomena.

The best view of these dark polar caps and bright belt was had on November 19, with a power of 1,000 diameters on the great telescope, and almost perfect seeing.

At this transit the satellite partly obscured its own shadow on the south preceding side, and was partly projected on the southern edge of the south equatorial belt, and partly on the bright region beyond.

With the fine seeing the satellite presented a beautiful appearance. It stood out in bold relief like a little globe. The polar caps were heavi.ly marked and quite dark, while the bright belt was very conspicuous.

The observation was perfectly satisfactory, and the second theory had become a fact.

I send a careful drawing of the appearance of the satellite on this date, which will give a good idea of the phenomenon. The shadow was larger than the satellite.

We have here established the discovery made 1890 September 8 with the 12-inch telescope, that this little attendant on Jupiter has distinct polar caps that are dusky like those of Jupiter, and that it has a bright equatorial belt, as bright as the brightest portion of Jupiter's surface. The conclusion is uncontestable, therefore, that the satellite also rotates on an axis nearly perpendicular to its orbit, as Jupiter itself does.

From the fact that the bright belt is not always exactly parallel to those on Jupiter, and the line between the polar caps is not always exactly perpendicular to the belts of Jupiter, there must be a slight inclination of the axis of rotation of the satellite. From the fact also that the south cap is sometimes apparently a little smaller than the northern one, its axis is probably tipped away from us at its southern end. It is also probably tilted towards the west by a few deg-rees. From peculiarities in the appearance of the belt, it is probable that the period of rotation on this axis is not coincident with the satellite's period of revolution about Jupiter.

I have data in my hands now that will after a few more observations, perhaps, settle th.e inclination of the axis, and probably give us the period of rotation.

I think the presence of these dusky poles and bright belt would rather imply that this satellite at least is in a physical condition not vastly different from that of Jupitm' itself.

Mount Hamilton, California:
1893 November 27.

Royal Astronomical Society Provided by the NASA Astrophysics Data System
1894MNRAS..54..134B
**********************************************************

Again, today we have the distinct advantage of knowing that Io's rotation period does indeed match its orbital period, and its axis is virtually parallel to Jupiter's, but I can't help but be impressed by Barnard's careful observations.

I'd love to hear of others repeating this observation. It's quite achievable (hell, I done it!), and extremely rewarding. How small a scope can this be observed in? We've got down to 6" so far... My 4.5's incapable, I believe, but another might be able...

J


Post Extras: Print Post   Remind Me!   Notify Moderator  
Eddgie
Postmaster
*****

Reged: 02/01/06

Re: Some thoughts on a Transit of Io new [Re: JasonBurry]
      #5559796 - 12/07/12 06:42 PM

Quote:

The best view of these dark polar caps and bright belt was had on November 19, with a power of 1,000 diameters on the great telescope, and almost perfect seeing.




Isn't this quite interesting.

In "Almost perfect seeing" the best view of the dark polar caps was seen with a power of 1000 diameters.

Interesting that in an 889mm aperture this power gives an exit pupil of .9mm.

Hmmmm. How very interesting... Could have used any power they wanted on a night of excellent seeing, but they felt that their best result was with an exit pupil that I myself often recommend as being about the best exit pupil for planetary observing where the goal is to see the most detail possible in a view.

Funny coincidence. But I guess that is all it is. I mean they were using a refractor and everything, so I would have thought 1750 diameters would have been better because that would be about 50x per inch.

Yeah, maybe just a funny coincidence that in a night of almost perfect seeing, their best result was with a .9mm exit pupil.

I don't know.. Head scratcher I guess.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Rick Woods
Postmaster
*****

Reged: 01/27/05

Loc: Inner Solar System
Re: Some thoughts on a Transit of Io new [Re: Eddgie]
      #5559804 - 12/07/12 06:50 PM

Quote:

Yeah, maybe just a funny coincidence that in a night of almost perfect seeing, their best result was with a .9mm exit pupil.

I don't know.. Head scratcher I guess.




Not at all.
You just have a good feel for the right exit pupil for the job.


Post Extras: Print Post   Remind Me!   Notify Moderator  
azure1961p
Postmaster
*****

Reged: 01/17/09

Loc: USA
Re: Some thoughts on a Transit of Io new [Re: Eddgie]
      #5559885 - 12/07/12 07:57 PM

That's a terrific illustration. Clear to the point and how I envisioned it from the thread, notably Eddgies and Normed description. An interesting question would be WHEN does this effect disappear and the moon is merelly a circle hoar airy disc? I'm confident my 70mm could not pull offthis kind of thing even at 300x.

I'm going to guess based on Normed effort its conceivable maybe that a 5" aperture might do it?

The 1000 diameters for a 36" telescope being optimum compared to 50x per inch would sound about right for it on a night of near perfect seeing while 50x per inch would probably suite a 6" scope. I don't think that the power per in h rule is a constant with apertures of greater and greater size where the atmosphere plays a greater and greater role in compromising the potential for full resolution. At some point even near perfect seeing is going to prevent this as aperture increases.

Pete

Edited by azure1961p (12/07/12 08:34 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: Some thoughts on a Transit of Io new [Re: azure1961p]
      #5560008 - 12/07/12 09:42 PM

Pete and Jason, in fact, Pickering urged amateurs to attempt it in 4 and 5 inch scopes! Why? Probably because he understood the angular size of Io would allow it. It get's increasingly difficult do do as Airy disc size approaches it's angular size. As the optical path difference drops to zero (each point is closer to the center of the Airy disc) strange things happen. At zero OPD, the Airy disc itself expands and brightens.

What's interesting, in the article one observer credited with resolving (sorry Eddgie ) a binary star at 0.2" arc in a 6" refractor also said he could detect no elongation of Io. One has to assume he was using a scope large enough to see the poles, then Io would indeed not be circular. But, that does not refute the observation that Io /appears/ elongated in some apertures.

Pete, this stuff is right up your alley. Jason, yes, we're not violating anything about Raleigh or Dawes. They define the limits of resolution as dark space between two point sources. This is not happening here, we are observing the effects on diffraction on extended objects. There is, of course, no dark space /between/ any of Io's points. There is dark space "around" them, however, which allows "detection" as one spurious disc invades that dark space. Everything Dawes and Raleigh say remains true. But, to call their work true limits is kind of misleading.

This is why the guy mentioned above could discern such a small 0.2" arc angular separation in a 6" refractor. I have a similar experience with elongating 72 Pegasi at 0.5" arc.

Edited by Asbytec (12/07/12 10:08 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
azure1961p
Postmaster
*****

Reged: 01/17/09

Loc: USA
Re: Some thoughts on a Transit of Io new [Re: Asbytec]
      #5560193 - 12/07/12 11:56 PM

Damn does this mean I can elongate 0.1? Funny because 0.1 was the first sign of elongation I could detect with seeing-free aberrator. Boy that'd be a 9/10 night or nothing.

Pete


Post Extras: Print Post   Remind Me!   Notify Moderator  
Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: Some thoughts on a Transit of Io new [Re: azure1961p]
      #5560391 - 12/08/12 03:55 AM

Dunno, don't let Dawes stop you from trying.

But, you know, once you start getting that close, strange things begin to happen. Up to a point where optical path is very small, the PSF remains relatively normal. You might even be able to detect two stars directly on top of each other by noting the Airy disc increases in size, pushing the first ring outward a bit. As I understand it, anyway.

"Resolution of two stars in coherent light at 1.22Lamda/D angular separation varies with the OPD between two sources. At zero path difference, the two patterns merge together, forming the central maxima of 1.83LambdaF in radius and 1.47 peak intensity. At pi/2 OPD the combined pattern is identical to that in incoherent light, and at OPD=pi; the two 1.11 maximas are somewhat more widely separated, with the intensity deep between them dropping to zero (Sparrow limit), the latter two indicating significantly better limiting resolution."

http://www.telescope-optics.net/telescope_resolution.htm

Bold is my edit. I think that is what he is saying.

Edited by Asbytec (12/08/12 07:18 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Dean Norris
Carpal Tunnel
*****

Reged: 11/05/08

Loc: Santa Cruz, Ca
Re: Some thoughts on a Transit of Io new [Re: Asbytec]
      #5561528 - 12/08/12 09:22 PM

This thread has been very informative for me. I enjoyed reading the discussions here and the historical accounts. I would love to see the albedo features on Io during transit. I look forward to a night when the seeing is perfect when I can boost the magnification to the max. Maybe a trip to Mt Hamilton. BTW I did have the opportunity to look through the 12 and 36 inch telescopes at Mt Hamilton years ago. Thanks to the people who have contributed to this great thread. There is an image by DesertRat that shows the Io transit that happened last night which I saw in ok seeing. His image shows the dark poles that are contrasted by the face of Jupiter. The other image linked to earlier by Mike Phillips is an excellent image as well.

Great thread. Thank you everyone. Dean


Post Extras: Print Post   Remind Me!   Notify Moderator  
Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: Some thoughts on a Transit of Io new [Re: Dean Norris]
      #5561633 - 12/08/12 10:48 PM

Dean, glad you chimed in. This observation sheds new light on Jupiter. I bet you will be amazed when you get a chance to attempt it.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: Some thoughts on a Transit of Io new [Re: Asbytec]
      #5562404 - 12/09/12 12:45 PM

Had to peek at Io again tonight. Very clear, pretty steady. Unfortunately, I have too file tonight's observation as inconclusive. Io would just not hold still long enough for a good look. Still had those moments where it appeared elongated, but just cannot rule out seeing as the cause. Both Io and Europa were just not steady for periods long enough to really make a call. But both in the same FOV for comparison.

Post Extras: Print Post   Remind Me!   Notify Moderator  
azure1961p
Postmaster
*****

Reged: 01/17/09

Loc: USA
Re: Some thoughts on a Transit of Io new [Re: Asbytec]
      #5562615 - 12/09/12 03:05 PM

I don't think there is a solar system body that's able to reveal albedo effects that's more seeing demanding than Io. Little tremors in seeing that might gently soften the planets temporarily obliterate everything . I envy hour views regardless based on your description elsewhere.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: Some thoughts on a Transit of Io new [Re: azure1961p]
      #5562773 - 12/09/12 04:52 PM

You can bet, gonna keep that one on the list.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: Some thoughts on a Transit of Io new [Re: Asbytec]
      #5563855 - 12/10/12 10:00 AM

Well, I am totally sold. I do n0t care what anyone says, Pickering's and Barnard's observations can be repeated in a 6 or 8" aperture. Io is just barely elongated, even against the black of space. It takes very excellent conditions to pull it off in a 6", but it can be done. Period.

Post Extras: Print Post   Remind Me!   Notify Moderator  
JasonBurry
sage


Reged: 04/27/12

Loc: Cape Spencer, NB, Canada
Re: Some thoughts on a Transit of Io new [Re: Asbytec]
      #5564036 - 12/10/12 12:01 PM

I'm sold too (ya know, what with having seen it in my own eye and such), though I long for a repeat of that first night's conditions so I can see it again. I've had the scope out only 2x since then, both days with 5-20 arcsecond seeing, so no chance.

J


Post Extras: Print Post   Remind Me!   Notify Moderator  
Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: Some thoughts on a Transit of Io new [Re: JasonBurry]
      #5564063 - 12/10/12 12:17 PM

Jason, now if other's would just do it, make it common knowledge. Eddgies "pearl" comment and your thread opened the door to nudge visual observing into a better place. I just hope my own observations put down a door stop to keep the door from slamming shut. Great thread, one that made a difference for those who care to observe such things. Scratch one more boring disc from the list of boring discs.

Edited by Asbytec (12/10/12 12:19 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | (show all)


Extra information
1 registered and 0 anonymous users are browsing this forum.

Moderator:  Rich (RLTYS), star drop 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 6386

Jump to

CN Forums Home




Cloudy Nights Sponsor: Astronomics