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Ceres and Jovian Moons

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#1 patindaytona

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Posted 02 September 2019 - 03:22 PM

I;m using a 10" dob. I've tried to resolve Jupiter's moons, but so far, it doesn't appear i can at least see them as a disk. They still appear as perfect points. Is it possible with a 10"?

What about Ceres? IF....i got lucky enough to locate it, would I be able to disern it as a disk at all, rather than a point?gvvvvvvvvvvvvvvvvvv.jpg screenshot-in-the-sky.org-2019.09.02-16_34_57.png

Noticed position of the star cluster M80. See how the location's vary from one to other? Both dates are correct. Suppose it's good enough...if...i can resolve Ceres into at least a tiny dot so i know it's what I'm looking at. 10" reflector.


Edited by patindaytona, 02 September 2019 - 03:40 PM.

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#2 aeajr

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Posted 02 September 2019 - 03:49 PM

Using a 12" they have only been star like dots to me. 



#3 happylimpet

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Posted 02 September 2019 - 04:56 PM

With my 12" and decent seeing, its pretty easy to distinguish Europa (tiny) and ganymede/callisto (bigger/greyer). I doubt you'd resolve Ceres, thats marginal for even top end imaging methods with a 12", and maybe a 10" on a perfect day.



#4 patindaytona

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Posted 02 September 2019 - 06:59 PM

With my 12" and decent seeing, its pretty easy to distinguish Europa (tiny) and ganymede/callisto (bigger/greyer). I doubt you'd resolve Ceres, thats marginal for even top end imaging methods with a 12", and maybe a 10" on a perfect day.

Ok, then all i could hope for would be estimate with some confidence that I am actually looking at Ceres and not just another star.

With a 10" is it still difficult to see any sign of a disk rather than a point with Ganymede/Callisto? While i'm on the subject, could i be able to see the G.B.Red Spot?



#5 Redbetter

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Posted 02 September 2019 - 11:59 PM

Yes, the large Jovian moons should be resolvable as disks in a 10" with sufficient magnification if the seeing is stable, but the smaller ones will not be as sharp because of diffraction blurring of the edges.  We are well past opposition now, and just a few days from quadrature for Jupiter, so the moons are not quite as large as they were earlier.  The relative sizes of their shadows are even more noticeable on the planet.

 

Ganymede, at 1.7", is not that much smaller than Neptune (2.4") when Jupiter is near opposition. 

 

Ceres is likely resolvable for a 10 inch in good seeing at high power near opposition.  This year it reached about 0.7" in size, but that was in May.  I didn't have very good seeing this year in May (which is usually near the peak seeing) but I could still see it as a discrete disk in the 20" in early June, with flares from seeing bouncing around it.  It has shrunk to under 0.5" at present.

 

If you are seeing bright moons as "perfect points" then you are likely not using sufficient magnification.  You should be using enough power that stars of the same magnitude would reveal an airy disk, and more than that is preferable if seeing allows.  For perspective, I have used a 10" to see the elongated shape of the airy disks of Zeta Bootes at 417x with only 0.36" of separation, in 4/10 seeing--I could detect it at 250x but higher power made things more apparent.  The diffraction rings were a mess due to the seeing.



#6 happylimpet

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Posted 03 September 2019 - 05:47 AM

This might be useful.  This was done with a 12", using an R filter. a 10" unfiltered under excellent seeing should give similar results (smaller aperture and shorter wavelength roughly cancelling out).

 

You can see the difference in appearance. It must be much more striking with the 20" as redbetter mentions.

 

https://www.cloudyni...s-a-disk-again/


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#7 patindaytona

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Posted 03 September 2019 - 08:52 AM

Yes, the large Jovian moons should be resolvable as disks in a 10" with sufficient magnification if the seeing is stable, but the smaller ones will not be as sharp because of diffraction blurring of the edges.  We are well past opposition now, and just a few days from quadrature for Jupiter, so the moons are not quite as large as they were earlier.  The relative sizes of their shadows are even more noticeable on the planet.

 

Ganymede, at 1.7", is not that much smaller than Neptune (2.4") when Jupiter is near opposition. 

 

Ceres is likely resolvable for a 10 inch in good seeing at high power near opposition.  This year it reached about 0.7" in size, but that was in May.  I didn't have very good seeing this year in May (which is usually near the peak seeing) but I could still see it as a discrete disk in the 20" in early June, with flares from seeing bouncing around it.  It has shrunk to under 0.5" at present.

 

If you are seeing bright moons as "perfect points" then you are likely not using sufficient magnification.  You should be using enough power that stars of the same magnitude would reveal an airy disk, and more than that is preferable if seeing allows.  For perspective, I have used a 10" to see the elongated shape of the airy disks of Zeta Bootes at 417x with only 0.36" of separation, in 4/10 seeing--I could detect it at 250x but higher power made things more apparent.  The diffraction rings were a mess due to the seeing.

Thanks for all that info! Right now, Jupiter is very low of course and the seeing is not good. I'll try to up the magnification though and see if i can get the moons.



#8 patindaytona

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Posted 03 September 2019 - 08:57 AM

From your comment above, i can resolve Neptune cleary as a disk with a 10"?

What about Uranus?



#9 Redbetter

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Posted 04 September 2019 - 05:42 PM

From your comment above, i can resolve Neptune cleary as a disk with a 10"?

What about Uranus?

Yes. 

 

You should download Stellarium or some other planetarium software that will show sizes and such.  Uranus is about 3.7" right now.  As with Neptune, Uranus doesn't change much in angular size because it is already so distant.

 

A lot of these questions are ones you can get an idea for/work through by understanding the resolution limit (specs and such are out there) and by looking up the information on the web.  But the best thing to do is to observe and use a wide range of magnifications to become familiar with key factors.  Experience will give you the best feel for what you can do with your scope.  Seeing is always a major player for high power observing and resolving small objects.

 

With regard to experience, I have the impression from you posts that you have never done star tests/examined the airy disk pattern.  Maybe you don't have the seeing for it, or have only been using mid/low power eyepieces, but until you become familiar with what it takes (magnification and appropriate star brightness) to see that pattern in focus, it is going to be difficult for you to anticipate what your scope should be showing.  That was why I asked about that sort of thing when you first described problems with your scope. 



#10 Napp

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Posted 04 September 2019 - 05:51 PM

As to Ceres sketch the position of Ceres in relation to the immediate surrounding stars.  Then observe a couple nights later to verify that the object you thought was Ceres moved or if one of the surrounding ‘stars’ is what changed position.  Whichever one moved is Ceres.


Edited by Napp, 04 September 2019 - 05:52 PM.

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#11 patindaytona

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Posted 04 September 2019 - 07:49 PM

Yes. 

 

You should download Stellarium or some other planetarium software that will show sizes and such.  Uranus is about 3.7" right now.  As with Neptune, Uranus doesn't change much in angular size because it is already so distant.

 

A lot of these questions are ones you can get an idea for/work through by understanding the resolution limit (specs and such are out there) and by looking up the information on the web.  But the best thing to do is to observe and use a wide range of magnifications to become familiar with key factors.  Experience will give you the best feel for what you can do with your scope.  Seeing is always a major player for high power observing and resolving small objects.

 

With regard to experience, I have the impression from you posts that you have never done star tests/examined the airy disk pattern.  Maybe you don't have the seeing for it, or have only been using mid/low power eyepieces, but until you become familiar with what it takes (magnification and appropriate star brightness) to see that pattern in focus, it is going to be difficult for you to anticipate what your scope should be showing.  That was why I asked about that sort of thing when you first described problems with your scope. 

I'll google airy disk pattern. It's a collimation test? I tried that but it's alot more difficult than collimating indoors which is bad enough!



#12 Redbetter

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Posted 04 September 2019 - 11:16 PM

The airy disk pattern is the most basic appearance of a focused, relatively bright star at high magnification (typically 25 to 50x/inch, 1mm to 0.5mm exit pupil.)   It is one of the most fundamental aspects in understanding resolution, basic evaluation of what a star image should look like, and for evaluating close double stars. 

 

Not knowing what the airy disk pattern is explains to me why you have been unable to tell the difference in appearance between Jupiter's moons vs. stars with 10" of aperture.  Neither are "perfect points."  With sufficient magnification, stars and very small apparent size objects look different than one another, even if the latter lack well defined edges.  This is more apparent in good seeing, but can be detected in mediocre conditions to some degree. 

 

One of the subtle things in poor seeing at lower power when comparing a moon/asteroid vs. stars is that extended objects display less twinkle/fluctuation in brightness.  Given the same magnitude, they look different, steadier.



#13 patindaytona

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Posted 05 September 2019 - 07:42 AM

The airy disk pattern is the most basic appearance of a focused, relatively bright star at high magnification (typically 25 to 50x/inch, 1mm to 0.5mm exit pupil.)   It is one of the most fundamental aspects in understanding resolution, basic evaluation of what a star image should look like, and for evaluating close double stars. 

 

Not knowing what the airy disk pattern is explains to me why you have been unable to tell the difference in appearance between Jupiter's moons vs. stars with 10" of aperture.  Neither are "perfect points."  With sufficient magnification, stars and very small apparent size objects look different than one another, even if the latter lack well defined edges.  This is more apparent in good seeing, but can be detected in mediocre conditions to some degree. 

 

One of the subtle things in poor seeing at lower power when comparing a moon/asteroid vs. stars is that extended objects display less twinkle/fluctuation in brightness.  Given the same magnitude, they look different, steadier.

I've seen the airy disk thru my telescope to check on the collimation and it looks good. When you say "airy disk pattern is the most basic appearance of a FOCUSED, relatively bright star at high magnification",  I don't understand how you can see an airy "disk" pattern, if it's in focus. I thought it supposed to be slightly out of focus to see it.

I understand about the WHY in stars vrs planets twinkling. A disk overlaps itself slightly, with any turbulence in atmosphere.



#14 patindaytona

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Posted 05 September 2019 - 07:59 AM

"If you are seeing bright moons as "perfect points" then you are likely not using sufficient magnification.  You should be using enough power that stars of the same magnitude would reveal an airy disk, and more than that is preferable if seeing allows.  For perspective, I have used a 10" to see the elongated shape of the airy disks of Zeta Bootes at 417x with only 0.36" of separation, in 4/10 seeing--I could detect it at 250x but higher power made things more apparent.  The diffraction rings were a mess due to the seeing"

 

I see what you're saying more clearly here. Enough power that stars of the same magnitude (zeta bootes with 417x magnification) would reveal an airy disk.

In other words, their IS an airy disk when it is in focus. I don't understand what you mean you saw the "elongated shape" (is zeta bootes an asteroid??).

 

The gest of this is that i should be seeing a star with similar magnitude at magnification when it begins to show an airy disk. THAT, is the magnification threshold to use?

Just seems it would be alot easier and faster to use various eye pieces until i do........see a disk with Jupiter's moons. It's the same thing, i'd think.

If i get a chance i'll crank up the magnification alot more to see what i get with the larger moons, if possible.

 

If i get an airy disk with a star, how do I know if that's all I'm getting when i view one of Jupiter's moon also? (rather than an "actual" disk of the moon).


Edited by patindaytona, 05 September 2019 - 08:02 AM.


#15 Redbetter

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Posted 05 September 2019 - 03:00 PM

I've seen the airy disk thru my telescope to check on the collimation and it looks good. When you say "airy disk pattern is the most basic appearance of a FOCUSED, relatively bright star at high magnification",  I don't understand how you can see an airy "disk" pattern, if it's in focus. I thought it supposed to be slightly out of focus to see it.

For simplicity I will quote Wikipedia:  "In optics, the Airy disk (or Airy disc) and Airy pattern are descriptions of the best-focused spot of light that a perfect lens with a circular aperture can make, limited by the diffraction of light."

 

It is fundamental, this is the pattern that focused point sources present through a circular aperture.  As long as the star is sufficiently bright and the seeing is sufficiently stable, at high magnification the central spurious disk will be seen along with a minimum followed by 1st diffraction ring and so on.  The second or third rings may or may not be seen depending on the nature of the scope/figure/brightness of the star, seeing, one's eye, etc.

 

Inside and outside of focus results in a broader pattern as diffraction rings are added.  This is helpful for star testing and collimating because the scale is easier to see and interpret, but it arises from the airy pattern caused by diffraction.  Indeed, as you turn the focus knob in and out you can watch the central spot disappear as it expands into a ring, then a new spot appears until it too forms a new ring, and so on.  The diffraction effects are visible at focus, but more so out of focus.

 

Stars don't remain the same size at focus as magnification is increased to the point that the airy disk can be resolved.  At roughly 1mm exit pupil, most observers can begin to see the 1st diffraction ring separate from the central spurious disk at best focus.  (I typically see it beginning at roughly 1.5mm exit pupil in good seeing if the star is the right balance of bright, but not too bright.)  As magnification is increased the pattern becomes larger.  This is fundamental to double star observing where much higher powers are sometimes employed to examine the spurious disks and look for "bumps" on diffraction rings, etc.  The following link has a lot of information on how this works:  link.



#16 patindaytona

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Posted 05 September 2019 - 03:14 PM

I know this is all interesting to know, as is alot of things to do with optics and the mechanics of telescopes.

But in practice, I will just use high magnification eye pieces that i have available to me and see with my own eyes which one (s) do the best job, depending on circumstances such as the seeing, transparency etc.

I'm primarily interested in finding out if i can see any of Jupiter's moons, Ceres, Neptune, Uranus.

It's all good to know though.



#17 Redbetter

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Posted 05 September 2019 - 03:46 PM

 

I see what you're saying more clearly here. Enough power that stars of the same magnitude (zeta bootes with 417x magnification) would reveal an airy disk.

In other words, their IS an airy disk when it is in focus. I don't understand what you mean you saw the "elongated shape" (is zeta bootes an asteroid??).

 

The gest of this is that i should be seeing a star with similar magnitude at magnification when it begins to show an airy disk. THAT, is the magnification threshold to use?

Just seems it would be alot easier and faster to use various eye pieces until i do........see a disk with Jupiter's moons. It's the same thing, i'd think.

If i get a chance i'll crank up the magnification alot more to see what i get with the larger moons, if possible.

 

If i get an airy disk with a star, how do I know if that's all I'm getting when i view one of Jupiter's moon also? (rather than an "actual" disk of the moon).

Zeta Bootes is a close binary star that is nearing its closest approach (when it won't be resolvable visually).  What I described was the appearance of the two overlapping airy disks.  They overlapped because they were closer than the resolution limit of the aperture.  In my 20" I could still resolve them individually with a gap between, but only when the seeing was good enough to see the spurious disks. 

 

As to how you know a diffraction pattern from a tiny disk:  experience.  I have explained some of the ways I can tell at the threshold and even beyond.  The lack of a first diffraction ring, instead seeing a somewhat blurred disk is a giveaway.  The same applies to double stars where one can interpret disturbances in diffraction patterns even when the seeing is not sufficient for resolution...how...by comparing to similar magnitude stars and recognizing odd interference patterns or irregularities.

 

The problem you have had is that it doesn't sound as though you have used enough magnification to notice there was an airy disk pattern on bright stars.  The eye needs some scale, typically 120 arc seconds of apparent size as a minimum, if not 180 arc seconds to better see something vs. detection only as a point.  Last night I did some backyard observation of Neptune and Triton to remind myself of the appearance in smaller apertures:

 

I took another look tonight with the 80ED and then the 10" Dob in town to remind myself how Neptune's disk appears in smaller apertures.  Skies were about 18.9 MPSAS (red/orange transition) and about 4/10 Pickering in the 10".

 

The strong color contrast was even noticeable in the 80, although there Neptune was more of a slate gray at 150x rather than a more obvious blue.  Even at 100x I could tell that Neptune was a tiny, barely resolved, soft disk.  At 150x and 200x there was some interesting comparison with the ~9 mag star to the south.   While Neptune's disk scaled larger in size, the comparison star had a noticeably smaller, more intense (but overall less bright) spurious disk.  Neptune was also more steadily seen compared to the fluctuations in brightness of the central spurious disk of the stars.

 

The 10" made Neptune brighter and easier to discern as a somewhat sharper disk at 208x, more so at 250 and 313x.  The blue was more readily seen than in the small scope.  Triton was visible with certainty after a bit of study at 250x.  It was less readily seen at 208, equally well seen at 313x with greater separation, and more difficult at 417x due to seeing disruption impact on the faint moon.



#18 Redbetter

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Posted 05 September 2019 - 04:08 PM

I know this is all interesting to know, as is alot of things to do with optics and the mechanics of telescopes.

But in practice, I will just use high magnification eye pieces that i have available to me and see with my own eyes which one (s) do the best job, depending on circumstances such as the seeing, transparency etc.

I'm primarily interested in finding out if i can see any of Jupiter's moons, Ceres, Neptune, Uranus.

It's all good to know though.

It isn't just "interesting to know" it is an explanation of how to see things and what you are actually observing.  You kind of need to know what a focused star should look like at high power to be able to get much out of your viewing or to even evaluate seeing one night vs the next.  The more you know about how this works, the more readily you know what magnification to use and what to look for.  What is the max magnification you have used on these objects?  I haven't read all of your many recent posts, but I don't recall you ever stating what magnifications were used.  That is kind of critical for someone trying to understand what you are seeing and trying to assist you.

 

If one doesn't know that a star will have a diffraction pattern and a sufficiently large moon/asteroid will not, then how would one tell them apart when the size of the diffraction pattern is nearly the same as the solar system body?  If large enough and with sufficient aperture at sufficient magnification then a person should recognize the disk, but if they never use enough magnification...then all they will see are points.

 

From what I recall when you first started asking questions about your scope, I asked if you had star tested it and what stars looked like at high power.  I didn't really get a response to that and it took hundreds of posts before that was sorted by others.  This is not an infrequent problem for new observers.  Folks don't know what magnifications to employ to get an idea whether their star images look right, or the impact that problems will have on a planetary image.  Collimation is a starting point, but what matters is how the focused star image appears because there can be other things impacting it (pinch, astigmatism, spherical aberration, thermals, bad seeing, etc.)  Defocused star testing is used to identify the types and source(s) of defects.

 

It can be overwhelming at first.  When you have had a chance to do more observing these sort of things will become second nature.

 

Good luck. 



#19 patindaytona

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Posted 05 September 2019 - 04:28 PM

It isn't just "interesting to know" it is an explanation of how to see things and what you are actually observing.  You kind of need to know what a focused star should look like at high power to be able to get much out of your viewing or to even evaluate seeing one night vs the next.  The more you know about how this works, the more readily you know what magnification to use and what to look for.  What is the max magnification you have used on these objects?  I haven't read all of your many recent posts, but I don't recall you ever stating what magnifications were used.  That is kind of critical for someone trying to understand what you are seeing and trying to assist you.

 

If one doesn't know that a star will have a diffraction pattern and a sufficiently large moon/asteroid will not, then how would one tell them apart when the size of the diffraction pattern is nearly the same as the solar system body?  If large enough and with sufficient aperture at sufficient magnification then a person should recognize the disk, but if they never use enough magnification...then all they will see are points.

 

From what I recall when you first started asking questions about your scope, I asked if you had star tested it and what stars looked like at high power.  I didn't really get a response to that and it took hundreds of posts before that was sorted by others.  This is not an infrequent problem for new observers.  Folks don't know what magnifications to employ to get an idea whether their star images look right, or the impact that problems will have on a planetary image.  Collimation is a starting point, but what matters is how the focused star image appears because there can be other things impacting it (pinch, astigmatism, spherical aberration, thermals, bad seeing, etc.)  Defocused star testing is used to identify the types and source(s) of defects.

 

It can be overwhelming at first.  When you have had a chance to do more observing these sort of things will become second nature.

 

Good luck. 

120x   to  200x magnificaton mainly, which i can see falls way short of an airy disk.

A "body" will not have a diffraction pattern?

I am reading up on airy disk right now.   All i have right now is the 7mm and the 12.5 and the 10mm eye pieces, but i do have the barlow 2x.  

Which magnification do you suggest?

The airy disk all depends on the atmospheric turbulence, doesn't it?



#20 Redbetter

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Posted 05 September 2019 - 04:45 PM

120x   to  200x magnificaton mainly, which i can see falls way short of an airy disk.

A "body" will not have a diffraction pattern?

I am reading up on airy disk right now.   All i have right now is the 7mm and the 12.5 and the 10mm eye pieces, but i do have the barlow 2x.  

Which magnification do you suggest?

The airy disk all depends on the atmospheric turbulence, doesn't it?

Remember, airy disk angular/apparent size with magnification is a function of the aperture.  200x in a 10" is 1mm exit pupil and should be sufficient to show the pattern assuming the seeing supports it, but it is not a guarantee.  Smaller exit pupils make it more readily visible.  Some of this has to do with experience and/or visual acuity.  Once you know what it looks like in your scope, you will likely find it easier to discern in the future. 

 

Bodies will have a diffraction pattern but because they are extended sources the pattern doesn't present discrete rings like a point source (star) does.  Instead it tends to create a fuzzy edge on small apparent size objects at high power--but lacking a distinct minima that a star has.  The smaller the apparent size of the body the less of disk and the more of a fuzzy zone there is to see. 

 

To understand what seeing does to the airy disk pattern of a focused star, go to Damian Peach's Pickering scale representations:  link.



#21 patindaytona

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Posted 05 September 2019 - 04:46 PM

I'm supposed to find the highest magnification that the first diffraction and the second diffraction ring remain perfectly round and visible (at least for a fair amount of duration due to the atmospheric turbulence), correct?

Once i go higher in magnification, and the secondary (first diffraction ring is still continuous) diffraction ring begins to break up (arc), then that is TOO high of magnification (at least for that particular night), correct?



#22 Redbetter

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Posted 05 September 2019 - 04:51 PM

There is no hard and fast rule on this.  Just use the magnification that the seeing allows and begin looking for the diffraction pattern at best focus.  Whether a second ring is seen or not can depend on several things, but once you are sure you see the first ring you will be able to distinguish extended objects from point sources.



#23 patindaytona

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Posted 05 September 2019 - 05:07 PM

There is no hard and fast rule on this.  Just use the magnification that the seeing allows and begin looking for the diffraction pattern at best focus.  Whether a second ring is seen or not can depend on several things, but once you are sure you see the first ring you will be able to distinguish extended objects from point sources.

Ok, to be clear then, I should first test this out on a star (any particular night) to look for a diffraction (1st) ring. Once I begin to see that with a point source (star), then I'm good, basically?

 

From there, use that magnification to view the planet. I know since the planet is not twinkling like a star because it's disk is overlapping itself continously. And that central airy disk will now appear more fuzzier and it WON'T have a diffraction ring around it.

Is this all their is to it?



#24 patindaytona

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Posted 05 September 2019 - 05:20 PM

This might be useful.  This was done with a 12", using an R filter. a 10" unfiltered under excellent seeing should give similar results (smaller aperture and shorter wavelength roughly cancelling out).

 

You can see the difference in appearance. It must be much more striking with the 20" as redbetter mentions.

 

https://www.cloudyni...s-a-disk-again/

I used photoshop a bit to enhance the diffraction ring on the star. I do see the differences.

Is any magnitude star valid to test it?

Once i "begin" to see a diffraction ring surrounding the airy disk, that is the magnification I should aim for when viewing a smaller body?



#25 patindaytona

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Posted 05 September 2019 - 07:38 PM

Was just outside....i used the 2x Barlow with the 7mm Ortho eye piece. And I still...could not detect an airy disk. No concentric i could see around the central part. It looked like a single point but with a bit of flair of course at that magnification.

That's as high as I can go with my current eye pieces.

I did try out focusing and the concentrics all looks good and circular with the minima in between showing distinctly.

The only thing though was that as I'm looking through my telescope, on the side of the concentrics that is to the right (towards the primary direction), this is a bit of a slice off. Not much, but it's there. Very little.




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