17 replies to this topic

[kingsbishop]

Title

[Redbetter]

I am curious as to why you would ask?  

 

I am trying to think of what exquisitely figured/polished 30mm scope would benefit from 100x/inch and somehow remove the physics-dictated diffraction blurring that comes along with such magnification.  Not to mention how unpleasant a 0.25mm exit pupil can be for most people's eyes on bright objects.  Of course, Jupiter's surface brightness would be cut by ~7 1/4 magnitude so it will be approaching mid-mesopic 13 mpsas levels.

 

"best"?  Certainly not, but I have to wonder what someone would be trying to see, I figure there is a reason for asking.  Jupiter isn't a double star so resolving diffraction patterns would not be beneficial.

[Sincos]

Sorry , the title might not be enough . Do you actually want a 0.25mm exit pupil , if so to detect what ? Are you actually talking about a 30mm finderscope or an eyepiece with your telescope that yields a 0.25mm exit pupil ? How hard would it be to locate so small an exit pupil ? Are you trying to split a particular double ? Interesting quest good luck , you got me thinking and that’s always enjoyable.

[kingsbishop]

Sorry , the title might not be enough . Do you actually want a 0.25mm exit pupil , if so to detect what ? Are you actually talking about a 30mm finderscope or an eyepiece with your telescope that yields a 0.25mm exit pupil ? How hard would it be to locate so small an exit pupil ? Are you trying to split a particular double ? Interesting quest good luck , you got me thinking and that’s always enjoyable.

i would like an eyepiece where i can see the airy disk clearly with Maximum resolution and see good resolution on any planet 0.25mm exit pupil does that fine but i want to know if i can use that on Jupiter and still get good results?

[Dave Mitsky]

Is the telescope's aperture really 30mm or is that a typo?

[kingsbishop]

Is the telescope's aperture really 30mm or is that a typo?

i want to know my telescopes capabiliies without the distraction of atmosphere distortion because i have high hopes on adaptive optics and what it can bring to amateur astronomers otherwise what is the fun of astronomy accepting seeing as a limitations is like saying hypothetically that humans can run a max speed of 20kmh and half the population can already run that fast but no one can run faster what is the point if you cant improve or get better results thats why i really Count on adaptive optics so you can always get better and better views

[Dave Mitsky]

i want to know my telescopes capabiliies without the distraction of atmosphere distortion because i have high hopes on adaptive optics and what it can bring to amateur astronomers otherwise what is the fun of astronomy accepting seeing as a limitations is like saying hypothetically that humans can run a max speed of 20kmh and half the population can already run that fast but no one can run faster what is the point if you cant improve or get better results thats why i really Count on adaptive optics so you can always get better and better views

How does that answer my query?

[Redbetter]

This is an unanswerable question since it depends on the observer and conditions that are unavailable (perfect seeing.)

 

The best one can do is to try to recreate the case and experiment with it to find out how their own eye handles it.   The overwhelming majority of people find 100x/inch, 0.25mm exit pupil to be poor for planets even with small refractors in very good seeing.  There are various reasons for this:  at 0.25mm exit pupil the resultant apparent surface brightness of bright planets is well into the mesopic range where color perception/contrast is poor, floaters/structure/small flaws in the eye become more noticeable at such small exit pupil, some will claim the eye should be able to see all of the detail at 25x/inch (1mm exit pupil), while in reality it takes more.

 

At any rate you are trying to equate examining the diffraction pattern for max information from it, with the appearance of an extended surface at the same magnification/pupil.  However, these are not at all the same thing.  It is like examining an impression painting up close to see the dabs of paint, vs. backing up sufficiently to see it at best scale to interpret the picture.  Only it is worse, because to simulate the effect that the reduced exit pupil has, the lighting would need to be dimmed to mid-mesopic level as one moved very close and increased as one moved away.  

 

The solution is for you to do our own testing.  Take some apo/ED refractor with good figure on a night of very good seeing, and stop it down to 30mm in the center.  (This should be in the sweet spot of the optic anyway.)  Then test with eyepieces providing exit pupils down to 0.25mm--this is not so hard to do starting with a 600mL focal length scope since you only need 120x, a 5mm eyepiece.  It will be a few months before Jupiter is well placed again, but the Moon is available more frequently.

[John Fitzgerald]

Trolling?  Really, 30mm adaptive optics?

[kingsbishop]

How does that answer my query?

because 30mm telescopes you wouldnt see atmosphere distortion which is just easier to say than what about any telescope with adaptive optics

[kingsbishop]

Trolling? Really, 30mm adaptive optics?

no no no i said 30mm because atmosphere distortion doesnt get in the way foe 30mm telescopes so it would be equivelent of saying a larger telescope with adaptive optics but people would then answer the question differently if i said that

[John Fitzgerald]

no no no i said 30mm because atmosphere distortion doesnt get in the way foe 30mm telescopes so it would be equivelent of saying a larger telescope with adaptive optics but people would then answer the question differently if i said that

A 30mm telescope has very limited resolving power. The smallest detail it can resolve is about 3.7 arc seconds.  So, no, there would be no benefit on Jupiter with any exit pupil smaller than 0.5mm.  There would likely be no benefit viewing Jupiter with any telescope having an exit pupil less then about 0.5mm.  Practically speaking, 0.75 mm is a better limit for Jupiter.

[Redbetter]

Atmospheric distortion (seeing) is a factor even in very small apertures--this is apparent naked eye in the form of twinkle with only 5-7mm of aperture.  A smaller aperture will suffer the least because of its inherently poor resolution, but when you use 120x for a 30mm, the physical (length) of the seeing is still 120x, the same as for a 60mm at 50x/inch.  Of course the 60 will show much more...brighter image and twice the resolution.  

 

Again, the way to find the answer is already in front of you, you will have to test it yourself to make a determination:  stop down a good refractor to your desired aperture, and test different small exit pupils on a night that has very good seeing (as measured with full aperture.)   

[kingsbishop]

Atmospheric distortion (seeing) is a factor even in very small apertures--this is apparent naked eye in the form of twinkle with only 5-7mm of aperture. A smaller aperture will suffer the least because of its inherently poor resolution, but when you use 120x for a 30mm, the physical (length) of the seeing is still 120x, the same as for a 60mm at 50x/inch. Of course the 60 will show much more...brighter image and twice the resolution.

Again, the way to find the answer is already in front of you, you will have to test it yourself to make a determination: stop down a good refractor to your desired aperture, and test different small exit pupils on a night that has very good seeing (as measured with full aperture.)

i would love to test but Jupiter is just too close to the sun right now and im quite impatient to test the actual resolution for different exit pupil since i didnt think of stopping down the appeture idea last year

[Redbetter]

i would love to test but Jupiter is just too close to the sun right now and im quite impatient to test the actual resolution for different exit pupil since i didnt think of stopping down the appeture idea last year

The Moon will be back in the way in a week or so...which is why I mentioned it earlier.

[kingsbishop]

This is an unanswerable question since it depends on the observer and conditions that are unavailable (perfect seeing.)

The best one can do is to try to recreate the case and experiment with it to find out how their own eye handles it. The overwhelming majority of people find 100x/inch, 0.25mm exit pupil to be poor for planets even with small refractors in very good seeing. There are various reasons for this: at 0.25mm exit pupil the resultant apparent surface brightness of bright planets is well into the mesopic range where color perception/contrast is poor, floaters/structure/small flaws in the eye become more noticeable at such small exit pupil, some will claim the eye should be able to see all of the detail at 25x/inch (1mm exit pupil), while in reality it takes more.

At any rate you are trying to equate examining the diffraction pattern for max information from it, with the appearance of an extended surface at the same magnification/pupil. However, these are not at all the same thing. It is like examining an impression painting up close to see the dabs of paint, vs. backing up sufficiently to see it at best scale to interpret the picture. Only it is worse, because to simulate the effect that the reduced exit pupil has, the lighting would need to be dimmed to mid-mesopic level as one moved very close and increased as one moved away.

The solution is for you to do our own testing. Take some apo/ED refractor with good figure on a night of very good seeing, and stop it down to 30mm in the center. (This should be in the sweet spot of the optic anyway.) Then test with eyepieces providing exit pupils down to 0.25mm--this is not so hard to do starting with a 600mL focal length scope since you only need 120x, a 5mm eyepiece. It will be a few months before Jupiter is well placed again, but the Moon is available more frequently.

yeah i really want to test it but unfortunately Jupiter is too low and i didnt think of stopping down thr appeture last year

[Sebastian_Sajaroff]

You can test it with Venus.
It’s half the apparent size of Jupiter but still a big bright target. It will give a good idea of what to expect.
Yesterday afternoon, I observed Venus with a 0.53 mm exit pupil (2x Barlow on a Morpheus 6.5 mm, 70 mm F/6 refractor) and it was more than acceptable.
Then I tried 0.37 exit pupil (Barlow on Morpheus 4.5 mm) and was really poor, amorphous melting blob.
Note that seeing was exceptionally good for Montreal standards (equivalent to OK for the rest of the world)

[havasman]

No.