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What does "the magic 2mm exit pupil" mean? And is 0.5 too small?

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#1 jgroub

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Posted 26 October 2014 - 05:13 PM

I've seen people make reference to it like this:  "in a 1200mm f/l 8-inch dob, the 12mm 100x EP will provide the magic 2mm exit pupil."  

 

I've also seen people refer to an exit pupil of under a 1mm as being too small.  

 

What does all this mean?  For example, what do people with Maks do?  A 127mm aperture 1500mm f/l  Mak basically starts off with a 2.7mm exit pupil with a 32mm EP, and it just keeps getting smaller from there.  Is 1mm the limit?  Because that would limit a 1500mm f/l scope to only 127x.  And if your Mak is larger/longer, are those people just screwed?  

 

How small can you before it gets uncomfortable?  How small can you go before you start degrading the view somehow?  Is there a practical or theoretical limit to the size of an exit pupil?  

 

 



#2 Alan French

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Posted 26 October 2014 - 05:51 PM

I find it useful and easy to think in terms of exit pupil. As you increase magnification lunar and planetary detail becomes easy to see, and deep sky objects show up better (the eye/brain tends to more easily perceive faint objects when they cover more of the retina).

 

At around 2mm there is a good match between the telescope's ability to show fine detail and the resolution of the retina (for bright objects). So it's close to the power you need to see all the telescope is capable of revealing. Many observers, however, find lunar and planetary detail best with an exit pupil around 1.0mm.

 

When you get smaller than 1.0mm the small bundle of light tends to reveal floaters (debris) in your eye, which is distracting and can obscure details. Generally I tend to be happiest with an exit pupil larger than about 0.8mm. Younger eyes may be able to handle smaller exit pupils. With their higher powers they might be helpful for splitting double stars, but are unlikely to show more planetary detail.

 

It's very much a personal thing, depending largely on your eye, so experiment. And, of course, larger scopes may be most limited by seeing, precluding the use of small exit pupils.

 

Generally, a 2mm exit pupil tends to be very agreeable.

 

Clear skies, Alan

 

BTW, Perhaps you know, but in case not, here's a nice shortcut. The exit pupil is equal to the eyepiece's focal length divided by the f/ratio of the telescope. So a 12mm eyepiece in an f/6 telescope gives a 2mm exit pupil and an eyepiece with a focal length equal to the telescope's f/ratio gives a 1mm exit pupil. (One more reason I like thinking it terms of exit pupil.)


Edited by Alan French, 26 October 2014 - 05:54 PM.


#3 jgroub

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Posted 26 October 2014 - 05:58 PM

When you get smaller than 1.0mm the small bundle of light tends to reveal floaters (debris) in your eye, which is distracting and can obscure details. Generally I tend to be happiest with an exit pupil larger than about 0.8mm. Younger eyes may be able to handle smaller exit pupils. With their higher powers they might be helpful for splitting double stars, but are unlikely to show more planetary detail.

Interesting, especially about the floaters.  So are owners of Maks sort of screwed in this department if they try to use high power?  



#4 Alan French

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Posted 26 October 2014 - 06:22 PM

 

When you get smaller than 1.0mm the small bundle of light tends to reveal floaters (debris) in your eye, which is distracting and can obscure details. Generally I tend to be happiest with an exit pupil larger than about 0.8mm. Younger eyes may be able to handle smaller exit pupils. With their higher powers they might be helpful for splitting double stars, but are unlikely to show more planetary detail.

Interesting, especially about the floaters.  So are owners of Maks sort of screwed in this department if they try to use high power?  

 

 

Any scope used with an eyepiece that provides a small exit pupil will reveal floaters. They are usually less of a problem in young eyes, but everyone is different.

 

With Mak-Casses you are generally stuck with smaller exit pupils, but a 2 to 3mm exit pupil is quite agreeable and should not show floaters. Even a 1mm exit pupil works well for many people.

 

Clear skies, Alan



#5 GeneT

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Posted 26 October 2014 - 06:43 PM

I find nothing magical about an exit pupil of 2. In fact, it is the least used factor in my viewing. I own a 12.5 inch, F5 Portaball. A 2 exit pupil puts me at a 10 mm eyepiece with a magnification of 152X and 12X per inch. The 10mm eyepiece is my least used eyepiece. 

 

My most used eyepiece is a 13 Ethos. It has an exit pupil of 2.7 and a magnification of 117X.  It gives me about 9.4X inch. My next most used eyepiece is a 21 Ethos. It has an exit pupil of 4.3 and a magnification of 73X. It gives me about 5.8X inch.  

 

I then go to an 8 Delos or 8 Ethos. They have an exit pupil of 1.7 and a magnification of 191X. They give me about 15X an inch. 

 

My 7 TMB Supermono has an exit pupil of 1.5 and a magnification of 218X. It gives me about 17X an inch.

 

My 6 Delos 6 Ethos have an exit pupil of 1.3 and a magnification of 254X. They give me about 20X an inch. 

 

My 5mm XO has an exit pupil of 1 and a magnification of 305X. It gives me about 24X an inch.

 

My 4.7 Ethos has an exit pupil of .98 and a magnification of 324X. It gives me about 26X an inch.

 

My 4 Radian has an exit pupil of .83 and a magnification of 381X. It gives me about 30 an inch.

 

In short, I normally go with a 21 or 13 Ethos for general sky sweeping with 4.3 and 2.7 exit pupils (73X and 117X magnifications).

 

For higher power viewing I most often use the 8 Delos and 8 Ethos and 6 Delos and 6 Ethos eyepieces (1.7 and 1.3 exit pupils and 254X magnifications).

 

When moving into the higher magnifications, I do find that the larger AFOV of the Delos (72) and Ethos (100) and decent eye relief (20mm Delos and 15mm Ethos) to be very helpful. Both factors combat eye strain when holding an object such as a planet in view for a long time. I like my 5 XO and 7 TMB Supermono eyepieces, but they have small AFOV and short eye relief.

 

What each person needs to do is try a variety of eyepieces and see what exit pupil, magnification, and field of view best suits one's needs. There is a large subjective factor in all this. Some people just like different parameters in their eyepieces. 

 

All that really matters is what you like.



#6 Jon Isaacs

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Posted 26 October 2014 - 07:22 PM

I've seen people make reference to it like this:  "in a 1200mm f/l 8-inch dob, the 12mm 100x EP will provide the magic 2mm exit pupil."  

 

I've also seen people refer to an exit pupil of under a 1mm as being too small.  

 

What does all this mean?  For example, what do people with Maks do?  A 127mm aperture 1500mm f/l  Mak basically starts off with a 2.7mm exit pupil with a 32mm EP, and it just keeps getting smaller from there.  Is 1mm the limit?  Because that would limit a 1500mm f/l scope to only 127x.  And if your Mak is larger/longer, are those people just screwed?  

 

How small can you before it gets uncomfortable?  How small can you go before you start degrading the view somehow?  Is there a practical or theoretical limit to the size of an exit pupil?  

 

 

Alan and Gene have provided some good information.  In my experience, there is no magic exit pupil, I use exit pupils as small as 0.3mm for splitting double stars and exit pupils as large as 7mm and even 8mm for view faint, diffuse objects.  It's all about the right exit pupil for the object and conditions.  It does require an exit pupil in the 1mm range for the eye to begin to resolve the Airy disk and more to see the diffraction rings so this puts something of an upper limit on what is useful.. 

 

Jon



#7 jgroub

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Posted 26 October 2014 - 07:39 PM

I use exit pupils as small as 0.3mm for splitting double stars and exit pupils as large as 7mm and even 8mm for view faint, diffuse objects.  It's all about the right exit pupil for the object and conditions.  It does require an exit pupil in the 1mm range for the eye to begin to resolve the Airy disk and more to see the diffraction rings so this puts something of an upper limit on what is useful.. 

 

Jon

 

You don't see those floaters at 0.3mm?  I guess there's hope for me yet.  

 

And isn't it counterproductive to use 7mm or 8mm exit pupils, because it's larger than your eye's fully dilated pupil, and that means that the image is actually darker because the light is being wasted on non-retinal parts of your eye?  Shouldn't you limit yourself to the exit pupil that essentially matches the full-dilation ability of your eye?  Especially if you're not a teenager anymore - and who among us are?  Supposedly, the full-dilation point shrinks down to 6mm in your 30s, and 5mm in your 40s-50s.  Seems like a lot of wasted light.  



#8 Alan French

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Posted 26 October 2014 - 08:34 PM

There is a fair amount of variation in a person's maximum, low light pupil size, even in older folks. The best way to know is to measure it.

 

As to large exit pupils, there is really no "wasted" light. If your telescope's exit pupil is larger than your eye's pupil, some light from the telescope will not enter your eye (just like stopping down the telescope), but it is still the brightest view you could possibly get at that magnification. In a refractor there are no worries, but in an obstructed telescope the central obstruction will take up a larger part of the exit pupil.

 

If possible, a higher magnification with the same true field (a wider field eyepiece) is probably better than stopping down the telescope. Higher powers tend to show more.

 

Clear skies, Alan



#9 Jon Isaacs

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Posted 26 October 2014 - 09:04 PM

 

I use exit pupils as small as 0.3mm for splitting double stars and exit pupils as large as 7mm and even 8mm for view faint, diffuse objects.  It's all about the right exit pupil for the object and conditions.  It does require an exit pupil in the 1mm range for the eye to begin to resolve the Airy disk and more to see the diffraction rings so this puts something of an upper limit on what is useful.. 

 

Jon

 

You don't see those floaters at 0.3mm?  I guess there's hope for me yet.  

 

And isn't it counterproductive to use 7mm or 8mm exit pupils, because it's larger than your eye's fully dilated pupil, and that means that the image is actually darker because the light is being wasted on non-retinal parts of your eye?  Shouldn't you limit yourself to the exit pupil that essentially matches the full-dilation ability of your eye?  Especially if you're not a teenager anymore - and who among us are?  Supposedly, the full-dilation point shrinks down to 6mm in your 30s, and 5mm in your 40s-50s.  Seems like a lot of wasted light.  

 

 

I definitely have floaters and sometimes they are a problem but they don't seem to be too much of a problem splitting tight double stars. Again, it depends on the situation.  

 

As far as the usefulness of large exit pupils:  As Alan said, it's best to measure it, we are all different.   I am 66 and last year I measured mine by using a camera to take a photo in a dark closet.  It's not the best technique, the camera emitted a small amount of light to focus but still, I measured my dark adapted pupil at 7mm.  My experience is that for large objects that fill a substantial portion of the field of view, sometimes the object will be more visible with an 8.2 mm exit pupil, typically this happens with O-III and H-beta filters.  I don't know quite why it happens, whether it's because the smaller size of the object is more easily seen or because the image is brighter or for some other reason, I only know what I see.  With each object, I compare what I see with different eyepieces, I pay attention to what I can see and leave the analysis and theory of why I am seeing what I am seeing for another time.

 

Jon 



#10 BDS316

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Posted 27 October 2014 - 12:30 PM

I find it useful and easy to think in terms of exit pupil. As you increase magnification lunar and planetary detail becomes easy to see, and deep sky objects show up better (the eye/brain tends to more easily perceive faint objects when they cover more of the retina).

 

At around 2mm there is a good match between the telescope's ability to show fine detail and the resolution of the retina (for bright objects). So it's close to the power you need to see all the telescope is capable of revealing. Many observers, however, find lunar and planetary detail best with an exit pupil around 1.0mm.

 

When you get smaller than 1.0mm the small bundle of light tends to reveal floaters (debris) in your eye, which is distracting and can obscure details. Generally I tend to be happiest with an exit pupil larger than about 0.8mm. Younger eyes may be able to handle smaller exit pupils. With their higher powers they might be helpful for splitting double stars, but are unlikely to show more planetary detail.

 

It's very much a personal thing, depending largely on your eye, so experiment. And, of course, larger scopes may be most limited by seeing, precluding the use of small exit pupils.

 

Generally, a 2mm exit pupil tends to be very agreeable.

 

Clear skies, Alan

 

BTW, Perhaps you know, but in case not, here's a nice shortcut. The exit pupil is equal to the eyepiece's focal length divided by the f/ratio of the telescope. So a 12mm eyepiece in an f/6 telescope gives a 2mm exit pupil and an eyepiece with a focal length equal to the telescope's f/ratio gives a 1mm exit pupil. (One more reason I like thinking it terms of exit pupil.)

This is my experience exactly and why I chose my current eyepiece lineup.  My 11mm Nagler gives me slightly less than a 2mm exit pupil with a very wide three quarter degree field at 109x.  This is the largest exit pupil that I can use without glasses because of astigmatism.

 

My ES 6.7mm gives me a 1.1mm exit pupil which is my smallest exit pupil before floaters become a problem. It's my favorite planetary eyepiece.  I use a Barlow to achieve smaller exit pupils when needed for double star splitting but I didn't want to purchase an eyepiece that would always show me floaters, hence no ep's less than 6.7mm for me


Edited by BDS316, 27 October 2014 - 12:31 PM.


#11 REC

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Posted 27 October 2014 - 12:48 PM

 

I find it useful and easy to think in terms of exit pupil. As you increase magnification lunar and planetary detail becomes easy to see, and deep sky objects show up better (the eye/brain tends to more easily perceive faint objects when they cover more of the retina).

 

At around 2mm there is a good match between the telescope's ability to show fine detail and the resolution of the retina (for bright objects). So it's close to the power you need to see all the telescope is capable of revealing. Many observers, however, find lunar and planetary detail best with an exit pupil around 1.0mm.

 

When you get smaller than 1.0mm the small bundle of light tends to reveal floaters (debris) in your eye, which is distracting and can obscure details. Generally I tend to be happiest with an exit pupil larger than about 0.8mm. Younger eyes may be able to handle smaller exit pupils. With their higher powers they might be helpful for splitting double stars, but are unlikely to show more planetary detail.

 

It's very much a personal thing, depending largely on your eye, so experiment. And, of course, larger scopes may be most limited by seeing, precluding the use of small exit pupils.

 

Generally, a 2mm exit pupil tends to be very agreeable.

 

Clear skies, Alan

 

BTW, Perhaps you know, but in case not, here's a nice shortcut. The exit pupil is equal to the eyepiece's focal length divided by the f/ratio of the telescope. So a 12mm eyepiece in an f/6 telescope gives a 2mm exit pupil and an eyepiece with a focal length equal to the telescope's f/ratio gives a 1mm exit pupil. (One more reason I like thinking it terms of exit pupil.)

This is my experience exactly and why I chose my current eyepiece lineup.  My 11mm Nagler gives me slightly less than a 2mm exit pupil with a very wide three quarter degree field at 109x.  This is the largest exit pupil that I can use without glasses because of astigmatism.

 

My ES 6.7mm gives me a 1.1mm exit pupil which is my smallest exit pupil before floaters become a problem. It's my favorite planetary eyepiece.  I use a Barlow to achieve smaller exit pupils when needed for double star splitting but I didn't want to purchase an eyepiece that would always show me floaters, hence no ep's less than 6.7mm for me

 

 

 

I find it useful and easy to think in terms of exit pupil. As you increase magnification lunar and planetary detail becomes easy to see, and deep sky objects show up better (the eye/brain tends to more easily perceive faint objects when they cover more of the retina).

 

At around 2mm there is a good match between the telescope's ability to show fine detail and the resolution of the retina (for bright objects). So it's close to the power you need to see all the telescope is capable of revealing. Many observers, however, find lunar and planetary detail best with an exit pupil around 1.0mm.

 

When you get smaller than 1.0mm the small bundle of light tends to reveal floaters (debris) in your eye, which is distracting and can obscure details. Generally I tend to be happiest with an exit pupil larger than about 0.8mm. Younger eyes may be able to handle smaller exit pupils. With their higher powers they might be helpful for splitting double stars, but are unlikely to show more planetary detail.

 

It's very much a personal thing, depending largely on your eye, so experiment. And, of course, larger scopes may be most limited by seeing, precluding the use of small exit pupils.

 

Generally, a 2mm exit pupil tends to be very agreeable.

 

Clear skies, Alan

 

BTW, Perhaps you know, but in case not, here's a nice shortcut. The exit pupil is equal to the eyepiece's focal length divided by the f/ratio of the telescope. So a 12mm eyepiece in an f/6 telescope gives a 2mm exit pupil and an eyepiece with a focal length equal to the telescope's f/ratio gives a 1mm exit pupil. (One more reason I like thinking it terms of exit pupil.)

This is my experience exactly and why I chose my current eyepiece lineup.  My 11mm Nagler gives me slightly less than a 2mm exit pupil with a very wide three quarter degree field at 109x.  This is the largest exit pupil that I can use without glasses because of astigmatism.

 

My ES 6.7mm gives me a 1.1mm exit pupil which is my smallest exit pupil before floaters become a problem. It's my favorite planetary eyepiece.  I use a Barlow to achieve smaller exit pupils when needed for double star splitting but I didn't want to purchase an eyepiece that would always show me floaters, hence no ep's less than 6.7mm for me

 

 Well I can see why my ES 6.7mm with my ED80 600mm f/7 = 1mm exit pupil at 90x is comfortable and when I use it in my C102 f/10 1000mm = .7mm exit pupil at 150x is a push on most nights and for only bright planets and the moon.  In that case I'll back off to my ES 8.8mm getting closer to a 1mm exit pupil at 114x.  Now it's making better sense to my 65 year old eyes:)



#12 ChristianG

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Posted 27 October 2014 - 05:14 PM


What does all this mean?  For example, what do people with Maks do?  A 127mm aperture 1500mm f/l  Mak basically starts off with a 2.7mm exit pupil with a 32mm EP, and it just keeps getting smaller from there.  Is 1mm the limit?  Because that would limit a 1500mm f/l scope to only 127x.  And if your Mak is larger/longer, are those people just screwed?  

 

How small can you before it gets uncomfortable?  How small can you go before you start degrading the view somehow?  Is there a practical or theoretical limit to the size of an exit pupil?  

 

 

 

Ha Ha! Screwed? Not if used as intended...

 

In the city from my balcony I mainly use a 127 mm f/12 Maksutov telescope to look at planets, Moon and Sun (folks have measured it to operate nearer f/13, by the way). So the limited range in exit pupils is not a problem. For instance, I will use a small set of 60-70 AFOV eyepieces: a 25 mm, 15 mm, 12 mm (or 10 mm), and sometimes 8 mm when seeing is good. I also have its bigger brother, 180 mm f/15 from Orion, but I found that my balcony/house is not steady enough for that large a focal length (2700 mm!!!!), so in frustration I often revert back to the 127 mm...

 

As you say, exit pupil is about 2 mm maximum, but for something bright this is not a problem. I can still fit the whole Moon or Sun in the field of view.

 

And--bonus--for me, below 8 mm (0.67 mm exit pupil), my telescope magically turns into a microscope! Showing me nice strings of dead red blood cells and other debris... People less afflicted by floaters might like exit pupils in the 0.5 mm range, but for me I it's a bit distracting.

 

For big deep sky objects, I like my 4" f/7 ED refractor. With it I have the full range of exit pupils available. But in the city, Maksutovs can be just fine!

 

--Christian



#13 Allan Wade

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Posted 27 October 2014 - 05:55 PM

An exit pupil of 1.5 to 2.5 is a range where a great many DSO appear at their best. Magnified enough to reveal detail, and still bright enough to clearly see the detail.

 

A good guide, if you enjoy DSO, is to spend the most money on an eyepiece focal length that is twice the f/ratio of your scope. That will give you the magic 2mm exit pupil.



#14 jgroub

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Posted 27 October 2014 - 06:10 PM

 

In the city from my balcony I mainly use a 127 mm f/12 Maksutov telescope to look at planets, Moon and Sun (folks have measured it to operate nearer f/13, by the way). So the limited range in exit pupils is not a problem. For instance, I will use a small set of 60-70 AFOV eyepieces: a 25 mm, 15 mm, 12 mm (or 10 mm), and sometimes 8 mm when seeing is good. I also have its bigger brother, 180 mm f/15 from Orion, but I found that my balcony/house is not steady enough for that large a focal length (2700 mm!!!!), so in frustration I often revert back to the 127 mm...

 

As you say, exit pupil is about 2 mm maximum, but for something bright this is not a problem. I can still fit the whole Moon or Sun in the field of view.

 

And--bonus--for me, below 8 mm (0.67 mm exit pupil), my telescope magically turns into a microscope! Showing me nice strings of dead red blood cells and other debris... People less afflicted by floaters might like exit pupils in the 0.5 mm range, but for me I it's a bit distracting.

 

For big deep sky objects, I like my 4" f/7 ED refractor. With it I have the full range of exit pupils available. But in the city, Maksutovs can be just fine!

 

--Christian

 

Christian, thanks so much for posting this.  I've been having a discussion about getting a 127 Mak on this thread, here:  

 

http://www.cloudynig...-2#entry6274572

 

and it's nice to hear from someone saying that they're good for looking at solar system objects right from the light polluted City.  That's the best recommendation I could ever get!   :waytogo:

 

I've been looking at the Celestron Nexstar 127 and the Orion Apex 127, and have been wondering how Orion gets away with charging $419 for just the OTA while Celestron sells what must be the same OTA for $429 and throws in the mount and Goto motor.  So, with that, I'm much more interested in getting the Celestron.  Which one do you have?  And what mount do you have it on?  How's your vibration?  I've heard bad things about the Celestron SLT mount as it comes straight from the factory, but nothing that can't be modded to be made much better.  

 

And any thoughts on which of the two set of specs is correct?  Celestron calls it a 1500mm f/12 (really f/11.8 if you do the math), while Orion calls it 1540mm f/12.1.  I think Celestron is probably rounding down, for whatever reason.  Meanwhile, you say it's really an f/13.  Where did you hear that?  

 

So, with the 10mm, you're getting it "only" up to 150x; do you find it's the atmosphere or the scope that keeps you from getting to use the 8mm and getting it up to about 190x?  Because I was looking around and thinking that if I were to get this scope, I would get a 7mm X-Cel LX to use with it.  But you're saying that the 8mm pretty much maxes it out.  Hmmm.  

 

Love to hear more from you about your 127 Mak, and thanks again!  



#15 ChristianG

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Posted 28 October 2014 - 12:18 AM

----Hi. Some answers:

 


I've been having a discussion about getting a 127 Mak on this thread, here:  

http://www.cloudynig...-2#entry6274572

 

----I read it. Comments from others are correct.

 

I've been looking at the Celestron Nexstar 127 and the Orion Apex 127, and have been wondering how Orion gets away with charging $419 for just the OTA while Celestron sells what must be the same OTA for $429 and throws in the mount and Goto motor.

 

----Weird? As far as I know they are the same OTA. Either way, the 127 Mak needs a good mount, equatorial and tracking if possible. No real need for Go-To in the city, planets and Moon are easy to find! But a Right Ascension motor is nice to have, with a rough pola alignment (no need to go crazy for visual).

 

So, with that, I'm much more interested in getting the Celestron.  Which one do you have?  And what mount do you have it on?  How's your vibration?  I've heard bad things about the Celestron SLT mount as it comes straight from the factory, but nothing that can't be modded to be made much better.  

 

----A used Orion 127 Mak with gray tube. Very good optics. For equatorial tracking I have it on an Omni CG-4 or an old Vixen Super Polaris. Both have only a single-axis motor drive. When just playing around, an AZ-4 alt-azimuth mount. All 3 share one feature: stiff tripod with steel or wooden legs. Aluminum tripods are not stiff enough for a 1550 mm focal length instrument and will just be a source of confusion.

 

And any thoughts on which of the two set of specs is correct?  Celestron calls it a 1500mm f/12 (really f/11.8 if you do the math), while Orion calls it 1540mm f/12.1.  I think Celestron is probably rounding down, for whatever reason.  Meanwhile, you say it's really an f/13.  Where did you hear that?  

 

----I think everybody agrees that they are the same instrument. As for the smaller actual f/ratio, it's just that the mirror size is the same as the corrector lens. But because the corrector lens is a somewhat diverging lens, some light misses the mirror and the system operates more around f/13 than f/12: like an 118 mm telescope. Both Celestron and Orion are the same in that regards. Finally, because focusing is done by moving the primary mirror, the focal length will depend somehow on the diagonal and eyepiece combination. So it's somewhere around 1500 or 1550 mm. All moving mirror Cassegrain telescope have an ill-defined focal length. Including Questar.

 

So, with the 10mm, you're getting it "only" up to 150x; do you find it's the atmosphere or the scope that keeps you from getting to use the 8mm and getting it up to about 190x?  Because I was looking around and thinking that if I were to get this scope, I would get a 7mm X-Cel LX to use with it.  But you're saying that the 8mm pretty much maxes it out.  Hmmm.  

 

----Good question. For my eyes, 8 mm seems to be the max magnification these days, but I have a lot of floaters--too many years doing martial arts maybe? But this is subjective. I routinely stick a 5 mm in it when 1/4 Moon is up and atmosphere is steady. So 7 mm would be just fine. Recently planets were so low on the horizon that seeing precluded really high magnification. On my 127 Mak the in-focus star test is very good indeed for a 400$ instrument, so high magnification is no problem. And for a f/12 or so instruments, 70$ eyepieces like X-Cel LX, AT Paradigms etc. will give you very good views. I use Agena Starguider Dual ED eyepieces, same as AT Paradigms, but in stock!

 

Love to hear more from you about your 127 Mak, and thanks again!  

 

----Great instrument. But let it cool down! Takes quite a while in the Winter. It's easy to check if it has reached equilibrium, though. And get some kind od dew/light shield for it! Some unwanted light can make it past the baffles. And do a search for 'painted sandpaper' mod for the primary baffle: a huge improvement for zero cost, and totally reversible. Cheers!

 

--Christian

 



#16 havasman

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Posted 28 October 2014 - 11:36 AM

My kit provides exit pupils down to 0.7mm in both Dobs and I am very pleased when conditions support those lowest value/highest magnification views. Most commonly, a 3.5mm exit pupil sees use as a galaxy finder ep and is followed by a 1.7mm exit pupil for observation and then steps down to find the usable limit for the object that night until the most revealing view is determined.

 

An aside regarding floaters if I may. I had a rapid onset of large and numerous intrusive floaters in my dominant eye about 8 years ago that my ophthalmologist warned could indicate an increased risk of retinal detachment. So far only good news. The floaters are gone, dissolved into the interoccular fluid and no further negative developments. So, floaters are not forever.



#17 lamplight

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Posted 28 October 2014 - 11:58 AM

this is a great topic, Im just learning to think in terms of exit pupil.



#18 gene 4181

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Posted 28 October 2014 - 05:02 PM

the magic exit pupil of 2mm might have been inferred from the backyard astronomer's guide by Dickson and Dyer, they quote this in the book. they mention the eyepiece that seemed to show the most was at a 2mm exit pupil. the only scope I ever thought that about was a 4in refractor at f 10, the 20 mm seemed to make that scope sing at 50 power. but that might have just been an illusion.



#19 galexand

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Posted 31 October 2014 - 01:51 PM

I don't have experience with a large variety of telescopes, but with the few that I've used, I do feel like a 2-3mm exit pupil lets me tease the most detail out of a diffuse DSO.

 

But there are so many other things to consider.  In my main telescope, a 3mm exit pupil also happens to correspond to about 50x magnification, which is enough magnification to cut through some light pollution but still provide a wide field of view.

 

Mostly I feel like exit pupils less than 1mm are dimmer.  I also notice floaters too.  It's not as enjoyable, I don't get as good a view.  But if I'm looking for a particular detail like a double star or a planetary feature, it is just like dealing with seeing.  You just wait for the moment when everything lines up and you can see that exact spot clearly.

 

So, like Jon says, it really depends on what you're looking at and what instrument you're using and everything else.

 

Given my preferences, there are a lot bigger problems than exit pupil size that turn me off of long focal length telescopes like casses and so on.  But if your preferences run the other direction, I think you can probably stand having a small exit pupil for your highest magnifications.



#20 penguinx64

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Posted 31 October 2014 - 04:47 PM

I wanted to try the 'magic' 2mm exit pupil.  In my Starblast 4.5 scope, that's an 8mm eyepiece.  So I ordered a Vixen NPL 8mm because it seemed to have good reviews.  It gave me only 56x magnification with a 50 degree AFOV and with very short 4.5mm eye relief.  There was nothing really 'magic' about it at all.  I get better views using a 3-4mm exit pupil, or a 0.6-1.3mm exit pupil.



#21 Dave Mitsky

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Posted 03 November 2014 - 10:58 AM

There is a physiological basis for the "magic" 2mm exit pupil.

 

https://www.astronom...t-pupils_t.aspx

 

A 2mm exit pupil most closely matches the area of highest resolution in your eye and gives you good detail for planetary, lunar, and globular cluster observing. The sharpness of those details is likewise improved by a 2mm exit pupil, as a smaller exit pupil minimizes astigmatism at the edges of your dark-adapted eye. Also, the visibilty of small galaxies and planetary nebulas is often enhanced by the darkening of the sky background with a 2mm exit pupil.

 

Mel Bartels discusses observing strategies based on various exit pupil ranges at http://www.bbastrode...com/visual.html

 

What magnifications should be used? I favor three strategies both based on exit pupil (the eyepiece's focal length in mm divided by the telescope's overall focal ratio [e.g., 24mm eyepiece on a F/6 scope produces a 4mm exit pupil]):

 

The first is based on Richard Berry's advice. Arrange your eyepieces so that they give exit pupils as following:
5-7mm Richest Field observing
3-5mm best deep sky observing
1-2mm best detailed observing (globulars, planetaries, lunar and planetary)

 

The second is based on Stephen O'Meara's comments (e.g., his Herschel 400 Observing Guide). He uses modest aperture (4 inches [10cm]) at low, medium and high powers. He takes his time studying the object carefully at each power. His low, medium and high exit pupils are:
4.4mm
1.4mm
0.96mm

 

If you are wondering who to look to for observing advice, pay attention to the top observers who use smaller scopes, like O'Meara.

 

The third is a strategy that I've developed in response to the super wide angle eyepieces available today. It allows me to see large scale objects otherwise too big for a given scope. I call this strategy “framing” or “composing” the view where the object is magnified to fill the eyepiece’s field of view as much as possible with a nice border around it for contrast. Increasing the apparent object size beyond this 'cut-off' results in a less pleasing more difficult view. Here, the widest possible field of view is important, even at the cost of more glass for the light to pass through. In this approach, I smoothly decrement the exit pupil. I use a set of exit pupils as follows (note that the typical set of eyepieces does not fit nicely):
5-6mm for largest scale objects
3-4mm for medium scale objects
1-2mm for small scale objects

 

Finally, poor seeing conditions especially with larger apertures will limit magnifications to 200-300x or 2-3mm exit pupil. 

 

Since the majority of deep-sky objects that are observable in amateur telescopes are rather small in apparent size, using a smaller exit pupil makes sense.  My experience has been that an exit pupil range of 1.5 to 2.5mm works very well in most cases. 

 

Dave Mitsky



#22 Jon Isaacs

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Posted 03 November 2014 - 05:33 PM

 

A 2mm exit pupil most closely matches the area of highest resolution in your eye and gives you good detail for planetary, lunar, and globular cluster observing. The sharpness of those details is likewise improved by a 2mm exit pupil, as a smaller exit pupil minimizes astigmatism at the edges of your dark-adapted eye. Also, the visibilty of small galaxies and planetary nebulas is often enhanced by the darkening of the sky background with a 2mm exit pupil.

 

Dave:

 

I have seen it said that a 2mm exit pupil provides the best match for the resolution of the eye but my experience suggests that is not the case.  Probably the best test of the eye's night time resolution is splitting double stars, this requires resolving the Airy disk.  A bit of background: Stars are points at infinity but resolve to a disk/dot whose diameter depends on the aperture of the telescope. That disk is known as the Airy disk and defines the resolution of the telescope. The bigger the aperture of the scope, the smaller the disk, the greater the resolution.  Once one can resolve the Airy disk, it's like seeing the pixels on your monitor, you can resolve no more.  

 

A 2mm exit pupil is 50x in a 100mm telescope, 100x in a 200mm telescope.  For planetary, lunar and other high magnification observation, all my experience says that this is not sufficient.  Under excellent seeing conditions, even under poor ones, I know of no one who observes Jupiter and Saturn at 50x in a 4 inch scope.  Viewing the double-double in a 4 inch scope, it resolves very nicely at 150x..(7mm exit pupil), at 100x quite nicely if the seeing is half decent and some can see a split at 50-60x but it is far from optimal.  Closer doubles require even great magnifications.

 

For me, the bottom line is that there is no magic exit pupil.  One really needs to have eyepieces that cover the entire range of possibilities.  For dim objects under dark skies, there are times when an 8mm exit pupil is optimal.. For close double stars in a small or medium sized scope, I will use exit pupils of about 0.3mm.. For the planets, generally somewhere around 0.5mm to 1mm.  But each object, each night, it's different. Fortunately, I can use my eye as the judge..

 

When I am riding to work on my bicycle, I face a variety of conditions, there is no magic gear, I want to have gears that work for me when I am climbing hills, gears that work for me when I am riding away from a stop sign and gears that are well suited for going down hill.. The "magic" gear may be the one that I use when I am riding along on the flat but regardless, the right gear is the one my legs tell me is the right gear..  The right exit pupil is the one my eye tells me is the right one.. 

 

Jon



#23 REC

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Posted 04 November 2014 - 09:15 AM

 

 

A 2mm exit pupil most closely matches the area of highest resolution in your eye and gives you good detail for planetary, lunar, and globular cluster observing. The sharpness of those details is likewise improved by a 2mm exit pupil, as a smaller exit pupil minimizes astigmatism at the edges of your dark-adapted eye. Also, the visibilty of small galaxies and planetary nebulas is often enhanced by the darkening of the sky background with a 2mm exit pupil.

 

Dave:

 

I have seen it said that a 2mm exit pupil provides the best match for the resolution of the eye but my experience suggests that is not the case.  Probably the best test of the eye's night time resolution is splitting double stars, this requires resolving the Airy disk.  A bit of background: Stars are points at infinity but resolve to a disk/dot whose diameter depends on the aperture of the telescope. That disk is known as the Airy disk and defines the resolution of the telescope. The bigger the aperture of the scope, the smaller the disk, the greater the resolution.  Once one can resolve the Airy disk, it's like seeing the pixels on your monitor, you can resolve no more.  

 

A 2mm exit pupil is 50x in a 100mm telescope, 100x in a 200mm telescope.  For planetary, lunar and other high magnification observation, all my experience says that this is not sufficient.  Under excellent seeing conditions, even under poor ones, I know of no one who observes Jupiter and Saturn at 50x in a 4 inch scope.  Viewing the double-double in a 4 inch scope, it resolves very nicely at 150x..(7mm exit pupil), at 100x quite nicely if the seeing is half decent and some can see a split at 50-60x but it is far from optimal.  Closer doubles require even great magnifications.

 

For me, the bottom line is that there is no magic exit pupil.  One really needs to have eyepieces that cover the entire range of possibilities.  For dim objects under dark skies, there are times when an 8mm exit pupil is optimal.. For close double stars in a small or medium sized scope, I will use exit pupils of about 0.3mm.. For the planets, generally somewhere around 0.5mm to 1mm.  But each object, each night, it's different. Fortunately, I can use my eye as the judge..

 

When I am riding to work on my bicycle, I face a variety of conditions, there is no magic gear, I want to have gears that work for me when I am climbing hills, gears that work for me when I am riding away from a stop sign and gears that are well suited for going down hill.. The "magic" gear may be the one that I use when I am riding along on the flat but regardless, the right gear is the one my legs tell me is the right gear..  The right exit pupil is the one my eye tells me is the right one.. 

 

Jon

 

 

Jon, did you mean .8mm exit pupil for 150x in a 4" scope and not 8mm?  I need my 6.7mm EP to split the double double @ 150x in my 4" f/9.8 C102.



#24 Dave Mitsky

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Posted 05 November 2014 - 05:53 PM

 

 

A 2mm exit pupil most closely matches the area of highest resolution in your eye and gives you good detail for planetary, lunar, and globular cluster observing. The sharpness of those details is likewise improved by a 2mm exit pupil, as a smaller exit pupil minimizes astigmatism at the edges of your dark-adapted eye. Also, the visibilty of small galaxies and planetary nebulas is often enhanced by the darkening of the sky background with a 2mm exit pupil.

 

Dave:

 

I have seen it said that a 2mm exit pupil provides the best match for the resolution of the eye but my experience suggests that is not the case.  Probably the best test of the eye's night time resolution is splitting double stars, this requires resolving the Airy disk.  A bit of background: Stars are points at infinity but resolve to a disk/dot whose diameter depends on the aperture of the telescope. That disk is known as the Airy disk and defines the resolution of the telescope. The bigger the aperture of the scope, the smaller the disk, the greater the resolution.  Once one can resolve the Airy disk, it's like seeing the pixels on your monitor, you can resolve no more.  

 

A 2mm exit pupil is 50x in a 100mm telescope, 100x in a 200mm telescope.  For planetary, lunar and other high magnification observation, all my experience says that this is not sufficient.  Under excellent seeing conditions, even under poor ones, I know of no one who observes Jupiter and Saturn at 50x in a 4 inch scope.  Viewing the double-double in a 4 inch scope, it resolves very nicely at 150x..(7mm exit pupil), at 100x quite nicely if the seeing is half decent and some can see a split at 50-60x but it is far from optimal.  Closer doubles require even great magnifications.

 

For me, the bottom line is that there is no magic exit pupil.  One really needs to have eyepieces that cover the entire range of possibilities.  For dim objects under dark skies, there are times when an 8mm exit pupil is optimal.. For close double stars in a small or medium sized scope, I will use exit pupils of about 0.3mm.. For the planets, generally somewhere around 0.5mm to 1mm.  But each object, each night, it's different. Fortunately, I can use my eye as the judge..

 

 

Jon

 

 

Jon,

 

I never said that there was an all-inclusive magic exit pupil of precisely 2mm*, hence my use of "magic".  What I mentioned, based on my experience, was a range of exit pupils from approximately 1.5 to 2.5mm.  That range applies to small, deep-sky objects.  Smaller exit pupils are, of course, required for observing objects that require higher magnifications.

 

Exit pupils in the range of 2mm are not ideal for planetary or tight double star observing but they work very well for most extended DSOs.  The explanation for why this is so can be found at http://bpastro.org/a...cting-eyepieces

 

You’ll find that a 2mm exit pupil will show the most detail in extended objects like galaxies and nebulae. While it’s true that the largest exit pupils produce the brightest images and, one would think, be the best for deep-sky observing, the background sky will be equally as bright and you’ll find that the overall image will be lacking in contrast, especially in light-polluted areas. With a 2mm exit pupil the overall image (the background sky and the object itself) will be equally darkened but the size of the image will be increased, and one of the properties of the human eye is that it’s easier to see faint things if they are large. At this point you can continue to decrease exit pupil size (increase magnification) to see if more detail can be drawn out. There are many diffuse objects that look better at higher magnifications but in general the 2mm exit pupil will show the most detail.

 

This will only optimize detail for extended, diffuse objects like galaxies and nebulae. It does not apply to bright objects like stars and planets. Since stars are points of light, they don’t appear to change in brightness as exit pupils get smaller. However, the background sky will get increasingly darker as exit pupils decrease, making it easier to see faint stars. This is valuable to observers who are attempting to split very faint double stars, for example, but not something that most novices will be concerned with. For now it’s enough to know that a 2mm exit pupil is an excellent all-around size for most observers and will be a favored eyepiece.

 

There's an interesting discussion of exit pupil ranges at http://www.rocketmim...Brightness.html

 

* http://www.rocketmim...Brightness.html

 

Meanwhile the smaller the exit pupil, the more the diffraction limit of the pupil impedes the resolving power of the eye. So there is an optimum point where the combined effects of spherical aberration, chromatic aberration, and diffraction effects reach a minimum.

 

It turns out that an exit pupil of about 2-3mm (2.4mm to be precise) is the optimum point for maximizing the resolving power of the eye. This is about where theory would put it, and it has been confirmed by observational studies.

 

Dave Mitsky



#25 REC

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Posted 07 November 2014 - 09:26 AM

Dave, nice description on why this 2-3mm is a very favorable range. Maybe that's why Meade and Celestron include a 25-26mm eyepiece with their scopes? Mostly in my 8 & 10" scopes.

 

My favorite EP in general is a 19-20mm SWA 68* .  I'll go up or down depending on the object I'm viewing. Also the LP does play a major part from where I live, Red zone and over 30mm the sky starts getting too bright and grey.

 

Bob




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