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2 mm exit pupil magnification

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#1 Swirl19

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Posted 18 April 2019 - 02:42 PM

Could you explain please what the difference between 2 mm exit pupil magnification and 7 mm for instance in relation of ability of an eye to perceive faintest objects? I am using 9 mm and 14 mm morpheus for observing moon and I noticed may be erroneously that 9 mm morpheus gives more bright moon then 14 mm one. I always thought that the more FL of an eyepiece more light it sends to your eyes and vice versa. Is that not so?

Edited by Swirl19, 18 April 2019 - 02:43 PM.


#2 bobito

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Posted 18 April 2019 - 02:49 PM

Yes. The 9mm would be a smaller exit pupil than the 14mm, so the 9mm should be more dim.  A guess would be that your experience was due to your eye being more dark adapted when you tried the 9mm?



#3 kathyastro

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Posted 18 April 2019 - 03:03 PM

A 2mm exit pupil is close to the upper end of the magnification range of a scope.  High magnification usually means a dimmer image, since the same number of photons are spread out over a larger part of the field of view.  A 7mm exit pupil is roughly the lower limit of magnification for a young person with healthy, well dark-adapted eyes.  The actual limit is determined by the pupil of your eye, which is why good dark adaptation is important.  With a lower magnification, the image will generally be brighter.

 

However, if you pupil is significantly smaller than 7mm, much of the light will be lost, and the image will be dimmer than it might have been.

 

The more focal length an eyepiece has, the higher the magnification, and therefore the dimmer the image.  So the relationship of brightness to focal length is the opposite of what you thought.


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#4 Swirl19

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Posted 18 April 2019 - 03:22 PM

Understood. May be that effect because of nonlinear effect of pupil size change? When the scene was brighter (14 mm) pupil shrinked much more then it did when 9 mm eyepiece was used and owing to this scene appeared to me less illuminated?

Edited by Swirl19, 18 April 2019 - 03:36 PM.


#5 Swirl19

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Posted 18 April 2019 - 03:26 PM

Yes. The 9mm would be a smaller exit pupil than the 14mm, so the 9mm should be more dim. A guess would be that your experience was due to your eye being more dark adapted when you tried the 9mm?

I tryed many times. Even first used 14 mm.

#6 Dave Mitsky

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Posted 18 April 2019 - 03:36 PM

The more focal length an eyepiece has, the higher the magnification, and therefore the dimmer the image.

Actually, it's just the opposite.

 

https://www.skyandte...elescope-owner/



#7 Dave Mitsky

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Posted 18 April 2019 - 03:42 PM

There's some useful information on exit pupil and magnification at https://starizona.co...serving-theory/


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#8 macdonjh

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Posted 18 April 2019 - 03:44 PM

Magnififation = (telescope focal length) / (eye piece focal length)

Exit pupil = (telescope aperture) / (magnification)

So eye pieces with shorter focal length give higher magnification and smaller exit pupil. For bright obje ts I routinely use magnification that results in an exit pupil of 0.7mm. Bright objects include the moon, planets, some planetary nebulae, globular clusters,etc.

As to why the moon seemed brighter in a 9mm eye piece than a 14mm eye piece... Perhaps you viewed through the 9mm first as has been suggested? Did you observe along the terminator with the 14mm such that a portion of the field was dark?

#9 Jon Isaacs

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Posted 18 April 2019 - 03:47 PM

Actually, it's just the opposite.

 

https://www.skyandte...elescope-owner/

 

:waytogo:

 

Shorter focal length eyepieces provide high magnifications, the light is spread out more and so the image is dimmer.

 

The moon is a poor target to investigate brightness versus exit pupil.  The moon is large enough and bright enough that the pupil contracts to day light levels.  That means the exit pupil of the eyepiece may be larger than your pupil, not all the light enters the eye. Furthermore the brightness affacts other aspects of dark adaptation.

 

Jon


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#10 dakinemaui

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Posted 18 April 2019 - 03:48 PM

LOL, it's complicated.

 

In addition to the brightness factor mentioned by others, a smaller pupil may cause objects to appear sharper as the rays are less distorted by aberrations in your eye. Often, eyeglass wearers don't need them at these small pupil sizes (small == good). On the other hand, the pupil may be more easily obscured by floaters in your eye (small == bad).

 

Then, there is pretty reliable research that says things are easier to resolve if they're larger (small pupil == large magnification == good) -- more than 10 arcmins apparent size.

 

The sensitivity of the receptors on your retina is also inversely tied to the brightness. This is one of the reasons it's difficult to separate binary stars when there is another bright object in the field of view. Similar when looking at the edge or shadow of the moon. Greater magnification makes the true field of view smaller so that bright object is not decreasing your sensitivity. 

 

Along similar lines, the sky glow is diminished by magnification. If those "faint objects" are stars, that's a good thing, since they can take a lot of magnification before they start spreading out (small pupil == good). On the other hand, it's a wash if you're trying to see details of a nebula since that would be equally dimmed.

 

IMO, it's best if you magnify as much as possible, stay dark adapted, use averted vision.


Edited by dakinemaui, 18 April 2019 - 03:53 PM.

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#11 Jon Isaacs

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Posted 18 April 2019 - 04:10 PM

waytogo.gif

 

Shorter focal length eyepieces provide high magnifications, the light is spread out more and so the image is dimmer.

 

The moon is a poor target to investigate brightness versus exit pupil.  The moon is large enough and bright enough that the pupil contracts to day light levels.  That means the exit pupil of the eyepiece may be larger than your pupil, not all the light enters the eye. Furthermore the brightness affacts other aspects of dark adaptation.

 

Jon

 

Just to clarify: the exit pupil might be 3 mm with one eyepiece and 2 mm in the other but if your eye is dilated to 1.5 mm, both will be equally bright.

 

And it does depend on the scope type.  The shadow of the central obstruction of a Newtonian or SCT has its own exit pupil, it's the exit pupil times the CO.

 

A 3 mm exit pupil with a 25% CO produces a shadow that's 3 mm x .25 = 0.75 mm.

 

A 2mm exit pupil with a 25% CO produces a shadow that's 2 mm x .25 = 0.5 mm

 

If your eye is only open to 1 mm, the larger shadow of the 3 mm exit pupil means the 3 mm exit pupil is actually dimmer.

 

Jon


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#12 havasman

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Posted 18 April 2019 - 04:24 PM

Image scale complicates the matter. At higher magnifications (smaller exit pupils) a larger image of an extended object will be contained in the smaller collimated pencil of light that enters your eye. The larger image will stimulate a larger patch of receptors on your retina than would a smaller image of that object. There is an interplay between the brightness of the reconstructed wavefront of light that passed through the scope aperture to the eye's entrance pupil and the size of the extended object contained therein. Some observers tend to prefer the brighter image with a smaller image of the primary object while others may sacrifice some brightness for the image scale gain. Some observations may be better facilitated by one strategy while others are best via more of the other.

A 2mm exit pupil is a good mid-point from which an observer can move in both directions to find what is preferred on a night for an object.


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#13 havasman

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Posted 18 April 2019 - 04:39 PM

‚ÄčHere are 3 basic arithmetic equations you may enjoy knowing. They demonstrate the interplay between EXIT PUPIL, MAGNIFICATION and TOTAL FIELD OF VIEW.

 

EXIT PUPIL = EYEPIECE FOCAL LENGTH / SCOPE FOCAL RATIO

 

MAGNIFICATION = SCOPE FOCAL LENGTH / EYEPIECE FOCAL LENGTH

 

TFOV = (EYEPIECE FIELD STOP DIAMETER X SCOPE FOCAL LENGTH) / 57.3

 

And this website is the most authoritative source of optical knowledge I am aware of. If some presentation disagrees with what's found here I look for the flaw in the presentation.  https://www.telescop...BLE_OF_CONTENTS



#14 Swirl19

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Posted 18 April 2019 - 04:42 PM

John Isaacs, I definitely feel that with morhpeus 9 mm moon seems much brighter. I thought what the heck with my eyes today. Appears it was the shadow you mentioned above. I thought something wrong with my eyes or with that eyepieces.

Edited by Swirl19, 18 April 2019 - 04:43 PM.


#15 gnowellsct

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Posted 18 April 2019 - 10:42 PM

Well folks YES, as you increase magnification objects dim out.  That's really true on planets, I've never liked pushing my 102mm refractor to its theoretical max of 2x per mm.

 

But if you're looking at NGC 6207 in a 3 or 4 inch refractor you might find it impossible to locate in a 4.5 degree field at 16x.  And I don't mean because you're "lost in the field of view."  I mean you don't see it.

 

Sometimes when you increase magnification you get a helpful effect: more receptors in the eye are activated.  Increasing magnification, when you are sure of the location, is an old trick for seeing a fainter galaxy or other object. 

 

With the MOON, though, there really is a pretty strong linear relationship, the lower the magnification, the more blindingly bright it is.  So I agree with that.

 

As to the superiority of the 9 mm Morpheus:  A master telescope maker once commented in forums that often what amateur astronomers mean when they say they like a new eyepiece is, they mean that the new eyepiece is CLEAN.  So maybe time to take a look at that 14 mm and whether some of the chicken drumstick grease you got on there six months ago but forgot about is still there.  Just a thought.  It's been known to happen.  I cleaned my oculars just the other day and they were way overdue.

 

GN



#16 Swirl19

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Posted 19 April 2019 - 02:39 AM

In 9 mm eyepiece moon occupies almost whole FOV (34'). In 14 mm one - only half diameter of FOV (54'). 14 mm eyepiece is clean. It means that whole light from the moon goes through eyepiece in both cases. Nothing lost. But 9 mm gives smaller exit pupil therefore smaller shadow from second mirror, like John Isaacs said. Where are receptors that cause retina to constrict situated? If they located on retina may be then they cause it to constrict to maximum degree and quantity of light that goes throuh pupil determined by size of that shadow? The moon was realy bright and I always ran into that some shadow every time I was trying to look through eyepiece. I watched without any moon filters. Without it you gather more realistic information about lunar surface. I saw two or three high mountains with craters upon peaks with lava trails. When I watched with filters I was seeing only craters.

Edited by Swirl19, 19 April 2019 - 11:40 AM.


#17 InkDark

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Posted 19 April 2019 - 09:25 AM

Does the moon fills up the FOV in the 9mm but not in the 14mm? If you have some dark space around the moon (in the 14 mm), it could be perceived as less bright total image than when the bright moon image saturate the entire FOV of the higher power 9 mm.

 

BTW, I have the 9 mm Morpheus. It is a nice and comfortable EP.



#18 Jon Isaacs

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Posted 19 April 2019 - 10:02 AM

John Isaacs, I definitely feel that with morhpeus 9 mm moon seems much brighter. I thought what the heck with my eyes today. Appears it was the shadow you mentioned above. I thought something wrong with my eyes or with that eyepieces.

 

What telescope are you using?

 

Jon



#19 rowdy388

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Posted 19 April 2019 - 10:04 AM

I believe Jon and Dick in posts #11 and #12 combined to give an excellent explanation on why the 9 Morpheus

is brighter than the 14 on the moon.


Edited by rowdy388, 19 April 2019 - 10:04 AM.


#20 Swirl19

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Posted 19 April 2019 - 11:39 AM

What telescope are you using?

Jon

Newton 10 inch, 1200 FL, f/4.8

#21 Swirl19

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Posted 19 April 2019 - 12:00 PM

Does the moon fills up the FOV in the 9mm but not in the 14mm? If you have some dark space around the moon (in the 14 mm), it could be perceived as less bright total image than when the bright moon image saturate the entire FOV of the higher power 9 mm.

BTW, I have the 9 mm Morpheus. It is a nice and comfortable EP.

Wondering if Nagler 3.5 mm will have the same contrast as Morpheus 9 mm in my scope? I never seen such contrasting scene on the moon before when I used filters and eyepieces like deep sky plano 6.5 mm and others. Those mountains resemble volcanic mountains on earth.

Edited by Swirl19, 19 April 2019 - 12:08 PM.


#22 Jon Isaacs

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Posted 19 April 2019 - 12:17 PM

Newton 10 inch, 1200 FL, f/4.8

 

That is certainly a scope that could be dimmer with the 14 mm than with the 9 mm. The numbers I provided are reasonably close.

 

Once the exit pupil is larger than your dilated pupil, it is only refractors that maintain a constant brightness. Telescopes with central obstructions become dimmer and dimmer as the focal length of the eyepiece is increased, the shadow of the secondary grows but your pupil masks the aperture.

 

Jon


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#23 Swirl19

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Posted 19 April 2019 - 12:30 PM

It seems so. Going to test these eyepieces today again.

Edited by Swirl19, 19 April 2019 - 12:31 PM.


#24 Swirl19

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Posted 19 April 2019 - 01:10 PM

Test was completed. The first I used 14 mm and only then 9 mm. Nevetheless, 9 mm seemed me brighter. In confirmation of the guess,the shadow of 14 mm is more disturbing. The shadow of 9 mm scarcely noticable.

#25 planet earth

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Posted 21 April 2019 - 08:09 AM

Swirl19

Here's site that you might like.

 

http://www.rocketmim...escope_eqn.html

 

Sam




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