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Date: 24 Sep 2006 22:13:52
From:
Subject: Exit Pupil Question
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Ok, so there is a theoretical relationship between the power of a
scope and the size of the objective. Some say 5mm, some say 7mm.
My question; If my scope has a exit pupil of 7mm with a 35mm lens.
(120mm 600 fl.17 power with a 35mm lens)
Will not the image be still brighter with a 40mm lens. Won't I be able
to see still fainter stars at even lower powers? And if so, what
purpose is calculating the exit pupil other than to fugure out how
much brighter the image would be than with the unaided eye?
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Date: 25 Sep 2006 10:23:02
From: Willie R. Meghar
Subject: Re: Exit Pupil Question
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larrymiracle@amicomm.net wrote:
>My question; If my scope has a exit pupil of 7mm with a 35mm lens.
>(120mm 600 fl.17 power with a 35mm lens)
>Will not the image be still brighter with a 40mm lens.
You will need to be *very specific* in how you define "image" and
"brighter" before I'll even attempt to answer that question. (Similar
questions have come up in the past!)
>Won't I be able
>to see still fainter stars at even lower powers?
That is an easy question to answer. The answer is: "NO."
A search unearthed the following relevant site that you may find more
convincing:
http://mysite.verizon.net/vze55p46/id9.html
I was pleasantly surprised to see Tony Flander's name when the page
loaded!
Willie R. Meghar
http://meghar.blogspot.com/
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Date: 25 Sep 2006 12:57:47
From: Willie R. Meghar
Subject: Re: Exit Pupil Question
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I wrote:
>A search unearthed the following relevant site that you may find more
>convincing:
>
>http://mysite.verizon.net/vze55p46/id9.html
>
>I was pleasantly surprised to see Tony Flander's name when the page
>loaded!
My apologies go out to Tony Flanders. It seems that I hit the
apostrophe key one letter too soon. Sorry about that.
Willie R. Meghar
http://meghar.blogspot.com/
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Date: 25 Sep 2006 10:06:11
From: canopus56
Subject: Re: Exit Pupil Question
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<larrymiracle@amicomm.net > wrote in message
news:8pheh21jkdbe9as123vgelov1k8fvhp5j7@4ax.com...
> [W]hat purpose is calculating the exit pupil other than to fugure out how
> much brighter the image would be than with the unaided eye?
Consider the other end oft the scale - extreme magnification. As the exit
pupil shrinks below 0.3mm, your eye can no longer perceive it.
Certain exit pupil diameters are better suited for observing detail in some
objects than others, e.g. - observing galaaxies, planetary nebula and
splitting double stars. Use a search engine on this newsgroup for Knisely's
Useful Magnifications Table in D. Knisely, 5/14/2004 sci.astro.amateur
Usenet post, Thread, "Eyepiece Advice." That contains a table by
magnification per inch of aperture, the resulting exit pupil, and what those
various levels of magnification are useful for.
Also note Whitaker's rule - at 25x per inch of aperture, diffraction
limited effects begin to be seen.
- Canopus56
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Date: 25 Sep 2006 16:35:13
From: Chris L Peterson
Subject: Re: Exit Pupil Question
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On Mon, 25 Sep 2006 10:06:11 -0600, "canopus56"
<canopus56@NOyahooSPAM.com > wrote:
>Consider the other end oft the scale - extreme magnification. As the exit
>pupil shrinks below 0.3mm, your eye can no longer perceive it.
How do you figure? In principle there is no minimum exit pupil size,
although in practice (with the human eye) issues such as floaters and
irregular areas of the cornea obviously become factors. But these things
are related to types of distortion; I don't see that a small exit pupil
affects your ability to perceive the image.
Of course there is always some extreme of magnification where you have
simply spread out too little light over too much area, and you drop
below the sensitivity or contrast threshold for the human eye, but
framing that in terms of exit pupil doesn't make much sense to me.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 25 Sep 2006 04:17:48
From:
Subject: Re: Exit Pupil Question
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larrymiracle@amicomm.net wrote:
> Ok, so there is a theoretical relationship between the power of a
> scope and the size of the objective. Some say 5mm, some say 7mm.
>
> My question; If my scope has a exit pupil of 7mm with a 35mm lens.
> (120mm 600 fl.17 power with a 35mm lens)
> Will not the image be still brighter with a 40mm lens. Won't I be able
> to see still fainter stars at even lower powers? And if so, what
> purpose is calculating the exit pupil other than to fugure out how
> much brighter the image would be than with the unaided eye?
Those are good questions.
If you decrease the magnification to the point that your scope's exit
pupil is larger the your eye's pupil you will be reducing your scope's
effective aperture. The sky background doesn't look any brighter once
you pass that point; you just get a wider field of view as you continue
to reduce the mag. In fact the sky always appears dimmer than with the
unaided eye due to light losses in the optics.
On the other hand, increased magnification darkens the field (its light
is more spread out) but because stars are point sources they don't tend
to look larger. In other words, fainter stars can often be more easily
seen against a darker background.
Even extended sources (nebulae and galaxies) are often easier to see at
higher mags because they now appear larger, even if their light is now
more spread out. However each object, telescope and observer is
different so finding the "best" magnification to use becomes with
easier with experience.
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Date: 25 Sep 2006 13:38:08
From: canopus56
Subject: Re: Exit Pupil Question
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Chris L Peterson wrote:
> How do you figure? In principle there is no minimum exit pupil size, . . .
> Of course there is always some extreme of magnification where you have
> simply spread out too little light over too much area, and you drop
> below the sensitivity or contrast threshold for the human eye, but
> framing that in terms of exit pupil doesn't make much sense to me.
Thanks Chris, you have more precisely defined the Blackwell curve and
how it relates to exit pupil.
http://clarkvision.com/visastro/omva1/plot.bl.surf.3d.s1.gif
I could have been more accurate by saying "you do not perceive any
useable detail" instead of "do not see anything."
On a bright object with a 4mm lens with a barlow in Knisley's useless
magnification range, you see a blurred featureless diffraction limited
field with your eye's floaters in the apparent foreground. On a dim
nebula at useless magnification, the dimness of the nebula fades to
black below the Blackwell surface detection limit. On a point stellar
object, you see the Airy disc with diffraction rings.
The Knisely useful magnification table based on exit pupil size per
inch of aperture has always been a very practical way for me to
initially select eyepieces by focal length when using my 10" inch Newt
and 5 1/4" refractor on various types of objects. In practice, it
works for me and is easy to apply.
Also easy to apply is the minimum detection magnitude (MDM) and optimum
detection magnitude (ODM) rules-of-thumb that we discussed in this
newsgroup in April 2005. These rules-of-thumb are similar to Knisely's
useful magnification but are based on the Blackwell surface. Looking
back to that April 12, 2005 thread -
Clark gives the following advice:
"Thus, the observing strategy to detect deep-sky objects, or detail
within objects, is to magnify those objects, or detail within the
objects. so they appear about 100 arc-minutes in size. For example, if
you are trying to detect a dark nebula in a galaxy arm, magnify that
dark nebula so that it appears about a degree across or more. . . . To
see all the detail in an object, use many powers, from very low to very
high, examining the entire object with each magnification."
http://www.clarkvision.com/visastro/m51-apert/index.html (accessed
4/2005)
Since most galaxies are about 4 x 5 arcmins, Clark is saying magnify
them 20-30x and then use a series eyeieces at increasing magnification
to till you see the best detail. That is the common sense method that
most amateurs intuitively develop on their own, but the method has some
theory behind it.
In this usenet group between 2000 and 2003, there were a series of
extended discussions about what is the "best" rule-of-thumb to the
optimal magnification. This is the kind of rule-of-thumb that gets
amateur astronomers going. Everyone has an opinion and I have no inside
straight on what is the "best" method.
The rule-of-thumb I extracted from those discussions and that I
personally use are the following:
1) Start at with Minimum Detection Magnification in Appendix "E" to
Clark's book. http://clarkvision.com/visastro/appendix-e.html (accessed
4/2005). Note that some of the MDMs in Clark's table, which are
mathematically derived, from astronomical catalogues are way off. But
generally, Appendix "E" gives you a good starting point.
Alternatively, use the ODM value listed in Rodman's Astroplanner
software. Another option is have Bartel's program on hand, compute the
ODM on your laptop and use that as the starting point. Understand that
use ODM, you theoretically will be closer to your final target
magnification. I perfer to use MDM as the starting point.
2) If MDM is used as the starting point, the background brightness of
the night sky in the eyepiece will be brighter than the absolute black
body of the barrel of the eyepiece seen on the fringes of the image.
3) Increase magnification until the background night sky brightness is
equal in brightness to the black body of the eyepiece barrel. Is the
same thing as increasing magnfication until the brightness of the
extended background night sky drops "below" the Blackwell surface
around MPAS 25.0, and can longer be seen by the human eye. From this
point, no matter how much you increase magnification, the background
night sky in the eyepiece will not appear any darker. It's too dim for
the human eye to detect. But the increasingly magnified object will
start to appear dimmer.
4) Then use one or two more levels of eyepieces interpolate around this
magnification point, either increasing or decreasing magnification to
maximize detail. Do this to personal viewing taste.
5) Pray you have enough aperature to sufficiently magnify the object to
the 100 arcminute size suggested by Clark and the chart before the
extended object dims to invisibility. -:)
Both the Knisely rule-of-thumb table and Clark's advice are probably
useful to beginner-intermediate amateurs like the top-poster.
- Canopus56
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Date: 25 Sep 2006 22:09:21
From: Chris L Peterson
Subject: Re: Exit Pupil Question
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On 25 Sep 2006 13:38:08 -0700, "canopus56" <canopus56@yahoo.com > wrote:
>Thanks Chris, you have more precisely defined the Blackwell curve and
>how it relates to exit pupil.
>http://clarkvision.com/visastro/omva1/plot.bl.surf.3d.s1.gif
>
>I could have been more accurate by saying "you do not perceive any
>useable detail" instead of "do not see anything."
>...
>The Knisely useful magnification table based on exit pupil size per
>inch of aperture has always been a very practical way for me to
>initially select eyepieces by focal length when using my 10" inch Newt
>and 5 1/4" refractor on various types of objects. In practice, it
>works for me and is easy to apply.
Okay, I think I understand your point. I have a problem with using a
minimum exit pupil in absolute terms (which is how you put it). But in
terms of exit pupil per unit of aperture, that makes perfect sense.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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