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  1. #1's Avatar Guest

    Default Exit Pupil Question

    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?

  2. #2's Avatar Guest

    Default Exit Pupil Question wrote:

    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.

  3. #3
    canopus56's Avatar
    canopus56 Guest

    Default Exit Pupil Question

    <> wrote in message

    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

  4. #4
    Willie R. Meghar's Avatar
    Willie R. Meghar Guest

    Default Exit Pupil Question wrote:

    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!)

    That is an easy question to answer. The answer is: "NO."

    A search unearthed the following relevant site that you may find more

    I was pleasantly surprised to see Tony Flander's name when the page

    Willie R. Meghar

  5. #5
    Chris L Peterson's Avatar
    Chris L Peterson Guest

    Default Exit Pupil Question

    On Mon, 25 Sep 2006 10:06:11 -0600, "canopus56"
    <> wrote:

    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

  6. #6
    Willie R. Meghar's Avatar
    Willie R. Meghar Guest

    Default Exit Pupil Question

    I wrote:

    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

  7. #7
    canopus56's Avatar
    canopus56 Guest

    Default Exit Pupil Question

    Chris L Peterson wrote:

    Thanks Chris, you have more precisely defined the Blackwell curve and
    how it relates to exit pupil.

    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." (accessed

    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. (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

  8. #8
    Chris L Peterson's Avatar
    Chris L Peterson Guest

    Default Exit Pupil Question

    On 25 Sep 2006 13:38:08 -0700, "canopus56" <> wrote:

    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



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