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  1. #1
    Barry Maxwell's Avatar
    Barry Maxwell Guest

    Default how much is the magnification of my barlow increased?



    I use a Dakin 2.4x barlow with my CCD camera. Until a week ago, the CCD
    focal point rested just at the end of the barlow length. Now that I've
    added a filter wheel before the CCD, it is now approximately 2 1/8" farther
    away from the end of the barlow. Instead of 2.4x amplification, what has it
    increased to?

    Thanks,
    Barry Maxwell



  2. #2
    Chris L Peterson's Avatar
    Chris L Peterson Guest

    Default how much is the magnification of my barlow increased?

    On Sun, 22 May 2005 14:55:54 GMT, "Barry Maxwell" <abcd@eg.hij> wrote:


    That is very hard to calculate, but very easy to measure (and I doubt it
    was ever exactly 2.4X). Just shoot a star field and solve the plate. It
    is a good idea when imaging to know your exact image scale, and
    measurement is really the only practical way to determine this.

    _________________________________________________

    Chris L Peterson
    Cloudbait Observatory
    http://www.cloudbait.com

  3. #3
    William Hamblen's Avatar
    William Hamblen Guest

    Default how much is the magnification of my barlow increased?

    On 2005-05-22, Barry Maxwell <abcd@eg.hij> wrote:

    The magnification equals 1 + D/F, where D is the distance from the
    negative lens to the focal point and F is the focal length of the negative
    lens expressed as a positive number. I think the focal length of a Dakin
    barlow is -30 mm, but that is from memory and I am likely to be wrong.
    If you increase D by 55 mm you increase the magnification plus 55/30. The
    new magnification becomes about 4.2x.


  4. #4
    David Nakamoto's Avatar
    David Nakamoto Guest

    Default how much is the magnification of my barlow increased?

    I have to agree with this. There are field of view calculators as freeware and
    otherwise, but in the end the only way to solve the FOV problem is to take a
    image of a dense star field that you know the angular separation of the stars
    and measure their distances in the image. Another fair way to accomplish this
    is to compare the FOV with an image generated from the Digitized Sky Survey and
    compare the two.

    Good Luck !

    --- Dave
    --
    ----------------------------------------------------------------------
    Pinprick holes in a colorless sky
    Let inspired figures of light pass by
    The Mighty Light of ten thousand suns
    Challenges infinity, and is soon gone

    david.nakamoto@verizon.net


    "Chris L Peterson" <clp@alumni.caltech.edu> wrote in message
    newsf7191hg21msg6iges11bbsgs3kfdkb4tn@4ax.com...



  5. #5
    canopus56's Avatar
    canopus56 Guest

    Default how much is the magnification of my barlow increased?

    Barry Maxwell wrote:

    As Chris notes, the practical method to determine magnification is by
    measuring the image recorded by the chip or on film. This reply goes
    to computing the approximate distance a negative barlow should set
    behind prime focus to achieve a desired magnification. This method will
    only provide an approximation and a means to plan an initial position
    to place your negative barlow and your camera's imaging device.

    In negative projection, the negative lens is placed between 0mm and one
    barlow lens focal length inside focus of prime focus. As the lens is
    moved between 0mm inside prime focus to one focal length inside prime
    focus, magnification increases.

    Normally, the computation of a desired linear image size on the chip or
    35mm film diagnol begins with the arcsecond or arcminute size of the
    object and proceeds through the telescope to the imaging plate (ccd
    chip or film diagnol).

    Based on your question, let's confine the computation to the placement
    of a 30mm focal length ("F") negative barlow lens inside of prime
    focus (the "A" distance) and corresponding the placement of the camera
    plane outside prime focus (the "B" distance) to achieve a desired
    magnification ("M").

    The following cheat sheets may help in defining terminology used in
    this reply vs. placing a lengthy text explanation here:

    http://members.csolutions.net/fisher...Barlowneg1.jpg
    http://members.csolutions.net/fisher...h/Negproj1.jpg

    First, you have to find out where real image at prime focus is located
    behind the objective. I'll assume you know how to measure that and
    will skip that step.

    The second involves confirming the focal length of your 30mm negative
    barlow. To do this, as explained in steps 3, 4 and 5 of:

    http://members.csolutions.net/fisher...Barlowneg1.jpg

    unscrew the negative lens from the back of your barlow. On one piece
    of paper measure to dots 20mm apart. On a second piece of paper,
    punch too small (1mm) holes into a piece of paper 10mm apart. Using
    measuring calipers or a ruler, under a collimated light source like
    the Sun, hold the paper with the punches on top of the lens. Project
    the two points of light through the lens onto the paper. Move the lens
    until the two points of projected light match the 20mm marks. Now
    measure the distance between the center of the lens and the paper with
    the 20mm marks. This distance is the focal length of your lens.

    It should confirm Bill Hamblen's 30mm report for the focal length of
    your particular barlow. If it does not confirm that focal length, you
    can recompute a table of A-B distances and the resulting magnification
    from the following formulae.

    What happens when you place the negative barlow lens a given "A"
    distance inside prime focus between 0mm and one focal length of the
    barlow?

    Assuming that you know the barlow's F (focal length in mm) and A
    (distance behind prime focus in mm), then M (magnification) is -

    M = F / ( F - A ) (1.0)

    The "B" distance between prime focus and your camera's image plane is:

    B = ( F x A ) / ( F - A ) (2.0)

    Also note that:

    M = B/A (3.0)

    For a 30mm negative Barlow lens, this works out to the following values
    (to view this table, you may need to switch to "original format" view
    for some newsreaders):

    F_mm A_mm B_mm M A/B
    30 2 2 1 1
    30 5 6 1.2 1.2
    30 8 10 1.3 1.2
    30 11 17 1.5 1.5
    30 14 26 1.8 1.8
    30 17 39 2.3 2.2
    30 20 60 3 3
    30 23 98 4.2 4.2
    30 24 120 5 5
    30 25 150 6 6
    30 26 195 7.5 7.5
    30 27 270 10 10 Practical limit
    30 28 420 15 15 ==========
    30 29 870 30 30

    These equations are not the same for projection magnification using a
    positive eyepiece lens.

    The linear size of your object recorded on your camera's detector ( ccd
    chip or film) is the linear size of the object at prime focus (the
    diameter of first image) times the magnification -

    M x Dia_1st_image = Linear_dia_on_film_chip (4.0)

    The positions in the above tables are only approximations. You still
    have to focus and then confirm the magnification by comparing your
    recorded image of a star field to those star's known positions using a
    planetarium program or astrometry catalogue.

    - Enjoy Canopus56

    Further reading:

    Edmund Sci. 1966-1998 Photography with your Telescope. pp. 19-20

    Edmund Sci. 1974-2001. Popular Optics. pp. 53 (A-B distance) and 78 (C
    distance).

    Brown, Sam. Edmund Sci. 1974-2001. All About Telescopes. pp. 142.

    Covington, Michael A. 1999. Astrophotography for the Amateur Cambridge
    University Press. ISBN 0-521-64133-0 (hardback), 0-521-62740-0
    (paperback) << http://www.covingtoninnovation*s.com/astro/ >>

    North, Gerald. 1997 (2ed). Advanced Amateur Astronomy. Cambridge
    University Press. ISBN: 0521574307 <<
    http://www.amazon.com/exec/obidos/AS...302643-9250225

    P.S. -

    For a 50mm, the A-B-M table is:

    F_mm A_mm B_mm M A/B
    50 5 5 1.1 1
    50 10 12 1.2 1.2
    50 15 21 1.4 1.4
    50 20 33 1.6 1.6
    50 25 50 2 2
    50 30 75 2.5 2.5
    50 35 116 3.3 3.3
    50 40 200 5 5
    50 45 450 10 10
    50 46 575 12.5 12.5 Practical limit
    50 47 783 16.6 16.6 ==========
    50 48 1200 25 25
    50 49 2450 50 50


  6. #6
    canopus56's Avatar
    canopus56 Guest

    Default how much is the magnification of my barlow increased?

    Barry Maxwell wrote:
    farther
    has it

    Barry, my prior post went to a basic negative barlow. It looks like
    the Dakin may be an erector consisting of a doublet of positive lenses.
    If so, please confirm. Then a different set of equations apply. -
    Canopus56


  7. #7
    canopus56's Avatar
    canopus56 Guest

    Default how much is the magnification of my barlow increased?

    Barry Maxwell wrote:

    Assuming your Dakin 2.4x barlow is a doublet erector, then positive
    projection applies. The "A" distance is measured from the center of
    the lens cell to the real image at prime focus. The doublet is treated
    as a single thick lens.

    In positive projection, the barlow eyepiece lens is placed between 1
    focal length and 2 focal lengths outside focus of prime focus. This
    barlow/erector lens placement differs from negative project where the
    "A" distance is measured from and the lens is placed inside focus. As
    the lens is moved from 2 focal lengths down to 1 focal length from the
    real image at prime focus, magnification increases.

    The corresponding equations and tables for positive projection
    magnification are:

    M = F / ( A - F ) (5.0)

    B = ( F x A ) / ( A - F ) (6.0)

    M = B/A (3.0)

    For a 30mm positive Barlow lens, this works out to the following values
    (to view this table, you may need to switch to "original format" view
    for some newsreaders):

    F_mm A_mm B_mm M A/B
    30 57 63 1.1 1.1
    30 54 67 1.2 1.2
    30 51 72 1.4 1.4
    30 48 80 1.6 1.6
    30 45 90 2 2
    30 42 105 2.5 2.5
    30 40 120 3 3
    30 38 142 3.7 3.7
    30 36 180 5 5
    30 35 210 6 6
    30 34 255 7.5 7.5
    30 33 330 10 10
    30 32 480 15 15
    30 31 930 30 30

    For a 50mm positive lens, the A-B-M table is:

    F_mm A_mm B_mm M A/B
    50 95 105 1.1 1.1
    50 90 112 1.2 1.2
    50 85 121 1.4 1.4
    50 80 133 1.6 1.6
    50 75 150 2 2
    50 70 175 2.5 2.5
    50 65 216 3.3 3.3
    50 60 300 5 5
    50 58 362 6.2 6.2
    50 56 466 8.3 8.3
    50 55 550 10 10
    50 54 675 12.5 12.5
    50 53 883 16.6 16.6
    50 52 1300 25 25

    See "Further reading" in the prior post for details.

    - Enjoy - Canopus56


  8. #8
    William Hamblen's Avatar
    William Hamblen Guest

    Default how much is the magnification of my barlow increased?

    On 2005-05-24, canopus56 <canopus56@yahoo.com> wrote:


    The Ralph Dakin barlow is a negative doublet. VERNONscope has them now
    for much bucks.


  9. #9
    canopus56's Avatar
    canopus56 Guest

    Default how much is the magnification of my barlow increased?

    X-No-archive: yes
    William Hamblen wrote:

    Thanks, Bill, then the negative lens equations do apply. - C


 

 

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