# Thread: how much is the magnification of my barlow increased?

1. ## 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. ## 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. ## 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

4. ## 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. ## 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

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

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

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. ## 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. ## 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

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. ## 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. ## 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