Path: newsfd02.forthnet.gr!HSNX.atgi.net!headwall.stanford.edu!newshub.sdsu.edu!postnews2.google.com!not-for-mail From: canopus56@yahoo.com (Canopus) Newsgroups: sci.astro.amateur Subject: Re: Image Luminosity vs magnification Date: 29 Aug 2004 23:21:28 -0700 Organization: http://groups.google.com Lines: 95 Message-ID: References: <1093598175.922815@athnrd02> <1093633886.990864@athnrd02> <1093641120.500668@athnrd02> <1093642681.105451@athnrd02> NNTP-Posting-Host: 216.190.205.196 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit X-Trace: posting.google.com 1093846888 8189 127.0.0.1 (30 Aug 2004 06:21:28 GMT) X-Complaints-To: groups-abuse@google.com NNTP-Posting-Date: Mon, 30 Aug 2004 06:21:28 +0000 (UTC) Xref: newsfd02.forthnet.gr sci.astro.amateur:518683 Ioannis wrote in message news:<1093642681.105451@athnrd02>... > The calculations still show that I should see M33, for example, 0.42 > times less bright in the 20x100 binos than in the 11x80, whereas in > reality I see it about 2-3 times as bright in the larger pair. > Why the apparent discrepancy? What you are trying to do is compare the unit brightness of images seen in telescopes of differing aperature and magnitude. The _relative brightness of the telescopic image_ is the ratio of the telescopic image brightness to the brightness of the naked eye image, given by the general equation - RB =( D_obj^2 * theta_transmission factor) / (D_exit_pupil)^2 * M^2 Eq. 1.0 In summary, expressed using constant lines of magnification applied to telescopes of varying size, the relative brightness of each image in the eyepiece, using the brightess image in the largest scope as a standard is: ================= Table 1 Relative telescopic brightness as a percentage of the brightess image within a magnification class Objective size D_obj_in 1 3 3.5 4 4.75 6 8 10 12 D_obj_mm 25.4 76.2 88.9 101.6 120.7 152.4 203.2 254.0 304.8 Relative Brgt 0.06% 1.56% 2.48% 3.70% 6.20% 12.50% 29.63% 57.87% 100% ================= These percentages apply if you comparing, for example, an 11x80 bino with 10" DOB dropped down to 10 magnification. How do you compare brightnesses of different aperatures and magnifications? Graphics showing the relative brightness of telescopes of various aperatures and magnifications are posted at: http://members.csolutions.net/fisherka/astronote/astromath/RelativeBrightness/RelativeBrightnessMagnificationPercentChart.gif http://members.csolutions.net/fisherka/astronote/astromath/RelativeBrightness/RelativeBrightnessMagnificationLogChart.gif (The project directory is at: http://members.csolutions.net/fisherka/astronote/astromath/RelativeBrightness/ The above files can be "surfed to" via this link.) A more detailed explanation of how these percentages were derived follows in the next long post, for those wanting to read further. One counterintuitive implication of the relative brightness equation (Eq. 1.0) is that the magnified image in the eyepiece is always dimmer - in terms of the brightness of a unit area - than that seen with the naked eye. The brightest image is that seen directly in the primary mirror with an eyepiece. The current full Moon can used to illustrate these effects. First, look at the light grasp brightness. Light grasp brightness can be perceived by removing the eyepiece from a Newtonian reflector and observing the brightness of the image of the full Moon in the primary mirror. Compare this to its naked eye brightness. Next, compare the brightness of the full Moon as seen with the naked eye, to its brightness as seen in the eyepiece. Note that: Light_grasp_brightness > Brightness_naked_eye > Brightness_magnified_telescopic > Why the apparent discrepancy? Again, this is on the basis of unit area brightness. The effect of having the magnified Moon expanded in the 50 degrees of apparent field in the eyepiece (and a correspondingly larger area of your eye's fovea) can lead to the errorenous impression that the magnified image is brighter than the naked eye image. For exploring relative brightness by aperature and magnification using the full Moon, compare the _unit brightness_ of image produced by a 10x50 binocular, a 20x70 binocular, a 6" DOB at 40x and 100x and the 6" the DOB's primary mirror. The ordering of brightness generally follows that suggested by Table 6 in the following detailed post. Again, a more detailed explanation of the relative brightness equation follows in the next long post, for those wanting to read deeper. The post is split into two parts as a courtesy to those who do not want the additional information. Enjoy - Canopus (Kurt)