[Chaos-l] Burgess Finders?

Thu Aug 23 21:29:29 EDT 2007

neat conversation.

I have a 6 x 30 finder. Is there any simple way to compare the field of view of the 8 x 50 with the smaller finder?

Being greedy, I would like to get 2.78 times the light gathering power and also a wider FOV.

Thanks to all for sharing their wisdom.
  ----- Original Message ----- 
  From: Mark South 
  To: Michael Hrivnak 
  Cc: Chapel Hill Astronomical Observation Society 
  Sent: Tuesday, August 21, 2007 10:27 AM
  Subject: Re: [Chaos-l] Burgess Finders?

  hi, 
  1 more correction to yesterday's email.
  I said the 50mm has 278 times more light gathering power.  Correction:  Based on the formula below, that is actually 2.78 times more...and that still is a 178% increase from 30mm to 50mm.  Remember, however, 50mm is still small aperture as a finderscope, so still don't expect to be "wowed" by this scope, unless you are in really transparent, dark skies.
  Kind regards,
  Mark
  On Aug 20, 2007, at 7:32 AM, Mark South wrote:

    One more thing as I state the obvious:  :)
    Note I said to square the radii, but used the diameters to calculate.  You get the same answer either way because of the ratios.  
    25^2/15^2 = 2.78/1
    thanks
    Mark
    On Aug 20, 2007, at 6:33 AM, Mark South wrote:

      Hi,
      Overall, I think you will find the Burgess RACI to be much better than your older Orion finder-scope.  It should provide more light gathering power.   I personally own an Antares RACI, so I don't have experience with a Burgess finder-scope.   However, I do own a Burgess 7mm eyepiece and have found it to be exceptional, so I would have to assume their RACI's would be exceptional as well.  

      The ratio of light gathering power = the square of the radius of the larger object/ the square of the radius of the smaller object.  
      In your case, (50mm)^2/ (30mm)^2= 2500/900 = 2.78/1 ratio.   So that's 278x more light gathering power, or 178% increase of the 50mm over the 30mm.  (change/original= % increase-- that is, [2.78-1]/1*100)

      With that said, in my experience 50mm will perform really well in dark skies, but can be a challenge in light polluted skies due to the low overall aperture, knowing which way is north, etc.   Personally, I find the RACI more helpful than a straight-through because my map will match my views.  I would consider a few things to help you  find the harder to locate objects:
      1)  When you purchase the finder-scope, try to determine your field of view (FOV).  Mine is 7-degrees. 
      2)  If you are using a traditional map, consider drawing a circle to match your FOV with a permanent marker on an overhead.  Cut out the overhead to a smaller square to fit your map.   For example for my 7-degree FOV, I was able to measure 7 degrees by measuring the declination on the right side of the map.   If you have program on your palm (i.e. Planetarium), you can program in the circle to the appropriate degrees.  
      3) Know which way is north and orient your map accordingly.   The way I determine North is slightly "nudge" your scope toward Polaris.  With practice, you can orient your map to match what you see in your finder-scope quickly so you can know where to hop next. 
      4) Look for star patterns and orient your map accordingly.   After finding that bright star, I like to look for triangles or other patterns and jump to them.   The patterns are also helpful to orient your map. 
      5) above is adapted from the Sky and Telescope.  consider taking a deeper look here:

        http://www.skyandtelescope.com/howto/visualobserving/Map_at_the_Telescope.html

      Best to you.  Hope for clear skies ahead so some of us can star-hop!
      Mark

      On Aug 20, 2007, at 2:55 AM, Michael Hrivnak wrote:

        Any experience or opinions on the Burgess 8x50 RACI finder?

        http://www.burgessoptical.com/Accessories/Finder8x50.html

        I realized that I don't use my Orion 6x30 finder very much, and I think it's 
        because I just can't see very well with it.  I can barely make out a few 
        brighter objects like M13, but it's a real stretch.  My hope is that since an 
        8x50 should gather 2.78 times as much light, I'll have a much easier time 
        identifying things in the finder, and thus it will be a more valuable tool.  
        Does your experience support that theory?  Do you prefer a different size 
        than 8x50?

        If I have my math right, I think objects in a perfect 8x50 should be about 
        1.56x brighter than in a perfect 6x30.  If only to help my own understanding, 
        I should quickly run through the math.  The saying is that doubling 
        magnification reduces brightness by a factor of 4, right?  I think that can 
        be formulated to say that the factor by which brightness will change from one 
        magnification to the next is equal to (old magnification / new magnification) 
        ^2.  In this case, we have (6 / 8)^2 = .5625.  Correct me if I'm abusing the 
        laws of physics here.

        Then the difference in aperture area = 25^2 / 15^2 = 2.78.

        We have two factors by which brightness will change, so we multiply them!  
        2.78 * .5625 = 1.5625.

        Object should be bigger, brighter, and thus much easier to locate.  What do 
        you think?

        Thanks,
        Michael
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