[Chaos-l] Burgess Finders?

[Mark South]

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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>>
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