[Chaos-l] Burgess Finders?
Mark South
md_south at mac.com
Fri Aug 24 08:59:42 EDT 2007
> [Bobi said]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?
Bobi and Michael:
Great question: I had the same question when I was purchasing my
finder-scope...
Although there is a simple formula to find the true field of view, it
is not helpful for finder-scopes generally. Let me explain:
TFOV= AFOV of eyepiece/magnification given by the eyepiece (where
TFOV = total field of view and AFOV = apparent field of view)
Unfortunately, finding the TFOV is difficult for a finder-scope
because the AFOV is usually not documented for finder-scopes. You
will know the magnifications however-- 6x and 8x respectively.
Note: You will usually find the AFOV with any eyepiece you buy to
help calculate TFOV with your regular telescope.
With that said, the following are 2 ways to get the TFOV:
1) Look at the specifications of the finder-scope on the web-page,
call the manufacturer, or check the manual. As I stated in a
previous email, knowing this info is critical for good star-hopping.
For example, the Antares RACI clearly stated that my field-of-view is
6-degrees. My orion finder-scope clearly said in the specs when I
bought it that the TFOV was 7-degrees. This is the easiest way to
compare, but probably not the most accurate (if you do the test below).
2) Find two bright stars that are around 6-degrees or 7-degrees
apart. Put them both in the finder-scope to estimate the distance in
degrees.
3) Drift test. Most accurate method to test if the manufacturer is
correct (below is adapted from Thompson's "Astronomy Hacks" ).
a) the star must drift across the diameter of the finder-scope
(not a chord)
b) time in seconds the time for the drift (you may let if drift
half-way of the diameter and then double the time) For us Dob users,
this may be easier!!!
c) Calculate TFOV= T * 0.2507 * Cos(Declination) TFOV is in arc-
minutes. T= seconds Make sure the declination is in a decimal value.
d) It is best to use a star on the celestial equator so that the
declination = 0, therefore cos (0)= 1, but any star as close as
possible to this equator is OK also.
Example used by Robert Thompson: Antares: suppose it drifts
for 4:43 minutes or 283 seconds. The Declination = -25 degrees 26'
or -25.433 degrees.
So: TFOV = 283 seconds * 0.2507 * Cos(-25.433 degrees) =
64.1 arc-minutes. Convert this to degrees and the TFOV = 1.07 degrees.
A finder-scope will take much less time to drift for 6 and 7
degree field of views.
I recommend getting the specs because that it is easiest. I've used
method #2 to estimate. However, Thompson has noticed manufacturers
are often wrong in the specs and that can really make a big
difference if you are really into star-hopping. If I get time at
MASP (www.masp.org) I'm going to do a drift test (method #3) to make
sure my scopes are truly spec'd as advertised.
Kind regards,
Mark
On Aug 23, 2007, at 9:29 PM, BOBI GALLAGHER wrote:
> 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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