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Old Mar 4, 2006, 7:10 PM   #1
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I am interesting in getting into digiscoping, and I am looking into this set up:

(1) Rubinar 500mm f5.6 (M42 threads)

http://www.rugift.com/photocameras/rubinar_500_lens.htm

(2) M42 to T2 (T-mount) adaptor

http://www.alpineastro.com/eyepiece_...ope%20Adapters

(3) ScopeTronix MaxView Plus eyepiece (fits T-mount adaptor of the Rubinar; also has T-mount threads to mate with camera)

(4) add a T-mount to 28mm adaptor to fit Coolpix 990 lens

I wonder what the final magnification would be?

Rubinar is 500mm

MaxView quotes about 6X magnification

Rubinar + MaxView = 3000mm (35mm film camera equivalent) ?

3000mm is the focal length for a 35mm film camera - it would be much higher for non-SLR digital cameras

Coolpix 990's lens is 8-24mm (equivalent to 38-115mm) - this indicates a "magnification" factor (corrected to 35mm film camera) of about 4.8X ?

Does this mean the "final" focal length equivalent (with respect to 35mm film camera we are more familiar with), would be 3000 X 4.8 = 14400mm

??

I am probably wrong!
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Old Mar 5, 2006, 9:24 AM   #2
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blindsight wrote:
Quote:
I am interesting in getting into digiscoping, and I am looking into this set up:

(1) Rubinar 500mm f5.6 (M42 threads)

http://www.rugift.com/photocameras/rubinar_500_lens.htm

(2) M42 to T2 (T-mount) adaptor

http://www.alpineastro.com/eyepiece_...ope%20Adapters

(3) ScopeTronix MaxView Plus eyepiece (fits T-mount adaptor of the Rubinar; also has T-mount threads to mate with camera)

(4) add a T-mount to 28mm adaptor to fit Coolpix 990 lens

I wonder what the final magnification would be?

Rubinar is 500mm

MaxView quotes about 6X magnification

Rubinar + MaxView = 3000mm (35mm film camera equivalent) ?

3000mm is the focal length for a 35mm film camera - it would be much higher for non-SLR digital cameras

Coolpix 990's lens is 8-24mm (equivalent to 38-115mm) - this indicates a "magnification" factor (corrected to 35mm film camera) of about 4.8X ?

Does this mean the "final" focal length equivalent (with respect to 35mm film camera we are more familiar with), would be 3000 X 4.8 = 14400mm

??

I am probably wrong!
I can't comment about calculations, but I looked this system up. The samples are not as good as that of a good spotting scope. On the other hand, the price is right...

http://www.digit-life.com/articles2/rubinar/

Dave
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Old Mar 7, 2006, 9:06 AM   #3
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Yes - wrong. The Nikon CP 990 has a 3X zoom. The "X" factor for a digicam's zoom range is calculated by starting with the wide angle then going to full telephoto. You begin with 8mm and end up with 24mm. That is three times 8 = 24 or "3X" (this has nothing to do with "power" calculations.
Quote:
Power is roughly correlated as follows:
Quote:
A 50mm lens is generally considered as life size. A 100mm lens is 2 power, a 200mm lens is 4 power. a 300mm lens is6 power, a 400mm lens is 8 power, etc. These are"rough" approximations as they might correlate to a telescope's "power factor".
That's a rather obtuse way of getting a decent digiscoping combination. Thinkinstead about using something like a Kowa spotting scope or a Meade ETX-90. Couple this with a 24mm William Optics or ScopeTronix eyepiece which have a28mmthread to match your CP990's 28mm filter thread. Witha combination such as this you will havea decent range of zoom and plenty of "power" to get really nice digiscoping resultswhile still letting in enough light to get decent images.

The "practical" limit in focal length for digiscoping with the 990 is around6000mm. Any morethan this and you won't have enough light to make a usable image.

Take the "power" factor of the scope combination (eyepiece and scope barrel) and multiply bythe equivalency in "power" of the 35mm equivalency at full zoom to arrive at an overall "power" factor. So the 35mm equivalency of the 990 is 114 or 115 mm (depending on which specifications you read). Let's call it 115mm. Then the "power" factor is about 2.3x. If your scope/eyepiece produces about 50 power thenwith the camera (CP990) attached at full zoom you would have somewhere around 115 power. In terms of 35mm equivalence focal length take the focal length of the telescope/eyepiece combination and multiply by the focal lengths of the camera at wide angle then at full zoom to arrive at 35mm equivalence focal length.

Lin
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Old Mar 7, 2006, 1:56 PM   #4
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Thanks for the explanation - I guess you use 50mm (standard lens) focal length as the starting point to estimate the magnification power of the 990's zoom.

This gives my setup a maximum focal length (35mm film camera) of 3000 X 2.3 = 6900mm, justbeyond the "practical" usable range of the 990.

Well at least I now know there is no point buying a higher power 1000mm lens to mate with the CP990!
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Old Mar 7, 2006, 4:10 PM   #5
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blindsight wrote:
Quote:
Thanks for the explanation - I guess you use 50mm (standard lens) focal length as the starting point to estimate the magnification power of the 990's zoom.

This gives my setup a maximum focal length (35mm film camera) of 3000 X 2.3 = 6900mm, justbeyond the "practical" usable range of the 990.

Well at least I now know there is no point buying a higher power 1000mm lens to mate with the CP990!
Actually you can, as long as the eyepiece will let you use less than full zoom without vignetting. Some eyepieces are made specifically for the exit pupil to match the cameras. For example with the William Optics 24mm attached to a 1250mm (native focal length) Meade ETX-90 you get 5989mm at full zoom but you can zoom back as far as 1100 mm and still have a vignette-free image. Having zoom range gives you lots more options for framing and lets you shoot in a bit lower light as well.

It's nice to have options....

Best regards,

Lin
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Old Mar 22, 2006, 5:11 PM   #6
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The magnification for the the telescope built by combining a Rubinar (500mm focal length objective lens) with MaxViewPlus(40mm focal length eyepiece) is (500/40)=12.5

Thus the maximum SLR focal length obtained when attaching the Nikon CP990 to this scope will be 12.5X114mm = 1425mm which is well within the range for good digiscoping pictures as far as overall magnification is concerned.

Dalcio
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Old May 24, 2006, 10:42 PM   #7
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------ sorry wrong post -----
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