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Barlow-to-focus distance

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#1 BillHarris

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Posted 31 August 2019 - 02:53 AM

Does anyone have the nominal Barlow to focus distance for the 1.5x and 2x Dakin Barlows mounted at the Axial port? I'm getting CCD cameras set up for Lunar and Planetary photography.

#2 Loren Gibson

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Posted 31 August 2019 - 07:26 AM

The barlow-to-focal plane distance isn't a fixed quantity, since focusing by moving the primary changes the relationship of the barlow with the incoming cone of light. The focal point moves to and fro along the optical axis beyond the barlow.

 

Am I understanding your question correctly?

 

Loren



#3 Optics Patent

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Posted 31 August 2019 - 07:32 AM

That makes sense. The Barlow is a negative focal length lens (diverging rays). So it must be closer that the pint of focus.

The focal point can be as far away as desired with or without it as one can focus to get the rays parallel.

The question might really be: “how close can the focus be with a Barlow?” Or: “how far back does a Barlow shift the close focus distance?”

#4 BillHarris

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Posted 01 September 2019 - 04:08 PM

Autocorrect left out hyphens, that first sentence should have been

"the nominal [optimum or designed] Barlow-to-focal point distance for the 1.5x and 2x Dakin Barlows"

For any Barlow there is one, and only one, distance from the barlow lens to the focal point at the designed amplification (1.5x or 2x). You can get 2x from the 1.5x Barlow or 1.5x from the 2x Barlow, but it isn't optimized for that amplification. This value will be a good starting point for setting up the optics for a CCD camera. For the pixel resolution of the camera I'm using, prime focus will be good, the 1.5x Barlow will be optimum, the 2x Barlow will push the envelope, and that Barlow used at 3x would be the limit. Photographically, the f/ratios will be f/14, f/21, f/28 and f/42, respectively.

I'll set up a target and measure that backfocus.

#5 Loren Gibson

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Posted 02 September 2019 - 06:03 AM

Clarifying what was meant by "nominal" makes me understand your question now, thanks. I don't know the answer. However, if you have those barlows already you might be able to get a good guess by measuring the distance from the len's position in the barlow housing up to the location that some eyepiece's field stop would be, when the eyepiece is secured in the barlow. (Of course, the position of the field stop can vary among eyepieces, so this suggestion is not totally helpful.)

 

Loren



#6 Loren Gibson

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Posted 02 September 2019 - 07:31 AM

(I should have been able to infer what you meant with the word "nominal"; my bad!)

 

Loren



#7 BillHarris

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Posted 02 September 2019 - 11:04 PM

I was hoping that somewhere along the line Questar had proclimated "use this Barlow-to-film distance for these Barlows. The measurement process will be done photographically: one photo of a distant target at prime focus amd then a series of photos with each Barlow at different Barlow-to-CCD distances, then measure the size of the target on the CCD image in pixels with Photoshop to determine the amplification. Then graph the amplification vs backfocus, and that line should be linear over the range we are looking at. My next "cloudy day" project.
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