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Optical path length for 2.5X Barlow

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#1 DoubleStaRR

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Posted 07 August 2020 - 07:19 PM

How would I determine the optical path length for a 2.5X barlow to get the correct back focus? Thank You



#2 ButterFly

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Posted 07 August 2020 - 08:58 PM

Few things going on here.

 

Barlows have a nominal power (here 2.5x).  The actual power one gets out of a barlow depends on the distance between the end of the barlow and either the imaging surface or the eyepiece focal plane.  The further away the two are, the greater the power of the barlow.  The actual focal shift depends on the actual power of the barlow as used.

 

Secondly, "correct back focus" is only relevant if the barlow is placed between a reducer and/or corrector and the image plane.  For all other uses, just use the focusing knob.  Barlow and corrector can be a logical combination, but barlow and reducer are usually not needed.  The corrector needs to see the scope's native focal length (f/ratio really) to work properly, so a barlow is best placed between it and the imaging surface.  Barlows themselves are huge black boxes.  They change the prime focus position and they have some physical length.  The easiest way to determine the combined effects of these two is to measure it directly. 

 

The measure the actual shift for a given barlow position, do the following.  With no barlow in place, point at the moon and focus its image on wax paper or some other translucent material taped to the end of the focuser tube.  Place a hash mark on the focuser drawtube showing how far out it is.  This is the scope's native focal plane.  Then place the barlow somewhere and repeat.  The difference between the two hash marks is the focal shift caused by the barlow when the barlow is placed at that position.  Changing where the barlow is placed can, and usually does, affect the focal shift because the actual power of the barlow changes..

 

There is no easy way to calculate how much that focal shift will be beforehand, or even whether it can come to focus, simply because the barlow focal lengths and physical sizes are usually not given.  Typically, when used around their nominal power they only change the focal position just a little so that the product can be used with many different scopes.


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#3 DoubleStaRR

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Posted 08 August 2020 - 02:51 PM

Easy as that, like everything else with this hobby that is just what makes it fun. Thank you for the detailed response it is helpful




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