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Back focus?

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3 replies to this topic

#1 madeinusa

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Posted 29 November 2020 - 12:58 PM

Finding my back focus is not a problem. I use a piece of paper to determine that without the blocking filter diagonal in place. That's where you would put your eyepiece or camera. What I don't understand if using a barlow where does that go in the image train and how does that work with the back spacing of your camera. Should I include the barlow in my paper test measurement? On my DS rig I measure the back spacing from the front of my .79 reducer to get this number. How do I do this with a barlow in the setup?

Edited by madeinusa, 29 November 2020 - 01:16 PM.


#2 sg6

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Posted 29 November 2020 - 02:06 PM

Adding a barlow basically wipes out the "back focus" specified distance.

It could be moved to (within reason) almost anywhere but usually further outwards.

 

Are you talking "back focus" or focal plane? First bits reads more of focal length rather the back focus.

 

What is the point of a reducer and a barlow. One decreases the image size then the next makes it bigger again. Best idea - take reducer and barlow off and just use the scope.

 

Back Focus is rather simply where the new image plane will be located and where the camera sensor has to sit from some datum face on the equipment. That can be almost anything you wish to define as "The Datum". 55mm is just the common value and is usually from a flat face on the rear of the reducer/flattener. You can measure it from where you like but it would then be a custom value for you.

 

And the Camera and Eyepiece would not be at the same position (back focus). The camera sensor would be at the back focus/image plane, the eyepiece would be beyond it. The object plane of the eyepiece would have to be at the back focus/image plane.



#3 madeinusa

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Posted 29 November 2020 - 05:30 PM

Yes, you are correct I should have said I know how to figure out the focal plane. Sorry for the confusion. So as for as the back focus is concerned the Barlow negates it within reason. I would think the Barlow should remain in when measuring the focal plane to get the true focal length.

So my image train would look like telescope, straight through blocking filter, extensions if needed (due to diagonal removal), Barlow, and camera. Is this a fair statement?



#4 rigel123

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Posted 29 November 2020 - 09:31 PM

Yes, you are correct I should have said I know how to figure out the focal plane. Sorry for the confusion. So as for as the back focus is concerned the Barlow negates it within reason. I would think the Barlow should remain in when measuring the focal plane to get the true focal length.

So my image train would look like telescope, straight through blocking filter, extensions if needed (due to diagonal removal), Barlow, and camera. Is this a fair statement?

I would say yes, that is the configuration you want.




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