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# Focal length of Celestron 2x Barlow

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7 replies to this topic

Sputnik

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Posted 20 October 2020 - 11:06 PM

I am trying to find out the focal length of the Celestron X-Cel LX 2x Barlow. I know that one does not normally need to know this, but I do need to know it. I want to determine the optimum distance between the Barlow and a webcam for planetary photography. The manufacturer nor any dealers do not specify this number. Perhaps someone on this forum will have measured it.

### #2 Ernest_SPB

Ernest_SPB

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Posted 20 October 2020 - 11:24 PM

FL 2x Barlow normaly can be evaluated with length (axial dimension) of the Barlow body.

### #3 sg6

sg6

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Posted 21 October 2020 - 02:13 AM

Doubt anyone will have tried to measure the focal length as it will be negative. The effect it will have on the image/scope depends on how far it is from the front objective, a difference in that value will mean a different change in size of the final image and where it falls.

The formula used, well the simple one is:

1/F = 1/Fo + 1/Fb -D/(Fo*Fb)

F= Final Focal Length, Fo = Objective Focal Length, Fb = Barlow Focal Length.

All gets complex and is why people say "My barlow is 2.3x not 2x" big amount depends on their scope and where to barlow is located. In effect no barlow is what it states on the sdie, something in that area perhaps. But only that.

The same "2x" barlow in a 400mm scope will be different to that in a 600mm scope

### #4 sixela

sixela

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Posted 21 October 2020 - 02:38 AM

Nope, because if you solve for the ratio between final and original focal length it only depends upon focal length of the barlow and distance to the new focal plane (or the old one, depending on the formula you want).

Edited by sixela, 21 October 2020 - 02:39 AM.

### #5 sixela

sixela

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Posted 21 October 2020 - 02:43 AM

FL 2x Barlow normaly can be evaluated with length (axial dimension) of the Barlow body.

If you know exactly where the principal plane of the barlow is, which is tricky for air-spaces three lens units. And impossible for my BarAdv, since even that moves when the barlow factor changes — to get correct values I have to assume a reference plane closer to the eyepiece for small barlow factors.
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Sputnik

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• Loc: Camarillo, CA

Posted 26 October 2020 - 11:07 PM

Doubt anyone will have tried to measure the focal length as it will be negative. The effect it will have on the image/scope depends on how far it is from the front objective, a difference in that value will mean a different change in size of the final image and where it falls.

The formula used, well the simple one is:

1/F = 1/Fo + 1/Fb -D/(Fo*Fb)
F= Final Focal Length, Fo = Objective Focal Length, Fb = Barlow Focal Length.

All gets complex and is why people say "My barlow is 2.3x not 2x" big amount depends on their scope and where to barlow is located. In effect no barlow is what it states on the sdie, something in that area perhaps. But only that.

The same "2x" barlow in a 400mm scope will be different to that in a 600mm scope

### #7 Ernest_SPB

Ernest_SPB

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Posted 27 October 2020 - 12:16 AM

See the picture

#### Attached Thumbnails

Sputnik

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• Loc: Camarillo, CA

Posted 27 October 2020 - 01:14 PM

Doubt anyone will have tried to measure the focal length as it will be negative. The effect it will have on the image/scope depends on how far it is from the front objective, a difference in that value will mean a different change in size of the final image and where it falls.

The formula used, well the simple one is:

1/F = 1/Fo + 1/Fb -D/(Fo*Fb)
F= Final Focal Length, Fo = Objective Focal Length, Fb = Barlow Focal Length.

All gets complex and is why people say "My barlow is 2.3x not 2x" big amount depends on their scope and where to barlow is located. In effect no barlow is what it states on the sdie, something in that area perhaps. But only that.

The same "2x" barlow in a 400mm scope will be different to that in a 600mm scope

I am not trying to determine the “true” magnifying factor of a Barlow. I am using the Barlow without an eyepiece — directly inserting nose piece of a webcam into the Barlow. The amplification, A, (not “magnification”) of the prime focus image size depends on the distance, d, of the webcam sensor from the Barlow lens, as well as on the focal length, f, of the Barlow. The equation (from Thierry Legault, Astrophotography) is : A= (d/f) + 1.

A negative lens does have a focal length regardless of whether it can be directly measured.

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