What is the Focal Length of a Barlow?

[Specman]

Hi,

Wonder if anybody can help with my problem

I understand that to find the viewing power you divide the focal length of the scope by the focal length of the eyepiece to find magnification, but what if you are using only a 2x Barlow and a CCD camera such as a NexImage?

Here's the issue:

I have a Celestron 4SE with a 1325mm focal length. I can't get a focus using my 2x Barlow and NexImage. I think I'm working beyond the useful power of the scope and the mag is too high but I can't confirm this because I can't work out the actual power Am I being thick here?

Does the Barlow double the focal length, if so how can I calculate the power?

The images I see through the EP is totally different to the image I get on my screen with the CCD. I can't focus it and see detail. I just get a blob!

I have a Revelation EP set that includes adaptors for shooting through the EP. I also have a T piece and ring to fit my Canon 1000D so various options are available.

Is it just me or am I working beyond the scopes ability?

Any advice much appreciated.

Clear skies

Mark

[Merlin66]

mark,

For imaging the traditional magnification doesn't mean anything...

It's the image scale that makes the difference, and this really depends on the focal length.

If you have a 1325mm fl scope and you use the barlow at the correct spacing for x 2 then the effective focal length will become 2650mm.

For a webcam say 2.8 x 3.7mm sensor with 5.6micron pixels, the FOV would be 3.6 x 4.8 min arc, and a scale of 0.44 sec arc/ pixel.

Does this help?

(Google CCDCalc for Ron's software)

[TheThing]

To make it slightly easier, I understand that webcams like the NexImage, Orion StarShoot and the Meade LPi are equivalent to a 6mm EP.

[Specman]

Thanks very much guys that is a great help and answers my questions. So the 2x Barlow does exactly what it says on the tin, its a doubler! Duuhhh (how thick am I)

If the CCD camera NexImage is about equal to a 6mm I think here lies my problem. Its just beyond the useful power of the scope. I have a Celestron XCel EP 8mm and I recon thats about tops. It would certainly explain my focusing problems,

TheThing - I notice that you use a 4SE. What is you take on using the NexImage or similar with it? Have you encountered any issues?

I salute you both Thanks again guys

Clear skies

Mark

[TheThing]

I use the 4SE for visual work only. I image with the 6" Soligor Newt.

[great_bear]

Not of any practical use, but here's some extra info in case you're interested:

Barlows are x2 or x3 etc. purely because of the spacing that the housing creates. There's nothing to stop a x2 Barlow being used as a x3 and vice-versa if the lens can unscrew from its housing. Just like telescope objectives though, the longer the focal length the better and (again, like objective lenses) achromatic ones work fine if they are long enough.

In terms of focal length, Barlows have a negative focal length. If you were to draw on paper how the edge of a parallel beam of light spreads out when leaving the Barlow, and then - with a ruler and pen - extend those lines back on themselves through the Barlow lens to the point where they cross, the distance between that point and the lens is the negative focal length of the Barlow.