Distance From Barlows

[Langy]

I've just had delivered my 2x Barlows and Adaptor to fit my DSLR to hopefully when the clouds clear start having ago at some photography through the scope properly.

This has got me thinking about what happens when the distance between the lens in the Barlows changes between itself and the sensor on the recording device.

For example fitting my DSLR to the adaptor and screwing that onto the Barlows will leave a fixed distance between the Barlows lens and the sensor probably some 40mm back from the front of the camera.

Now if I change that distance what actually happens? If the sensor is moved further from the Barlows does that mean that after refocusing the FOV or the light will be exactly the same or will the light still be spreading wider the further the sensor moves back meaning that the FOV on the sensor actually narrows resulting in a larger image hitting the sensor?

Now as it stands my DSLR will be fixed to the Barlows, but I have a cheap webcam stripped down ready to try, but the board on this is just slightly to wide to fit inside a 1.25" tube meaning that the board would sit flush on top of the Barlows. If I was to carefully remove the corners (hopefully not wrecking the board) I may be able to move it closer down the tube, alternatively mount it at the end of a 1.25" tube that could then be inserted in the Barlows and then being able to move the sensor even further away.

[Alien 13]

Hi Paul, the magnification factor increases with distance the orion adapter  that i use has a sliding tube to change the magnification.

Alan

[Leveye]

What adaptor did you get? Most DSLR's need 55mm from the sensor

[Leveye]

I have not been able to find a really affordable proper eyepiece clamp that will hold a full size DSLR and a lens at the proper length. Most better ones are a few hundred dollars.

[Langy]

I called Scopes and Skies and they sold me the Antares Antares Deluxe 2x Barlow lens with T-thread and Antares T-mount adaptor ring for SLR cameras (Canon EOS AF).

[glowingturnip]

my 2" 2x barlow comes with what is effectively a hollow tube between lens and camera - that can be unscrewed, and just the lens part screwed into a regular nosepiece, reducing it down to about 1.5x

[Leveye]

I have a 3x Tele Vue and a 3mm eyepiece. Must find a solution without using an expensive flip mirror attachment for now. Let me know how that adaptor handles the weight of a full size DSLR with a 1.25 fitting.

[Leveye]

http://www.ebay.com/itm/Canon-EOS-Adapter-4-Telescope-1-25-Eyepiece-VARIABLE-Projection-photography-wT2-/251426367067?pt=LH_DefaultDomain_0&hash=item3a8a2de65b

[Langy]

OK so another part to this, what is the loss in light strength as you move further away from the Barlows?

I've got a cheap webcam which I still haven't tested to see if it works but also have the xBox webcam on order to mod myself. Today I've just got some 1.25" tube from work which I can then cut at any length meaning I can try different distances from the Barlows, but with moving it further away must reduce the quality of light as you gain magnification.

[NickK]

It depends. I've been doing a bit of research for my heliospectragraph.

A Cannon 2x Teleconverter for the old FD lenses keeps the 42mm back focus.

From my limited research a 2x TV PM gives 2x back focus.. so if I used a 42mm FD lens it would be 84mm. I may be wrong and haven't managed to test this yet.

I may have the chance at the weekend to test.

[wxsatuser]

OK so another part to this, what is the loss in light strength as you move further away from the Barlows?

I've got a cheap webcam which I still haven't tested to see if it works but also have the xBox webcam on order to mod myself. Today I've just got some 1.25" tube from work which I can then cut at any length meaning I can try different distances from the Barlows, but with moving it further away must reduce the quality of light as you gain magnification.

I have done a bit of EP projection, camera mounted on a Baader Hyperion Zoom and the amount of light does seem to fall off quite quickly.

I have'nt quantified how much but there must be a formula for it somewhere.

So far for me the light appears more diffuse, a bit like being out of focus although I believe I was in focus.

Tracking is a nigthmare of course.

[ronin]

This has got me thinking about what happens when the distance between the lens in the Barlows changes between itself and the sensor on the recording device.

If you alter the barlow lens to sensor distance you get an out of focus image.

To alter the seperation between them and still get an in-focus image you have to alter the prime image to barlow lens distance also. Depending on the actual lens used the movement to get a bigger image may be inwards or outwards.