Question about optics : Barlow and extension tubes [Solar System] Acquisition techniques · Jérémie · ... · 4 · 98 · 1

Hi All,

I hope some people having good knowledge in optics will help me : I have bought a 4x Powermate and need to use some extension tubes to be able to focus.

On my setup I have connected the Barlow directly to refractor, and then the filter wheel and camera : that’s how I saw I could not focus (except when removing the camera and handling it further away from the Barlow), and needed some extension tubes.

But then I saw several setups where astrophotographers put the extension tubes first and then connect the Barlow, and then the filter wheel and camera. See picture below.

My question is : is that the right setup, putting extension tubes between the refractor and the Barlow ? I have read the magnification of the Barlow increases when you put extension tubes between the Barlow and the eyepiece (or the camera I guess ?). See this website : http://w1.411.telia.com/~u41105032/barlow/Barlext.htm

What is the right order ? Extension tubes first, between refractor and Barlow, or extension tubes after the Barlow, between the Barlow and camera ?

Any explanations or advices ? (Of course, I could try myself both but (i) I did not receive my extension tubes yet and (ii) I ordered M42 extension tubes while my Barlow is threaded M48, which force me tu put the tubes between the Barlow and the camera... but I can return them and order M48 ones if that’s the proper setup !)

Thanks fot tour help !

It appears to depend a.lot on the Barlow.

FWIW I have a Barlow with an RC8. The achieve focus the Barlow sits in the extension tubes I already needed to get a 254mm back focal distance for my camera - but at a distance that is slightly closer to the main mirror (about 245mm behind the back of the mirror).  My camera is attached to the focusser behind the Barlow, about 65mm from the back of the Barlow.

Thanks guys !