Barlow lens question

[blinky]

I bought a 3X TeleVu from Rus a week or 2 ag. I finally managed to get some time this morning to play around with it. When I stick it in my focuser and slot in a EP it seems that instead of my eye resting on the eye relief of the eyepiece it now needs to be a few centimetres away from it. IF i put my eye right up to the EP like I normally do I can only see a small 'tube' of light that is out of focus. I take it this is normal? and is there anything that can be done?

[GordonCopestake]

Barlows do move the focal point of eyepieces but they should maintain the eye relief. Which eyepiece were you using? Televue are renound for their high quality products and I doubt the barlow is at fault.

[blinky]

I used a 40mm and 25mm plossl that I got from Ebay, they seem pretty good (to my uneducated self anyway) I also tried a 30mm Plossl that Chewy gave me when I got the 'scope and it seemed to be the same.

[GazOC]

The eyerelief will be the same using the barlow as without it. How far away is the object you are focusing on Blinky?

[blinky]

4 or 5 miles But like I say if i put my eye right up all i see is an out of focus blob in the middle surrounded by blackness

[FLO]

Actually, a barlow does tend to push the eye-relief further back. The longer the eyepiece focal length, the further out it will be pushed.

<edit> Televue Powermates are different - they are designed to overcome this effect and keep the eye-relief where the eyepiece designer intended and so prevent vignetting on longer focal-length eyepieces.

[blinky]

That explains it then. Maybe using the 40mm was too much. I will likely be using it for Webcaming of Saturn and also for visual of Saturn using somthing like my 12.5mm.

[FLO]

A 40mm in a regular negative-element barlow will vignette.

[blinky]

Steve, that reply is way way way to techie for me :nono: Negative elements vignetting

[FLO]

Sorry

Suffice to say that a regular barlow will increase eye-relief. On short focal-length eyepieces, the effect is negligible but on longer focal-lengths it can be quite noticeable and cause the image to darken around the edges.

TeleVue Powermates are designed differently - the eye-relief is unchanged so no edge-darkening.

[blinky]

So whats going on with it then? Is it just that I am using a 40mm and 25mm EP? or that it is not pointing at something at infinity? or are my cheap EP's to blame? It really is quite a bit out from the eye relief that I have to be with it.