Trouble with old cheap Barlow

[CaptainShiznit]

I dug out some old accessories and found my cheap x3 Barlow lens so I had a little try looking at Venus after sunset tonight. It was out of focus range and I wanted to ask why, for when I buy a new and better Barlow sometime soon.

I could see the light from Venus out of focus in the shape of the primary mirror with the secondary mirror's shadow on it and couldn't focus far enough. I don't really understand the mechanics of focusing yet so if anyone had any links or explanations, I'd be grateful.

I was using a 20mm Plossl > 3x Barlow > 1.25" to 2" tube > 130 PDS.

Thanks

[Rob Sellent]

I think a 3x Barlow would make a f/5 scope behave like a f/15. In effect, tripling magnification, reducing TFoV by a third and likewise the exit pupil. Depending on the Barlow you may need more out focus to accommodate for the narrower light cone being produced. A possible sollution is to unlock the Barlow-EP combo from the focuser and slowly bring it out and see if you can reach focus.

[CaptainShiznit]

3 hours ago, Rob Sellent said:

A possible sollution is to unlock the Barlow-EP combo from the focuser and slowly bring it out and see if you can reach focus.

Thanks, I didn't think of doing that. Always learning.