Diagonals and Barlows

[Rusty Strings]

Is it true that where you place barlow affects it's performance. I've read in the Backyard astronomers guide that placing the barlow before a diagonal increase its amplification by around 50% turning a 2x barlow into a 3x. Also does this affect focus? I tried a 2x barlow before a diagonal and I couldn't achieve focus.

John

[Towa]

With my scope, I have to use about 150mm of extension tubes, before I can get focus when using my 2x Barlow before the diagonal.

[Rusty Strings]

So I do need an extention tube, how do I know how long is necessary?

John

[red dwalf]

didn`t know that John, nice bit of imformation, i`ll try it on my sct, should get focus with it and would be useful for planetary imaging

[LukeSkywatcher]

I was under the illusion that a Barlow can only be inserted into a star diag........and the EP inserted into the barlow.

On my reflector (Dob) the Barlow goes into the focuser and the EP goes in the Barlow. So you are suggesting:

Remove the star diag (if your scope has one) and inserting the Barlow into the optical tube........then inserting the star diag into the Barlow and finally the EP?.

Think i'm confused.

Scope>Barlow>Star diag>EP

??????????????

I can see how it works........i just dont know if it would make a difference.

Interesting.

I have tried my Dob with this config (using the diag from my refractor):

Focuser>diag>Barlow>EP.

Didnt work well.

However if i swap the position of the diag and Barlow..................

That may work.

Would things appear the "right way up"????

[Towa]

So I do need an extention tube, how do I know how long is necessary?

John

It will most likely vary from set-up to set-up. What I did was to focus on a distant object during the day with the diagonal, barlow and eyepiece in the normal position/way. Then removed them from the focus mount, inserted the barlow first. Then holding the barlow and Diagonal in my hand simply moved it away from the focus mount

until I got a focus. The distant between your focus mount and hand will give a idea of how much extension you will need.

[Rusty Strings]

That sounds like a plan, I'll give it a go, I did think that there would be some maths to work it out.

Luke Skywatcher - I was puzzled by how putting the barlow before the diagonal works, but seeing as the focus point has changed, I'm assuming, by increasing the focal length, then I guess it works.

Anybody out there care to explain why?

John