Barlows, Powemates, extensions?

[iceman]

Hi guys.

I'm not sure if i've been under the mistaken impression that pulling my barlow (both my GSO 2x and my Intec 2.4x) out of the focuser a bit, as well as pulling the ToUcam out of the barlow a bit, gives a longer focal length (and therefore larger image scale)..?

I read somewhere recently that only the 2.5x powermate and 5x powermate work with extension tubes, giving extra magnification when used in this way - whereas the other powermates don't provide that same feature.

How does this work, and does it work with all barlows or only barlows/powermates of a certain design?

I'm fairly sure that while imaging Jupiter over the last season, that pulling the barlow and ToUcam out as far as they'll go while still allowing me to rack the focuser in far enough to come to focus, has given me longer focal length and larger image scale. However I tried it as usual with my new Antares 3x the other day, and didn't notice any appreciable difference in image scale.

So now, combine that, with the article I read the other day, and i'm questioning my original assumptions. It could have been that the larger image scale I was observing (while imaging) was simply due to Jupiter coming closer to opposition.

I would've thought that bringing the focal point further away from the mirror would most certainly be increasing focal length, but then at the same time i'm also having to rack the focuser all the way in to come to focus - so maybe i'm offsetting one with the other?

Thoughts please?

[gbeal]

Hi Mike,
I too have heard that theory, but decided to see if I could locate what you were after.
Not sure if I have, but try http://www.televue.com/engine/page.asp?ID=243#Powermate
Gary

[slice of heaven]

http://www.atmsite.org/contrib/Carlin/barlow/
Barlow extensions
1 good eyepiece,1 barlow and multiple lengths of tubes.

[acropolite]

Barlows work by extending the focal length of you scope therefore the longer the optical path the greater the focal length. In another thread http://www.iceinspace.com.au/forum/showthread.php?t=2142&highlight=barlow in this forum there was a discussion on the reasons and some links to explanations as to why it happens. Here's a link explaining why placing a barlow after your diagonal gives greater magnification than placing it before the barlow, basically the same effect you are seeing. http://www.astunit.com/tutorials/barlow.htm

[RAJAH235]

That explains it very nicely Slice. Reading Mike's set-up procedure, it sounds like he was sort of defeating the purpose, so to speak. Pulling one out & then having to focus in. Photons & glass,eh! Still got lots to learn..... Ta. :thumbsup: :D L.
ps. Mike, just a thought, have you tried measuring/scaling the images?

[slice of heaven]

Lucky I organized my links after installing Firefox (thanks Laurie) or I wouldnt have found it so quickly.