lower mag barlows

[raymo]

There must be plenty of people out there that would like to have one or
more barlows of magnifications that nobody seems to produce, such as
1.25x, 1.5x, or 1.75x. I imagine that many scope owners would like a
bit more power. I have a 2" 1.5x barlow that screws into the eyepiece,
and I use it a lot. Does anybody produce odd mag barlows?
raymo

[janoskiss]

Antares used to have a 1.6x 2" barlow. On the GSO 2x 1.25" barlow (the basic 2-element achromatic one) the lens element unscrews and has standard 1.25" EP filter threading. If you screw that on the end of a 1.25" EP that will give you around 1.5x. Televue also used to make a nice 1.8x 1.25" barlow.

You can always increase the magnification of any barlow by adding a barrel extension between EP and barlow. (Even just pulling the EP out of the barrel a bit will give you a little extra power.) With some barlows you can also swap the tube between the EP holder section and lens element section for a shorter one to lower the magnification. I believe that with the GSO 1.25" 2x you can remove the tube altogether and screw the lens element into the EP holder section to create a low magnification ultra-shorty (?). Note however that shorter barlows tend to exhibit more aberrations than long ones.

[Merlin66]

The best "Barlow" I have is a TMB x1.8.....absolute majic!

[AEAJR]

Other than the 1.5 screw on piece I can't imagine why I would want an odd mag barlow. I mean, hey, cool, another toy. But why?

For my 8" 1200 mm FL I have a 2" 2X barlow that does all my eyepieces.

For my short FL, 400 mm, refractor I have a 2X barlow and a 3X barlow. I guess I could have a 2.5 but would not see a need.

Just not enough of a market I would think, so they would be costly. Why pay twice as much for a 1.7 barlow as opposed to a 2X and a 3X for the same total.

[dannat]

you could use one of the power switch devices they make for binoviewers, you can also get the 1.25x & 1.75x mag correctors made for binoviewers, just screw onto your eyepice

[raymo]

Thanks for the comments everyone; I was actually thinking from the point of view of framing of various sized DSOs. I am aware of the different ways of using barlows to vary the magnification, a good example being whether it is put before or after a diagonal. What about a zoom
barlow that telescopes and has a twist lock that you nip up at the
desired position, you'd only need one 1.25" and one 2" to cover all your needs. Just thunking [dangerous, I know].
raymo

[janoskiss]

There are several manufacturers that make purpose built zoom/variable magnification barlows. The better ones range from expensive to hideously expensive. And none of them are as easy as just twist and click afaik. And it's inherent in the barlow's simple (but effective) optical design that you will have to refocus between different magnifications.

You also may need to add extra focuser travel beyond what your focuser normally accommodates to cover a broad range of magnifications. (So you'd be back to having to use extension tubes again.) A telescoping barlow design was incorporated into the Speers Waler ultra-wide zoom EPs. I don't think they are made any more though. And the design is somewhat flawed because the telescoping barrel does not fit in the focusser and that requires a lot more in-focus than normal EPs.
http://www.scopestuff.com/ss_sw2xz_L.jpg

I successfully and happily use simple barrel extensions; not screw-ins but drop-ins like e.g. http://www.andrewscom.com.au/images/products/telescopes/accessories/gs_extensiontube.jpg to fine tune the magnification of my long barlows (I also use inexpensive barlows with he lens group removed as extension tubes). There is a good S&T article on this topic. I'll try to find a link.

... Here (maybe he did not publish it in S&T; he published about several other things though):
http://w1.411.telia.com/~u41105032/barlow/Barlext.htm
The author's Nils Olof Carlin's home page is here: http://web.telia.com/~u41105032/

[raymo]

Thanks for the information Steve, very interesting.
raymo

[ausastronomer]

I have had one of these for about 15 years. It's a very high quality Japanese made barlow. Unfortunately it uses set screws instead of a compression ring. I fixed that by re tapping and using 4mm nylon set screws.

While Sky Instruments in Canada (Antares Optical) has closed down there are still a few dealers in the USA with stock of these for about $US90.

If this is the sort of thing you are after you won't do any better quality wise. I also have a 1.8x 1.25" Televue barlow which I have had for over 20 years and it's pretty good too.

That all having been said the 2x TV powermate sees more use than the other too but it might be different if I didnt have most eyepiece focal lengths covered with native eyepieces. The 31mm Nagler T5 works exceptionally well in the 1.6x Antares for about a 19mm eyepiece.

Cheers
John B

[raymo]

Hi John, thanks for the info. I have an Antares 2" to 1.25" compression
ring adaptor, beautifully made.
raymo