upper barlow limit

whats the formula, or rule of thumb for working out whats the upper limit of the barlow you can use on the scope? will any scope be too small that a 4x barlow would be a burden rather than a benifit? i have a 2x and im wondering for if i got a 4x barlow, although id increase magnification, would there be other factors limiting teh 4x on my 6" scope, thus rendering a 4x not so good?

The surface quality of your primary mirror will ultimately determine the upper limit on what useful magnification is possible with your scope. If you have a really good, difraction limited surface then you can expect (on a night of good, steady seeing) to squeeze at least 50x magnification per inch of aperture... so about 300x for your 6".

With a really excellent surface (difraction limited and exhibiting no micro-ripple under test) and under really, really steady skies (not here in other words!) you can well exceed 300x per inch of aperture. I have occasionally experienced up to a few seconds of this kind of seeing here in Ireland when I have pumped up the magnification on Jupiter or Mars, but I generally had to wait for ages at the eyepiece for those few seconds to come along!

When the seeing allows I often use two 2x Barlows to crank up the mag., so a 4x or 6x is certainly usable even if you won't get to use it all that often. You would probably be better served by getting a range of good eyepieces so that the magnifications increase in nice discrete steps when used in conjunction with a 2x Barlow. If you still want more mag then get a second 2x Barlow which will be cheaper than a 4x or 6x.

Hope this makes sense!

Phil.

PS - One major benefit of using a Barlow (or two) is that it preserves the eye-relief of the eyepiece used with it. Short focal length eyepieces usually have very short eye-relief, meaning that your eyeball has to nearly touch the lens to see the full field of view. A longer FL eyepiece will have a longer eye-relief even if it is giving double or quadruple its normal magnification!

Hi Phil
As always your knowledge and advice of optics is excellent. In conjunction with fguihen question I was thinking of investing in a Televue Powermate 2.5X, would this be the way to go for planetary imaging rather than purchasing for want of better words a "standard Barlow". Budget restrictions aside (or indeed pushing your budget restrictions) would it be a good investment for long term use? I have a Celestron f10 Nexstar 8!

Clear skies
Mike

whts the difference between a 4x barlow, and a 4x powermate? i have had a look online and can only see powermate's by televue. is it just a "fancy barlow" from televue that delivers great results ( at astronomical prices!)?

*Moved to the Beginners & "How To" forum*

IF you want to go the route of using Barlows rather than shorter focal length eyepieces there aren't very many available above 3x so you may have to stack. 2x Barlows are the most common, 3x can be found and the only higher mag I've heard of is the 5x from Antares. At 79 Euro it's a lot cheaper than the 5x Powermate.

Personally I find it more difficult to place my eye when using a Barlow, which is why I've collected two sets of eyepieces - 4mm to 32mm Plossls and 3.5mm to 24mm Hyperions. I also have three 2" eyepieces but because of telescope limitations they don't as much use.

This review of the 5x Powermate may help explain the difference between Powermates and Barlows. I use mine with my digital camera rather than visually.