Re: Barlow vs. Powermate

This thread appeared just as I was about to ask this group some similar questions.

Here's my situation. I just got a TV Ranger for use as a terrestrial spotting scope. (I've got other, larger scopes for astronomy.)

I'm interested in getting the widest, clearest, "picture window" view out of the Ranger that I can at 40-75x. With a focal length of only 480mm, it seems like the best way to do this is to use a suitable barlow and suitable plossl. (I'm really not terribly interested in expensive eyepieces that essentially include their own barlow, etc. since I've already got a collection of "Super Plossls". (Besides, my other Maks seem to do best with simpler e.p.s like plossls.)

So, the question boils down to: Does it make a difference which barlow or Powermate I use? (Assuming that we're talking about same power factors.) Is the Powermate still the preferred choice for this application?
Also, are there any barlows that don't "stop down" the light path?

Thanks,
Mike Swaim

I would say that the Powermate is preferred for the Ranger because of the Ranger's focuser design. Having a parfocal option (one that doesn't require refocusing much when you swap it in) is disproportionately valuable with helical focusers.

No decent Barlow stops down the light to any point in the view. The cause of "vignetting" when Barlows are used with long focal-length Plossls and such is divergence, which is a consequence of the way any Barlow works in the first place. The vignetting occurs (or not) in the eyepiece depending on how well the eyepiece design is able to handle it. Long focal length Plossls are, unfortunately, not very good at handling it. Powermates avoid the problem by using massive divergence after the first lens group and then cancelling it with the second lens group after the divergence has had the desired effect. With conventional Barlows the problem is minimized by using a longer focal length Barlow, but for a given magnification factor this results in a much taller unit that is nowhere near to being parfocal when used in star diagonals. Again, we're back to recommending a Powermate with the Ranger.

The existence of a built-in Barlow in an eyepiece or not is a red herring. You say you want the widest, clearest, 'picture window' view out of the Ranger, but this will be achieved, IMO, with with an ultrawide or at least superwide eyepiece, and the best of these have built-in Barlows (or something like that). But this is a matter of trivia only. The eyepieces are designed to work as indivisible units only.

I think you might very much enjoy a 9mm Nagler T6, which will provide about the true field of a 15mm Plossl, or a 12mm Nagler Type 4, which will provide a true field of view about the same size as a 20mm Plossl. Soon there will be an 11mm Nagler T6 to choose from also. If cost is the issue, then with knowledge of the budget perhaps some more affordable, best-available options could be found.
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Mike