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Originally Posted by MarkN
Interesting as ever to read your posts John.
Would like to know your views about Barlows in various combinations of scope and ocular design that you've tried. Are there some "marriages" which just don't work and are there some that do?
Mark.
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Hi Mark,
I do have some thoughts on barlows.
Let me say two things for starters. In the last 15 years, barlows have improved immensely. Historically, even the best, most expensive barlows could cause serious image degradation and in many cases, vignetting. The modern top quality barlows are very good and that brings me to the second point, "buy the best barlow you can afford". Like most things astronomical you get what you pay for. Another generalisation you could make is that a "long" barlow will always be better optically than a "short" barlow. However, it is not physically as convenient and easy to use, compared to a short barlow. This particularly applies to scopes necessarily using a diagonal like refractors. SCT's and Maksutovs. I will also add that with these scopes you can change the amplification of the barlow by using it "ahead" of the diagonal, as opposed to "after" the diagonal. A 2X barlow used "ahead" of the diagonal will give an amplification of 2.5X to 3X. You can also achieve this "additional" amplification with an extension tube in any of these scopes, as well as in a newtonian. It's also worth mentioning that a barlow with a greater clear aperture will have less tendency to vignette than a barlow with restricted clear aperture. This particularly applies to longer focal length eyepieces which naturally can have a tendency to vignette, like long focal length plossls. Any traditional barlow will increase the eyepieces native eye-relief. In most cases with eyepieces having inherently short eye-relief this is an advantage. For this reason many people prefer medium focal length orthos and plossls in a good barlow for high power planetary work as the native plossls and orthos have very short eye-relief. This eye-relief extension becomes a dissadvantage when using eyepieces having long native eye-relief. It is actually extended "too far" in many cases making it more difficult to hold the exit pupil, particularly for less experienced observers.
My preference is for the Televue Powermates as they preserve the eye-relief in all cases. Optically they are very good, albeit a tad behind the best barlows optically. the reason I like them is because they preserve the eye-relief and are transparent in use. You don't notice that "something else" is in the optical train which you usually do with the very best barlows. The powermates just "disappear" on you.
The very best barlows that I have used are the following, in no order:-
1.25" Televue 1.8X barlow (no longer available FI. I owned 1 and foolishly sold it)
1.25" Televue 2X barlow
1.25" Vernonscope Dakin Barlow. (The company that makes Brandon eyepieces for Questar telescopes)
1.25" Televue 2.5X Powermate
2" Astrophysics 1.7X Barcon
2" 2X Televue Big Barlow
2" 2X Televue Powermate
The best "value for money" barlows I have used are the Orion Ultrascopic, which I believe is now sold under a different name and the Orion Shorty Plus/Celestron Ultima. Two barlows which some rate highly that I have issues with, are the University Optics 2.8X Klee and the Meade #140 APO Barlow. The Klee whilst excellent on axis, can vignette with some eyepieces and I have seen odd samples of the Meade #140 noticeably dim the image.
CS-John B