Quote:
Originally Posted by Merlin66
Bob,
Interesting results....
You don't mention which barlow you used - Standard long focus, Shorty, triplet etc.
Notwithstanding....
In "Telescopes, Eyepieces Astrographs" by Smith, Ceragioli & Berry, p 374-388 they analyse the design and performance of two barlows and the Powermate.
They say that the typical (good) barlow will control longitudinal and lateral colour, spherical aberration and the amplification factor, but do little for coma, astigmatism and field curvature (p376)
When the barlow is used with slower telescopes >f6 they also say that "slowing the f ratio..... reduces the angular aberrations inherent in the eyepiece" (p388)
My practical experience with barlows has been with spectroscopes (to obtain f ratios >f7) and solar imaging. My preference has been the TMB x1.8 ED barlow or the Powermates.
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Thanks Ken,
I used a Celestron, short barlow. I can't say too much about it as it was given to me some years ago and I am normally disappointed when I use it.
Yes, the literature certainly would have me believe other than what I saw for myself. In Rutten & van Venrooij's Telescope Optics, it is stated (P155): "... a Barlow lens often permits the use of relatively poorly corrected eyepieces.."
Furthermore, I have read elsewhere that wide-field EPs often have a negative lens or built in barlow and this is vital to their success.
So in essence, I am still trying to match up what I understand of theory with my own observations. Currently, it doesn't make sense!
I can only now say that a barlow increases the magnification, is useful for better ER and sometimes causes vignetting.