Re: [ATM] Barlow quality/testing

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Tony,
" I'm especially gun shy when a manufacturer claims to reduce aberrations 
when used with certain ocular designs. For CCD work that last thing you'd 
want is a barlow that's introducing aberrations of an a sign opposite that 
of the typical ocular in an effort to obtain better visual performance."

The mere fact that the effective f/ ratio is doubled (typically) should mean 
that the aberrations of oculars can be considerably reduced - whatever 
aberrations, if any, that the Barlow itself may add. But the geometry of the 
situation is considerably altered - the effective entrance pupil seen by the 
eyepiece is not one f.l. away, but somewhere about 2 Barlow's focal lengths 
away, with clearly divergent, not essentially parallel, beams of rays.

"What the imager needs is a pure barlow with good color correction and no 
introduction of spherical aberration. I've star tested several and none so 
far can really meet that standard.Any thoughts out there on this? I hate the 
idea of sweating blood to make a great planetary mirror only to be undone by 
the barlow."

Interesting - what aberrations did you find?

At least Uncle Al is eloquent about the televue powermate concept:
http://www.televue.com/engine/page.asp?ID=42

But perhaps it is not a good idea to judge the performance of a Barlow for 
CCD work by its interaction with EPs. Have you tried testing with the Barlow 
+ CCD? It ought to show a useful star test pattern (ala Suiter) to tell you 
about any potential problems in actual use.

just some thoughts

Nils Olof 


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