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Double up Barlows


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I didnt know you could double barlows !

You can use a stack as long as you can afford. Just remember that each one spreads out the light, making the image fainter; magnifies the imperfections in the optics and in the turbulent air through which the light comes; and contributes its own aberrations.

For visual work you don't really need a barlow at all, even with a short focus Newtonian ... an eyepiece of the right focal length is a better solution, and good quality planetary eyepieces with good eye relief are available right down to 3mm, which is plenty even for a f/4 scope.

For imaging, the compromises entailed by stacking are too much. You do need a barlow / Powermate type device to get sufficient focal length to get full resolution but the 5x Powermate is more than enough for most scopes, and far too much for many.

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For visual work you don't really need a barlow at all, even with a short focus Newtonian ... an eyepiece of the right focal length is a better solution, and good quality planetary eyepieces with good eye relief are available right down to 3mm,

...and these use a negative group which, in effect, functions like a barlow at the field side (except it usually introduces aberrations corrected by the rest of the eyepiece).

In fact, many of them have more lens groups and glass-air transitions than an ortho or Plössl in a barlow.

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