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You are here: Experts > Science > Space and Astronomy for Kids > Astronomy > built-in barlow
Expert: Paul Wagner - 10/29/2009
Question
Dear Mr. Wagner:
I recently purchased a celestron powerseeker 127 Eq. The label on the scope says 1000mm but the scope is 18" long. I found the reason for this is that a barlow lens is builtinto the bottom of the focuser thereby doubling the focal length. Is this good or bad? Without the built-in barlow lens the sscope would be a fast F/5 500mm wide-field scope. Does the addition of the barlow lens denigrade the image quality, or because it would normally admit alot of light because it's a fast scope that the barlow would not provide any significant image denigration?
Thanks
Randy
Answer
Hi Randy
the short answer here is that if the scope was designed to incorporate this Barlow into its focal system, then it probably works best that way. But if you are a dedicated fiddler like I am, then there is no reason not to explore the question. Yes, your scope would have lower magnification and a wider field of view without the Barlow. And yes, the Barlow probably introduces a bit more distortion (maybe 5%? Barlow's are pretty simple, and they work with long FL ratios, so they aren't too bad).
But the real question is whether your scope will even come to focus without the Barlow. The Barlow achieves its greater magnification by extending the apparent focal length of the objective. If you take it out, you may find that your focal point is too close to the objective...and you won't be able to get anything in focus.
That would be a problem.
The other issue I would raise is a question about the overall performance of the scope. At 18 inches or so of FL, your magnification without the Barlow is going to be from about 15X to 60X max, and at 60X you will have very little eye relief. That means that you are going to using this scope mainly between about 15 and 30X--at which point it becomes a pretty nice half of a good set of binoculars. Nice, but once you have seen the ten objects in the sky that really fill that field of view, your are going to be frustrated by the image size of everything else.
Bottom line? If you have three other scopes, then play around with this one to see which configuration you really like. If this is your only scope, then I would leave it pretty much as is. The benefits won't outweight the liabilities as a general use scope.
Paul Wagner
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