Telescope Reviews: ETX 80 Built In Barlow Question

Once you understand the way a Barlow works, some of your questions may answer themselves.

A Barlow injects a new series of optics into the path of light, optically "stretching" that path by the power of the Barlow (in the case of the 80's built-in Barlow, two times, or 2x.) This has the effect of increasing the focal length of the instrument (and hence, the magnification), but it also increases your f/ratio by the same power. Meaning, in this case, you go from an f/5 to an f/10.

This would explain why it got a lot darker. And as your magnification goes up, your field of view is decreased, which is why your "view range" appeared smaller.

Now, to be perfectly honest (and I love my 80), they could have left the built-in Barlow off entirely and I would have been perfectly happy. Other than visually assessing when I looked through the diagonal without an eyepiece that the Barlow element was actually engaging and disengaging, I've never used it and probably never will. I have a perfectly functional Barlow tube that will not alter the focus as dramatically as the built-in one does.

As far as seeing eyelashes...if you were attempting to use a low eye-relief (typically higher powered) eyepiece, you may have been pressing your own eyelashes up against the top element of the eyepiece in an attempt to see something... anything...through the scope. It sounds almost like you might have pushed the effective magnification limit for the scope past its boundary. In other words, if you were trying to engage the Barlow AND use a high powered EP, it would take you beyond the optical resolving limits of the scope. For all practical purposes, the upper limit is generally around 50x per inch of aperture (or around 2x per millimeter) under ideal seeing conditions. For some reason, Meade states in its specs that it's somewhere around 230x, but it would be a rare night indeed to have conditions that allowed that with any amount of clarity.

By all means, test the scope during the day! This is how you'll get familiar with it! In fact, to train and calibrate the motors, it's highly recommended that you do so while focusing on a distant, earth-bound object like a sign-post or telephone pole during daylight hours. This will give you an opportunity to test that Barlow out if you like, although keep in mind, what you see in broad daylight with the Barlow engaged may not reflect similar results you'll get on celestial objects at night. In all candor, you'd be better off leaving the built-in Barlow disengaged and investing in a decent quality Barlow tube. I wouldn't recommend going above a 2x, but that's just me.

Try using your low power (26mm) EP during the day with the Barlow and let us know how that works for you.

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Tim

Champion of small aperture scopes everywhere!

Meade ETX-125PE
Meade ETX-80
Celestron FirstScope 76mm Mini-dob

Updated: 09/16/2009