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17mm Aperture stop on "22mm" CA Celestron BV's

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#1 dcoyle


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Posted 27 November 2012 - 10:22 PM

I just recieved my new BV's from Celestron, who advertise it as a 22mm clear aperture. Instead, it has a 17 or 18 mm diameter stop screwed in about a half inch into the 22 mm ID nose piece.

What's up with that?



#2 GlennLeDrew


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Posted 28 November 2012 - 12:24 PM

The 22mm aperture as claimed is to allow the use of some lower power eyepieces without edge-of-field darkening to become too evident. So in a sense the 'spec' is correct. An aperture stop well ahead of the focus had an impact on the fastest scope useable. For example, if we assume the optical path length between the 17mm stop and the focus to be 100mm (it's likely different--this us merely an example), the fastest scope which can be used before aperture reduction occurs is 100 / 17 = f/5.88.

It's nice to have extra aperture at the front end, of course, as this increases the size of the circle of full illumination at the focus. However, this is not as important as suppressing internal reflections, which I suspect that 17mm stop is there for.

In the end, as long as you get full illumination on axis you're doing well. Off-axis fall-off due to the front end stop will almost certainly NOT be detectable, and if reflections are indeed supressed or lessened, so much the better.

Unless you're trying to fit the BV to a faster scope (e.g., f/5), fear not!

#3 dcoyle


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Posted 29 November 2012 - 12:59 PM

Thanks for taking the time to respond, Glenn.

I was hoping someone would say "Oh, we take that out."

Instead, you have the message that things won't be that bad. :)

I'm quite disappointed with Celestron. I have no doubt that if Celestron spent the money to put it there, it was needed.

The last thing I need using a binoviewer on a small scope is further reduction of light throughput.

I'm returning them.

Thanks again.


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