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Barlow and focal ratio question

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4 replies to this topic

#1 cpsTN

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Posted 02 July 2020 - 04:54 PM

<p>I know that if you use a 2x barlow on an f/4 scope, you get the power of an f/8 scope, but does it perform like an f/8 scope in every way? I've never had two scopes that were like this side-by-side to see if it did this so I just thought I would ask. I assume that it would not.&nbsp;</p>

#2 howardcano

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Posted 02 July 2020 - 04:58 PM

No.  If it's a Newtonian, it will still have the coma from the F/4 mirror.  If it's a refractor, it will still have the chromatic aberration of the F/4 lens.


Edited by howardcano, 02 July 2020 - 05:00 PM.

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#3 photoracer18

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Posted 02 July 2020 - 05:03 PM

If you want a better result you need to use a tele-extender instead. The extra elements are designed to shape the light cone so its closer to the resulting F-ratio not the original one.



#4 SteveG

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Posted 02 July 2020 - 06:19 PM

If you want a better result you need to use a tele-extender instead. The extra elements are designed to shape the light cone so its closer to the resulting F-ratio not the original one.

That doesn’t change the answer though. A reflector will still have the same amount of coma, and a refractor will still have the same amount of CA.



#5 cpsTN

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Posted 02 July 2020 - 07:31 PM

I was thinking this would be the answer but I couldn't remember for sure. I should have though because I'm well aware of coma and CA. Maybe I've been sleeping too much or not enough for something.Thanks to all. 




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