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Can a Barlow increase image circle in a Cassegrain?

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#1 Nicole Sharp

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Posted 04 December 2018 - 07:45 AM

Can using a Barlow lens increase the size of the image circle for a prime-focus camera in a Cassegrainian OTA with a small baffle tube?  E.g. would using a 2X Barlow on a C90 (baffle tube diameter of 15 mm) double the diameter of the image circle to 30 mm (in a 32-mm focuser), to work with a Canon APS-C DSLR camera at prime focus with a reduced resolution of 1080p for video (an effective sensor size of 26 mm)?  Or would the image circle still be 15 mm even with the 2X Barlow?


Edited by Nicole Sharp, 04 December 2018 - 07:49 AM.


#2 jallbery

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Posted 04 December 2018 - 10:27 AM

You can't assume a C90 evenly illuminates a 15mm image circle, just because it has a 15mm rear baffle.  Nor does the frame suddenly go dark when you hit the 15mm mark.   It's just a question of how much vignetting you are willing to accept.

 

But assuming the barlow doesn't add additional vignetting, yes, it will increase the size of the image circle.  Doubling the image circle diameter will decrease brightness by a factor of 4, though.

 

And while the sensor in 16:9 format may have an effective diagonal of about 26mm, it's only going to effectively be about 12.6mm tall and 22mm wide sensor.  Do you even care if the corners are dark?


Edited by jallbery, 04 December 2018 - 10:28 AM.

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#3 Nicole Sharp

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Posted 04 December 2018 - 10:55 AM

You can't assume a C90 evenly illuminates a 15mm image circle, just because it has a 15mm rear baffle.  Nor does the frame suddenly go dark when you hit the 15mm mark.   It's just a question of how much vignetting you are willing to accept.

 

But assuming the barlow doesn't add additional vignetting, yes, it will increase the size of the image circle.  Doubling the image circle diameter will decrease brightness by a factor of 4, though.

 

And while the sensor in 16:9 format may have an effective diagonal of about 26mm, it's only going to effectively be about 12.6mm tall and 22mm wide sensor.  Do you even care if the corners are dark?

 

I was thinking of this setup specifically for filming of Lunar occultations.  The Barlow would be useful to zoom in on the star, and it would be nice (though admittedly not necessary) if the Lunar surface was not vignetted.

 

But if the Barlow can double the size of the image circle, then that could be useful for lots of other things as well, particularly for general close-up Solar and Lunar imaging where vignetting is more noticeable.  At full resolution with an APS-C DSLR though, the image is already oversampled, so a Barlow would not add any useful magnification unless it is used for video instead of still photos.


Edited by Nicole Sharp, 04 December 2018 - 11:00 AM.



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