If you have a long focal length scope, you can trade focal length for more field of view with a focal reducer.
I'm wrapping up a project to explore the limits of focal length reduction in the Questar. I've added an overexposed shot of M7 and Austin's sky glow to show the expansion of the field of view in the Questar and vignetting and summarized my results in my report on the 172mm focal reducer with an APS-C size sensor.
The focal reducer adds some useful field of view in the short (vertical) dimension but images are compromised by vignetting in the long dimension. Flat frames can compensate for this, but the image sensitivity will be reduced in the corners.
The bottom line is that the Questar was designed for a full frame 35mm camera at prime focus. Using a mild 172mm focal reducer for about 30% increase in the field of view, gives an APS-C sensor camera about the same FOV as a full frame sensor. Significant corner vignetting indicates that there is little value in further focal reduction with a large sensor.
See the full report above for the details. If you have a different sensor or scope, the full article will show techniques that you can use to get the most from your setup.
Content created: 2016-08-02
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