Sky & Telescope - April 2021 - 66

Imaging Science

S SMALLER FIELDS Both of these hydrogen-alpha images of NGC 6823 were captured with the same FLI ML16200M CCD camera, with the image at left recorded with the 106-mm f/5 refractor, while the image at right was captured using the 10-inch f/4.6 Newtonian. Although the Newtonian
records about ¼ of the field of the refractor, it resolves finer details.

SX FOCAL LENGTH ADVANTAGE Stepping up to a larger telescope
not only permits you to shoot smaller objects to advantage, but it also
allows you to revisit targets you recorded in the past and shoot them at
higher resolution. Compare these images of M31 taken with an FSQ106EDXIII at f/5 (above) and a close-up recorded with a 11-inch SCT
at f/7 (right). The wide-field shot highlights the bluish outer arms of the
spiral galaxy, while the high-resolution close-up focuses more on the
dust lanes in the galaxy's inner regions.

that of the Newtonian's due to a narrower field of view. At
the same time, it's as heavy as the Newtonian and doesn't
yield a meaningful resolution gain, taking into account the
atmospheric seeing limit of 1 arcsecond.
Can one reach the goal of 1 arcsecond per pixel with a
small refractor by choosing a sensor with smaller pixels?
While the idea looks attractive, this approach doesn't quite
work. Due to the wave nature of light, in practical terms the
spot size (star-image size) produced by a diffraction-limited
OTA depends only on the instrument's f/ratio, and thus does
66

APRIL 2 021 * SK Y & TELESCOPE

not scale down as the OTA is scaled down at the same f/ratio.
Too small a pixel can emphasize optical abberations in the
scope. The end result is that scaling down both the telescope
and the camera pixels while retaining the same focal ratio
results in the inevitable loss of resolution.

Final Considerations
In short, a small, high-quality refractor is a great choice for
imaging larger nebulae and a few nearby galaxies, but the
number of available targets is small when compared to a



Sky & Telescope - April 2021

Table of Contents for the Digital Edition of Sky & Telescope - April 2021

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