Astronomy - USA (2020-08)

54 ASTRONOMY • AUGUST 2020

that I have owned for many years. This

avoids the smaller field of view typical of

most DSLR camera chips, and also enables

me to take advantage of individual R, G,

and B filters to minimize the chromatic

aberration typical of achromatic lenses

(including the Pentax 67), such as those

used with most consumer-grade DSLR

cameras. This is not to say that one cannot

obtain a pleasing wide-field backdrop

using a one-shot color-cooled CCD cam-

era, or even using a DSLR with a fixed

focal length lens, but I’ve always found

that monochrome CCD cameras, com-

bined with broadband or narrowband

filters, have yielded the best results in

my hands.

My setup is not exactly “plug and play,”

but the results are worth it. The U16M

CCD camera requires an adapter to con-

nect the filter wheel to the Pentax lens, and

was manufactured to my specifications by

PreciseParts (www.preciseparts.com). The

length of my custom adapter for the

Pentax 67 lens to the Apogee FW50-7S/9R

Series filter wheel was 29.5 millimeters,

but your setup could be different. I use a

RoboFocus attachment that I tailor-made

with sheet metal and a few odds and ends

from the local hardware store, and I

obtained the rubber belt and timing

sprocket from http://www.robofocus.com.

Using a RoboFocus is critical to achiev-

ing good focus with this system, especially

since most filters are not exactly parfocal,

and I wanted my setup to be automated. I

also found that with the Pentax lens, the

sweet spot for obtaining sharp images

with a reasonable aperture was f/5.6.

Building a composite image

I won’t review all of the usual steps in

image processing here, since these have

been well covered in other articles on the

subject. Rather, let’s focus on steps that

A spectacular finished image of the Cepheus field

containing IC 1396 and Sh 2–129 shows the

effectiveness of this inventive technique, and how

it brings life and detail into wide-field astroimages.

A typical image from the author’s setup shows the region of IC 1396 in and
around Sharpless 2–129 Cepheus. This represents eight hours of an HαRGB
exposure, after calibration, stretching, and gradient removal.

After aligning previously made, higher resolution images of the two principal

targets in this field, IC 1396 and Sh 2–129, the author placed the higher resolution

images into the wide field at this stage, making sure to carefully align them.