My
Orion 120 mm F5 ST achromatic refractor OTA was purchased for visual
use. I wanted something portable for rich field deep sky viewing and
at 25 inches long and 8.6 lbs this seemed like a good choice. I
liked being able to reach to magnitude 12.2 and I liked the 2 inch
rack and pinion focuser. The fact that it was an achromat did not
bother me since deep sky objects are faint visually.
The views I got of deep sky objects which were comparatively bright
and contrasty were very pleasing and the sky background was very
dark. Stars were very crisp essentially across the whose field. In
addition to stars and deep sky objects, I’ve occasionally
looked at the moon and planets with the scope, but it really isn’t
designed for high power viewing. However, it can be pushed to
something like 120x or 150x with a respectable image. Purple
fringing around bright objects is present but is not too
objectionable (at least to me).
After
owning the scope for a while, I decided to try some color deep sky
imaging with it. My thought was that maybe problems caused by
chromatic aberration inherent in essentially all short focal length
achromats might be able to be tamed in image post-processing. Also,
I was very attracted to the speed at which faint images can be
recorded with aperture and focal length.
I
put the OTA on my Losmandy G11 mount and took 30 minute images of a
couple of bright deep sky objects (M27 and M31) using my Canon Rebel
DSLR at ISO 1600. I used the native 600 mm focal length for M31 but
added a 1.5x barlow for M27 to increase image scale. The images
were made in unguided 3 minute subexposures which were then
registered and stacked. I discovered that astronomy tools (plug-ins
and actions) available for Photoshop go a long way to correcting the
bluish violet halo’s around brighter stars typical of achromat
images, sharpening stars, removing noise. and increasing contrast.
In my opinion at least, such tools begin to make the achromat an
attractive choice for low cost but respectable deep sky color
imaging. A photo of the OTA on my Losmandy G11 mount and imaging
results for M27 and M31 are shown below:
Orion 120 ST on
Losmandy G11 mount.
Shown with Celestron 80
mm guidescope and Canon Rebel DSLR.
Dumbell Nebula (M27)
taken with the Orion 120 ST at FL=900 mm. 3 min subs x 10
Andromeda Galaxy (M31)
taken with Orion 120 ST at FL=600 mm. 3 min sub x 10
Conclusions:
I have really started
to love this small, portable, lite weight, and very capable
telescope. It is well worth the approximately $320 or so that it
currently sells for (October 2009). It is really fantastic for
visual observation of the deep sky and I am beginning to discover
that it has some potential as an imaging scope as well, provided that
steps are taken in post-processing to correct for purple halos around
brighter stars. I think that this scope is a great bargain for the
price and I plan to have mine in regular use for many years to come.
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