Siebert
Optics 34mm Observatory Series:
Some Impressions
Eyepiece
The Siebert 34mm
Observatory Series eyepiece is the target of observations in the
following text. This eyepiece weighs between 8 and 9 ounces, and is
advertised as having a 70-degree apparent field of view. The list
price for the eyepiece is $199 (US).
As seen in the image
above, the Siebert 34mm has an “industrial” finish. By this, I
mean that it has a brushed aluminum exterior with no rubber grip.
Admittedly, I like the look of other lines of eyepieces better. In my
opinion, however, good looks are not necessary for a good viewing
experience.
Setting
Over the course of four
nights, I evaluated my 34mm Observatory Series eyepiece. On three of
the four nights, I used a 16” Meade f/4.5 dob with an OMI-Torus
refigured primary mirror. I used a 150mm Celestron f/8 achromat
refractor on night four. Two nights had a bright moon, and two nights
had no moon (including the night with the refractor). Seeing was
typically 3/5 on the clear sky clock, with 4/5 or better transparency
and cloud cover. Temperatures ranged from 10 degrees F to 30 degrees
F. Under no moon and average transparency, my rural Iowa skies have a
limiting magnitude of between 5.5 and 6.0 at zenith with no yard or
street lamps within 0.75 miles.
In evaluating the
eyepiece, I tried to balance extreme test cases (e.g., very bright
objects, such as Sirius) with targets that were not as extreme (e.g.,
M41 and M42). This balance, along with the range of sky conditions
described above, was intended to permit a thorough evaluation of the
eyepiece. My observations summarizing the sessions follow. Some
involved head-to-head comparisons with other eyepieces, and others
did not.
Strengths
1.
Compared to an older 32mm Celestron plossl and a 26mm Meade Series
4000 plossl, individual star colors were much more obvious in the
34mm Observatory (especially in the double cluster and M41) when
viewing through the dob. I did not look for such details with such
higher-end eyepieces as Panoptics or Naglers. Individual star colors
were present to a lesser degree in the refractor, though its smaller
aperture reduced the contrast between the Siebert and plossl
views.
2. Compared to the plossls referenced above, structure
within regions of nebulosity in M42 and the Pleiades was more
prominent in the 34mm Observatory. To account for different fields of
view between the Siebert and the plossls, I verified that this
conclusion could be reached for the central portion of the fov as
well. There were very distinct billowing clouds of nebulosity in M42,
as well as nebulosity around many members of the Pleiades. This
nebulosity was very obvious, both in the refractor and in the dob.
But, the refractor’s smaller aperture reduced the contrast between
the Siebert and plossl views.
3. Under skies with no moon,
there was very slight field curvature in my dob, as on-axis best
focus put edge-of-field focus roughly 1/20 of a JMI-DX1 focuser knob
turn out of focus. Under moonlit skies, this effect was not apparent.
This test was not conducted with the refractor.
4. Edge of
field sharpness in the 34mm Observatory was virtually identical to a
35mm Panoptic when using the f/4.5 dob. Note, I do not use a coma
corrector in the dob. The 35mm Panoptic was not available for use in
the refractor. So, I can only compare Siebert sharpness in the
refractor to Seibert sharpness in the dob. Siebert edge of field
views in the refractor were consistent with Siebert views in the dob.
Tests for edge of field sharpness were conducted by passing bright
objects (e.g., Sirius and Saturn) out of the field of view, as well
as observing star characteristics in the Pleiades, the Double Cluster
in Perseus, and the Trapezium as they passed through the field of
view. For instance, I could pass the Trapezium out of the field of
view and maintain distinctly separated A, B, C, and D elements to the
same degree in the Siebert as in the 35mm Panoptic.
5. On-axis sharpness
was virtually indistinguishable between the 35mm Panoptic and the
Siebert in the dob. Siebert on-axis sharpness in the dob was
consistent with Siebert views in the refractor.
6. I found the
Siebert easy to use with glasses on, regardless of scope.
Furthermore, the sweet spot for eye positioning was easy for me to
attain and hold. Ease of use was similar to the 35mm Panoptic in the
dob, and much easier than I experienced when using a 22mm T4 Nagler
in the dob. Easy positioning for the sweet spot was critical in the
refractor when viewing near the zenith.
7. There was no
evidence of astigmatism nor chromatic aberration apparent to me when
using the Siebert in the dob. Because the refractor was an achromat,
I did not look for CA when using it for testing…I have a hard time
distinguishing between EP induced CA and that inherent in the scope.
However, I did notice a similar absence of astigmatism in the
refractor.
8. In both scopes, panning across star fields gave
rise to linear paths being followed by all stars, rather than curving
inward or outward. This indicated an absence of pincushion
distortion.
9. In theory, the true fov in the 34mm
Observatory and the 35 Panoptic should have been the same to two
decimal places (roughly 1.29 degrees in the dob and 1.98 degrees in
the refractor, obtained by dividing apparent field of view by
magnification). I did not do a timed transit. I could not visually
identify a discrepancy between their true fields of view.
10. Though not as much
of an issue in my dob (it has a bit of stiction on the alt axis),
optical tube assembly (ota) balance is always an issue in the
EQ-mounted refractor. If switching between the 28oz 35mm Panoptic and
the much lighter Siebert in the refractor, I would have to attend to
ota rebalancing.
Weaknesses
1.
When introducing a very bright object into the field of view, a
single very faint ghost (reflection) appeared in the Siebert. The
ghost disappeared when the bright object was centered in the field of
view, but diverged from the primary target as the primary was moved
off-axis. This was observed both in the dob and in the refractor. I
removed my glasses to verify that I was seeing a reflection in the
EP, rather than on my eye glass lens.
The ghost was absent
when viewing such targets as Alnitak and associated nebulosity in
that region, as well as absent when viewing the region surrounding
M42. The Pleiades showed no ghosting, either. It was only apparent
when viewing very bright objects that had dark backgrounds, such as
Sirius and Saturn under no moon. This ghost may be an issue for some,
but it did not bother me for two reasons. First, it was very faint.
Second, my viewing practices rarely require the use of an EP like the
34mm on extremely bright objects.
2. When placing Sirius
just outside of the field of view, I observed a very faint flare.
This flare extended roughly 15% into the field of view, and was quite
narrow. The flare was absent on dimmer targets. Note, this flare was
observed in both scopes. As with the ghost referenced in weakness 1,
the flare was so subtle that I did not find it distracting.
Both of the weaknesses
described above were not present in the 35mm Panoptic.
For Further Review
I spent little time
looking for galaxies with the Siebert. This was because I am more
interested in star clusters and nebulae than looking for galaxies.
Still, the absence of reporting on galaxies is a notable limitation
to this review. I plan to go hunting for galaxies in due time.
Personal impression
I really enjoyed using the 34mm Observatory. Taking my personal
preferences, intended uses, and eyepiece strengths/weaknesses into
account, I believe that my Siebert 34mm Observatory Series was money
very well spent.
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