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Siebert Optics 34mm Observatory Series


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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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