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First Impressions of the Siebert 5.9mm StarSplitter Modified Ortho

Posted by William Paolini   03/26/2006 12:00AM

First Impressions of the Siebert 5.9mm StarSplitter Modified Ortho
About me…

By profession I work in the field of computer science, and I have been involved in astronomy, photography, and optics as an amateur for 45 years. I have spent the greater time with photography gaining what I characterize as a thorough operational understanding of light, resolution, and contrast. As far as astronomy is concerned, over the years I have owned 90mm refractors; 4.25”, 8” (home built), and 10” reflectors; and 4” and 8” Schmidt-Cassegrains. My current instrument in the f4.7 Orion 10XT Intelliscope Dobsonian, my first venture away from (and I may never go back) the equatorial mount. My observing location is the suburban Northern Virginia in the Washington, D.C. Metropolitan area.

Equipment and Goals…

The equipment I used for this initial comparison was:
Orion f4.7 10” XT Intelliscope (a fast Dobsonian reflector with Orion’s GOTO automation)
Siebert 5.9mm Modified Orthoscopic Star Spliiter eyepiece
University Optics 6mm ABBE HD Orthoscopic eyepiece
TeleVue 4.8mm Nagler Type-1 eyepiece
University Optics 2.8x Klee Barlow

To date, my University Optics ABBE HD Orthoscopics have been a wonderful observational eyepiece. They are sharp, contrasty, bright, color neutral, well made, have tight (but not overly so in my opinion) eye relief, and have a small but aesthetically sufficient field of view. All-in-all I am the most satisfied with them over any other design. They would not replace the other eyepiece (ep) designs I use since each design has its own unique strengths, depending on the mix of observing conditions. For instance, while the contrast is excellent in the UO HD, the original smooth side TeleVue Plossl (the original version with the circle “U” with a slash thru it optical manufacturers mark on the side) provides some of the best light throughput I have come across in my inventory. This TeleVue series can be consistently counted upon to reveal the dimmest magnitude stars I can observe. The UO HD is close, but these TeleVues edge them out ever so slightly. Plus the TV’s provide a slightly warmer view which at times seems to accentuate atmospheric banding on planets as well as color rendition on red stars. Similarly, I find the Edmund RKE series provides the most pinpoint star image of any ep design in my inventory. But overall, I consider the UO HD series the ones I am most pleased with overall and best performing in all evening observational categories -- especially if you can get past the prejudices that a larger apparent field of view (afov) eyepiece tends to psychologically invoke.

My goals in attaining a Harry Siebert Star Splitter series eyepiece were two-fold. First I wanted a larger field of view for very high power observations. While I love my UO HDs, when I barlow the smaller focal length ones (e.g., the 5mm or 6mm) with my 2.8x Klee, I may only have 15 or so seconds of observing time with the 45 degree afov they provide before I have to “nudge” the Dobsonian to get the object back in the field. This actually hasn’t proven to be much of a bother. However, with that smaller afov and such high power (561x @ 56x/inch in my 10XTi), it becomes somewhat difficult to find the target if not exactly centered in the afov prior to barlowing the ep. Secondly, my goal is what I think is common to most amateur astronomer enthusiasts, always being ISO something a little better (i.e., satisfying desires). Like many of us, I am always reading reviews and tests of different eps and scanning the forums for user’s impressions for something perhaps a little bit better so I can perhaps see something not yet glimpsed.

In my Internet research of reviews, test reports, and user forums, I browsed through everything I could find on various competitors to the UO HD, such as the Baader, Takahashi, Pentax, Zeiss, TMB/BO Planetaries, and TMB Monocentrics. In addition, I spoke and corresponded directly with others who have used some of these eps. After sifting through it all, when I only used apples-to-apples reportings (i.e., use of these eps through fast focal ratio reflectors) it became apparent that the difference, if any were noted at all, were relatively minor. Most amateurs reported that for the most part, differing resolution between all of these could be detected only after repeated A-B comparison over the course of an evening. Areas of difference related to contrast difference or light throughput differences were somewhat more marked between some of these competitors in the reports I read, but no difference in terms of an order of magnitude. Now before anyone starts writing me a rebuttle to this, yes there are posts where an individual describes a seemingly significant difference. My point here is that in the collective of all the many reports I have read, especially when we restrict the field to those reports relative to those eps used on fast Newtonians, the marginal difference between any of them seems to be, well, marginal. However, price is not marginal.

So, with all that in mind, did I want to shell out the exorbitant amount of money for a Pentax or Zeiss Ortho if all I’m getting for 3+ times the price of a UO HD is a marginal increase in resolution under perfect seeing conditions? Given that I could use that extra money to invest in a different optical design ep to have its strengths available in my inventory, it made no sense. So given that the collective of these report by other observers seemed to show them significantly better than a UO HD, and indeed many of them reviewed out worse, it became apparent that the field from which to choose from for something better than the UO HD was fairly non-existent, more so given their price-point. So that left me to look towards custom houses (like Rini and Siebert Optics) or moving to other optical designs like the Monocentric for something better than the Orthoscopic. I actually considered deeply the TMB Monocentric as it is not overly expensive if it would be such a step up. Many have touted its resolution and more so its contrast. But then again, they were mostly APO users. Most of the fast Newtonian users reported slightly better resolution and markedly better contrast. But I myself am skeptical on the contrast issue since the Mono’s have such a narrow afov -- this viewing condition tends to just look more contrasty since your eye sees more of the surrounding totally black field surrounding the visual afov of the ep. I find this true with almost any ep in my inventory. As I go down in afov for any given focal length, the field “appears” more contrasty. But it always is just an impression of contrast and most of the time I can’t actually discern a real difference in the space between the stars. So my opinion here is that it’s just the overall (and correct) impression of your mind because you are seeing not only the star field in these narrow afov designs, but also the jet-black surrounding area to the afov where no light is coming in from. So with so much jet-black in the field of view, overall you register a sense of more contrast.

Eventually, I gave up on trying out the Monocentric because the only real gain there would seem to be was in what I’ll call “apparent contrast.” Plus the even narrower afov would make barlowed high power work all but impossible using my Dobsonian. Additionally the Monocentrics used in fast Dobs were showing softness off-axis in reports. So with such a limited afov, I certainly don’t want that for more money than a UO HD! The Burgess Planetaries were tempting also, but most reviews reported similar off-axis issues, no real gain in resolution, and less light throughput than the UO HDs. So they did not seem to be a good candidate since the UO HD got higher marks everyplace except in afov and eye relief relative to the Burgess Planetaries.

At this point, I was down to two real options, go for a really custom design, that is have someone build me something not available which on the optical drawing board would provide me higher resolution (I actually found a technical design or two with sub-one micron spot sizes for an ep), or consider some small optical producer like this new Star Splitter series from Siebert. So I took the easier route of course (and less expensive I’m sure). Unfortunately there was zero out there about the Star Splitters, only stuff on the Siebert Optical site, and I usually don’t consider anything written by the producer or marketer, since in the end, it is marketing. For the other optical designs Siebert had, there were user reviews, and they seemed to be mixed. Some people loved them, others hated them. Unfortunately most of these reviews lacked any real specifics like instruments used or observation techniques used to determine whether it was better or worse in performance. That being the case I could not really draw any conclusions. Therefore, I figured the only way to test out this ep, which could possibly provide some strength over the UO HD, would be to purchase one. However, $99 is a lot for an experiment (I’m not Bill Gates). So I did the only thing left…I called Harry Siebert to get a “feel” for him, what he’s doing relative to this design, and then make a value judgement if it’s worth the risk of purchase -- even though they offer a 30 day money back guarantee, I am often reluctant to do this. So we talked, he seemed credible, so I made the purchase.

First Impressions and First Light…

After several weeks (Harry had to construct it since on wasn’t available on the shelf), I received the Siebert 5.9mm Star Splitter Modified Orthoscopic. It is touted to have a 65 degree afov with 10mm eye relief. When it arrived is appeared just as the photo on their website – hand machined aluminium body with an inset on the top in a darker rubbery material which housed the lens. The machining was done well (i.e., uniform and accurate) and buffed leaving the small lathing lines from the machining (i.e., the external housing is not buffed to a high shine high smoothness like you see on the typical chromed brass barrels of standard production eps). The ep was large relative to the UO HD, fitting in the full width of my palm. Optical elements showed a nice purple/violet coating with the overall ep feeling light for it’s size (actually a welcome thing since overly heavy eps tend to be less favorable on a Dobsonian because there are no counterweights to offset significant weight additions). The bottom had a large field element which I presume is a built-in barlow for the ep, similar to what TeleVue does on some of theirs, as well what we see on the Orion Stratus and similar wide-field designs – an effective way to achieve better eye relief for a given focal length. Overall it looked and handled very good.

OK, we are finally getting to first light. So on a relatively dark evening in March I took my f4.7 Orion 10” XTi Dobsonian out to compare the new Siebert 5.9mm Star Splitter (SS) to the 6mm UO HD. I also brought out the 4.8mm TV Nagler for a general impression difference. I didn’t want to make too much of a comparison to the Nagler since they were different focal length eps. So I placed the scope and eps outside and left them there for an hour thermal adjustment period. It was 70 degrees in the house and probably 45 degrees outside, so I’d need at least an hour. I had also collimated the Dob the weekend before.

Once outside I set up the Orion’s Intelliquest (GOTO system), then checked out my diffraction rings on a bright star. I did this to ensure the allighment was still good, and I also do this to determine how well the mirror and other optics are thermally adjusted as you can easily see the thermal ripples in the diffraction pattern if the scope is not in thermal equilibrium with the environment. The star test confirmed the allighment and thermal balance were good.

So what should I test as a first object to compare the UO HD with the SS? Saturn was high overhead. Perfect. The Saturn test was excellent. The UO HD provided an outstanding image as usual, Cassini division clearly visible around the majority of the ring (sometimes difficult in front portion of the ring) and some good cloud belts visible. The planet’s shadow on the rings was also very clear, giving the nice three-dimension impression to the planet orb. I now popped in the SS. It only required a minor refocus over the UO HD. I didn’t know what to expect, hoping for the best but prepared for the worst. Actually I was just hoping it would be equal in terms of resolution to the UO HD. If it was equal, then for just $20 more than the HD I could move from and afov of 45 degrees to 65 degrees, and that would be great.

I made the initial focus of Saturn using the rack and pinion, then moved to the fine focus afforded by the Clear Skys Fine Helical Focuser Adaptor. This adaptor fits in 2” focusers and provides fine focus for 1.25” eps only. I have found this affordably priced accessory to be the best accessory I have for the scope. I was astonished how easy, fast, and precisely it allows you to find the exact focus for an ep. Invaluable for any high power work. Bottom line is lots more satisfying observing and almost zero focus time since I got this accessory. With Saturn quickly at fine focus in the field of view, the first impression was…it’s sharper! The Cassini Division was nice and clear fully throughout the visible ring system. Boundaries between illuminated planet and shadows cast on the rings were more distinct, and cloud banding more pronounced than in the UO HD.

I was really surprised. I had not expected it to best the UO HD, not in resolution anyway. But it did. How much? At the moment, with this initial impression I would say moderately significantly. Not what I might characterize as an order of magnitude, so perhaps by a half an order of magnitude. To explain better what I mean when I subjectively say it was half an order of magnitude better, what I mean is as follows. When I am observing I do notice a resolution difference between the TV Plossl and the RKE and the UO HD when they are compared to each other relative to Saturn. But after the observing session is over, the impression that is left with me is that I recall they were different, but I can’t recall the experience to say that it was a real “remembered” difference. The picture does not remain in my memory. So they were really objectively probably just close…a tad better resolution to the UO HD…enough to be obvious while observing and comparing. The difference between the UO HD and the Siebert SS however was more than that. Well after the observing session it is easy to recall the image of Saturn with its greater detail. So somewhere around a moderate increase in apparent resolution. Good!

Next I moved to splitting some doubles. First off I chose Polaris. Not because it is difficult, but because the companion is so dim relative to the primary. Polaris is magnitude 2 and its companion is magnitude 9. So a nice light throughput comparison. The UO HD showed the companion fine; dim, but there. The SS showed it as well. I could not discern any significant difference. Perhaps a little better in the SS, but can’t say. So I’ll leave that determination to an evening with the Trapezium and trying to catch component F, which is usually difficult in my light polluted observing area given how close it is to its brighter component. So this was good also, equal to the UO HD.

Next I moved to Mizar as well as several of the doubles in Bootes (using the Intelliquest to get there). In Bootes I checked Iota, Kappa, and Xi. Xi is the closest of the bunch with a 6.9 arc second separation. All split easily in both the UO HD and the Siebert SS. This, of course I expected after viewing Polaris. While I didn’t think there would be any problems splitting these doubles, I was surprised to notice that the Siebert SS provided a more pinpoint star image. I guess that should have been expected given it resolved more on Saturn. But I have noticed that this is not always the case. I have found that an ep will do a better job on Saturn even if it is not as sharp in resolution if it has better color correction. Recently I did a bench test with several eps using a photo resolution test target. The ep which resolved the most lines per millimeter on the test target performed miserably on Saturn. It was then that I noticed that this high resolution ep (high resolution for black and white) had noticeable color aberrations both on and off axis. From that point I learned the importance of color correction to resolution on extended objects such as planets.

Next I moved to M36 and M37 in Auriga. They were available between two large trees from my yard. Using the Intelliquest I dialed them in and pushed to scope to the coordinates with ease. Both the UO HD and the Siebert SS provided good views. I think perhaps the sky background between the stars seemed a tad darker in the UO than in the SS, but it was difficult to determine if that was really true. It appeared a slight bit darker, but I’m not sure if it was because of the optical phenomenon I described earlier relating to contrast appearances when switching from a narrower to a wider afov ep. Anyway, at this point in the evening the Orion Nebula was well behind an offending tree so I will save a field darkness or contrast assessment until an evening when I can target that. I like using the Orion Nebula for this purpose because of the many fine gradations in the whisps and molts of the nebula. Just makes it easier for me to make those types of determinations.

Getting back to M37 with its many fine dim stars throughout the cluster, what was a particular advantage to the Siebert SS over the UO HD was the larger field of view. It was quite pleasing to see so much of the cluster at this high power (200x). The finer pinpoints of the SS also made it a better observing experience. And when I barlowed the SS up with the Klee (561x), even at this extreme power it was still a pleasant view into the star cluster. It was quite a pleasant surprise to be able to work at such a high power on an open cluster. I attribute the 65 degree afov the SS has to this.

So in my barlowing of the SS (and the UO HD), it was apparent that both of the eps barlow extremely well. Light throughput was not diminished beyond what was expected from the higher power, and the images held up extremely well with stars remaining very point-like, especially in the SS. With the Barlow attached, I moved the scope back to Saturn to see how it fared under barlow at this extreme power setting. As expected, the image was very much larger and a bit softer. I expected a softer image since this put me at 56x per inch of aperture, so in excess for this night’s seeing, which was good but not excellent. Plus 56x per inch is pushing it for any scope, especially a fast production Newtonian. But it was pleasing that the image was still very good. The Cassini gap being extremely pronounces plus other markings now visible along the outside of the rings. Overall I would put the SS as moving from moderately significantly better in resolution over the UO HD, to being a “tad” or noticeably better under barlow.

The final test I wanted to do was an edge-of-field star test. The UO HD performs extremely well here. It does get some bloating of the star image towards the very edge, but not much, maybe 10% bloating at the extreme edge. With the larger afov of the Siebert SS, I expected nothing so good, especially since this was on a fast scope at f 4/7. I picked a random medium bright star in the field near Saturn (off towards the Beehive), and tracked it through the field of the SS. To my extreme surprise there was only minor bloating as well, maybe 10% towards the last 5-10% or less of the field. I was actually in shock it was so minor. And that minor bloat was not all the way around. So at some parts of the field the star exhibited no bloat right to the edge. So quite impressive! Going back to M37, I re-verified this. So with the bloating so minor, as it is in the UO HD, it was not even apparent looking over the stationary cluster throughout the field of view that any star was anything but a point. So an extremely satisfying view in this very fast scope all the way to the edge.

Overall, the Siebert Star Splitter excels the UO HD, and it does so significantly when we factor in the larger afov and all for only $20 more in cost. Again, let me caution that these are “first light” impressions over a weekend of use for the 5.9mm specimine only. As I said before, each optical design has its own unique strengths. So perhaps over the next few weeks these strengths will play out between the UO HD and the Siebert SS. But for now, in a first impression, I would say that the Siebert Star Splitter series has the potential to “replace” my University Optic HD Orthoscopics! What it definitely means at the moment though, before doing any actual replacing of ep lines, is that I can confidently move on to purchasing a few other focal lengths of the Star Splitters. And if each of these similarly outperforms their UO HD counterparts, there may be something very special here in the Siebert Star Splitter Modified Orthoscopic design for fast Newtonian users.

By-the-way, I did do a very cursory comparison of the SS to the 4.8mm Nagler. As expected, since I know the differences well between the Nagler and the UO HD, overall the Nagler produced a less resolute and less contrasty image exhibiting less light throughput than the other designs I have. The Nagler is slightly less resolute than the UO HD, which means a lot less (but still good) when compared to the SS. The edge of field performance on the Nagler is also not as tight at the UO HD or the SS. But the Nagler still has some intrinsic unique characteristic strengths in the image it produces, like the Plossls and RKEs and Erfles each also do, which means for me that this Modified Orthoscopic design should not replace the Nagler. But overall and in general, the Siebert 5.9mm Star Splitter ranks much higher in observational qualities to the 4.8mm Nagler Type-1.

Next month I will post an update to this First Light review as I get more experience with the Star Splitter under my belt. In that update I’ll pay more attention to color correction (which seemed perfect), actual afov size (which seemed a little less than 65 degrees), and other performance characteristics. To end the viewing this evening however, I wanted some observational “desert” be the last taste for my night of observing. So I turned the scope back to Saturn, took the Star Splitter and placed a Burgess Optics #82A light blue filter on the bottom, and took in some last views of that exquisite planet for the evening’s memories. And it was…spectacular! Being in a moderately light polluted area, I find the light blue color filter does wonders as it darkens the background sky without losing me many (or any) stars from the minimally reduced light throughput. I find it very much more pleasing than a pollution filter since it gives me a much brighter image. Plus the light blue doesn’t detract from most images. So there she was, absolutely stunning with her atmospheric bands and ring system in all their splendor and much accentuated with the filter in place. A most wonderful last image for the evening with this pleasantly surprising new eyepiece.

Happy Observations!

ps – FYI, I have no interest or relation to Siebert Optics or any other private or commercial photographic, optical, or astronomical entity. I am simply a visual astronomy hobbyist enjoying the hobby from his back yard.