9.9mm Siebert Star Splitter Orthoscopic comparison test


9.9mm Siebert SS-review



9.9mm Siebert Star Splitter Orthoscopic comparison test
on 8.5-inch f/7.5 Newtonian




Siebert Optics purchased direct - $79 + $26 shipping + £20 duty + VAT. Total cost £80

Eyepieces compared:

STAR SPLITTER 9.9mm x 60º 2-1 SIEBERT OPTICS x164 22'arc fov Er 18mm
ORTHOSCOPIC 9mm x 45º 3-1 EDMUND x180 15'arc fov Er 8mm
HD ABBE 9mm x 45º 3-1 Orthoscopic - UNIVERSITY OPTICS x180 15'arc fov Er 7.5mm
BRANDON 8mm x 45º 2-2 Orthoscopic - VERNONSCOPE x200 13'arc fov Er 6.8mm
BO/TMB 9mm x 60º Planetary Series 1-1-2-1 wide angle x180 20f'arc fov Er 16mm

Discussion:

Advertised as a 60° five element design in shorter focal lengths. A 2-1 three element design - i.e. reversed Kellner type in 8.9 & 9.9mm focal lengths, intended for focal ratios slower than f/6.

Harry Siebert supplies the eyepiece as standard in a semi-polished Duralamin 1.25-inch push fit barrel. I ordered mine in a semi-matt hard black anodized finish. It was supplied in a screw top plastic bolt case without body caps.

The lens assembly is well recessed into a dead matt black rubber eyecup that may be folded down. The ID of the 1.25-inch sleeve was also painted dead matt black. There is a matt black filter thread about 1/4-inch depth. The rest of the ID, roughly 1-inch depth is smooth matt finish. The lens assembly is in a separate sleeve piece screwed into place and retained in the barrel by three grub screws. The eye lens end of the barrel is 1 5/8-ins OD x 1/2-inch long. The external wall of the barrel is black hard anodized in Harry's own workshop.

The eyepiece is very light, weighing no more than 1 & 3/4 ozs. The field doublet is 5/16-inch dia. and the eye lens 3/8-inch dia. Eye relief is just sufficient to allow the eyecup to nestle in the eye socket. The apparent field of view (afov) in the 2-1 design I estimated to be 60°, slightly greater than a Brandon.

There was a very slight blue-green internal fringing to the field stop when held up to bright light indicating slight overcorrection of lateral colour. The field stop was slightly out of focus.

The air-glass surfaces are MgFl2 bloomed. Internal reflections are well suppressed. I could detect no internal reflections off either the barrel ID or the lens edges.

This is a mechanically well designed and well machined planetary eyepiece.

Observational Tests:

8.5-inch f/7.5 NEWTONIAN _ 4-vane intersecting spider _ 25% c-o
28DEC2006 18h:30m to 19h:30m UT Seeing III (Antoniadi) Light Cloud _ 3 octas
MOON Age 8 days; Capella & Iota Cas.

Compared 9.9mm Siebert Star Splitter 2-1 60° Ortho (x164) with a traditional 9mm 3-1 45° volcano top Ortho (x180)

Estimated afov of 9.9mm to be the claimed 60°, although the field stop was slightly out of focus. Examined Clavius at sunrise, the Straight Wall, Birt and Rima Birta, and Archimedes in both eyepieces.

The field was dark, detail crisp over 2/3rds the fov. The outer 1/3rd was slightly aberrated, a combination of astigmatism and lateral colour, red inwards, indicating slight overcorrection. The inner edge of the field stop was tinged greenish-blue. No evident ghosts or internal reflections or scatter.

The volcano top gave a marginally less crisp image, had no perceptible outfield aberrations, but there was a very narrow citron-yellow fringe to the edge of the field stop.

The eye relief of the Star Splitter was adequate, and to my liking. Comparable to the traditional volcano top 9mm standard Ortho.

Next, Capella. With Capella centred the diffraction pattern was clean, crisp, and contrasty. Displaced to 20° apparent field radius, the Airy disc remained unaberrated. The edge of the field was marred by lateral colour and astigmatism, which became barely noticeable immediately beyond the 20° app radius.

I detected one ghost, very faint and highly defocused. When Capella was held roughly 10° off axis (apparent), a faint purple disc could be descried opposite the field centre. When Capella was put just behind the field stop, there was a short faint radial flare detectable. No other flare or ghosting was noticed.

Then onto Iota Cas a very nice triple, cleanly resolved in the average seeing with the image centred.


8.5-inch f/7.5 Newtonian - 4 vane intersecting spider - 25% c-o
22JAN2007 20:00 - 22:30 UT Seeing: II-III Conditions: none T = 5 W = 0
M42; M45; Rigel; Castor; M35; Saturn; Double Cluster
Observers: Terry Devon, Bob Rutter, Chris Lord

Eyepieces compared

9.9mm Siebert Star Splitter Ortho 3-1 afov 60°
9mm Edmund Ortho (volcano top) 3-1 afov 45°
9mm UO HD Abbe Ortho 3-1 afov 45°
8mm Brandon Ortho 2-2 afov 45°
9mm TMB/BURGESS Ortho 1-1-2-1 afov 60°

Findings:

EYEPIECE

AXIAL DEF

EDGE DEF

ASTIG

LATCOL

9mm Edmund

vg

vg

no

no

9mm UO HD

ex

vg

no

no

8mm BRANDON

ex

ex

no

no

9mm TMB/BURGESS

ex

vg

v slight

v slight

9.9mm SIEBERT SS

ex

vg

v slight

v slight


EYEPIECE

GHOSTS

DEFOCUS

FIELD STOP

9mm Edmund

no

no

sharp

9mm UO HD

no

no

sharp

8mm BRANDON

no

v slight

v slight

9mm TMB/BURGESS

specula

no

sharp

9.9mm SIEBERT SS

off axis

no

slight defocus


The traditional volcano top Orthoscopic worked equally well to all the rest except in terms of scatter around Rigel & Castor. All gave clean Airy discs with dark fields & well defined diffraction rings. The UO HD Orthoscopic we would rate best, What let the Brandon down was very slight defocus at the field edge. The TMB/BURGESS Ortho retro-fitted with a flat black spacer ring, performed equally well to the Siebert Star Splitter. The TMB/BURGESS Ortho is a very comfortable eyepiece to use compared to the others. The Siebert had an unobtrusive defocused ghost, round when Castor was slightly off axis, becoming much fainter and elongated as Castor was displaced towards the field boundary, always opposite and equidistant from the field centre. The TMB/BURGESS exhibited a pin point specula ghost that flitted about the field as the observer's eye moved. Getting one's eye right on axis eliminated it. Fortunately the adjustable eye cup enables the observer to hold his eye precisely on axis.

Saturn was well defined, and Dione was close to the northern edge of rings. We thought the 9mm UO HD Ortho showed it most clearly, followed closely by the 8mm Brandon Ortho and the 9.9mm Siebert Star Splitter.

We also observed the Double Cluster & M35 using the 9mm UO HD Ortho; 9.9mm Siebert SS & 9mm TMB/BURGESS Ortho. Our impression was this is where the TMB/BURGESS Ortho came into its own. The 60° afov is comfortably accessible because of the generous eye relief, and the stars appeared like diamonds sprinkled on black velvet.

We also compared the Edmund volcano top Ortho, UO HD Ortho & Siebert SS at f/15 using a x2 Irving Barlow. At f/15 there was nothing to choose between any of the three, and the Siebert came into its own, the outfield astigmatism and lateral colour vanished. And so the well worn adage, there is no problem in eyepiece design that cannot be remedied by increasing the focal ratio was proved once again.

Conclusions:

The 9.9mm Siebert Star Splitter works well over 50° of its 60° afov. The outer 5° field radius is marred by moderate but not detrimental astigmatism and lateral colour. Personally I prefer Orthoscopic eyepieces that exhibit neither, and at long focal ratios (greater than f/10) have a visually unaberrated field up to the field stop. This would be the case if the afov were restricted to 50°.
The Star Splitter is advertised as an advanced Orthoscopic design that embodies the properties of the Zeiss Abbe Othoskop with a longer eye relief and a wide apparent field of view. This may be so for the shorter focal length five element design, but was not my finding with the 2-1 design, which performed no better than a standard 45° Orthoscopic.


On the Siebert Optics website a comparison is also made with the University Optics HD Abbe style Orthoscopic. I have a full set of UO HD Ortho's, The 9mm has afov 45°, only 0.8Fe eye relief, functions to a critical focal ratio f/6.5, and with my f/7 TEC140APO exhibits very slight outfield astigmatism and slight defocus, but no lateral colour.

The 9.9mm Star Splitter is a more comfortable eyepiece to use compared to the UO HD 9mm, but ironically not the traditional volcano top 9mm standard Ortho (as sold by Edmund Scientific during the 60's and 70's). Both the UO HD 9mm and 9.9mm StarSplitter gave equally well defined axial images.

My personal preference of the three is the UO HD Abbe, but I observe with a driven equatorial with the object centred. The reason I also prefer an equally crisp outfield, is so Saturn can be placed just behind the field stop when looking for Mimas or Enceladus. No need for an occulting bar. In this respect the slightly out of focus field stop might not serve so effectively.

I can see the incentive to opt for the Siebert 60° afov Star Splitter Orthoscopic if one observes with an undriven telescope. However I have to question the rationale of choosing an undriven telescope for hi-res Lunar & Planetary and double star work where high powers are necessary.

The 9.9mm Star Splitter does live up to the maker's claims, except for the outermost field where definition falls off due to slight astigmatism and lateral colour. It is well designed and made, and constitutes excellent value.

How does the 9.9mm Siebert Star Splitter rank against rival Planetary eyepieces?:

Is it the sharpest, and highest quality Orthoscopic? How does it rate with other high end Planetary eyepieces? How does it compare to the Edmund-Rank 2-1 RKE reversed Kellner?

Dealing with these comparisons in reverse order as they function with my TEC140APO (f/7) :-

i) 8mm RKE 2-1 45° afov; 0.9Fe eye relief; no lateral colour; slight outfield astigmatism; slight defocus; no ghosts.

ii-a) 8mm Brandon Orthoscopic 2-2 45° afov; 0.8Fe eye relief; no lateral colour; no astigmatism; flat field; no ghosts.

ii-b) TMB 8mm Super Monocentric 30° afov; 0.8Fe eye relief; no outfield astigmatism; no lateral colour; slight field curvature; no ghosts.

ii-c) 3/8" Monocentric by Ronald N. Irving c 1960 28° afov; 0.6Fe eye relief; no lateral colour, slight outfield astigmatism; very slight outfield defocus; slight ghosting.

ii-d) 9mm TMB/BURGESS Orthoscopic 1-1-2-1 60° afov; 18mm eye relief; very slight lateral colour; very slight outfield astigmatism; no defocus; off axis ghosting.

iii) 9mm Zeiss Abbe Orthoskop 35° afov; 0.8Fe eye relief; no outfield astigmatism; slight defocus; no lateral colour; no ghosts.

My personal preference of these, and the 9mm UO HD Abbe Ortho, taking into account image sharpness and contrast at the field centre and field edge and darkness of field, rank as follows:

1) 9mm Zeiss Abbe Orthoskop (not a/r coated)
2) 9mm UO HD Abbe Ortho (multicoated)
3) 8mm TMB Super Mono (multicoated)
4) 8mm Brandon Orthoscopic (bloomed MgFl2)
5) 9.9mm Siebert Star Splitter Ortho (bloomed MgFl2)
6) 9mm TMB/BURGESS Orthoscopic (multicoated)
7) 8mm RKE (bloomed MgFl2)
8) 3/8" Irving Monocentric (not a/r coated)

but there is little to choose between any of these eyepieces.

Value for money:

If you are looking for a basic set of Orthoscopic eyepieces, there is so little between the performance of the volcano top Orthoscopics and the rest of the pack, that they would serve well. If you can afford a set, then opt for the UO HD's. The TMB/BURGESS & Siebert Star Splitter for Orthoscopic performance across a 60° afov, and of the two, I would say the TMB/BURGESS is better overall, but at slightly greater cost. The least value for money is the Brandon. You're paying for an all American made eyepiece with purer optical glass polished to a higher scratch/dig ratio. Brandons are noted for freedom from off axis ghosts and flaring (something the TMB/BURGESS notably falls down on). However on the range of objects we observed, it was outperformed overall by the UO HD Abbe, which we would say represented the best value for money.

Discussion

There is a growing consensus that apparent field of view is the most important facet of any eyepiece. Amateur astronomers who predominantly observe deep sky objects with a Dobsonian mounted alt-az short focus Newtonian appreciate wider real fields at medium and high powers. It enables them to observe for longer before their undriven telescope needs shoving up-down & sidewards.

It is unfortunate this mindset has become so entrenched it now encompasses all eyepiece preferences and observing subjects. If your observing bias is hi-res work, by which I mean, Lunar, Planetary, Solar and double stars, you should not be using a Dobsonian mount or a short focus Newtonian.

The ideal telescope for hi-res work is either a Cassegrain, Maksutov-Cassegrain, long focus Newtonian (c-o less than 25%), or either a long focus achromatic refractor or a long to medium focus apochromatic refractor. It should be mounted on a substantial equatorial, have slow motions and be driven, preferably in both RA & DEC.

When you have the correct telescope and equatorial mount for hi-res work, at high power you do not need eyepieces with wide apparent fields of view. Eyepieces always perform best on axis (except the Plossl which possesses a 30% zonal correction). If you centre the image, and can keep it centred, all that matters after that is Orthoscopy. The ideal hi-res eyepiece should have a flat, uniform, unaberrated, undistorted field of view, free from internal reflections, scatter and either axial, abaxial or out of field ghosting.

A wide apparent field of view is the least important facet of a hi-res eyepiece. This is just as well because it is optically impossible to design an eyepiece with spherical lens surfaces that is truly Orthoscopic when the apparent field of view exceeds a radian.

There are two types of distortion, angular magnification, and rectilinear. Astronomical eyepieces are designed to give a uniform magnification across their apparent field of view. Binocular and military eyepieces, intended for terrestrial viewing, are designed to keep straight shapes, straight, whether at the field centre or the field edge.

Angular magnification distortion is a function of the tangent of the field radius angle. Rectilinear distortion is a function of the field radius angle itself. Because the tangent of an angle cannot equal the angle itself (the angle expressed in radian measure - 1 rad 57°.3 ), if the lenses have spherical surfaces, it is impossible to correct for both at the same field radius angle.

If you restrict the eyepiece design to spherical surfaced lenses, then the only way to maintain true Orthoscopy (freedom for either angular magnification or rectilinear distortion), is to restrict the apparent field of view to less than a radian.

It matters not how many elements the eyepiece has, how many lens groups, or what type of optical glasses are used. If the lens surfaces are all spherical, distortion will appear when the apparent field is wider than a radian. You either have to correct for angular magnification distortion and put up with rectilinear distortion, or vice versa.

When you correct for either type of distortion, the correction entails an optical trade off. Distortion is measured across the field radius. But an eyepiece designed for an astronomical telescope is by definition supposed to be interchangeable. It is designed to match a flat telescope focal plane. When either type of distortion is minimized, what the designer is doing is forcing the eyepiece's focal surface away from the ideal flat surface, onto a curved surface. In a binocular design this would be the objective's Petzval surface. The designer cannot do this with an astronomical eyepiece because it is intended for use on a wide range of focal ratios and Petzval field curvatures. The designer is obliged to force the focal surface onto either the sagittal or tangential field surface or somewhere in between. The consequence of making this compromise is astigmatism.

Strictly speaking Orthoscopy is correction for the "Offence against the Sine Condition" - OSC. It includes correction for spherical aberration and coma, not astigmatism. An optical system that is Orthoscopic is also aplanatic. An optical system that is both Orthoscopic and free from astigmatism is termed Anastigmatic. However it is generally understood that an Orthoscopic eyepiece is also free from field curvature and the astigmatism associated with field curvature.

If an eyepiece is to combine both Orthoscopy in the accepted sense of the term when applied to eyepiece design, and a wide apparent field of view (wider than a radian) the only option open to the designer is one or more aspheric lens surface.

The way forward lies in the precision manufacture of aspheric surfaces in optical glass, and the technology is becoming well established. Many photographic camera lenses incorporate aspheric elements and have done so for 30 years. (For instance the Canon 55mm f/1.2 S.S.C. c1976)

It is possible to rework existing Orthoscopic designs substituting one or more aspheric surfaces to widen the apparent field of view with no other detrimental effects.

I for one am tired of seeing traditional designs re-branded and touted as something new and wonderful (e.g. "Super Plossl" - read Erfle type II). What is actually required is a fundamental shift in design that involves aspheric elements, or at least one element with one aspheric surface. There are several recent patents registering aspheric design variants of the Abbe style 3-1 Orthoscopic, e.g. US6011655 04JAN2000 MORIYASU KANAI.

Zeiss made a 25mm microscope eyepiece 2-1 aspheric Orthoscopic. It was sold a few years ago remounted in a standard 1.25-inch sleeve by Marcus Ludes, and a fine eyepiece it is too.

How much longer I wonder will it be before aspheric Orthoscopic astronomical eyepieces become the norm rather than the exception?

Chris Lord

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