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BO/TMB Planetary Series Comparison


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BO/TMB Planetary Series comparison test

BO/TMB Planetary Series comparison test
on TEC140APO f/7 Fl 980mm

Burgess Optical purchased through SCS Astro - £69.

Eyepieces compared:

  • BO/TMB 9mm x 60º Planetary Series 2-2-2 wide angle x109 33'arc fov Er 16mm
  • E-Z VIEW 9.5mm x 50º 2-1-2 compensating - APOGEE x103 29'arc fov Er 8mm
  • HD ABBE 9mm x 40º 3-1 Orthoscopic - UNIVERSITY OPTICS x109 22'arc fov Er 7.5mm
  • BRANDON 8mm x 45º 2-2 Orthoscopic - VERNONSCOPE x123 22'arc fov Er 6.8mm
  • TMB SUPER-MONO 8mm x 32º -3- triplet Monocentric - APM x123 16'arc fov Er 6.8mm

 

Introduction:

Thomas Back has introduced a wide angle "Planetary Series" eyepiece as a bridge between his discontinued triplet Monocentric and ultra-wide angle types. It is a 2-2-2 design, having 16mm eye relief (to be increased to 20mm in the next production run) and a 60º apparent field of view (afov). The eyepiece barrel has a retractable eyecup with a folding rim. To decrease eye clearance the eyecup is twisted anticlockwise. Eye clearance is adjustable between 15mm (eyecup fully home with rim folded down) and 1mm (eyecup fully retracted with rim flipped up).  All 6 air-glass surfaces are anti-reflection (a/r) multicoated, and to judge by the greenish caste to the reflection it appears to be a double layer "quarter-half" broad band a/r coating.

The body is black hard anodized with a grip and the focal length in bold silver making it easy to distinguish in subdued light and red safelight. The barrel's internal wall is chased and the retaining rings are matt black. The lens assembly is held in a separate cylindrical sleeve. The field stop at the end of the lens assembly is so placed within the barrel, as to prevent light reflected off the filter thread from entering the lens train. The eye lens is surrounded by a dead flat black retaining ring. The 1.25-inch sleeve is hard nickel-chrome finished with a recessed shoulder. The filter thread has a matt black finish.

 

The eyepiece is fitted with caps, and is supplied in a robust cardboard box.

Daylight visual inspection:

The eye can be readily located at the eye point because of the adjustable eyecup. Eye point location proved critical. The field stop can be seen in its entirety only when the eye is located precisely at the eye point. There is little margin for error. The field stop was fringed with greenish/cyan light when held up to daylight, indicating slight over-correction. When I moved my eye slightly off axis I could see a tell-tale 180º bright thin arc just outside the afov, caused by light reflecting off the end face of the 1.25-inch hard chromed sleeve.

Observational Test

TEC140APO 07DEC2005 17h:30m to 19h:30m UT Seeing III-IV (Antoniadi) Excellent Transparency, no wind.

MOON at 1st Quarter, Mars and M45.

 

I loaded all five eyepieces into a TEC 5 port turret. I never wear my spectacles when observing. With the eyecup fully home (as is supplied) and the rim flipped up, I could position my eye comfortably but there was still a small clearance. I prefer to have the eyecup nestled firmly into my eye socket. I find it helps fix my eye at the Ramsden Disc, and also prevents stray light shining across my line of vision. I regard these conditions very important because when met, it is far easier to continually fixate on the object of interest. By retracting the eyecup 2mm (half-turn a/cw) I could obtain a hard seat between the rim of the eyecup and my eye socket.

I began by looking at the First Quarter Moon. The Triesnecker rhille system was well placed just within the terminator. The 33'arc fov just accommodated the whole disc nicely, with the terminator running through the field centre. The limb, adjacent to the field stop exhibited a thin greenish/cyan fringe.

Provided the image was centred and all contained within the fov, contrast and sharpness were very good, and the outfield dark. With the sunlit hemisphere placed behind the field stop so as to observe Earthshine, contrast was poor and the field milky due to flaring. Flaring could be reduced by shifting my line of vision, effectively looking obliquely to the optical axis. Even so contrast in this condition was noticeably poorer than the other dedicated planetary eyepieces.

I then examined a faint star about 1/4º from the bright limb, again with the Moon placed just outside the fov. Flaring produced a milky field and poor image contrast, hindering my attempts to find the faint star. Contrast was improved marginally by skewing the direction of vision off the optical axis.

On Mars and the brighter stars in M45, lateral colour (over-corrected, red outwards) began @ 20º field radius, accompanied by slight defocus, and slight tangential astigmatism at the field boundary. So although the afov is 60º, only 40º of it is useable for detecting the finest and lowest contrast Lunar or planetary detail. Contrast was very good, star images fairly crisp, with some slight hint of narrow angle scatter. There was no ghosting or internal reflections, even when Mars or one of the Pleiads was placed immediately behind the field stop.

The 9.5mm E-Z View also exhibited lateral colour, beyond 15º field radius; stronger over-correction than the BO/TMB 9mm. However it and the other eyepieces all rendered noticeably better contrast, darker fields and no flaring. Earthshine on the 1st Quarter Moon, and the faint star off the bright limb, could be seen readily, regardless of the placement of the bright hemisphere within or without the fov. Detail in the Triesnecker rhille system was equally good in all the eyepieces, when it was placed in the centre of the field. Overall I reckon the Brandon had the edge over the UO HD Abbe and the TMB Mono, but it was a marginal call.

I like the design of the barrel, apart from the hard nickel chromed 1.25-inch sleeve. The end face reflects light into the fov when bright objects are just outside it. It would have been better to have an all hard black anodized finish, and a matt end face as well as filter thread. The sleeve also has the now almost ubiquitous recessed shoulder to prevent the eyepiece slipping out if the lock screw loosens. However if your telescope's rackmount has a lock ring, the reduced diameter prevents the lock ring being fully tightened, and if it has a collet, the shoulder is superfluous. I have had to modify my TEC 5 port turret so the lock rings will grip a shouldered sleeve. The snag in doing this is that it causes the lock rings to close up, and when the lock screw is undone, the lock ring does not fully expand past the shoulder and the eyepiece sometimes gets jammed. An example of an idea that looks good on paper but causes more problems than it was supposed to address.

Is a 60º afov needed to observe a planet? I have been observing the planets since the late 1960's, and fail to see why a wide angle eyepiece is advantageous. The Abbe & Brandon Orthoscopics and the TMB Monocentric performed better on axis, and off axis. Given that only 40º of the 60º afov was useable (in terms of sharpness), even though the field fully accommodated the Moon at x109, only the central two-thirds was crisp.

Personally I prefer the TMB Monocentric and Brandon Orthoscopic. They are both superb planetary eyepieces with comfortable working eye clearance (provided you do not wear spectacles). Faint stars and isolated mountain peaks lying just beyond the Lunar terminator were noticeably more point like.

What is evident is that the optical and mechanical design is well thought out, but the optics are a compromise. However at only $99 / £69 this eyepiece is good value. The TMB Super Monocentric and the Brandon Orthoscopic cost about $225. The UO HD Abbe though is better than the BO/TMB Planetary Series in my opinion; not in terms of eye relief or afov, but where it matters, in terms of image contrast, freedom from flaring, and darkness of field, and it costs only $80.

 




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