BO/TMB Planetary Series comparison test


BO-TMB-review



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 1-1-2-1 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 1-1-2-1 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 8 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 inwards) 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.

Subsequent to my posting this comparison test report, I received the following commendation in an e-mail advertisement from APM on 19th January 2006:


Question APM to TMB at 16. November 2005:
Thomas,
To be honest, I did not followed any review of that eyepieces yet, so maybe you can describe me shortly what is the advantage of that eyepieces and how do they compare quality wise against other high end eyepieces?


Answer TMB to APM at 16. November 2005:
Markus,
They are very comfortable, with 16mm of eye relief in all focal lengths. The axis on sharpness is as high as the best ortho, because I designed the spots sizes at f/4 to be 1/3 the size of the Airy disk. At f/7, it is about 10 times smaller than the Airy disk. There is no lateral color, like other high end eyepieces have, and very low distortion. Even the edge of the field is sharp at the 60 degree field stop. The scatter, with the new baffle, is quite low, like a good ortho. Not as good as the Super Monos for sure, but for the low price an eyepiece, they are the best eyepieces on the market, and in fact, are now the largest selling eyepiece in the US right now, more then even any other high end eyepiece. We are coming out with a 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, and 9mm. Then 2" versions.



Which clearly flies in the face of my findings, for the 9mm at any rate. To be charitable perhaps the out-field performance is better on the TMB 100mm f/8 SD apo. But it is my judgement that they are by no means the best eyepieces on the market in their class.

The previous week SCS Astro sent a replacement lens retaining ring with a chased internal diameter, with instructions as to how to remove the existing one (located behind the Smyth converter) and replace it. It was suggested that the ring be removed with a screw driver, rather than a ring spanner, which I fortunately possessed. These retaining rings were issued to all purchasers of the BO/TMB Planetary Series eyepieces, delivered prior to 16th December 2005. However even with the substitute lens retaining ring there was still that tell-tale thin 180° arc of bright ligth when the eyepiece was held uo to the daylight sky. The flaring I noted will not be elliminated by the chased (and hence flattened) lens retaining ring because it is caused by reflection off the nickel-chromed end face to the 1.25-inch sleeve.

I received the follwoing post on Cloudy Nights:

A thorough inspection of my 9mm shows no possible involvement of the chrome "sleeve". Perhaps you can provide a diagram that shows how you think this might be taking place.

I see five POTENTIAL sources of glare in the 9mm. The design has a 2-element air-spaced field group followed by a 2-1 positive assembly. It is thus a 1-1-2-1 design (I have disassembled it to determine this).

1. The most significant observed glare is the confirmed result of low-angle specular reflection off of the smooth field group retaining ring. The new retaining ring is effective in reducing this to the point that it is reasonable to say that it is elminated.

2. A second potential source of glare is the narrow spacer ring in the field group.

3. The third potential source of glare is related to the spacer. The eye-facing surface of the field lens is concave and flat ground on the inside edge to abut the spacer ring. The unpolished and uncoated glass on this flat-ground lip "lights up" when illuminated, and the spacer ring doesn't mask it off entirely. This is the primary source of what appears to be a bright specular reflection around the exit pupil that one sees when holding up the eyepiece at a distance from the eye and looking through it in a well-lit room.

4. The outside flange of the field group is slightly beveled but appears to have about 0.75mm wide surface parallel to the optical axis. Although a knife-edge is probably not practical here, the best-baffled negative-positive designs reduce this to somewhat less. I don't think this is a significant source of glare, but if you wanted to make everything as good as it could possibly be, you'd make this surface more narrow.

5. Lens edges in the field group. Other than the specific surface noted in #3, I don't think this is significant.

What one wants to see is the exit pupil surrounded by blackness. Because of sources 2 and 3, one sees what appears to be a specular reflection surrounding the exit pupil. Once the glare from source 1 was corrected, this proved to be of small significance in my testing, but I was able to make an incremental improvement by opaquing the flat-ground eye-facing edge of the field singlet. I cannot recommend this activity since it was about the hardest thing I have ever done to an eyepiece to blacken this and also leave the useful surface completely clean for reassembly.

AFAIK, the 4mm and 5mm have cemented field groups, so the baffling situation is much simpler there. I don't know about the "garage-sale" 6mm.

Mike Hosea

To which I responded:

Mike,

Thank you for your input on this. I erroneously judged it to be a 2-2-2 design as you know. A 1-1-2-1 design is not good for a planetary eyepiece, having 8 air-glass surfaces. I came to my conclusion by examing reflections of a light bulb off the lenses.

When I substituted the lens retaining ring, I made a note beforehand to check the 180° arc of light visible when the eyepiece is held up to the eye and the open sky observed, and then compared it afterwards with the chased retaining ring fitted. I could see no substantial difference except the bright arc of light became a series of thin concentric arcs! Presumably reflections off the crests of the chasings.

The inside of the 1.25-inch nickel-chromed sleeve is threaded and flattened, but the filter thread has a chamfer, which is not flattened. It is my conclusion that when the Moon is placed behind the field stop (just), light is reflected off the inside edge of the chamfer onto the field lens cell and then into the lens array.

I would not dispute your findings in any way, other than your conclusion on POTENTIAL flare, item 1, for the afforementioned reason.

When I fit a flock tube to the inside diameter of the 1.25-inch sleeve, with its lip protruding slightly forward of the sleeve's end face, the thin bright arc is no longer present.

If you take a line from the lip of the filter thread chamfer to the front edge of the field lens it does not seem possible for light reflected off the chamfer to produce flaring. What I suspect is occuring is that when the Moon is deliberately placed 1/2° or thereabouts, off-axis, the out-of-focus image is reflected off the inner crest of the chamfer, on the opposite side, and from there into the field lens, from where it is imaged as an out-of-focus diffuse glow.

When I observed the Moon in my preliminary comparison test, this glow was quite bright. Sufficiently bright to mask Earthshine, or make it possible to acquire a faint star near the bright limb (unless my eye was skewed off axis).

After I substituted the replacement lens retaining ring, and observed the waning crescent Moon earlier this week, there was no significant improvement.

In my opinion it would have been better to make the diameter of the field lens stage bigger than the optical design called for, which would put the inside diameter of the lens retaining ring well outside the lightpath, and effectively occulted by the field stop.

Flattening the filter thread chamfer with matt black made no difference to the bright arc, or the bright ring surrounding the exit pupil.

What I did notice, after Mike Hosea pointed it out to me, is that when the field lens group located within the 1.25-inch sleeve is removed and held up at arm's length to a bright light, with the eye surface facing, as Mike said, the rear ground seating face of the lens, facing the eye, "lights up". With the field lens group removed, when you look into the 2-1 eyepiece section from the eye surface, held at arm's length, there is no bright ring surrounding the exit pupil.

I tried fitting a thin flock ring to the inside diameter of the retaining ring, thinking it would both more effectively flatten the ring i.d. and block the light reflected off the ground seating face. But, it made little difference. There was still a bright ring surrounding the exit pupil.

BOTMB9mm.jpg
With the eyecup fully withdrawn, the 180° arc cannot be seen when the eye is placed at the eye cup. However when it is screwed home, as it would be with a spectacle wearer, it can still be seen.

This is clearly a mechanical design fault with this particular eyepiece.

Then there followed this amazing example of censorship by a Cloudy Nights moderator:

I'm sending this PM to you to let you know that I have gone ahead and edited your post to remove the quotes that you had in there to TMB/APM and some of the surrounding text.

BO/TMB Planetary Series

Thanks for your contributions to the thread. I realize that your post was intended to be helpful, and was done with the best of intent. Please refer to the Terms of Service on the use of quotes and e-mails if you are interested.

Clear Skies, and thanks in advance for your understanding!

To which I responded:

Since the information was sent to me by Markus Ludes, and it was the stated opinion of Thomas Back, which I am sure he will stand by, I see no justification for your censorship.

There is clearly a problem with this particular eyepiece, as Mike Hosea concurrs, if not for precisely the same reasons.

May I suggest you obtain one of these 9mm BO/TMB eyepieces and star test it. Thomas Back's claim that the images are tack sharp across the fov and free from lateral colour in my opinion are open to question. They certainly fly in the face of my findings. It may be the problems I noted are peculiar to the 9mm, or that they miraculously vanish on a TMB 100SD f/8 semi-apo. Nevertheless, sweeping the issue under the carpet through a misplaced sense of fairness to all, is not going to help the manufacturer deal with the situation effectively.

I also contacted Thomas Back and had the following exchange:

Dear Thomas,
On Jan 28, 2006, at 17:31, TMBoptical@aol.com wrote:
Dear Chris,
I recently purchased a 9mm Burgess Optical/TMB Planetary Series eyepiece.

I would appreciate it if you would take the time to read a comparison
test I conducted, published on my website:

http://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/BO-TMB- review.html


Yes, I read your review. One issue I really take is that the Brandon's show finer detail on the Moon, than the TMB Super Monos.

Well I did add it was a marginal call.

There remain problems with this eyepiece that defy resolution by an amateur telescope maker without risking damaging it. Do you intend modifying the mechanical / optical design of the 9mm Planetary Series eyepiece to effectively elliminate the bright ring around the exit pupil and the bright 180 arc visible at the eye point when the eye is displaced slightly off-axis; one or both of which seem to be the cause of flaring when the Moon is located behind the field stop so as to observe Earthshine? If so I would appreciate being sent a replacement.

First, I have never tested a 9mm Planetary as of yet. When I wrote the post, I only had the 4mm and 6mm, with the new retaining rings, and the improved coatings. This is due to the fact that Bill Burgess is in complete control of the production, and sales of the eyepieces, and at the time, he did not send me a 9mm. In fact, just two days ago, he called me, and he promised to send next day air, a new 9mm eyepiece. He did not carry through on his promise.

I have all the focal lengths from 2.5mm, 4mm, 5mm, 6mm, 7mm, 8mm and 9mm on my optical software, fully optimized, and raytraced so there would not be any of the reflection problems that you are seeing in the 9mm. The 4mm and new 6mm, are almost totally free from all the aberrations that you speak of, that are in the 9mm, per your tests. I fully stand behind my statements about performance, but again, I only have the 4mm and 6mm to make these judgments from.

The manufacturer must have changed the design I sent to them, if you are seeing these problems, or you have received a defective unit. It is quite possible, as I have not heard from anyone that owns the 9mm, to have the problems you are seeing. I will talk to Bill Burgess, and have him read your review, and again, try to get my hands on one or more of the 9mm Planetary eyepieces, and contact the manufacturer to make sure that future production is made to my specifications, if I find the same results that you have -- which may not be the case.


If you read Mike Hosea's comments about my 9mm comparison test, it is apparent his 9mm exhibits the same problems. The attached jpeg shows the bright ring around the exit pupil which appears to be produced by the ground seating face on the eyeward surface of the second lens in the 1-1 negative group. I failed to notice this at first, or to be correct I noticed it but it didn't register. Now that Mike Hosea has pointed it out to me, I can understand the point he is making (item 1 in his comments). I have never seen this in any other eyepiece. I tried fitting a thin flock ring to the id of the replacement retaining ring thinking it would also mask the ground surface of the second lens, but it didn't, and in the end made no difference. Flatting the chamfer on the filter thread made a slight difference to the flaring problem, but most of the scattered light visible when the Moon is placed just outside the fov would seem to come from the ground surface of the second lens' seating face. The significant thing is that when I look into the eyepiece with the 1-1 negative group removed, there is no bright ring surrounding the exit pupil (although it fills the eye lens).

As Burgess Optical is the seller of the BO/TMB Planetary eyepieces, when and if there is a replacement (and remember, there is a no charge refund policy on these eyepieces, for the first buyer, so you can always send them back to Burgess Optical for a full refund), Burgess Optical will ship it out to you.

I purchased mine from SCS Astro in the UK. I assume the same conditions apply.

Thank you for your e-mail.

Best Regards,

Thomas M. Back
Owner and Designer

TMB Optical
P.O. Box 44331
Cleveland, OH 44144
(216) 524-1107



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