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AndyPandy

"TMB Design" versus "TMB Planetary II Clones"

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Apologies. I have taken this thread off topic, because I am concerned that that people think the clones are somehow similar to the authorised TMB Planetary eyepieces. There follows a short comparison that I wrote last year comparing the TMB clone (called TMB in the following because that is what I then believed it to be), with the "TMB Design" which better addresses the point in the orignal post.

I have in front of me two sets of 4mm and 9mm eyepieces, one set marked TMB Optical Planetary II SW and the other marked 1.25"-58 degree wide angle (apparently manufactured by Barsta and commonly called TMB designed) eyepieces. See respectively:

http://www.astronomi...ics&keyword=tmb

1.25" & 2" Eyepieces

The design of all these eyepieces is with a Konig eye set (see: Eyepiece - Wikipedia, the free encyclopedia) which has three lenses in two groups. This is identical for all focal lengths. The field set is different for the different focal lengths and is generally a cemented doublet. In this format, 5 lenses in 3 groups, it is just about the minimum number of lenses for a well corrected wide angle eyepiece with good eye relief (of around 12mm). Thomas Back wrote that his design works well with telescopes as fast as f4.

Taking the two 4mm eyepieces first, these are apparently the same optical design, having very similar dimensions, but they differ enough to suggest that they come from different production lines. For example the eye lens in the TMB is slight larger, there are mechanical differences and the lens coatings are different with the TMB appearing to be green and the Barsta, magenta.

The two 9mm eyepieces are not to the same optical design. On the TMB 9mm, there are apparently three lenses in two groups in the field set. In the Barsta 9mm the field lens is a bi-concave doublet, the eyepiece is shorter and has a different focal point (the ranges in general are not parfocal).

Physically there are things I like and dislike about both designs. The Barsta design feels more robust and the screw up top has a nicer motion than the TMB. However the Barsta has a chrome barrel with a heavy undercut, whereas the TMB has a, preferable, black satin barrel and undercut with a shoulder. Interior blackening is not perfect in either set of lenses, particularly when compared with (more expensive) Vixen NLV eyepieces.

In use, both sets have sharply defined field stops and give sharp flat images. The TMB eyepieces have less glow round bright objects and consequently show more contrast, though the 9mm shows less contrast than an Antares Ortho (similar to BGO). The large eye lens can show glint (a problem for me observing with spectacles) and this is more pronounced in the Barsta than the TMB. Overall I prefer the TMB models.

Edited by cs1cjc, 15 July 2011 - 08:52 AM.

Despite this positive review, I became unhappy about various issues with the clones. I have four but they are little used now being replaced by ES82s and a 1.5X barlow element.

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Well, all I can say is I have what i suspect is a clone of the 6mm TMB Planetary II Series, despite of this it's an excellent eyepiece! I was able to put 208x on my 3in f/16 refractor which means x70/in and obtained astounding images of Jupiter disc, full of details I'd never seen with my 9mm Celestron Omni Plössl. So I certainly recomend thses planetary eyepieces. At first the long eye relief is somewhat annoying but you have to adjsut the plastic cup distance rotating the barrel. Ove a comfortable distance is obtained it's a superb eyepiece! Tested on the Moon and also gave superb images. Double satrs are a real pleasure too! Very warm colours on Castor, liked it!

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I don't really want to get embroiled in the argument about TMB clones etc, but I believe this a TMB planetary, but I don't know where it fits in the scheme of things. I am a bit disappointed with it for picking out detail in dso's and separating doubles, but don't really know what to expect. To get any appreciable improvement would I be better off upgrading this 8mm, or complementing it with a BST 12mm. The other e.p I use most is the Meade 5000 26mm.

Thanks Jason

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Here's an image of my "TMB" Planetary  II Series. No TMB logo, just 6mm, so it's a clone... Paid about US$72 here in Brazil, but you have to consider it has almost 100% import taxes ( ohhh blessed government we have...) so I believe its real cost was about US$35 or less. Anyway it performs extremly well. As I've said, astounding images of Jupiter with my old (1970?) 3-in f/16 refractor. Nothing to complain about.
ibsg.jpg

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