"TMB Designed" 8mm

[gooseholla]

Hi

Just purchased one of these of astro buy and sell for £18. Are they any good? And how exactly are they "designed"?

Thanks

John

[John]

The original of this type of eyepiece was designed by the late Thomas Back of TMB and manufactured and marketed in partnership with Bill Burgess of Burgess optical. It was therefore known as the Burgess / TMB Planetary. Since then there have been a Mk II version which is sanctioned by TMB and numerous clones which are not. There have been some quite lengthy threads on this on the forum - try the "search" facility to find them.

Most of the ones sold now are clones of the Mk II. People seem to like them well enough so your investment is probably OK.

[cs1cjc]

And how exactly are they "designed"?

Thomas Back wrote this about his 2005 design:

> This is the first wide angle, long eye relief eyepiece that has the sharpness,

> contrast, and lack of scatter that the best orthos have, but without the limited

> eye relief, edge correction, [...] that planetary orthos have.

The eyepieces generally have 5 lenses in three groups. The field group is a negative doublet (three lenses in two groups in the 9mm) Smyth group acting like a barlow which is different for each focal length. The remaining lenses form a reverse Kellner, a doublet followed by a singleton eye lens, which are identical for all the focal lengths up to 9mm. The overall eyepiece was designed to work well at focal speeds at fast as F/4 with the field group compensating for the limitations of the reverse Kellner.

Note that in some versions, the 9mm has a doublet Smyth lens group like other focal lengths. The additional 15mm, 20mm and 25mm in some sets follow a similar design pattern, but without interchangeable parts.

[Karen]

I have a TMB "clone". I can't fault it. On doing some research it seems these are made by Lacerta Optical of Austria at a factory in Hungary.

[John]

I think Lacerta is just another re-branding to be honest. Telescope Services in Germany do the same thing. There seem to be a number of Chinese manufacturers now making this pattern and the mk II one as well. If you ordered enough you could have your own brand name etched on them !.

Nothing wrong with this - it's the same with many eyepiece types nowadays including the UWAN / Nirvana's, the Extra Flat's and a number of of others.

[johninderby]

If you ordered enough you could have your own brand name etched on them !

I think you need to order a minimum of 100.

John

[gooseholla]

Well, I used it last night. Is a brilliant eyepiece compared to say my 9mm gso plossl. Don't know how it compares to a BST or TMB - I suspect there are other threads that have dealt with that.

[Capricorn]

I know that GSO's have a good name but the TMB's and BST's surpass them so for the extra money they are worth considering.

You just need to know what you want/need, and there are so many out there.