JAC51

They are indeed excellent telescopes. I have aTMB 130/1200LW version from 2005, lens number 005. It has a draw tube and 2" Feather light focusser which I think is to make it bino friendly.

The contrast as already said is superb , the tube dews up but the lens at the business end never has so far in 10 years of ownership and it seem to have very little in the way of tube currents when cooling. The f9+ focal length is very forgiving of any eyepieces used and the cool time down is no where near as bad as I feared (45min?+) perhaps due to thinner glass  in the more gently curved lenses compared to shorter focal lengths.

Below is things that I have read online and perhaps half remembered , if I'm wrong I apologise and please contact me..

OK4 glass by accounts  I have read is very close to FPL53.

Now I wish I had the link to this but here goes..

Years go I remember reading, I think on APM website that their 6" ED F8 doublet using FPL51 glass was equivalent to an achromat of F30.

They also produced for a short while a 6" LZOS F8 doublet using OK-4 which was equivalent to a F45 achromat.

I think my memory is accurate in which case the above would suggest that OK-4 is optically  superior to FPL51, though of course the mating elements matter a great deal as well.

Now for complete speculation from me. It is possible that FPL51 and OK4 might be equivalent in how easy they are to work . FPL53 is I believe said to quite  soft and difficult to work with, something  I have not heard about OK-4. 

This by the way is my first post after saying hello, please forgive any typos.

John

I have been reading some of the excellent threads on here for years.

Thank you for that now it's about time I took part.

John