A friend reminded me of the above option, it's a 127mm f/7.5 ED triplet.
Aperture is 127mm and focal length is 952mm.
Dew shield retracted, it is ~840mm long and when extended, it's like 1020mm long, the weight is like 6.5kg, not very heavy considering that it's a triplet. The diameter of the dew shield is slightly more than 150mm, i.e. my Cheapy-ERF will still be large enough, finally at f/7.5, the PST etalon module should be happy with it.
Last but not least, it's pretty inexpensive, and in case there's not enough back focus, I can always ask my local friend to cut it... since it's cheap and so I won't hesitate to cut it. :-)
1. Solar imaging:-
- to be used with the Cheapy-ERF, PST etalon, BF10, this will ensure internal reflection free; since it's also less than 1000mm, the BF10 will still be big enough to cover the full solar disc, maybe good for DSLR at prime focus
- to be used with my Herschel Wedge, for excellent white light observation
- to be used with my Herschel Wedge + Baader CaK, for the best possible CaK
2. Deep sky scanning:-
- to be used with my Unistar Lighter at lower power, with my Pentax 40mm XL, it yields 2.73 degree which is very nice!
3. Planet imaging:-
I guess it's do-able with my LXD55
With the very long tube, I guess I can only hide it behind the door, maybe to put the front inside a big plastic box with some dessicant inside