To me, I want to remain light weight and absolute portable, and of course, I want to keep the cost down as well.
Several candidates from my mind:
1. Takahashi FC-100DC, f/7.4 at 2.8kg only
2. Borg 100ED, similar to the above
3. Canon EF 300mm f2.8L (USM), very light and fast too, but double the price of the above
4. Canon FD 300mm f2.8L, cheaper than all the above but have to modify myself
5. Chinese ED refractor, like Skyrover, even cheaper than the above one but far heavier
So, the best balance points are Canon EF 300mm f2.8L without USM (the problem is that Canon is not going to fix them in case of any problem) and the Canon FD 300mm f2.8L which got no problem about maintenance since it has nothing to repair basically.
For the simplest modification, check this out:
After removing the rear mounting assembly which also contains the aperture control, the thread is M77 P0.75 so it's relatively easy to find suitable adapters. And the best length from the M77 thread to the EF adapter seems to be 51mm according to the above site.
To get such an adapter with proper length, we have "大口径 中号调焦筒" from taobao, adapter are available from M62 to M42 featuring adjustable length from 28-62mm, very flexible indeed.
By connecting a M77 to M62 step down ring, the modification is done. Other cheaper adjustable length adapter could be used, they're of smaller aperture.
I hope that it could be used as a small telescope and telephoto lens. Time to search for those online reviews. Tempting.
On the next morning, I searched around my cabinet. The following adapters are useful:
First is a M42 to Canon mount adapter, should be useful if I want to couple the converted lens into a Canon EF lens, of course, manual focus only, no aperture control since it's removed.
Second is a M60 to 1.25" barrel adapter, useful to use the converted lens as a telescope. 1.25" is wide enough for a 300mm lens, 2" is probably over kill. But who don't want wider field?
So after the adjustable tube, I will need stepping ring with M60 male thread to connect an eyepiece adapter, and I will want M42 male thread to conect with a EF lens (mechanically only!)
Reverse macro rings could be used for EF lens coupling too.
And since 1.25" is smaller than 42mm, so it's safe to use a M42 adjustable tube with male thread as the output interface from the lens.
On choosing the adjustable tube, we need to consider the stepping rings.
M77 male from the lens --- adjustable tube -- M42 for EF lens OR M60 for eyepiece
M60 is not very common, so I will need one more ring...
Oh no.... simplifying the whole thing, I have an EF to 1.25" adapter! So, once the lens is adapted as EF lens use, I could use an eyepiece without any other concern:
So the solution boils down to...
M77 male from the lens --- adjustable tube -- M42 for EF lens
So I will need stepping rings from M77 to M42, to connect to a both side M42 adjustable tube and then to the EF lens adapter!
The optical path length of a 1.25" diagonal is like 7-9cm, so even the shortest adapter (i.e. a M77 reverse macro adapter, male-male need to resolve too) won't work. ... need to try!
1. Buy a suitable FD 300mm 2.8, wait for a good deal
2. Buy a M42-M42 adjustable tube, and then output EF mount, to use eyepiece, use existing adapter
Phase Two: buy a reverse 77mm tube to couple with existing eyepiece adapter, to see if I can attached 1.25" eyepiece without barlows....
No no no ....
I found that I could simply do the following:
1. Buy a 77mm female to female adapter (possible two step rings, one step up and one step down again, 82mm seems a good bridge)
2. Buy a 77mm reverse macro ring
So the connection will be like this
M77 male thread from lens ->
M77 female thread from step up ring say to M82 ->
M82 step down ring output M77 female thread -->
Reverse macro ring
So the above coupling will be shortest, and then I could use my existing pure mechanical macro extension tube (combinations) to give around 51mm and then the FD lens will be able to reach focus with EF camera.
To use eyepiece, the extension tube could be removed to give more in-travel to accommodate a diagonal.
Very simple now, just a reverse macro ring plus two stepping rings!
I guess I don't need two M77 ring.... maybe stepping down a bit before going to a reverse adapter a good.
Final solution after looking at those stepping rings:-
1. EOS - M42 (female) adapter
2. 42mm male - 58mm female adapter
3. 58mm male - 77mm female adapter
Should be shortest, cheapest and thinnest with off the shelf components.
I believe that I will need to use a couple of macro extension tubes to create proper flange distance. I really hope that it can reach focus without a barlows, then it will be a great rich field telescope too! In that case, even better than a 300mm f2.8L IS USM!
Guess what, I got one off ebay at less than 1K USD.
Expected delivery on 7th of April... should proceed to order adapters from taobao.