There is one issue to keep in mind.  If you use an autofocus teleconverter
on a manual focus (non-electronic) lens, then the TC will close the
microswitch in the EOS mount that tells the camera that a lens is present.
When you take the picture, the camera will hang.  You'll either need a
non-electronic TC, or you have to press the lens release and rotate the TC
slightly toward releasing from the camera to disconnect the switch.  You
might want to use two tripods, one for the lens and one for the camera to
reduce the risk of the lens falling off.  EOS cameras meter nicely in Av and
M modes with manual lenses.

I have used the 2x TC with my F/8 mirror and the results are not great.  The
TC gives you F/16 which causes the viewfinder to not be very bright and the
depth of field is not shallow enough at long distances to judge the plane of
best focus manually very well.  It is easier with larger apertures and long
focal lengths and closer distances where the focal plane is easily visible.
Aside from difficult focussing, the pictures are too soft for almost
anything.  Even 4x6 prints aren't great.   Adding the 2x TC to the mirror
lens doesn't give better results than just cropping 1/2 the picture out.

A mirror lens has the benefit of no chromatic aberation but otherwise I
don't think you will be pleased.  A Nikon-EOS adaptor is fairly expensive
also as is a good quality TC. I am sure that the Nikkor is much better than
the Quantaray mirror that I have but still I would think that the money
would be better put toward a refracting lens.  The Sigma 600mm F/8 mirror is
fairly well liked also and includes a hood and case for $449.
http://www.amazon.com/Sigma-600mm-F8/dp/B000GFZNEW/sr=8-1/qid=1159515434/ref=pd_
bbs_1/102-8780229-4918504?ie=UTF8&s=photo

I suspect that this Samyang/Opteka/Phoenix 650-1300mm F/8-16 lens would get
you better results than a mirror with TC:
http://www.opteka.com/productdetail.asp?productid=4127
It even includes a carrying bag and a T-Mount adaptor.

Some pictures from this lens are here:
http://www.fortunecity.com/greenfield/porton/464/id37.htm

Pictures of it mounted on a 20D are here:
http://photography-on-the.net/forum/showthread.php?t=103319&page=24
Another case where I think two tripods are needed.  There is a couple of
moon pictures and one of Mars taken with this lens.  They didn't put up any
pictures with the 2x TC though.

With a bit of noise reduction (for the high ISO's needed), contrast
adjustment and careful sharpening, you could probably get some ok pictures
from it.

Consider also the Russian manual focus lenses (Kiev has the best prices):
http://www.rugift.com/photocameras/canon_cameras_lenses.htm
http://www.zenit-camera.com/canon_cameras_lenses.htm
http://www.kievcamera.com/cameras.php?cat=14

They have a few mirror lenses for sale including a 500mm f/5.6, 500 f/8,
1000mm f/10.  The Peleng 8mm f/3.5 circular fisheye is a lot of fun too and
a good lens for the price.

With a 500mm F/5.6 mirror, and the M42-EOS adaptor with focus confirmation
($95):
http://www.rugift.com/photocameras/canon-eos-adapter-focus-confirm.htm

you would get the autofocus confirmation light during manual focussing which
may help a lot.  According to Bob Atkins, it works well.
http://www.bobatkins.com/photography/reviews/m42_eos_focusing_adapter.html

If you want the to take the kind of pictures that Roger has on his site,
you'll probably have to invest in a more serious lens.

I posted this earlier:
"You can adapt Nikkor lenses to Canon bodies, and since the adapter
doesn't allow for auto-diaphragm operation and mirrors don't have
auto-diaphragms, this can work well. The older-version 500mm f8
is quite sharp at distances out to infinity, unlike most others - so sharp
that it works well on the Nikon TC14B, giving a good 700mm f11
('course the "real" stop is almost one stop less, as is generally true with
mirrors...). [with some editing...]"
Note that I specify the Nikkor TC14B converter - the Nikkor TC14A,
TC14C, and likely the newer AF 1.4x, do not work as well (someone
else commented about the possibility of an AF converter causing the
Canon body to hang, also...). Unfortunately, The TC14B in nice
codition may cost more than the lens in similar condition...
--
David Ruether
ruether@verizon.net
rpn1@cornell.edu
http://www.ferrario.com/ruether

The best name brand mirror (apart from maybe Zeiss) would be Olympus's
OM 500mm which come up from time to time on Ebay at around $600/ea.
The finest mirror lens of all time was the Questar 700 from Questar
corporation.  The $100-$150 no-names out there are pure garbage.  IMO,
if you must use a mirror lens and you don't anticipate needing high
speed results and if you can use a tripod, would be a good telescope,
like a Meade ETX optical tube assembly.   However, they are slow lenses.

Peter,
That's great.  When I got my 500, it was a "life changing" lens
because it opened up so many new possibilities.

One thing I found with such a lens, is that you need very
good tripod support.  I had bogen 3021 tripods.  I even modified
them to reduce vibration.  Not good enough.  Vibrations
reduced image sharpness (why use such a lens if you can't
get sharp pictures?).  I found, as many others before me
have, that you need carbon fiber.  The lowest end carbon fiber
tripod for the 500 is the Gitzo 1228 (about $600).  Then I found a ball
head or pano head can work but I quickly tired holding the
lens up, and tracking action was very difficult without
the lens falling over.  So, as others before me found, the choice
for such a large lens is the full Wimberly head (another $600).
Then add in plates for a couple hundred more for total
(minimum) for supporting the lens of about $1400.
(prices may be a little off; I bought this stuff years ago.)
Oh, and then you need a large photo backpack to carry it in,
another couple hundred.
Sorry for the bad news.  But after getting over the sticker shock,
I have never regretted it spending the money.  Best investment
in photo gear I ever made.   (There is a reason why on a wildlife
photo shoot, you encounter a lot of professionals and retired
professionals, like doctors and dentists: they have the money for
this stuff.)  ;-)

Roger
Photos at: http://www.clarkvision.com

Peter,
Are you getting an IS lens, or older non-IS?  I would not
recommend non IS in lenses this size.  Professional
wildlife photographers have switched from Nikon to canon
because Nikon does not have super telephotos with
VR (Nikon's IS).  The IS is critical for sharp
images when following action.  You can get good
images without IS, but it is much harder.

Roger