#1. "RE: Would this work for me?" In response to Reply # 0
Ely, Cambridgeshire, GB
Using a mirror lens on a DSLR can yield good results and it will give you a cheap way into long tele lenses and the techniques needed to use them. There are threads on the MF forum about them.
As far as this one is concerned I have never heard of the make Telesar so cannot comment on this one in particular.
However there is a thread on the MF forum that shows a comparisson between a poor mirror lens and a good one - there is a massive diffrence.
Specific makes to go for would be the Nikon and the Tamron 500mm f8.
I use the Tamron - picked it up for UK£60 so they are very cheap. The shots of the stonchat and gannets in my gallery are taken using this lens on a D50 - fully manual as there are no electronic connections in this lens.
#2. "RE: Would this work for me?" In response to Reply # 0
Livermore, CA, US
If you're looking for a lens to shoot birds in flight, then I'd recommend you say away from manual focus lenses. I expect you'll spend this money now, and very quickly decide to spend more on an AF lens, so it may be cheaper in the long run to buy something you'll be happy with.
Used versions of these lenses might seem like a steal, but be sure to check that a used lens has the built-in motor to allow AF on your D40. There are many older versions of 70-300 zooms that won't AF.
Unfortunately you won't get longer than 300mm and AF on a D40 without spending about $900 new, and it looks like you can find a used Sigma 50-500 for <$700.