One cheap option to consider and I will be able to provide a review shortly to help, is Pentax 67 lenses and a Pentax 67 lens to Canon EOS adapter.
These lens are older but from what I can gather are superb astrophotography performers and nothing like the prices for these L series Canon etc.
They cover a 6 x 7cm film and have a backfocus of 85mm compared to something like 46mm for EF style lenses on APS sized sensors.
So it will not vignette your 5D and hopefully is super sharp.
Here is a site that reviews each of these lenses and rates them to help tell the good ones from the not so good.
I have ordered a 165mm F2.8, 300mm F4. I am trying to pick up a narrow focus lens for super widefield. All are quite "cheap" (less than a Nagler).
http://www.antiquecameras.net/pentax6x7lenses.html
I did a post on the equipment forum giving examples for these lenses using CCD cameras which is my intended use but it seems they can also be used on an EOS Canon or Nikon with an adapter that costs about AUD$60 or so.
There are quite a few examples on the net of these lenses being used for astrophotography.
The 300 F4 67 EDIF lens seems one of the best but I have not been able to find one for sale. I foound a 400mm but it was US$3650.
You'd get a Tak scope before you'd get a lens like that but I imagine the 300mm ED may go for US$1500 or less. They are getting older now.
Greg.