[Ian Robinson]

I looked at the 400D and 40D , and considered the 450D , decided on a 40D because :

- better build quality than 400D and 450D (Mg Alloy & Stainless Steel vs Stainless Steel & Engineering Plastic) , feels more solid too.
- better snr in chip and wider ISO range (3200 vs 1600 for 400D and 450D)
- I can change the view screens ( I will install the precision focusing screen )
- I don't think an extra 2Mpix I could get from the 450D was worth sacrificing the extras in the 40D
- I can lock up the mirror and have live view if I want it
- integrated cleaning system
- accepts a wider range of Canon and aftermarket camera accessories than the other.
- better shutter.

I think of the 3 , the 40D is the better camera for MY purposes (astro , scientific / engineering , and general holiday and terrestrial photography).

I've also decided while I'm at it to get :
- angle viewer C (focusing microscope)
- body armour (to protect that investment)
- all three available focusing screens
- a 2nd (spare) battery
- a car adapter cable (those long exposures will be rough on the Li-ion batteries)
- 4 Gbyte memory card
- TC 80N3 remote (better than the RC 80N3) has self timer, interval timer, long exposure, exposure counting ... not cheap !!! .... YOU NEED A REMOTE (is indispensible in astrophotography !!!!!!)
- EFS 17-85mm f4 - 5.6 IS USM zoom lens (will be great for general photography)
- EF 70-200mm f2.8 L IS USM zoom lens (will be good for general and low light engineering and astro photography) ... not cheap !!! I was tempted by an even bigger one but that was super expensive (a similar sized APO with telecompressor/field flattener would be cheaper ... maybe in 3 or 4 years when I've saved a few more pennies.

That will keep me happy for a while I think.
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I might get a reconditioned secondhand 350D or 400D body and mod it for astrophotography later or just go for a real ccd astrocamera with 8 or 10 Mpix LATER .