what lens for a canon 400d

im seriously conisdering a canon 400D, hopefully before the eclipse this month. I want to try piggybacking it to try some basic deep sky so im wondering what is the minium focal length of a lens i should be looking for? I know the camera comes with a 17-55mm. Argos have a 90-300mm canon lens for about 200 quid. is 300mm enough, or should i be thinking more in the region of 500mm?

Well, how deep do you want to go? Generally, using this example, If you want to get the orion nebula and it's surounding area, a 100mm lens will do fine, but remember, that will be 160mm using the 400D as the sensor size on the canon dslr's are 1.6 times smaller than an old 35mm film frame.

Also, what is *very* important is your ability to track on a motorised mount, the longer the focal length, the more critical your tracking will have to become.

Keith..

but remember, that will be 160mm using the 400D as the sensor size on the canon dslr's are 1.6 times smaller than an old 35mm film frame.

Keith..

Im aware of the crop factor/ Focal length multiplier, but if im buying a lens designed for a digital camera(not an older lens designed for 35mm camera) with a FLM of 1.6 like the canon 400d, thought the focal length written on the side was showing the equivalent of a 35mm camera, so , for example, even though a lens is actually only a 250mm lens at the telephoto end, it is the equivalent of a 400mm lens on a 35mm camera, and 400mm is written on the lens.

So, I would only have to take the 1.6 rule into account if i was using older lenses designed for 35mm cameras. is this an incorrect assumption?

I had the Canon 90-300mm EF - it's OK but nothing to write home about. I recently traded it in for a Sigma 70-300mm APO DG (Macro) F4-5.6. Now this is a nice lens for the price (about €210 from the Camera Centre in the Square). The focal length range of this lens is ideal for wide field deep sky stuff and at 300mm you'll get a fairly decent size moon.
A 500mm would be great for moon shots but again longer focal lengths mean better tracking is required (though for short exposure on the moon this is not a big issue). Above 300mm the price jumps exponentially, especially if you start looking for small F numbers (a 500mm F2.8 will cost in the thousands).

cool. I was looking at wellbuttie's horsehead nebula with a 500mm lens. thats where i picked 500mm from.

(see thread www.irishastronomy.org/boards/viewtopic.php?p=62131#62131 )

for astrophotography, do you want high f values ( smaller apature) or low f Values? I know higher f value means smaller apature and better dof, but less light so longer exposures. what is generally the rule of thumb for astronomy?

So, I would only have to take the 1.6 rule into account if i was using older lenses designed for 35mm cameras. is this an incorrect assumption?

Incorrect! The C size sensor in the 400D etc is smaller than a full size sensor of a 35mm by a factor of 1.6. Therefore it has the effect of magnifying by a factor of 1.6 any lens used. Therefore a 300mm lens will be the equivalent of a 480mm when used on any SLR with a C size sensor.