Widefield Astrophotography

[pvaz]

I guess it will depend on pixel size. In my 60D (18MP) the 375/focal lenght seams about right on the few tests I've done.

If I use the 600/focal lenght rule, I should be able to get 37s with the Canon 16-35, however I get noticeable trails at 30s.

Anyway I'm no expert, but it seams logical if 2 APC-S sensors have the same physical size, but different MP count, that the one with more pixels will have smaller pixels.

[RikM]

Yes, but the pixels are distributed over a different area - it doesn't matter about the phyiscal size of the individual pixels, only their spacing. Think of it in terms of a star point moving over the surface of the chip - let's say it moves 1mm on a crop camera sensor. To traverse the same number of pixels, the star dot would have to move 1.6mm on the full frame sensor.

The pixel size and spacing is the same, the 5D-II just has more pixels to give a physically bigger chip. If a star moves 1mm, it will traverse 154 pixels on either chip.

[jgs001]

There is another factor to take into account, not just the focal length, although that's a useful guide. Where on the sky you are imaging has an effect on the max useable sub length. It's similar for imaging in alt az. Low in the east and west allows for longer exposure times. Whereas, north and south and up to the zenith reduces the useable sub length before trails set in.

[Ags]

Sorry I thought they had the same MP count. OK, but now display or print an image from the two cameras side by side at the same size: the 5D image effectively has the trails scaled 1.6 times smaller. If you do a 100% zoom, then indeed the trails will be identical. But nobody actually looks at a picture at 100% zoom, except the photographer :-)

[RikM]

I think John makes a very good point. I normally just use the longest time I can get away with by taking test shots of the targets.

[Ags]

Me too!

[JamesF]

The pixel size and spacing is the same, the 5D-II just has more pixels to give a physically bigger chip. If a star moves 1mm, it will traverse 154 pixels on either chip.

That was exactly my thinking, Rik. And with the same lens it wouldn't therefore matter which camera you were using, the star trail would still move the same number of pixels in the same amount of time.

James