This is really nice.
So what I understand is
1. the aperture should be at least f/4 if not larger
2. Should be away from city lights and a moonless night
3. 25s (or max 30s) exposure
4. Widest focal length possible.
5. You must stack the images. (Is this really necessary?)
Is 40mm f/2.8 or 50mm f/1.8 preferred to 24-105 f4?
If your camera is on a stationary tripod, then the stars will elongate due to the rotation of the Earth. This is
(a) why you want to collect as much light as you can in a short amount of time (and that's why low focal ratio lenses are a huge benefit)
(b) you want to use a very short focal length (ultra-wide angle) because it takes longer to notice the rotation at very wide angles.
As for the exposure length, divide the focal length of your lens into 600 if using a full-frame camera or divide 600 by your crop factor and divide into that (you could also just multiply your focal length by the crop-factor of your sensor and divide that result into 600. That gives you the number of seconds you can likely expose without the stars appearing to elongate in your images and grow star trails.
Both 40 and 50mm focal lengths would be very long if you are on a stationary tripod. E.g. even if you had a full-frame body with a 40mm lens then you'd only get 15 seconds. On an APS-C crop-frame body it's just shy of 10 seconds. That's not very long so you'd need to crank the ISO up (and that means more noise.)
If, on the other hand, you had a 10mm lens... or maybe the Rokinon 14mm f/2.8... a 10mm on an APS-C body is 37 seconds (you might be able to push that to 40 if you wanted) or the 14mm would get you about 27 seconds (and you might be able to push that to 30). Some imagers are bit more conservative round down just to make sure they don't have any elongation on the stars.
If you have a tracking head, you can go as long as you want BUT... if there's landscape in the image then the land is going to blur. Also if you have a tracking head you can take lots of images and that allows you to stack. Stacking has lots of advantages w.r.t. to noise suppression and also the ability to suppress things that only show up in one frame (satellite trails, aircraft trails, etc. can all disappear if you take enough frames and then use a sigma clipping algorithm to stack because the pixels where the trails show up become outliers and the stacking algorithm is smart enough to realize that it should reject outliers.)
If you want a landscape shot with Milky Way then you're better off looking for a very short focal ratio lens. I use a Canon EF 14mm f/2.8L IS II... but that's an expensive lens. The Rokinon 14mm f/2.8 is MUCH more affordable (I think the Rokinon, Samyang, and Bower 14mm f/2.8 lenses are actually all the same lens with different branding.)
This is what happens to the foreground when using a tracking head on a long exposure (I thought I had posted this previously, but now that I'm looking for it, I can't find it):
The camera is angled because this was just a test (I didn't care about the land) and I'm framing for the Orion constellation in the center of the frame (I switched to a 135mm f/2 lens to get much greater detail in that area after aligning the head.)
This was an 8 minute exposure at f/10 but I am using an f/2.8 lens. Had I shot this at f/2.8 it would only have needed to be 40 seconds long to get the equivalent exposure.
I took this shot in January in Science City on Haleakala in Maui. BTW, the observatory you see in the foreground (blurred) is this one: http://www.universetoday.com …es-atop-hawaiian-volcano/ The observatory is still under construction at the time of this image.
The star just above the observatory dome is Sirius (in Canis Major - the "Big Dog")... apart from the Sun that's the brightest star in the sky. It's not technically a particularly bright star ... it's just a little brighter than average but it is particularly close (only about 8 light years away) so it "seems" really bright to us. In the bottom center of frame is Procyon (in Canis Minor - the "Little Dog"). In the extreme lower left corner are Pollux and Castor (the Gemini twins). Pollux is the lower and slightly yellower of the two. Castor is higher but farther left in the frame and it's also slightly bluer. Near the top edge (left half of the image) is Aledberan (in Taurus) - the red angry-eye of Taurus the bull. Roughly near the center, the bright yellowish star is Betelgeuse (in Orion) - a red-giant star that is expected to go supernova soon (likely within the next thousand years). In this image Orion is laying on it's side. To the right you can see the three stars of Orion's belt (from bottom-to-top they are Alnitak, Alnilam, and Mintaka) and Orion's "sword" region where the brightest point in that area is not a single star... it's the Orion nebula -- which is why it looks a little fuzzy.) Slightly above and to the right of the nebula is Orion's knee ... the bright blue star Rigel.