10-25-2017 10:48 AM
10-25-2017 01:27 PM
It depends on how much view you want.
Your 18-55 should be fine for wide-angle shots, but 300mm is not enough for large magnifications.
This was at 150mm and is Orion:
This was at 500 mm:
10-25-2017 08:21 PM
There are a few variations on “astrophotography”.
Some people are thinking of doing nighttime landscape photos with the stars in the sky (e.g. Milky Way photos, etc.) and for these you generally want a very wide-angle lens with a very low focal ratio.
And there are those (like me) who tend to photograph sections of the sky with deep-sky objects ... and might use either a longer lens or attach the camera to a telescope. But for these shots, the camera is “tracking” the sky (more accurately... the Earth spins from west to east so the camera is mounted on a special tracking head which rotates from east to west at the same speed and this cancels out the motion of the Earth. The result is that you can take long exposure images of deep sky objects with gorgeous results (and need not use a wide-angle lens).
If you do NOT have a tracking head, then you want very wide angle lenses with very low focal ratios.
For example, Robinon makes a very low-cost 14mm f/2.8 lens which is completely manual everything (manual focus, manual aperture) their optical quality is good IF you get a copy with nicely centere optics but their manufacturing consistency is poor so it’s very common to get a copy of the lens with de-centered optics. The result is that when you’ve focused as well as possible, part of the image is nicely focused, and another part of the star field is soft (out of focus). So it’s cheap but... you get what you pay for.
There are some slightly longer focal length choices that actually have lower focal ratios and when you do the math, these lenses ultimately collect more light. For example.... Canon makes a 24mm f/1.4 (an L series lens... not cheap). But even though it’s 24mm, it’s 2 full stops faster than an f/2.8 lens so it literally collects FOUR times more light per second than an f/2.8 lens. An f/2.8 lens would need to be 6mm to compete (and nobody makes a 6mm lens)
For tracking heads (these require a quality photo tripod... something solid that isn’t going to vibrate as you use it during long exposures) the two major vendors are Sky Watcher (Star Adventurer head) and iOptron (SkyTracker Pro head). The rotation axis of the head has to be aligned with Earth’s celestial pole, but the camera can be pointed in any direction. (Lots of tutorials are available on YouTube, Vimeo, etc. that demonstrate how these things work.)