Capturing Galaxies & Nebula, vs capturing Planets
...are two very different things, as it turns out.
In fact, these two things seem to me to be almost two different branches of astrophotography when it comes to what you need, and what technics to use. You do use some similar skills, but wow, surprising how different.
Why?
Well.. most people who have never done astrophotography, or tried to find both of these types of objects in powerful telescopes before, would think that planets like Venus, Mars, Jupiter, and Saturn, being so very bright in the sky, would probably be larger and easier to capture than very distant objects like Galaxies and Nebula.
The interesting thing is, Jupiter for example, while it's WAY brighter by I don't know how many magnitudes than the Andromeda Galaxy, is actually way smaller visually than Andromeda. When I say way smaller.... of course it's way smaller... Jupiter is just a planet, and Andromeda is a galaxy full of billions of stars and planets... but what I mean, is VISUALLY smaller in our sky, even though Jupiter is far closer than Andromeda. So why can't we see Andromeda dominating the sky? Because Andromeda is far... far dimmer than Jupiter. It's so dim, while you can see Andromeda with the naked eye, you can't see it if you are within city lights, and even under a dark sky, you have to know where to look, and then it just looks like a small fuzzy star, as all you are seeing is the center core.
Here are some good visual images to explain better.
This image below I have linked to (not one of mine) is a composite image showing relative sizes of the moon in the sky, to the Andromeda Galaxy. This is what it might look like if the Andromeda galaxy was similar in brightness to the Moon. Kind of surprising, isn't it?
Below is two images I took, taken with my D5300 with a 70-300mm Tamron lens. Both images are at 300mm, but the Andromeda image was created with a stack of light frames, with long exposures, about 2 minutes each, while the Jupiter image was shot around ISO 400 I think, something like 1/60 of a second. This should help explain that I needed to absorb a great deal more light for Andromeda in order to create this image, as well as doing a lot more post processing.. Jupiter on the other hand, is a single image... and there really wasn't a lot of point in post processing, as it's just a white dot at 300mm...LOL.
It is important to note, that Andromeda is visually the largest galaxy in our sky, and definitely one of the larger deep sky objects. It's often the first target people choose when going after deep sky objects... with good reason! It's really awesome!
There are many Galaxies and Nebula that are much smaller than Andromeda... but there are a LOT of targets you can get with a focal length of anywhere between super wide like 11mm (star trails, or Milkyway shots) to 300mm... and you will get even more options upto 600mm.
Jupiter, Saturn, Mars, and Venus won't look much different at 600mm. If you want to shoot planets, you need much longer focal lengths. Really, while a telescope with a camera mount is useful for all targets... if you want to get some detail images of planets, a strong telescope is pretty much necessary for planets.
Differencies in imaging Galaxies and Nebula, vs imaging planets.
I am not going to write about this much, because I don't know a lot about creating great images of planets yet. I hope to go down this road, but right now, I can only share what I have read on the topic, not what I have done.
When Imaging planets, it seems like the popular method is to use a camera that can take video with pretty good resolution... and then take video of a planet through a strong telescope. The video settings should be set at something like 60 fps. Then software is used to divide those 60 frames of video into separate images, and stack them. The software will use the best frames, and produce a better image than seen in the video. In 2 seconds, you could have 120 frames to stack. I am guessing there is a lot more too this, but you will need to do some additional research. I do know one issue with imaging some of these planets, is unlike galaxies or nebula that look pretty much the same as they would have when you looked at them last year... planets rotate. Some of them pretty quickly. Jupiter, for as big as it is, has a 10 hour day. Hence, it's surface features are travelling across it's surface more than twice as fast as on earth. Hence, if you took images for several hours, and tried to stack them all, it would be a mess. Fortunately, as mentioned earlier, Jupiter is much brighter than those other deep sky objects, so as mentioned earlier, the technic is to take video at 60 fps. In a few seconds you have hundreds of frames. If you want to image longer, you could produce a bunch of images over the night, showing the rotation of Jupiter.
When Imaging Galaxies and Nebula, these images are far, far dimmer than planets. Fortunately, the actual physical size of these super large, very distant objects, is incomparable to a planet, such that rotation or overall motion of these objects, other than with the rotation of the entire sky is a non-issue. You can, and people do all the time, stack images taken weeks or months apart, all together, to make an improved image of a galaxy or nebula target. What is required instead, is very long exposures, and often much higher ISO settings than what imaging a close planet requires.
Images can be produced of these targets without a tracking mount, but when that is done, very high ISO settings are required, because shutter time will be very limitted.... so this usually means a ton of images are needed to stack, in order to get a good quality image.
Using a tracking mount, that will track your camera with the sky motion, is a game changer.
Unguided tracking (not using a guiding scope and camera with software, and only doing a manual polar alignment) still can help you get 2 minute of longer exposures even when zoomed in. If you go the extra for a guiding scope and camera, and polar align using software, you can get super long exposures.
Long exposures, means you can get away with lower ISO settings, making a cleaner image. Still, you will need a lot of exposures, as well as taking dark frames, flat frames, and bias frames for the best image. You can spend an entire night, or even several nights imaging ONE target. Unless you are impatient like me... I tend to try for atleast 2, maybe 3 targets in a night if I have the time, which means I sacrifice a little quality for a bit more variety.... but again, because they of the type of object, I can take more images another night, and stack them into the same pile :)
Summary:
So in short, with Galaxy and Nebula, you can get a lot of targets with not that much focal length, but it requires a LOT of images, and either very high ISO settings with the longest shutter you can get away with, without tracking... or use of a tracking mount, longer exposures, and lots of post processing.
Planets require much longer focal lengths... really, they require a good strong telescope. Instead of taking long exposures, you take high resolution video, at a high frame rate, and divide the frames, and stack those.
I hope this helps explain the difference. Again, this is not meant as a full technical explanation, just a general one. I am sure I got some of the details incorrect on the planet side of things, but hopefully it is helpful on pointing you into the right general direction :)
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