I recently bought an entry level camera, Canon Rebel T6 EF-S 18-55 IS II Kit. Im slowly learning on my own more about the camera and its settings, but one of the main reasons i wanted to start this photograpy journey was to do some Astrophotography (the milky way and such). I know the lenses I have aren't suitable for my goals (EFS 18-55mm)(75-300mm) and im aware I will need a very sturdy tripod, but if I could get some advice on a lens that would suit my needs for these shots I would be very grateful. As you know my camera is a cropped sensor and it would be a better choice to have a full frame camera but money is an issue and again im a beginner. Im trying to achieve a full frame shot with a cropped sensor camera, was wondering what lens would best suit my situation, fish eye lens? 35mm lens? im willing to pay good money on a lens that'll help me achieve my Astrophotography goals. Should i stay with a canon lens or another brand lens that'll do the same job? Anyways, thank you in advance for you help. Much love.
Astrophotography has "tiers" of complexity.
At the simplest level... put the camera on a stationary tripod and use a wide-angle lens. The exposure calculation is based on the "500 rule" -- so named for 35mm film cameras. That rule says that if you divide 500 ... by the focal length of your lens... then the result is the number of seconds that you can expose without noticing elongated stars due to the rotation of the Earth.
In other words if you had a full frame camera with a 24mm lens, then it would be 500 ÷ 24 = 20.8 seconds (about 21 seconds). If you push it much longer than that... you'll notice the stars aren't round.
But you don't have a "full frame" or 35mm film camera. Your camera has a crop-factor of 1.6 (meaning if you divide the diagonal measure of a full-frame sensor (roughly 44mm) by 1.6, you end up with the diagaonal measure of your camera's sensor. It's about 27mm.
So you can either divide 500 by 1.6... or you can multiply the focal length of the lens by 1.6... either would get to the right answer.
500 ÷ 1.6 = 312.5. Supppose you had a Rokinon 10mm f/2.8 lens (with Canon mount)... that would get you to a 31 second exposure.
It turns out the 24mm f/1.4 collects more light because AT f/1.4 you collect 4x the amount of light then you do at f/2.8. But it's not 4x the focal length... it's a little over double the focal length. So 312.5 ÷ 24 = 13 seconds... (you could get away with rounding that to 15 seconds). But in that 15 seconds you would collect what the 10mm f/2.8 lens would collect after a full 1 minute.
You an see how "low focal" ratios have an advantage because they collect more light in less time. Excessive "noise" in an image means your "signal to noise" ratio was poor. The more "signal" you can collect (more real photons of light) the better the image results and the less "noise" you'll have. There is a technique involing lots and lots of images and then "stacking" them to get a better result. The science of that technique is all around using more data to improve the signal to noise ratio (SNR).
I don't typically do Milky Way photos because I don't live near a good area for it. Ideally you want to shoot these on clear moonless nights and you want to be miles and miles away from any light pollution. The moon is a huge source of light pollution if it happens to be in the sky.
The Rokinon lenses are popular for this because they tend to have fairly good optics (with one major caveat I'll mention in a moment) and they're completely manual everything... manual focus & manual aperture. It turns out in astrophotography you have to do manual focus anyway (the stars aren't bright enough for auto-focus to work) and you typically just shoot at a low aperture (but if you have a tracking head and can shoot longer then you can get sharper stars by stopping down slightly.)
The caveat... is that Rokinon lenses often suffer from problems with "de-centered" optics. In other words the lenses weren't properly made or properly installed and the lens suffers from various issues that ultimately results in a non-flat focus field or issues like astigmatism or coma. So when you get one of these lenses, you really need to test the optics to make sure you have a good copy. But if you DO get a good copy, they are usually quite good.
Taking a photo of the moon is also extremely easy. The moon follows something called the "Loony 11" rule. That rule says that if you use f/11 as the focal ratio, then the correct shutter speed is simply the inverse of the ISO setting. E.g. at ISO 100 then it's a 1/100th sec exposure. At ISO 200 then it's a 1/200th sec exposure... and so on. The reason they use f/11 is becuase it's the only f-stop where the relationship between ISO and shutter speed is a simple inverse. But if you know how to trade stops of exposure, you don't have to use f/11.
E.g. suppose you use ISO 100 at f/11 then it's 1/100th sec... but NOW suppose we use f/8 (twice as much light gather as f/11) so that would be a 1/200th sec exposure. Or at f/5.6 it's 1/400th sec... at f/4 it's 1/800th sec.... and so on. So you don't have to use f/11... it's just there because it's the easiest to remember.
There's a slight nuance in astrophotography called "extinction" and that's the notion that as light passes through thicker amounts of atmosphere, you loose more light (atmospheric conditions and transparency also contribute to extinction). It's why the sun appears dimmer at sunset. The moon also appears dimmer near moonrise / moonset. The Loony 11 rule is based on the moon being high in the sky.
Based on the sensor size for your camera, a focal length of about 1000mm (even 1200mm would be fine) would give you a large moon that mostly fills the frame. Straight out of the camera you get something like this:
That might seem a bit underexposed, but it's actually accurate. The moon is not nearly as bright as one might expect. It's true brightness is roughly that of a worn asphalt road. The key is you want to make sure it isn't over-exposed... then you can tweak it and turn it into something with more pop.
Here's one that has been processed.
Beyond this... you need a bit of gear. Either a telescope on a tracking mount or a camera tracking head. The tracking heads mount to a regular photographic tripod (preferably a nice solid tripod that wont shake during long exposures). They tend to cost in the $300-400 category. I use a Losmandy StarLapse head (about $800) but I don't know that Losmandy even makes that head anymore.
The popular heads are the Sky Watcher "Star Adventurer" or "Star Adventurer Pro" and the other brand is iOptron who makes the "Sky Tracker" and "Sky Guider" heads. (both companies make less-beefy / more-beefy version -- it's really all about how much weight it needs to hold). The head is aligned so that it's rotation axis is parallel to Earth's celestial pole. As the planet spins from West-to-East, the head spins from East-to-West at the exact same rate. And since the axis of the head is parallel to Earth's axis (basically you point it to the Earth's north celestial pole and it comes with an alignment aid to help you set it up), it cancels out the movement of the Earth and you can take a nice sharp image.
This is a section of sky I shot using a 135mm lens (Canon EF 135mm f/2L USM) on my tracking head using my Canon 60Da camera:
That image is the lower region of Orion ... including the Orion nebula, the Running Man nebula, the Horsehead nebula, the Flame nebula, and M78 (a dark nebula).
It was shot at ISO 800 using my Canon 60Da but it's a series of exposures at f/2... some are 2 minutes, some are 1 minute, some are 15 seconds, and some are 3 seconds... all combined to create this result after processing. But I have taken 8-minute exposures and have no star trailing (nice tack-sharp stars.)
Everything to this point can be done using camera lenses (alhtough my moon photos were done through a telescope .. they could have been done with camera lenses).
As you get to deep-sky objects, it starts to become necessary to use a telescope. Here's a shot of the Andromeda galaxy and this was done with a telescope (540mm focal length) ... but clearly you do this with a 500mm lens and get the same result.
But keep in mind that the Andromeda galaxy is about 6x wider than the width of the moon (moon is about 1/2° wide in angular dimension... Andromeda is about 3° wide in angular dimensions).
These images are heavily processed... you take LOTS of exposures (preferably a few hours worth or more) and then combine them using image stacking software.
The telescope (a TeleVue NP101is apochromatic refractor) was on an equatorial tracking mount (a Losmandy G11 mount) to catpure the image data for that image.
The galaxy photo require a lot of 8 minute long exposures. 8 minutes is a long time to shoot and expect to have clean images (no tracking errors). So typically when these types of exposures are taken... there's a second scope & camera called a "guide scope" and "auto-guider". While your main imaging camera is taking an 8 minute expousre... the auto-guider is taking very short expoures (perhaps 2 second expousres) of just one star. Each frame is compared to the master frame to test if that star shows any signs of drifting. If it is drifting, the guide-software sends a correction to the telesocpe mount to put it back on target. Basically it is sensitive to detect even sub-pixel drift and correct before things get bad enough to harm the main (long duration) exposure.
There is a website called "Photographing Space" and they tend to be pretty good with tips and how-to tutorials.