I'm a person who's in love with the night sky, so that I want to capture some beautiful moments I experience. I'd like to buy a new DSLR , but I already have a Nikon D5200 and I I tried so much to get a clear view of the sky and it didn't work so much.( I tried to capture the Milky Way. Even though I focused to infinity the stars weren't clear and due to high ISO the picture was not in a very good condition ) Is there any DSLR specialized for astrophotography?! If so I'd like to know them plus the features needed to do astrophotography. (I thought of buying Nikon D810)
I already have a Nikon D5200 and I I tried so much to get a clear view of the sky and it didn't work so much.
The camera you have is a fine one to start with. You should spend some time working on technique before you worry about switching to a different camera.
Astrophotography is all about capturing tiny amounts of light compared to daytime photography, but the tradeoffs aren't all that different. You want a long enough exposure that the subject is well exposed, but not so long that noise or motion ruin your shot. In the daytime, you'd worry about the camera or the subject moving; at night you'll have the camera fixed on a solid tripod or other mount, and your chief concern is the motion of the earth creating star trails. In the daytime you'd pick a location with good lighting conditions; at night you want a clear sky with as little light pollution as possible.
There's plenty to learn about astrophotography, and it's worth getting a book or hopping online to find good advice that'll get you started. Doing that will make a much larger difference in your results than changing your gear.
Is there any DSLR specialized for astrophotography?! If so I'd like to know them plus the features needed to do astrophotography.
There are DSLR's that are specialized for astrophotography. Most DSLR's have an infrared filter covering the image sensor, which is helpful for most daytime photography but not necessarily ideal for astrophotography. Some people remove this filter (or have it removed by someone who knows what they're doing) to make their camera more sensitive to IR light, and a few models (Canon 60Da, Nikon D810a) have different filters the allow more IR light. However, if you're not already getting good night sky shots, these cameras aren't going to help you.
In addition to what Caleb suggested, you can improve your images in a number of ways. You didn't specify what type of astrophotography you're doing. There's landscape astrophotography, where you're taking pictures of the land and sky together. There's also deep space astrophotography, where you attempt to capture very dim objects like nebula and galaxies in clear detail. Each of these things requires different gear and techniques.
For landscape astrophotography where you want to capture your earthly surroundings with the sky as a foreground or background subject, you'll need the widest lens you can get. Wider lenses can be left open for a longer period of time before the stars appear to start streaking. You can use the rule of 600 to find the amount of time you can capture with a lens of a given focal length before streaking starts. You'll also want to get as wide an aperture as possible to let in as much light as possible - preferably f/2.8 or wider.
On the other hand if you're doing deep space astrophotography, you'll want a longer lens (or telescope mount) to capture a smaller part of the sky in more detail.
You can also reduce noise in your photos by stacking images using a tool like the free StarStax application. If you take several photos of the same area of the sky, you can blend them together to reduce the noise and bring out the signal.
I know people who do landscape astrophotography with stacking and sometimes even panoramas. They take images where the land is stationary and stack them to reduce the noise in the land parts. Then they take images of the sky and stack those together to reduce the noise. Then they blend the 2 together keeping the stationary parts of each and discarding the parts that moved. But there is an easier option:
For deep space astrophotography you'll need some sort of mount that can move your camera with the stars. I've not used any, so don't have a specific recommendation, but there are plenty of web sites that describe and review them. You can search for them on Amazon or other online shops.
Finally, get out to the darkest areas near you. There are dark parks that are in areas with the lowest amount of light pollution, and they might not be that far from you! If you're in the US, there are several out west, and there's even one in Michigan!
i'm a person who's in love with the night sky
Well, it's a cheap date, so that's good. :-)
, so that I want to capture some beautiful moments I experience. I'd like to buy a new DSLR , but I already have a Nikon D5200
I think you don't need a new DSLR. The D5200 uses a sensor that has a very low level of noise. There was a significant leap in high ISO noise when the sensor in the D5000 came out and it's improved a bit since then. As people were happily shooting with much older DSLRs, I doubt you'd need a new camera.
and I I tried so much to get a clear view of the sky and it didn't work so much.( I tried to capture the Milky Way.
Oh, for a decent clear dark sky. :-)
Even though I focused to infinity
Careful. How did you focus to infinity ? Typically people will do this by moving the focus as far as it will go, but some lenses focus past infinity and the correct infinity focus is just short of the furthest the focus ring will go.
As the link above explains a technique for getting good focus for astrophotography I'd encourage you to read that.
the stars weren't clear
That's not very clear either. Maybe elaborate on that or post an example photo.
and due to high ISO the picture was not in a very good condition )
Cameras have feature like long exposure noise reduction buried in the menu system for this sort of problem. You need to use this.
Most of the noise is thermal noise (random) and to reduce the noise using the long exposure noise reduction setting a camera will take two exposures of equal duration. The first is the "normal" photo, and the second is shot taken with the shutter closed (!) (all done automatically). The second shot is used (automatically) to do dark frame subtraction which removes a lot of that kind of noise.
The more ambitious can try using RAW and doing their own dark frame subtraction. This isn't something I'd recommend unless you're very serious.
Is there any DSLR specialized for astrophotography?!
There are some DSLRs with the optical UV/IR modified or removed (at the factory) which are better for shooting at these wavelength. This is pretty specialized and not something most hobbyists or even many professionals need.
There are third parties who can modify a camera in that way as well.
Some DSLRs have no anti-aliasing filter which can, in extreme situations, give slightly more detail. I've never found this a make-or-break thing and have an AA filter is useful for general shooting, IMO.
If so I'd like to know them plus the features needed to do astrophotography. (I thought of buying Nikon D810)
The only advantage is about a stop to a stop and a half better noise handling, but I think in practice you'd find it overkill when you have learned to use long exposure noise reduction and, if needed, software noise reduction.
Software like IRIS (free) has many specialized tools and is designed for astronomy. Software like this can be worth investigating and learning to use if you're serious.