I am planning to take a bridge camera, which apart from the usual personal events usage would be used for astrophotography. I have seen the models and most of them have decent zoom to start off with. What other factors/ features of the bridge camera should be considered?
"More Zoom" really isn't much of a feature to consider when using a bridge camera for astrophotography. The small sensors and narrow apertures at maximum "zoom" found in bridge cameras pretty much preclude using them for long focal length astro work. By the time you've exposed long enough to capture a usable amount of light, the stars have rotated halfway across the field of view at longer focal lengths.
Normally the way to combat this is to place the camera on an equatorial tracking mount that moves to counteract the Earth's motion and keeps the camera pointed at the same point in the sky while also preventing the the stars near the edge of the frame from rotating around the center. But a good equatorial tracking mount can cost an appreciable percentage of the price of most bridge cameras.
In general bridge cameras aren't particularly well suited to astrophotography. The areas where they are most capable is for wide field work. The following things are most helpful:
- Sensor size. The larger the sensor (or more specifically the size of the pixels on the sensor), the higher the full well capacity before the pixel well reaches full saturation. The higher the full well capacity, the brighter the exposure before you start blowing all of the stars out as pure white instead of the various colors they are. The increased dynamic range allowed by larger pixels also allows for a better signal-to-noise ratio. This allows your images to look cleaner and less grainy. Most bridge camera have relatively small sensors because it allows them to have more "zoom" with a smaller lens.
- Maximum aperture. The larger the lens' maximum aperture (the lower the f-number), the shorter the exposure times you can use to get the same amount of light into the camera OR the more light you can allow into the camera in the same amount of time. This is important because the Earth is rotating beneath the sky. Exposing for very long means stars begin appearing as blur trails instead of fixed points. The maximum exposure length before motion blur appears is based on the field of view you are capturing. The wider the field of view, the longer the maximum exposure before stars begin to trail. The narrower the field of view (the longer the focal length - the more you "zoom"), the shorter the maximum exposure time before stars begin to trail. Tracking mounts are a way to extend exposure times, but the good ones are very expensive.
- Maximum usable ISO. The higher ISO you can use, the shorter you can expose. For why this is important, see directly above.
- RAW file capability. The significantly more information contained in a raw image file compared to a compressed JPEG created from that raw information is critical to doing astrophotography. Cameras aren't very good at processing shots of the night sky. Post-processing applications can produce much better final results when the raw image data, rather than an in-camera produced jpeg, is used.
- Maximum exposure time. Due to the small sensor found in bridge cameras, many of them limit the longest exposure to times as short as 30 seconds or less, sometimes a LOT less. Many of them also decrease the maximum exposure time as the ISO setting is increased. They do this to prevent the user from making excessively noisy images.
- Low geometric distortion lens. This is not a particularly strong suit of most bridge cameras, particularly those with a lot of "zoom". To make a lens with such varied focal lengths, designs have to be compromised and one of the things that is often traded for a wide focal length range is the amount of geometric distortion that is tolerated. A bridge camera with less "zoom" will usually demonstrate less geometric distortion than a bridge camera with a "superzoom" lens.
One way astrophotographers increase the quality of their images is to do what is known as 'image stacking'. Several (sometimes hundreds) of discrete frames are combined to increase image quality and reduce the effects of noise and shooting at high ISO. To do image stacking well the camera needs to move with the sky for each successive exposure (There's that pesky need for an expensive tracking mount again!). In the absence of a tracking mount, stacked images can be aligned with the differences in field of view resulting in cropping the edges of frames to include only the parts of the field of view that is included in all of the shots. For stacking to work, though, the stars need to be the same distance apart in every frame. The 'warping' or 'stretching' caused by geometric distortion makes stacking very difficult to do and less effective.
In the end you really should consider whether a bridge camera is the best choice for your intended usage. Astrophotography is a very demanding type of photography with regard to the technical capabilities of the equipment. It can be a rabbit hole of ever increasing spending to get better gear that never ends, but everyone has to start somewhere. For most, though, an interchangeable lens camera with an APS-C or larger sensor is probably a much better place to start than a bridge camera. Even entry level DSLRs that cost about the same as many bridge cameras will be much better for astro work.
For further reading:
What lens characteristics are important in star photography?
Longer exposure & lower ISO or shorter exposure & higher ISO - what gives better results when photographing stars?
How can I do astrophotography with my 70-300mm lens?
Why do I have blurry star trails?
How to have colors in Milky-way?
How to take the night skies and meteor showers?
What should I look for in a camera for shooting in bulb mode for astrophotography?
Astrophotography exposure setting for noise reduction
What caused my astrophotography images to come out hazy?
How do I get a shot of both the foreground and stars with astrophotography?
How to enhance a cameraphone photo so you can see stars?
How do star trackers work to take a photo for 5 minutes without blur, and are they worth it?
Black sky for night long exposure
What is the "Rule of 600" in astrophotography?
Can I get longer than 15 second exposure with my canon SX 50?
How to set white balance in a photo of stars?
Why should I use the widest aperture for star photography?
How do I focus in low light for long exposures?
Star photography: what aperture
Whether it is a camera or a binocular, a bigger aperture is required to collect more light from the night sky. In fact many star gazers prefer wider aperture to longer zoom. So you have to check the max aperture of the lens/ fixed lens camera. Also, a bulb mode is a preferred feature.