Is is possible to make astro-images with entry level digital point-and-shoot cameras?
The answer to that question is a reluctant “somewhat”. With a basic camera it is indeed possible to shoot decent astro-images but the objects are rather limited: the moon and star constellations.
It is not about pixels
Basic astrophotography is not about Mega-pixels. Good images can be taken with cameras of 4 MP, or even less. What is important are three vital camera features, without them astrophotography will become a gamble. These features are:
- Manual focus
- Capability to preset exposure time (Tv)
- Capability to delay exposures.
Another component is of great importance: a solid tripod. The magic word here is “solid”.
Manual focus is important because of the way cameras perform auto focus. Some compare contrast changes. Sharp images have more pronounced contrast changes between adjacent pixels, unsharp images deliver more gradual changes. Other cameras compare bit patterns in specials sensors (phase detection). The patterns are shifted when the image is out of focus.
Either way, both methods are not really helpful when imaging a quasi black dark sky. Furthermore, if there is any contour visible in the image (f.e. a tree in the foreground), automatic focus will jump right at it, putting the actual celestial object out of focus. Astrophotography objects need to be focused manually to infinite.
Manual Time Setting (Tv)
Time setting is important because the amount of light gathered by the CCD is only a tiny fragment compared to that of daylight images. This means, the exposure time need to be long. Typical exposure times for imaging stars are between 1 second and 30 seconds.
Tripod – Solid
A solid tripod will keep the exposing camera steady in position. With long exposure images, any vibration will be clearly visible in the images. The camera has to be absolutely still. For noise reduction purposes we need to take a series of at least 10 images, ideally 30-50. More on this subject later.
Even if the camera is firmly mounted on a tripod, pushing the exposure button will cause slight vibrations. The result is star streaks in the image. Exposure delay prevents this effects. Many cameras have a built in 2 seconds or 10 seconds delay. When the button is pushed, there are still initial vibrations, but the delay allows mount and camera to stabilize. The result will be significantly sharper images.
Zoom – Better Not
Some cameras have a zoom feature. Unless you are shooting the relatively bright moon with a very short exposure time – just forget the zoom feature of the camera. Why? Because the Earth rotates. This will show badly in the images in form of elongated stars. Please try to follow the short calculation below – it is indeed eyeopening.
The earth rotates once in 24hours, one rotation equals 360 degrees. That means, in one hour the rotation angle is 360/24=15 degrees, and in one minute it is 15/60 = 0.25 degrees, right? A quarter of a degree does not sound a lot.
True, but… Lets say we want to expose a the constellation Orion for 12 seconds. The angle the earth moves during this period is 0.25/5=0.05 degrees. A 10x zoom would increase the apparent angle by the same factor of 10. Within 12 seconds the image would shift by 0.5 degree.
One might think, that still seems negligible. Does it really have an effect? – Yes it does, and very much so. Picture the moon. The angle of the moon is, well, 0.5 degrees. That’s right, within 12 seconds exposure using 10x zoom, stars in our image would become as long as the diameter of the moon is; definitely not what we are looking for.
So, how are images with a high power telescopes possible?
Astrophotography with high power telescopes requires special mounts; they are called German Equatorial Mounts (GEM). These mounts have gear and electronically controlled motors that move the telescope exactly so that it perfectly compensates for the Earth’s rotation. You have probably guessed it: these mounts are rather expensive. Price depends on their carrying capability and accuracy. Entry level models that can be used for basic astrophotography start at about $500 ($300 used), and with growing demands, mounts can reach quickly true astronomical prices.
- Make sure the battery is fully charged and the memory card offers enough space.
- Reduce the brightness of the camera display to minimum. This helps to keep / maintain the night vision.
- Mount the camera on the tripod.
- Choose your object
- If possible set your camera to the highest ISO speed.
- Manually focus to infinity.
- Set exposure delay to 2 (or more) seconds.
- Set exposure time (Tv) to 5 sec.
- Take your first test shot. You can see if the object is framed right and the image is in focus.
- You might need to play with ISO speed and exposure time to optimize image exposure.
- Once done and you are satisfied, take a series of at least 10 images of your object (recommended 30-50).
Note: photos of stars look always quite dark in the camera monitor. It is often advised to increase the brightness of the image later during post processing.
Post processing (very basic):
This following description is for images with stars (not applicable for moon shots).
- Load your images to your computer and inspect every single image
- Sort out wiggly images, and such that have unwanted artifacts like plane or satellite trails
- Stack the remaining images with DeepSkyStacker (DSS) – setting: average
- Once DSS has created an image, optimize it with the build-in post processing tool
The advantage of stacking a series of astro-images (rather than using just one image), is that the noise portion will be significantly reduced, and the lunimance and saturation of the actual objects (stars) are emphasized. Since we are working usually with high ISO speeds, noise is much more present in astro-images than it is in daylight images.
- Catching the Light – Great site on Astrophotography with a DLSR by Jerry Lodriguss. Noise reduction in astronomy images
Deep Sky Stacker tutorial