Whats the deal with webcam astrophotography with and without using eye pieces?
Question by planck12:
I am trying to understand the reason of using a eye piece to take a picture of planet and at times removing the eyepiece and sticking the camare right into the eyepiece holder to take a picture.
I am trying to understand the reason of using a eye piece to take a picture of planet and at times removing the eyepiece and sticking the camare right into the eyepiece holder to take a picture.
How does the former differ from the latter ? I am just wanting to use webcam astrophotography to see how things work out, what is a cheap webcam (I suppose with CCD sensor as opposed to CMOS) in the market that would give good results with a newtonian 5"inch ape and 650mm focal.
thanks
July 1st, 2008 at 3:32 am
There are digital camers specifically designed to take pictures in a telescope at prime focus (with no eyepiece). They are usually called CCD cameras, even though all digital cameras use one or more CCDs. These cameras have very sensitive CCDs though and are often actively cooled to keep their tempurature down to reduce noise in the image.
Telescopes focus light into points. Our eyes are not able to process this focussed light directly, so the eyepiece reformats this light and recollimates it, which is essentially "defocusing" the light back into parallel beams. The lens of our eye then refocuses these beams so that they converge on a point on our retina. Most cameras have their own lenses as well and therefore need the aid of the eyepiece before being able to capture an image. The CCD camera however has no lense, just the CCD device. It can take the focussed light at prime focus (the point where the light comes to focus in the telescope) and create images that way.
CCD cameras can be pretty expensive and are good for long-exposure astrophotography. Regular high-quality digital cameras can be used for this with similar success, but they are often used afocally (using an eyepiece). Webcams on the other hand are often used for photographing bright objects. What is often done is the webcam takes a video of an object, like a planet. Then you use a program like Registax which will process that video into individual frames. You pick out the frames that look the best and Registax can combine them into one really nice image.
July 2nd, 2008 at 1:53 pm
An eyepiece magnifies the telescope's image, how much depends on the focal length of the eyepiece and the focal length of the telescope. If you have, say, a 6" f/8 telescope (focal length 1200mm), a 20mm eyepiece gives a 60X (60 power) magnification. Without an eyepiece, it's like using a 1200mm camera lens, and the field of view depends on the size of the imaging chip.
You'd use a webcam with an eyepiece (called "eyepiece projection") to take images of planets, and other fairly small things that require a lot of magnification to see detail on and that are bright (the more you magnify the image, the more light you lose). With eyepiece projection, you have to also have the regular lens on the webcam in place, focused at infinity.
You'd use the webcam without an eyepiece for wider-field images, of galaxies or nebulae. In that case, you remove the webcam's lens, and the telescope itself acts as one big lens for the webcam.
Webcams are good at the first kind, and not so good at the second kind (they're not very sensitive, they have small imaging chips, and they're quite noisy).
The Philips webcams are by far the most popular with planetary imagers — easy to use and known for fairly low noise and ease of adaptation to telescopes.