My First Set of Gear
My first six months of observational astronomy were with that 8-inch Schmidt-Cassegrain on an alt-az mount. These together are great for visual observing - large aperture in a relatively lightweight and small package (it didn't seem to small at the time, but compared to what I have now, it's absolutely portable!) - but are not exactly meant for astrophotography. My camera was a Nikon D3100, an entry-level DSLR that at least has the minimum requirement for AP: Bulb mode, and manual settings for ISO, white balance, etc.
Schmidt-Cassegrain reflectors are difficult to image through due to their long focal length. My C8 has a focal length of 2032 mm (that's over 6-1/2 feet folded up into a footlong tube!) and an aperture of 203.2 mm, giving a focal ratio of f/10. If you are not a photographer or astronomer, then I will tell you that that is a rather high ratio, which means rather dim images. (Focal ratio is the focal length divided by the aperture). For astrophotography, this means that you have to image an object much longer in order to get the same amount of light in each pixel as a lower f-ratio telescope. Having such a long focal length does have one upshot though - small field-of-view (FOV). Well, this is an upshot if you like looking at galaxies and planetary nebulae, at least.
Alt-az mounts are lightweight and easy to set up - no polar alignment or counterweight-balancing required. You just throw it on the tripod, give it two or three stars, your location, and the time of day, and you're off to the races. Alt-az mounts do not follow the motion of the sky, however - they move up-down, left-right, with relation to the Earth. They will of course track something across the sky quite readily, but it has to move both axes to do it, and over the course of the evening, the object you're observing will appear to rotate - this is called field rotation. The effect is negligible when doing visual observing, but becomes readily apparent when taking images - for example, this uncropped image of M42, the Great Nebula of Orion.
This is a stacked image (more on that later), and you can see how each successive frame is rotated. Each frame is only 20-30 seconds long, and it doesn't take long to start seeing the effect. This is one reason why you are limited to very short exposures (30 seconds or less) on an alt-az mount.
DSLRs have a number of limitations as well (although I have gotten some fantastic images with mine, I could be doing even more amazing things if I had a CCD camera). These include:
- Small pixels. Ordinary daytime photographers always demand more pixels, so many pixels are packed onto the same size chip - meaning they are getting smaller and smaller. My 14 MP D3100 has 5 um (micron) pixels, while my 24 MP D5300 has 3.9 um pixels.
- Bayer filter. In short a Bayer filter is what makes a color camera able to shoot in color. It's a filter that lies over the top of the pixels and has a color pattern, typically RGGB. In other words, it takes four pixels to make one image pixel - one red, two greens, and a blue. (The green is duplicated because it is the wavelength that your eye is most sensitive to). So it cannot resolve as fine of detail as a monochrome camera can.
- IR filter. DSLRs, in order to more closely mimic the visible spectrum that the human eye is sensitive to, have another filter on them as well, one that blocks IR and a decent portion of red light, since our eyes are not very sensitive to red. Unfortunately, many astronomical objects shine brilliantly in the red, so it is much harder with a DSLR to collect that light.
- High noise. Every kind of sensor convolves a little noise with the signal. Uncooled DSLRs tend to have high noise profiles, both in thermal noise (when ambient heat causes a pixel to fire), and in read noise (noise that is introduced when the camera reads the data off the chip). This is greatly reduced in (much more expensive) CCD chips, particularly ones that are cooled (as most astronomy CCDs are).
Now, it would seem that I am painting a pretty grim picture here. And if I knew all of these things when I started, I might have given up! But in spite of all of these things that make imaging with the equipment I had difficult, I nonetheless managed to acquire some good images - with some help from digital image processing. But that topic deserves several posts dedicated to it, so keep reading!
I have discovered, however, that you don't necessarily have to break the bank to get high-quality astro images on fancy equipment. I've got another post coming up about a relatively cheap setup that I've gotten some fantastic images out of so far as well. Stay tuned!
M42 Orion Nebula, taken on my C8 on its alt-az mount.