Afocal astrophotography involves using an ordinary digital camera to capture an object through a telescope eyepiece. Instead of looking though the eyepiece, the camera lens replaces your eye. This is the simplest way to capture the some targets like the moon with an ordinary consumer digital camera particularly those with fix lenses.
Some people says that afocal imaging is a waste of time, I disagree. Many targets can be captured with afocal method. While it is true that afocal imaging does have some drawbacks, it also have some advantage.
Disadvantage:
- Lower image quality because there are a lot of glass between the sensor and the light source
- Vignetting - darkening of the corner due to the round exit pupil
- Flat field is more difficult to achieve. Stars near the edge will appear distorted.
Advantage:
- Easiest and fastest method depending on the target
- Low cost
Low magnification lunar and solar Image
Requirements:
- Telescope
- Low-Medium magnification eyepiece
- Digicam with manual control
- Photo editor (Photoshop, Gimp, etc.)
The sun, moon and during certain times Venus is the easiest afocal target. The basic step is to hold the camera lens against the eyepiece or attached it with a holder. Adjust the exposure length to the desired exposure. Focus is very important as any amount of processing cannot fix a lousy focus. For this type of target, post processing may not be needed at all. Good lunar shot can be done without processing the image. But processing will definitely improve the detail in most shots.
High magnification afocal imaging
Requirements:
- Telescope with tracking
- High magnification eyepiece or Medium magnification eyepiece plus barlow (recommended)
- Digicam with video mode
- Camera holder
- Video editor (I use Virtualdub for converting video format)
- Registax
- Photo editor
Unlike the moon, high magnification image(for planetary, crater, sunspot) in general, requires a lot of images stacked to improve detail and reduce noise. This method helps us counter the effect of seeing through the atmosphere. You will notice a "bad seeing", when you look at some lunar craters or planet at high magnification and the image appears unsteady. The effect is similar to a distant object seen behind the steam from a pot of boiling water.
The effect of atmospheric "Seeing"
An easy way to capture a lot of image is to capture a video clip of the target. If you have a camera that can capture 30 frames per second of video, then you can have a thousand frames by just capturing 34 seconds of video clip. Imagine trying to shoot 1000 images manually, it will probably take days or at least several hours of pressing the shutter button! Not to mention the sore finger that you'll eventually get. Capture at least 30 seconds of exposure for good amount of details to be captured.
Once the video clip is captured, you can stack it with a free software package called Registax. Note that some camera output cannot be processed directly by Registax. Canon Powershots for example, record videos in AVI but uses Motion JPEG compression. For my camera (Canon Powershot A540), I use VirtualDub, another freeware package, to convert the video clip to uncompressed AVI first before I can open it in Registax.
Once Registax finished stacking the video clip into a single image, post processing can be performed.
Clockwise form top-left: Raw frame, Result after stacking 1000+ frames, wavelet adjusted, post processed. Notice how much details can be acquired by image processing! This is one reason why professional observatory with large telescopes imaging planets using film 20 years ago cannot compete with modern day digital age amateurs with relatively small telescopes. The image above was taken with a digicam on a small 4 inch (102mm) Maksutov-Cassegrain telescope on a low tech and low cost EQ-1 mount. Planetary images with a larger amateur telescopes in the range of 8-16 inches can usually blow film-base professional planetary image in the early 80's away.
As a comparison, here's "one of the best" ground based observatory image of Jupiter in 1983. I took this image from an old book (Lunar and Planetary Observatory, courtesy of University of Arizona).
Compare it to images captured by Damian Peach, an amateur planetary imager using some medium amateur size scope. (Just search for his work on the net)
Deep sky imaging
Requirements:
- Telescope (for smaller deepsky image)
- Mount with accurate tracking (highly required)
- Low magnification eyepiece
- Camera with manual control (at least 10 seconds of exposure required)
- Camera holder
- Photo editor (I use Gimp)
- Imaging stacking application (if you prefer dedicated software to simplify your task)
Deep sky imaging requires long exposure because most deep sky objects (DSO) are difficult to see not because they are small but because they are dim. An example is the Andromeda galaxy, with an apparent size of more than 3 full moons in the sky, it should be easily visible right? Wrong. It is big but very dim. More than a thousand times dimmer than a full moon. Actually, the Andromeda galaxy is still relatively bright compared to other DSOs. On a clear dark sky, one can barely see it as a smudge of light with the naked eye.
It is possible to capture some large deepsky image with just a camera. Some examples of targets for a camera includes: Our Milky way galaxy, Orion complex, etc. However this is no longer considered afocal imaging. ;)
Most digicams with manual controls are limited to 15 seconds of exposure, but some can go up to 30 seconds. Canon Powershots with CHDK firmware hack can go as high as 64 seconds or more. Accurate tracking is essential because a slight tracking will cause background stars to trail. This trailing is caused by the rotation of our planet. Tracking motors are basically there to counter the effect of earth's rotation.
To capture DSOs, the telescope must be polar aligned to achieve accurate tracking. Any misalignment will make a star trail image. The greater the misalignment, the shorter the amount of time before the star image begins to trail. Set the camera to manual control mode and try to capture the target with exposures as long as possible depending on the accuracy of the mount, alignment, etc. Also make sure that the aperture of the digicam is opened to the widest (lowest F number) to allow the greatest amount of light to fall in. Focusing really depends on the camera, some works in macro mode, while others work by setting focus to infinity. For best focus, I set the camera focus to infinity then adjust the telescope focuser while looking at a digicam preview screen. Use the timer to start a capture, this should eliminate the initial shake caused by pressing the shutter button.
Similar to a planetary image, capture as many as you can then stack the images to reveal more detail. But since we use manual control and long exposure, video mode is not applicable. Use Gimp or another photo editor application with layer support to stack the images. Manual stacking is done by setting each frame as a layer and adjusting its opacity proportional to how many frames you have, then merging the layers/images together. Some people use automated tools to stack images like MaximDL, Iris, DeepSky Stacker, etc.
Once stacking is done, post process the image with your favorite program.