Compared to traditional photography, astrophotography is more aligned with radio astronomy in that it has more to do with signal processing than lighting and composition.
This is where the road splits with traditional photography and astrophotography.
Traditionally, you can take only one photograph and if your settings are correct, you will get a nice image.
The best way to get a clear image is to increase exposure until enough light exposed the film. There are advantages and disadvantages with film and digital imaging techniques. This page will be focused on digital imaging techniques.
In digital astrophotography, there are many different types of shots that you need to take. Here is a list of the digital imaging types which may or may not be used depending on your image processing software:
- Light Frames
- Bias Frames
- Dark Frames
- Flat Frames
- Dark Flat Frames (optional)
Along with the Light Frames, these are other types of images needing to be captured to help remove artificial noise, light, and artifacts so that the final image is as close as possible to the real thing. As if we were in a spaceship observing it with our own eyes.
You can use a DSLR body to capture these images or use a dedicated astrophotography camera attached to your telescope. Or you can use your DSLR as normal with a camera lens.
There are different techniques in how you can capture your images. Long exposures with star tracking hardware or mount. Or a technique called Lucky Imaging which consists of capturing many more shots with overall shorter exposures.
Here are more details on each frame type:
1. LIGHT FRAMES
These shots are captured like a normal photograph. Given that the targeted celestial body is very dim, you will need to figure out what settings are the best to use.
ISO or Gain – Determines how sensitive your hardware is to the available light. This set too high will introduce unnecessary noise to be captured. Set too low and you will not capture anything. The exact setting will be based on the equipment you use.
» Long Exposure ~ Start with 800 ISO or depending on your dedicated astrophotography camera (DAPC) a gain setting at medium-high.
» Lucky Imaging ~ Same or less ISO as long exposure or gain set to a lower value. Generally, image sensors have a sweet spot for the minimal amount of noise. Try starting with 640 ISO or a Gain value of 20.
Aperture – DSLR lenses have the advantage of being able to adjust their focal ratio. A low aperture value allows more light to be gathered but may introduce aberrations or distortions. Telescopes have fixed focal ratios but adding a focal reducer/corrector will increase its light-gathering capabilities. A general rule with aperture is the higher the aperture, the less light but easier for the light to be able to be focused onto the sensor or film focal plane.
» Long Exposure ~ DSLR lens can start with an aperture around ƒ/4.0
» Lucky Imaging ~ Can be a higher focal ratio (less light) like ƒ/5.6 or even higher like ƒ/8. Keep in mind each f-stop is effectively halving (ƒ/4.0 -> ƒ/5.6)or doubling (ƒ/5.6 -> ƒ/4.0) the amount of light.
Exposure – How much time the exposure is taken. Longer exposure equals more light. Exposures over 30 seconds will show star trails or your target will move out of the frame if you are using a high focal length lens or telescope.
» Long Exposure ~ Requires star tracking hardware or computerized telescope mount. Start with 30 seconds for DSLR or DACP.
» Lucky Imaging ~ Start with 5-second exposures for DSLR or DACP
DSLR -> Long Exposure: ISO 800, ƒ/4.0, 30-second exposure
DSLR -> Lucky Imaging: ISO 640, ƒ/5.6, 5-second exposures
DAPC -> Long Exposure: Gain medium-high, 30-second exposure
DAPC -> Lucky Imaging: Gain maximum, 5-second exposure
Long Exposure: 45 shots
Lucky Imaging: 250 shots
Note: Make sure to not adjust the focus of your telescope or DSLR lens after taking your Light Frames as this is an important setting that is reused for other frames.
2. BIAS FRAMES
Bias frames are ISO or Gain dependent but can be reused respectfully. You take these frames with the lens or telescope cap is on and the fastest exposure time possible. These shots can be taken at any time.
DSLR -> Lens cap on, 1/8000 exposure, ISOs 640, 800, 1000.
DAPC -> Telescope cap on, equivalent settings for exposure and gain.
Long Exposure & Lucky Imaging: 50 shots per ISO or gain setting
3. DARK FRAMES
Dark frames are captured immediately before or after you capture all of your Light Frames. Dark Frames must also have the exact settings as your Light Frames but with taken with the lens or telescope cap on. These are used to isolate and eventually remove hot pixels or amp-glow from your Light Frames.
DSLR -> Long Exposure: ISO 800, ƒ/4.0, 30-second exposure
DSLR -> Lucky Imaging: ISO 640, ƒ/5.6, 5-second exposure
DAPC -> Long Exposure: Gain medium-high, 30-second exposure
DAPC -> Lucky Imaging: Gain maximum, 5-second exposure
Long Exposure & Lucky Imaging: 20 to 30 shots
4. FLAT FRAMES
Flat Frames are almost exactly like Dark Frames but instead of putting the cap on your lens or telescope, you use white cloth (i.e., t-shirt) instead. You then use a bright even and defused light to illuminate the sensor. Additionally, you will need to reduce your exposure time so that the histogram of your Flat Frames is between 1/2 and 2/3. These frames are gray-white looking and are used to isolate and eliminate any dust or scratches in your optical hardware or sensors. Again, focus settings need to match Light & Dark Frames.
DSLR -> Long Exposure: ISO 800, ƒ/4.0, 1-second exposure or less
DSLR -> Lucky Imaging: ISO 640, ƒ/5.6, 1-second exposure or less
DAPC -> Long Exposure: Gain medium-high, 1-second exposure or less
DAPC -> Lucky Imaging: Gain maximum, 1-second exposure or less
Long Exposure & Lucky Imaging: 25 shots per ISO or gain setting
5. DARK FLAT FRAMES
Some techniques call for the use of Dark Flat Frames. These are almost exactly like Flat Frames but with the lens or telescope cap on. These can be considered optional.
DSLR -> Long Exposure: ISO 800, ƒ/4.0, 1-second exposure or less
DSLR -> Lucky Imaging: ISO 640, ƒ/5.6, 1-second exposure or less
DAPC -> Long Exposure: Gain medium-high, 1-second exposure or less
DAPC -> Lucky Imaging: Gain maximum, 1-second exposure or less
Long Exposure & Lucky Imaging: 25 shots per ISO or gain setting
Please note: This page is a work in progress and is generally being used as a place to document my research into becoming an astrophotographer.