Astrophotograpy captures small specks of light on an (ideally) black sky. As such, this poses significant challenges to a digital camera. The black regions of your image will have very little detail, since you haven't exposed very long there (you have bright stars that are going to wash out otherwise). The way we overcome this is to take lots and lots of pictures of the sky and "stack" them on top of each other. This allows us to add more and more faint details in the darker parts of the image and slow accumulates an image there, while the stars just remain white, but not blown out.
There is specialized software (PixInsight, DeepSkyStacker, etc.) for stacking that aligns the images so that they perfectly overlay each other and then does some more processing and spits out a composite/stacked image.
This software can also handle additional calibration images, which we also capture with our camera. Digital sensors have a certain noise level that needs to be accounted for, the lens and sensor might have dust specks on it, etc. "Calibration" frames capture how our camera operates, some depend on the outside conditions and should be taken as part of your imaging session, others will be done inside at any time. See below for more details on how to shoot these.
This guide assumes you are using a standard general-purpose digital SLR or mirrorless camera and lens, i.e. not a dedicated astrophotography camera.
- Lens: generally, the faster (smaller f-number) the better, as more light is captured
- Intervalometer: allows you to automatically take pictures at certain intervals. Some cameras have this built-in.
- check out OpenMemories: Timelapse for older Sony cameras (A7/S/R I/II)
- Dew heater: prevent lens from fogging up (depends on your climate)
- TODO (optional): tethered to Windows / Linux computer (APT, Ekos, etc)
- Ideally close to new moon, unless you have a narrowband setup
- Check weather: rain, cloud cover, seeing, wind
- Meteoblue's astronomy forecast works worldwide and has been reliable for me in Australia
- Clear Outside (worldwide) astronomy forecast designed by astronomers for astronomers. Has been fairly reliable for me in Germany.
- Darksky source of Clear Outsides data, with viewable map.
- Specific regions (US/Europe) may have better sources - add here.
- Use Stellarium or similar software to choose appropriate target(s) visible for at least a few hours above 30 degrees altitude, or higher if you have obstacles
- OAT Usage
- Use the time between sunset and astronomical twilight to polar align, test the mount, calibrate guiding and so on. You can also take "twilight flats", see "Calibration" below.
These are the four main types of frames you need to shoot, two of which you should do in the field:
Purpose: These are the regular images that you are taking with your camera.
Method: Use a good exposure level that gives good stars that are not overexposed or blown out. Usually 60s or more with aperture at the sweet spot of your lens.
Purpose: These capture what an image with zero light looks like. This can be used to eliminate "thermal noise" of the sensor, including "hot" pixels.
- Take 30-50 images at the same exposure duration as your light frames.
- Take them at the same temperature conditions as your light frames. Best done in the field, before or after your imaging session.
- For a DSLR it's best to take the lens off and put the cover on the body, and also cover the viewfinder to prevent light leaks. You can also just put the lens cap on, but then make sure you're in a dark room and the camera is covered with some dark material.
- For a DSLR, set the ISO at the same level as your Lights.
Purpose: These capture the flaws of the optic pathway, like lens vignetting, uneven sensor response, dust or dirt on or in the lens and dust on the sensor. They are used to "normalize" each of the light frames (compensate for the uneven response to light)
1.Use the camera in the lights configuration, but cover the lens with something like a white T-shirt (no wrinkles/lumps) and point at an even light source (a clear blue sky works, but not directly at the sun or lightbulb). You can also take these during twilight. Avoid clouds if taking sky flats.
- Take 50 or more exposures at the same focus as your lights
- For DSLRs, use a low ISO (ISO 100).
- For cooled CMOS cameras, expose for at least 2s, long enough to have the histogram be 1/3 - 1/3 of the way to the right.
- For DSLRs and daytime/twilight flats, keep the exposure time under 1/400s and use the same exposure time for all your flats. Wait for twilight if histogram shows over exposure. These are essentialy light frames, make sure they are correctly exposed (not over or under exposed). Overexposed flats will not compensate for vigneting.
Flat Darks (darks for Flats, sometimes called Dark Flats)
Purpose: These are dark frames taken to calibrate the flat frames taken above. Think of flats as another set of "light" frames that need their own separate calibration before being used to "normalize" the light frames.
- Remove the lens and cover the camera with a body cap and take 100 or more at the same exposure time you use for your flats.
- For DSLRs, use the same ISO as your Flats
- For cooled CMOS cameras (where you can modify both the gain and offset), use the same gain, offset, and temperature as used to capture the Flats.
-Biases - deprecated for CMOS/DSLRs- DO NOT USE.
These are "zero" exposure frames that in theory capture the "zero-exposure" noise (the read noise) of the sensor. Since CMOS sensors cannot read out the chips without exposing, in practice these are taken with the minimum exposure time possible. Each set of light/flat exposures (as in same ISO (gain/offset)/exposure time/temperature) needs their own separate set of bias frames. In theory they can be used to "scale" dark frames so they can be used for different exposure time darks. For CMOS sensors, especially for cheap sensors with "amp glow", they are detrimental as lead to incorrect math (double subtraction). They are not necessary since darks already contain the same information. Do not use instead of Dark Flats.
This page explains calibration frames well. Darks, Flats and Flat Darks frames are the most important three. Biases are deprecated, do not use for DSRLs or cooled CMOS sensors. Always take darks and flat darks, even if you have a low noise sensor and lots of exposure time, the calibration math will not be correct otherwise and will lead to hard to remove artefacts (banding).
- Disable any automatic noise reduction e.g. high ISO NR, long exposure NR. These slow you down and are unnecessary as you will capture your own calibration frames and you don't want to throw away data early.
- Disable image stabilization
- Set camera to save RAW images
- Change camera mode to manual (M)
- Find your best ISO setting(s) to make a good trade off between dynamic range and noise. You can search this online (google: [camera] astrophotography iso) or use the graphs here. Newer cameras are 'dual-gain' which means there are two optimal ISO settings and the others will just reduce dynamic range which can make you lose star colour.
- Start at your widest aperture. You may want to stop down (increase f-number) later if you aren't satisfied with the lens performance wide open. Check the edges of the frame for vignetting, coma (comma shaped stars) and purple fringing.
- Set lens to manual focus (MF). Point at a bright star, and use a 3D printed Bahtinov mask to focus (best) (check example image here) or zoom in as far as you can in live view and try to make the stars as small as possible.
- Start taking test images starting from 10-30s. You may need BULB mode to go longer than 30s. Open the histogram display on your camera. Note where the peak of the histogram is. Increase or decrease the shutter speed until the peak is approximately 1/4 to 1/3 of the way from the left of the histogram.
- Zoom in to your test image to make sure the stars are round and not making trails - you may have to calibrate the OAT here.
- Set your (built-in) intervalometer, PC software, etc to the shutter speed you determined above and fire away! Aim for at least an hour of exposure in total; more is of course better but there are somewhat diminishing returns.
- As described above, you may want to take dark frames while you're out, as the need the same temperature as your light frames. Flats and accompanying flat darks (darks for flats) can be taken before or after your imaging session.
- Set up guiding for better long exposures especially at longer focal lengths.
- If guiding, make sure to enable dithering so fixed pattern noise, hot pixels etc are 'averaged out'. This is also useful if you want to use drizzle to increase resolution when undersampling or imaging smaller targets (more info from DeepSkyStacker).
- Easiest: Sequator
- Alternatives: DeepSkyStacker, Siril
- Advanced: PixInsight (paid)