PHOTOGRAPHING THE NIGHT SKY
There’s a tidal wave of information on the Web about astrophotography, but this item covers most of the basic stuff you need to get started . “Youtube” has hundreds of articles on post processing , mostly in Photoshop, but also using specialist software like Deep Sky Stacker ( which is free) and PixInsight (which isn’t).
First- don’t expect too much. Just using a camera, you can get decent large images of the moon, but all other Solar System objects except the sun* will always be points of light or very small circles at best. Wide field images, taking in a big area of the sky, especially with “light painted” scenery as foregrounds, are often the most impressive astrophotos outside the really specialist “deep sky” stuff.
*Solar photography is a specialist subject. Doing it wrongly can destroy a camera or your eyesight. Seek specialist advice.
This item is about photography without a telescope. Telescopes are pricy and some of the priciest may be unsuited to photography. You would need a T-mount to connect the two optics – basically a ring that fits the lens mount of your camera and screws into a tube, usually 1.25″ or similar diameter which slides into the telescope eyepiece mount tube. You should talk to a specialist before buying anything. There are web forums where you can get advice about what and where to buy. “Stargazers Lounge” is a good one. Lots of friendly, UK centred advice to be found there.
When using a camera on its own, don’t use long zoom or telephoto lenses, except for the Moon. Use a wide angle or short zoom and go for wide field images- at least to begin with. Trying to find faint “deep sky” objects with a long lens can be very frustrating. If you do try something like The Orion nebula or Andromeda galaxy, start with a wide angle, locate the target, centre it, then swap to the longer lens.
With or without a telescope, you need a solid mount, A decent tripod is fine for the moon, but the moon moves pretty fast when seen through a tripod mounted lens. So does everything else in the sky- and everything else is also very faint.
To solve this, you can use a driven telescope mount, or “tracker mount”– basically a motor, geared to turn at the same angular speed as the sky. This fits onto a tripod and the camera (or telescope) is attached to the mount, usually via a ball and socket head. Once the mount has been “Polar aligned” (ie pointed precisely at the celestial north pole- which is not exactly at the pole star), the mount will keep any object in the sky in your camera’s viewfinder, if necessary for hours, except satellites ( Yes, the Moon is a satellite- but a decent quality driven mount has a special setting for the Moon).
You don’t use a driven mount to do star trails– in fact it makes them impossible, unless you switch the drive motor off, as it’s the rotation of the sky that creates star trails in the first place. What a driven mount does is allows long exposures of faint objects while keeping them centred. But! The sky turns 15 degrees per hour, so if you use a driven mount in reverse, you can do star trails twice as fast, at least in theory, though you may find gaps in them.
Focussing a modern digital camera in the dark can be harder than with an older, film camera. Auto focus may cope with the moon, (Contrast based systems seem best), but for anything else, you will need manual focus at infinity. While manual lenses had an infinity mark, modern autofocus zooms often do not and focussing on a star can be fiddly. Use the Moon, or a distant street or house light. Lock the focus if you can. Do this before you start taking any actual pictures and check for blurring in live view or after your first image. “Trailing” of stars means the polar alignment is not accurate enough; the rotation axis of the camera is imperfectly aligned with that of the Earth, so the stars become elliptical or stretch into lines. Keeping exposure lengths under 10 seconds reduces this, but better alignment is needed.
White balance. “Daylight” is usually OK, but if you shoot raw images, you can edit afterwards, which you will probably do anyway.
It’s dark at night! Can you find your camera controls with your eyes shut? It’s surprisingly easy to fall over a tripod-mounted camera in the dark, even one you just set up yourself , doing costly damage to it and to you. It’s even easier to fall over someone else’s, which will make you no friends.
Carry a torch. A small, cheap LED one is fine. (You may have to hold it in your mouth sometimes). Get a red ink marker and scribble on the lens so it emits a dull red light- enough to see camera controls by, but not enough to dazzle anyone you are with. If you want to “light paint” scenery around you, take a larger , bright torch- but be careful where you point it. Remember spare batteries. Head torches are ok, but don’t look at any companions while talking to them, as you will wreck their night vision, which takes 15 minutes or more to recover fully.There’s a certain etiquette about this which avoids you having your lights punched out. Going out at night alone has obvious risks though. At least let someone know where you will be and when you should be back.
Let someone know where you are. Take a cellphone and check for a signal. It can double as a flashlight.
Dress warmly. You will be doing this at night, remember, probably in winter. It gets cold.
Have spare batteries for everything and ensure rechargeables are recharged! Batteries die early in the cold.
Let your camera cool down well before you use it – otherwise, condensation may form inside the camera when it moves from indoors to outdoors (or vice versa). You won’t see it, but it will fog the sensor. If going out at night, put the camera gear (without the battery) in your car, or a garage, or other unheated place an hour or two before you go. Keep the battery in a warm pocket and replace it once you get there. Carry a spare battery, or three. Don’t forget it! A spare memory card won’t hurt either.
Turn off any motion or anti vibration sensors in your camera. They probably won’t help and can actually create vibration. Try on, then off and compare results. Go with what’s better. These systems improve all the time, but sometimes get in the way.
Keep your paws off. Using a multiple time exposure function, or a remote control, or even a self-timer delayed shutter release will reduce camera vibration, as will not dancing around , while trying to keep warm. Remember you are probably in a muddy field and mud moves when danced on. Walk gently away from the camera while it does it’s stuff.
In theory, with a perfectly aligned driven mount, you could take a twelve hour time exposure. In practice, you can’t, because your battery will die, an owl will nest on your camera, thermal noise will turn the image into something like an explosion in a purple paint factory, a car headlight will swing over you, or the sun will come up and ruin everything , but you’ll have frozen to death by then anyway. What you should have done is…
Take lots of short exposures (8-10 seconds is common without a driven mount) and “stack” them in a computer. That way you can ditch those exposures when clouds went over, or a 747 . You won’t get as much thermal noise and if you are on a tripod with no drive , or a misaligned driven mount, you won’t get “trailing” .On a fixed mount, 30 seconds is the absolute limit before visible trails form, with a wide angle lens, maybe 10 seconds with a short telephoto. With a driven mount, shots of sixty seconds or longer are possible, but for much over five minutes, very precise alignment is needed- and any external light will ruin this. (Security lights are bad, as are passing aircraft, car headlights, fireworks etc).
This is a link to Deep Sky Stacker, a free-to-download program for stacking astroimages.
Noise can be reduced by taking frames with zero actual exposure (ie lens cap on) , then using a computer algorithm to subtract the thermal noise pattern from the images. This is a “darkfield” image. Some cameras can do this automatically, but that turns every 8 second image into a sixteen second pair- so you must allow extra time. Don’t take darkfield images while doing star trails, or you will in effect be taking time lapse images of a rotating light source, so you get “star dotted lines” instead of trails. Take your dark field shots when you are done with the star trails- using the same exposure settings as for the trail shots, but with the lens cap on.
ISO for astrophotography depends on the camera and lens, but 1600 is a common place to start, except for the moon, which is far brighter – see below. Open the lens to maximum aperture. Depth of field isn’t a consideration as you are working at infinity, usually on point light sources. For foreground or horizon scenery it’s best to take separate images and blend them in software. (You can even do this in daylight for a bizarre effect). One problem with a driven mount is that while the sky is held still, the background and foreground move, relative to the camera, so scenery may have to be separately shot and cunningly pasted in after the star shots are done.
The Moon is the big exception. Use the longest lens you have. Try autofocus once. If it works, fine. If not, go to manual. A full moon is a big, largely black, rock in very bright sunlight – so it approximates well to an 18% grey card. It’s also far brighter than you might think. Use a daylight white balance. Try about 1/125th second at 100 ISO to start with. Use spot metering if possible. You may have to underexpose by a couple of stops unless you have a lens big enough to fill the frame. If you use automatic metering with a wide angle lens, you are asking the camera to meter correctly for a small, bright object against a big black background. It will overexpose wildly, so the moon will be washed out in the viewfinder. Best to set the camera to manual and meter yourself. If you do use a long lens, you will appreciate why astronomical telescopes have driven mounts. Seen in a narrow arc of sky, the moon moves fast.
For shadow detail of lunar craters and mountains, full moon is actually the worst time possible, as sunlight is then directly overhead on the moon from our point of view. Most professional “full moon” shots are stitched composites taken at different times in the lunar cycle, overlaid in software and corrected for varying light levels as you would with a stitched panorama.
For photographing anything other than the moon, it’s best to pick a night with no moon as moonlight will wash everything out.
Try taking a couple of dozen images, then stack and average them in Photoshop to reduce noise.
Long exposures of stars, lasting several minutes, using a driven mount, are interesting and give quite bright images without stacking. Imperfect polar alignment will show as ” star trailing” , where stars are short blurry lines rather than points. Light pollution and in-camera “noise” will give the image a reddish or purple cast, unless you are in a totally dark area, It’s worth trying though, to get an idea how your camera behaves .
Without a driven mount, If aligned on the north star, this sort of long exposure will give circular star trails. Hiding the pole star behind a steeple or tall building creates a “star halo” round the building. Nice if a reflective pond is available.
All the above is fine in theory and I intend to try it out if it ever stops raining.
Edited 3rd Feb 2016.