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Transcript
ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
ASTROPHOTOGRAPHY
A BEGINNERS PROGRESS
STEVE BALDWIN
STEVE BALDWIN: WGCPC: 5th January 2016
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
INDEX:
• Introduction
• What is there to capture?
• Where and when to take the images?
• Planning
• Equipment and settings (for imaging different astronomical subjects)
• General equipment and settings issues
• Dew formation
• Post processing
STEVE BALDWIN: WGCPC: 5th January 2016
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Introduction:
I have been taking photographs of the night sky since December 2012 after attending a short “Astrophotography for Beginners” course at the Royal Observatory in Greenwich.
I am still a beginner – and this evening I hope to pass on some of what I have learnt so far and impart some of the enthusiasm I have for this genre of photography. But first a health warning:
• It is not sociable – unless you are with like minded folk
• It can involve a lot of driving
• You can be waiting around in the dark for hours
• It can be damned cold
• It will result in severe loss of sleep
• It can be extremely frustrating
However, all of these are forgotten (but good to brag about) when you feel proud of a finished image!
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Introduction (continued):
Astrophotography can be spit into two main disciplines:
• Photographs taken with a camera attached to a telescope.
• Photographs taken with only a DSLR camera and tripod.
The latter can be further categorised as either wide field or deep sky astrophotography. Deep sky astrophotography can be carried out with and without tracking, i.e. mounting the camera on a device which tracks the stars making corrections for the rotation of the earth.
I do not own a telescope and whilst I now have a tracking mount this talk is about my attempts at taking images of the night sky using a static DSLR camera and a tripod, i.e. wide field astrophotography. Thus far, apart from specific applications i.e. star trails, my images are obtained without “stacking” in post processing. While the use of stacking is used in all forms of deep sky astrophotography it can be used as a means of capturing deep sky objects without the need for a tracking device and is a subject in itself – perhaps for another evening!
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
What is there to capture?
There is a significant list of subjects that fall under the umbrella of astrophotography, not all of them visible after dark:
• Lunar images *#%
• Starscapes (landscapes taken after dark) and star trails *#%
• Milky Way *#%
• Planetary conjunctions *#%
• ISS and satellites *#%
• Meteor showers *#
• Noctilucent clouds*
• Aurora *#
• Zodiacal light *#
• Eclipses, both solar and lunar *#
• Deep sky objects such as galaxies, comets and nebulae
• Planetary transitions across the sun *
*
# %
Images can be taken without stacking Images I have attempted to date
Guidance on taking images is provided as part of this talk
Subjects in italics are not strictly wide field astrophotography but are included for completeness.
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Where and when to take the images?
Astrophotography in general relies on having clear skies which in the UK can be problematical. I have been known to travel many miles to have the opportunity to capture specific astronomical events such as last years solar eclipse.
Weather aside, we in the UK suffer from the effects of light pollution which often restricts our view of the night sky even when the sky is cloud free. So getting to a dark (or darker) sky site is important. An important aide in this quest is the Philips Dark Sky Map of Britain and Ireland available from bookshops and online resources such as the Dark Sky Discovery website at:
http://www.darkskydiscovery.org.uk/
In the south east we have locations such as the Dengie peninsular in Essex (1.5hr drive) and the North Norfolk coast (2.5hr drive). Further afield the Purbeck peninsular in Dorset (3.5 hr drive) can provide magnificent views of the central band of our galaxy, the Milky Way. There are of course many other locations and dark sky parks such as those in the Northumberland and the Brecon Beacons National Parks which are more distant but well worth a visit.
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Where and when to take the images?
Philips Dark Sky Map of Britain and Ireland :
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Where and when to take the images (continued)?
The dark sky map is simple to read. The darker the colour of the overlay the darker the skies and the more stars that should be visible to the naked eye!
The colours indicate the brightness of the dimmest star that can be seen with the naked eye at that location. The brightness scale can be confusing with the brightest objects having a –ve magnitude and the dimmest a +ve magnitude. The sun, the brightest object in the sky, has a brightness magnitude of ‐26.7, the full moon at ‐12.5, Venus, the brightest planet in our solar system at ‐4.4. When it comes to stars, Sirius A the brightest star in the sky has a magnitude of ‐1.5, Betelgeus, the star forming the right shoulder of Orion, has a brightness magnitude of 0.8 and the pole star, Polaris, a brightness magnitude of 2.1.
In the centre of London the dimmest star that can be seen on a good day when away from street lights is of magnitude 3.7 whereas stars as dim as magnitude 6 can be observed with the naked eye in the darkest sky locations in the UK and Ireland. This is not a linear scale and the difference in brightness of these stars is a factor of 8, for those interested in the maths, (6.0‐3.7)2.5=8.
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Where and when to take the images (continued)?
Dark sky contours for the south east of England:
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Where and where to take the images (continued)?
Dark sky contours for the south west of Cornwall:
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Where and when to take the images (continued)?
Dark sky contours for the west of Ireland near Galway:
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Where and when to take the images (continued)?
Remember, once you get to a dark sky site give your eyes time to adjust. The dimmest stars visible at the chosen site will not be apparent for about 20 minutes after escaping from artificial light.
So once at the selected dark sky site make sure you use a red torch to assist when setting up your gear – this will ensure that you preserve your night vision for the duration of the shoot.
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Where and when to take the images (continued)?
The dark sky contours provide guidance as to what you might see in a particular location if the viewing conditions are correct. The atmosphere is often clearest after heavy rain and when it has been windy. I have taken some of my best images of the moon this winter – every cloud has a silver lining!
It is also often better to image the moon when it is high in the sky as this limits the amount of the earths atmosphere that you have to shoot through.
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Planning?
Planning is one of the most important aspects of successful astrophotography and there are many aides to assist the photographer:
• Planetarium software such as Stellarium which is free to download at: http://www.stellarium.org/en_GB/
• Ordnance survey maps
• Compass
• Lunar phase calendar (available online, free)
• Sunrise ‐ sunset calendar (available online, free)
• Weather forecast ‐ Clear Outside at: https://clearoutside.com/forecast/51.82/‐0.10
• NASA website for sightings of the ISS (International Space Station) at:
http://spotthestation.nasa.gov/
• Sightings of satellites including iridium flares at Heavens Above website:
http://www.heavens‐above.com/
• Sky At Night Magazine – news on forthcoming astronomical events including monthly calendar of possible photographic opportunities
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Planning (continued)?
Stellarium screen capture showing planetary conjunction last October: STEVE BALDWIN: WGCPC: 5th January 2016
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Equipment and Settings ‐ Lunar Photography:
Let us start with Lunar photography. Equipment you will need to get started is little more than you will need for other photography genres:
• DSLR camera with manual mode option
• Tripod
• Cable release
Photographs of the lunar surface can be taken with a moderate telephoto lens. I use a 100‐400mm lens at 400mm on a full frame Canon camera but have also achieved reasonable results with a 300mm lens on a camera with an APS‐C cropped sensor.
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Equipment and Settings ‐ Lunar Photography (continued):
The moon is a bright object and hence an shutter speed of between 1/125s and 1/250s and f8 at ISO between 200 and 400 would be a good starting point depending on the moon phase. Remember to bracket your exposure. The moon is generally bright enough that your auto focus should still work. Remember to set your Autofocus mode to single point and ensure that the selected point is located over the surface of the moon.
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Equipment and Settings ‐ Starscapes:
Starscapes require little more equipment than necessary for lunar photography except that now a standard or wide angle lens is necessary and the camera should have a manual or bulb facility to allow long exposures and the lens should have a manual focus option and ideally, the camera should have a live view facility.
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Equipment and Settings – Starscapes (continued):
For starscapes focusing must be carried out manually. Setting the lens to infinity as shown in the following slide is a good starting point. A camera with live view would be beneficial to focus on the brightest star or some other focal point in the field of view at a suitable distance from the camera. Some cameras have a magnify function as part of live view which is of great assistance to the process especially considering that a wide angle lens may be being used.
Sometimes it is necessary to use a powerful torch to illuminate the foreground to facilitate manual focusing.
Focusing is particularly important recognising that in low light situations the lens is probably going to be close to fully open. Fast lenses with maximum apertures of f1.4 or f2.8 are ideal in low light situations but mean that the depth of field is particularly shallow making focusing more critical.
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Equipment and Settings – Starscapes (continued):
Setting lens to focus at infinity, manual focus:
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Equipment and Settings – Starscapes (continued):
In order to avoid star trails shutter speeds must be limited to not more than as predicted using the 500 rule, i.e. shutter speed  500/focal length. Remember that if using a camera with a cropped sensor the shutter speed calculation should be corrected to account for this. The following table gives maximum recommended shutter speeds to avoid star trails:
Lens Focal Length (mm)
Maximum Shutter Speed (s)
Full Frame
APS‐C Cropped Sensor
35
13
8
24
20
15
16
30
20
10
50
30
The above table assumes an equivalent focal length of 1.6 x full frame focal length which I believe this is suitable (but perhaps conservative) for all Nikon, Canon and Sony cameras with APS‐C cropped sensors.
The 500 rule is approximate and accuracy depends in which direction you are shooting relative to the celestial poles. Always check your results as you go!
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Equipment and Settings – Starscapes (continued):
Having set the shutter speed to the longest possible to avoid star trails and having set the aperture to fully open we now have to select the ISO setting for the sensor. I would recommend that you start with ISO of between 400 and 800 dependent upon your camera. Trial and error is the best way to get the most pleasing results. Use the histogram to check the exposure and if possible avoid it being weighted to far to the LHS (dark side). Often in order to increase the exposure while avoiding star trails the only option is to increase the ISO setting further. The higher the ISO setting the greater the noise and where available it is advisable to use the long exposure noise reduction facility in your camera. This has the disadvantage of doubling the time to capture each image.
If your camera does not have a long exposure noise reduction function then take a dark frame after each exposure and keep for noise reduction in post processing if necessary.
A dark frame is simply another exposure at the same setting but taken with the lens cap on. This will indicate the “noise” in the image and can then be used to subtract the noise from the image in post processing.
Samples of the noise in images are shown in the next slide taken in a Canon 350D which is now some 10 years old. Modern sensors give less noisy results.
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Equipment and Settings – Starscapes (continued):
The following images show noise in images taken in a Canon 350D taken at 30s at ISO values of 100, 800 and 1600:
ISO 100
ISO 800
ISO 1600
These dark frames are used in post processing to extract noise from the image if it cannot or was not done in camera during shooting.
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Equipment and Settings – Star Trails:
As the earth rotates the stars will appear to drift across the sky. Whereas in capturing starscapes we did not want to see any star trails in our image we are now setting out to create an image showing the stars tracking across the sky!
There are two ways of creating star trails, taking one long exposure of say 20 minutes or longer, or taking multiple images with a shorter exposure time and then stacking them on top of one another to create the trails. I prefer the latter approach because this will reduce the risk of over exposing any foreground and minimises the influence of light pollution.
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Equipment and Settings – Star Trails (continued):
In order to create start trails we therefore need a camera that has a bulb setting and an intervalometer which will allow you to programme the camera to take a series of images, maybe 50 or more, that can then be stacked to create the star trail. An intervalometer such as marketed by Hahnal, see details via the link below, is not strictly necessary as once set up the release button on a digital cable release could be locked down to allow capture of star trails.
http://www.hahnel.ie/index.cfm/action
In general terms we can use the same settings (shutter speed, aperture and ISO) adopted for our starscapes for creating star trails. However in order to minimise the “gap” in the trails (between images) when stacked we cannot use the long exposure noise reduction facility of the camera and must rely on using one or more dark frames in the stacking process.
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Equipment and Settings – Star Trails (continued):
Image stacking is carried out using software such as the free download from:
http://www.startrails.de/html/software.html
This is simple to use and allows you to load your images and dark frames. Star trails can also be created within Photoshop but I have always used this specific star trails application. There are others available. One downside of the software is that it only uses .jpg formats. It does however facilitate the production of both still images and time‐lapse video in AVI format.
You may wish to light paint your foreground while shooting one of the images to complete the picture. STEVE BALDWIN: WGCPC: 5th January 2016
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Equipment and Settings – Milky Way:
The equipment needed for capturing images of the Milky Way is identical to that required for starscapes. The differences are in the camera settings and the need to get to a dark sky location. Whilst stars can be seen even in built up areas the Milky Way will not become visible to the naked eye unless you get away from light pollution. From here the Dorset coast is probably the closest location where you have a good chance of seeing the centre of our galaxy near the constellation of Sagittarius above a southern horizon during May – October.
While the Milky Way can be clearly seen above your head along the North Norfolk coast it is always brighter in southern skies when viewed from the UK.
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Equipment and Settings – Milky Way (continued):
Shutter speed is again limited by the 500 rule and the camera is normally wide open, ideally at f2.8 or larger. The principal difference is in the ISO used. An ISO of 3200 or even 6400 is necessary to bring out the detail in the central band of our galaxy. Noise reduction in the form of the long exposure noise reduction facility in camera if available or taking a dark frame and removing the noise in Photoshop.
As for startrails and starscapes you may wish to light paint your foreground while shooting to complete the picture. Foreground interest is an important means of putting you astrophotography into context.
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Equipment and Settings – Planetary Conjunctions:
The equipment needed for capturing images of planetary conjunctions is similar to that required for starscapes. The differences are in the choice of lens. In some situations I have used a 24‐
105mm lens at 100mm to capture the moon in conjunction with Venus and Mars. Shutter speed is again limited by the 500 rule and if shooting at twilight or dawn the camera need not be wide open. If fact using a smaller aperture of say f8 will give you a starburst effect, particularly with a planet as bright as Venus.
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ASTROPHOTOGRAPHY: A BEGINNERS PROGRESS
Equipment and Settings – Planetary Conjunctions (continued):
Planning is also important. If you can capture the moon in conjunction with the planets of our solar system when in either waxing or waning crescent phase you stand a chance of capturing “earthshine” when the lunar surface not illuminated by the sun can be seen by light reflected from the earth. Obviously by reducing the aperture the ISO may have to be set slightly higher ‐ the correct value is often found through trial and error!
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Equipment and Settings – ISS:
The equipment and set up needed for capturing star trails is essentially the same as that needed for imaging the ISS.
Stacking your images in the same way as you would stack your star trail images can give some interesting results. Allow plenty of time to get set up and be sure you are confident where the ISS will appear in the sky as the maximum time it will be visible is about 5 minutes.
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General Equipment and Settings – White Balance:
Thus far white balance has not been mentioned. If asked for a recommendation it would be to shoot in daylight mode as all the light comes from our sun, our star, albeit that it may be reflected by other celestial bodies, or come from other more distant stars.
The exception would be if shooting when there are significant low clouds around which are illuminated by light pollution and therefore appear very red, particularly close to the horizon. In these situations I have resorted to using tungsten mode which tends to reduce turn red tones to blue and can give a more pleasing result.
As ever the back stop is to shoot in RAW where corrections can be made afterwards.
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General Equipment and Settings – Lens Coma:
Having purchased a new Canon 16‐35mm f2.8 lens you can imagine my disappointment when I found stars in the corner of my images were not pin point sharp – but instead looked liked kites! This it turns out is the product of lens coma and is caused when a lens does not contain aspherical elements. A good article on this can be found at:
http://intothenightphoto.blogspot.co.uk/2013/02/overcoming‐coma‐
aberration‐part‐2.html
The effect can be eradicated by reducing the aperture but this rather reduces our options in astrophotography! The only way forward was to invest in Samyang lenses and I now use a 24mm f1.4 and a 14mm f2.8 for much of my wide angle astrophotography. These lens are, thankfully, reasonably priced but mainly because they are manual focus only – not a problem when used for astrophotography.
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General Equipment and Settings – Battery Life:
You should be aware that in cold evenings camera battery life is often compromised. The number of images that can be captured per battery charge is significantly reduced at lower temperatures. Remember to take several batteries with you on a shoot and keep them warm in your coat for as long as possible.
If shooting an extended set of images for star trails it may be advisable to fit a battery grip which contains an additional battery. Hence if the first runs down you do not have to remove the camera from the tripod to replace the battery mid shoot! The grip may also facilitate use of AA batteries as a back up. Worth considering – but not cheap.
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General Equipment and Settings – Live View:
Live view is a valuable tool for astrophotography. We have already discussed its use in focusing but it can be helpful in other ways.
When taking an image with a traditional DSLR camera to take the image the mirror has to be moved out of the way. On some cameras this mechanical movement can shake the camera enough to cause stars to appear like commas in your picture. Live view prevents this as the mirror is already out of the way when the image is taken.
A downside of live view is that its extensive use will inevitably result in a warmer sensor – this could result in more noise that may have to be dealt with in post processing.
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General Equipment and Settings – Tripod:
Finally, a very basic precaution. Make sure your tripod is set up on firm ground. This is of particular importance if you are setting up to shoot start trails over a relatively long time. Setting up on a beach in sand the tripod may move, possibly only very slightly, but it may be enough to develop a rather peculiar shape star trails!
If it is windy you may want to hang a counterweight under your tripod to make it as stable as possible during the shoot.
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Dew Formation:
Photography at night, especially when it is cold or particularly humid, can be curtailed prematurely by dew formation on the lens. Without wishing to purchase a dew heater and the associated power source, nor carry it to my desired location, I use a lens muff to keep dew at bay. This is simply a cloth holder for three hand warmers which wraps around the lens. Details can be found below: http://www.kadamsphoto.com/catalog/digital‐after‐dark‐lensmuff‐keeping‐
your‐lens‐p‐85.html
I have found this works pretty well and although it comes from the US hand warmers available in Blacks and Halfords which will last up to 8 hours fit perfectly:
http://www.blacks.co.uk/mens/124850‐hot‐hands‐hand‐warmers‐5‐
pack.html/724130/?istCompanyId=d92b362f‐ac8a‐4a8a‐87ca‐
c56eafad7955&istItemId=raxlqtxal&istBid=tziq&gclid=CJD_o5DzjcoCFQMcGwo
dV9IG2w
You need to make sure the lens muff is on almost straight way otherwise, in my experience, you will be fighting a losing battle with dew formation on the lens.
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Post Processing:
Thus, apart from noise reduction, I have attempted little post processing. When capturing star trails and stacking them using www.startrails.de software dark frames have been taken and loaded as part of the stacking process. Dark frames can also be used in Photoshop to reduce noise. This tutorial should help you if you decide to go down this route:
http://starcircleacademy.com/2012/10/darkframes/
Post processing images of the moon can be carried out using the levels facility in Photoshop. Tutorials are available in back issues of The Sky at Night magazine.
If you want to post process your Milky Way photos there are plenty of tutorials on You Tube. I like this one where there are some relatively simple editing methods in the camera raw plug‐in available in Photoshop without having to go into Photoshop proper (even though this is covered in the tutorial): https://www.youtube.com/watch?v=FOAmP7A_x6c
In my view the darker the sky and the higher the ISO you camera can cope with it will reduce the amount of post processing you have to do.
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