Introduction to Astrophotography

Transcription

1 Introduction to Astrophotography The art and science of photographing the night sky with a digital camera Keerthi Kiran M Bangalore Astronomical Society

2 What is Astrophotography? Astrophotography involves taking photos of the sky and the various objects in sky. ASTROPHOTOGRAPHY IS THE ART AND SCINCE OF CAPTURING PHOTONS INTO PIXELS AS THEY COME THROUGH AN ATMOSPHERE HAVING A VARIABLE REFRACTIVE INDEX AS WE MOVE FROM WEST TO EAST, GRABBING THOSE PHOTONS USING A MAN MADE MACHINE CALLED MOUNT.

3 The Challenge Stars trail in the sky (due to earth s rotation) Targets are very faint and need long exposures Light Pollution

4 Tracking the stars - FoV Stars move in the sky at the rate of 15 arc secs / sec One circle = 360⁰. One degree = 60 (arc min) One arc min = 60 (arc sec) Examples: FoV of a 50mm lens on a full frame body = 40⁰ by 27⁰ FoV of a 600mm lens on a Canon crop body = 2⁰ by 1.5⁰

5 Fixed Tripod Limit the exposure time 600 rule is the starting point Max Exposure = 600/(Effective focal length) seconds Example: For a 18mm lens on a Canon crop body, exp = 600/(18*1.6) = 21 seconds 600 rule is useful for web sharing 600mm rule is only a rule of thumb. Other factors like pixel size also affect the max exposure.

6 Skytracker Portable setup Can carry upto 3.5kg weight (even more with some modifications) Can be used with lenses upto 200mm Exposure times of upto 600s with wide angle lenses

7 Polar alignment of Skytracker For proper tracking, the Skytracker has to be aligned with the celestial pole. Accurate alignment increases the maximum exposure we can achieve. Bad alignment will cause drift in north-south direction (declination) The Lat, Long for Bangalore is 13 and 77.5.

8 Other Skytrackers Vixen Polarie Astrotrac

9 Astronomical Mounts Equatorial Mounts Computerized with a database of thousands of objects Support GoTo functionality Can carry upto 25kg payload Support Autoguiding for accurate tracking Examples: Skywatcher NEQ6 ioptron ieq45 Orion Sirius G Celestron CGEM

10 Astronomical Mount Two types of Mounts Alt-Az Equitorial Equatorial Mount is recommended for Astrophotography Aligned to the celestial Pole Tracks the stars using a stepper/servo motor No field rotation

11 The Cameras for Astrophotography DSLR Cameras Nikon D810A Canon 60DA Canon 6D Sony A7S2 Pentax K-1 Dedicated Astro Cameras CCD Cameras (SBIG STF- 8300) CMOS cameras (ZWO ASI 1600) Use Intervalometer Control the camera through laptop

12 Digital SLR Camera Advantages over point and shoot Bigger Sensor Interchangeable Lens Can be coupled with a telescope Programmable exposure time Raw Image Capture

13 Some things to note Choose a camera with tilt-screen. Your back will thank you. Turn off Long exposure noise reduction and High ISO noise reduction. Always shoot in RAW Sony cameras have a Star-eater problem (may be resolved in 2018 cameras) Old Nikon Cameras did not have true raw output. So, some of the details would be lost. Nikon and Sony cameras have in-built intervalometer. Sony cameras can even generate timelapse video or startrail images automatically. Pentax K-1 has an inbuilt Astrotracer feature which will move the sensor in the direction of the stars thus tracking the stars (for short durations of time). Nikon D810A allows more red nebula than other cameras.

14 RAW vs JPEG JPEG uses lossy algorithm Removes all faint data not readily visible ALWAYS shoot in RAW Raw format preserves every bit of information Very important for bringing out details during post-processing

15 What is different in an Astro Camera

16 Astro modification of camera Remove the IR Block Filter Use a replacement filter to block all IR except H- alpha frequency.

17 Astrophotography Tool Backyard EOS ImagesPlus MaximDL Camera Control

18 Lenses and Telescopes Always use prime lenses Sigma Art Lenses Samyang 135mm f2 Nikon 180mm ED AI-S Canon 200mm f2.8 Day photography lenses will not give pin-point stars. Also they have distortions. Telescopes Refractors Achromats Apochromats Reflectors Newtonians Cassegrain Telescopes

19 How do telescopes work Camera lenses and apochromats have multiple glass elements with antireflection coatings. Apochromats are optimized to shoot the sky. They are designed to minimize distortions thereby producing pin-point stars. Newtonians have two mirrors. A concave mirror at the bottom of the tube to collect light and a flat mirror to reflect the concentrated beam towards the camera.

20 Autoguiding for High accuracy in tracking

21 Now that we have all the equipment, it Mount - $1700 OTA - $670 Camera - $700 Guidescope - $100 Guide Camera - $280 Battery - $100 Laptop Dew Heaters - $100 Add shipping plus customs is time to shoot

22 Manual Focus The camera auto-focus system does not work on stars. Live View Zoom focusing can be used. Tape the lens after focusing (take care of lens creep). APT provides tools to achieve the best focus. live_view.mp4

23 Capturing the images Capturing faint astronomical objects requires long exposure upto several hours. Single exposure tends to be noisy. Also, a single exposure for several hours is not practical because of light pollution. Solution: Stack multiple images to increase the signal and decrease the noise.

24 Example: M81 and M82 galaxies Telescope: Skywatcher 8 f4 (800mm focal length) with Baader MPCC Coma Corrector Mount: ioptron ieq45 50mm guider with StarShoot Autoguider camera Canon 6D Exposure details: ISO1600 Calibrated with flats and bias Processed in Photoshop and PixInsight

25 It s all about data Main Aim Increase Signal to Noise Ratio What is Signal? Astronomical Object What is noise? Unwanted artifacts Lets take an example

26 An example of Signal and Noise 32 images 128 Images 1024 images

27 Sources of Noise Heating of the image Sensor Readout Error Light pollution and sky background Cosmic Rays Pixel Defects Hot Pixels Cold pixels

28 Noise in images DARK NOISE : aka noise produced by hardware during exposure leading to extra electrons READOUT NOISE : aka Bias or Read noise, error of reading due to poor amplifier PHOTON NOISE aka POISSON NOISE :Random arrival time of photons Random noise: insignificant in our cameras and ccds but assumed due voltage fluctuation in camera circuitry

29 Different types of frames captured Light Frames: These are individual sub-exposures of the object These frames (obviously) help collect the data Dark Frames These are images captured with the same settings as the light frames but with the lens cap on. These frames help remove the dark current noise from the light frames Flat Frames This is an image of an evenly illuminated surface taken with the same camera-lens combination. These frames help to overcome the effects of Vignetting, dust specs on the camera sensor and so on. Dark Flats These frames help remove the dark current noise from the flat frames Bias Frames (Offset Frames) These are images taken with (ideally) zero exposure with the lens cap on. These images help in optimizing the dark frames.

30 Different Steps involved in Image Create Master Dark Calibration Average the dark frames Create Master Flat Average the flat frames Subtract the Master Dark from individual light frames Divide individual light frames by master flat

31 Image Alignment The light frames will not be aligned due to tracking errors or polar misalignment Detect the stars in the images Align the stars by using translation and rotation

32 Image Integration or Stacking Average the pixel values of aligned images Remove artifacts created due to cosmic rays, satellite trails etc. Detect Hot and cold pixels and remove them

33 How many images? When Average combination is used, the noise decreases by the order of square root of the number of images.

34 An Example Light Frame

35 An Example Flat Frame

36 An Example Dark Frame

37 An Example - Stacked Image

38 DSS to the rescue Registering Stacking Simple post-stacking processes to quickly view the final result. Saving the resulting image to a TIFF or FITS file (16 or 32 bit)

39

40 Registration Parameters

41 Stacking Parameters Choose Average method You could also use Kappa-Sigma clipping to iteratively remove bad pixels like cosmic rays, satellite trails etc Choosing Super Pixel mode will result in a 4-times smaller image but of a better quality. Use drizzle option if the object of interest is very small (Ex: a planetary nebula). This will work better if you have used dithering while guiding. Use custom-rectangle in conjunction with drizzle to make the computation faster.

42 Additional Settings

43 Basic Post Processing

44 Save Picture to File Two Options 16-bit TIF Supported by most image processing Software 32-bit TIF Supported by 64-bit Photoshop Supported by specialized Software like PixInsight, ImagesPlus etc. Recommended for Astro Image processing as this allows more flexibility during image processing

45 Post Process your image with a specialized Software Adobe Photoshop Nebulosity ImagesPlus PixInsight GIMP AstroArt

46 Finally let s see some images

47 Star Trails

48 Star trail basics Use low ISO to preserve star colors. Choose the exposure time based on the sky brightness (amount of light pollution). Under expose the images. When stacked, they automatically become brighter. Before stacking the images, preprocess the images by increasing star colors. My Movie2.mp4

49 Startrails Demo

50 Milky way

51 Shoot the foreground and background separately and merge Tracking the stars makes the foreground blurry Without tracking, the stars trail

52 Deep sky Objects

53 Leo Triplet Processing Demo

54 Moon and Planets

55 Sun

56 And So on

57 Image Showcase

58 Aperture, Shutter Speed and ISO Aperture: Diameter of the lens/mirror of a lens/telescope. Bigger the aperture, faster the image can be captured. Shutter Speed/ Exposure Time: The time for which camera shutter is opened and the image sensor continues to gather light. Longer the exposure time, brighter the image. Focal Length: aka Zoom More the focal length, more magnification. F-ratio of f-number: (Focal Length)/(Aperture)

59 Seeing and Transparency Effects Image motion Blurring Scintillation Fat stars

60 Seeing and its effects

61 SEEING and ITS EFFECTS

62 SEEING and its EFFECTS

63 HOW TO PREDICT SEEING CUMULUS CLOUDS IN AFTERNOON BAD SEEING FOR AT LEAST FEW HOURS AFTER SUNSET HIGH ALTITUDE CIRRUS AND STEADY 30to 40 Km WINDS PREDICT GOOD SEEING ALWAYS CHECK FOR TWINKLING

64 Overcoming seeing Take pictures from above the atmosphere-uh Make the air stand still- not possible High altitude improves seeing 30 metres above the ground has great effects Equal to ft: Publications of Astronomical society of pacific Voll117,pg 408 Sources that are near the ground Heat source

65 Effect of light pollution

66 Effect of light pollution