An Introduction to DEEP SKY ASTROPHOTOGRAPHY

Definition  The Art and Science of capturing Photons in a static form into a light bucket as it comes through a medium of ever changing refractive index, when captured from a moving platform with a man made machine to counteract the movement is astrophotography.

Hardware  Imaging device  CCD or DSLR  Tracking device  Mount  Optics  Telescope or Telephoto lens

The Challenge  Moving platform – Earth  15 arc secs / sec 0r 23 hrs and 56 minutes is One Sidereal Day  Patience – loads of it ! And let us think about revolving platform

OSCILOSCOPE How Mounts Work  worm and worm gear

Orion Nebula

WHY WORM and WORM GEAR ?  DESIRABLE PROPERTIES !  TORQUE MULTIPLIER  HIGH RATIO SPEED REDUCTION  Starting point : Stepper motor with spur gear  First level reduction, next the worm is driven and then worm gear.  Worm can turn the gear,reverse is not true.

Star No Drift Mount Pointing at NCP NCP Earth Telescope

Star Mount Pointing at East NCP Earth Telescope Telescope Trace Drift Down

Earth Star Mount Pointing at West NCP Earth Telescope Trace Drift Upward Telescope

Star No Drift Mount Pointing Correctly NCP Earth Telescope

Star Drift Up Mount Pointing Up NCP Earth Telescope

Star Drift Down Mount Pointing Down NCP Earth Telescope

Pause to think  Why choose the star near the meridean for e/w error identification ?  And similarly why at the horizons for north south error identification?

Why choose a star at the Meridean  Maximum drift for east west error  Occurs at the Meridean  Maximum drift for North south error  Occurs at the eastern/ western horizon

OSCILOSCOPE Periodic error

Periodic Error  Periodic error-correctable(Integer fundamentals) ie even number of times  Occuring once in a worm period 1x fundamental frequency or 1x fundamental  If smooth – long exposure guided pictures  If jerky – long exposure is not possible  Random error-Not correctable(non integer fundamental)  Properties:Frequency,Amplitude,Phase and Roughness  Usage of Frequency Fourier transformation ie  Data points ofTime+location into a frequency spectrum  ? Why cant non integer fundamentals not be corrected

Time+location data point into a frequency spectrum

Effect of PE on your image

PEMPRO software: corrects refraction errors+ PE

Review of drift,PE and effect on ccd/cmos

Polar alignment Real time 

NGC 253

Pause to think  Why are observatory domes placed in the region of 30 metres or 90 ft elevation?  Why do u have a cooling fan for larger mirrors and newtonians ? Other than cooling ? Why cool anyway ?

SEEING and TRANSPARENCY  THEY ARE NOT THE SAME !  TRANSPARENT SKY MEANS :  DARK SKY WITH LITTLE OR NO HAZE  THE SEEING COULD STILL BE BAD !

Seeing and Transparency  You cant image what u cant see  Atmosphere  Turbulence  Vertical mixing of air  Lateral movement  Variable RI

Seeing and Transparency Effects  Image motion  Blurring  Scintillation  Fat stars

Seeing and its effects

CALIFORNIA NEBULA

SEEING and ITS EFFECTS

SEEING and its EFFECTS

Seeing based on latitude

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

Diagnosis of seeing problems  Tube currents: Dark or thin white line moving over a bright defocussed star  Local ground currents: Swarms with wrinkles across the view of the scope  Atmospheric seeing: as blur of the focussed star, etc

NORTH AMERICA NEBULA

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

Boundary Layer

SEEING  Review of Seeing

M51

How to choose Imaging Equipment based on seeing  Pixel Size – arc sec/ pixel =  Image scale:206xpixel width/focal lenght  Seeing Conditions using FWHM  Nyquist Theorem Oh no I only know Pythogoras  8  FOV in degree= 57x size of chip dimension in mm/focal length in mm  Eg for my scope camera combination  57x(22x15)/600 = degrees  BOTTOM LINE: THINK FOV not Image scale

F ratio  All other factors being equal F Ratio matters most !  If two scopes of equal focal length but different aperture size, both will require same duration for getting equal pixel SNR

FWHM on a STAR PROFILE

Autoguiding Mechanism  Methods  OAG  Guide scope  Advantages and disadvantages  0.25x cosine of DEC= sidereal rate

Basic Principle using OAG  The distance between the guiding chip to the pick off prism should be the same distance to the main camera chip.  After approximating the guide camera, if there is a requirement for inward focus then u must move the guide chip closer and vice versa.

Working Example  Main camera: 0.84’/pixel  Guidecamera: 1.28’/pixel  Max guiding error: 75% of image scale of main camera  75% main camera= 75%of 0.84=0.63 px p to p .translate to guide camera 0.63/1.28 =0.49 px peak to peak,so min motion should be 0.49/2= 0.24

OSCILOSCOPE Guide Scope Stepper Motor Autoguidin Auto Guiding

Auto-guiding settings  Min motion  Max movement= 75% of imaging camera  Exposure settings: 5 sec duration  Aggressiveness  IF Autoguiding can overcome error why do a PEC at all ?

Collimation

Othogonality Error correction  For thos e who like Math !! ( not me)  OE is othogonality error in DEGREES  X is distance between the front and rear scope attachment rings  X tan*(OE)= additional shim thickness in mm  How to find out the error in degrees?  For people like me: Simply note the distance between the star and cross hair. Correct half the error with shims ; repeat till no more error !

Performance Checklist  Check flexure  Balance your system –How is it done ?  Polar align your mount  Perform periodic error correction  Collimate if SCT or Newtonian and focus carefully  Check for orthogonality error (for accurate meridean flip)  Determine Imaging conditions  Optimize guider settings  Plan your target,Image analyse and calibrate

JUPITER

How cameras/CCD work  Photons fall on pixels –converted to electrons in the pixel (wells)  Fill Factor- portion that records light  Electrons are counted and the camera s computer stores as ADU s- in linear fashion for RAWs  Electronics for amplification,NABG circuitry and Bayer Matrix cuts the photons and hence QE is diminished

Noise and SNR  Random arrival time of light quanta:light is noisy  Shot Noise  Noise is sq root of signal( hence)  Stacking increases SN ratio  Methods to increase SN ratio- a)duration,b) more subs,c)dark skies,d)focus,e)calibration

Why All this theory  Think Exposure duration terms of hours  Not minutes  Try to get the longest sub exposure your back ground light allows for individual exposure  Or till your stars just begin to saturate

Duration of your subs  Photon noise must be 10 times the read noise  Photon noise limited  Beyond this point no great advantage  Bottom line : 5 to 15 min from semi dark skies  20 – 30 minutes from very dark skies and seeing

Signal to Noise Ratio

Dynamic Range and Gain  The difference between brightest and dimmest that can be simultaneously recorded by CCD camera.  Gain or ISO settings  Gain is set by output electronics  Gain determines how many electrons per ADU  Full well depth divided by gain= dynamic range

System gain  Gain of 2 means 2 electrons per ADU  If Fwd is 64 K then 32 k counts  Ie each pixel is represented by 32 k ADU or shades of gray  Hence Higher Gain means lower dynamic range

CCD Jargon Review  Review of CCD Jargon

Summary  Polar alignment  Critical focus using fwhm in software or bahtinov mask  Guide star  Subs or the main photo sequence

Image Calibration  Removing all sources of Electrons other than  Photons  Lights or Subs : Long individual exposure  Multiple exposures upto few hours  Stacking : to increase the signal to noise ratio

DARKS  It is electron build up in pixels when exposure  Is on whether there is light or no light ! This is due to heat build up when camera works  Removal : Cooling and Darks subtraction  Cooling can reduce the rate by a factor of 100  Hence CCDs are cooled, for DSLR we can do external cooling.  Remaining electrons are removed by Dark Subtraction.

5 min dark at 70 F

Darks AT MINUS 15 C 5 MIN EXPOSURE qhy 10 ccd

FLATS  Eliminate Dust Motes, Vignetting and optical variations  Also Removes pixel non uniformity  T shirt flats  Histogram from 1/3to ½

Flats showing vignetting

Bias Frame aka Read Noise  Take the shortest possible exposure between 9-30, average combine and subtract from dark to get a Thermal exposure !  To measure Read noise subtract a bias from the combined bias.  Charge is too small, so amplification is required which is the major noise source

WHAT ISO ?  Jerry Lodriguss Suggests high ISO for  Warm temperature eg ISO 800 and 1600  For cooler temperature shoot at ISO 200, which is closer to Gain =1

Stacking  Acquire  Stacking and its advantages.  Stacking Increases Signal to Noise Ratio  Calibrate  Final Image

Image Processing  5 S s !  Signal(signal to noise ratio)  Subtract-Digital Subtraction  Stacking  Stretching the data  Saturate colour

My favourite websites and books and forums  Starrywonders.com  Petesastrophotography  Allaboutastro.com  Photoshop Astronomy by Dale Ireland  Guide to astrophotography –Jerry lodriguss  BAS and astronomyactivities  Digital astro  Autoguiding  Qcuiag

What is the main ingredient  Passion,Patience, A Supportive family and a good set of friends.  Thank you