Narrowband Imaging, or How to Image in Downtown Portland Duncan Kitchin OMSI Astro Imaging Conference 2010
Agenda Sources of noise in astro images Selectively removing light pollution Narrowband filters and imaging Accurate polar alignment methods
Sources of Noise in Astro Images
Sources of Noise in Astro Images Important noise sources (not an exhaustive list): Camera induced noise Readout noise Dark current noise PRNU (photo-response non-uniformity aka fixed pattern noise ) Inherent noise Shot noise from imaged object Shot noise from light pollution Even with a theoretically perfect camera, there will still be noise Depending on the conditions, different noise sources may dominate
Shot Noise Shot noise is the random variation inherent in anything which consists of an aggregate of independent discrete events Light Electric current Has nothing to do with the performance of the equipment Fundamental physical constraint
Properties of Shot Noise Magnitude of noise increases with the square root of the signal Signal to noise ratio also therefore increases with the square root of the signal Absolute magnitude of noise is increasing, just not as fast as the signal
Shot Noise Magnitude Mean: 10 Standard Deviation: 10
Implications for Unwanted Signals Unwanted signal Dark current in the camera Background sky glow Subtract the mean value by shifting the black point The noise that comes along with the unwanted signals remains Impact of Light Pollution is Sometimes Catastrophic Excess Noise
Noise Conclusions Need to fix the biggest sources first In heavy light pollution, skyglow dominates Light pollution filters can help Narrowband filters can help a great deal After fixing that, next dominant source is likely to be dark current Cooled CCD can help
Selectively Removing Light Pollution
Selectively Removing Light Pollution Exploit one of two things: Light pollution does not have uniform wavelength distribution In some cases, target object does not have uniform distribution LPS filter solution Notch out the wavelengths where light pollution dominates Narrowband solution Notch in the wavelengths of interest
Light Pollution Dominant Wavelengths Hg Hg Hg Hg Na Hg 320 370 420 470 520 570 620 670 720
IDAS LPS Filter Response
Effects of LPS Filter In LP limited conditions, IDAS LPS filter provides an improvement of approximately 2x 3x YMMV What this indicates: LP is not confined to just the dominant wavelengths Has significant distributed component (presumably from incandescent lighting)
Effects of a Narrowband Filter Pass just a single wavelength of interest H-alpha, [OIII], [SII] most commonly used Much stronger selectivity for good signal versus background sky glow But Wanted signal also has substantial distributed component We re reducing the wanted signal significantly too Requires substantially longer exposure time
My Motivation
Imaging in Downtown [anywherearoundhere] Narrowband imaging is the biggest hammer in the toolbox In heavy light pollution, nothing else will quite do but you gain several new problems that weren t there before
Narrowband Filters & Imaging
Narrowband Filters & Imaging Considerations in choosing a narrowband filter Narrowband imaging issues Increased exposure time Emission line brightness Assembling false color images
Choosing Narrowband Filters Multiple manufacturers (non-exhaustive list) Baader Astrodon Astronomik Different filter pass band 3nm 35nm Wide variation in price Cost per filter ranges from ~$120 to $400 for a single 1.25 mounted filter Variation in quality
Implications of Pass Band Narrower pass bands have greater rejection of light pollution Increased signal to noise ratio for a given exposure time Very narrow pass bands can resolve additional spectral lines Price increases significantly as pass band reduces
Coating Issues Some filters have been known to exhibit edge issues Imperfect coatings near the edge cause leaks which can be seen distinctly on flats
Narrowband Flat With Artifacts
Narrowband Flat Artifacts Bright artifacts resulting from leaks are a function of the light across a large image area Flats are uniformly bright, and show very bright leak artifacts Starfield images show much smaller artifacts Nebula images show intermediate artifacts Because these artifacts are not just an uneven illumination issue, calibration with flats does not rectify the problem
Calibration Example 1
Calibration Example 2
Dealing With Edge Issues Two options: Get a filter with no such artifacts Stop down the filter Strong recommendation: As soon as you get a new narrowband filter, take flats Look for artifacts, post for others to look at If you have such artifacts, and you are stuck with that filter, stopping down may reduce or eliminate them At the expense of increased vignetting
Filter Aperture Stop
Narrowband Imaging Issues Longer exposure time Narrowband filter dramatically improves contrast with respect to skyglow but the signal is very much reduced Best case is with a monochrome CCD May require exposure times of 20 30 minutes
Narrowband Filter With a DSLR One Shot Color Camera Has Substantially Reduced Sensitivity
Emission Line Brightness Most commonly used (brightest) emission lines H-alpha Oxygen III Sulfur II Bright and planetary nebulae will have strong emissions in h-alpha, but may not have strong emissions in the other wavelengths Good data can be hard to find
NGC1499 H-alpha, 2 hours
NGC1499 [OIII], 2 hours
Assembling Narrowband Images If you have three channels: Hubble palette: [SII] Red H-alpha Green [OIII] Blue Opinions vary If you have two channels Consider LRGB assembly, using H-alpha as L Duplicate h-alpha as red No fixed rules requires experimentation
NGC1499 Two-Color
Accurate Polar Alignment
Accurate Polar Alignment Theory Analyzing drift and frame rotation Practice Polar scope methods Iterative alignment Automated alignment Drift alignment
M97, One Hour, Guided, Poor Alignment
Analyzing Drift & Frame Rotation North/south drift and rotation of objects in the field of view, resulting from polar misalignment Relation between polar alignment and degree of drift and rotation is very complex, and depends on the location of the object in the sky and the hour-angle of the error Autoguiding can fix drift, but can t fix the frame rotation With long duration exposures (20+ minutes) frame rotation can become a significant problem Effect of frame rotation depends on the distance between the center of the image frame and the guide star; significantly worse with off-axis guider
Frame Rotation Spreadsheet Parameters Time 20.0 minutes Time angle t 5.00 degrees (0.087266 rad) Error e 10 minutes (0.002909 rad) Pixel Size p 5.40 microns Offset o 17.4 mm Rotation limit rhom 0.000310 rad Declination Max Rotation 0 (0.0000000 rad) 0.000254 5 (0.0872665 rad) 0.000255 10 (0.1745329 rad) 0.000258 15 (0.2617994 rad) 0.000263 20 (0.3490659 rad) 0.000270 25 (0.4363323 rad) 0.000280 30 (0.5235988 rad) 0.000293 35 (0.6108652 rad) 0.000310 40 (0.6981317 rad) 0.000332 45 (0.7853982 rad) 0.000360 50 (0.8726646 rad) 0.000396 55 (0.9599311 rad) 0.000444 60 (1.0471976 rad) 0.000510 65 (1.1344640 rad) 0.000604 70 (1.2217305 rad) 0.000747 75 (1.3089969 rad) 0.000990 80 (1.3962634 rad) 0.001484 85 (1.4835299 rad) 0.003009 89 (1.5533430 rad) 0.017417
Iterative Alignment Only works with a GoTo mount Works best with Polaris visible Can work in other conditions, but may be ineffective depending on what is visible Procedure: 1-star align on anything between 6 and 24 hours RA GoTo Polaris Adjust polar axis to remove 2/3 of the error Repeat
Drift Alignment Cons Time consuming compared to other methods More complex Pros Very accurate given enough time Works even when Polaris not visible Does not require GoTo capability
Drift Alignment: How it Works Drift at celestial equator on east or west horizon depends only on polar misalignment in altitude Drift at celestial equator on meridian depends only on polar misalignment in azimuth By looking for drift north or south in two different locations, can tune polar axis
Example: Drift On Eastern Horizon True NCP Mount Movement Star Movement Mount NCP If mount altitude is too low, star drifts south
My Drift Alignment Method: What You Need Some kind of video-like monitoring capability PHD Guiding works well for this http://www.stark-labs.com/phdguiding.html Monitoring crosshair capability highly recommended StarTarg available at Andy s Shot Glass for $19.95 http://www.andysshotglass.com/startarg.html Instrumenting the mount very useful, but not essential
StarTarg Overlay
Instrumenting the Polar Axis
Drift Alignment Procedure Use the longest available focal length Repeat for each of east/west and south Step 1 : Get oriented Step 2 : Locate any star in the center Step 3 : Run for 5 minutes Step 4 : Adjust Repeat With instrumented mount Start with 64 tick movement After first direction change seen, divide movement amount by 2 each time
Getting Oriented Switch off tracking and watch which way the star moves It always moves west Dependent on your configuration, this will give NSEW directions Dependent on true or reversed image Diagonal or OAG will give reversed image
Getting Oriented True Image Reversed Image N N E W W E S S
Wrap Up & Key Takeaways In heavy light pollution, noise from LP dominates LPS filters effective for moderate LP, narrowband effective in heavy LP Narrowband filters introduce new issues Filter artifacts Much reduced signal; dark noise can become dominant Longer exposures; more accurate alignment
Contact: duncan@kitchinonline.com http://duncankitchin.zenfolio.com