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A flat fielding primer Pete Kalajian NEAIC 2010. My interests Exoplanet transits Oph Arcturus Spectroscopy Cataclysmic Variables.

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Presentation on theme: "A flat fielding primer Pete Kalajian NEAIC 2010. My interests Exoplanet transits Oph Arcturus Spectroscopy Cataclysmic Variables."— Presentation transcript:

1 A flat fielding primer Pete Kalajian NEAIC 2010

2 My interests Exoplanet transits Oph Arcturus Spectroscopy Cataclysmic Variables

3 Part 1 What does a flat do?

4 What is a flat field frame? Camera/OTA exposed to a uniform illumination source

5 CCD review Noise (quantum mechanics) Pixel-to-pixel variation (manufacturing) Vignetting (optics) Dust (environment) Flats take care of the last three!

6 Next 5 images courtesy Steve Mazlin

7

8

9 No flat

10 After flat Magic!

11 Part 2 The math!

12 CCD calibration math raw frame - dark frame ( flat frame - dark frame) Final frame = Remove dark current noise Very simple equation! Brightens weak pixels, dims strong pixels Normalized

13 Normalization Average value = 100 (Flat frame-dark frame) pixel values pixel value average value Normalized value = Done automatically in your image processing software! Assumes that flat light source is even!

14 Applying the normalized flat to your image frame (Raw frame - dark frame) pixel values Calibrated frame ÷ By normalized flat

15 Importance of dark subtraction Assume 10 ADU of dark noise in the flat frame Average: 110 Raw flat frame Overcorrected images! Flat frame-dark frame Average: 100 Subtract dark Normalized values too low!

16 Importance of staying in linear regime If non-linear, pixel values will read less than actual value Normalized flat pixel value too small Flatted image pixel value too large: Overcorrected images! # of photons arriving at detector ADU values Non-linear linear

17 Characterizing linearity Aim at 6-9th mag star near the zenith Expose series of images with increasing exposure length Measure flux inside aperture Divide flux by exposure time to get flux/sec Will be similar at each exposure length in the linear regime

18 Figure 3. Detector linearity test. The normalized flux rate is linear to 1% up to maximum pixel values of around 23 kADU.

19 Noise considerations Make master bias/dark (s/n improves as the square root of the # of frames combined) Dark OR bias correct flats Million photon flats –10 6 /avg ADU = # of frames = 40 frames! No matter what, flats add some noise to final calibrated image

20 Part 3 How to get good flats

21 Acquisition methods All sky flats Light box Twilight flats Dome flat Electroluminescent panel

22 What makes a good flat? Evenness of illumination ADU values at upper range of linear regime of CCD detector Longer than 2 seconds to eliminate shutter effect Many dark subtracted sub frames Repeatable filter wheel positioning

23 The rotation method for evaluating flats Expose / rotate 90˚/ expose Dark subtract and use second set as flats - flatted flat Look at histogram Analyze standard deviation ( )

24 Basic statistics Poisson distribution of ADU values centered on a mean value Width of distribution measured by standard deviation, 99.7% of all values lie within 3 of the mean 3

25 Statistics II For a given light source, range of values is constant regardless of mean value! / % uniformity 3 x better! Histogram of flat (mean 10k ADU) / % uniformity Histogram of flat (mean 30k ADU) Standard deviation is a measure of evenness of illumination!

26 How many ADU is enough? Maximum value of any pixel must be in the linear regime of the chip. Anti-blooming chips go non-linear somewhere mid-range Non-linear pixels in flat will result in incorrect normalization Funny artifacts in flatted images Good StatisticsNon-linear pixels

27 All sky flats Sum lots of images dithered to get enough ADUs for good stats. Can be important for photometry or back illuminated chips because spectral response matches raw images Star artifacts difficult to remove completely Tough with wide field images/big non-linear stretches

28 Light boxes Needs proper baffling and reflective illumination Careful attention to corner shadows Bulky and difficult to use robotically

29 Twilight flats Racing against the clock Neutral point in sky is not fixed Virtually guaranteed to have gradients in wide field images Possible star artifacts Can you get all filters covered in one twilight? Quality is not repeatable!

30 Twilight flat case study April Average transparency (clear sky clock) No visual signs of cirrus 12.5 RCOS with ST2000 (identical setup) Moon below horizon

31 Twilight flatted flat at Galaxy Quest Standard deviation = 171 ADU

32 Dome flats Painted section of dome illuminated by light source Difficult to eliminate gradients Requires careful set up and testing

33 Dome flatted flat at SSRO Standard deviation = 187 ADU Data courtesy Jacob Gerritsen, SSRO

34 Electroluminescent panels Proper design ensures excellent flatness Easy to diffuse Compact Not all panels are broad spectrum Variation in manufacturing Stability of power supply Alnitak Astrosystems!

35 Flat-Man XL case study

36 A dark subtracted sigma combined master flat

37 Flatted flat Standard deviation = 9.5 ADU!

38 Flat-Man XL Statistics For our test case: mean= ADU Range of values 2 x 3 = 57 ADU 57/24271 x 100% = 0.23% variation in brightness!

39 Flat quality comparison Flat illumination sourceStandard deviation Twilight flat171 ADU Dome flat187 ADU Flat-Man XL9.5 ADU !!! Repeatable!

40 Spectrum of AA el panels

41 Ha Flats Courtesy Doug Baum (Flip-Flat owner) at Nightvision Astronomy

42 DOs DONTs Use even illumination Master dark/bias subtract individual flat frames Sigma combine lots of calibrated flat frames Check your flat quality with the rotation method Overexpose into non-linear regime Apply noise reduction or smoothing Stretch histogram

43 Flat -Man XL $100 off on XLs today and tomorrow

44 Flip-Flat $50 off today and tomorrow

45 13 diameter EL panel Wall or manually mounted USB controlled For telescopes Shipping by May 1 $50 off today and tomorrow Flat-Man L


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