RHO CCD. imaging and observa3on notes AST aug 2011

Transcription

1 RHO CCD imaging and observa3on notes AST aug 2011

2 Camera Specs & Info 76 cm Telescope f/4 Prime focus (3.04 m focal length) SBIG ST- 8XME CCD Camera Kodak KAF- 1603ME Class 2 imaging CCD Built- in TI TC- 237H CCD autoguider Capable of TDI Imaging 10- filter filter wheel (with 5 filters) Johnson- Cousins B,V,R,I Clear filter with Blue- blocking

3 CCD Specs & Info Kodak KAF- 1603ME Size: 1530 x 1020 pixels (9µm pixels) Plate Scale: 0.61 per pixel Read Noise: 15 electrons RMS Frame Download Time: 3.7 seconds Gain = 2.54 e/adu Guiding camera is a TI TC- 237

4 QE number of signal electrons created per incident photon

5 Charge Transfer: Two- phase CCD Picture Credit & More info about this configura3on: hep:// micro.magnet.fsu.edu/ primer/digitalimaging/ concepts/twophase.html Java Applet: hep:// micro.magnet.fsu.edu/ primer/java/digitalimaging/ ccd/shigregister/index.html

6 JC- B,V,R,I Transmission Curves

7 Field of View FOV: 10.5 x 15.5 (blue rectangle)

8 Vigneing

9 Coma3c Aberra3on This is really only problema3c near the outer edge of the image, or when the camera is out of focus. Usually, if the image is properly focused, the seeing is worse than the coma3c aberra3on.

10 What s wrong with this image?

11 Calibra3on Images An observed CCD image includes signals from three sources: 1. Photons imaged onto CCD by telescope Sources: Astronomical objects and sky Generated by photoelectric effect 2. Dark current Source: thermal electrons collected in each pixel during the exposure 3. Bias A signal added to each pixel during readout Must subtract off dark current and bias. Courtesy: John Oliver

12 Calibra3on: Dark Frames What: Exposures taken with the closed (hence dark ) Darks should have the same exposure Gme as your science images. Darks should also be taken at the same temperature as your science images (why?). You want to take mulgple dark images and (median or average) combine them. Reduces the noise Eliminates spurious pixels in individual frames Combined image typically called the master dark. Purpose: Remove the dark current from thermally excited electrons. Applica1on: The master dark frame is subtracted from the science image.

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14 Calibra3on: Bias Frames What: Zero second dark exposures You want to take mulgple bias images and (median or average) combine them. Reduces the noise Eliminates spurious pixels in individual frames Combined image typically called the master bias. Purpose: Remove the bias signal The bias is an offset level introduced when the detector is read out. Even if the output from the CCD were 0 electrons, there would sgll be a bias signal that varies from pixel to pixel in a repeatable fashion. This is an addigve offset. Applica1on: The master bias frame is subtracted from the science image.

15 Calibra3on: Flanields What: Exposures of a uniformly illuminated source Types of flarields: Dome, Twilight, Sky You want to take mulgple flarield images and median combine them. Reduces the noise and eliminates spurious pixels in individual frames. Eliminates objects for twilight and sky flats Must normalize all images to same mean value before combining Combined image typically called the master flat. Normalized to have a mean value of 1. Taken in each filter For dome and twilight flats, you want to have as many counts per pixel as possible to reduce noise, but not so bright as to saturate or have concerns about nonlinearity. Purpose: Correct for pixel- to- pixel sensigvity variagons Each pixel has a different response Quantum efficiency variagons, dust on the opgcs, vigne[ng,... If illuminate all pixels with the same brightness, then you can measure and correct for these variagons Applica1on: Divide the science frame by the master flat.

16 Out of focus dust Vigneing

17 Calibra3on: Flanields This flat was not taken at RHO, but is an extreme example of a flat field. Courtesy: John Oliver

18 Calibra3on: Flanields Dome Flats Images of a uniformly illuminated white screen inside the dome. Twilight Flats Images taken at twilight, when the sky is much brighter than astronomical sources. Effec3vely using the sky as your uniformly illuminated screen. Sky Flats Median combina3on of many science exposures during the night, again using the sky as your uniformly illuminated screen....each has advantages and disadvantages...

19 Calibra3on: Standard Stars What: Images of stars with well- known magnitudes (and colors) Only relevant when the weather is photometric Each standard star should be observed mulgple Gmes during the night at different airmasses Purpose: To enable photometry Need an object with known magnitude to determine the magnitudes of other sources that are your science targets Landolt 1992, AJ, 104, 340

20 Courtesy: Dr Steven R. Majewski

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23 Some Manuals and Spec Sheets for Instruments at RHO Overview of the SBIG ST- 8XME CCD Camera : hep:// ST8XME_overview_ pdf and a spec sheet: hep:// SBIG also has a general informa3on sheet about CCDs: hep:// AboutCCD_ pdf The spec sheet for the CCD (a Kodak KAF- 1603ME) can be found here: hep:// 1603MELongSpec.pdf The filter wheel informa3on is available here: hep:// Filter- Wheel.html. The manual for using the camera can be found here: hep:// Maxim- DL Manual (imaging sogware we use with the camera) is available here: hep:// DL.htm.

24 Ɣ e - From Kodak s CONVERSION OF LIGHT (PHOTONS) TO ELECTRONIC CHARGE conversionoflightphotons.htm.

25 Problems and Solu3ons Cosmic Rays Hot Pixels Unevenly lit CCD Readout Noise Dark Current Out of Focus Dust Rings Co- add Frames Dark Frames Flat Frames Bias/Dark Frames Dark Frames Flat Frames the handouts for CTO!)