Polaroid ST-7E, ST-8E, ST-9E manual Taking a Good Flat Field, Building a Library of Dark Frames

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Section 5 - Advanced Imaging Techniques

5.4.Taking a Good Flat Field

If you find that flat field corrections are necessary due to vignetting effects, CCD sensitivity variations, or for more accurate measurements of star magnitudes, try either taking an image of the twilight sky near the horizon or take an image of a blank wall or neutral grey card. The Kodak CCDs may have a low contrast grid pattern visible in the sky background. A flat field will eliminate this.

Finding areas of the sky devoid of stars is very difficult after twilight. Therefore, you should take flat field images of the night sky after sunset, but long before you can see any stars. If this is not possible, take an image of a featureless wall or card held in front of the telescope. However, if using this second method, be sure that the wall or card is evenly illuminated. Appendix D describes how to do this. You will know if the flat field is good if the sky background in your images has little variation across the frame after flat fielding, displayed using high contrast (a range of 256 counts is good for showing this).

If you plan on flat fielding Track and Accumulate images you should also refer to section 6.8. Since the same flat field is added to itself a number of times, be sure that you do not saturate the flat field image by starting with pixel values too high. Typically try to keep the pixel values between 10% to 20% of saturation for this purpose. For single flat field images, try to keep the values to approximately 50% of saturation.

5.5.Building a Library of Dark Frames

The ST-7e/8E/9E have regulated temperature control, and therefore it is possible to duplicate temperature and exposure conditions on successive nights. You can set the camera TE cooler temperature to a value comfortably within reach on your average night, and then take and save on disk a library of dark frames for later use. This is a good project for a rainy night. We recommend you build a file of 5, 10, 20 ,40, and 60 minute dark frames at zero degrees Centigrade for a start. Otherwise you will find yourself wasting a clear night taking hour long dark frames!

Note: Dark frames taken the same night always seem to work better. The adaptive dark subtract will help if the ambient temperature changes slightly.

5.6.Changing the Camera Resolution

The Camera Setup command allows you to select the resolution mode you wish to use for taking and displaying images. The ST-7E/8E/9E cameras have High, Medium, Low and Auto modes. The High Resolution mode is the best for displaying the greatest detail since it utilizes the maximum number of pixels for your particular camera. The Medium Resolution Mode operates by combining 2x2 pixels giving the same field of view as High Resolution Mode, but with 1/4 the resolution. This results in significantly faster digitization and download times. Also, in Medium Resolution Mode, with larger pixels and comparable readout noise there is a better signal to noise ratio for very dim diffuse objects. This improved signal to noise ratio combined with faster digitization and download times makes Medium Resolution Mode ideal for finding and centering dim objects, and for imaging most objects. Additionally, a Low resolution mode is provided which bins the CCD 3x3 before readout. Low resolution mode is sensational for displaying faint nebulosity with short exposure times. In Auto Resolution Mode, the camera and software will always use High Resolution for all imaging and display

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Contents Operating Manual Page Table of Contents Technique Quick Tour IntroductionRoad Map of the Documentation Ccdops Software Ccdops for Windows or MacintoshCCD Camera Page How CCD Detectors Work Introduction to CCD CamerasCameras in General Full Frame and Frame Transfer CCDs Camera Hardware ArchitectureCCD System Block Diagram CCD Special Requirements CoolingDouble Correlated Sampling Readout Dark FramesFlat Field Images Pixels vs. Film GrainsGuiding Electronic Imaging Black and White vs. Color Page Attaching the Camera to the Telescope At the Telescope with a CCD CameraStep by Step with a CCD Camera Focusing the CCD Camera Establishing a Communications LinkCamera Back Focus Displaying the Image Finding and Centering the ObjectTaking an Image Processing the ImageSub-Frame Readout in Focus Crosshairs Mode Photometry and AstrometryAdvanced Capabilities Track and Accumulate Autoguiding and Self GuidingAuto Grab Color ImagingAt the Telescope with a CCD Camera Connecting the Relay Port to the Telescope Connecting the PowerConnecting to the Computer Camera HardwareUsing Mechanical Relays Push to Make Switch Modification Modular Family of CCD Cameras Joystick ModificationSystem Features 20.6x sizeum Focal length cm Camera Hardware Connecting the older model CFW-6 filter wheel to the Camera Battery Operation Page Deep Sky Imaging Advanced Imaging TechniquesLunar and Planetary Imaging Terrestrial ImagingBuilding a Library of Dark Frames Changing the Camera ResolutionTaking a Good Flat Field Flat Fielding Track and Accumulate Images Tracking Functions Advanced Imaging Techniques Page Accessories for your CCD Camera Cooling BoosterFocal Reducers Tri-color ImagingCamera Lens Adapters and Eyepiece Projection AO-7 and Lucy-Richardson SoftwareWindows Software Third Party Products and ServicesSGS Self-Guided Spectrograph Image Processing SoftwareSbig Technical Support Common Problems Common Problems Common Problems Page Glossary STV Glossary Glossary Sbig Tracking Interface Cable TIC-78 Appendix a Connector ad CablesAppendix a Connector Pinouts Figure A1 CCD Connector for TIC Mating Regenerating the Desiccant Appendix C MaintenanceCleaning the CCD and the Window Page Appendix C Capturing a Good Flat Field TechniquePage Index IBM PC Separations
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