Home Observatory Equipment Images Technical Notes

Image Acquisition

Updated: 02/04/2010


Are Dark Frames Needed? (6/2/2007): This page discusses the underlying issues surrounding dark frame subtraction and offers some areas for further investigation.

CCD Camera Cooling Considerations (10/1/2007): This paper discusses a method to determine the camera cooler set point, based on noise considerations.

Dithering Techniques (2/17/2006): A new algorithm improves dithered image separation for better artifact rejection.  Also provided are some recommendations on how to set your dithering magnitude.

Short Sub-exposure Application (7/14/2004): In an attempt to make the case for short sub-exposures a little more understandable and without all the math in the original paper, I have written a brief application note on the topic.

Sub-Exposures and dark frames (6/29/2004, updated 2/4/2010):  Based on some work by Stan Moore in quantifying the master dark contribution, I have put together a spreadsheet that applies this work, along with my concepts outlined in this paper.  This spreadsheet should give you the tools you need to determine both your sub-exposure and how many darks to run for a given noise scenario and camera operating temperature. 

Sub-Exposures and Signal-to-Noise Considerations (5/9/2004, updated 2/4/2010):  A paper is presented that analyzes the various random and non-random noise that occurs when multiple sub-frames are acquired for a given target.  Techniques for dealing optimally with both types of noise are presented with along with recommendations for determining sub-exposure time and the number of sub-frames.  My recommendation is basically to take 2n+1 sub-exposures of t/2 duration, as opposed the n sub-exposures of t duration. Here, n is the number of sub-exposures and t is the exposure that is calculated to overwhelm read noise. Additional requirements are to take a significant number of frames, at least 10, use dithering and use outlier rejection combination techniques. This approach has been used on all images I have taken since the end of January with good results.  The original empirical tests that gave rise to this paper are here.

Dithering (9/18/2003): An introduction to dithering and its effect on imaging is presented, along with an automated method of acquiring AO7-guided images.

ST-10XME/CFW-8A Background Fogging (6/10/2003): A discussion and and approach to solving mysterious gradients in color frames taken with this equipment combination is presented.

Light Box Design (5/21/2003): Here is a page on a light box I built for use with my RC.  You may be able to get some ideas for your own needs.  The high intensity white LED's provide a good balanced light source to keep the exposure times for the blue flat field reasonable.

Exposures, Signal-to-Noise and other factors: A while back, Stan Moore had a sequence of threads that gave some great insights on sub-exposures, SNR and other factors as a function of camera type. Please note that they are unedited from the SBIG Yahoo group, so be sure to read through to the end for later corrections. A lot of excellent insight from Stan is compressed in this thread.  One example is how to determine the minimum exposure time.  Basically, you want the sky noise to be sufficient to override the readout noise.  For my ST-8E USB camera, I have a gain of 2.6 e/ADU and a readout noise of 15e.  This translates into a minimum background level of 770 ADU.  With my RC operating at F/7.2, I hit this level with around 5-7 minute sub-exposures.  Once you know the calculated minimum ADU, experiment with your exposure lengths to get to the targeted ADU.

Color Calibration (8/13/2002): I used the information from some very interesting emails to determine the exposure ratios of my ST-8E.  SBIG recommended RGB exposure ratios are 1:1:1.6.  After calibrating using the procedures outlined by Bart Declercq in "Calibrating RGB Color Part 1" and "Calibrating RGB Color Part 2" along with the warnings noted by Stan Moore in "True Color and Ratio", I found my exposure ratio to be more like 1:0.9:1.4.  This is for a star at the zenith and should be adjusted for atmospheric extinction.  Al Kelly described the calibration process for AIP4WIN in "Solar Analog Stars".  It is relatively easy to develop the ratios for your own camera following the approach outlined in the above links.  Also, Don Goldman has done an incredible amount of comparative measurements on color camera and filters.  See his page on "Color Exposure".

FocusMax and Scripting (Expose 2.02): I have modified my script to allow FocusMax to refocus the system at the start of each exposure sequence. This is a more closed loop approach in that no predicted temperature offsets nor filter offsets are required. By taking advantage of FocusMax dim star feature, I am able to focus without slewing to a brighter star. I am still testing this script but it may be viewed here. Through a judicious choice of exposure sequences and types, it is possible to minimize focus point changes without unduly lengthening the time of the complete exposure sequence. Due to the high level of integration with FocusMax, I have removed the ability to work with CCDSoft. I have also allowed certain FITS entries to be made under script control.  Click on the title or here to see the source script.

Scripting (Expose 1.04): Frequently I use a script file to take unattended exposures for the evening. I generally start tracking while the object is low on the horizon. I then set an exposure delay for when I want the sequence to start. After the delay, the script takes whatever series of exposures I want. The telescope is then parked and the script takes whatever set of dark frames is programmed. Finally, the script brings the cooler to room temperature and shuts down the cooler. During the expose sequence, the script corrects the focus point on my refractor for any temperature changes and slightly offsets the focus point to take into account focus points for different filters. These offsets are programmable. The script works with MaximDL/CCD v3.xx or CCDSoft 5.00.04x. It requires the ASCOM platform for a delay function, Orchestrate to control TheSky and RoboFocus v3.13 for temperature compensation. It is offered not as a finished product but as a work in process to give you some ideas of scripting possibilities. It may be viewed or downloaded here. Please note that it is a .txt file and the extent should be changed to .vbs to work as a script. Eventually I would like to expand it to acquire and track a guide star and be able to go from object to object.

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