All information in this page is Copyright (c) Carsten A. Arnholm
(This tutorial is based on K3CCDTools, a software program authored and owned by Peter Katreniak. For legal information relating to use of K3CCDTools, see http://www.pk3.org/K3CCDTools/.)
Personal, non-commercial use of tutorial allowed: You may freely use this tutorial for non-commercial purposes, but you are strictly required not to make any form of profit (monetary or otherwise) related to the use of this tutorial. You may not use this tutorial in relation to commercial product(s) you may be marketing.
Commercial use of tutorial prohibited: Any person, company or other entity that markets astronomical products for the purpose of making profit are commercial users. Such persons, companies or other entities are prohibited from using, copying or linking to this tutorial without an express written permission from the author.
If you have detailed questions about the program, its theory or implementation, I suggest you contact Peter directly. If you have suggestions for how this tutorial can be improved, you are very welcome to send that to me. This is version 2 of the tutorial, dated Nov 27. 2002. It is based on K3CCDTools version 22.214.171.1241
You can create your own long exposure AVI's, but for the purposes of this tutorial, my M57 (Ring Nebula) files can be found in . It is a 1.49MB zip file containing a M57 video (68 exposures, each 30 seconds long), a dark frame video and a flat field bitmap image.
Please note: In order to facilitiate easy download over the internet, the supplied AVI files have been compressed using DivX codec version 5.0.2. In order to read the AVI files with K3CCDTools, you must install the DivX codec, available for free at http://www.divx.com/divx/. Compressing the files using DivX codec has degraded the image quality somewhat, but it is still very usable as a tutorial example. The original M57/dark frame video size was about 20MB, too big for easy download.
The raw videos were created August 7, 2002 in Norway. At that time of the year we do not have complete darkness, only astronomical twighlight. I used a C8 SCT mounted on a Vixen Super Polaris GEM and a concrete pillar. The focal length was 2032mm (F10). No autoguiding or periodic error correction was in use, and the raw frames suffer from significant periodic error due to imperfections in worm shaft and brass gears between MT-1 motor and worm shaft. The camera is a Phillips Vesta 675K SC, i.e. the simplest no amp-off modification. It was "T-shirt conditions" during the capture, so there are many hot pixels in the images.
captured frame = real frame + dark frame
real frame = captured frame - dark frame
We can figure out what the dark frame is by creating an image whithout allowing any light to touch the CCD chip. This is why we cover the front of the telescope while shooting dark frames. All other conditions, especially temperature, should be the same as when making the real exposures. By subtracting the dark frame from the captured frames, we can make a very good estimate of what the "real frame" should be.
The experts say that, as a rule of thumb, you should capture at least 5 raw dark frames, but the more you capture, the better (especially true for very faint objects). The dark frames will be averaged to form a master dark frame which we subtract from the real frames. The length of each dark frame exposure should be the same as each real exposure, in order to generate the same level in the dark frame images as when you do the real exposures. The master dark frame will contain only averaged internal noise generated by the camera itself. When we subtract this information from the real exposures, the signal to noise ratio will go up, and our image will look much better.
Task: Start K3CCDTools and open the dark_divx.avi video (File | Open video ...).
Here, you see a number of identical looking images with white dots in them. These dots are not stars, they are hot pixels. Hot pixels are a characteristic of CCD cameras, the higher the temperature is, the more hot pixels you get. That is why many cool their cameras with fans or peltier devices. An important aspect of dark frame subtraction is to "remove" the hot pixels in the final image.
Task: Create the master dark frame:
Task: Rough alignment:
Task: Select dark frame:
Result = (Frame - DarkFrame) * FlatFieldMax / FlatField
One can make a flat field by imaging a uniformly lit area. I didn't do that in this case. Instead, I "faked" a flat field by trying to compensate for the remaining amplifier glow (this procedure is formally incorrect, the dark frame subtraction should remove the glow, but somehow it wasn't quite enough). It turns out that in this case, it works well to make a flat field image based on the dark frame we made earlier. I simply entered the dark frame into Photoshop, and used the levels tool to increase the brightness to a fairly high level. Then I used Gaussian blur to turn it into a completely smooth image and saved it as a normal 24 bit BMP file.
The resulting flat field image is shown below (as JPG). As you can see, it is geared towards fixing the most annoying problem in our raw data, the amplifier glow.
Task: Select flat field:
Task: Perform fine alignment:
Task: Integrate the aligned images:
Task: Ajust the histogram:
Notice that from version 126.96.36.1991, it is possible to zoom the histogram. This is very convenient for selecting the proper levels. Zooming can be done by clicking the histogram picture using the mouse. The left mouse button performs zoom-in, the right button zoom-out. You can scroll through the histogram by means of the horizontal bar above the histogram. The number in the bar indicates the level of the left edge of the histogram window. The trackbar on the right side of the histogram window enables zooming in y-axis of the histogram. The green and red triangles, which define low and high histogram levels, can easily be moved to currently visible histogram window by clicking to the left or right part of their path.
At this stage, this is what we have:
Task: Save integrated image to BMP:
Task: Photoshop adjustments:
Color balance dialog in Photoshop.
Brighness/Contrast dialog in Photoshop.
Gaussian blur dialog in Photoshop.
Unsharp mask dialog in Photoshop.
Curves dialog in Photoshop.