Using the ST-i Color Planetary Imaging Software

Transcription

1 Using the ST-i Color Planetary Imaging Software INTRODUCTION The ST-i color software is a utility developed specifically for planetary and lunar imaging with a ST-i Color Camera. Its main purpose is to automatically grade and stack images, but it is also handy for viewing objects in color at the telescope. This is a first release of the software. Please let us know of any issues you might find with the software and we will try to incorporate solutions in future updates. Some choices are intentionally limited to simplify operation. For example, when capturing images the camera automatically takes a dark frame first. For long exposures of deep sky objects this would not be done, but is desirable for planets. This instruction manual describes the use of the software in more detail. INSTALLATION To install the software, download the folder labeled ST-i Color Installer and put it in a directory on your PC. Next, open the folder and double-click SETUP.EXE. This should install the program. Find the new program in the program list (selected by clicking the START button in the lower left corner of your PC s screen), and select it to start the program. You can also right click it to create a shortcut on your desktop. The opening screen for the software is shown below, with the addition of a Jupiter image. The software has three modes of operation, one as a viewer for seeing images in color at the telescope, one for automatically collecting a series of images, and another for grading and stacking images captured with CCDOPS using the AUTOGRAB command (in CCDOPS) or ST-i Color Stacker. It will only open ST-i files saved as SBIG or FITS files since it expects a certain file dimension. 1

2 To become familiar with the software select FILE I/O, and select LOAD SBIG ST-i Image, and navigate to the JUPITER.SBIG image included with the setup files. After it loads you can change the background and range settings, and the Red, Green and Blue multiplying factors in the upper right area to see how they change the image. Select REFRESH COLOR IMAGE to apply your corrections and see the result. You will find the color is quite sensitive to the color multipliers. In this mode the Sharpness Multiplier has no effect. CAPTURING AND VIEWING IMAGES To capture and view images with the software, first get an ST-i working using CCDOPS. You will have to install the drivers before connecting the camera. After you have verified it works with CCDOPS, SHUTDOWN the camera there and exit CCDOPS and, within the ST-i Color program under the ST-i CAMERA heading select ESTABLISH LINK. It should give you a message if the camera is not found, but no message if it is. You can then use TAKE EXPOSURE to verify the camera is connected and to determine an appropriate exposure for your scene. Then go to CONTINUOUS UPDATE to repetitively see the image. Note you must perform a TAKE EXPOSURE first to lock in an exposure time and other settings. Each subsequent image taken in CONTINUOUS UPDATE mode will then use the Background, Range, and color settings entered before. 2

3 CAPTURING AND SAVING IMAGES Another option under the ST-i CAMERA menu is CAPTURE AND SAVE A SERIES. You will be prompted for the exposure time, number of exposures and file name, and a sequence of images will be captured to disk in SBIG format. When you are asked to enter a filename you can select a different directory. As the software collects images it appends a file number to them (from 001 to 999) and saves that also. Use a filename that is new each time to avoid overwriting files or confusing different image sets. When the data is being collected hit the ABORT button once on the lower right of the screen to stop the process. It may take a few seconds to actually stop, so be patient. GRADING IMAGES You can grade and stack a series of images very easily. After saving a sequence of about 50 to 100 images to disk, during initial testing, first load the first image using the LOAD SBIG ST-i image again, and load the first image of the sequence. Adjust its appearance to pleasing values, and then select PROCESS FOLDER under the FILE I/O menu. Initially leave the Sharpness multiplier to its initial setting of 1.05, and let it run. The software will automatically load each frame of the sequence, grade its sharpness, decide whether it is over the Sharpness Threshold or not, and, if it is, register and stack it. At the end a box will pop up saying NO MORE FILES. Close it and then the stacked color image will appear on the screen. At this point you can save the stacked image two different ways. You can save what you see on the screen as an 8 bit png file, which is a lossless compressed bitmap format that Paint and Photoshop can open, or as a SBIG color file format, in Bayer matrix form. The SBIG file doubles the files dimension to avoid any smoothing of the final data. This format can then be converted to other types of files by other astronomy programs such as CCDSOFT, Maxim, etc. We recommend saving a good stacked image in both png and SBIG formats. SHARPNESS MULTIPLIER The SHARPNESS MULTIPLIER parameter deserves some explanation. When the program is going through a series of files, it keeps the running average of the sharpness. The threshold of sharpness a file has to have to be used is based on the average sharpness value. If you select a SHARPNESS MULTIPLIER of 1.0 (the average) it will use about half the files. At 1.05 about 40% of the files are used. At 1.15 I have found about 15% of the files are used. At a setting of 0.1 they will ALL be used. The text box at the end of a stacking run will show the file number and the sharpness parameter for each file if you want to select and copy this data and export it into a spreadsheet program to study it. 3

4 The way the program works is as follows. Each image is decomposed into three wavelet transforms and a remainder, and the power in the second wavelet component used as a sharpness metric. If an image is selected the second wavelet component is correlated to that for the aggregate image to determine an X and Y offset, sharpened heavily, and added to the aggregate array. You need to include at least 10 or more images in the final result or you will not like the excessive graininess of the image. I see no advantage with adding more than about 30 to 50 images total. We are starting with 16 bit data, not 6 bit data like a video camera, so we don t need to stack so many images. One determines how many images to stack using the SHARPNESS MULTIPLIER parameter, and some experimentation. In my experience, the result will be about as sharp as your best image sharpened hard, but not grainy. The program is doing a LOT of calculation, and that is why it becomes quite slow when it finds a sharp image. It is calculating a large correlation matrix of all of the bright regions of the image with the accumulating saved image in order to determine the optimum offset. The center of the correlation matrix is shown in the scrollable window. No movement from frame to frame will result in the max value of 100 appearing at location 16,16. On my computer it takes a few seconds per frame to stack each planetary image and, if it is a lunar scene that fills a frame, up to 10 seconds per frame. Sorry! For best results, capture about 500 to 999 images in a sequence. The seeing does vary a lot minute to minute, in most cases. It is best with Jupiter to capture as many images as you can in 2 to 3 minutes since the planet rotates fast. With the moon and other objects you have more time. OPTIONS You will discover that it is almost impossible to have a completely clean ST-i CCD at such long focal lengths and large F/numbers. The software can apply a flat during the stacking process. In fact, even if the chip is completely clean there are pixel variations that are corrected by a flat field you will see the difference in the result. The way to apply a flat is capture it beforehand by pointing the telescope at a white screen and taking a number of dark subtracted flat fields. Take at least 5. Then average them in CCDOPS to create a master flat. To apply the flat, when starting up ST-i Color, load the flat first, and then select USE IMAGE AS FLAT under the TOOLS menu. From then on any image loaded will have the flat applied. It will not be applied to images captured and saved by the program to avoid confusion. It is ONLY applied to images loaded from disk. That way you can take a flat after an image sequence and still use it. Under the DISPLAY menu you will see commands to display the components of an image. After a stack is performed the data displayed will be the stacked data. You can also adjust the contrast of the displayed image using ADJUST CONTRAST. 4

5 The ST-i Color Stacker program will also work with monochrome cameras. Of course you will not end up with any color but you can at least experiment with the stacking and sharpening results. If you are doing this, under the TOOLS menu select MONOCHROME IMAGES. HINTS At the telescope: when imaging the planets, use a focal length of at least 120 inches (3 meters). It is a single shot color chip with a Bayer matrix, so you want an image scale that oversamples the seeing. Focal lengths beyond 240 inches (6 meters) are probably not advantageous. Flats: When taking flats use a light source that is white LED or fluorescent lamp based. If you have a flat field box using incandescent bulbs, throw it away. Stacking deep sky objects: the program does not work since the sharpness metric is tuned for planetary scenes. Also, I have found that if the individual frames are not at least 30 seconds long there is virtually no improvement regarding signal to noise ratio in the final result. Watch out for dew: the sharpness metric is proportional to brightness. If the brightness goes down the sharpness will also. What this means is if your telescope corrector plate or lens is dewing over during a series, the declining brightness will mean that only your first few dozen images are used. So, check the corrector plate before starting a long series. You would be amazed how little effect slight dew has on image quality, so the brightness fading is your first indication of a problem. 5