BinStar V 1.0
Binary Star Measurement Software, developed by Ed Hitchcock
This is free, experimental software. It comes with no guarantee, express or implied. Use it at your own risk.
BinStar was developed for the purpose of quickly and easily measuring the separation and position angle of double stars, using a telescope-mounted webcam. The software uses the rate of drift and the declination of the star in order to calibrate the measurements. At present, this software works only with AVI files.
Before you Begin...
Before using the software, you will need one or more AVI files of binary star drift. In order to do this, locate a suitable double star, move the scope in RA so the star is just off the east side of the field of view, and shut off the RA drive (assuming it is a driven scope). As the double star appears, start the video capture, and stop it as the pair is about to drift off the west side of the FOV. If you start too early or let it go too late, it is not a serious issue – allowances are made in the application for starting and ending at any frame within the file. You should name the AVI file for the binary star being captured for easy reference. Take several captures of the same star at different focal lengths (eg add a Barlow) – you can name the files sequentially (eg STF222-a.avi, stf222-b.avi etc.).
Don’t worry about aligning the camera perfectly – the entire purpose of BinStar is to calibrate automatically so you don’t have to worry about it in the field.
If you are shooting through a diagonal, do not align the diagonal and camera at an odd angle - you will only confuse the software when it tries to figure out the RA axis from the video...
As you read through the following, please refer to the screenshot below. In order to use the software, follow the flow of controls from the top to the bottom as described in the following sections.
Loading an AVI
Use File -> Load AVI to load you video sequence into BinStar.
Setting the Star Data
Enter the designation of the star (ie the catalogue name) – if you forgot which star you were doing, the filename is listed in the status bar at the bottom of the window. That’s why it is useful to use the star name as the filename. Enter the RA and Dec values for the pair. RA is entered in HH MM SS, Dec may be entered as Deg (°) Min(') Sec(") or as decimal minutes – eg either 31 52 30 or 31 52.5
Only the declination is required for calculations. The RA is for recording the results only, it is not required for any calculations. To facilitate entry, you can TAB from designation, through RA and DEC boxes.
The observation date defaults to the date and time of the video capture. Add or subtract the appropriate number of hours (using the h+ and h- buttons) to set the time to UT. The epochs (both Julian and Besselian) are calculated automatically.
Use the video controls to play, pause, rewind, step forward or step back. If you choose not to use the entire video, decide where you want to start in the video sequence, and press the Start Frame button. The frame number will be displayed. Repeat this process for the End Frame. It is always a good idea to preview your video before running the analysis, to ensure the target does not drift too far off screen. You can also use the slider tool at the bottom of the video window to find suitable start and end frames.
The Reset button will return the AVI to your start frame. It will also clear any recorded values - though at this point we haven’t recorded any values.
Measurement Frame Parameters
This is where you may want to practise to get a feel for the settings.
Select an appropriate box size in pixels. Use the + and – buttons to change the box size in increments of 2 (minimum 4 and maximum 40). The Primary bullet should be selected in the Select Star radiobox. As a general rule, you want a box that is large enough to encompass the star, but not so large that they primary and secondary boxes overlap.
On the display, click on the image of the primary star. A box will appear and centre itself on the primary. The Select Star selection will automatically jump to Secondary. Choose an appropriate box size and click on the image of the secondary star. If the stars are quite close, the secondary box may latch on to the primary. If so, you may need to drop the box size of the secondary and repeat.
After the secondary star is selected, a blue box (internally called the System Box) will appear surrounding the primary and secondary boxes. If you are not happy with the sizes of your boxes, simple click the Reset button, select the Primary radio button, adjust your box size and start again.
Two methods of determining star positions are available. The Peak Search method is preferable. This method automatically identifies points representing the peaks of each star image, and then uses a circumference of one standard deviation to determine a mean centre value. The Weighted Average method works fine for well separated stars, and simply takes the weighted average value for each star image contained within the primary and secondary boxes. Both methods use the System Box to help locate the pair within the frame. Noisy images, or poor seeing, can confuse either method in some frames.
The Orientation selection is for selecting whether you are using a diagonal, or if you are shooting straight through the scope.
Running the Video Analysis
When you are content with your boxes, click the big GO! Button. Alternatively, the manual step buttons (<- and +>)can be used to analyse each frame one at a time.
BinStar will step through each frame, first locating the binary pair in the system box, then positioning the primary and secondary boxes, and then calculating the centre of each star image. Each image is converted to greyscale in order to iliminate colour effects. As it progresses, the frame number, time (in ms), separation in pixels and position angle (in screen coordinates) is displayed.
When the end frame is reached, the trail of centre points is displayed. The slope of the trails is calculated using least squares linear regression, and the average for the two values is used as an east-west reference line. The position angle between the primary and secondary is calculated in reference to this line.
The length of the trail in arcseconds is calculated using the time (in ms) and the declination - so it is important to have the correct declination value! The number of pixels per arcsecond is then calculated, and used to determine the separation of the two stars, using the average separation from all frames. (In fact, since there is some wandering, it is not the first and last frames that are used, but an average of the first 12 and last 12 frames).
If you are using the manual step controls, you can select the Calculate button as soon as there are more than 15 good frames. You can also recalculate using this button if you change the declination value (it is very easy to forget to do this!) or if you activate or deactivate individual frames (see Selecting Frames below).
Note that the centre point data for individual frames may be made inactive (ie hidden) if the separation value is far off of the initial value, or if it detects a copied or "dropped" frame.
Chooosing the Zoom checkbox causes a zoom window to appear, letting you watch an enlarged version of the action as each frame is analysed.
Using the slider tool and selector buttons below the video frame you can go through the frames individually to verify that the centre points have been selected appropriately. Using the Zoom window will facilitate this. If the software has miscalculated the centrepoints in a given frame, (this can happen due to noise, blurry seeing, or too much overlap between the stars) you can uncheck the Active checkbox for that frame. You can also activate frames that appear inactive to see where the points were placed. Note that dropped or doubled frames are made inactive automatically by the software in order to not skew the results. Once you are happy with the selection of frames, you can use the Calculate button to recalculate the values.
The final calculated results are shown in the pale blue boxes at bottom right. The full set of results, including any notes you care to jot in the Notes box, can be saved to the results file by clicking the Save Results button. The results are stored in TAB delimited format in the file RESULTS.TXT in the BinStar home directory. This can easily be loaded into a spreadsheet for later use.
Should you wish to uninstall BinStar, Simply use the Add/Remove software feature of your computer's control panel to completely remove the software. BinStar does not make any registry changes on installation, nor does it install DLL files, so uninstalling should be clean.
This application is in the early stages. Things I am working on – when I have the time – are:
Saving and loading project files
Improved methods of graphical analysis of close binary pairs
Reference database of binary stars (probably from the WDS catalogue) So I can be lazy and don’t have to enter the RA and DEC manually.
If you have any other suggestions, or if you enjoy programming in Delphi and would like to help out, I can be contacted by email.