Measuring variability of celestial objects brightness is one of the most rewarding task, that can be picked up by astronomy amateur. Variable stars are perfect example of dynamics in the Universe, and the timescale of this changes may be years, days but also minutes! But these are not only stars that varies – we can also measure exoplanet transits or asteroids. In this latter case basing on asteroid lightcurve both rotational period and shape can be determined.
In this entry I will try to describe how to create lightcurve of single object using C-Munipack free software pack available here. Before you start playing with photometry I would like to encourage you to read AAVSO CCD photometry guide. This is not required to operate with C-Munipack, however this is required if you would like to know, how all these things work and how to get most reliable results.
At this moment I assume you already have:
- astroimaging setup with camera that you know how to use
- installed Muniwin software that is a part of C-Munipack package
- calibrated series of frames where you have your object to measure. These frames may be from one night session, but also can be from several nights spread over very long period (actually you can also calibrate frames in Muniwin, but if you already know and use other program for processing, you do not need to change it)
Whole proces contains several steps. At the beginning lets take a look into Muniwin and create our hardware profile. In the Tools select Edit profiles. Then select Light curve profile and save under new name.
Then you need to adjust Camera settings to the values of your camera. In Source frames you may narrow Max.pixel value down to the value you know your camera is still linear. Safe value here will be 50,000-55,000. You can also add your data in Observer section. All other values you can adjust during work with actual project. Once you know them, you can enter them here as well. Don’t forget to save your profile after editing.
Next step is to create project. In Project menu select New, enter project name (like your object name), select profile and you can also change default project files location. Once you have project you can add files to the project using Frames -> Add individual frames menu command. We will not calibrate frames in Muniwin, so make sure the files you add are already calibrated. You can add more frames to the project in any moment later.
Then new frames needs to be processed using command Reduce -> Process new frames. This and following steps you need to do each time you add more frames to the project. Next step is Reduce -> Fetch/convert files. In this and all next steps you can always press More options… button. Then project settings window will be opened, and you can adjust options for this particular step. These options are copied from the profile you selected when new project was created. After pressing OK button files will be converted to Muniwin format.
Then next thing to do is actual photometry. To trigger it you need to call Reduce -> Photometry. This process is pretty time consuming, so it is better to adjust options at single frame before you start process on whole set. To do this you need to select one frame, then click Reduce -> Photometry, and in the window check Selected file only option and start process. After few moments process will complete on this frame, and the number of detected stars will be displayed next to the frame in the list. If you think there is too many or too few stars detected (compared to what you see in the frame) you can adjust settings. You may start with Detection threshold. Quite often at default settings there are too many stars detected, so you need to increase this threshold to limit this number.
Once photometry settings are adjusted you may run photometry on all frames (select All files option). It will take a while, and then all frames in the set should have assigned number of detected stars. If the number of detected stars in some frames differs much (like more than 50%) from other frames you may exclude this frame from set, because it may be frame of poor quality.
There is one more step ahead before we can plot our first lightcurve. Muniwin needs to match stars across frames, so it will know what object to follow when plotting lightcurve. To do it we should open Reduce -> Match stars process. There will be list of frames ordered by number of detected stars. We select Stationary target as target type, and Frame from current project as reference frame and start the process. Now you need to notice how many stars are matched for each frame. If the percentage of matched stars is at level of 80-90% or more, then everything is good. If the percentage varies much, drops below 60% or even there are some unmatched frames you need to break the process and adjust matching algorithm settings. You find it in the Reduce -> Match stars window under Options button. You may start with increasing the Read stars parameter and/or with increasing the Clipping factor parameter.
After all this tasks you may eventually create light curve of your object 🙂 After clicking Plot -> Lightcurve in the lightcurve window you may check Compute air mass coefficient option. Ensemble photometry option allows you to select more than one comparison star. After clicking OK button new window will open, where you need to choose stars. You can select a view of raw image, detected stars or mixed. Then you need to identify you variable star at the image (you may pan and zoom). Click on the star and select Variable from context menu. In the same way you need to select one or more Comparison stars – you need to know its brightness. And it is worth to select at least one Check star. Then you may save this selection before you proceed to next step, so it will be easy to get back here and change any of selected objects.
Then we click OK button and there is another window opened, where we need to select measuring aperture. It is usually the best option to select the aperture, where Std. dev. parameter reaches its minimum. If there are two apertures with the same deviation, you may select the smaller one. Then finally after clicking OK button lightcurve should be shown.
Example lightcurve above was made for (13) Egeria asteroid. The default view presents on the Y axis the difference between variable and comparison star (V-C) – but you can change it to see other plots. It is worth to check also C-K plot, so the difference between reference star and check star – this way you can check what is the deviation you achieved, and if the reference or check star is not itself a variable. Exposures were made with QHY163M camera and 130mm refractor with V photometric filter. At the Lightcurve window you have many options – you can pan and zoom the plot, display table data, select another aperture to see how it affects the errors (check Error bars in View menu) and export data both as plot and text file, for example to import in AAVSO database. Some of these possibilities I will describe in next blog entry about photometry with Muniwin.
So, that’s all folks 🙂 At the moment. In case you have not yet read AAVSO CCD photometry manual, here are some important points to know about photometry in general:
- be patient and take care about data quality and calibrate frames, so the data you will get will be reliable
- know your camera and setup, so you will not overexpose target and comparison star – you cannot do photometry with overexposed images
- select exposure time to maximise signal to noise ratio, but not overexpose star image. You must not reach nonlinear area of your camera (safe limit is 50,000 – 55,000 ADU)
- prepare to session, make sure you have target and comparison stars in the frame
- you may do some lightcurves without filters (like measuring exoplanets or asteroid rotation curves), but for variable stars it is recommended to use photometric filters. Most common is V visual filter. Second one could be R or B photometric filters
- select targets available to your setup. If you have modest aperture then do not hunt for both faint and fast changing targets. You will not be able to achieve good data on them, because for faint objects you need to do long exposures to get high SNR ratio. And long exposures mean, that it is not possible to catch fast changing lightcurve in good resolution. Go for brighter targets, or slower changers
Photometry is real fun, and the reward is almost immediate! And there are plethora of objects to analyse.