Guide to Light Curve Reduction in Tangra

(Including Step-by-Step Reduction Examples)

• General recommendations for light curve reduction
• Measuring a star that is very close to a brighter star
• Measuring a very windy video
• Measuring a drift through video
• Measuring a faint star using software integration and pre-processing
• Reprocessing of a light curve file produced by someone else
• Important configuration settings

General recommendations for Light Curve Reduction

First of all if you are a new user of Tangra and you have used LiMove before you should know that Tangra is a lot different than LiMovie in a number of ways and there are important parts of setting up the measurement objects that do not exist in LiMovie. You should learn about those differences and use them in your advantage. It is fairly important to choose the correct Light Curve Reduction settings for your video for the success of your measurement. As of version 1.4 Tangra supports two types of measurement - Tracked Asteroidal Occultation and Untracked Measurement. Please note that Tracked Mutual Satellite Event is not supported at the moment but support will be added in future as we approach the next season of mutual events involving the satellites of Jupiter.

All light curve reductions in Tangra start with the "Light Curve Reduction" screen where you can choose the type of measurement and other settings. When you measure occultations you will want in 99% of the cases to use Tracked Asteroidal Occultation measurement. This type of measurement is suited for objects that do not have considerable motion relative to the stars but undergo small to considerable brightness variation.

On the Light Curve Reduction screen you should also provide any additional information using the check boxes under the measurement type radio buttons. Here "Field rotation" happens to Alt-Azimuth mounted telescopes. It is a good idea to try and memorize the flags so you can immediately tell whether one of them applies to your next video measurement. For example if you know that there is "Wind, shaking or movement" flag and you do notice wind in your video you would immediately correlate to Tangra this and would not forget to use the flag.



Once you have got the reduction flags to correctly indicate the situation in your video the next important step is to provide sufficient number of good quality guiding stars. As a general rule you should always try to use 2 or 3 guiding stars whenever possible. If you need to measure one comparison star that is fainter then you can use 2 guiding stars and 1 comparison star. If you don't need a comparison star (in many cases you don't) or if all guiding stars are not particularly bright then use 3 guiding stars. Remember that guiding stars are also measured and can be used for comparison. Comparison stars on the other hand cannot be used to improve the tracking.

Setting up the measurement objects and doing the measurement should be intuitive if you know these facts:

• You must select an "Occulted Star" to start the measurements
• Use guiding stars to improve the tracking of the occulted star
• Guiding stars are also used for comparison
• Tangra doesn't start the measurement immediately but first runs a few refining frames. So always set up your objects at least 2-5 sec before the expected occultation and preferably longer than that


Now that we have covered the basics lets have a look at some special measurement cases. It is best to go thought all the examples in the order they appear below.



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Measuring a star that is very close to a brighter star Video File: 071231_Nealley 41m25s_copressed.zip (right click and choose "Save Target As...") Size: 9.4 Mb (zipped) Codec: XviD Tracked asteroidal occultation Full or almost full disappearance


One of the unusual cases is when our occulted star is very close to another brighter star. This doesn't happen very often but if it happens to you, you may find it difficult to measure. One of the difficulties comes from the object auto-centering feature of Tangra where if you click anywhere in the video, Tangra will locate the closest (in a reasonable area) object to the position where you clicked and will center and select this object. In the case where you have a brighter star next to an occulted star this creates a problem because Tangra will be positioning on the brighter star when you actually want to select the fainter star next to the brighter one,

So let's get started. Open up the video in Tangra and choose "Light Curve Reduction" (in case it doesn't show automatically when you open the video). Now choose "Tracked Asteroidal Occultation" and also choose "Full or almost full disappearance" as this is a full disappearance video. If Tangra tells you that the selected photometry method does not work with Full Disappearance and suggests using Aperture Photometry then agree to this by pressing OK. You will not get asked this question unless you have modified your default configuration.

Our next task is to select and add the occulted star for measurements. To do this click on the video where the occulted star is. What will happen is Tangra will selected the much brighter star to the left instead (see first image below). Now to select the fainter occulted star you need to hold down the Ctrl key and then click with the mouse on the occulted star. Also note that you can do this on the zoom window at the top right corner. Once you have done this Tangra will now have the fainter star selected (see the second image below). Now press the Add Object button and by default the selection will look something like the third image below.



There is a number of changes we need to do. First of all mark this as "Occulted Star" and choose "Manually Positioned" aperture. This will make Tangra not try to locate the star on each frame by looking for the brightest object in the close proximity but will derive the occulted star location from the relative position to the guiding stars that we are going to add later. If we didn't do that the tracking would have been unsuccessful because Tangra would have always picked the very close bright star instead. The next step is to position the "Manually Positioned" aperture. For this use the U, D, L and R buttons to move the aperture pixel-by-pixel up, down, left or right until the aperture is centered nicely on the occulted star. Finally we would like to decrease the aperture size a little bit so it reduces the amount of light that will be coming from the adjacent star. Now press "Add" to add the occulted star.

We have successfully added the occulted star and is now time to add our guiding stars. We are going to use two guiding stars and one comparison star. The first one should be the bright star very close to the occulted star. Simply click on it again and press "Add Object" leaving the default settings as shown below.



We have a good number of stars to choose from for the other two stars. For example use the bright star that is up and right from the occulted star as shown on the image below. Click "Add Object" and use the default settings.



Finally lets pick a comparison star for example as shown below. Select the star and click "Add Object". When we add a comparison star we can also choose between Auto-Centered or Manually Positioned. It all depends on the brightness of the star. If you have a star that is faint and you have bright guiding star(s) when you may be better off using the Manually Positions aperture. In this case the star is probably bright enough for Tangra to be able to Auto-Center it but I decided to go safe and use manually positioned aperture as shown below.



We are now ready to go, so press the "Start" button and Tangra will start the measurement. It will first take a couple of refining frames (frames during which it will not actually measure) that are necessary to determine better the position of the guiding stars as well as to determine the brightness variation and potential drift through motion (in a case of a drift through video). Once the refining is completed you will see Tangra taking measurements. You can either press "Stop" to stop the measurements at any time or Tangra will stop automatically when it reaches the end of the video. In our case lets wait for the video to finish as it is not long. Once the measurement has finished Tangra will ask you to enter the times of two frames from the OSD time-stamps of those frames. You will be automatically positioned to the first frame. If the time-stamp is not readable of if you want to add the time of another frame you can use the "-1Fr" and "1Fr+" buttons bellow. The "Show Fields" button will help you separate the two interlaced fields so you can read the time-stamp better (when needed).



After you enter the first time-stamp press "Next" and also enter the second time-stamp. Once ready press "Finish". As soon as you have done this you will get a warning message from Tangra that the duration of the measured part of the video calculated based on the number of frames and the frame rate recorded in the video does not correspond to the time difference derived from the entered time-stamps. Sometimes the error may be due to incorrectly entered start and end times but it could be also a problem with the video file.

Grabbers used to digitize video and DVRs control the value for the frame rate recorded in the AVI file. The valid frame rates for PAL and NTSC videos should be 25.00 and 29.97 respectively. However any frame rate may end up being set in the AVI file. In this example the warning message refers to a frame rate of 26.899 which is not standard. This alone is an explanation of the warning. Sometimes however there may be also duplicated or dropped frames that contribute to the issue and also a strange frame rate like this can be caused by dropped frames during the grabbing process.

It is important to note that even if you have dropped or duplicated frames this is not a problem for measuring and timing your events because every single frame has a time-stamp which is certainly correct. However if you get this warning message you will need to read the OSD time-stamps when timing your events rather than using the automatically calculated time by Tangra. For this example answer "No" to the warning message to load the light curve. As Tangra opens the light curve it will warn you one more time about he problem and will suggest using the time-stamps on the frames for timing the events. It will also tell us that the time difference is 956ms (almost a second or about 24 frames).



Once you press "OK" Tangra will open the Light Curves for you as shown below. If you select a random frame to the right Tangra will display the pixels with the apertures. The apertures will be positioned exactly as they were when the measurements have been made and this is a very easy way to determine whether a single measurement is good. Tangra may also show information and warning messages for each of the objects for the selected frame. In our case the occulted star and the comparison star have a warning that informs us that those objects have been measured with manually positioned apertures. Such apertures are not verified i.e. Tangra doesn't check whether the expected object falls within the aperture. From the "Data" menu item you can change the view displayed to the right from measured pixel areas to PSF view or background histogram view. When you select a frame Tangra will also display at the bottom the frame number and the time as derived from the entered frame times and number of measured frames. If the time is displayed in red as it is now, this means that you should use the time-stamps on the video frames to time events.



We can see immediately that there is an occultation. To get a better view of the light curve we can remove the guiding and comparison star from the plot. To do this click on the "Include Objects" button at the status bar of the form and then remove all the objects but the light blue occulted star. Now select the point of the light curve that is in the middle of the disappearance slope. As you do this Tangra will also position the video at that frame. Now read the time-stamp for the disappearance and you should get the D at 10:41:24.95. Now select the point where the reappearance begins (frame 232) and write down the time-stamp from the video. You should get the R at 10:47:27.15. If you now want to send the light curve plot to someone then click on the light curve and press "Esc" to remove the selection then choose File -> Export Light Curve -> Copy Plot to Clipboard. Now paste the copied light curve in your favorite image editing program. You should get something like this:



Before you exit make sure you save the light curve using "File" -> "Save Light Curve" from the menu. Tangra will save a file with extension ".lc" that will contain all measurements as well as an area of 35x35 pixels around each measured object in each measured frame. You will be able to send this file to anyone else for a review. They will be able to see your measurements, the positioning of the apertures and will even be able to re-measure your data with slightly different settings. The size of the file will be also very reasonable. In our case the .lc file is 1.27 Mb. The file size only depends on the number of measured objects and number of measured frames and contain the best quality pixel data available from the video. So if you have measured 4 objects in 300 frame from your 2Gb video you will still get a 1.27 Mb file.



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Measuring a very windy video Video File: 090926_Dione.zip (right click and choose "Save Target As...") Size: 238 Mb (zipped) Codec: No Codec Required Tracked asteroidal occultation Wind, shaking or movement Strong flickering



In this example you will learn how to best handle very windy videos. You are also going to learn a few "tricks". First of all download the video and play it to get an idea of how much wind is in there. There are periods that the wind is so strong that there are no stars visible at all but rather some blured lines. Those frames cannot be measured of course but the goal will be to set up the stars once and get Tangra measure the whole video without interruptions. At the end we would like to get a light curve and we will expect that some periods will not be measured but we will have a reading for all measurable frames.

So let's get started. Before we begin we would like to adjust the tracking settings to best handle this video. For this open up the Settings form and move to the Tracking tab. By default "Try automatic recovery from lost tracking" check box is checked. Just ensure that it is still checked. The second change is to decrease the number of refining frames to 4. We want to do this because the shaking will make it difficult for Tangra to complete the initial refining. Remember these are rather extreme conditions. In all other cases it is still best to use 16 refining frames. Read more about adjusting the refining frames below. Finally for very windy videos we want to set the "Recovery Tolerance" to the right most setting - "Slower & more tolerant". With this Tangra will use more time to try and recover from lost tracking, for example after everything has been completely blurred, but will have a better chance of finding the stars being more tolerant to where they are and how blurred they are.



Now that we have configured the tracking options let's open up the video. On the Light Curve Reduction form choose "Wind, shaking or movement" and the "Strong flickering" option. We are going to use the strong flickering for two reasons. First of all when the stars are slightly blurred they will look fainter. By choosing strong flickering we are telling Tangra to accept the recovered stars as correct mostly based on their relative position rather than their brightness. Secondly we are going to use the following trick in this measurement. Because there are only 2 bright stars and one of them is the star to be occulted we cannot have a good tracking in such windy conditions by using a single guiding star. At least 2 guiding stars are required for a good tracking in windy conditions. So what we are going to do is we are going to track the occulted star as "Guiding Star" because it is bright enough to improve the tacking. However in case of an occultation we would like to tell Tangra to tolerate bigger brightness variations and this is why we will choose "Strong flickering". Finally to start a measurement we need an occulted star. As we are going to use the star to be occulted as Guiding Star, we need another object for Occulted Star. For this we can choose any other fainter star or even simply click somewhere in the noise and configure this as manually positioned occulted star. This is really a trick to get around the problem of tracking in windy conditions where we don't have two real guiding stars.

Before you continue on the "Reduction Settings" tab make sure you have selected "Aperture Photometry" as reduction method. Otherwise you will not be able to use "Manually Positioned" apertures. If you only see one tab on the "Light Curve Reduction" form then press the "More Options" button at the bottom to get the extra tabs.



So let's do the configuration as discussed. First click on the first bright object and configure it as a guiding star accepting the default options.



Then select the second star. This is the star that can be occulted but we are going to configure it as a guiding star. After you press "Add Object" Tangra will probably suggest to use the star as Comparison Star by default as it is slightly dimmer than what Tangra believes the brightness of a Guiding Star should be. In our case we want to use it as a Guiding Star anyway so we mark it as "Guiding Star" and press "Add".



Finally let's add a fictional occulted star. Simply click anywhere on the video and press "Add Object". Then configure this to be "Occulted Star" with "Manually Positioned" aperture. If you don't have the "Manually Positioned" aperture settings available then you are probably doing PSF Photometry. If this is the case you will need to start again and make sure you have selected "Aperture Photometry" as the reduction method. Before pressing "Add" we want to increase the tolerance of the position of the occulted star to the maximum 3.5 pixels (see third image below). This is another trick that will help for better tracking in windy conditions.

The tolerance, configured next to the occulted star, has the meaning of what difference in the distances between the tracked objects Tangra will accept as being okay. In a case of a strong flickering or wind relaxing the value will result in less frames being marked as suspect as it is more likely the centers of the fitted PSFs to wobble around because of the non perfect shapes of the PSFs. So in our case we want to use the maximum allowed value of 3.5 pixels.



Well now we are ready so press the "Start" button and watch the show. The processing will be rather slow because Tangra will try really hard to locate the stars as the wind moves them around and blurs them. At the top right corner you will see the "Stripes" that represent the measurements. They are only rough estimate of the measurements and each line has the brightness of the pixel that is in the center of the placed aperture. When Tangra is unable to locate the guiding stars and marks the frame as suspect, instead of a gray line it will put a colour line with the colour of the corresponding object. You will likely see something like the first image below which means that some frames are not measured successfully. And this is not something we didn't expect. Once the processing is completed you will see that there will be some Unsuccessful frames (second image below). In my case 72 frames were marked as unsuccessful and they are in the most severe wind.

Here instead of entering times we can press the "Continue with no times" button and get the light curve immediately. I usually do this on those videos where I haven't seen an occultation on the screen. This way I get the light curve faster but in a case of an occultation I can still read the time from the time-stamped frames. This video was recorded away from the predicted center line and the most likely result is no occultation recorded.



Shortly after we press the "Continue with no times" button Tangra displays the following light curve (see below). You will notice that some of the lines and dots on the plot are gray. These are all measurements where the tracking was not successful and the frame was marked as suspect. In such a case Tangra still makes a measurement using the last known position of the objects but marks the measurements as most likely bad. A review of the positioning of the apertures after the measurement can tell whether Tangra was right marking the frame as suspect or whether the measurements can be still useful in some way. If you select one of those frames as shown below, to the right Tangra will show the positioning of the apertures and will show the warning signs. If you hover the mouse over the signs, Tangra will tell you more information about the warning. In our case we get two messages - what happened is Tangra failed to locate the object where it expected them to be, and also failed to recover them in a different position. Sometimes you can see a warning that the objects were not initially found but later were recovered after looking "more widely". A full list of all warning messages can be found on the Tracking page.



Now we remove the green object as we know that it is fake and we get the light curve below. What is interesting about this light curve is that, apart from the two areas where the wind was too severe, we have reasonable measurements for all other frames and there is no sign of an occultation. Of course there is still a possibility that an occultation occurred during one of those severe wind blows.





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Measuring a drift through video Video File: 20081019_Prokne_DWD_C.zip (right click and choose "Save Target As...") Size: 198 Mb (zipped) Codec: DVC/DV Video Tracked asteroidal occultation Full or almost full disappearance Drift through


We are going to measure a drift through video where our occulted star is rather faint as well. In the first reduction we are going to measure it directly and then in a second reduction we are going to use software integration and pre-processing to improve the S/N ratio of the occulted star.

So open up the video and choose "Tacked Asteroidal Occultation" (selected by default) and then choose "Full or almost full disappearance" and "Drift through".



We are going to select one guiding star and setup the occulted star to use Manually Positioned aperture. Looking at the video frame we can tell that the focusing is not great and stars are rather big. Generally the size of the stars (FWHM of the point spread function) should be the same for all stars regardless of their brightness. So in our case we are going to find what is a good size aperture for the bright guiding star and then use the same size aperture for the very faint occulted star. By doing this we ensure that we include the right amount of light from the very faint object.

So click on the bright star to the left to select it and press "Add Object". Then keep increasing the aperture until it nicely contains the full star. You will probably get somewhere to 5.5 pixels aperture radius. Don't worry if the star is not perfectly centered in the aperture. This is not so important for Auto-Centered objects.



Next try to select the occulted star. It is barely visible so simply click somewhere close to the location shown on the screenshot below. Tangra will automatically center the selection on the brightest objects in the area. Then click "Add Object" and select Occulted Star. Now we want to configure our occulted star to use "Manually Positioned" aperture and we also want to set the aperture size to 5.5 pixels which was the value that worked well for our guiding star. If there is enough light for an object Tangra will do a better PSF fit and the automatically determined aperture will fit much better by default. However for very faint objects and out of focus objects like the ones we have here we should set the aperture size based on the aperture determined for other stars. Finally use the U, D, L and R buttons to move the Manually Positioned aperture around and center it. If you look at the guiding star and occulted star images carefully you will notice that they look similar because of the same optical imperfections that make them not perfectly rounded. For best results when centering the manually positioned aperture try to position it the same way the aperture was automatically positioned for the guiding star. In this case the brightest part of the star is not exactly in the middle but is a few pixels up. Position the aperture around the faint occulted star the same way and press Add to add the object,



Now we are ready to measure so press the "Start" button. When you do this Tangra will show you the following warning message that may look strange to you if you haven't seen it before:



Probably the biggest surprise for you will be how a black and white video camera can produce a colour video? The answer to this question is that it doesn't. However in the grabbing process of digitizing the video there have been a colour space conversion. There are two ways to present colour in digital world - RGB and YUV. The first one uses values for Red, Green and Blue channel. If you have a black and white video then for any pixel the three values R, B and G will be exactly the same. The second colour scheme uses a Luminance channel (Y) and two other channels to present colour - Hue and Saturation. The colour in your video is created as an artifact while doing conversion between the two colour spaces and this is because there are only 256 discrete values for any of the R, G, B, Y, U or V channels. The conversion from RGB to YUV uses formulas that produce floating point numbers. Then those numbers are rounded to integers because Y, U and V can only be integers. Then if we now convert the same Y, U and V numbers back to RGB we will also need to use rounding and this time the R, G, B numbers may be slightly different from what we started. They will also not be exactly the same i.e. R channel will not have the same value as the B and G channels for the same pixel. This is what makes the video a "colour" video.

As far as the reduction is concerned for black and white videos all the three R, G and B channels are the same and we only need to measure one of them. By default Tangra measures the R channel. However if we have a colour video, the best thing to do is to use a derived GrayScale value that is computed from all R, G and B values. This will represent the actual luminance of the object the closest. So in this case we press "Yes".

Tangra will start measuring and after not too long it will complete the measurement of the video. One thing you may notice is that there are significant number of frames that Tangra marked as suspect. This is due to the too large brightness variation of the guiding star (i.e. flickering). You will likely get the number of suspect frames down if you select the "Strong flickering" check box in the reduction settings form.

Now we need to enter the times of two frames. I don't know exactly what was the time-inserter used in this video but seems that there is a time-stamp on each field and the two time-stamps overlap. This is why they appear blurred. So to read the time-stamps we need to press the "Show Fields" button and then enter the time for the first frame. For this I will use the time of the top field. When ready press the "Next" button and Tangra will position to the last frame and ask you for the time. We will also enter the time of the top field. When ready we press "Finish" and Tangra will show you a time inconsistency warning.



When we look at the frame rate in the video - it is 29.89 fps and this is NOT a standard frame rate so it may be wrong. Usually not many videos (at least not many of my videos) have this problem and if I get this warning I always first press "Yes" and go and enter the times again just in case I have made a mistake during the entry of the times. In this case we press "No" as we know that there is a problem. Tangra then plots the light curves and warns us again, this time giving the exact time difference which is 166.5 ms in this case. When you get in such a situation you should always use the time-stamps on the video when deriving the D and R times.



So this is how our light curve looks like but we can't see much from this plot so lets remove the guiding star from the plot (which is pretty bright).



Do this by clicking on the "Include Objects" button in the status bar of the form and then deselect the Blue star.



Now we get a cleaner light curve and it looks like that there may be an occultation but not very conclusive.



Let's see if we can improve the S/N by using some pre-processing (additional video processing before measuring).



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Measuring a faint star using software integration and pre-processing



We are going to re-measure the same video but this time using pre-processing. Tangra supports a number of pre-processing options (for more details see Pre-Processing) and we are going to use software integration and stretching for this example.

So if you haven't yet done so, close the light curve and the video and re-open it. Still choose the same reduction options on the first tab - that is "Tracked Asteroidal Occultation" with "Full or almost full disappearance" and "Drift through". If "General" is the only tab you see on this form then press the "More Options" button. Now move to the "Integration" tab to configure the software integration. You can use different settings but most times you will want to use "Stepped Averaging" using a "Mean" value. Usually integrating 3 frames will give a S/N boost without affecting the timing precision too much. When you choose the integration option Tangra will open up a preview window and you will see immediately how the background is now less grained i.e. the S/N ratio has improved.

To make the star a bit brighter as well we can use Stretching or Brightness/Contrast adjustments. In this example we are going to use a simple stretching with a factor of 2. Again when you select this option from the Pre-Processing tab you will see in the preview window that stars become brighter.



Now lets add the guiding and occulted star again. Do as directed in the previous example. Only this time you will notice that the occulted star is much more 'visible' and that Tangra will have centered the aperture well by default when adding the object:



When ready press the "Start" button and answer "Yes" to the question whether to use GrayScale for this video only. You will notice that this time there are no frames marked as suspect (the stripes only contain gray lines from the measurements and contain no colour lines). You will also notice that the measurement runs slower now because of the integration and preprocessing used by Tangra. On the status bar we see that a 3 frame integration is used.



Once the measurement has finished repeat the entering time procedure from the previous example and acknowledge the time inconsistency warning. You will then get the following light curve:



The much lower levels of noise are immediately evident and if we remove the blue guiding star again we end up with this light curve:



Now this looks much more like an occultation so lets try to extract the D and R times of the event. Before we start remember that this was a 3 integrated frames measurement so our timing resolution is now 0.033 sec * 3 = 0.1 sec. Also because of the timing inconsistency we need to use the time-stamps from the video frames. For the disappearance and reappearance events I've got these numbers: D at 10:44:43.4 and R at 10:44:56.7.



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Reprocessing of a light curve file produced by someone else


The last example will show you what you can do if someone else sends you their Tangra light curve (.lc) file. For this you can use my Prokne-DriftThrough.lc file (1.93 Mb). So download the file and open it up in Tangra. There are a number of differences that you will noice. First of all even that you may have on your hard drive the same video file that I measured it is likely saved in a different location so Tangra cannot find it. Because of this no frames will be updated when you select points from the plot. However Tangra will show you an image with the position of the measured stars from the first measured frame. This will help to verify that the right objects were measured in the first place. You will see the stars marked with their respective colour:



There are a number of things that you can do on the Light Curve forms such as normalizing and binning but what I would like to show here is how to re-process the data. With the light curve file (.lc file) Tangra saves all pixels in an area of 35x35 around all measured objects and does this for all measured frames. This means that by having the .lc file you also have the actual measured areas in the video and you can re-measure them with different settings. To do that use the "Data" -> "Quick Re-Process" menu item from the Light Curve form. Because Stretching pre-processing has been used Tangra will warn you that only the original non pre-processed data are available for measurement.



There is not much you can do here other than agree to re-process the original data (still the integrated frames). Let's change some of the parameters and this time use a Low Pass Filter (LP) and also reverse an encoding gamma of 2.22. By doing this we will be effectively applying a gamma of 1/2.22 = 0.45 = Low Gamma. Applying gamma is a pre-processing that magnifies certain dynamic range of the video and applying a Low Gamma (as you probably know from using it in your video camera) makes the fainter stars more visible. This is what we want to achieve here.



After you have selected the re-processing options press "OK" and shortly Tangra will show you the new light curve. If you now remove the guiding star from the plot you should see something like this:



This is a very convincing light curve consistent with an occultation event. Not only that we have a well visible drop but also the disappearance and reappearance events are very short and steep.



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Important Configuration Settings


Setting up the OSD area

Occultation videos are often time-stamped with a device such as IOTA VTI, KIWI OSD and others, which put a time-stamp on each video frame. This method of time-stamping is often called on-screen-display (OSD) time insertion. For best tracking results Tangra needs to know where is the time-stamp on your video. This is why it is recommended to set up correctly your OSD area for the equipment you are using. To do this you need to do the following:

- Open up a video and choose "Light Curve Reduction"
- From the toolbar under the main menu click on the "OSD" button



Once you have done this a blue rectangle with thick borders will show up on the screen. This is your current OSD area and your time-stamp should be fully contained in the rectangle. By default the OSD area will be probably larger than your actual time-stamp. You can adjust the borders by selecting them with the mouse and moving them. As you do so the cursors will change and the colour of the border will change to purple. Once you have finished moving the line and when you release the mouse button the border colour will change back to blue. With this the area is saved immediately and you don't have to press a Save button.

By default the OSD area is set to cover a large part of the bottom area of the video which is usually where time inserters put their time-stamp. However if you process videos from various time inserters you can check the OSD area for each of them and set up an OSD area in Tangra that covers the combined areas from all time inserters.


Refining frames

Before Tangra begins to track and measure the selected objects it will run a for a number of frames to refine the positions of the objects and determine a number of parameters for them including the amount of flickering, the FWHM of the fitted star model and the motion of the object in a case of a drift-through video. Knowing the values of those parameters is important for a good tracking and successful measurement. By default the number of refining frames is 16 and this is configured from the Settings -> Tracking tab.



The number of refining frames are the total number of successfully processed frames rather than total number of frames since you pressed "Start". Because of this in some rare cases if you have a very noisy video without bright guiding stars Tangra may have problems locating the objects during the refining process and it may take more frames to complete the refining. First of all if you have similar problems you should always try to use bright guiding stars or software integration when you are measuring light curves. If nothing else works however you can try to decrease the number of refining frames.