Tangra 1.4


 
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Guide to Video Astrometry in Tangra

(Including Step-by-Step Reduction Examples)


Depending on how fast an object is moving the Astrometry in Tangra is done slightly differently and there are different things to look for. The two examples given below cover slow moving objects (typically motion < 2"/min) and fast moving objects such as Near Earch Asteroids (NEA) that encounter close approaches to the Earth and could be moving with thousands of arcsecs per minute.

Video astrometry of those fast moving asteroids is not a trivial task and could become particularly hard if the observer/measurer is not familiar with the foundations of astrometry and/or video observing and timing. I recommend to everyone that hasn't done both video observing and astrometry to read the Video Astrometry Guide put together by Dave Herald before Tangra existed. The guide describes the reduction process using FITS files in Astrometrica but also goes through the important concepts about using video cameras and doing astrometry in general.

Currently Tangra allows you to do astrometry of both slow and fast moving asteroids without leaving the software and it does everything for you, however the measurer still needs to follow a guided process when reducing the video to ensure the maximum precision is achieved and no systematic timing errors are introduced. The purpose of this guide is show you how to do this and give you some suggestions as well.

Since version 1.3 when starting a new astrometric measurement Tangra will suggest to adjust the levels of the video so the effective dynamic range of the pixels strech to the full dynamic range. For all of the examples below you can choose to ignore the suggested level adjustments.

There are three examples below:


Astrometry of Fast NEA Flyby

Successful observation of a fast moving asteroid requires both a good quality observation data and correct data reduction. Fast moving objects are particularly difficult to record because the exposure has to be right (when using integrating video cameras). On one hand you need to have enough light collected from the object so it is visible on the image but on the other hand you don't want the image to be trailed. Also if the object is moving too fast there still may not be enough light to produce a well detectable object. In some cases the object may be just too faint for your aperture. Probably the best advice for recording a NEA on a video for astrometry is that every object is different and needs some thought and careful planning for your observation to be successful.

Standard video runs with 25 (PAL) or 29.97 (NTSC) frames per second. There are video cameras that can record more frames per second than this but for the purpose of this guide by non integrated video we will refer to 25 or 29.97 frames per second. There are cases where NEAs can be recorded with no integration but those cases are probably the minority of all observable NEA approaches. If the asteroid gets brighter than magnitude 12 then there are chances it can be recorded without integration on 20cm scope. Doing so will be the preferable way to record it provided there is sufficient signal coming from the object.

The majority of the NEAs observable with Video are however in the range 13 - 16 stellar magnitude and will require integrating video camera such as WAT-120N. Integrating video cameras offer a range of exposures (typically from 1 to 256 frames) which would make a fast moving object to appear from underexposed, to well exposed and trailed. To be able to make a decision what exposure (integration) to use when observing a NEA you first need to know the pixel size in arcseconds of your video. This can be determined using simple calculation from your FOV size and video frame size in pixels (not the CCD cell dimensions) or you can open and solve a video with Tangra and it will display the pixel size for you. On the screenshot below the pixel size is 1.5 arcsec per horizontal pixel and 1.4 arcsec per vertical pixel.



Once you know your pixel size in second of arc you can then use the tables below to find out what is the maximum integration that can be used for a given NEA sky motion. The values in the table are approximate but should be good enough. They are calculated on the basis of the maximum wanted elongation (trail) on the image due to the fast movement to not exceed 3 pixels.


Maximum useful video integration for PAL/CCIR video (25 fps)

  1.2"/px 1.4"/px 1.6"/px 1.8"/px 2.0"/px 2.2"/px 2.5"/px 3.0"/px
256 frames <=   20"/min <=   25"/min <=   30"/min <=   30"/min <=   35"/min <=   40"/min <=    40"/min <=    50"/min
128 frames <=   40"/min <=   50"/min <=   60"/min <=   60"/min <=   70"/min <=   80"/min <=    80"/min <=   110"/min
 64 frames <=   80"/min <=  100"/min <=  110"/min <=  130"/min <=  140"/min <=  150"/min <=   180"/min <=   210"/min
 32 frames <=  170"/min <=  200"/min <=  230"/min <=  250"/min <=  280"/min <=  310"/min <=   350"/min <=   420"/min
 16 frames <=  350"/min <=  400"/min <=  450"/min <=  500"/min <=  560"/min <=  620"/min <=   700"/min <=   850"/min
  8 frames <=  680"/min <=  800"/min <=  900"/min <= 1000"/min <= 1100"/min <= 1200"/min <=  1400"/min <=  1700"/min
  4 frames <= 1400"/min <= 1600"/min <= 1800"/min <= 2000"/min <= 2300"/min <= 2500"/min <=  2800"/min <=  3400"/min
  2 frames <= 2700"/min <= 3200"/min <= 3600"/min <= 4100"/min <= 4500"/min <= 5000"/min <=  5600"/min <=  6800"/min
  1 frame <= 5400"/min <= 6300"/min <= 7200"/min <= 8100"/min <= 9000"/min <= 9900"/min <= 11000"/min <= 14000"/min


Maximum useful video integration for NTSC/EIA video (29.97 fps)

  1.2"/px 1.4"/px 1.6"/px 1.8"/px 2.0"/px 2.2"/px 2.5"/px 3.0"/px
256 frames <=   25"/min <=   30"/min <=   35"/min <=   40"/min <=    40"/min <=    45"/min <=    50"/min <=    60"/min
128 frames <=   50"/min <=   60"/min <=   70"/min <=   75"/min <=    80"/min <=    90"/min <=   100"/min <=   130"/min
 64 frames <=  100"/min <=  120"/min <=  140"/min <=  150"/min <=   170"/min <=   190"/min <=   200"/min <=   250"/min
 32 frames <=  200"/min <=  250"/min <=  270"/min <=  300"/min <=   350"/min <=   400"/min <=   450"/min <=   500"/min
 16 frames <=  400"/min <=  500"/min <=  550"/min <=  600"/min <=   700"/min <=   750"/min <=   850"/min <=  1000"/min
  8 frames <=  800"/min <=  900"/min <= 1100"/min <= 1200"/min <=  1400"/min <=  1500"/min <=  1700"/min <=  2000"/min
  4 frames <= 1600"/min <= 1900"/min <= 2200"/min <= 2400"/min <=  2700"/min <=  3000"/min <=  3400"/min <=  4000"/min
  2 frames <= 3200"/min <= 3800"/min <= 4300"/min <= 4900"/min <=  5400"/min <=  6000"/min <=  6800"/min <=  8100"/min
  1 frame <= 6500"/min <= 7500"/min <= 8600"/min <= 9700"/min <= 11000"/min <= 12000"/min <= 14000"/min <= 16000"/min

To demonstrate how to determine the best integration lets go through the following example: Let's assume that we have a NEA that will be moving with 150"/min and will be 13.5 mag. If our pixel size is 1.4"/px then looking at the second column from the PAL/CCIR table we determine that we cannot use integration higher than 32 frames for 150"/sec. This is because the max speed for 32 frames is 200"/sec and the max speed for 64 frames is 100"/sec. So we cannot use 64 frames integration but can use 32 frames integration. Now we try to use the highest integration, smaller or equal than 32 frames, that will give sufficient signal. If we have a 20cm telescope at magnitude of 13.5 we should probably use integration of 32 frames. However if we have a 35cm telescope then 16 or 8 frames integration would probably give sufficient signal and will therefore be a better choice. Using no integration would be the best option but for this we'll need a 50-60cm telescope.

Looking at the table it is evident that the larger the arcsec per pixel value the higher integration can be used. This could make it possible very fast objects to be observed with smaller apertures (if they are bright enough) but this also comes at the cost of a lower timing resolution because of the coarser pixels.

Another very important thing to look for is to have sufficient number of reference stars (try for at least 7 stars if you can) as the more reference stars the better the astrometry. So it is really a balance of getting enough signal so the asteroid is visible, enough exposure to get more reference stars but at the same time not too large integration so the object is not trailed. It is a good practice to record with a number of different integrations and then see which video provides best data during the measurements. Try to record 1 minute with each integration setting if possible and also bear in mind that sometimes your telescope aperture will simply be too small to get enough signal for some asteroids.

Assuming that you have now successfully recorded a video of a NEA, the next step will be to reduce it with Tangra. To illustrate this process I will use a real video of the asteroid 2003 UV11 obtained by my friend Dave Gault. The asteroid reached magnitude ~12 while moving with ~200"/min. The video file is 193Mb and can be downloaded below.


2003 UV11 Video

Video File: 2003 UV11 - (23h26m38s 22d49'40'').avi (right click and choose "Save Target As...")
Size: 193 Mb
Calibration Type: Three Star Fit (See image on the right)
Video Camera: WAT-120N+ (PAL)
No Flip
Focal Length: 1250
Limiting Magnitude: 14
Field Center: RA=23h 26m 38s DE=+22d 49' 40"
 

Unless your name is Dave Gault or you have exactly the same optics, video hardware and grabber as him, you will first need to calibrate the video. Once you have it open, choose "Astrometry" from the menu and choose a new configuration and give it some name. The camera should be WAT 120N+ (PAL), the limiting magnitude - 14 and no flipping options need to be selected. Make sure the "Calibrate Now" checkbox is checked and click OK to continue. If you haven't done a calibration in Tangra before then you should read the
calibration guide to become more familiar with it. When you are ready to calibrate choose the field and focal length as shown below on the "Solve Configuration" form. The calibration stars identification chart is shown above for the UCAC3 star catalog. If you have UCAC2 or NOMAD instead you'll need to find the field in a planetarium program and identify the stars manually to complete the calibration.

     

After you have completed the calibration you'll need to reload the video. Before starting the actual astrometric measurement there is one more thing to do. We are going to use Tangra's automated object recognition which uses the MPC web service. For this you'll need to be online and also need to configure coordinates or MPC observatory code. You can do this from Settings -> Star Catalogue & Location tab. This video has been recorded from the E28 observatory so put "E28" in the MPC Observatory Code box and hit OK. Next select "Video Actions" -> "Astrometry" again and this time pick the calibrated configuration which should look something like this (shown to the right below)

     

We then enter the UT date of the record which is 29 Oct 2010 and also fill in the UT time as read from the timestamp. You then have two options to either put the object designation which is 2003 UV11 and use the "Known object in the field" setting or you can directly enter the coordinates of the field using the "Known approximate field center" and then identify the asteroid manually. The "Known object in the field" will require internet access and will query an MPC web service to identify the object. For this to work you will need to have configured the MPC observatory code in the Tangra settings because of the close approach the position of the asteroid will be significantly different from different locations on the Earth.

   

If you have used the field center then after the field is solved you should see something as shown below. If you have used the known object option you should also see the selected object below to have a label of 2003 UV11.



To begin the measurements you need to select the object (as shown above) and then click the "Multi-Frame Astrometry" button to the right. The "Multi-Frame Measurements" form will show up and first we need to select the expected motion, which in our case is "Slow Flyby (2.0"/min -> 200"/min)". The video was recorded using 16 frame integration. For a correct measurement it is essential to position the video to the first frame of a new integration interval and also enter the timestamp from this frame. So we first select "First frame of integrated interval" option from the second dropdown and this will make visible more controls under it. If you have the "Detect Used Integration" button visible then use it. If the integration is detected successfully then Tangra will automatically position the video to the first frame of the next integration interval when you press the "Accept" button on the "Detecting Video Integration" form. In all other cases when the detection is not successful or the automatic detection is not available you must go back and manually move the video to the first frame of the next integration interval. To do this you will need to press Cancel on the "Multi-Frame Measurement" form and then use the "-1Fr" and "1Fr+" buttons to move the video frame by frame until you recognize the first frame of the next integration field. Once you are certain you have selected the correct frame you need to select the object again and press the "Multi-Frame Astrometry" button to start again.

     

Lets assume that you have positioned to the first frame on an integration interval automatically or manually. You now need to confirm or enter the timestamp of the current frame and this is essential for a correct measurement. If you have entered the first frame time correctly then Tangra should have automatically put the time for you but sometimes there may be rounding differences of 1ms or sometimes there may be larger differences due to dropped frames in the video. Once you have checked and modified the time as necessary you now may need to enter the instrumental delay of your camera. Using the right value for the instrumental delay is also essential for a correct measurement. For WAT 120N and WAT 120N+ cameras Tangra will automatically set the instrumental delay when you change the integration and press Enter or when the integration has been automatically detected and accepted by you. For the instrumental delay Tangra uses the tables provided by Gerhard Dangl. If you are using integrating camera different than WAT120 you must determine the instrumental delay of your camera before you can do high precision astrometry of fast NEAs. For all other camera models, than WAT120N, Tangra will use as a default value an assumed instrumental delay of exactly half integration frames but this is not guaranteed to be correct and may introduce a constant systematic error.



Once you have sorted the integration, timestamp and instrumental delay press "Next" to continue. On the next page you configure how many and which frames should be measured. In a case of a NEA recorded with integrating camera Tangra will determine the position of the object in all measured frames in each integration interval and then derive a median value for the middle of the integration interval. So the many frames we measure the better, because of this we will measure every frame and use the default value of "Measure every frame" and click "Next". If you also want Tangra to measure the magnitude of the object then check the "Fit and Compute Stellar Magnitudes" checkbox. Measuring magnitudes will make the measuring process slower and if there are many tens of reference stars in the video the measurements may be significantly slower. I our case we choose to measure the magnitudes and leave the default settings.

     

Now we are ready to begin the measurements and press "Start". Tangra will plate solve each frame, compute the positions and magnitude of the selected object, then derive a single value for each integration frame and assuming a linear motion in both RA and DEC it will fit the motion curve. All those measurements will be displayed in the blue and green areas on the right. The fitted line will be drawn in bold and the +/- one sigma lines will be also drawn as a thinner lines. You can leave Tangra to measure the selected number of frames or press "Stop Measurements" at any time.



Now you can click anywhere in the RA and DEC areas and Tangra will compute and display the position of the object from the fitted motion line at the closest normal position to the location where you have clicked. The MPC observation format only allows 6 decimal places in the fractions of the reported day which corresponds to 0.086 sec or 2.16 PAL frames. For very fast moving NEAs this may introduce random time and position errors to the measurements and because of this MPC allows reporting of normal place positions which are the computed RA/DEC at any 0.0000001000 day. Tangra computes normal positions for all NEA measurements regardless of whether integration has or hasn't been used. The text "normal" appearing in brackets after the displayed time indicates whether this is a normal place position or not.

   

The last step is to put the measurements into a report file. To do this click the "Add to MPC Report File" button. Tangra will first ask you to enter or confirm the designation of the object. You can enter the common name/designation as 2003 UV11 to the left or you can enter the packed MPC format K03U11V to the right. Once you have confirmed the object designation and if you haven't selected a report file yet, Tangra will ask you to select a report file where to add the observation. Select "[New Report File]" and select a location where to save the file. Then Tangra will ask you to confirm the report header. You need to make sure the values of the individual lines are correct and in accordance with the MPC requirements as Tangra doesn't validate the header.

   

The information below shows the positions reduced from the same video with Tangra and reported to MPC in the beginning of Nov 2010 as well as the residuals compared with residuals of other observations made in the same period.
     K03U11V  n2010 10 29.58673223 26 37.62 +22 49 40.1          11.6 R      E28
     K03U11V  n2010 10 29.58679023 26 36.94 +22 49 39.7          11.6 R      E28
     K03U11V  n2010 10 29.58690123 26 35.62 +22 49 39.5          11.6 R      E28

20101028 J08 (2.5- 1.4+)                     20101029 E28 0.5+ 0.1-                      20101029 157 0.7+ 0.1+  
20101028 J08 (1.7- 0.6+)                     20101029 E28 0.6+ 0.2-                      20101029 157 1.2+ 0.1+                           
20101029 E12 0.7+ 0.0                        20101029 E28 0.6+ 0.0                       20101029 157 0.8+ 0.1+                       
20101029 E12 0.5+ 0.3-                       20101029 E28 0.5+ 0.1-                      20101029 B66 0.5+ 0.6+                         
20101029 E12 0.2+ 0.3+                       20101029 E28 0.4+ 0.4-                      20101029 510 0.4- 0.0                                 
20101029 E12 0.3+ 0.1+                       20101029 E28 0.7+ 0.4-                      20101029 510 0.4- 0.1+                                            
20101029 E12 0.4+ 0.0                        20101029 157 1.4+ 0.1+                      20101029 510 0.3- 0.2-                            
	

Astrometry of Standard Comet or Asteroid

With medium sized telescope and integrating video camera it is easy to go down to magnitude 14-15. This gives an opportunity to video observers to do astrometry of slow moving objects such as comets and asteroids. The motion of the comets is affected by non gravitation forces and astrometry of comets is actually a very useful thing to do. Also if one wants to submit observations to MPC an observatory code is first required and in order to get an observatory code one need to observe and submit astrometry of known numbered asteroids. Tangra can do all this and in the next two examples we will do a reduction of a video of the asteroid (453) Tea which I used when I obtained my new MPC observatory code and the comet 103P/Hartley.


(453) Tea Video

Video File: (453) Tea - (14h44m50s -20d52'25'').avi (right click and choose "Save Target As...")
Size: 10 Mb
Calibration Type: Three Star Fit (See image on the right with UCAC3 star numbers)
Video Camera: WAT-120N+ (PAL)
No Flip
Focal Length: 1000
Limiting Magnitude: 16
Field Center: RA=14h 44m 50s DE=-20d 52' 25"
 

The first thing you'll need to do is calibrate the video because you are almost certainly not using the same optics, camera and video recorder as me. So open up the video and choose "Astrometry" from the menu. Then create a new configuration for WAT 120N+ camera as shown below. Make sure "Calibrate Now" checkbox is checked and click "OK". If this is your first time doing a calibration in Tangra then you should read the
calibration guide to become more familiar with it before you continue. In this example I am also going to demonstrate another aspect of the calibration process - adjusting the star depth during a calibration. Before clicking "Calibrate Configuration" check the "Adjust Star Depth" checkbox under the button.

     

You should now see the form shown below where Tangra shows you the brightest stars from the image which it will use during the image alignment and calibration process. You can use the "<" and ">" buttons to adjust the number of stars Tangra will use during the image alignment. By default it will use a number higher but close to 25. In this example we will increase the star depth so we can calibrate using some fainter stars. To do this click the ">" button until Tangra shows you a depth of 4 and the star configuration shown below. Then press "OK". Then select the coordinates and limiting magnitude as shown to the right below.

   

Now use the identification chart shown above in the area where you downloaded the video from to complete the calibration.



Once you are calibrated you will need to reopen the video as directed by Tangra. Now choose "Astrometry" from the menu again and this time select the calibrated configuration name. This observation was done on 28 Aug 2010 so choose the date/time settings as shown below. In this example we are also going to find the field by specifying the asteroid number and using the MPC web services to locate the field. So choose "Known object in the field", type "453" as shown below and click "OK".

   

You will need active internet connection for this to work and after a little while Tangra should return back with solved plate and a circled object marked as (453) Tea.



The next step is to measure the position of the asteroid. Because this is not a fast moving asteroid we are going to derive a single position from a large part of the video by computing a median position. Because the used integration is quite high (128 frames or 5 sec) we would like measure at least a few integrated frames and take the median position.

So the next step is to select the asteroid and click the "Multi-Frame Astrometry" button. We then select as expected motion [ Slow < 0.2"/min ]. Because this is an integrated video we select that the current frame is "first frame of integration interval". Then press the "Detect Used Integration" and let Tangra figure out the integration for you. It will eventually come back with a suggested integration of 128 frames starting as frame 96.

 

After you click "Accept" Tangra will actually move the video to the 96-th frame and will adjust the current (96-th) frame time. In this case there will be a rounding error of 1ms which you may or may not correct. The 1 ms difference will have effectively no effect on the final astrometry in the slow motion case. You will also notice that Tangra has configured a 64 frames instrumental delay.



We then click "Next" and configure the number and types of frames to measure. As we said early we would like to measure many integrated intervals and because of this will choose to measure every 8-th frame which means to measure 16 frames from each 128 frame integration interval until the end of the video, which will correspond to a total of 152 measured frames. We then click "Next" again and choose the photometry settings. When done press "Start" to begin the measurements.

     


Tangra will begin to do measurements and show the measured RA and DE positions. You can wait until all 152 frames are measured or you can press "Stop measurements" at any time to stop the measurements. After the measurements have been stopped (the middle image below) we have the distribution of the RA and DE measurements and as we can see they are sort of consistent but still there is some variation. We now need to click on both the RA and DE panels individually and select which part of the measurements should be used to derive the final position. Tangra will also display the magnitude and time under the DE panel (the last image below).

   

We are now ready to add this measurements to an existing or a new MPC report file by clicking the "Add to MPC Report File" button. We then follow the prompts to add the observations to the report file. As I said early this observation was reported to MPC as shown below along with the residual. As you can notice while the position is exactly the same as what we measured above, the time is slightly different (28.40829 instead of 28.40838). The reason is that in the initial reduction I have probably measured every 4-th frame and got less frames in total with different middle time. To get an idea of the reported timing precision the measured interval in our case was about 40 sec which means the reported time was +/- 20 sec which corresponds to 0.0002 days so there is no to real discrepancy between the two different times.
00453         C2010 08 28.40829 14 44 49.57 -20 52 25.9          14.6 R      XXX

    20100828  E24  0.0   0.2+



103P/Hartley Video

Video File: 103P (07h34m48s -17d52'01'') .AVI (right click and choose "Save Target As...")
Size: 6.8 Mb
Calibration Type: Three Star Fit (See image on the right with UCAC3 star numbers)
Video Camera: WAT-120N+ (PAL)
No Flip
Focal Length: 1000
Limiting Magnitude: 15.5
Field Center: RA=07h 34m 48s DE=-17d 52' 01"


 

NOTE: The first frame of this video is black so you'll need to press "1Fr+" to move to the next frame before you begin.

The first thing you'll need to do is calibrate the video because you are almost certainly not using the same optics, camera and video recorder as me. If you have already calibrated the previous (453) Tea video you can use this calibration but if you haven't you should do a new calibration. See the
calibration guide for more information and the details on the screenshots below to complete the calibration.

     

Now use the star identification chart shown above in the area where you downloaded the video from to complete the calibration.



Once you are calibrated you will need to reopen the video as directed by Tangra. Now choose "Astrometry" from the menu again and this time select the calibrated configuration name. This observation was done on 10 Dec 2010 so choose the date/time settings as shown below. In this example we are also going to find the field by specifying the comet designation and using the MPC web services to locate the field. So choose "Known object in the field", type "103P" as shown below and click "OK".

   

You will need active internet connection for this to work and after a little while Tangra should return back with solved plate and a circled object marked as 103P/Hartley.



The next step is to measure the position of the comet. Because this is not a fast moving object we are going to derive a single position from a large part of the video by computing a median position. Because the used integration is quite high (64 frames or 2.5 sec) we would like measure at least a few integrated frames and take the median position.

So the next step is to select the comet and click the "Multi-Frame Astrometry" button. We then select as expected motion [ Slow < 0.2"/min ]. Because this is an integrated video we select that the current frame is "first frame of integration interval". Then press the "Detect Used Integration" and let Tangra figure out the integration for you. It will eventually come back with a suggested integration of 64 frames starting as frame 50.

     

After you click "Accept" Tangra will actually move the video to the 50-th frame and will adjust the current (50-th) frame time. In this case there will be a rounding error of a few ms which you may or may not correct. The time difference will have no effect on the final astrometry in the slow motion case. You will also notice that Tangra has configured a 32.75 frames instrumental delay.

We then click "Next" and configure the number and types of frames to measure. As we said early we would like to measure many integrated intervals and because of this will choose to measure every 4-th frame which means to measure 16 frames from each 64 frame integration interval until the end of the video, which will correspond to a total of 142 measured frames. We then click "Next" again and choose the photometry settings. When done press "Start" to begin the measurements.

   


Tangra will begin to do measurements and show the measured RA and DE positions. You can wait until all 142 frames are measured or you can press "Stop measurements" at any time to stop the measurements. After the measurements have been stopped (the middle image below) we have the distribution of the RA and DE measurements and as we can see they are not strictly consistent. For comets this happens as result of the error when determining the center of the object which is not a dot. We now need to click on both the RA and DE panels individually and select which part of the measurements should be used to derive the final position. In this case with the selection we exclude parts (shown in orange) which are not consistent with the median mesaurement. Tangra will also display the magnitude and time under the DE panel (the last image below).

   

We are now ready to add this measurements to an existing or a new MPC report file by clicking the "Add to MPC Report File" button. We then follow the prompts to add the observations to the report file. The observation line that is generated by Tangra looks like this:
0103P         C2010 12 11.54588 07 34 48.28 -17 52 01.2          13.1 R      E24