Showing the FoV of your Telescope & Camera in Aladin

New 20th July 2015

Last modified 2nd December 2019


When I have used Aladin in the past I realized that you could show the field of view (FoV) of various telescope and instrument combinations on images (try the HST Wide Field Camera to see how small it is) but I did not realise that you can create and save your own FoV and save them.


One way of doing this is described in the Aladin FAQ however this seems quite difficult to use. However there is a better way that was described by Adam Block in the August 2015 edition on Astronomy magazine. This method does not seem to be in the Aladin Manual or FAQ despite the tool being created in 2008.

Before you start you need to find the 'Plate Scale' of your Telescope and CCD and also the plate scale of and physical offset your offset guider chip if you have one. These measurements need to be in arc seconds. There are several tools on the web that enable you to calculate the FoV of a telescope & CCD combination or you can try my Excel Spreadsheet that allows you to calculate the FoV and other parameters for you telescope. This file is an Excel Template and you will need to save it to your computer with an appropriate name when you have filled the details in.

The next step is to download Aladin to your computer (if you have not already done this) and open it as normal.


1: Before you start select an object in the normal way - it does not matter any of the Messier objects are a good choice - here I chose M82. There is no real need to do this it just make showing the FoV of an instrument on a sky image easier.

2: From the Aladin menu select 'File' -'Load Instrument FoV' and the following window should open


Aladin - Instrument FoV selector


As you can see the FoV for a number of Ground and Space based telescopes can be loaded as well as the Schulman32 created by Adam Block and is the subject of the article in Astronomy mentioned above. For this example highlight Schulman32 and then click on Submit (not load it as you might expect !). The overlay showing the FoV will then be loaded into Aladin. As normal graphics are transparent and are loaded as images on top of the base image. The Aladin screen should now look something like this:


Aladin with Schulman32 overlay



The inner square shows the FoV of the camera used on the Schulman32 telescope overlaid in this case on a DSS image of M82. The small rectangle shows the FoV of the off axis guider chip and the two circles show the area where the guide camera could be rotated to to find a bright enough star to guide on. In this case there are two candidates at 3 and 5 o'clock and another at 1:30 which seems to require M82 to be moved a little off center.


The normal Aladin tools can be used to show the brightness of the guide stars either hovering the cursor over the image of the star, the cursor tip should show you the star ID & magnitude else load the Sinbad over lay and select the star of interest when the details will be shown below the image as normal.


A couple of tips at this point before we move onto creating your own FoV.

  1. If you select properties of the Schulman32 layer you can than select a roll angle and click apply and the FoV layer will be rotated. In the case above I rolled the FoV by 219 degrees to center the star at 1:30 to the position of the guide CCD.

  2. You can also move the overlay relative to the image to show what centering the guide star does to your image and how much you might lose. To do this click on the select button, then the Schulman32 layer and finally the very center of the overlay. If you have clicked in the correct place a green dot surrounded by 4 green squares should appear. You can then move the relative position of the overlay by clicking  and dragging. The instructions state you can rotate the overlay using the green squares as well but it does not work for me. To rotate the image you need to move the mouse pointer to one of the corners, it will then turn into a rotating arrow and you can click and drag the overlay to rotate it. If you move the cursor to one of the sides it will turn into a hand and you can move the overlay.

  3. Having moved and rotated the overlay to your satisfaction the position of the centre of the overlay and the roll angle can be read off from the properties page.

  4. You can change the colour, opacity and roll angle of the overlay from the properties page.

Creating your own FoV indicator(s)


The easiest method to create a FoV overlay is to use the 'Create your own' button on the Aladin Instrument Field of View page shown above. This should online footprint editor shown below.


The first thing to do is to edit the footprint name to something memorable preferably containing the telescope and CCD details. NB don't include double quotes and possibly other special characters in the name You will get a Java error when you try to load it into Aladin.


Instrument Footprint editor - initial page


You now need to enter you first shape. I have a Canon 40D on a 203mm (8") f10 telescope which has a FoV of 2304 x 1532 arc seconds. To enter the FoV click 'New Rectangle' and enter the width and height in the box. There is no need for an offset as your main CCD will probably be centered in the telescope view. The offset is useful for adding other CCDs such as the guide CCD shown in the image above. The roll angle is not really important for the main CCD as you can alter the displayed roll when you load the FoV.


Clicking apply will generate the FoV as shown below.


Footprint editor with FoV for 8" f10 with Canon 40D


You can now save the FoV or test it in Aladin first if you wish. The file will be saved with the extension .VOT, you may need to rename it and then move it into a folder where you keep your Aladin auxiliary files.


The other functions allow you to add extra FoVs for the guide chip and circles -useful for showing the swept area for the guide CCD as in the Schulman32 example above.


Having saved the .VOT file you can then use the 'Load it' button in the Instrument field of view to load it. All you need to do is then select it in the normal way and it will be shown in Aladin. The final result is below showing the FoV of a Canon 40D on an 8" f10 telescope. Here is a link to my FoV file for a DSLR with 60% sensor (compared to 35 mm film) on an 8" (203mm) f10 SCT


M31 with 8" f10 & Canon 40D Overlay


A tip don't do as I did above and include double quotes in the Footprint Name - this causes a Java exception when it tries to load and the load fails. I have tried editing the Instrument, Telescope, Description & Author fields so they make more sense in TopCat but Aladin fails to read the modified .VOT file correctly. It appears that editing the filename itself is not a problem.


Update - 2nd December 2019


Following an email from the designers of Aladin and the footprint editor they have confirmed that TopCat changes the structure of the file when it saves the VOT. As a result the best way to edit the file is to change the 'values' in the VOT with an ASCII text editor.


For Windows users Notepad will not work as it does not understand the Linux line endings and shows the file as one long stream making it very difficult to edit. The best option for Windows users is to use notepad++ which understands the file format and shows the data & structure elements in different colours similar to the Linux editor below. It also works with Linux line endings. (It does lots of other things as well visit their website to find out more and to download a copy of this freeware programme for windows.



For Linux (based on Linux Mint) the standard character based text editor 'Pluma' understands that the VOT is written in XML and colours the fields and variables so it is clear what can be edited.


The following image shows the footprint VOT opened for edit in Pluma. Which ever editor you use you should only edit the value field for TelescopeName, InstrumentName, InstrumentDescription and Origin. You need to edit the ID="MyFootprint" if you did not change this in the footprint editor as these need to be unique.


You should enter the same description in the origin field in all your FoV files - I suggest your name or initials. That way you can sort by this to get all your FoV files shown together in the 'Instrument fields of view' window if you have multiple FoV files loaded.


As above it is Ok to change the filename to allow you to load the correct footprint. The line endings do not seem to matter either Linux or DOS(Windows) formats work ok.


Footprint VOT file opened in Pluma Editor

Loading your FoV fields into Aladin at automatically at startup

If you are like me with a couple of telescopes and a few cameras from web-cams to DSLRs you will need end up with quite a lot of FoV files for different telescope / lens / camera combinations. When you have a few files it is easy to use the 'load it' button or else type 'load FoVfilename.vot' in the command line. However it would be a lot easier if all the FoV files were loaded on startup so they appear on the list with the professional telescopes. There is a way of loading the FoV files on startup using an Aladin Script file (these have the file extension .AJS), while this solves the immediate problem you have to create the script file and edit it every time you create a new FoV file. To get round this I have written a Windows Batch file that creats an Aladin script file 'on the fly' before Aladin Starts and then opens Aladin which executes the script file and loads all the FoV files.

The following is the batch file I use:

REM This file loads the FoV files named FoV*.vot in the Aladin Working folder E:\Aladin+Sinbad\FoV
REM by creating an Aladin Script file on the fly

REM Change to the Aladin Folder and delete the old Aladin ajs script file if it exists
CD \Aladin+Simbad
If exist FoV.ajs (delete FoV.ajs)

REM Change to the FoV folder - has to be a sub folder of Aladin Working folder
CD \Aladin+Simbad\FoV

REM This section constructs the aladin script file containing all the FoVTest*.vot files in the directory
REM The next line is used to create the file and provide the Aladin Script Identifier
ECHO #AJS > FoV.ajs

REM tell the user what is happening
Echo # This file is auto generated and will be deleted and recreated the next time Aladin is run >> FoV.ajs

REM Create first message for user
Echo info loading the instrument FoV files >> FoV.ajs

REM the carat quotes the backslash so it is echoed
for %%A in (FoV*.vot) do echo load %cd%^\%%A >> FoV.ajs

REM counter used to count number of FoV files loaded
set counter=0

REM Count the number of files
for %%B in (FoV*.vot) do (set /A counter+=1)

REM Tell the user how many user FoV files have been loaded
echo info %counter% user FoV files loaded >> FoV.ajs

REM Move the script file to the Aladin working directory
Move FoV.ajs E:\Aladin+Simbad\

REM Load Aladin with 64 Bit Java and 2 GByte Memory
REM updated for JRE8 20140329
CD C:\Program Files\Java\jre8\bin
java -Xmx2048m -jar C:\Java\Aladin.jar %1 FoV.ajs


  1. My Aladin working folder is E:\Aladin+Simbad - you need to change this to match your setup

  2. The Aladin footprint files are in E:\Aladin+Sinbad\Fov

  3. All my Aladin footprint files start FoV  and end in .vot

  4. Several on the lines starting 'ECHO' create fixed text in the Aladin Script fie

  5. The line 'for %%A in (FoV*.vot) do echo load %cd%^\%%A >> FoV.ajs' creates a line in the Aladin script file in the form 'Load Path \ FoVfilename.vot '

  6. The Aladin Script file FoV.ajs is created in the FoV folder and has to be moved to the Aladin working folder.

  7. The line starting java starts Aladin with 2GB of memory, the %1 allows a filename or command to be passed to Aladin to execute on start-up and FoV.ajs is the script file created by the batch file.

My automatically generated Aladin script file using a number of test field of views looks like this:

# This file is auto generated and will be deleted and recreated the next time Aladin is run
info loading the instrument FoV files
load E:\Aladin+Simbad\FoV\FoVTest1.vot
load E:\Aladin+Simbad\FoV\FoVTest2.vot
load E:\Aladin+Simbad\FoV\FoVTest3.vot
load E:\Aladin+Simbad\FoV\FoVTest4.vot
load E:\Aladin+Simbad\FoV\FoVTest5.vot
info 5 user FoV files loaded


  1. The lines starting with # are comments

  2. the comment #AJS tells Aladin it is a script - it should detect this but Aladin recommend you add this to the first line

  3. The info messages show up below the main Aladin window as it loads - you have to be quick to see the first one !

  4. The info message telling you how many files have loaded is not very reliable as it counts the number of FoV*.vot files in the folder rather than detecting they have actually loaded. However if all is working correctly it should be correct.

  5. When you open the Instrument field of view tab your FoVs may not appear together as the list appears to be sorted by default in Telescope order. You can click on the column headings to sort by other orders - possibly the most useful is the 'Author' field. If you used the same Author Name in the origin field all your FoV's will be grouped together.

  6. NB: The thing to be careful of as stated above is that the 'Footprint Name' otherwise known as the Resource ID has to be unique for each file otherwise subsequent ones loaded with the same name overwrite the earlier ones.


John Murrell



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