<sect2 id="ekos-align">
    <title>Align</title>
    <indexterm>
        <primary>Tools</primary>
        <secondary>Ekos</secondary>
        <tertiary>Align</tertiary>
    </indexterm>
    <sect3 id="ekos-align-introduction">
        <title>Introduction</title>
        <screenshot>
            <screeninfo>
                Ekos Align Module
            </screeninfo>
            <mediaobject>
                <imageobject>
                    <imagedata fileref="ekos_astrometry.png" format="PNG"/>
                </imageobject>
                <textobject>
                    <phrase>Ekos Align Module</phrase>
                </textobject>
            </mediaobject>
        </screenshot>
        <para>
            Ekos Alignment Module enables highly accurate GOTOs to within sub-arcseconds accuracy and can measure and correct polar alignment errors. This is possible thanks to the astrometry.net solver. Ekos begins by capturing an image of a star field, feeding that image to <ulink url="https://astrometry.net">astrometry.net</ulink> solver, and getting the central coordinates (RA, DEC) of the image. The solver essentially performs a pattern recognition against a catalog of millions of stars. Once the coordinates are determined, the true pointing of the telescope is known.
        </para>
        <para>
            Often, there is a discrepancy between where the telescope thinks it is looking at and where it is truly pointing. The magnitude of this discrepancy can range from a few arcminutes to a couple of degrees. Ekos can then correct the discrepancy by either syncing to the new coordinates, or by slewing the mount to the desired target originally requested.
        </para>
        <para>
            Furthermore, Ekos provides a Polar Alignment Assistant Tool to correct polar alignment errors. It takes three images, slewing between the images, and calculates the offset between the mount axis and polar axis. It feeds back to the user the altitude and azimuth adjustments needed to align these axes. These images are typically taken near the celestial pole (Close to Polaris for Northern Hemisphere) but can work well taken from anywhere, usually starting near the meridian and slewing either East or West.
        </para>
        <para>
            At a minimum, you need a CCD/Webcam and a telescope that supports Slew &amp; Sync commands. Most popular commercial telescope nowadays support such commands.
        </para>
        <para>
            For the Ekos Alignment Module to work, you have an option of either utilizing the <emphasis>online</emphasis> astrometry.net solver, <emphasis>offline</emphasis>, or <emphasis>remote</emphasis> solver:
        </para>
        <itemizedlist>
            <listitem>
                <para>
                    <guilabel>Online Solver</guilabel>: The online solver requires no configuration, and depending on your Internet bandwidth, it might take a while to upload and solve the image.
                </para>
            </listitem>
            <listitem>
                <para>
                    <guilabel>Offline Solver</guilabel>: The offline solver can be faster and requires no Internet connection. In order to use the offline solver, you must install astrometry.net in addition to the necessary index files.
                </para>
            </listitem>
            <listitem>
                <para>
                    <guilabel>Remote Solver</guilabel>: The remote solver is an offline solver the resides on a different machine (for example, you can use Astrometry solver on StellarMate). Captured images are solved on the remote machine.
                </para>
            </listitem>
        </itemizedlist>
    </sect3>

    <sect3 id="ekos-align-get-astrometry">
        <title>Get astrometry.net</title>

        <para>
            If you are planning to use <emphasis>Offline</emphasis> astrometry then you need to download astrometry.net application.
        </para>
        <note>
            <para>
                Astrometry.net is already shipped with StellarMate so there is no need to install it. Index files from 16 arcminutes and above (4206 to 4019) are included with StellarMate. For any additional index files, you need to install them as necessary. To use Astrometry in StellarMate from a remote Ekos on &Linux;/&Windows;/&MacOS;, make sure to select <guimenuitem>Remote</guimenuitem> option in Ekos Alignment Module. Furthermore, make sure that the <guimenuitem>Astrometry</guimenuitem> driver is selected in your equipment profile.
            </para>
        </note>
        <screenshot>
            <screeninfo>
                Ekos Remote Astrometry
            </screeninfo>
            <mediaobject>
                <imageobject>
                    <imagedata fileref="ekos_remote_astrometry.png" format="PNG"/>
                </imageobject>
                <textobject>
                    <phrase>Ekos Remote Astrometry</phrase>
                </textobject>
            </mediaobject>
        </screenshot>
        <variablelist>
            <varlistentry>
                <term>&Windows;</term>
                <listitem>
                    <para>
                        To use astrometry.net under &Windows;, you need to download and install the <ulink url="https://adgsoftware.com/ansvr/">ANSVR Local Astrometry.net solver</ulink>. The ANSVR mimics the astrometry.net online server on your local computer; thus the internet not required for any astrometry queries.
                    </para>
                    <para>
                        After installing the ANSVR server and downloading the appropriate index files for your setup, make sure ANSVR server is up and running and then go to Ekos Alignment options where you can simply change the <guilabel>API URL</guilabel> to use the ANSVR server as illustrated below:
                    </para>
                    <screenshot>
                        <screeninfo>
                            ANSVR Parameters
                        </screeninfo>
                        <mediaobject>
                            <imageobject>
                                <imagedata fileref="astrometry_windows_ansvr.png" format="PNG"/>
                            </imageobject>
                            <textobject>
                                <phrase>ANSVR Parameters</phrase>
                            </textobject>
                        </mediaobject>
                    </screenshot>
                    <para>
                        In Ekos Align module, you must set the solver type to <guimenuitem>Online</guimenuitem> so that it uses the local ANSVR server for all astrometry queries. Then you can use the align module as you would normally do.
                    </para>
                    <para>
                        Remember as indicated above that StellarMate <emphasis>already</emphasis> includes astrometry.net. Therefore, if you would like to use StellarMate remotely to solve your images, simply change solver type to <guimenuitem>Remote</guimenuitem> and ensure that your equipment profile includes <emphasis role="bold">Astrometry</emphasis> driver which can be selected under the <guilabel>Auxiliary</guilabel> dropdown. This is applicable to all operating systems and not just &Windows;.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term>&MacOS;</term>
                <listitem>
                    <para>
                        Astrometry.net is already included with &kstars; for &MacOS;, so no need to install it.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term>&Linux;</term>
                <listitem>
                    <para>
                        Astrometry.net is already included with &kstars; bleeding version. But if astrometry is not installed, then you can install it by running the following command under &ubuntu;:
                    </para>
                    <para>
                        <command>sudo apt-get install astrometry.net</command>
                    </para>
                </listitem>
            </varlistentry>
        </variablelist>
    </sect3>

    <sect3 id="ekos-align-download-index-files">
        <title>Download Index Files</title>
        <para>
            For offline (and remote) solvers, index files are necessary for the solver to work. The complete collection of index files is huge (over 30 GB), but you only need to download what is necessary for your equipment setup. Index files are sorted by the Field-Of-View (FOV) range they cover. There are two methods to fetch the necessary index files: The new download support in Align module, and the old manual way.
        </para>
        <sect4 id="ekos-align-automatic-download">
            <title>Automatic Download</title>
            <screenshot>
                <screeninfo>
                    Astrometry.net Indexes Download
                </screeninfo>
                <mediaobject>
                    <imageobject>
                        <imagedata fileref="astrometry_indexes_settings.png" format="PNG"/>
                    </imageobject>
                    <textobject>
                        <phrase>Astrometry.net Indexes Download</phrase>
                    </textobject>
                </mediaobject>
            </screenshot>
            <para>
                Automatic download is only available for Ekos users on &Linux; &amp; &MacOS;. For &Windows; users, please download ANSVR solver.
            </para>
            <para>
                To access the download page, click the <guibutton>Options</guibutton> button in the <link linkend="ekos-align">Align module</link> and then select <guilabel>Astrometry Index Files</guilabel> tab. The page displays the current FOV of your current setup and below it a list of available and installed index files. Three icons are used to designate the importance of index files given your current setup as follows:
            </para>
            <itemizedlist>
                <listitem>
                    <para>
                        <inlinemediaobject><imageobject><imagedata fileref="security-high.png" format="PNG"/></imageobject></inlinemediaobject> <guilabel>Required</guilabel>
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <inlinemediaobject><imageobject><imagedata fileref="security-medium.png" format="PNG"/></imageobject></inlinemediaobject> <guilabel>Recommended</guilabel>
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <inlinemediaobject><imageobject><imagedata fileref="security-low.png" format="PNG"/></imageobject></inlinemediaobject> <guilabel>Optional</guilabel>
                    </para>
                </listitem>
            </itemizedlist>
            <para>
                You must download all the required files, and if you have plenty of hard drive space left, you can also download the recommended indexes. If an index file is installed, the checkmark shall be checked, otherwise check it to download the relevant index file. Please only download one file at a time, especially for larger files. You might be prompted to enter the administrator password (default in StellarMate is <userinput>smate</userinput>) to install the files. Once you installed all the required files, you can begin using the offline astrometry.net solver immediately.
            </para>
        </sect4>
        <sect4 id="ekos-align-manual-download">
                <title>Manual Download</title>
                <para>
                    You need to <ulink url="http://data.astrometry.net">download</ulink> and install the necessary index files suitable for your telescope+CCD field of view (FOV). You need to install index files covering 100% to 10% of your FOV. For example, if your FOV is 60 arcminutes, you need to install index files covering skymarks from 6 arcminutes (10%) to 60 arcminutes (100%). There are many online tools to calculate FOVs, such as <ulink url="http://starizona.com/acb/ccd/calc_pixel.aspx">Starizona Field of View Calculator</ulink>.
                </para>
                <table frame="all">
                    <title>Index Files</title>
                    <tgroup cols='3' align='left' colsep='1' rowsep='1'>
                        <thead>
                            <row>
                                <entry>Index Filename</entry>
                                <entry>FOV (arcminutes)</entry>
                                <entry>Debian Package</entry>
                            </row>
                        </thead>
                        <tbody>
                            <row>
                                <entry>index-4219.fits</entry>
                                <entry>1400 - 2000</entry>
                                <entry morerows="11" valign="middle"><ulink url="http://data.astrometry.net/debian/astrometry-data-4208-4219_0.45_all.deb">astrometry-data-4208-4219</ulink></entry>
                            </row>
                            <row>
                                <entry>index-4218.fits</entry>
                                <entry>1000 - 1400</entry>
                            </row>
                            <row>
                                <entry>index-4217.fits</entry>
                                <entry>680 - 1000</entry>
                            </row>
                            <row>
                                <entry>index-4216.fits</entry>
                                <entry>480 - 680</entry>
                            </row>
                            <row>
                                <entry>index-4215.fits</entry>
                                <entry>340 - 480</entry>
                            </row>
                            <row>
                                <entry>index-4214.fits</entry>
                                <entry>240 - 340</entry>
                            </row>
                            <row>
                                <entry>index-4213.fits</entry>
                                <entry>170 - 240</entry>
                            </row>
                            <row>
                                <entry>index-4212.fits</entry>
                                <entry>120 - 170</entry>
                            </row>
                            <row>
                                <entry>index-4211.fits</entry>
                                <entry>85 - 120</entry>
                            </row>
                            <row>
                                <entry>index-4210.fits</entry>
                                <entry>60 - 85</entry>
                            </row>
                            <row>
                                <entry>index-4209.fits</entry>
                                <entry>42 - 60</entry>
                            </row>
                            <row>
                                <entry>index-4208.fits</entry>
                                <entry>30 - 42</entry>
                            </row>
                            <row>
                                <entry>index-4207-*.fits</entry>
                                <entry>22 - 30</entry>
                                <entry><ulink url="http://data.astrometry.net/debian/astrometry-data-4207_0.45_all.deb">astrometry-data-4207</ulink></entry>
                            </row>
                            <row>
                                <entry>index-4206-*.fits</entry>
                                <entry>16 - 22</entry>
                                <entry><ulink url="http://data.astrometry.net/debian/astrometry-data-4206_0.45_all.deb">astrometry-data-4206</ulink></entry>
                            </row>
                            <row>
                                <entry>index-4205-*.fits</entry>
                                <entry>11 - 16</entry>
                                <entry><ulink url="http://data.astrometry.net/debian/astrometry-data-4205_0.45_all.deb">astrometry-data-4205</ulink></entry>
                            </row>
                            <row>
                                <entry>index-4204-*.fits</entry>
                                <entry>8 - 11</entry>
                                <entry><ulink url="http://data.astrometry.net/debian/astrometry-data-4204_0.45_all.deb">astrometry-data-4204</ulink></entry>
                            </row>
                            <row>
                                <entry>index-4203-*.fits</entry>
                                <entry>5.6 - 8.0</entry>
                                <entry><ulink url="http://data.astrometry.net/debian/astrometry-data-4203_0.45_all.deb">astrometry-data-4203</ulink></entry>
                            </row>
                            <row>
                                <entry>index-4202-*.fits</entry>
                                <entry>4.0 - 5.6</entry>
                                <entry><ulink url="http://data.astrometry.net/debian/astrometry-data-4202_0.45_all.deb">astrometry-data-4202</ulink></entry>
                            </row>
                            <row>
                                <entry>index-4201-*.fits</entry>
                                <entry>2.8 - 4.0</entry>
                                <entry><ulink url="http://data.astrometry.net/debian/astrometry-data-4201-1_0.45_all.deb">astrometry-data-4201-1</ulink> <ulink url="http://data.astrometry.net/debian/astrometry-data-4201-2_0.45_all.deb">astrometry-data-4201-2</ulink> <ulink url="http://data.astrometry.net/debian/astrometry-data-4201-3_0.45_all.deb">astrometry-data-4201-3</ulink> <ulink url="http://data.astrometry.net/debian/astrometry-data-4201-4_0.45_all.deb">astrometry-data-4201-4</ulink></entry>
                            </row>
                            <row>
                                <entry>index-4200-*.fits</entry>
                                <entry>2.0 - 2.8</entry>
                                <entry><ulink url="http://data.astrometry.net/debian/astrometry-data-4200-1_0.45_all.deb">astrometry-data-4200-1</ulink> <ulink url="http://data.astrometry.net/debian/astrometry-data-4200-2_0.45_all.deb">astrometry-data-4200-2</ulink> <ulink url="http://data.astrometry.net/debian/astrometry-data-4200-3_0.45_all.deb">astrometry-data-4200-3</ulink> <ulink url="http://data.astrometry.net/debian/astrometry-data-4200-4_0.45_all.deb">astrometry-data-4200-4</ulink></entry>
                            </row>
                        </tbody>
                    </tgroup>
                </table>
                <para>
                    The Debian packages are suitable for any Debian-based distribution (Ubuntu, Mint, &etc;). If you downloaded the Debian Packages above for your FOV range, you can install them from your favorite package manager, or via the following command:
                </para>
                <para>
                    <command>sudo dpkg -i astrometry-data-*.deb</command>
                </para>
                <para>
                    On the other hand, if you downloaded the FITS index files directly, copy them to <filename role="directory">/usr/share/astrometry</filename> directory.
                </para>
                <note>
                    <para>
                        It is recommended to use a download manager as such <ulink url="https://addons.mozilla.org/en-US/firefox/addon/downthemall/">DownThemAll!</ulink> for &firefox; to download the Debian packages as browsers' built-in download manager may have problems with download large packages.
                    </para>
                </note>
            </sect4>
        </sect3>

        <sect3 id="ekos-align-how-to-use">
            <title>How to Use?</title>
            <para>
                Ekos Align Module offers multiple functions to aid you in achieving accurate GOTOs. Start with your mount in home position with the telescope tube looking directly at the celestial pole. For users in Northern Hemisphere, point the telescope as close as possible to Polaris. It is not necessary to perform 2 or 3 star alignments, but it can be useful for some mount types. Make sure your camera is focused and stars are resolved.
            </para>
            <itemizedlist>
                <listitem>
                    <para>
                        <guilabel>Capture &amp; Solve</guilabel>: Capture an image and determine what region in the sky the telescope is exactly looking at. The astrometry results include the equatorial coordinates (RA &amp; DEC) of the center of the captured image in addition to pixel scale and field rotation. Depending on the Solver Action settings, the results can be used to Sync the mount or Sync and then Slew to the target location. For example, suppose you slewed the mount to Vega then used <emphasis>Capture &amp; Solve</emphasis>. If the actual telescope location is different from Vega, it will be first synced to the solved coordinate and then Ekos shall command the mount to slew to Vega. After slew is complete, the Alignment module will repeat Capture &amp; Solve process again until the error between reported and actual position falls below the accuracy thresholds (30 arcseconds by default).
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <guilabel>Load &amp; Slew</guilabel>: Load a FITS or JPEG file, solve it, and then slew to it.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <guilabel>Polar Alignment Assistant</guilabel>: A simple tool to aid in polar alignment of German Equatorial Mounts.
                    </para>
                </listitem>
            </itemizedlist>
            <warning>
                <para>
                    <emphasis role="bold">Never</emphasis> solve an image at or near the celestial pole (unless Ekos Polar Alignment Assistant Tool is used). Slew at least 20 degrees away from the celestial pole before solving the first image. Solving very close to the poles will make your mount pointing worse so avoid it.
                </para>
            </warning>
        </sect3>

        <sect3 id="ekos-align-alignment-settings">
            <title>Alignment Settings</title>
            <screenshot>
                <screeninfo>
                    Astrometry.net Settings
                </screeninfo>
                <mediaobject>
                    <imageobject>
                        <imagedata fileref="astrometry_settings.png" format="PNG"/>
                    </imageobject>
                    <textobject>
                        <phrase>Astrometry.net Settings</phrase>
                    </textobject>
                </mediaobject>
            </screenshot>
            <para>
                Before you begin the alignment process, select the desired CCD &amp; Telescope. You can explore astrometry.net options that are passed to the astrometry.net solver each time an image is captured:
            </para>
            <itemizedlist>
                <listitem>
                    <para>
                        <guilabel>CCD</guilabel>: Select CCD to capture from.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <guilabel>Exposure</guilabel>: Exposure duration in seconds.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <guilabel>Accuracy</guilabel>: Acceptable difference between reported telescope coordinate and actually solved coordinate.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <guilabel>Bin X</guilabel>: Set horizontal binning of the CCD.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <guilabel>Bin Y</guilabel>: Set vertical binning of the CCD.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <guilabel>Scope</guilabel>: Set the active telescope in case you have different Primary and Guide scopes. FOV is re-calculated when selecting a different telescope.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <guilabel>Options</guilabel>: Options that are passed to the astrometry.net solver. Click the edit button <inlinemediaobject><imageobject><imagedata fileref="document-edit.png" format="PNG"/></imageobject></inlinemediaobject> to explore the options in detail.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <guilabel>Solver</guilabel>: Select solver type (Online, Offline, Remote). The remote solver is only available when connecting to a remote device.
                    </para>
                </listitem>
            </itemizedlist>
            <para>
                By default, the solver will search all over the sky to determine the coordinates of the captured image. This can <emphasis>take a lot of time</emphasis>; therefore, in order to speed up the solver, you can restrict it to only search within a specified area in the sky designated by the <guilabel>RA</guilabel>, <guilabel>DEC</guilabel>, and <guilabel>Radius</guilabel> options above.
            </para>
        </sect3>

        <sect3 id="ekos-align-astrometry-net-settings">
            <title>Astrometry.net Options</title>
            <para>
                Options for offline and online solvers.
            </para>
            <screenshot>
                <screeninfo>
                    Astrometry.net Options
                </screeninfo>
                <mediaobject>
                    <imageobject>
                        <imagedata fileref="astrometry_settings.png" format="PNG"/>
                    </imageobject>
                    <textobject>
                        <phrase>Astrometry.net Options</phrase>
                    </textobject>
                </mediaobject>
            </screenshot>
            <para>
                Most of the options are sufficient by default. If you have astrometry.net installed in a non-standard location, you can change the paths as necessary.
            </para>
            <itemizedlist>
                <listitem>
                    <para>
                        <guilabel>WCS</guilabel>: World-Coordinate-System is a system for embedding equatorial coordinate information within the image. Therefore, when you view the image, you can hover it and view the coordinate for each pixel. You can also click anywhere in the image and command to the telescope to slew there. It is highly recommended to keep this option on.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <guilabel>Verbose</guilabel>: If the solver repeatedly fails to solve, check this option to enable <emphasis>verbose</emphasis> output of the solver to help you identify any problems.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <guilabel>Overlay</guilabel>: Overlay captured images unto the sky map of &kstars;.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <guilabel>Upload JPG</guilabel>: When using online astrometry.net, upload all images are JPEGs to save bandwidth as FITS images can be large.
                    </para>
                </listitem>
            </itemizedlist>
        </sect3>

        <sect3 id="ekos-align-solver-options">
            <title>Solver Options</title>
            <para>
                Ekos selects and updates the optimal options by default to accelerate the performance of the solver. You may opt to change the options that are passed to the solver in case the default options are not sufficient.
            </para>
            <screenshot>
                <screeninfo>
                    Solver Settings
                </screeninfo>
                <mediaobject>
                    <imageobject>
                        <imagedata fileref="solver_settings.png" format="PNG"/>
                    </imageobject>
                    <textobject>
                        <phrase>Solver Settings</phrase>
                    </textobject>
                </mediaobject>
            </screenshot>
        </sect3>

        <sect3 id="ekos-align-capture-and-solve">
            <title>Capture &amp; Solve</title>
            <para>
                Using Ekos Alignment Module, aligning your mount using the controller's 1, 2, or 3 star alignment is <emphasis>not</emphasis> strictly necessary, though for some mounts it is recommended to perform a rough 1 or 2 star alignment before using Ekos alignment module. If you are using EQMod, you can start using Ekos alignment module right away. A typical workflow for GOTO alignment involves the following steps:
            </para>
            <orderedlist>
                <listitem>
                    <para>
                        Set your mount to its home position (usually the NCP for equatorial mounts).
                    </para>
                </listitem>
                <listitem>
                    <para>
                        Select <guilabel>Slew to Target</guilabel> in the <guilabel>Solver Action</guilabel>.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        Slew to a nearby bright star.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        After slew is complete, click <guibutton>Capture &amp; Solve</guibutton>.
                    </para>
                </listitem>
            </orderedlist>
            <para>
                If the solver is successful, Ekos will sync and then slew to the star. The results are displayed in the <guilabel>Solution Results</guilabel> tab along with a bullseye diagram that shows the offset the reported telescope coordinates (&ie; where the telescope <emphasis>thinks</emphasis> it is looking at) vs. its actual position in the sky as determined by the solver.
            </para>
            <para>
                Each time the solver is executed and returns successful results, Ekos can run on the following actions:
            </para>
            <itemizedlist>
                <listitem>
                    <para>
                        <guilabel>Sync</guilabel>: Syncs the telescope coordinates to the solution coordinates.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <guilabel>Slew to Target</guilabel>: Syncs the telescope coordinates to the solution coordinates and then slew to the target.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <guilabel>Nothing</guilabel>: Just solve the image and display the solution coordinates.
                    </para>
                </listitem>
            </itemizedlist>
        </sect3>

        <sect3 id="ekos-align-polaralignment">
            <title>Polar Alignment</title>
                <para>
                    When setting up a German Equatorial Mount (GEM) for imaging, a critical aspect of capturing long-exposure images is to ensure proper polar alignment. A GEM mount has two axis: Right Ascension (RA) axis and Declination (DE) axis. Ideally, the RA axis should be aligned with the celestial sphere polar axis. A mount's job is to track the star's motion around the sky, from the moment they rise at the eastern horizon, all the way up across the median, and westward until they set.
                </para>

                <mediaobject>
                    <videoobject>
                        <videodata contentdepth="595" contentwidth="800" fileref="https://www.stellarmate.com/images/videos/polar_align.webm"/>
                    </videoobject>
                    <caption>
                        <para>
                            <phrase>Polar Alignment Assistant</phrase>
                        </para>
                    </caption>
                </mediaobject>

                <para>
                    In long exposure imaging, a camera is attached to the telescope where the image sensor captures incoming photons from a particular area in the sky. The incident photons have to strike the same photo-site over and over again if we are to gather a clear and crisp image. Of course, actual photons do not behave in this way: optics, atmosphere, seeing quality all scatter and refract photons in one way or another. Furthermore, photons do not arrive uniformly but follow a Poisson distribution. For point-like sources like stars, a point spread function describes how photons are spatially distributed across the pixels. Nevertheless, the <emphasis>overall</emphasis> idea we want to keep the source photons hitting the same pixels. Otherwise, we might end up with an image plagued with various trail artifacts.
                </para>
                <screenshot>
                    <screeninfo>
                        Polar Alignment
                    </screeninfo>
                    <mediaobject>
                        <imageobject>
                            <imagedata fileref="alignment_polaris.png" format="PNG"/>
                        </imageobject>
                        <textobject>
                            <phrase>Polar Alignment</phrase>
                        </textobject>
                    </mediaobject>
                </screenshot>
                <para>
                    Since mounts are not perfect, they cannot perfectly keep track of object as it transits across the sky. This can stem from many factors, one of which is the misalignment of the mount's Right Ascension axis with respect to the celestial pole axis. Polar alignment removes one of the biggest sources of tracking errors in the mount, but other sources of error still play a factor. If properly aligned, some mounts can track an object for a few minutes with the only deviation of 1-2 arcsec RMS.
                </para>
                <para>
                    However, unless you have a top of the line mount, then you'd probably want to use an autoguider to keep the same star locked in the same position over time. Despite all of this, if the axis of the mount is not properly aligned with the celestial pole, then even a mechanically-perfect mount would lose tracking with time. Tracking errors are proportional to the magnitude of the misalignment. It is therefore very important for long exposure imaging to get the mount polar aligned to reduce any residual errors as it spans across the sky.
                </para>
                <para>
                    Before starting the process, point the mount as close as possible to the celestial pole with the counterweights down. If you are living in the Northern Hemisphere, point it as close as possible to Polaris. If Polaris is not visible (&eg; blocked by trees or  buildings) you may point elsewhere, preferably near the Meridian. Make sure there is at least 30-60 degrees of sky viewable in an arc East or West of the Meridian from the position you choose. Select the direction of free sky, the number of degrees for each of two slews, the mount slew speed, and whether the mount will be slewing automatically (recommended) or manually.
                </para>
                <para>
                    The tool works by capturing and solving three images. After capturing each, the mount rotates by the fixed amount you entered and another image is captured and solved. If you chose manual, you will need to slew the mount by roughly the angle chosen.
                </para>
                <screenshot>
                    <screeninfo>
                        Polar Alignment Assistant
                    </screeninfo>
                    <mediaobject>
                        <imageobject>
                            <imagedata fileref="polar_assistant_main2.png" format="PNG"/>
                        </imageobject>
                        <textobject>
                            <phrase>Polar Alignment Assistant</phrase>
                        </textobject>
                    </mediaobject>
                </screenshot>

            <sect4 id="ekos-align-platesolvecorrection">
                <title>Plate Solve Correction Scheme</title>
                
                <para>
                  The images below show the workflow when the <emphasis>Plate Solve</emphasis> correction technique is used. The image below shows a display after the 3 measurement images are captured and solved. It shows an error of almost 18' in altitude and that the mount's axis needs to be moved up. Similarly it shows an azimuth error of almost 15' and that the axis needs to be moved to the right (as viewed from behind the telescope). 
                </para>
                <screenshot>
                    <screeninfo>
                        Polar Alignment Assistant
                    </screeninfo>
                    <mediaobject>
                        <imageobject>
                            <imagedata fileref="polar_assistant_ps1.png" format="PNG"/>
                        </imageobject>
                        <textobject>
                            <phrase>Polar Alignment Assistant, Plate Solve Workflow 1</phrase>
                        </textobject>
                    </mediaobject>
                </screenshot>
                <para>
                  If your error is low enough (&eg; less than an arc-minute) then you don't need to make any adjustments. Simply press stop and you're done.
                </para>
                <para>
                  If you will be making corrections to your mount's axis, you should select the adjustment approach (we're using Plate Solve in this example), and how often the system should recapture images to re-measure the polar alignment error. The refresh interval should be frequent, but it doesn't make sense to make it faster that your CPU can capture and plate-solve the images. We're using 2s in this example. Then press the <guibutton>Refresh</guibutton> button to begin the correction process.
                </para>
                <para>
                  The system will capture images, and re-estimate the polar alignment error after each image. You can try to reduce the error by adjusting the Altitude and Azimuth correction knobs on your mount. The image below shows the screen after the altitude error has been almost zeroed. See the difference between the  <emphasis>Measured Error</emphasis> row, which shows the originally measured error after the original 3 captures, and the  <emphasis>Updated Error</emphasis> row which shows the current error estimate.
                </para>
                <screenshot>
                    <screeninfo>
                        Polar Alignment Assistant
                    </screeninfo>
                    <mediaobject>
                        <imageobject>
                            <imagedata fileref="polar_assistant_ps2.png" format="PNG"/>
                        </imageobject>
                        <textobject>
                            <phrase>Polar Alignment Assistant, Plate Solve Workflow 2</phrase>
                        </textobject>
                    </mediaobject>
                </screenshot>
                <para>
                  Below the user has also adjust Azimuth to reduce the error further. Now the error is very low and the process is done. The user should press the stop button.
                </para>
                <screenshot>
                    <screeninfo>
                        Polar Alignment Assistant
                    </screeninfo>
                    <mediaobject>
                        <imageobject>
                            <imagedata fileref="polar_assistant_ps3.png" format="PNG"/>
                        </imageobject>
                        <textobject>
                            <phrase>Polar Alignment Assistant, Plate Solve Workflow 3</phrase>
                        </textobject>
                    </mediaobject>
                </screenshot>
                </sect4>
                <sect4 id="ekos-align-move-star-correction">
                <title>Move Star Correction Scheme</title>

                <para>
                  We also have an alternative schemes for correcting polar alignment. Two variations are <emphasis>Move Star &amp; Calc Error</emphasis> and <emphasis>Move Star</emphasis>. When you select this scheme, the system place a yellow/green/violet triangle on the screen. The triangle can be moved by clicking near a star, and the yellow/violet corner is moved to that star. In this scheme the user corrects polar alignment by first adjusting the mount's azimuth knob so that the selected star moves along the yellow side of the triangle. Once the star is near the next vertex, the azimuth knob should be adjusted so that the star moves along the green side of the triangle. Once the star is moved to the green/violet vertex, the mount is polar aligned, and the user can click <emphasis>stop</emphasis>.
                </para>
                  <para>
                    The difference between <emphasis>Move Star &amp; Calc Error</emphasis> and  <emphasis>Move Star</emphasis> is that in the former, the system attempts to track the star the user has selected, and places a circle around that star. In that scheme it also attempts to update the <emphasis>Updated Err</emphasis> row. If the star tracking isn't reliable, simply ignore it or use the <emphasis> Move Star </emphasis>scheme and move the star by-eye until it's close to the final target. An example of using this technique is shown in this video: 
                    <ulink url="https://www.youtube.com/watch?v=iOp7hrxw0oU">https://www.youtube.com/watch?v=iOp7hrxw0oU</ulink>
                </para>
            </sect4>
        </sect3>
</sect2>
