Quick photometry ################ Photometry on hand-picked objects of a single image *************************************************** .. |quickphoto-button| image:: ../_images/icons/photometry_dark.svg The **quick photometry** button |quickphoto-button| is a button located in the toolbar and used to perform a photometry of the stars, this is generally the simpliest way to proceed. .. tip:: If the star is in the middle of several stars and the tool fails to point to the right star, an alternative solution is to draw a selection around the star and then right-click and click on :guilabel:`PSF`. It may also be interesting to know that :kbd:`Ctrl`-middle click (or :kbd:`Cmd`-middle click on MacOS) draws a selection of a recommended size for PSF/photometry (based on the configured outer radius). .. tip:: When photometry is performed on the RGB layer, the results are actually calculated on the green layer. To obtain photometry on the red or blue layers, you need to work on the corresponding channels. .. admonition:: Siril command line :class: sirilcommand .. include:: ../commands/psf_use.rst .. include:: ../commands/psf.rst Click on this button to change the image selection mode, then click on a star. The photometry and the `PSF (Point Spread Function) `_ of the star are computed, giving plenty of details. Two models are used for the calculation of the PSF, which can be selected by the user in the :ref:`Dynamic-PSF ` window (:kbd:`Ctrl` + :kbd:`F6`). .. figure:: ../_images/photometry/photometry_result.png :alt: photometry result :class: with-shadow :width: 100% Photometry results window. The result of the photometry and the associated PSF are displayed in the form: .. code-block:: text PSF fit Result (Gaussian, monochrome channel): Centroid Coordinates: x0=5258.25px 09h25m34s J2000 y0=2179.72px +69°49'31" J2000 Full Width Half Maximum: FWHMx=7.13" FWHMy=6.79" r=0.95 Angle: 82.87deg Background Value: B=0.000874 Maximal Intensity: A=0.628204 Magnitude (relative): m=-2.3948±0.0014 Signal-to-noise ratio: SNR=28.9dB (Good) RMSE: RMSE=1.890e-03 #. The fit was done with the **Gaussian** fitting function so no additional parameters are needed. However, if Moffat was used, the following output will be shown: .. code-block:: text PSF fit Result (Moffat, beta=2.9, monochrome channel): #. **Centroid Coordinates** gives the coordinates of the centroid in pixels. However, like in the example above, if astrometry was set on the image, Siril gives coordinates in the `World Coordinate Systems `_ (RA and Dec). #. **Full Width Half Maximum** (`FWHM `_) is returned in arcsec if the image scale is known (obtained from its header or from the GUI :menuselection:`Image information --> Information`) and in pixels if not. The roundness *r* is also computed as the ratio of :math:`\frac{\text{FWHMy}}{\text{FWHMx}}`. #. **Angle** is the rotation angle of the *X* axis with respect to the centroid coordinates. It varies in the range :math:`[-90°,+90°]`. #. **Background Value** is the local background in the :math:`[0,1]` range for 32-bits images and :math:`[0,65535]` for 16-bits images. This is a fitted value, not the background computed in the aperture photometry annulus. #. The **maximum Intensity** value is also a fitted value and represents the amplitude. It is the maximum value of the fitted function, located at the centroid coordinates. #. The **magnitude**, given with its uncertainty, is the result of photometry. However, if for some reasons the calculation cannot be done (saturated pixels or black pixels), an uncertainty of **9.999** is given. In this case, the photometry is flagged as invalid but a magnitude value is still given, although it should be used with caution. #. An estimator of the **signal-to-noise ratio** is shown in the results. Its value is calculated from the following formula and given in `dB `_: .. math:: :label: SNR \text{SNR} = 10 \log_{10}\left(\frac{I}{N}\right) where *I* is the net intensity, proportional to the observed flux *F* and *N* the total of uncertainties as expressed in :eq:`sigma_mag`. For easier understanding, it is associated with 6 levels of quality: #. Excellent (SNR > 40dB) #. Good (SNR > 25dB) #. Satisfactory (SNR > 15dB) #. Low (SNR > 10dB) #. Bad (SNR > 0dB) #. N/A This last notation is displayed only if the computation failed, for one reason or another. #. Finally, **RMSE** gives an estimator of the fit quality. The lower the value, the better the result. .. _PSF_more_details: When the image is plate-solved, the button :guilabel:`More details` at the bottom of the window links to a page on the SIMBAD website with information about the selected star. However, it is possible that the page does not give any additional information if the star is not in the SIMBAD database. .. figure:: ../_images/photometry/simbad.png :alt: photometry result :class: with-shadow :width: 100% More details about the analyzed star. Click on the picture to enlarge. Quick photometry on sequences ***************************** Quick photometry can also be performed on a sequence. This is generally intended to obtain a light curve as explained :ref:`here `. To proceed, you must **load a sequence**, make a selection around a star, then **right click** on the image. .. tip:: Ideally, the sequence must be registered without interpolation so as not to alter the raw data. For example, use the **global registration** with the option **Save transformation in seq file only**. .. note:: Make sure the inner and outer radii for the background annulus are adapted to the star and sequence being analyzed. Some images may have much larger FWHM than the reference image, because of sky conditions or bad tracking. They can be changed in the :ref:`preferences ` or with the ``setphot`` command. At the end of the process, Siril automatically opens the plot tab showing computed curves. It is possible to click on several stars to reproduce the calculation, however the first star keeps the particular status of *variable*, and the others serve as *references*. This is important in the calculation of the light curve. .. figure:: ../_images/photometry/photometry_example.png :alt: Photometry on sequence :class: with-shadow :width: 100% In this example, 3 stars have been analyzed. The first one is used as variable. The others are references. Computing true magnitudes ************************* The calculated magnitude is only meaningful if it is compared to others in the linear image. Indeed, the value given does not correspond at all to the true visible magnitude of the star, it is uncalibrated, also called relative magnitude. Siril provides tools that can be used to calculate an approximate apparent magnitude. This requires knowing the magnitude of another star visible on the image that will act as reference. It is currently possible to use only a single star as reference, hence the *approximate*. For a greater precision, use a star of similar color and magnitude as the star(s) you want to measure should be chosen, and its provided magnitude should be in adequation with the filter used to capture the image. Catalogs contain magnitudes computed using a `photometric filters `_, which is generally not what amateur use to make nice pictures, this adds another approximation. * Do a quick photometry on a known star, the given relative magnitude is ``-2.428``. It is possible to find out the actual visible magnitude by clicking on the :guilabel:`More details` button as explained above. Let's say the value found is ``11.68`` (make sure you use a value corresponding to the spectral band of the image). * Once done, keep the star selected, then enter the following command in Siril .. code-block:: text setmag 11.68 That will output something like .. code-block:: text 10:50:49: Relative magnitude: -2.428, True reduced magnitude: 11.680, Offset: 14.108 .. admonition:: Siril command line :class: sirilcommand .. include:: ../commands/setmag_use.rst .. include:: ../commands/setmag.rst * Now, all calculated magnitudes must have values close to their true visual magnitude. However, this is especially true for stars whose magnitude is of the same order of magnitude as the star taken as reference. .. figure:: ../_images/photometry/photometry_setmag.png :alt: photometry setmag :class: with-shadow :width: 100% Photometry results window with true magnitude set. * To unset the computed offset, just type .. code-block:: text unsetmag .. admonition:: Siril command line :class: sirilcommand .. include:: ../commands/unsetmag_use.rst .. include:: ../commands/unsetmag.rst .. tip:: The same commands exist for the sequences. They are ``seqsetmag`` and ``sequnsetmag``. It is used in the same way when a sequence is loaded. .. admonition:: Siril command line :class: sirilcommand .. include:: ../commands/seqsetmag_use.rst .. include:: ../commands/seqsetmag.rst .. admonition:: Siril command line :class: sirilcommand .. include:: ../commands/sequnsetmag_use.rst .. include:: ../commands/sequnsetmag.rst