# Frame merging

The frame merging makes one or more resulting CCD frames by combining a set of source frames. This function is usable for example when the very faint objects are observed, thus demanding very long exposure durations to obtain desired accuracy. The exposure duration is limited in practice due to inaccuracy of the telescope driving. The merging can virtually extend the attainable exposure time, because the image shift can be corrected.

The implementation used in the current version of the C-Munipack project requires calibratted source frames and corresponding photometry files with information about frame offsets. These are produced by the matching process, no special option is required.

Corresponding pixels are summed and the result is divided by the total number of source frames. Pixels, which are not covered by all source frames or which contain bad pixels, are set to zero. The exposure duration is computed as sum of the durations of source frames. The observation time is computed as arithmetic mean of times of source frames. The Muniwin program allows automatized or manual splitting of the set of input files and making a single or a set of result frames.

## Before you start

Before you start the reduction of your own CCD frames, you may need to perform several pre-processing steps. Though it isn’t necessary, combining the several correction frames into so-called “master” ones is advisory, because it reduces the noise and makes the result more precise. The method of making master correction frames is described in separate chapters.

## Creating a new project

First of all, we will create a new project. To begin with processing of a variable star observation, we create a new project. To do so, open the Project menu and activate the New item. A new dialog appears.

The dialog for making a new project.

Fill in a name that will be assigned to the project. Because the name of the file that keeps track of the data related to the project, the project file, is derived from the name, some characters cannot appear in the project name, do not use: / ? % * : | “ < and >.

The field Location shows a path to the directory where a new project will be created. Edit the path to change the location, you can also click the Browse button to select a directory in a separate dialog.

Select a profile (3) to specify an intial set of configuration parameters into a new project. When you confirm the dialog, you should be in the main window again now. The table of input files shown there is empty.

The main application window with the table of input files, now empty.

See also

New project (dialog) and Main window

## Input files

Now, we are going to tell the program which files we are going to work on. These files are called the input files. Their list is displayed in the table in the main application window. When the application is closed, the list of files are saved to the disk and it is restored back when the program is launched again.

Supposing that the table now consists of files from your previous task, let’s get rid of them. Please, use Files ‣ Clear files to start a new task instead of just removing the files from the table. Besides the clearing the table of input files, this function resets all internal variables, too.

Now, we need to populate the table with the CCD frames we’re going to reduce. There are two methods how to achieve that - adding a individual files or adding all files from a folder. Which way is the best for you depends on organization of your observations on the disk. I’d suggest you to make a folder for each year, a folder for each night in it, the a subfolder for a name of object or another view field identification and finally a subfolder named upon the color filter (if you use more of them). In this case, the “Add frames from a folder” method is more convenient.

Click on Files ‣ Add frames from folder in the main menu. A new dialog appears. In the dialog, find a folder where the inputs files are stored in. Click on an entry in the Places pane to go to one of a preselected folders, double click in the middle pane enters the folder. The buttons in the upper part of the dialog shows your current position in the directory tree, you can use them to go to one of the parent folders. Enter the folder with the input files - you should see them in the middle pane. Then, click on the Add button to add files to the table of input files. The program shows the number of added files in the separate dialog. The Add frames from folder dialog is not closed automatically and allows a user to continue. Click on the OK button to close the dialog and return to the main window.

The “Add folder” dialog with the place selection box (left), the file selection box (middle) and the preview panel (right).

If you want to reduce only a subset of files from a folder, click on Files ‣ Add individual frames in the main menu. A new dialog appears, similar to the previous one. In the dialog, find a folder where the inputs files are stored in. Click on an entry in the Places pane to go to one of a preselected folders, double click in the middle pane enters the folder. The buttons in the upper part of the dialog shows your current position in the directory tree, you can use them to go to one of the parent folders. In the middle pane, select the files using the `Ctrl` modifier to include and exclude a single file and the `Shift` modifier to include a range of files. Then, click on the Add button to add selected files to the table of input files. The program shows the number of added files in the separate dialog. The Add individual frames dialog is not closed automatically and allows a user to continue. Click on the OK button to close the dialog and return to the main window.

The “Add files” dialog with the place selection box (left), the file selection box (middle) and the preview panel (right).

See also

Add frames from folder (dialog) and Add individual frames (dialog)

## Frame reduction

Reduction of CCD frames is a process that takes source CCD frames, performs their conversion and calibration, detects stars on each frame and mearures their intensity and finally finds correlation (match) between objects that were found in the data set. The process of reduction prepares the data that are necessary for making a light curve or a variable star.

The reduction consists of several steps - conversion, calibration, photometry and matching. They can be invoked step-by-step manually. The preferred way is to use the Express reduction dialog that allows to perform these steps in a batch. Using the menu, activate the Reduce ‣ Express reduction item. A new dialog appears. The dialog has several options aligned to the left, Each of them relates to an optional step in the reduction process.

The dialog for setting parameters of the reduction process

Fetch/convert files

Check the Fetch/convert files. In this step, the program makes copy of the source CCD frames. This is necessary, because the following calibration steps will modify them and we don’t want the program to change our precious source data.

Dark-frame correction

A raw CCD frame consists of several components. By the calibration process, we get rid of those which affect the result of the photometry. In some literature, the calibration is depicted as the peeling of an onion. There are three major components which a raw frame consists of - the current made by incident light, current made thermal drift of electrons (so-called dark current) and constant bias level. In standard calibration scheme, which we will demonstrate here, the dark-frame correction subtracts the dark current and the also the bias. Because of the nature of the dark current, it is necessary to use a correction frame of the same exposure duration as source files and it must be carried out on the same CCD temperature, too. Thus, the properly working temperature regulation on your CCD camera is vital.

Flat-frame correction

Then, we have to compensate the spatial non-uniformity of a detector and whole optical system. These non-uniformities are due to the fabrication process of a CCD chip and they are also natural properties of all real optical components, lenses in particular. The flat-frame correction uses a flat-frame to smooth them away. The flat-frame is a frame carried out while the telescope is pointed to uniformly luminous area. In practice, this condition is very difficult to achieve, the clear sky before dusk is usually used instead.

Photometry

The photometry is a process that detects stars on a CCD frame and measures their brightness. Unlike the previous steps, the result is saved to a special file, so-called the photometry file. There are a lot of parameters which affect the star detection and also the brightness computation. In this example, the default values work fine, but I would suggest you to become familiar with at least two of them - FWHM and Threshold - before you start a real work. Check the Photometry option.

FWHM

  The FWHM parameter specify the expected width of stars on a frame. The value is the Full Width at Half Maximum in pixels. The parameter controls the behavior of the low-pass digital filter, which is used in the star detection algorithm.

Threshold

  The Threshold parameter specify the lowest brightness of detected stars. Fainter objects are considered to be background artifacts and thus sorted out. The value is dimensionless coefficient.

Once you tune up the parameter for your environment, usually it is not necessary to adjust them for every task, unless the quality of your images varies considerably. In the first iteration, you can use the default values (FWHM = 3.0 and Threshold = 4.0) and do the photometry. Click on the Reduce ‣ Photometry item in the main menu and confirm the new dialog by the OK. Then, by double click on a frame in the main window open the preview window and check the results. If there are stars which have been detected as a close binary although it is not true, you should increase the FWHM value. If the stars you are interested in are not detected, try decrease the Threshold value. If it doesn’t help, decrease the FWHM. If there is a lot of background artifacts detected as a real stars, increase the Threshold. By several iterations, adjust the parameters, so all the stars you are interested in are detected and there are no false binaries.

Matching

The previous command treated all source files independently. As a result of this, a star #1 in one file is not necessarily the same as a star #1 in another file. The matching is a process which finds corresponding stars on source frames and assigns an unique identifier. Check the Matching option.

It is necessary to select one frame from the set of source frames that all other frames are matched to, this frame is called a reference frame. In my experience, the frame with the greatest number of stars works the best. Back to our example, let’s pick up the first one.

Invoking the reduction process

In previous steps, we have configured parameters of the reduction process and we are ready to start it. Click the OK button. During the execution a new window appears displaying the state of the process; all the information is also presented there. This window will be automatically closed after finishing the process. Wait for the process to finish.

The dialog displayed during time demanding operations.

After finishing, the icon in the file table changes; the information about the time of observation, the length of the exposition and the used filter is filled in. In case some of the frames could not be processed successfully, the entry is be marked with a special icon and in the Status column the error message is indicated.

The main application window after the recution.

See also

Express reduction (dialog)

## Making combined frames

Open Make menu, select the Merge frames item. A new dialog window appears. If you are going to make a set of resulting frames, select the Split frames and process each group separately option. Set the parameters for automatic splitting frames to groups; one resulting frame will be made for each particular group. Fill the maximum number of frames in a group to the Merge every … frames field. Set the maximum time span between the first and the last frame in a group. Enter the minimum number of frames (Min. frames). Confirm the dialog by the OK button. The standard dialog for saving files is opened. Find a directory, where the output files shall be stored to and enter the file name prefix. For example, if you enter the crcas prefix, the resulting files will be named `crcas00001.fts`, `crcas00002.fts`, etc. Close the dialog by the Save to start the merging.

It is possible to split the files manually. In the Input files table, select the frames, which shall be combined to a single resulting frame. In the Make menu, select the Merge frames item. The same dialog window as in previous case is opened. In the upper part of the dialog, select the Process selected files only option and in the lower part, select the Merge all source frames to a single output frame item. Confirm the dialog by the OK button. The standard dialog for saving files is opened. Find a directory, where the output file shall be stored to and enter the its name. No numbers are added to the name in this case. Close the dialog by the Save to start the merging. Repeat the action subsequently for all input files.

If the frames have been aligned (transformed) already, you can run this step after the frame conversion. You have to check the option “Do not transform the source frames”. If this option is checked, the software does not attempt to do any alignment of the frames, so if they do not match, the objects will be deformed or copied on the combined frames.

See also

Merge frames (dialog)

## Processing merged frames

Merged frames can be processed in the same way as common CCD frames with an exception that no calibration (bias, dark and flat correction) is applied to them. Follow instructions in the Light curve section.