One of the most delicate and mysterious in a way that approaches for the beginner to astronomy is the construction of digital MASTER FLAT FIELD. In a previous post we have seen that there are basically two types of flat field: the Flat-Dome, made from an artificial light source is near the telescope, and the Sky-Flat , obtained by illumination the sky background during the hours of dawn or dusk. The latter type of flat field has the advantage of not requiring special equipment for recovery but requires some dexterity and speed of recovery of the frames during the short period of time, and some more attention in the processing of frames to create properly MASTER SKY FLAT.
Let us see what are the main steps to follow.
Before the shooting sequence of the flat frame, always remember:
1) use the exact same optical configuration used for the resumption of "raw" image: the same fire and the same filter.
2) Check the temperature of the CCD camera is stabilized and / or the fixed point set in the case of thermoelectric temperature control of the CCD.
3) to avoid gradients in the flat frame point the telescope at the zenith and hold off the chase
The first step is then to experiment to determine the correct exposure time based on the brightness of the sky: the goal is to shoot as many frames as possible in the flat short time before the complete darkness of the sky (if you are working during the sunset) and possibly a signal large enough: if we use a 16-bit CCD camera can impose, for example, an average value of the flat frame at least more than half of the range of linearity the CCD camera. In our case, having determined the linearity limit at 60000 ADU, just the flat frame are between 30000 and 60000 ADU. If you think about it is not a simple thing: we need evidence and a bit 'of experience to make the most of the time available (usually only a few tens of minutes). Of course, As the sky darkens and the average signal decreases may be necessary to increase the exposure time, in which case you must take note and remember to take a series of dark frames for the exposure time is longer.
We finally have a series of sky-flat frame like the one shown below:
Here's what a typical sky-flat frame: there are strips of some stars and even the passage of a weak satellite. Note in the status bar below the average backgroud (B = 31,456) in ADU. As you can see there is no need to keep on tracking telescope on the contrary, keeping it off we make sure that there will be no overlap between a star and the other frame and we can effectively employ the combined median frames.
Note: still not uncommon to read that the number of flat frames or dark frames should be composed of an odd number of images to be combined mode median. This is really a thing of the past when the first astronomical image processing software algorithms implementavono limited and incomplete. The complete algorithm as implemented in modern software Astroart , but also considers the case of an even number of frames. In particular, it establishes that, to calculate the median of n data:
1. n you sort data in ascending or descending order;
2. if the number of data is odd the median is the middle value, whichever is occupying the position (n + 1) / 2.
3. if the number of data n is even, the median is estimated using two values occupying the position (n / 2) and (n / 2 + 1) (generally choose their arithmetic mean). The combination
median is essential for the success of SKY MASTER FLAT: it is the only combination that eliminates the streaks of stars or satellites or other steps "defects" at random and different on each sky -flat frame. To function, however, requires a fundamental requirement: All frames must have approximately the same average value, is said to be "normalized .
If we had taken the flat frame under a constant light source (as in the case of Flat-Dome) this requirement would undoubtedly have been reached without any further steps and we could obtain the master flat field immediately using the appropriate options in the dialog Pretreatment of Astroart. Unfortunately, in our case, the brightness of the background sky changes rapidly during the minutes of twilight and the consequence is that all the sky-flat frame has a mean value different from each other: it is therefore necessary to normalize the same average value of reference.
To proceed with the normalization we have to perform two additional steps.
First subtract each sky-flat frame MASTER DARK FRAME corresponding individually and save the sky-flat frame as correct.
The first page "File" box Pretreatment of Astroart take a look something like this:
In our example we have 10 sky-flat frame (taken with the red filter) and 10 dark frames . Note that in order to save individual sky-flat frame, these should be included in the "Images" rather than in that of the "Flat Field". While the second page "Options" should be configured as follows:
you click on "OK" we get in our 10 new workbook file with the same file name of the sky-flat frame a suffix " _P" file to indicate that they are "processed" means the program will automatically calculate the MASTER DARK FRAME, it will be removed from the individual sky-flat frame, and saved the resulting files on your hard disk by affixing the suffix indicated. We can now normalize these 10 rows processed. We need to create a macro and apply it (as shown in the windows below) to the group of files with the suffix "_P".
Use the online help to create and apply macros Astroart. 
The "Normalize background" as used in macro takes one argument, a frame of reference from which obtain the normalization constant: just before running the macro, open one of the sky-flat frame processed Astroart desktop and select it when prompted during the launch of the macro. Always following the setting of our example, we have now in 10 new workbook file named "NormFlatRxxx" where xxx is a sequential number. They are now our standard flat-sky that can be included in the "Flat Fields" box Pretreatment of Astroart (always remembering to select the "median" of the flat field on the "Options" in the same window):
The end result: the MASTER SKY-FLAT perfettamente corretto senza alcun residuo di stelle:
