In previous arguments we have identified a number of digital elements that are normally used for astrophotography: the LIGHT FRAME and FLAT FIELD , with their dark frame, bias frame and . These four types of digital elements, they are actually arrays of numbers which, when combined, will produce the final image is correct (which we call SCIENCE FRAME). The term "correct" does not mean of course only from the aesthetic point of view but also from the photometric point of view (which is what most interests us).
photometrically corrected image is necessarily "better" because "pure", but the opposite is true: on the Net you see thousands of stunning astronomical images, but the vast majority of "impure", ie an end in themselves, generate ( often damaged) after hours of violence by various image-editing program. We amateur astronomers have the dangerous presumption to be able to overcome our shortcomings and technical equipment with a heavy and massive intervention of "image processing". To do what? Photo galleries of boring to be published with pride on our personal Web page. After all this is the ultimate goal of so many amateurs in the best case, however, after taking dozens of times throughout the Messier catalog along with more exotic and spectacular NGC, surely "get tired" and here, the amateur astronomer becomes a virtuoso "amateur astronomer" to search for the "photon lost."
It's time to do as the preprocessing classico di una serie di immagini riprese su una singola banda fotometrica: la banda V (visual) . Per le nostre riprese infatti utilizzeremo un filtro centrato all'incirca sulla banda V del sistema fotometrico di Johnson -Cousins: l'argomento filtri fotometrici è abbastanza complesso ma importante e merita una riflessione più dettagliata che faremo in futuro. Per il momento ci basti sapere che il nostro filtro V seleziona una banda di lunghezze d'onda centrate sui 522 nm e con un'ampiezza a metà altezza ( FWHM ) di 90 nm. In pratica stiamo selezionando una banda dello spettro elettromagnetico centrata sul colore verde della luce visibile.
L'oggetto astronomico al centro of our times is a famous planetary nebula: M27.
When point after the object and accurate focus can take a series of light frame as the one shown in the figure below.
The number of LIGHT FRAME is to shoot at their own discretion for the time we need to know that the greater the number of LIGHT FRAME and less "grainy" will be the final image (ie the better the signal-to- Final noise). The argument "signal to noise ratio is very important and will be addressed soon. In our case, were taken 5 LIGHT FRAME with an exposure time of 240 seconds each. Also for the exposure time there is no fixed rule adaptable to all situations: the only sure thing is that even in this case, the longer the exposure, the greater the signal to noise ratio of single LIGHT FRAME. Rather, the upper limit of exposure time may be dictated by the presence or absence of stars easily saturable just as you can see in the image above: The two brightest stars, one to the left and bottom right of M27 have an early blooming, clear signal that you have exceeded the limit of linearity CCD sensor (for aesthetic purists will see that you can correct this type of defect). Immediately after the
LIGHT FRAME will be the turn of the dark frames and bias frame: these are not strictly necessary unless, as in our case, the CCD camera has no problems with temperature stability during shooting of the different frames. The presence of bias frame in the window of the preprocessing of Astroart automatically instruct the program to make an optimization of several dark frames before creating the master dark frame to subtract the LIGHT FRAME.
Finally, it should return the flat field, taking care to maintain the optical train (same filter and same fire V) used during the resumption of LIGHT FRAME. In our case we used the technique of Master Sky Flat then we have ready for use FLAT MASTER FRAME.
Once you have all of our frames, we can finally fill in the file File window Pretreatment of Astroart: selezionamo the file folder and drag the same with a simple drag & drop in the corresponding boxes as indicated in the figure below:
At this point we have to tell the program how we want to do pre-processing: This stage is set in the Options folder window Pretreatment .
First we show that we want to "mediate" the 5 5 BIAS and DARK FRAME FRAME to create the MASTER DARK FRAME. Successivamente vogliamo allineare i 5 LIGHT FRAME (scegliendo l'opzione " Auto allineamento " con il metodo " Tutte le stelle ") per poi sommarli insieme ed ottenere l'immagine finale. La cartella Opzioni apparirà come nella figura qui sotto:
Ora possiamo lanciare il pretrattamento facendo click sul pulsante OK: in pochi secondi avremo sul nostro desktop la finestra con l'immagine finale aperta (SCIENCE FRAME) e 3 immagini ridotte ad icona in basso a sinistra: BIAS00.FIT, DARK00.FIT e FLAT00.FIT che sono rispettivamente i nostri MASTER BIAS FRAME, MASTER DARK FRAME e MASTER FLAT FIELD utilizzati nel processo: se desideriamo possiamo salvarli per utilizzarli in altri processi (il MASTER FLAT FIELD non ha comunque subito alcuna processo in quanto è lo stesso file salvato che avevamo trascinato nella finestra "Flat Fields" di prima ).
Tutto questo può sembrare un po' complicato descritto solo a parole: qui sotto tutti i passaggi sono mostrati in un breve video.
Abbiamo così creato il nostro primo SCIENCE FRAME dell'oggetto M27 in banda V: si tratta della somma di 5 esposizioni di 240 secondi, dunque l'integrazione totale equivale a 5 x 240 = 1200 secondi = 20 minuti. Se nella cartella Opzioni della finestra Pretrattamento avessimo scelto una option of combining different images, or " Media" or "Median " or " Sigma," the total integration time of the final image would always be equivalent to that of a single exposure, or 240 seconds .
