Method and astrophotographic apparatus for acquiring images of targets in sky area
Disclosed are a method and an astrophotographic apparatus for acquiring images of targets in a sky area. The method includes: step 1 , an image acquisition device is driven through a controlled rotatable component to point to the position near a target in a sky area, and images of the sky area are acquired through the image acquisition device; step 2 , the images of the sky area are analyzed to obtain the right ascension coordinates and the declination coordinates of the center point of images for synchronizing the coordinates to the controlled rotatable component; step 3 , the image acquisition device is driven by the controlled rotatable component to point to a target position of the target right ascension coordinates and the target declination coordinates corresponding to the target celestial body to acquire the images; and step 4 , an image processing is performed on the images to obtain the processed images.
1 . A method for acquiring an image of a target in a sky area, comprising:
step 1 , driving an image acquisition device through a controlled rotatable component to point to a position near the target in the sky area, and acquiring an image of the sky area through the image acquisition device according to a target right ascension coordinate and a target declination coordinate corresponding to a target celestial body received;
step 2 , analyzing the image of the sky area to obtain a right ascension coordinate and a declination coordinate of a center point of the image of the sky area for synchronizing the right ascension coordinate and declination coordinate to the controlled rotatable component, and determining that the image acquisition device is directed toward a position at which the right ascension coordinate and the declination coordinate are located;
step 3 , driving the image acquisition device by the controlled rotatable component to point to a target position of the target right ascension coordinate and the target declination coordinate corresponding to the target celestial body for the image acquisition device to acquire the image of the target in the sky area according to the target right ascension coordinate and the target declination coordinate corresponding to the target celestial body, as well as the right ascension coordinate and the declination coordinate currently pointed by the image acquisition device; and
step 4 , performing an image processing on the image of the target in the sky area to obtain a processed image of the target in the sky area;
wherein, the controlled rotatable component is an equatorial mount, and step 1 comprises following steps:
A, performing a polar axis alignment to the equatorial mount of an astrophotographic apparatus to enable a right ascension axis of the equatorial mount to be parallel to the axis of rotation of the Earth;
step A comprises the following steps:
S1, taking a first image through the image acquisition device, analyzing the first image to obtain a first coordinate file, and establishing a mapping relationship between a pixel coordinate of the first image and an equatorial coordinate through the first coordinate file;
S2, acquiring a second image by rotating the image acquisition device to a certain angle around a polar axis, analyzing the second image to obtain a second coordinate file, and establishing a mapping relationship between a pixel coordinate of the second image and the equatorial coordinate through the second coordinate file;
S3, calculating a pixel coordinate of an image rotation center according to the first coordinate file and the second coordinate file;
S4, converting the pixel coordinate of the image rotation center into an equatorial coordinate of the image rotation center;
S5, converting the equatorial coordinate of the image rotation center into a horizon coordinate of the image rotation center; and
S6, calculating a difference between the horizontal coordinate of the image rotation center and a horizontal coordinate of a celestial pole, and determining whether the difference is lower than a preset first threshold; if the difference is lower than the preset first threshold, terminating the polar axis alignment; if the difference is greater than the preset first threshold, adjusting a pointing direction of the equatorial mount according to the difference, and acquiring an n th image through the image acquisition device, calculating the difference between the horizontal coordinate of the image rotation center and the horizontal coordinate of the celestial pole, and determining whether the difference is lower than the preset first threshold.
2 . The method according to claim 1 , wherein after step A, the step 1 further comprises the following steps:
B, photographing the sky area and acquiring the image of the sky area through the image acquisition device installed on the equatorial mount after the equatorial mount rotates for a certain angle;
wherein the step 2 comprises the following steps:
C, matching stars in the image of the sky area with stars in a reference star catalog through a matching algorithm to obtain the right ascension coordinate and the declination coordinate of the center point of the image of the sky area for synchronizing the right ascension coordinate and the declination coordinate to the equatorial mount, and determining that an orientation of the image acquisition device in a celestial coordinate system are the right ascension coordinate and the declination coordinate.
3 . The method according to claim 2 , wherein the step C comprises the following steps:
step C1, randomly selecting two stars in the image of the sky area, and then constructing a circular area by taking the connecting line between the two selected stars as a diameter of the circular area, selecting another two stars in the circular area to form a first four-star combination, and calculating a first geometric hash code corresponding to each first four-star combination;
step C2, dividing the reference star catalog into grids, screening a plurality of stars with the highest brightness from each grid, forming a second four-star combination by grouping four screened stars, and calculating a second geometric hash code corresponding to each second four-star combination; and
step C3, successfully matching the first four-star combination and the second four-star combination when a difference between the first geometric hash code and the second geometric hash code falls within a preset second threshold, determining that the image are within a corresponding sky area when the number of successfully matched second four-star combinations and first four-star combinations in a grid exceeds a preset third threshold, and obtaining the right ascension coordinate and the declination coordinate corresponding to the center point of the image of the sky area through a corresponding relationship between the successfully matched first four-star combination and second four-star combination.
4 . The method according to claim 1 , wherein the step 4 comprises the following steps:
E1, performing an image rectification processing on the image of the target in the sky area through images taken by the image acquisition device in a dark field, a bias field and a flat field, separately, to obtain a rectified image of the target in the sky area;
E2, performing a noise reduction processing on the rectified image of the target in the sky area to obtain a noise-reduced image of the target in the sky area;
E3, performing a multi-frame stacking processing on the noise-reduced image of the target in the sky area to obtain the image of the target in the sky area with enhanced celestial targets but a weakened background; and
E4, performing a nonlinear stretching processing on the image of the target in the sky area after the multi-frame stacking processing to obtain the image of the target in the sky area in which pixels are nonlinearly stretched.
5 . The method according to claim 4 , wherein the step E2 comprises the following steps:
E21, determining a pixel point greater than a preset multiple of a sum of four pixel points as upper, lower, left, and right as a thermal noise point; and
E22, filling an average value of the four upper, lower, left, and right pixel points as a pixel value of the thermal noise point.
6 . The method according to claim 4 , wherein the step E3 comprises the following steps:
E31, determining star points among the image of the target in the sky area;
E32, determining a triangle with the star points as vertices;
E33, responding to the number of similar triangles in two images being greater than a preset threshold to obtain a mapping relationship of the star points in the two images, wherein a mapping function is F(x, y)=a1+a2*x+a3*y+a4*x*y, whereby the parameters a1, a2, a3, a4 are obtained through a matrix method by inputting the pixel values of 8 star points matched as similar triangles into the mapping function;
E34, inputting all pixel values of the image of the target in the sky area to the mapping function to obtain an aligned image of the target in the sky area; and
E35, performing stacking of multiple aligned images of the target in the sky area and then dividing by the number of stacked images to obtain the image of the target in the sky area after the multi-frame stacking processing.
7 . A method for processing an image of a sky area, comprising the following steps:
performing an image rectification processing on the image of the sky area, rectifying the image of the sky area and images taken by an image acquisition device in a dark field, a bias field and a flat field, separately, to obtain a rectified image of the sky area;
performing a noise reduction processing on the rectified image of the sky area to obtain a noise-reduced image of the sky area;
performing a multi-frame stacking processing on the noise-reduced image of the sky area to obtain an image of the sky area with enhanced celestial targets but a weakened background; and
performing a nonlinear stretching processing on the image of the sky area after the multi-frame stacking processing to obtain an image of the sky area in which pixels are nonlinearly stretched.
8 . An astrophotographic apparatus, comprising:
a main body;
a storage medium; and
a processor, wherein the processor is connected to the main body, wherein the storage medium stores program instructions, and the program instructions, when being executed by the processor, cause the processor to execute the method according to claim 1 .