CN114860196A - Telescope main light path guide star device and calculation method of guide star offset - Google Patents

Abstract

The invention relates to a telescope main optical path guide star device, which comprises a telescope main focus detector and a guide star detector, wherein the telescope main focus detector is positioned on a telescope focal plane, and the telescope main focus detector comprises: the guide star detector is arranged near the telescope prime focus detector, is in a focal plane with the prime focus detector, and shares the same optical path with the telescope prime focus detector. The invention realizes the guiding of the main light path, so as to eliminate the position and angle errors of the guiding detector and the main detector and improve the guiding and tracking precision of the telescope. The invention also discloses an algorithm of the vector offset, the vector camera sends the formed image to a program for processing in a certain exposure time, the program finds a star body in the image data by using a method in the field of image processing, and obtains XY (row and column) offset according to the historical record of the star body position, and then the offset is returned to a moving mechanism of the telescope to make the telescope perform correction movement, thereby achieving the purpose of compensation and forming a more reliable closed-loop control system.

Description

A kind of telescope main optical path guide star device and calculation method of guide star offset

technical field

The invention belongs to the technical field of telescopes, and in particular relates to a design of a guide star detector on the main optical path of a telescope, and according to a plurality of bright stars in a guide star image, an algorithm such as image processing is used to solve them to obtain a bright star. The position offset is used to correct the movement of the telescope's right ascension, declination or the horizontal and pitch axis of the telescope's main optical path guide device and the calculation method of the guide star offset.

Background technique

The guide star of the astronomical telescope is a function of observation. The guide star is mainly used in the following occasions: (1) When tracking the star, it is used for long-term tracking continuous imaging shooting or long-exposure imaging shooting. Due to the Earth's autobiography, if the telescope does not move, the star will move from east to west at 15ʺ/s in the celestial coordinate system along an axis parallel to the Earth's equator, and the right ascension axis of an equatorial telescope is parallel to the Earth's axis of rotation. In order to offset the Earth's rotation axis Due to the influence of rotation, its right ascension axis should move from east to west at a speed of 15ʺ/s, so that the image of the target object is fixed in the field of view. In the same way, in order to offset the rotation of the earth and ensure that the observed target is fixed within the field of view, another type of horizon telescope also needs to measure the ground of the horizon type telescope when exposing the target for a long time or tracking it for a long time. Correction adjustments are made to pan and pitch to avoid the target smearing in the image.

(2) For fast-moving celestial objects such as solar system asteroids, comets and low-orbit satellites, due to the fast moving speed of the target, the phenomenon of trailing is easy to occur even in the case of short-time exposure, and the guidance of the telescope must be used. The star device calculates corrections for telescope pointing and directs the controls of the telescope (equatorial or horizontal) mount to counteract the relative motion between the telescope and fast-moving objects.

The guide camera is an imaging device installed on the optical axis of the telescope. At present, most of the guide star imaging detectors are located outside the main focus, and some are installed near the lens barrel of the telescope and are called offset guide stars. That is, the guide star detector is not on the main optical axis of the telescope, as shown in Fig. The guide camera system is designed to be parallel to the main optical path outside the main lens barrel, which is equivalent to an independent small telescope system. The advantage of this method is that it is easy to install and debug the single guide telescope system. However, since the optical path of the guide star and the optical path of the telescope are separated from each other, position and angle errors will inevitably occur on the optical path, which will affect the guide and tracking of the telescope. precision.

SUMMARY OF THE INVENTION

The present invention provides a main optical path guide star device of a telescope and a method for calculating the guide star offset, which effectively improves the above problems.

To achieve the above object, the present invention adopts the following technical solutions:

A telescope main optical path guide star device, comprising a telescope main focus detector and a guide star detector. The telescope main focus detector is located on the focal plane of the telescope, wherein: the guide star detector is installed near the telescope main focus detector, and is connected to the main focus detector. The detector is in one focal plane and shares the same optical path with the telescope's main focus detector.

Further, the guide star detector is composed of several guide star cameras, and the telescope main focus detector includes several main focus scientific imaging detectors and wavefront imaging detectors. The guide star camera is installed around the main focus scientific imaging detector, and the wavefront detection The detector is also set up around the main focus scientific imaging detector.

Further, the number of guiding cameras is four, and the guiding cameras are installed around the main focus scientific imaging detector at equal intervals.

Further, the main focus scientific imaging detector is a spliced CCD with a large target surface and high resolution, which is used for precise scientific imaging of astronomical scientific targets.

For the calculation method of the guide star offset, the guide star device of the main optical path of the telescope is used, and the following steps are performed:

Step 1. When the telescope is working, the guide star detector continues to obtain image data from the optical path of the telescope.

Step 2: Select a predetermined number of brightest stars as guide stars in the first frame of image data, and determine the coordinates, brightness, full width at half maximum value FWHM and time information of the guide stars,

Step 3: Extract the predetermined number of brightest stars in the next frame of images as the updated guide stars, and determine the coordinates, brightness, FWHM and time information of the updated guide stars,

Step 4: Pair the updated guide star with the guide star of the previous frame, calculate the offset on the guide star coordinates, and return the offset to the motion mechanism of the telescope to make correction motion;

Step 5. Repeat steps 3 and 4 until the guide process ends.

In the image data obtained by the guide star detector, the star used as a guide star is a bright star. When determining the guide star coordinates, the center of the guide star is taken as the guide star coordinate. Before extracting the guide star coordinates, the point spread function of the image is detected. Whether the PSF and FWHM are within the preset range, if they are within the preset range, it is judged that the image focusing is appropriate, and the guide coordinates are extracted.

In step 4, when the updated guide star is paired with the guide star of the previous frame, if the offset of the coordinates of a certain updated guide star exceeds the offset threshold, it is judged that a new bright star is added, and the The image of the previous frame is invalid, and the image of the current frame is taken as the first frame, and step 2 is performed.

Select the three brightest stars as guide stars. Correspondingly, in step 4, determine the offsets of the three bright stars in the current frame and the three bright stars in the previous frame in the X and Y directions, and record them as star 0_x, star 0_y , star 1_x, star 1_y, star 2_x and star 2_y, a total of six quantities, if there are offsets that exceed the offset threshold in the six quantities, it is determined that the newly captured image contains one or more than For the first three brighter stars in the last time, the image of the previous frame is invalid, and the image of the current frame is taken as the first frame, and step 2 is performed.

The beneficial effects of the present invention are:

1. In the present invention, the guide star is directly designed near the main focus detector of the telescope, which is the same optical path as the main optical path, so as to realize the guide star of the main optical path, so as to eliminate the position and angle errors of the guide star detector and the main detector, and improve the performance of the telescope. guide and tracking accuracy.

2. The invention also discloses the algorithm program of the guide star offset. The guide star camera performs target imaging according to its own optical design (including target surface, focal length, field of view, etc.) The resulting image is sent to the program for processing. The program finds the star in the image data by using the method in the field of image processing, and obtains the XY (row and column in the image) offset according to the historical record of the position of the star, and then this offset The amount is returned to the motion mechanism of the telescope to guide it to make corrections, so as to achieve the purpose of compensation and form a more reliable closed-loop control system.

3. The present invention also solves two important problems in the algorithm program of the guide star offset: (1) The problem of focusing the image, in the process of extracting the center coordinates of the bright star, the accuracy of the center extraction due to the quality of the image It has a great influence. Before extracting the center coordinates, the present invention judges the quality of the image and the quality of focusing, and uses the FWHM (full width at half maximum) value of the PSF (point spread function) as the basis for judging whether the focusing of the image is appropriate. , to ensure the accuracy of the guide star coordinates, (2) After processing an image to extract the brightness of three bright stars and their respective coordinates, a new bright star is captured when processing the next image, and this The brightness of the bright stars is in the top three in this image, which is a very special but existent situation. The present invention can determine whether there is a new brightest star by adding an offset threshold. The image is invalid, does not participate in the calculation, and starts a new calculation with the current image (including the newly added bright star) as the first frame. This is a relatively "safe" way of working, which guarantees the correctness of image matching.

4. On the basis of the astronomical image processing algorithm, the present invention uses the magnitude and brightness as the standard to sort and identify the celestial objects in the guide star, and by extracting the center coordinates of the image spot, FWHM (full width at half maximum) and brightness information, matching. and calculate the position offset of the guide target in multiple images. This algorithm can satisfy the guide star to correct the pointing of the telescope in real time, quickly and accurately, and the final offset result can be quickly returned to the telescope control system for processing and correction.

Description of drawings

FIG. 1 is a schematic diagram of a reflective main telescope and a refracting star guide mirror as a star guide device in the prior art.

FIG. 2 is an optical path diagram of the main optical path guide star mode of the present invention (taking the Cassegrain system as an example);

Figure 3 is a schematic diagram of the focal plane distribution of the telescope's main focus detector;

Figure 4 is a schematic diagram of a bright star in a group of guide star images;

Figure 5 is the FWHM (the last item of the numerical sequence) and the contour curve under the condition of unsatisfactory focusing;

Figure 6 is the FWHM (the last item of the numerical sequence) and the contour curve under the ideal case of focusing;

Fig. 7 is the schematic diagram of bright star matching error;

FIG. 8 is a flow chart of a program for calculating the offset of the guide star image.

The reference signs are: telescope main focus detector 1 , wavefront imaging detector 2 , guide star detector 3 , guide star camera 31 , and main focus scientific imaging detector 4 .

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings.

As shown in Figures 2-3, a telescope main optical path star guide device of the present invention includes a telescope main focus detector 1 and a guide star detector 3. The telescope main focus detector 1 is located on the focal plane of the telescope, wherein: the guide star The detector 3 is installed near the main focus detector 1 of the telescope, is on the same focal plane with the main focus detector, and shares the same optical path with the telescope main focus detector 1 .

The guiding star detector 3 is composed of several guiding star cameras 31 , the telescope main focus detector 1 includes several main focus scientific imaging detectors 4 and wavefront imaging detectors 2 , and the guiding star cameras 31 are installed around the main focus scientific imaging detector 4 , the wavefront detector 2 is also arranged around the main focus scientific imaging detector 4 .

The number of guiding cameras 31 is four, and the guiding cameras 31 are installed around the main focus scientific imaging probe 4 at equal intervals.

The main focus scientific imaging detector 4 is a spliced CCD with a large target surface and high resolution, which is used for precise scientific imaging of astronomical scientific targets. The wavefront imaging detector (2) (designed with four) is a kind of A technique for imaging on the wavefront by active optical adjustment of the primary mirror. The purpose of active optics is for a telescope system to keep its primary mirror in optimal imaging condition against environmental forces such as wind, sag, thermal expansion, and deformation of the telescope axis. The wavefront detector can measure the wavefront error of the pupil plane of the imaging system in real time, and then convert these measurement data into control signals of the active optical system, and control the optical characteristics of the imaging system in real time. The purpose of the guide star detector is to correct and guide the movement axis of the telescope in real time when the telescope is in constant motion or when the target is fast-tracked or long-exposure shooting.

The invention is practically applied in the 2.5m wide field telescope (WFST) of the Purple Mountain Observatory of the Chinese Academy of Sciences. The WFST telescope is a horizontal large field telescope, and its focal plane detector consists of 9 main focus scientific imaging detectors, 8 wavefront imaging detectors and 4 guide star detectors (the guide star detector is shown in Figure 3). Dotted box), the primary mirror of the telescope is 2.5 meters in diameter, the effective focal length of the system is 6.2 meters, and the focal ratio is f/2.48. The main focus detector adopts CCD290-99 detector, and the single-piece imaging target surface is 92.2mmx92.4mm. The guide star detector adopts CCD47-20 detector, and the single-piece imaging target surface is 13.3mmx13.3mm. Combined with the focal length of the system, it is concluded that the field of view of the main focus scientific camera is about 3˚x3˚. The field is 7.4ʹx7.4ʹ, and the spatial pixel resolution is 0.43ʺ/pixel. Different from general telescopes, the guide star of the WFST telescope involved in the present invention is directly located near the main focus detector, and is designed to transfer four CCD frames around it. The design of multiple guide star detectors increases the probability that the guide star can catch bright stars, and the second is to comprehensively calculate the data of the four guide stars to make the data for calculating the offset of the guide stars more accurate. .

In addition to the guiding optical device of the main optical path, the present invention also relates to a specific algorithm of the guiding star offset.

The image of the guide star is generally a group of continuous multi-frame images taken when the telescope is tracking. The image processed in this experiment is a group of exposure images of 5 consecutive frames. The image name is:

HT0001316-2.fits,HT0001317-2.fits,HT0001318-2.fits,HT0001319-2.fits,HT0001320-2.fits,

The exposure time for each image is 30 seconds, and the time interval between images is also approximately 30 seconds.

The general idea of the guide offset extraction method is to use the Sextractor tool to extract the bright source of a fits image through programming, and arrange the images according to the bright source after extraction. In order to reduce the time consumed by the image algorithm, in the program Only the brightest three stars in the images are arranged to determine the brightest three stars in these five images as star0, star1, star2 (the brightness decreases in order), as shown in Figure 4. After determining the brightest three stars, you can determine that star0 in multiple pictures is the same star, and the same is true for star1 and star2, and then extract the center coordinates of star0, star1 and star2 in each picture according to the method of center coordinate extraction. At the same time, the time information and the XY coordinate values of bright stars are extracted from the header file of each fits format file and recorded. The program is written in python language, and finally constitutes the time-position values and two-dimensional motion trajectories of the three bright stars.

There are two special cases in this method that require special handling:

(1) Image focusing and extraction of bright star centers

After determining the paired target star, extracting the coordinates of the center position of the star is another problem. Due to the different magnitudes and distances of the guide target, the size of the image spot (circle) of the guide target in the CCD image is also different. Therefore, it is necessary to accurately extract the center position coordinates of the entire image spot as its position information. deal with.

In the process of extracting the center coordinates of bright stars, the quality of the image has a great influence on the accuracy of center extraction, so before extracting the center coordinates, it is necessary to judge the quality of the image and the pros and cons of focusing. Fig. 5 and Fig. 6 are the FWHM and its flux profile curves when the focus is not optimal and when the focus is optimal, respectively.

The present invention uses the FWHM (full width at half maximum) value of the PSF (point spread function) as the basis for judging whether the image focusing is appropriate. If the FWHM value is too large, it means that the focusing does not meet the requirements. At this time, the calculation program is terminated and the operator is reminded to perform focusing processing. When the FWHM value is in a reasonable range, the program continues to extract the information of the next image for processing. The present invention sets the range of the FWHM as a threshold variable in the program, which can be set and modified to adapt to different detectors and optical systems.

(2) A special target matching error case

After processing one image and extracting the brightness of three bright stars and their respective coordinates, a new bright star is captured when processing the next image, and the brightness of this bright star ranks among the top three in this image. bit, which is a very special case. As shown in Figure 7, if a brightest star (star D') is newly added in image 2. According to the flow of the program, the first three bright stars in image 2 become D', A' and B' in turn, and the offsets calculated by pairing them with image 1 become AD'_X, AD'-Y , BA'_X BA'_Y, CB'_X and CB'_Y This is obviously a wrong pairing, and the offset calculated by this is also wrong due to the mismatch. In order to correct this erroneous match, the program sets another threshold, that is, the offsets of the three bright stars in the current frame and the three bright stars in the previous frame in the X and Y directions are recorded as star 0_x, star 0_y, There are six amounts of star 1_x, star 1_y, star 2_x, and star 2_y. According to the setting of the 2.5m WFST exposure time, for example, the exposure is set to 5 seconds, if the telescope does not track the shooting, the position of the target will change to 75 arc seconds. It is known that the spatial resolution of the guide star detector is 0.43”/pixel, then the change of the star in the CCD coordinate system within 5 seconds is 175 pixels, that is, the maximum change value of the above six quantities is 175. If there is a larger offset than 175 in the six quantities, it is possible that the newly captured image contains one or more stars that are brighter than the previous three bright stars. As shown in the above figure, if AD is calculated '_x and AD'_y are significantly larger than AA'_x and AA'_y, and D' is a newly added bright star. At this time, the program will suggest to the observer to check whether there is a new bright star added to the image queue , and it is recommended to restart the calculation, invalidate the last image, do not participate in the calculation, and start a new calculation with the current image (including the newly added bright star) as the first frame. This is a relatively "safe" way of working. The correctness of image matching is guaranteed. If the newly captured image contains new bright stars, but the new bright stars are not as bright as the last three bright stars, the newly added bright stars have no effect on the calculation results of the program and can be ignored Under its influence, the program continues to run. The program sets the maximum change value of the above six quantities as a modifiable threshold variable, which can be calculated and set reasonably according to the exposure time or the spatial resolution of the detector.

(3) Speed of program operation

Since the purpose of the guide star is to correct the pointing of the telescope in real time, the algorithm and processing of the guide star are required to be fast and accurate, and the final offset result can be quickly returned to the telescope control system for processing and correction.

In order to meet the above requirements, the present invention uses the magnitude brightness as the standard to sort and identify the celestial objects in the guide star on the basis of the astronomical image processing algorithm, and uses the SExtractor tool to extract the center coordinates of the image spot, FWHM (full width at half maximum value) ) and brightness information, and match and calculate the position offset of the guide target in multiple images through Python programming.

Compared with the method of calculating the WCS (world coordinate system) coordinates of each image and then extracting the coordinates of the stars in the coordinate system, the method of sorting according to brightness and extracting the center avoids the process of compiling the star table and greatly reduces the It shortens the time consumed by the calculation and improves the real-time performance of the calculation results.

The data processing program calls the mature and widely used photometric data processing software Source Extractor (Sextractor) to perform photometry on the image, including source detection and positioning, source flow calculation, etc. Compared with the traditional commonly used image processing toolkit IRAF, Sextractor is easy to call, fast, and easy to integrate into data processing programs, and Sextractor can complete source detection and photometry at one time, without needing to perform multiple steps, and can also perform multiple operations at the same time. Various modes, multiple aperture metering, high efficiency.

The flow chart of the program is shown in Figure 8. After all image processing is completed, the program directly prints out the scatter plot of the XY coordinates and time of the first three brightest stars in each frame of image and outputs it.

The above are only preferred embodiments of the present invention, and the protection scope of the present invention is not limited to the above-mentioned embodiments, and all technical solutions that belong to the idea of the present invention belong to the protection scope of the present invention. It should be pointed out that for those skilled in the art, some improvements and modifications without departing from the principle of the present invention should be regarded as the protection scope of the present invention.

Claims (8)

1. The utility model provides a telescope main light path guide star device, includes telescope principal focus detector (1) and leads star detector (3), and telescope principal focus detector (1) is located telescope focal plane, characterized by: the guide star detector (3) is arranged near the telescope main focus detector (1), is positioned on a focal plane with the main focus detector, and shares the same optical path with the telescope main focus detector (1).

2. A telescope main light path guide star device according to claim 1, characterized in that: the telescope main focus detector (1) comprises a plurality of star guide cameras (31), the telescope main focus detector (1) comprises a plurality of main focus scientific imaging detectors (4) and a wave front imaging detector (2), the star guide cameras (31) are installed around the main focus scientific imaging detectors (4), and the wave front detector (2) is also arranged around the main focus scientific imaging detectors (4).

3. A telescope main optical path guide star device according to claim 2, characterised by: the number of the guide star cameras (31) is four, and the guide star cameras (31) are installed around the main focus scientific imaging detector (4) at equal intervals.

4. A telescope main optical path guide star device according to claim 3, characterised by: the main focus scientific imaging detector (4) is a large-target-surface high-resolution spliced CCD and is used for accurately and scientifically imaging an astronomical scientific target.

5. The method for calculating the guide star offset is characterized by comprising the following steps: the telescope main optical path guiding star device as claimed in claim 1, wherein the following steps are carried out:

step one, when the telescope works, the guide star detector (3) continuously acquires image data from the optical path of the telescope,

step two, selecting a preset number of brightest stars in the first frame image data as the guide stars, determining the coordinates, the brightness, the full width half maximum value FWHM and the time information of the guide stars,

step three, extracting the brightest star bodies with the preset number in the next frame image as the updated guide star, determining the coordinate, the brightness, the FWHM and the time information of the updated guide star,

matching the updated guide star with the guide star of the previous frame, calculating the offset on the guide star coordinate, and returning the offset to the motion mechanism of the telescope to make the telescope perform correction motion;

and step five, repeating the step three and the step four until the guide process is finished.

6. The method of claim 5, wherein: in the image data acquired by the guide star detector (3), a star body used as a guide star is a bright star, when the guide star coordinate is determined, the center of the bright star is taken as the guide star coordinate, before the guide star coordinate is extracted, whether the point spread function PSF and the FWHM of the image are within a preset range or not is detected, if the point spread function PSF and the FWHM are within the preset range, the image is judged to be properly focused, the guide star coordinate is extracted, if the point spread function PSF and the FWHM are beyond the preset range, the image is judged to be improperly focused, and the image is discarded and a warning is sent.

7. The method of claim 6, wherein: and step four, when the updated guide star is matched with the guide star of the previous frame, if the offset of the coordinate of a certain updated guide star exceeds the offset threshold, judging that a new bright star is added, abandoning the image of the previous frame, and taking the current frame image as the first frame, and executing the step two.

8. The method of claim 7, wherein: selecting three brightest stars as guide stars, correspondingly, in the fourth step, judging the offset amounts of the three bright stars in the current frame and the three bright stars in the previous frame in the X and Y directions to be recorded as six amounts of star 0_ X, star 0_ Y, star 1_ X, star 1_ Y, star 2_ X and star 2_ Y, if the offset amount exceeding the offset threshold value is solved, determining that one or more stars brighter than the three bright stars in the previous frame are included in the newly shot image, invalidating the image of the previous frame, and executing the second step by taking the current frame image as the first frame.

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