A telescope main optical path star guide device and a calculation method for star guide offset

Abstract

The present invention relates to a telescope main optical path star guiding device, including 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, is on the same focal plane as the main focus detector, and shares the same optical path with the telescope main focus detector. The present invention implements main optical path star guiding to eliminate the position and angle errors of the guide star detector and the main detector, and improve the star guiding and tracking accuracy of the telescope. The present invention also discloses an algorithm for star guiding offset, and the guide star camera sends the resulting image to a program for processing within a certain exposure time. The program finds stars in the image data by using methods in the field of image processing, and obtains XY (rows and columns in the image) offsets based on the historical records of the star positions, and then returns the offsets to the telescope's motion mechanism to make it perform correction motion, thereby achieving the purpose of compensation and forming a more reliable closed-loop control system.

Description

Technical Field

[0001] The present invention belongs to the technical field of telescopes, and in particular, relates to a telescope main optical path guide star device and a method for calculating the guide star offset, which comprises designing a guide star detector on the main optical path of a telescope, and solving a plurality of bright stars in a guide star image by using algorithms such as image processing to obtain position offsets of the bright stars so as to correct the movement of the right ascension, declination or horizontal pitch axis of the telescope. Background Art

[0002] Star guiding is a function of astronomical telescopes during observation. It is mainly used in the following occasions: (1) Observing while tracking stars, taking long-term tracking continuous imaging or long-exposure imaging. Due to the rotation of the earth, if the telescope does not move, the stars will move from east to west at 15ʺ / s along an axis parallel to the earth's equator in the celestial coordinate system. The right ascension axis of the equatorial telescope is parallel to the earth's rotation axis. In order to offset the influence of the earth's rotation, its right ascension axis should move from east to west at a speed of 15ʺ / s to keep the image of the target celestial body fixed in the field of view. Similarly, in order to offset the rotation of the earth and ensure that the observed target is fixed in the field of view, the horizontal and pitch of the horizontal telescope also need to be corrected and adjusted when taking long-term exposure or long-term tracking images of the target to avoid the target appearing as a tail in the image.

[0003] (2) For fast-moving celestial bodies such as asteroids, comets and low-orbit satellites in the solar system, the target is moving at a high speed, and even with a short exposure time, it is easy to produce a tailing phenomenon. It is necessary to use the telescope's guiding equipment to calculate the correction amount for the telescope's pointing and guide the control of the telescope (equatorial or horizontal) frame to offset the relative motion between the telescope and the fast-moving celestial body.

[0004] The guide camera is an imaging device installed on the optical axis of the telescope. Currently, most of the guide imaging detectors are located outside the main focus. Some are installed near the telescope barrel, which is called offset guide. That is, the guide detector is not on the main optical axis of the telescope. Figure 1 Take the reflective main telescope plus the refracting guide mirror as an example. The guide camera system is designed to be parallel to the main light path outside the main mirror tube, which is equivalent to an independent small telescope system. The advantage of this method is that it is easy to install and debug for this independent guide telescope system, but because the guide light path and the telescope light path are separated from each other, position and angle errors will inevitably occur on the light path, affecting the guiding and tracking accuracy of the telescope. Summary of the invention

[0005] The present invention provides a telescope main optical path star guide device and a method for calculating the star guide offset, which effectively improve the above-mentioned problem.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A telescope main optical path star guide device comprises a telescope prime focus detector and a guide star detector. The telescope prime focus detector is located on the focal plane of the telescope, wherein the guide star detector is installed near the telescope prime focus detector, is on the same focal plane as the prime focus detector, and shares the same optical path with the telescope prime focus detector.

[0008] Furthermore, the guide detector is composed of a number of guide cameras, the telescope prime focus detector includes a number of prime focus scientific imaging detectors and wavefront imaging detectors, the guide cameras are installed around the prime focus scientific imaging detectors, and the wavefront detectors are also arranged around the prime focus scientific imaging detectors.

[0009] Furthermore, the number of guide cameras is four, and the guide cameras are installed at equal intervals around the prime science imaging detector.

[0010] Furthermore, the prime focus scientific imaging detector is a large-surface, high-resolution spliced ​​CCD, which is used for precise scientific imaging of astronomical scientific targets.

[0011] The calculation method of the guide star offset is to use the telescope main optical path guide device and perform the following steps:

[0012] Step 1: When the telescope is working, the guide detector continuously acquires image data from the telescope optical path.

[0013] Step 2: Select a predetermined number of brightest stars in the first frame of image data as guide stars, and determine the coordinates, brightness, full width at half maximum (FWHM), and time information of the guide stars.

[0014] Step 3: extract a predetermined number of brightest stars in the next frame of image as updated guide stars, and determine the coordinates, brightness, FWHM and time information of the updated guide stars.

[0015] Step 4: Pair the updated guide star with the guide star of the previous frame, calculate the offset of the guide star coordinates, and return the offset to the telescope's motion mechanism to make it perform correction motion;

[0016] Step 5. Repeat steps 3 and 4 until the guiding process is completed.

[0017] In the image data acquired by the guide star detector, the star body used as the guide star is a bright star. When determining the guide star coordinates, the center of the guide bright star is taken as the guide star coordinates. Before extracting the guide star coordinates, the point spread function PSF and FWHM of the image are detected to see if they are within the preset range. If so, the image focus is judged to be appropriate and the guide star coordinates are extracted. If it exceeds the preset range, the image focus is judged to be inappropriate, the image is discarded and a warning is issued.

[0018] 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 has been added, the image of the previous frame is invalidated, the current frame image is taken as the first frame, and step 2 is executed.

[0019] The three brightest stars are selected as guide stars. Accordingly, in step 4, 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 determined and recorded as six quantities, namely star0_x, star0_y, star1_x, star1_y, star2_x and star2_y. If the offsets of the six quantities exceed the offset threshold, it is determined that the newly captured image contains one or more stars brighter than the first three bright stars in the previous frame, and the image of the previous frame is invalidated. The current frame image is taken as the first frame, and step 2 is executed.

[0020] The beneficial effects of the present invention are:

[0021] 1. The present invention designs the guide star directly near the telescope's prime focus detector, which is in the same optical path as the main optical path, to achieve main optical path guiding, thereby eliminating the position and angle errors of the guide star detector and the main detector, and improving the guiding and tracking accuracy of the telescope.

[0022] 2. The present invention also discloses an algorithm program for guide star offset. During a certain exposure time, the guide star camera performs target imaging according to its own optical design (including target surface, focal length, field of view, etc.) and telescope pointing, and sends the resulting image to the program for processing. The program finds stars in the image data by using methods in the field of image processing, and obtains XY (rows and columns in the image) offsets based on the historical records of the star positions. The offsets are then returned to the motion mechanism of the telescope to guide it to perform corrective motion, thereby achieving the purpose of compensation and forming a more reliable closed-loop control system.

[0023] 3. The present invention also solves two important problems in the algorithm program of the guide star offset: (1) the problem of image focusing. 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. Before extracting the center coordinates, the present invention makes a judgment on the quality of the image and the quality of the focusing, and 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, thereby ensuring the accuracy of the guide star coordinates. (2) After processing an image to extract the brightness of three bright stars and extract their respective coordinates, a new bright star is captured when processing the next image, and the brightness of this bright star is in the top three in this image. This is a very special but possible situation. The present invention can determine whether a new brightest star has been added by adding an offset threshold, and by invalidating the previous image and not participating in the calculation, a new calculation is started with the current image (including the newly added bright star) as the first frame. This is a relatively "safe" working method that ensures the correctness of image matching.

[0024] 4. Based on the astronomical image processing algorithm, the present invention uses the magnitude brightness as the standard to sort and identify the celestial body targets in the guide star, and extracts the center coordinates, FWHM (full width at half maximum) and brightness information of the image spot to match and calculate the position offset of the guide star target in multiple images. This algorithm can meet the needs of 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The present invention is a schematic diagram of a reflective main telescope plus a refracting guide mirror as a guide device in the prior art.

[0026] Figure 2 The main light path guiding method of the present invention is shown in FIG. Cassegrain system for example);

[0027] Figure 3 This is a schematic diagram of the focal plane distribution of the telescope's prime focus detector;

[0028] Figure 4 A schematic diagram of bright stars in a set of guide star images;

[0029] Figure 5 It is the FWHM (the last item in the numerical sequence) and contour curve diagram when the focusing is not ideal;

[0030] Figure 6 The FWHM (the last item in the numerical sequence) and contour curve diagram under ideal focusing conditions;

[0031] Figure 7 Schematic diagram of the error in matching bright stars;

[0032] Figure 8 Flowchart of the guide image offset calculation program.

[0033] Among them, the reference numerals are: telescope prime focus detector 1, wavefront imaging detector 2, guide star detector 3, guide star camera 31, prime focus scientific imaging detector 4. DETAILED DESCRIPTION

[0034] The present invention will be further described below in conjunction with the accompanying drawings.

[0035] like Figure 2-3 As shown, a telescope main optical path guide device of the present invention includes a telescope prime focus detector 1 and a guide star detector 3. The telescope prime focus detector 1 is located on the focal plane of the telescope, wherein: the guide star detector 3 is installed near the telescope prime focus detector 1, is on the same focal plane as the prime focus detector, and shares the same optical path with the telescope prime focus detector 1.

[0036] The guide star detector 3 is composed of several guide star cameras 31, and the telescope prime focus detector 1 includes several prime focus scientific imaging detectors 4 and wavefront imaging detectors 2. The guide star cameras 31 are installed around the prime focus scientific imaging detectors 4, and the wavefront detectors 2 are also arranged around the prime focus scientific imaging detectors 4.

[0037] There are four guide cameras 31 , which are installed around the prime scientific imaging detector 4 at equal intervals.

[0038] The prime focus scientific imaging detector 4 is a large-surface, high-resolution spliced ​​CCD used for accurate scientific imaging of astronomical scientific targets. The wavefront imaging detector (2) (four in number) is a technology for imaging at the wavefront in order to achieve active optical adjustment of the primary mirror. The purpose of active optics is to enable the telescope system to maintain its primary mirror in the best imaging condition to resist the influence of environmental forces such as wind, sag, thermal expansion and deformation of the telescope axis. The wavefront detector can measure the pupil wavefront error 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 motion axis of the telescope in real time when the telescope is in constant motion or when the target is being quickly tracked or when long-exposure photography is being performed.

[0039] The present invention is actually 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 prime focus scientific imaging detectors, 8 wavefront imaging detectors and 4 guide star detectors (guide star detectors such as Figure 3The telescope has a primary mirror aperture of 2.5 meters, an effective focal length of 6.2 meters, and a focal ratio of f / 2.48. The prime focus detector uses a CCD290-99 detector, and the single-chip imaging target surface is 92.2mmx92.4mm. The guide star detector uses a CCD47-20 detector, and the single-chip imaging target surface is 13.3mmx13.3mm. Combined with the system focal length, the field of view of the prime focus scientific camera is about 3˚x3˚, the imaging field of the single-chip guide star detector is 7.4ʹx7.4ʹ, and the spatial pixel resolution is 0.43ʺ / pixel. Different from ordinary telescopes, the guide star of the WFST telescope involved in the present invention is directly located near the prime focus detector, and is designed with a total of four CCD frame transfer detectors around it. The design of multiple guide star detectors increases the probability that the guide star can capture bright stars, and secondly, the data of the four guide stars are calculated comprehensively, making the data for calculating the guide star offset more accurate.

[0040] In addition to the guiding optical equipment of the main optical path, the present invention also involves a specific algorithm for guiding star offset.

[0041] The guide star image is usually taken as a set of continuous multi-frame images when the telescope is tracking. The image processed in this experiment is a set of 5 consecutive exposure images. The image names are:

[0042] HT0001316-2.fits,HT0001317-2.fits,HT0001318-2.fits,HT0001319-2.fits,HT0001320-2.fits,

[0043] The exposure time for each image was 30 seconds, and the time interval between images was also approximately 30 seconds.

[0044] The overall idea of ​​the guide star offset extraction method is to use the Sextractor tool to extract the bright source from a fits image through programming, and arrange the image according to the bright source after extraction. In order to reduce the time consumed by the image algorithm, the program only arranges the three brightest stars in the image to determine that the brightest three stars in the five images are star0, star1, and star2 (the brightness decreases in sequence), such as Figure 4 As shown. After determining the brightest three stars, we can determine that star0 in multiple images is the same star, and the same goes for star1 and star2. Then, we can extract the center coordinates of star0, star1 and star2 in each image according to the center coordinate extraction method, and extract the time information and the XY coordinate values ​​of the bright star from the header file of each fits format file and record them. The program is written in Python, and finally forms the time-position values ​​and two-dimensional motion trajectories of the three bright stars.

[0045] There are two special cases in this method that require special handling:

[0046] (1) Image focusing and bright star center extraction

[0047] After determining the paired target star, extracting the coordinates of the star's center position is another problem. Due to the different magnitudes and distances of the guide star targets, the image spot (circle) size of the guide star target in the CCD image is also different. Therefore, it is necessary to accurately extract the coordinates of the center position of the entire image spot as its position information for processing.

[0048] 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 quality of the focusing. Figure 5 and Figure 6 They are the FWHM and flux profile curves when the focusing is not optimal and optimal, respectively.

[0049] The present invention uses the FWHM (full width at half maximum) value of the PSF (point spread function) as a basis for judging whether the image focusing is appropriate. A larger FWHM value indicates that the focusing does not meet the requirements. At this time, the calculation program terminates and reminds the operator to perform focusing processing. When the FWHM value is within a reasonable range, the program continues to extract the information of the next image for processing. The present invention sets the FWHM range as a threshold variable in the program, which can be set and modified to adapt to different detectors and optical systems.

[0050] (2) A special case of target matching error

[0051] After processing an image and extracting the brightness and coordinates of three bright stars, a new bright star is captured when processing the next image, and the brightness of this bright star is in the top three in this image. This is a very special case. Figure 7As shown, if a new brightest star (star D') is added to image 2. According to the program flow, 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. Obviously, this is an incorrect pairing. Due to the matching error, the offset calculated by this is also wrong. In order to correct this incorrect matching, 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, star 1_x, star 1_y, star 2_x and star 2_y, a total of six quantities. According to the setting of the 2.5-meter WFST exposure time, for example, it is set to 5 seconds of exposure. If the telescope does not track and shoot, the position of the target changes by 75 arc seconds. The spatial resolution of the guide star detector is known to be 0.43” / pixel, so the change of a 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 the calculated offset of the six quantities is greater than 175, it is possible that the newly captured image contains one or more stars that are brighter than the first three bright stars in the previous image. For example, if AD'_x and AD'_y are calculated to be significantly greater than AA'_x and AA'_y in the above figure, D' is a newly added obvious bright star. At this time, the program will suggest the observer to check whether there is an obvious bright star newly added to the image team. The program will be added to the column and it is recommended to restart the calculation, invalidate the previous image and 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, which ensures the correctness of image matching. If the newly captured image contains a new bright star, but the new bright star is not as bright as the first three stars last time, the newly added bright star has no effect on the program calculation results, and its influence can be ignored, and 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 reasonably calculated and set according to the exposure time or the spatial resolution of the detector.

[0052] (3) Program operation speed

[0053] Since the purpose of guiding is to correct the pointing of the telescope in real time, the guiding algorithm and processing must be fast and accurate, and the final offset result can be quickly returned to the telescope control system for processing and correction.

[0054] In order to meet the above requirements, the present invention uses the magnitude brightness as the standard to sort and identify the celestial targets in the guide star based on the astronomical image processing algorithm, uses the SExtractor tool to extract the center coordinates, FWHM (full width at half maximum) and brightness information of the image spot, and matches and calculates the position offset of the guide star target in multiple images through Python programming.

[0055] Compared with the method of solving the WCS (world coordinate system) coordinates of each image and then extracting the coordinates of the star in the coordinate system, this method of sorting by brightness and extracting the center avoids the process of star cataloging, greatly shortens the time consumed by calculation, and improves the real-time performance of the calculation results.

[0056] The data processing program calls the mature and widely used photometric data processing software Source Extractor (Sextractor) to measure 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 the data processing program. Moreover, Sextractor can complete source detection and photometry at one time without multiple steps. It can also perform photometry in multiple modes and apertures at the same time, which is more efficient.

[0057] The flowchart of the program is as follows Figure 8 As shown in the figure, after all the 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 the image and outputs it.

[0058] The above are only preferred embodiments of the present invention, and the protection scope of the present invention is not limited to the above embodiments. All technical solutions under the concept of the present invention belong to the protection scope of the present invention. It should be pointed out that for ordinary technicians in this technical field, 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

1. A method for calculating a guide star offset, characterized by: A telescope main light path guide device is used to achieve this, the telescope main light path guide device comprising a telescope main focus detector (1) and a guide detector (3), the telescope main focus detector (1) is located on the focal plane of the telescope; the guide detector (3) is installed near the telescope main focus detector (1), is on the same focal plane as the main focus detector, and shares the same optical path with the telescope main focus detector (1); The method for calculating the guide star offset comprises the following steps: Step 1: When the telescope is working, the guide star detector (3) continuously acquires image data from the telescope optical path; Step 2: Select a predetermined number of brightest stars in the first frame of image data as guide stars, and determine the coordinates, brightness, full width at half maximum (FWHM) and time information of the guide stars; Step 3: extract a predetermined number of brightest stars in the next frame of image as 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 of the guide star coordinates, and return the offset to the telescope's motion mechanism to make it perform correction motion; Step 5. Repeat steps 3 and 4 until the guiding process is completed.

2. The method for calculating the guide star offset according to claim 1, wherein: In the image data acquired by the guide star detector (3), the star body used as the guide star is a bright star. When determining the guide star coordinates, the center of the bright star is taken as the guide star coordinates. Before extracting the guide star coordinates, it is detected whether the point spread function PSF and FWHM of the image are within a preset range. If they are within the preset range, it is determined that the image focus is appropriate and the guide star coordinates are extracted. If they are beyond the preset range, it is determined that the image focus is inappropriate, the image is discarded and a warning is issued.

3. The method for calculating the guide star offset according to claim 2 is characterized in that: 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 determined that a new bright star has been added, the image of the previous frame is invalidated, the current frame image is taken as the first frame, and step 2 is executed.

4. The method for calculating the guide star offset according to claim 3, wherein: The three brightest stars are selected as guide stars. Accordingly, in step 4, 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 determined and recorded as six quantities, namely star0_x, star0_y, star1_x, star1_y, star2_x and star2_y. If the offsets of the six quantities exceed the offset threshold, it is determined that the newly captured image contains one or more stars brighter than the first three bright stars in the previous frame, and the image of the previous frame is invalidated. The current frame image is taken as the first frame, and step 2 is executed.

5. The method for calculating the guide star offset according to claim 1, wherein: The guide star detector (3) is composed of a plurality of guide star cameras (31), the telescope prime focus detector (1) includes a plurality of prime focus scientific imaging detectors (4) and a wavefront imaging detector (2), the guide star cameras (31) are installed around the prime focus scientific imaging detectors (4), and the wavefront imaging detectors (2) are also installed around the prime focus scientific imaging detectors (4).

6. The method for calculating the guide star offset according to claim 5, characterized in that: The number of the guide cameras (31) is four, and the guide cameras (31) are installed at equal intervals around the primary focus scientific imaging detector (4).

7. The method for calculating the guide star offset according to claim 6, characterized in that: The prime focus scientific imaging detector (4) is a spliced ​​CCD with a large target surface and high resolution, and is used for performing precise scientific imaging of astronomical scientific targets.

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