tn-0311 lower gos alignment - dkist · 2020. 1. 22. · lower gos alignment tn-0311, revision a...

Project Documentation Document TN-0311

Revision A

Lower GOS Alignment

Rachel Rampy, Stacey Sueoka, Scott Gregory, Austin Kootz, Chris Runyan, Andy Ferayorni

PA&C Team

December 2018

Lower GOS Alignment

TN-0311, Revision A Page ii

REVISION SUMMARY: 1. Date:

Revision: Changes:

Lower GOS Alignment

TN-0311, Revision A Page iii

Table of Contents

1. INTRODUCTION ................................................................................................... 1 2. WHEEL ALIGNMENTS & MEASUREMENTS ...................................................... 3 2.1 LOWER GOS APERTURES RELATIVE ALIGNMENT ..................................................... 4 2.2 APERTURE MOUNTING SURFACES ............................................................................ 5 2.3 OCCULTER LOCATION CHECK .................................................................................. 8 2.4 DEPLOYED POSITION AND TIP/TILT WITH TOOLING BALL TARGET ............................... 9 3. PINHOLE CO-ALIGNMENT ................................................................................ 13 3.1 PINHOLE VENDOR METROLOGY ............................................................................. 13 3.1.1 0.2” and 1.68” Pinhole Metrology ................................................................................ 13 3.1.2 Inverse Pinhole Metrology ........................................................................................... 13 3.2 MICROSCOPE PINHOLE CO-ALIGNMENT .................................................................. 14 3.2.1 Alignment microscope setup ....................................................................................... 14 3.2.2 Use of transparent grid for real time alignment ......................................................... 14 3.2.3 Verify repeatability of measurement ........................................................................... 15 3.2.4 Determine the center of each pinhole ......................................................................... 15 3.2.5 Determine the diameter of each pinhole ..................................................................... 17 4. CONCLUSIONS .................................................................................................. 18 4.1 WHEEL APERTURES ALIGNMENT ............................................................................ 18 4.2 PINHOLE CO-ALIGNMENT ....................................................................................... 18

Lower GOS Alignment

TN-0311, Revision A Page 1 of 19

1. INTRODUCTION

The Gregorian Optical Station (GOS) provides apertures and calibration optics that will be used to align the telescope optics and ultimately define the telescope’s boresight. It must therefore be carefully positioned at the Gregorian focal plane as defined by the primary and secondary mirrors (M1 and M2) upstream. This document describes efforts to establish internal alignment of the GOS components at the NSO lab in Boulder, Colorado.

The lower GOS wheel contains 12 apertures, 10 of which require alignment relative to the rotational axis of the wheel. The wheel installed within the lower GOS box is shown on the left in Figure 1, and on the right is a zoomed in view of one of the empty apertures. The adjustment screws visible in the upper right allow rotational clocking of the mounting surface and changes in the distance between the aperture center and the center of the wheel. The aperture to the left is one of the Limb Tracking occulters, and is covered with blue tape for protection in this image.

The requirements for the relative alignment of the apertures state that when deployed each optic should have less than +/- 100 microns of x/y decenter, and any combination of x/y tilt and relative z displacement (i.e. height above the wheel plane) should be within +/- 130 microns. Additionally, the two pinholes and the inverse pinhole must be co-aligned to +/-38 microns.

Section 2 of this document describes use of the CMM arm and SpatialAnalyzer® (SA) software to:

1) Position dowel pins on each empty apertures mounting surface to within 100 microns of their intended location as determined from the CAD model.

Figure 1: The Lower GOS wheel is shown on the left, and a close up of one of the empty apertures is on the right.

Lower GOS Alignment


2) Check the flatness and location below the main wheel surface of the empty apertures mounting surfaces.

3) Determine the center of the two occulters and co-planarity of their top surfaces with the main wheel plane.

4) Verify the location of the tooling ball target after installation into each of the aligned empty apertures and adjust the motor units of the wheel such that the deployed position of each aperture is consistent with the CAD position to within requirements.

Section 3 details how a microscope was used to achieve the required alignment between the pinholes and the inverse pinhole. Section 4 presents conclusions from both efforts.

Lower GOS Alignment


2. WHEEL ALIGNMENTS & MEASUREMENTS

The CMM arm was mounted inside the lower GOS box using its magnetic base. This configuration is shown in Figure 2.

To locate the center of rotation of the wheel and the plane of its top surface, CMM arm measurements were taken along the accessible portions of the outer edge and top surface with the wheel in 2-3 separate locations. The wheel was rotated between each position, and the motor was engaged at each to hold it in place. Each set of outer edge and top surface measurements were fitted to circles and planes, respectively, and compared. Deviations between the results of each measurement were determined to be within the accuracy of the CMM arm (~50 microns). All measurements of the outer edge and plane were then combined to create a “master circle” and “master plane”. The circle points were projected to the plane, and the frame coordinate system was established with the center of the circle along the z-axis, and the wheel plane containing the x and y axes. Figure 3 shows a screenshot of the measured points and coordinate system in SA. This process was repeated on each new day that measurements were collected to ensure the coordinate frame was as accurate as possible.

Figure 2: The CMM arm was mounted within the lower GOS box.

Lower GOS Alignment


2.1 LOWER GOS APERTURES RELATIVE ALIGNMENT

The mounting surface of each aperture has two dowel pins for locating the optic, as shown below in Figure 4. These were selected as the most appropriate fiducials to use to position each aperture mounting surface relative to the center of the wheel. This was because the optical elements that will later be installed in the apertures were manufactured with tight tolerances between their critical features and these pins.

Figure 3: Measurements of the outer edge and top surface of the wheel were used to create a coordinate system in SA.

Figure 4: Close-up view of an empty aperture showing the mounting surface and dowel pins.

Lower GOS Alignment


Precision guage blocks were used to roughly center the rotational position of each aperture. After the correct distances from the center of the wheel were established for one aperture based on CMM measurement, all remaining apertures were roughly positioned to this same distance. These steps significantly reduced the time required to achieve correct relative alignment of all apertures.

All empty apertures were initially aligned on November 15-16 then rechecked on November 19-20. The pin farthest from the wheel center was designated “Pin F” and the nearer one “Pin N.” These are on the left and right, respectively, in Figure 4. Note that apertures 3 and 9 contain the occulters, which do not have external alignment mechanisms.

Table 1 presents the positions of each dowel pin and their deviations (deltas) from the CAD model positions for both the initial and rechecked measurements. Only Aperture 12 (red text) was found to have drifted to a location beyond 100 microns from the CAD position and required tweaking of the alignment during the recheck. On average, the position of the pins was observed to drift by ~40 microns, which is within the measurement accuracy of the CMM arm.

2.2 APERTURE MOUNTING SURFACES

The mounting surface of each empty aperture (see Figure 4) was probed with the CMM arm to determine both its flatness and distance below the top plane of the wheel. Measurements were taken in a consistant pattern on each aperture. The initial point was centered with the adjusters (top middle in Figure 4), and subsequent points were collected at ~5mm intervals moving clockwise around the inner edge of the aperture mounting surface. This method resulted in ~65 data points for each aperture.

If the flatness profile of an aperture showed significant structure, the measurement was repeated. This was the case for apertures 4, 7, 8, and 12. The statistical properties and values of interest for each measurement are presented in Table 2. The CAD value for this distance is 4.5 millimeters. Aperture 5 appears to have the minimal depth for reasons that are not well understood.

Table 1: Positions of the 2 dowel pins for each of the empty apertures were measured and compared on different dates.

Lower GOS Alignment


The points collected in each measurement were plotted to produce a surface profile. Figure 5 displays these graphs for each aperture, including the repeat measurements on apertures 4, 7, 8, and 12, which are labelled “a” and “b.” In every case the repeated measurement showed the same structure as captured in the initial profile. The vertical scale in all graphs is millimeters.

Although the “potato chip” shape of the aperture mounting surfaces noted above was unexpected, it isn’t anticipated to affecte the coplanarity of the installed optics with the wheel top surface. This is because in all cases the high points are approximately symmetric to within ~20 microns. Bolting the stiff surfaces of the optics against these surfaces may also cause them to flatten to a more level shape.

Table 2: Measurement RMS and values of interest. These indicate Aperture 5 has the least depth below the plane.

Lower GOS Alignment


Figure 5: Around half of the aperture mounting surfaces showed significant structure in their flatness profiles with peak-to-valley differences on the order of 100 microns. The shape of these profiles was consistent between multiple measurements.

Lower GOS Alignment


2.3 OCCULTER LOCATION CHECK

The two occulters do not have adjustment mechanisms for fine-tuning of their position within the wheel. Fortunately, measurements indicate their installed location is within specification. All measurements were taken with the occulters rotated into the deployed position. Decenter values are reported relative to the center of the deployed position in the CAD model using a synonymous coordinate system, and the height is relative to the top plane of the wheel.

To determine the location of their centers, their individual cable ribbon tray covers were removed and the outer edge of the inner tray boarder was probed with the CMM arm. This surface was manufactured to be concentric with the circular portion of the occulter opening. The 5 arcminute occulter, with cable cover removed, is shown in Figure 6.

The portion of each occulters surface that is anticipated to come into contact with sunlight, plus a few millimeters of excess margin along the outer edge, was coated with carbon nanotubes to provide maximal absorption. These are fragile and easily damaged through contact. However, this is the surface that needed to be contacted to determine the height of this optic with respect to the wheel plane. To minimize risk and impact to this coating, only 4 points were touched with the CMM arm and the 2mm diameter probe tip, and only along the inner edge of the inner cable tray boarder (i.e. outside the field of view).

The above measurements determined that the 2.8 arcminute occulter has decenter in x and y of 0.8 microns, and 13.3 microns, respectively, and a height of 9.5 microns. The 5 arcminute occulter was found to have 27.1 microns and 27.5 microns of decenter along x and y, respectively, and to have its surface located 84.5 microns above the wheel plane.

Figure 6: The 5 arcminute occulter is shown here with cable cover removed to allow for measurements of its position within the lower GOS wheel.

Lower GOS Alignment


2.4 DEPLOYED POSITION AND TIP/TILT WITH TOOLING BALL TARGET

The reference frame coordinate system used both in the occulter alignment check and measurements described in this section was established using the same fiducials within the CAD modal and SA measurement screen. External references to the wheel were selected to be the top edge of the lower GOS box, and the two dowel pins that will locate it to the upper GOS box. Coordinate frames were established with z along the wheel rotational axis, x and y contained within the wheel top plane, and the x-axis parallel to the line connecting the two dowel pins on the top of the lower GOS box. With the coordinate frames oriented in this way, the center of a deployed optic should bisect the positive xy-plane. Figure 7 depicts this configuration in the CAD model, with the deployed position in the upper right quadrant, and the pink line connecting the box dowel pins.

The tooling ball target was installed in each of the ten empty apertures, and each was rotated into the deployed position. The installed target is shown in Figure 8. CMM measurements of the ball surface were used to determine the distance of the aperture center from the CAD deployed position, check its location relative to the wheel center and height above the wheel plane. Measurements on the flat surface were used to determine the tilt across the aperture.

Figure 7: The coordinate frames in the CAD and measurement screen were oriented such that the deployed position bisects the xy-plane, x is parallel to the line adjoining the box dowel pins, and z is perpendicular to the wheel plane.

Lower GOS Alignment


Based on the measured tooling ball positions, the motor units (MU) value cooresponing to “deployed” for each aperture was adjusted and set in the Property Database through the PA&C Engineering GUI. The final MU values and measured positions for all apertures are reported in Cartesian coordinates for the above described reference frame in Table 3.

Figure 8: A tooling ball target was installed within each empty aperture and measured with the CMM. This allowed tuning of the wheel deployed positions relative to the CAD model and final verification that all apertures were aligned within specifications.

Table 3: Final x, y, and z, locations for each aperture are reported here, along with the deployed motor units (MU). Aperture 4 appears to be slightly outside specifications, however, this is resolved when polar coordinates and relative values are considered.

Lower GOS Alignment


Measurement of the tooling ball installed in aperture 5 indicates that its mounting surface is still considerably higher than the others. This coroborates similar findings from Section 2.2. After those measurements, an attempt was made to mitigate this height discrepency by dissasembling, inspecting, and reassembling all subcomponents with an effort to seat them more firmly. However, the persisting offset is not anticipated to be a problem as this aperture was serendipitously assigned to the Dark Shutter, whose performance will be insensitive to this type of misalignment. Initial inspection of the values in Table 3 also appear to indicate that the x-component of aperture 4 is slightly beyond the +/- 100 microns requirement, and its z displacement is just beyond the +/- 130 micron limit with respect to the CAD model. Both of these are resolved when the deltas are calculated relative to the average positions of all apertures instead of to the CAD. Also, for fine tuning of the wheel position and aperture radial positions, measurements were transformed to radial and azimuthal offset values. These are reported for each aperture relative to mean values in Table 4.

The flat surface of the tooling ball target was also measured for each aperture in the deployed position. This information was used to determine the magnitude of any tip/tilts with respect to the top plane of the wheel. Angles between the two occulters top surfaces and the wheel plane were determined from the 4-point measurements described in Section 2.3. These values are reported in Table 5, along with a caclulated “height” due to angular deviations, the sum of these and measured z displacements (“Total Z”), and a recaclulation of deltas in height from the mean value.

Table 4: The final measured positions of all apertures reported in radial and azimuthal components, and relative to the mean measured offsets from the CAD. The height of aperture 5 is still not within specifications.

Lower GOS Alignment


The “Height” column in Table 5 was caclulated by using the small angle approximation, with the RSS of the measured angles, across half the 76.2 millimeter extent of the 5 arcminute field of view at the Gregorian focal plane. The “Total Z” column is then the sum of these caclulated heights and the measured z displacements shown in Table 3. The rightmost column in Table 5 contains the distance in microns from the median value from the “Total Z” column (excluding aperture 5), which is 31.3 microns. These results also indicate that only aperture 5 is located outside of specifications.

Table 5: Angular deviations were measured from the flat surface of the tooling ball target for each of the empty apertures, and using 4 points outside the FOV for the occulters. Heights due to these tip/tilts were calculated, summed with measured z displacements, and presented as deltas from the mean total height. Aperture 5 remains outside of the specification.

Lower GOS Alignment


3. PINHOLE CO-ALIGNMENT

3.1 PINHOLE VENDOR METROLOGY

3.1.1 0.2” and 1.68” Pinhole Metrology

The metrology report from Oxford Lasers provides images and information on the actual laser drilled holes for the 0.2” and 1.68” pinholes. The 0.2” pinhole was prescribed to be a 0.52mm drilled hole, measurements show the measured diameter to be 51.1 micrometers. The 1.68” pinhole was prescribed to have a 0.429mm diameter, measurement show the measured diameter to be 436.3 micrometers.

Figure 9: Metrology of the 0.2” pinhole (pinholecv/Aperture#1) from Oxford Lasers. Diameter measured to be 51.1micrometers.

Figure 10: Metrology of the 1.68” pinhole (pinholewf/Aperture#6) from Oxford Lasers. Diameter measured to be 436.3micrometers

3.1.2 Inverse Pinhole Metrology

Metrology from Front Range Photomask (see vault PA&C/Testing/GOS/Optics/GOS Targets Masking Vendor Measurements.txt) indicates the inverse pinhole diameter averages out to 76.28 micrometers which meets the specification of 76.2 micrometers +/-12.7 micrometers.

Commented [AEF1]: Metrology from Front Rang Photomask (see vault PA&C/Testing/GOS/Optics/GOS Targets Masking Vendor Measurements.txt) indicates the inverse pinhole diameter averages out to 76.28um which meats the spec of 76.2 um +/- 12.7 um. I would include that info here.

Lower GOS Alignment


3.2 MICROSCOPE PINHOLE CO-ALIGNMENT

3.2.1 Alignment microscope setup

Sony XC-HR58 camera has sensor format 767x580 and pixel size of 8.3 micrometers. Using a 10x objective, and tube length of roughly 150mm, the spatial sampling in the object space is 0.833 micrometers, which is sufficient for sensing misalignment on the order of 38 micrometers.

The image size of the pinholes on the camera sensor will be approximately 760 micrometers (0.3” Inverse Pinhole), 4290 micrometers (1.68” Pinhole), and 520 micrometers (0.2” Pinhole). The sensor size is about 6.37mm by 4.81mm so each of the pinhole images will fit on the detector. In addition to the sensor, a frame grabber card was needed to save images from the microscope on a connected laptop. The frame grabber card used was an Epiphan DVI2USB3.0 and the Epiphan Capture Tool software on a laptop.

Figure 11: Microscope setup on the lower GOS box. Vertical translation of the microscope was necessary between pinhole measurements because the height of apertures in between (in particular the occulters) would result in collision.

3.2.2 Use of transparent grid for real time alignment

To visually aid the alignment process, a transparent grid was taped on to the laptop screen as shown in figure 12. The grid was centered on the location of the 1.68” pinhole in the deployed position. This location was determined by the CMM in Table 3. The GOS wheel was then rotated such that the 0.2” pinhole is in the deployed position. The aperture radial adiustment was used along with the GOS wheel rotary encoder position to center the 0.2” pinhole image on the target grid. These same steps were repeated with the inverse pinhole image.

Commented [AEF2]: Which is which? Suggest adding respective pinhole names.

Commented [AEF3]: Please add model of grabber (Epiphan DVI2USB3.0) and indicate what software you used to display images (Epiphan Capture?)

Lower GOS Alignment


Figure 12: What it looked like on the screen. A transparent target, half the sheet has square gridlines, the other half has concentric circles. This was taped onto the laptop screen to make the alignment process easier. The target was centered on the 1.68” pinhole, then the 0.2” and inverse pinholes were moved in order to co-align them with the 1.68” pinhole using the target. Radial adjustment came from the setscrew on the aperture, tangential movement was performed with the rotation of the wheel.

3.2.3 Verify repeatability of measurement

Repeatability measurements were performed with the different pinholes multiple times because there is some error in the repeatability of the microscope position when moving the microscope vertically before wheel rotation. The vertical movement of the microscope is required in order to avoid collision with the occulters when moving from one pinhole to another. Measurements of the 0.2” and 1.68” pinholes were taken intermittently before, during and after the alignment of the inverse pinhole on December 12th. The deviation from the average position of the center of these measurements are tabulated in Table 6. Repeated measurements were taken when moving the microscope vertically for both the 0.2” and 1.68” pinholes. The repeatability of the measurement setup is well within tolerance of the pinhole alignment.

Table 6: Measurements to determine precision of the microscope setup.

Measurement # Deltas from mean for the 0.2” pinhole (micrometers)

Deltas from mean for the 1.68” pinhole (micrometers)

1 before inverse PH align 0.71 1.99

2 middle inverse PH align 2.31 N/A

3 after inverse PH align 2.65 1.99

Standard deviation 1.03 2.05

3.2.4 Determine the center of each pinhole

Once the pinholes were aligned, a centroid function was used to determine the location of the center of each pinhole. Figure 13 shows the image of all three pinholes along with the circle determined by the fitted diameter, and a colored spot in the middle to show the center of the

Commented [AEF4]: Suggest mentioning why this had to be done (to avoid occulters)

Lower GOS Alignment


pinhole. Figure 14 overlays the 0.2” pinhole image with the diameters and center spots of all three pinholes.

Figure 13: Images from Microscope with overlayed computed centroid for all three pinholes. Top left is the 0.2” pinhole, top right is the 1.68” pinhole, bottom is the inverse pinhole. The 1.68” pinhole appears slightly wider in the horizontal direction, but only by a few pixels.

Figure 14: Image of 0.2” pinhole, Red, green and blue circles represent the outer diameter of each of the pinholes of the 0.2”, 1.68” and inverse pinhole respectively. The spots in the center represent the center of each pinhole.

Lower GOS Alignment


3.2.5 Determine the diameter of each pinhole

The fitted diameter was calculated using the mean of the semiaxes of each pinhole. The 1.68” pinhole diameter measured in pixels on the sensor was used with the vendor diameter data to create a conversion factor for the other two pinholes. The calculated pinhole diameters are in table 7. The 0.2” pinhole diameter calculated with the conversion factor is about a micrometer off from the vendor measured value (51.1 micrometers measured by vendor, 52.24 micrometers measured by microscope). The inverse pinhole diameter calculated with conversion factor is 2.49 micrometers larger than the vendor measured value (76.28 micrometers measured by vendor, 78.77 micrometers measured by the microscope). These values are of the same magnitude of the errors that result in the repeatability measurements.

Lower GOS Alignment


4. CONCLUSIONS

4.1 WHEEL APERTURES ALIGNMENT

The relative alignment between all apertures within the lower GOS wheel was established based on feedback from CMM arm measurements. The locations of the 10 empty aperture holders were optimized by measuring the optic locating dowel pins and adjusting the mounting surface clocking and radial distance from the wheel center. The locations of the two Limb Tracking occulters were checked (these do not have adjustments) to ensure they did not deviate significantly from the CAD. A tooling ball target was then installed in each empty aperture, and the wheel motor was used to rotate that aperture into the “deployed” position. Measurements of the surface of the tooling ball were used to refine the wheel motor units corresponding to the deployed position, such that deviances in the center location for all apertures were within the +/-100 micron requirement. The tip and tilt of each aperture relative to the top plane of the wheel was measured from the flat surface of the tooling ball target for the empty apertures, and by probing 4 points outside the FOV for the occulters. Maximal height deviations due to discrepancies in co-planarity were summed with the measured z displacements, and deviations from the mean total vertical displacement were calculated. With the exception of Aperture 5, all are within the requirement of having combined x/y tilt and relative z displacement within +/- 130 microns. Aperture 5 is only ~60 microns out of specification, and no functional issues are anticipated due to this misalignment because it will be occupied by the Dark Shutter.

4.2 PINHOLE CO-ALIGNMENT

The final position of the three pinholes was determined. The largest misalignment of 5 micrometers is between the inverse pinhole and the 0.2” pinhole, this is well within the +/-38 micrometer specification from the DRD. The diameters of the pinholes were confirmed as well with this measurement technique. Table 7: Final measurements of the 3 pinholes. Measured position of the center of each pinhole on the detector in pixels, difference in absolute position from the 0.2” pinhole, the diameter of each pinhole (*the vendor data for pinhole 1.68” was used to create a conversion factor for the other two pinholes. The ratio is 436.3micrometers/446pixels), and the final deployed position for the GOS wheel. Denoted in red, only the inverse pinhole position was changed for alignment, the other two remain the same as in table 3.

Aperture Measured position on detector (pixels)

Deltas from Pinhole 0.2” RSS

Diameter Deployed position

Name # X Y (micrometers) (micrometers) (motor units) Pinhole 0.2” 1 380.75 287.31 52.24 62978075 Pinhole 1.68” 6 380.59 288.53 1.20 436.3* 35015296 Inverse Pinhole 8 384.68 290.83 5.16 78.77 23831952

Lower GOS Alignment


Figure 15: Co-alignment of the pinholes brought the 1.68” pinhole center to within 1.2 micrometers of the 0.2” pinhole center. The inverse pinhole center was aligned to be 5.2 micrometers from the 0.2” pinhole center.

tn-0311 lower gos alignment - dkist · 2020. 1. 22. · lower gos alignment tn-0311, revision a...

Documents