Internal alignment between Filtergram (FG) and Spectro-Polarimeter (SP), image scale

Internal alignment between Filtergram (FG) and Spectro-Polarimeter (SP), image scale

T. J. Okamoto (NAOJ/Kyoto Univ.)Y. Katsukawa, T. Shimizu, K. Ichimoto, Y. Suematsu,

S. Tsuneta, T. Tarbell, and SOT team

We examined the relative alignment among the instruments on SOT. --- with a test data set obtained in the natural sun-light test in May 2005, --- to an accuracy of better than 0.1 pixel corresponding to 0.02”.

It is important to know relative displacements and plate scales of these instruments for accurate alignment of observational data.

Abstract

The main contents in this presentation are the following:

1. Relative alignment of Filtergram (FG) images a. Displacements and plate scales among Broadband Filter Imagers (BFIs) and Narrowband Filter Imagers (NFIs) b. Dependence on focus position

2. Relative alignment of Spectro-Polarimeter (SP) Images a. Displacements and plate scales between SP and NFI b. Uniformity of scanning steps

Overview

3883 5172 6302

6684

4305

5550

3968

4504

5250

6302

6563

5576

5896

BFI NFISPFG

alignment

(Angstrom)

The grid pattern was located at the secondary focus of the simulated Optical Telescope Assembly (OTA).

The images of the pattern were taken through all the filters of FG.

We measured positions of all the crosses in the grid pattern, and derived relative displacements and plate scales.

1024pix

2048 pix

1(a). Relative alignment of FG images

FG image (BFI)

FPP

Solar light

Pinhole-array

Re-imaging lensgrid

overview

CLU(Collimator Lens Unit)

CN bandhead 3883

Ca II H line 3968

G band 4305

blue cont 4504

green cont 5550

red cont 6684

(Focus = 2009) (date = May 26)

Relative alignment of FG images

N

E

1024pix

2048 pix

(1) Determine grid positions for each filter

(2) Plot relative displacements of the grid positions with respect to the positions on the 5550A image

(3) Calculate average displacements (DX, DY) between two filters by averaging the relative shifts of the grid positions

(4) Correct the average displacements, and then plot the grid positions again

(5) Get difference of the pixel scale among the images with respect to the 5550A image

(BFI)

BFI images

x

y






Relative alignment of FG images (BFI)







3883 3933 4305 4504 5550 6684

DX -0.2 -2.0 -1.6 -0.8 0.0 1.1

DY -5.4 -4.7 -1.6 2.1 0.0 -1.9

(BFI)

These shifts are due to wedge of each filter. (because only filters are different in the optical system)

This can be easily corrected.







3883 3933 4305 4504 5550 6684

DX -0.2 -2.0 -1.6 -0.8 0.0 1.1

DY -5.4 -4.7 -1.6 2.1 0.0 -1.9

wavelength-dependent image shift (averaged) removed

(BFI)

color aberration

difference of plate scale


(2) Plot relative shifts of the grid positions with respect to the positions on the 5550A image




3883 3933 4305 4504 5550 6684

in X 1.0049 1.0050 1.0047 1.0039 1.0000 0.9966

in Y 1.0045 1.0048 1.0045 1.0037 1.0000 0.9969


color aberration (difference of plate scale)

Relative plate scale

(BFI)

X-axis Y-axis

Grid position in X (pix) Grid position in Y (pix)Gri

d po

siti

on d

ispl

acem

ents

Gri

d po

siti

on d

ispl

acem

ents

As the wavelength is longer, the plate scale is smaller.

5172 5250 5576 5896 6302 6563

in X 1.0014 1.0008 1.0000 0.9984 0.9975 0.9971

in Y 1.0012 1.0008 1.0000 0.9981 0.9972 0.9969

5172 5250 5576 5896 6302 6563

DX 0.0 -0.2 0.0 -1.0 1.4 1.7

DY -0.3 -0.6 0.0 -1.2 0.9 0.7

Relative alignment of FG images (NFI)

Average displacements

Relative plate scale

The grid images taken with NFI were processed with the same procedure as with BFI.

Similar to BFI, as the wavelength is larger, the plate scale is smaller.

wavelength-dependent image shift (averaged) removed

displacement in X -15.6

displacement in Y -16.1

0.6732 (designed: 0.6750)

Relative alignment of FG images (between BFI and NFI)

The relative displacement of central grids of BFI 5550A with respect to NFI 5576A.

The relative plate scale among the images of BFI (5550A) and NFI (5576A).

BFI NFI

!!! Remaining issue: absolute plate scale

1(b). Alignment depending on Focus Position

The following figures show the displacements of the grid points in the X-direction as a function of the focus positions. The displacements represent the shifts from the grid positions obtained at focus position 2009. The images taken with BFI 5550A and NFI 5576A are used here. Only the grids in the middle row are plotted in these figures.

We notice 2 features from the figures:

(i) periodic variations with an amplitude of several pixels in a period of 18.

(ii) change of plate scales: the distance between two grids of the images becomes smaller as the focus position is larger. This means the plate scale is larger at the larger focus positions.

BFI NFI

This variation is due to change of shift of lens as the position of focus is moved. However, it is regardless because it is rarely moved during observations.(it is needed to correct the alignments again when we adjust the focus)

(arbitrary position)

right

left

2. Relative alignment of SP images

We examine the plate scale and alignment of SP along the slit and in the direction of scanning with respect to NFI.

(1) Distortion of NFI image

(3) Uniformity of scanning steps(2) Rotation

SP data NFI data

0 500 1000-1000 -500

・・・・・・・・・

1024pix

101 pix012 43 5・・・ 101

SP data is scanned on the FOV with 101 steps (slit position: -1000~1000 at intervals of 20). We select one wavelength of SP and make an image that has 101x1024 pix size.

This figure indicates an NFI image that is scanned on the FOV of NFI 6302A image with 101 uniform steps at intervals of 20.

According to these figures, the scanned steps of SP are non-uniform and the plate scale of SP and NFI is different because of …

FOV

(1) distortion of NFI image

Relative alignment of SP images

From NFI image, we can see it distorted by analyzing details. So, we compared observed grids with even interval grids.

even interval grids

(1) distortion of NFI image


Figure shows the degree of distortion of NFI as a function of a distance from the central point (1023.5, 511.5) of the image.

Distortion is larger near the edge of FOV.

The NFI image is rotated with respect to SP. The angle is 0.2 degree counterclockwise.

(2) rotation

It is needed to correct when we analyze the data.0.2°

r

r+dr

(r+dr)/r

SPNFI

(even interval grids)

(obs.)

r

(3) uniformity of scanning steps


Using vertical lines, we examined the position on NFI image of each slit of SP. The result is shown.According to these figures, we can see

the non-uniformity of scanning steps --- the periodic residual component.

To measure them with higher accuracy, it is necessary to analyze data having smaller step size than 20 steps.

SP data

NFI data

These results in this presentation are obtained with the data set in May 2005. We have new data set in 2006 with new optical system. We are analyzing them and the new results will be shown in the next time.

alignment (platescale, displacement)

Summary

We know the platescale and displacement of each filter,it is possible to align all data each other.

There is a remaining issue: “absolute platescale.”

3883 5172 6302

6684

4305

5550

3968

4504

5250

6302

6563

5576

5896

BFI NFISPFG

rotationdistortion

displacement platescale

X X

YY







integration of intensity along the vertical axis on each horizontal point

40 pix

2nd order fitting(to an accuracy of better than 0.1 pixel)

(BFI)







(1013, 478)

grid center

8 pix circle

(BFI)







(211, 179)

(202, 450)

(192, 720)

(483, 188)

(473, 459)

(463, 730)

(753, 198)

(743, 469)

(733, 739)

(1023, 207)

(1003, 748)

(1293, 217)

(1283, 488)

(1274, 758)

(1563, 226)

(1554, 497)

(1544, 767)

(1834, 235)

(1825, 506)

(1815, 777)

(1013, 478)

grid center

8 pix circle

(BFI)

Internal alignment between Filtergram (FG) and Spectro-Polarimeter (SP), image scale

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