Tech Note 91: Error contribution of motor movements during image transfer in MarkIV images

Robert Creager
December 1, 2002
Keywords: electronics

While implementing the ability to move RA/DEC position during image readout (to the memory card) on ROB (Berthoud, CO), it became apparent that moving the motors during the readout was causing shifts in the image data. My guess is that a reference voltage used for the analog to digital conversion is shifting, but having no background in electronice, I'll just present some light numerical analysis of the shift. The sifts seen are not significant, so all images presented here have been histogram equalized to enhance the visual effects. The control images for the two "Standard" sets are the same two images, yet different sections are analyzed based on the motor effects present in the movement images. These images were taken with 60 second exposures, at night in a dark room with the MarkIV pointing primarily at the ceiling. The "Stepped" images are also 60 second exposures and were taken during the day with opaque optical cloth covering the CCD's. Finally, the "Whole image analysis" are 60 second dark exposures, with the opaque optical cloth in place.

Image groups:

• Standard RA control image
• Standard RA move image
• Standard DEC control image
• Standard DEC move image
• Standard RA/DEC move image
• Stepped RA control image
• Stepped RA speed move image
• Stepped RA speed move image with shutter
• Whole image analysis with RA movement
• Conclusion

Standard RA control image

	I object with no movement (Control) Image Red Box Yellow Box Red-Yellow Samples 4204368 56056 411411 Mean 37112 37116 37115 0.5 Min 18514 37097 37096 1.0 Max 37140 37132 37132 0.0 Sigma 62 5 4 0.1

	V object with no movement (Control) Image Red Box Yellow Box Red-Yellow Samples 4204368 56056 411411 Mean 38834 38837 38836 1.2 Min 28822 38814 38810 4.0 Max 38885 38868 38869 -1.0 Sigma 38 7 7 0.3

Standard RA move image

	I object with 1.7 second 64x RA move (Red) Image Red Box Yellow Box Red-Yellow Samples 4204368 56056 411411 Mean 37113 37123 37116 7.7 Min 18514 37107 37096 11.0 Max 37150 37141 37132 9.0 Sigma 61 5 5 0.1

	V object with 1.7 second 64x RA move (Red) Image Red Box Yellow Box Red-Yellow Samples 4204368 56056 411411 Mean 38834 38855 38836 18.7 Min 28822 38824 38811 13.0 Max 38897 38885 38870 15.0 Sigma 38 8 7 0.8

Standard DEC control image

	I object with no move (Control) Image Red Box Blue Box Yellow Box Green Box Red-Green Blue-Green Yellow-Green Samples 4204368 361361 441441 341341 401401 Mean 37112 37116 37115 37115 37114 1.5 0.8 0.4 Min 18514 37097 37095 37093 37095 2.0 0.0 -2.0 Max 37140 37132 37132 37130 37130 2.0 2.0 0.0 Sigma 62 5 4 4 4 0.4 0.2 0.1

	V object with no move (Control) Image Red Box Blue Box Yellow Box Green Box Red-Green Blue-Green Yellow-Green Samples 4204368 361361 441441 341341 401401 Mean 38834 38837 38835 38835 38834 2.3 1.0 0.5 Min 28822 38810 38809 38810 38810 0.0 -1.0 0.0 Max 38885 38869 38867 38866 38866 3.0 1.0 0.0 Sigma 38 7 7 7 6 0.6 0.2 0.1

Standard DEC move image

	I object with 8.9/10.7/8.7 second DEC clamp/drive/clamp move (Red/Blue/Yellow) Image Red Box Blue Box Yellow Box Green Box Red-Green Blue-Green Yellow-Green Samples 4204368 361361 441441 341341 401401 Mean 37098 37086 37102 37084 37114 -28.1 -12.0 -29.9 Min 18514 37060 37084 37064 37094 -34.0 -10.0 -30.0 Max 37138 37102 37118 37101 37130 -28.0 -12.0 -29.0 Sigma 63 4 4 4 4 0.0 -0.4 -0.0

	V object with 8.9/10.7/8.7 second DEC clamp/drive/clamp move (Red/Blue/Yellow) Image Red Box Blue Box Yellow Box Green Box Red-Green Blue-Green Yellow-Green Samples 4204368 361361 441441 341341 401401 Mean 38820 38807 38824 38807 38834 -26.8 -10.1 -27.6 Min 28822 38784 38800 38784 38810 -26.0 -10.0 -26.0 Max 38886 38835 38850 38835 38867 -32.0 -17.0 -32.0 Sigma 40 6 6 6 6 0.1 -0.0 0.0

Standard RA/DEC move image

		These images present a combination of moves, with the DEC move first, followed by the RA move. The moves are the same as in the previous images.

Stepped RA control image

This series presents another set of control and movement images. The images are from a different run (during the day with CCD's covered with opaque optical cloth) which is the reason for a different set of control images. The images were taken with the RA drive moving at various siderial rates: 64x (Aqua), 32x (Red), 16x (Yellow), 8x (Blue), 4x (Green), 2x (Purple) and 1x (White).

I object with no move (Control) Image Aqua Box Red Box Yellow Box Blue Box Green Box Purple Box White Box Aqua-White Red-White Yellow-White Blue-White Green-White Purple-White Samples 4204368 91091 151151 151151 151151 151151 151151 151151 Mean 37119 37122 37122 37121 37121 37121 37120 37120 1.5 1.2 0.8 0.4 0.2 0.1 Min 34102 37101 37103 37103 37103 37102 37102 37102 -1.0 1.0 1.0 1.0 0.0 0.0 Max 37145 37140 37141 37140 37139 37138 37141 37140 0.0 1.0 0.0 -1.0 -2.0 1.0 Sigma 57 5 5 5 5 5 5 5 -0.0 -0.0 -0.0 -0.0 -0.0 0.0

V object with no move (Control) Image Aqua Box Red Box Yellow Box Blue Box Green Box Purple Box White Box Aqua-White Red-White Yellow-White Blue-White Green-White Purple-White Samples 4204368 91091 151151 151151 151151 151151 151151 151151 Mean 38839 38842 38842 38841 38840 38840 38839 38839 2.9 2.3 1.3 0.7 0.4 0.1 Min 37053 38816 38814 38815 38812 38811 38810 38813 3.0 1.0 2.0 -1.0 -2.0 -3.0 Max 38890 38871 38871 38871 38870 38870 38870 38869 2.0 2.0 2.0 1.0 1.0 1.0 Sigma 35 8 8 8 8 8 8 8 0.4 0.3 0.1 0.1 0.1 0.0

Stepped RA move image

I object with 64x, 32x, 16x, 8x, 4x, 2x and 1x sidereal rate moves (Aqua/Red/Yellow/Blue/Green/Purple/White) Image Aqua Box Red Box Yellow Box Blue Box Green Box Purple Box White Box Aqua-White Red-White Yellow-White Blue-White Green-White Purple-White Samples 4204368 91091 151151 151151 151151 151151 151151 151151 Mean 37120 37133 37127 37124 37122 37121 37121 37120 12.1 6.7 3.2 1.5 0.7 0.2 Min 34092 37116 37110 37106 37099 37103 37103 37103 13.0 7.0 3.0 -4.0 0.0 0.0 Max 37158 37147 37146 37144 37140 37141 37140 37140 7.0 6.0 4.0 0.0 1.0 0.0 Sigma 58 4 4 5 5 5 5 5 -1.0 -0.9 -0.1 -0.0 -0.0 -0.0

V object with 64x, 32x, 16x, 8x, 4x, 2x and 1x sidereal rate moves (Aqua/Red/Yellow/Blue/Green/Purple/White) Image Aqua Box Red Box Yellow Box Blue Box Green Box Purple Box White Box Aqua-White Red-White Yellow-White Blue-White Green-White Purple-White Samples 4204368 91091 151151 151151 151151 151151 151151 151151 Mean 38841 38857 38852 38846 38842 38840 38839 38839 18.3 13.1 7.1 3.2 1.5 0.3 Min 37041 38833 38825 38818 38814 38815 38811 38813 20.0 12.0 5.0 1.0 2.0 -2.0 Max 38897 38883 38878 38875 38872 38872 38869 38870 13.0 8.0 5.0 2.0 2.0 -1.0 Sigma 35 6 8 8 8 8 8 8 -1.2 0.2 0.7 0.4 0.2 0.1

Stepped RA move image with shutter

		These images present a the same RA steped moves as above, with the addition of shutter open/close after the RA moves.

Whole image analysis with RA movement

This table presents an image to image analysis using sets of three images in a sequence. The first image used (A) is an image with no RA movement. The second image (B) is a dark image taken with RA moving at the indicated speed in one direction, with the third image (C) is a dark image taken with RA moving in the opposite direction. This set of three images is repeated for each speed. Each row in the following table is then either the results of B-A or C-A at an image level.
Contrary to what I believed from earlier image analysis in this document, it is apparent from this table that slower RA speeds add a larger offset than faster RA speeds. There might be a slight trend in the sigma, but since the I image sigma's reduce with speed, while the V image sigma's increase with speed, I wouldn't pronounce any trending until many more images are examined.

Band	Speed	Mean	Sigma	Min	Max
i	-1	-18.72	6.03	-58.00	12.00
i	1	-23.98	5.98	-66.00	6.00
i	-2	-14.22	6.09	-44.00	23.00
i	2	-17.77	6.04	-48.00	13.00
i	-4	-13.11	6.06	-43.00	16.00
i	4	-14.16	6.06	-45.00	19.00
i	-8	-12.57	6.02	-43.00	19.00
i	8	-12.94	6.03	-45.00	20.00
i	-16	-10.85	5.97	-39.00	17.00
i	16	-11.13	5.97	-40.00	18.00
i	-32	-5.58	5.96	-36.00	25.00
i	32	-5.69	5.97	-40.00	25.00
i	-64	-3.85	5.93	-33.00	31.00
i	64	-3.12	5.94	-32.00	33.00
v	-1	-16.47	9.29	-75.00	28.00
v	1	-20.33	9.27	-78.00	25.00
v	-2	-11.92	9.42	-61.00	30.00
v	2	-13.98	9.34	-65.00	28.00
v	-4	-10.64	9.49	-67.00	34.00
v	4	-11.17	9.47	-64.00	31.00
v	-8	-10.57	9.48	-67.00	33.00
v	8	-10.77	9.47	-65.00	33.00
v	-16	-8.74	9.50	-64.00	37.00
v	16	-9.12	9.49	-71.00	41.00
v	-32	-4.97	9.52	-62.00	43.00
v	32	-5.08	9.55	-63.00	40.00
v	-64	-2.74	9.47	-62.00	55.00
v	64	-3.23	9.44	-60.00	52.00

Following are the images associated with the above table. There there may be some noise patterns associated with moving at faster speeds on the I camera, but not the V camera. The effect may not be real, but it is somewhat convincing as the noise does appear to increase with speed. There also appears to be a gradient when moving at speeds 1x and 2x, which will be investigated further in the near future.

Conclusion