Author Topic: Two-point calibration for tilted image  (Read 675 times)

Changkun

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Two-point calibration for tilted image
« on: May 11, 2018, 11:04:43 am »
Two-point calibration does not work for tilted image.
Please see attached image.

Since two reference points are enough to covert cordinate, two-point calibration is expected to work for any tilted and shrinked images.
Of course three-point calibration could be more accurate than two-point calibration.

Best,
Changkun

John Donovan

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Re: Two-point calibration for tilted image
« Reply #1 on: May 11, 2018, 11:21:38 am »
Two-point calibration does not work for tilted image.
Please see attached image.

Since two reference points are enough to covert cordinate, two-point calibration is expected to work for any tilted and shrinked images.
Of course three-point calibration could be more accurate than two-point calibration.

Hi Changkun,
Two calibration points are fine if they are defined in an XY diagonal *and* the sample is not rotated with respect to the image.  If the sample is rotated compared to the image, then 3 calibration points are necessary.  Otherwise there will be a significant distortion in the stage coordinate system.   

I guess one could use just 2 points if the degree of rotation was also specified, but just utilize three calibration points and all will be well.  You might also consider using the 90 degree CW or CCW rotation dialog in PictureSnapApp.  You still need to confirm the calibration using three points but it will get you closer to begin with.

One thing I do to make life easier with an uncalibrated image, is to start with the two point calibration, then click the Calibrate Image button. Then while the image is roughly calibrated, double-click to move the stage to the third calibration point. Finally click the three point calibration option, then calibrate a third calibration point. Finally again click the Calibrate Image button and you should be good to go!
john
« Last Edit: May 11, 2018, 02:12:23 pm by John Donovan »
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Changkun

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Re: Two-point calibration for tilted image
« Reply #2 on: May 11, 2018, 11:33:54 am »
One thing I do to make life easier with an uncalibrated image, is to start with the two point calibration, then click the Calibrate Image button. Then while the image is roughly calibrated, double-click to move the stage to the third calibration point. Finally click the three point calibration option, then calibrate a third calibration point. Finally again click the Calibrate Image button and you should be good to go!

Oh, it is!
« Last Edit: May 11, 2018, 02:12:44 pm by John Donovan »

John Donovan

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Re: Two-point calibration for tilted image
« Reply #3 on: June 07, 2018, 10:17:26 am »
Speaking of tilted samples, the latest version of PictureSnapApp (v. 1.5.7) now checks for the stage tilt and if greater than 0.5 degrees pops up a warning like this:



The calibration is still saved, but it warns the user just in case one forgets to update the z-positions for one of the calibration points.  Of course this only apples when one is utilizing three calibration points.
John J. Donovan, Pres. 
(541) 343-3400

"Not Absolutely Certain, Yet Reliable"