Hi everyone,
I would like to share here a methodology that I developed to check tilt in our optical systems.
With the arrival of the new CMOS cameras, with those really small pixels, tilt and other issues have become more noticeable.
I suffered tilt (or I noticed it) when I switched from DSLR to CMOS and was dealing with it for months. It was a nightmare, and the main difficulty that I found was how to analize the results with the tools I used. I tried both CCD Inspector and PixInsight FWHMEccentricity, wonderful tools that provide several measurements, but they didn't helped too much. Maybe because I'm not expert in optics, or because they return values measured on percentage, or even worst, in unitless values. That's why I developed a mechanical aproach to a mechanical problem.
The metodology is based on the measurements taken by a Bahtinov Mask, combined with the software Bahtinov Grabber, that gaves measurements in microns (a distance). In my opinion this gaves an advantage over other methods that return FWHM values (that can be affected by other kind of aberrations). In any case, after being participating in many discussions about tilt I learned that thing can become very complicated in some optical configurations, so I left the discussion opened to those who can contribute in order to refine this methodology.
The procedure is quite simple, and I'm going to summarize here the most important steps. If you want to read all the details, you can visit my original article here:
https://aiastro.wordpress.com/2019/01/24/sensor-tilt-adjustment/
First thing we must do is to identify the problem. Tilt is a mechanical problem produced when our sensor is not squared to the focal plane.
This would lead to areas on the image that will be defocused, but the reality is a little bit more complicated as we have to take into account other aspects, as the field curvature and the Critical Focus Zone (CFZ). Field curvature would make impossible to achieve perfectly focused images in the whole FOV, even in a squared sensor, while CFZ will give us some tolerance in the adjustment.
It is important to clarify that this statements are assuming an homogeneus curvature.
As stated, a tilted system will show areas out of focus, and this is specially noticeable in the corners of our images, so to check tilt we're going to do the following:
1.Center a bright star in the center of the FOV.
2.Focus it with the bahtinov mask.
3. Without refocus, we will move the star to every corner and check how much the stars is out of focus
If our system is tilted we will probably see something like this:
The bottom-left corner has an obvious problem that we may want to fix.
Before doing the adjustment procedure, there are some aspects to consider:
1. Unless your system has 0% curvature, it is impossible to achieve a focused stars in all 4 corners, so the best result we can achieve is to have the same amount of defocus on the corners.
2. The analisys of the focus by naked eye is very difficult so we are going to use Bahtinov Grabber. This tools gives a quantitative measurement of the defocus in microns. (This is important, a mechanical aproach to a mechanical problem)
3. I strongly suggest to use some kind of notation to not to get lost. I use the following:
In my website you can find an example of adjustment.
It is important to note that to make adjustments your system has to include a tilter. Otherwise you would need to put shimms where necessary.
Hope it helps, and as commented, any suggestion will be welcome!
Clear skies to evreyone and Merry Christmas!!
I would like to share here a methodology that I developed to check tilt in our optical systems.
With the arrival of the new CMOS cameras, with those really small pixels, tilt and other issues have become more noticeable.
I suffered tilt (or I noticed it) when I switched from DSLR to CMOS and was dealing with it for months. It was a nightmare, and the main difficulty that I found was how to analize the results with the tools I used. I tried both CCD Inspector and PixInsight FWHMEccentricity, wonderful tools that provide several measurements, but they didn't helped too much. Maybe because I'm not expert in optics, or because they return values measured on percentage, or even worst, in unitless values. That's why I developed a mechanical aproach to a mechanical problem.
The metodology is based on the measurements taken by a Bahtinov Mask, combined with the software Bahtinov Grabber, that gaves measurements in microns (a distance). In my opinion this gaves an advantage over other methods that return FWHM values (that can be affected by other kind of aberrations). In any case, after being participating in many discussions about tilt I learned that thing can become very complicated in some optical configurations, so I left the discussion opened to those who can contribute in order to refine this methodology.
The procedure is quite simple, and I'm going to summarize here the most important steps. If you want to read all the details, you can visit my original article here:
https://aiastro.wordpress.com/2019/01/24/sensor-tilt-adjustment/
First thing we must do is to identify the problem. Tilt is a mechanical problem produced when our sensor is not squared to the focal plane.
This would lead to areas on the image that will be defocused, but the reality is a little bit more complicated as we have to take into account other aspects, as the field curvature and the Critical Focus Zone (CFZ). Field curvature would make impossible to achieve perfectly focused images in the whole FOV, even in a squared sensor, while CFZ will give us some tolerance in the adjustment.
It is important to clarify that this statements are assuming an homogeneus curvature.
As stated, a tilted system will show areas out of focus, and this is specially noticeable in the corners of our images, so to check tilt we're going to do the following:
1.Center a bright star in the center of the FOV.
2.Focus it with the bahtinov mask.
3. Without refocus, we will move the star to every corner and check how much the stars is out of focus
If our system is tilted we will probably see something like this:
The bottom-left corner has an obvious problem that we may want to fix.
Before doing the adjustment procedure, there are some aspects to consider:
1. Unless your system has 0% curvature, it is impossible to achieve a focused stars in all 4 corners, so the best result we can achieve is to have the same amount of defocus on the corners.
2. The analisys of the focus by naked eye is very difficult so we are going to use Bahtinov Grabber. This tools gives a quantitative measurement of the defocus in microns. (This is important, a mechanical aproach to a mechanical problem)
3. I strongly suggest to use some kind of notation to not to get lost. I use the following:
- The rectangle represents the sensor and the outside notation (TL TR BL BR) in gray, corresponds to the name of the corners (Top-Left, Top-Right, Bottom-Left, Bottom-Right).
- The values in black are the amount of defocus on each point in microns (the calculated absolute focus error). Note that the value of the center hasn’t been writen. The reason is because it will always be 0 as we always start the analisys by focusing a star in the center of the image.
- Below them, in gray, there are the corresponding 15s average values in pixels, as mentioned before.
- The outer values in red are the gauge applied, in milimeters.
- The delta value is the difference from the previous step (that means, how much gauge has been added or removed)
In my website you can find an example of adjustment.
It is important to note that to make adjustments your system has to include a tilter. Otherwise you would need to put shimms where necessary.
Hope it helps, and as commented, any suggestion will be welcome!
Clear skies to evreyone and Merry Christmas!!
