How To Measure Tilt In Your System

How To Measure Tilt In Your System

Collimation woes: Continue

Hamza Touhami

Just Another Amateur

As per our last venture with trying to collimate the OOUK AG16 as close to perfection as possible, we have explored all the avenues possible. Some issues we thought about are:

  • The lenses inside the coma corrector were move for lack of tightness
  • The focuser was moving
  • The mirrors were moving

To verify the issue we thought were responsible for the misalignment we were having, I performed the following:

  • I removed the coma corrector and gently started moving it side to side, this effectively allowed me to hear a small rattle inside of it. So I ensured all the retaining ring were tight from both sides. After discussing my discovery with OPT, this coma corrector is going back to England for proper collimation.
  • I had to continue and check the rest. To check the mirrors, I removed the coma corrector and installed the camera directly in the focuser. I slewed the big 16 inch towards the four quadrants of the sky and noticed no change in star elongation

scripting help

For this session of collimation, I used the "Measure Field Tilt and Curvature" tool in PRISM to really understand how the CCD plane is tilted. For this you need:


Head over to PRISM and open the tool uder Analysis > MEasure Field tilt curvature


Find your telescope's focal ratio ( focal length / diameter of the telescope)

Example: 1500mm / 400mm = F/3.75


Multiply the focal ratio times 25x or 30x. Example: F/3.75 X 30 = 112.5 mm,

this will tell you how much you need to move between exposures


Get the focuser to starting position, snap a picture. Then move it 112.5mm and snap another picture. repeat this process to obtain at least 10 images.

PRISM has a built in scripting language that is both easy to use and very powerful, it is called PGM. I have obtained a great script that automates acquisition of the images required to measure tilt. The script can be downloaded here: Tilt Test Script

You can then execute the script by going to File > Execute Script or press F5 then follow the instruction in the log window.

Once the script is executed, the images are saved into a folder of your choosing. After this is completed, head over to PRISM and choose Analysis > Measure field tilt curvature

Load the images from the folder you choose as such:

PRISM will autofocus on 500 stars in your image and measure tilt and curvature

After the operation completes, you will get detailed measurements in the log window. You can actually know to the 100th of a millimeter how much error is in your system. This is the power of PRISM, analytics.





We publish frequent video tutorials to make sure our users are as successful as possible.

Aug 7th 2020 Hamza

Recent Posts