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How to Identify Telescope Optical Tilt in 3 Easy Steps



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Telescope Optical tilt is one of the most complicated issues amateur astronomers can face in any type of telescopes. This is easily identifiable in PRISM and can be fixed using some simple tools and techniques.

What Do We Mean by Optical Tilt?

Telescope collimation is a technique used to align of all the optics to the focal plane of the detector. There are numerous tools to helps us perform this including lasers, Cheshire eyepieces, autocollimators and even live star alignments.

It is no secret that your imaging train is susceptible to the slightest amount of optical tilt, this can be caused by thumbscrews, crocked/crossed threads, out of round elements and more.

But how do you figure out how much optical tip do you have in your telescope system? How do you go by figuring out in what direction do you counteract it?

The goal is to get stars across the field that have the same “roundess” or “ratio”, this will make for a “flat field” and produce quite pleasing images. In other words, aims for an elongation less than %20 to %30. It is hard to perceive elongation that is lower that %20.

in PRISM, a star’s roundness is expressed as percentage ( e.g: %9 ) difference between the long and short axes as depicted in the image below. The angle is also a helpful measure when talking about optical tilt.


Find Out If You Have Tilt in Your Imaging Train in 3 Easy Steps

One of the most advanced functions in PRISM is the tilt/curvature analysis tool. It allows us to narrow down the the a tenth of a millimeter.

It is important to understand how much and in which direction to tilt the system using the primary, secondary, tip/tilt ring or even the focuser in some telescope types.

To get started, from the menu on the top, click on Analysis > Measure field tilt and curvature.

1. Prepare Your Images

We are presented with a popup and will need to enter some images.

  1. Calculate the focal ration of your telescope using this formula: focal ration = focal length / aperture.
    1. Example 1: Refractor with a 530mm and an aperture of 106mm with have a focal ratio of 5
    2. Example 2: a Newtonian telescope with a 1600mm focal length and an aperture of 400mm ( also called 16 inch ) will yield a focal ration of F3.9
  2. Calculate the required focuser “offset”. Use the focal ration you figured out in step and multiply it by 30x
    1. Example: The focuser shift for the Newtonian in example 2 above would be F3.9 x 30 = 117mm.
  3. Focus your system using the automatic focus or direct focus routine in PRISM. Or, you may use a focusing aid such a bahtinov mask.
  4. In the focuser controller window, enter the 0.117mm number in the focuser and take an image. Rinse and repeat.
  5. It is helpful to save theme images in a folder that is easy to find (Example: On the desktop).

2. Load Your Images in the tilt/curvature tool

Now it is time to open the tool and load the images collected earlier. Head to the top menu then click on Analysis > Measure field tilt and curvature.

The tool open with some defaults settings, it is highly advised to use it as and just load your images. PRISM will read the focuser position from every image by extracting the information from the FITS headers.

PRISM v10 analysis menu, tilt and curvature analysis tool.a function to measure your telescope's optical tilt

3. Analyze the Output Graphical Representations

After a few minutes of processing, two images are displayed. The display switched to a rainbow histogram to clearly show the optical errors analyzed.

This image shows your telescope’s optical tilt and its direction, you can easily see

Telescope optical tilt represented in a graphical representation

The next image show the amount of curvature calculated.

Telescope optical tilt represented in a graphical representation

A Few Ways to Correct Tilt in your Telescope

Thankfully, there are few very effective solution to fix your telescope’s optical tilt. Let’s start from easiest to more complex

  • Verify all the suspected “flexible” areas. For example: thumbscrews, set screws, threads, the squreness of your focuser.
  • Adjustements are also possible on the front of some cameras such as ZWO cameras.
  • Use a tilt adjustment ring for M42and M54 threads. Larger diameter tilt adjusters are also available : M68 tilt adapter


Telescope optical tilt can be present right out of the box in a brand new telescope, this is easily remedied using collimation. But in some exceptional cases, it can also manifest itself in the camera chip or housing as well.

The main piece of advice we give out is to always make sure you use the uttermost care in installing your components and to always opt for a solidly threaded connections versus thumbscrews. Use high quality nosepieces and prefer self-centering components.

With the help of PRISM’s incredible tilt identification tool, one can quantify, understand and develop an attack plan to remove tilt and start producing exquisite images that are flat corner to corner.

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