Actual Step

Step 3a: Secondary Mirror Collimation using 
OCAL Electronic Collimator PRO

updated: 2024-05-18


Aligning the secondary mirror with the optical axis of the telescope

Previous Step

Step 2: Check and correct Mirror Spacing

Next Step

Step 4: Adjusting the Primary Mirror


M42 x 0.75 female threads are located on OCAL after opening the top cover. We recommend the following methods for initial OCAL setup.

  1. If your focuser has an M42 x 0.75 male thread, simply screw OCAL directly onto the thread on the focuser directly.
  2. If there is no thread at the end of the focuser and it uses a compression ring or screw fixed brass ring, please use some kind of extension adapter, such as 2-inch to m42, screw OCAL to the coupling threads of the coma corrector lens, and insert the coma corrector lens directly into the focuser. This method will achieve more precise collimation, but it will cause the image to be blurred. 
  • If possible remove the internal baffle on the main mirror that blocks obliquely incident light (this is possible e.g. on the TSO 203/1624 RC PRO)

Please choose according to your own situation.

Focus position 
(Eye Point Distance)

We define the distance from the secondary mirror to the OCAL as the Eye Point Distance

If the concentric circles have too little distance to be able to judge the alignment well, it is possible to work with spacer rings. This means that the individual circles have more distance between them, which makes it easier to assess the alignment.

The Eye Point Distance distance point of image A is shorter than that in image B. We will see a large gap between the red and green circle.

This means that the red and green circles can be easily distinguished. 

Note that when we are closer to the actual focus point (B), the distance between the rings becomes smaller. This makes adjustment more difficult, but also more accurate. 

To get the best result from your collimation, Type B is recommended.

Attach a White Paper

or place a flashlight inside the telescope 

Tape an appropriately sized piece of white paper behind the secondary mirror on the side wall of the telescope tube. This will make the shape of the secondary mirror easier to observe.

or place a flashlight in the telescope tube to illuminate the back of the OCAL camera at the end of the telescope so that the black dot of the camera sensor is clearly visible

Enter Calibration Code

  • Each OCAL collimator has a center calibration code. First look for the printed serial number on the back side of your OCAL camera (e.g. SN = 1B1716)
  • Download the actual OCAL center calibration codes OCAL2.0 Center Calibration Code.xlsx or goto the OCAL page: to locate your calibration code file.
  • Open the excel file and search for your OCAL SN number
  • Locate and open the focus.txt file in the OCAL installation folder: (in my case, it's on: drive:\Kits\astro\Collimation\Ocal)
  • Paste the Center Calibration Code (0 0 1620.35 1247.94) into your focus.txt file. Save it.

Start the OCAL App

  • activate the camera ("Turn on Camera")
  • Click "Camera Control". Try to adjust Focus and Exposure to suitable value.

OCAL Settings

Use these settings (for the actual RC telescope)

Camera Orientation

Prior to the collimation of the telescope, we will define the orientation of the telescope for better understanding.

Center Offset Function
Before  adjustment of center offset

After adjustment of center offset

Activate the green circle

Due to manufacturing and telescope adapter tolerances, the OCAL program reticule may not be perfectly centered; we use the Center Offset function to make a slight adjustment.


The adjustment method is very simple. For RC telescopes, use the outer edge of the primary mirror (A) and align the green circle with it. 

IMPORTANT:If the green circle has moved out of the center after adjusting the mirror, always recenter the circles by keeping the green circle on the outer edge of the primary mirror!

Inital Center Settings

Then use it as a reference to adjust the offset value (Save Config).

Misaligned Secondary Mirror

This image shows the initial collimation state of the telescope. Both mirrors are currently out of alignment.

The aim of this step is to bring the small black dot (this is the reflection of the OCAL camera lens) exactly into the center of the red area. The red area in turn is the reflection of the OCAL camera housing. In order to see this clearly, sufficient light must fall into the telescope from the front. This is best achieved by placing a piece of white paper in front of the telescope at a distance of 10 cm so that the collimation screws of the secondary mirror can still be easily reached, and illuminating this paper screen from behind.


  • Use the 4mm Allen wrench on the 3 push/pull adjustment screws X and Z (don't touch the adjustment screw Y to avoid mirror distance changes!) of the secondary mirror to move the black spot to the center.
  • When finished , carefully retighten the center screw if necessary.

Well Aligned Secondary Mirror

When the adjustment is complete, the black dot should be right in the center of the dark circle of the 2nd mirror center mark.

IMPORTANT: If, after adjusting the mirror, the green, blue or red circles have moved out of the center, as in this picture, always recenter the circles by keeping the green circle on the outer edge of the primary mirror!


When finished, carefully try to retighten the center screw if necessary. Check that the mirror is still in place.

Next Step

Step 4: Adjusting the Primary Mirror

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