Starguide - A Practical Guide to Astro-Photography

Actual Step

5a: Real Star Collimation with Eyepieces

updated: 2024-04-17

Purpose

In the final stage of the process, the telescope's optics are refined using a real or artificial star. This enables the stars to appear as points again, rather than as distorted or elongated blobs.

Previous Step

Step 4: Check and correct Mirror Distance

Next Step

Gear Installation an Calibration (Northern Hemisphere)

Step

5.1: First rough collimation of the secondary mirror

5.1.1

Choose a bright star with magnitude 0 or 1

5.1.2

Do not use a star diagonal when performing this procedure. Instead, place an eyepiece directly into the 1.25" adapter in the focuser. It should be an eyepiece that provides moderate to high magnification (at around the same value as the telescope diameter, e.g. Ø 200mm => 200x). The eyepiece required for a focal length of 1624mm => ~ 8mm.

5.1.3

Defocus the star until it appears as a doughnut, as in the photo below.

5.1.4

The black center hole (which is the shadow of the secondary mirror) must be perfectly centered

5.1.5

If the black center is not 100% centered, adjust your secondary mirror to move the black center until it is perfectly centered.

(which is quite difficult with a real star and the telescope pointing up, you may need very long arms to manipulate the secondary mirror while looking through the eyepiece)

Step

5.2: Intra and Extra Focal Alignment (2nd Mirror)

5.2.1

Choose a bright star with magnitude around 2 and near the zenith (directly overhead) rather than on the horizon to minimize atmospheric distortion. Using Polaris as the target star can be helpful, as minimal drift adjustments are required.

5.2.2

Do not use a star diagonal when performing this procedure. Instead, place an eyepiece directly into the 1.25" adapter in the focuser. It should be an eyepiece that provides a high magnification (at around the 2x or 3x the value as the telescope diameter, e.g. Ø 200mm => 400x to 600x). The eyepiece required for a focal length of 1624mm => ~ 2.5 to ~ 4 mm.

Do not hesitate to enlarge the image as much as possible to better see collimation errors.

5.2.3

Center the star in the field of view. Slowly defocus the image with the focusing knob until you see a series of concentric diffraction rings forming around the dark disk in the center. This dark disk is the shadow of the secondary mirror. 

The star is slightly defocused back and forth (intra- and extra-focal patterns). A complex system of rings and a central white point appears (see figure below).

5.2.4

By slightly moving the focuser in and out the system of rings must open and close concentrically and symmetrically and the bright spot must always remain in the center.

5.2.5

If this is not the case (lower series), adjust the collimation screws of the 2nd mirror on the side of the asymmetry as in the first step.

Step

5.3: Focus Alignment

5.3.1

Stay with the star from step 5.2 (e.g. Polaris).

Note: This last step needs good conditions of turbulence. If no Airy pattern (figures below) can be discerned, no high resolution result can be expected.

5.3.2

Careful focus the star from steps 5.2.x

5.3.3

An Airy pattern (see figures below) should appear as a doughnut.

If no Airy pattern is visible, do not expect a high resolution result.

5.3.4

5.3.5

In a well collimated telescope (Fig. A), the diffraction rings should appear round and concentric, with the dark disk exactly in the center. If the dark central disc is off-center (like in Fig B - D), the scope is out of collimation. The worst misalignment (fig. D) represents only a fraction of turn of a collimation screw of primary mirror.  On this type of instrument, the transition from fig. A to fig. B represents less than 1/20th of a turn, a modification of the orientation of the optical tube can be sufficient to produce this effect. It is evident that the precision of alignment increases with each step. 

Adjust the collimation of the secondary mirror, and only the primary mirror if necessary, while observing the defocused star until the dark central disk is exactly centered in the diffraction rings.