Original French version http://www.optique-unterlinden.com/howto/fcl90col.htm

[translated by Richard Lawler - I take no responsibility for any errors in translation nor errors in the original document. Follow these instructions at your own risk]

How to collimate the FCL-90

Using the same principles as other instruments

The FCL-90, travel-scope par-excellence, more than any other optics in the Takahashi line is at risk of being knocked out of alignment at some point following rough handling. When this happens there are two solutions: return the scope to your distributor for service or fix it yourself.

In the first case, you deliver your optical tube to us and we adjust it for you on our bench. The method we use is simple but beyond the capabilities of most amateurs: the optics are installed in an autocollimator (in front of a 400 mm diameter Zeiss plane mirror) and a white or monochromatic light point source transverses the optics 2 times simulating a nearly perfect star without atmospheric turbulence. This type of service can be ineffective in some cases due to uncontrollable nature of shipping. The vibrations could destroy precisely adjusted alignment.

In the second case, if you are resourceful and have the nerve we suggest you follow the method below in order to restore optimal collimation. This is obviously a delicate operation because of the F/ 5.6 focal ratio of the optics, that requires a lot of care and practice. The action on the 3 adjustment screws must be minutely adjusted using 1/8th turns of each key (1/16th turn toward the end). Nevertheless a knowledgeable amateur with previous experience collimating telescopes will have no insurmountable problems completing the FCL-90 collimation procedure. On the whole you have to just go for it, there is great joy in mastering the art of fine tuning of your telescope!!!

Step-by-step instructions:

(good for all the instruments, except for some minor differences) (lenses or telescopes do not have central reference marks)

The most effective method is to use a real or artificial star *.

- Install your scope on its equatorial mount, align the mount and start the motor.

- Once your optics have reached thermal equalibrium, point the scope at a star of average magnitude which shows a perfect diffraction image throughout the ranges of focus (both in-focus and out-focus). Choose a star near zenith to minimize atmospheric turbulence.

- Use a short focal length eyepiece (for example the OR-2.8mm for the FCL-90) to obtain a magnification close to 2X the diameter in mm (avoid using a barlow which can cause de-centering of the optical axis).

- Ideally it is better not to use a diagonal. But for practical reasons you may still want to use one. In which case in order to collimate on the real optical center it will be necessary to know how much you diagonal shifts the optical field off-axis. The real optical center is often shifted off-axis when viewed through a diagonal. It is essential that your equatorial mount be aligned and running in order to keep the reference star in place during the adjustments.

- If the image of diffraction is quite stable you can begin the analysis of the maladjustment. Locate the direction that the Airy disc is off center compared to the diffraction rings. Note the direction while repeatedly moving the focuser through the range of both in-focus and out-focus until you are sure of the actual direction of de-centering.

- Now tighten or loosen the adjustment screws to center the Airy disc. (** see the method below to determine which screws adjust in which direction).

While observing the objective with the Allen wrenches installed from the point of view of the eyepiece, tighten the screws located opposite from the direction to the de-centering of the Airy disc 1/8th turn: the result of this action will be immediately observable and the Airy disc should move toward the center. If it does not you must undo the previous step by loosening the screws on the off-centered side (the screws opposite of those that your have tightened). That will release the tension that is preventing the tightening action from working.

Observe the result after each action on the image of diffraction and repeat in the same manner until the Airy disc is perfectly centered in the diffraction rings.

- Having done this, very carefully observe the first diffraction ring, this one must be evenly illuminated over its entire circumference. As long as this is not the case continue the adjustments even more finely and gradually. Make sure the 3 screws are tight after each iteration because you can not tighten them after you have completed the adjustments. And if they are not tight your collimation adjustments will not last very long.

 

[photos here]

[caption]

FCL-90 in the autocollimator (or with a star at Zenith)

FCL-90 Collimation:

1st - unscrew the dew shield.

2nd - locate the 3 collimation screws.

3rd - use three 1.5 mm Allen wrenches.

4th - center a magnitude 1 star with using a magnification of 160x.

5th - adjust very finely the collimation screws (loosening and tightening).

6th - finish by perfectly centering the Airy disc within the diffraction rings which are evenly illuminated and without visible aberrations throughout the focus range -- both in-focus and out-focus. The 3 screws must be tight at the end of the procedure.

[illustrations here]

[upper left]

- objective cylinder is misaligned

- screw to loosen

[lower left]

- objective aligned

[middle]

focal plane

[upper right]

- collimation screw

diffraction images as seen through the ocular

[lower right]

cylinder seen against a grid

[link] to see whether it's necessary: information on diffraction and collimation

http://www.optique-unterlinden.com/pratique/diffract.htm [not translated]

* How to make an artificial star:

- Find a ball with a nice surface and a diameter of 4 to 6 mm from a ball bearing.

- Carefully place it on top of a long hex wrench (for example) clamped to a stable support, place the whole layout in front of a black cloth which will increase the contrast of image.

- Carefully place this assembly at minimum 50 meters from your telescope (100 meters if possible) (Theoretically 1000X the focal length of your scope in meters is needed for true infinity focus)

- The small disc of the Sun reflected in the ball will be your artificial star seen through the eyepiece of your instrument. In cloudy weather use a powerful halogen lamp located 500 mm from the ball

- The thermal radiation from the ground greatly influences the quality of the image obtained (so setup over grass lawn rather than asphalt if possible)

** How to determine which screws act in which direction (especially useful for a telescope):

Note the direction of offset of the Airy disc compared to the diffraction rings and from the direction which the star moves towards the edge of the field of the eyepiece with respect to the R.A. and Dec movements of the mount. It is necessary of course to start by loosening the locking screws or safety nuts before operating the adjustment screws. Collimation consists of centering the star by tightening the screw (or screws) on the side opposite from the off-center Airy disc. The one (or two) opposite screws must be loosened for the action to be effective. After many small iterative tries you must center the star in the center of the field of the eyepiece and perfectly center the Airy disk in the diffraction rings.