Tutorial for Collimating a Fast Newtonian Telescope

Started Oct 19, 2015 | Discussions thread
RustierOne
OP RustierOne Veteran Member • Posts: 4,401
Re: Tutorial for Collimating a Fast Newtonian Telescope

swimswithtrout wrote:

RustierOne wrote:

This thread is in response to a request for a "quick and easy" method for collimating an f/3.9 Newtonian. I'm afraid the process is not necessarily quick or easy. But all these steps are necessary, particularly as the f-ratio gets smaller. There is a large difference in difficulty between f/6 and f/5 - more so between f/5 and f/4. Often the difficulty lies in getting the secondary mirror correctly positioned before even trying to adjust the primary mirror. I would suggest the following.

You must get the secondary mirror adjusted correctly first!

For the secondary mirror:

  1. Accurately mark center of the primary mirror with a black spot or better yet a doughnut (like those stick-on reinforcements for a paper-punch hole).
  2. Mark the center of your secondary mirror using black thread held along the edges of that mirror with tape, forming a black thread crosshair at the mirror's center.
  3. If you want to apply secondary mirror offset, you can offset the center mark by the amount given in the link above. I don't worry a lot about secondary mirror offset since the only consequence of not doing so is that the fully illuminated FOV is not exactly centered on the focuser tube. But with a very fast f/3.9 system, you might want to apply the correction.
  4. Using a Cheshire eyepiece sight-tube, adjust the secondary mirror so that the mirror's thread crosshairs are aligned underneath the eyepiece crosshairs. This adjustment may require moving the secondary up or down the optical tube along with adjusting its tilt.
  5. Ensure that the center of the primary mirror is aligned with with the secondary mirror's center mark. This is accomplished only by adjusting the secondary mirror tilt. Ignore the reflection of the secondary mirror in the primary mirror (i.e don't adjust the primary mirror at this stage).

At the end of steps 4 and 5, the secondary mirror's adjustment will be correct only if both of the following are true:

  • The Cheshire eyepiece crosshairs are pointed at the secondary mirror's crosshairs.
  • The center of the primary mirror (seen reflected by the secondary mirror) lies behind the crosshairs of the slight-tube and of the secondary mirror.

If the above two conditions are not met, go back to steps 4-5 until they are achieved. Once they are correct, you can ignore the secondary mirror from now on. Just ensure that all the adjustment screws for the secondary mirror are tight to avoid problems in this area due to something moving when transporting the optical tube. It might be good to check the secondary mirror's adjustment from time to time. But if it is well locked in place, your future collimation efforts will be confined to adjusting the primary mirror.

Now for adjusting the primary mirror:

  1. Remove the thread crosshairs and tape on the secondary mirror.
  2. Adjust the primary mirror so that the reflection of the secondary mirror lies behind the crosshairs of the Cheshire sight tube.
  3. Under the stars, look at a medium brightness star (out of focus) at medium power - best with a reticle eyepiece that has the center of the FOV identified with crosshairs. Further refine the primary mirror's adjustment by centering the shadow of the secondary mirror in the bright out-of-focus disc of the primary mirror. This determination must be made while the star is centered in the FOV.
  4. Using high power center the out of focus star in the FOV. Adjust primary mirror to center the secondary's shadow when the star is at the center of the FOV.
  5. Progressively bring the star toward focus, while continuing to tweak the primary mirror to center the secondary mirror shadow in the bright disc.
  6. As you continue doing what is described in steps 4 and 5, just before focus is achieved, you should have a small bright doughnut with a tiny black spot exactly centered. It is important once again that the star be at the center of the FOV. At this stage it is easy to spot very slight offsets of the black spot. The thin doughnut of bright light must be of the same thickness all around.
  7. When the star is in focus, there should now be a nice star image with no extraneous spikes poking out at odd angles - just the 4 spikes from the secondary mirror spider. If you didn't do step 1 above, there could be some extra spikes due to the tape and threads on the secondary mirror.

Yes, there is no "quick and easy" way to collimate a fast Newtonian telescope. This is particularly so if the initial collimation is way off. Also a very fast f/3.9 system is particularly difficult to collimate compared to even an f/5 system. But if you follow the steps above, you'll achieve good collimation. Just make sure you do each step in sequence as described, before moving on to the next step. I'm afraid that is the price one must pay in reaping the benefits of a comparatively low-cost, fast, wide angle optical tube.

I might add that you can also use a laser collimator in lieu of the Cheshire eyepiece. Just follow the directions supplied with the laser. For this method to work well, you must again ensure the secondary mirror's adjustment is correct before moving on to the primary mirror adjustment.

If anything is not clear in my tutorial above, let me know and I'll try to clarify. I wish you success in your efforts. Also you can refer to the following link for good advice on collimation:

How To Align Your Newtonian Reflector Telescope

Thanks Russ ! With any luck, my OTA is now a lot closer. I'm waiting for the next clear night to check things out.

I hope the tutorial has made things better. Your images are great even with a few whiskers on bright stars. We're looking forward to more of your excellent photos!

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Best Regards,
Russ

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