This method of collimation is suited to Schmidt Cassegrain style telescopes (or "SCT"), with one adjustable mirror [secondary mirror] and one fixed mirror [primary mirror]. See the image below for a basic layout...

Fig 1.

One mirror adjustment refers to the secondary mirror being adjustable, and the primary mirror being fixed. To some extent this method will work for systems with adjustable primary mirrors too, however some iterations of the process is necessary and will not be explained here.

To say the least, this is a very simple method, with no analysis software required that measures FWHM, mirror tilt, field flatness, etc... If you feel inclined to do these measurements, you can do a before and after sample to check what the result was. All you need is you normal capture software to make the analysis and changes.

I guess many people are familiar with the single defocused star technique for collimating a telescope, and I admit this multiple defocused star technique is a clear variant of that approach. If you are not familiar to using the single defocused star technique, it involves the centring of a single bright star upon your field of view [or FOV], then you defocus the star to make a large donut shape like shown below.

Fig 2.

To correct your collimation, you adjust the tilt of the secondary mirror until that star appears nice and concentric [as shown above]. Some people expand the process by moving the defocused star into the four corners of their FOV to see if the star remains concentric at all positions [to try and ensure flatness across their FOV], see below...

Fig 3. Four separate images, placing the defocused star at each point to gauge FOV flatness

But there is a problem with this approach, in that you cannot easily check that the collimation centre point is centre to your FOV, as you are only taking images of a single defocused star at one time. In fact, you may later find that the collimation point is off centre. This means, the stars in that off centre position are most round, and then stars to the opposing side of the FOV will be more out of collimation. The above image (Fig 3) also assumes you have a near perfectly flat field, meaning that wherever you place the star, it will appear nice and concentric. Well, most telescopes will not have this luxury, unless you are using a relatively small pixel camera to the telescopes imaging circle. For example, the flatness of an image taken with a GSO 10" RC and a camera with the KAF8300 chip will be flatter than one taken with the KAI11002M chip.

Fig 4.

Collimation of a telescope can be a challenging task, especially for astrophotography with the precision required. You really need to do the collimation through your camera to get it right. Using alternate methods should be finally done with the camera of your choice through your telescope. I know some are happy with visually doing their collimation, so that is their choice.

After trying many methods, some using software to assist, I found a simple answer was to image an open cluster with the stars defocused. Then analysing the images to detect if the centre collimation point existed in the FOV. With the open cluster nicely centred in your FOV, you need to take a 10-20sec exposure to illuminate the defocused stars sufficiently at say 4x4 binning. You will need to test for the right exposure length, but just make sure not to over saturate the defocused stars. The aim is to have a nice spread of stars evenly illuminated across your FOV. Between adjustments of the secondary mirror, you need to make sure you allow your telescope sufficient time to settle before reviewing the next image. Immediately you can see where the current collimation point is, as the central collimation point will be represented by defocused stars being concentric, whilst the surrounding stars will point to this central point, elongating towards the central point. See the image below, taken with a STL-11000 on a Planewave 12.5...

Fig 5. Click on to enlarge

Just to reiterate that even on a very flat fielded scope like a Planewave, you can see stars across the field are not all concentric. It is easily definable where the central collimation point is, and I have marked it out on the following image..

Fig 6. Click on to enlarge

The idea now is to move this centre point of collimation to the centre of the FOV. Here you will have to experiment to see how the image moves when you alter your secondary collimation screws. What I did, was that I made a guide sheet to help me remember what way the image moves when I move my screws in or out (your camera must be in the same orientation every time relative to the top of the optical tube). It makes future adjustments faster to complete. See mine below...

Fig 7. Click on to enlarge

Every time you make an adjustment to the tilt of your secondary mirror, you need to re-centre the open cluster to your FOV. So keep your mount's hand controller close at hand while standing at the front of your telescope. It is therefore also important to be able to see you computer screen during this process to ensure you are moving your secondary in the correct direction.

Which way to move the secondary mirror?
When looking at your first images, you need to determine where the central collimation point is, and where it needs to move to? As a simple rule, you need to look at the defocused stars and their shapes, as this is the key to which way to tilt your secondary. Look above again to my guide sheet. Here I have a picture of a defocused star at the bottom, elongated to the one side. To remove the elongation, and to get the star concentric, you need to move the star in the direction of the fat side of the donut (as indicated in my guide sheet).

Now, you will be faced with stars elongating in all directions depending on where they are in your FOV. You need to pay attention to the centre of your FOV. In MaximDL, you can turn on a cross hair to find this centre point. Here is where you must study the star shapes. Make your assessment, which direction the stars must move, and then set your capture software into continuous capture mode. From the front of your optical tube, make your secondary mirror adjustments, and watch which direction the star pattern moves. Re-centre the open cluster with you mount controller, confirming position off your computer screen and the last image taken.

Once the stars have moved a test distance, check you are going the right direction and how big the movement was per the adjustment. Then review where the centre collimation point is. Did it move towards the centre of the FOV? Using this new information, continue until you feel confident the centre collimation point is aligned to the centre of the FOV.

Below is a near correct collimation, with concentric defocused stars existing at the centre of the camera's FOV, with outer stars elongating or pointing to this centre point. I did this quickly, within 10 mins, just for a demonstration...

Fig 8. Click on to enlarge

How to do the process again...
To make the process a quick procedure, I found that once you have the telescope on an open cluster, to:

1. Angle your computer screen towards the position you can see it from the front of your telescope [where you can alter the secondary mirror alignment screws].
    
2. Set your capture software to capture consecutively sub exposures of 10-20sec [depending on your binning rate and your optical system].
    
3. Have your telescope controller close at hand, so you can realign you image after a movement of the secondary, as it is important to keep the stars reasonably well spread across the FOV.
    
4. It is good to save you images to a set directory so you can analyse these once you have finished.
    
5. At the end of the process, refocus the telescope and image the open cluster at critical focus to see if stars are nicely pin point across the FOV.
    
6. If all looks good, you are finished.

Lastly, seeing conditions have little effect on this method. This is a great benefit. Of course, you should take a number of images just to be sure variation between shots is acceptable.

So give it a try, I think you'll be pleasantly surprised.  

Steve Mohr