Experiences with collimating a Ritchey-Chrétien Telescope

In September 2021 I acquired an 8 inch GSO Ritchey-Chrétien.  These are sold under a wide range of brand names but are all optically identical.  Its previous owner had produced a document on collimating one of these telescopes so I had hoped that it would have been in perfect collimation.  Sadly this was not the case as I found out when I did some first tests.  The stellar images were pretty terrible, but I was able to make a flat frame which pleasingly showed that, using the Micro 4/3  sensor camera I am using, there was virtually no vignetting. 

It is generally regarded that Ritchey-Chrétien telescopes are the trickiest to collimate and there are many articles and YouTube videos as to how to carry this out.  The problem is that there does not seem any real consensus as to how to do it and what tools are required.   One possible reason is the fact that there is known design flaw in the solid tube designs in that the focuser assembly is attached to the mirror cell.  This means that if one has to adjust the primary mirror after having correctly adjusted the secondary – which is the first step – the initial secondary alignment will be upset.  I thus believe that, if the primary needs to be adjusted, one need to take an iterative approach to achieve perfect collimation.

I found an excellent pdf manual relating to the Orion Richey-Chrétien telescopes which describes how to collimate them.  All these Richey-Chrétien telescopes are made by Guang Sheng Optical (GSO) no matter which of the many brand names they are sold under.  Mine is a First Light Optics, Stellar Lyra, telescope.  The link is:

https://www.telescope.com/assets/product_files/instructions/29361_06-15.pdf

I essentially followed these instructions but carried them a bit further as will be described.

On Nov 2nd 2021 an excellent YouTube video was posted that explains exactly what I had finally found worked.

https://www.youtube.com/watch?v=oYZJsOc-vRg

The collimation is done using a ‘Cheshire Eyepiece’.  This has a sight hole through which to view the interior of the telescope which passes through a 45 degree ‘mirror’ which is used to reflect light into the telescope so one can see its interior.  Mine does have cross wires which can help when collimating other telescopes but are a slight annoyance for this application. [Angle the side slot, perhaps toward a window, to get plenty of light into the telescope tube.]

As the instructions describe for this collimation procedure, the focuser should be attached directly to the rear of the telescope without use of the extender rings.

When I looked through the Cheshire Eyepiece I could see that a black spot was not in the centre of  visible image; surrounded by  concentric rings of light and dark.  (I believe that this the sight hole of the Cheshire Eyepiece appearing black within the reflecting, 45 degree,’mirror’ surface.)  As I was sure (as the instructions indicate) that it should lie in the precise centre I adjusted the secondary mirror tilt using the three adjustment bolts and, finally, was able to bring the black spot dead centre.  

The instructions say that one should then adjust the screws controlling the primary mirror after first unscrewing the smaller locking screws. These are adjusted until the view through the Cheshire eyepiece is perfectly symmetrical.  One can see 4 square mounting points at the outer ends of the secondary spider and these should be equally visible. I did see a faint outer ring – marked ‘Optical Axis’ in the Orion Image and this should of uniform thickness – it appeared thicker than in the diagram.

This is where I added to the Orion instructions.

As I had expected, the primary mirror adjustments very slightly affected the view of the central dark spot so I then made some further, very minor adjustment, to the secondary mirror.  I did suspect that I might then have to make some small final adjustments to the primary mirror followed by a further tweak to the secondary – an iterative approach.  In fact no further adjustment to the primary was required.

So this is then an ‘iterative’ procedure which I have only seen described elsewhere in the YouTube video linked to above.

I was able to carry out a further test using what was described as a ‘Next Generation Laser Collimator’. They appear to be available from a number of suppliers including Amazon from around £30. The body is 1.25 inch diameter, but it comes with a 2 inch adapter as seen below. It has 7 intensity levels but I used it on the brightest. If all is well, the returned laser beam should ‘disappear’ back into the hole from which the laser beam was emitted from. This was virtually perfect so I felt that I had been able to achieve a near perfect collimation.

Rather than making any further adjustments to the scope at this point, I then wanted to do a ‘star’ test to see how an out of focus star appeared and see if adjustments to the secondary would improve it.  The forecast implied that there would be no clear skies in the immediate future so I carried out a test in daylight imaging the branches of a distant tree.  It turns out that small gaps between the leaves form artificial stars – though moving around in the wind!  I was first able to determine the focus point of the camera – well out from the back plate of the telescope so requiring all three of the spacers (2 of 25 mm length and one of 50 mm length) which are supplied to allow the focuser to be placed away from the back plate so that any further extension needed to gain focus is minimised to avoid camera droop.  However, I still needed to use a 2″ focal extension tube beyond the focuser to obtain focus.  The in-focus view did appear to be quite clear showing that the collimation could not be too far off and the wavering, out of focus, ‘stars’ seemed to be symmetrical.

The final collimation test along with, perhaps, making some minor adjustments to the secondary, can be made using an artificial star or when tracking a real star at a reasonable elevation when the seeing is good.  One can purchase artificial stars or make one by covering a LED light with silver foil and making a very small pinprick.  As these scopes are generally used for astroimaging, most owners will have a camera and it is then best to use this to produce an image of the out of focus ‘star’ and observe it on a laptop screen facing the front of the scope so that the effects of any small adjustments can be easily seen and one can easily reach the adjustment bolts.  However, as each adjustment moves the star’s image away from the centre of the field, one will need to manually adjust the telescope’s pointing.  If one is tracking a star it should be at an elevation of 45-60 degrees so as to give as clean as possible image but also enabling one to easily make the adjustments.  [Note: the video suggests a star at very high elevation.  There are two problems; the first is that with a big mount, as I have, one may not be able reach the adjustment bolts on the secondary mirror and the second is that the adjustment allen key could easily fall into the telescope and hit the mirror!]

 

One advantage of using an image on a computer screen is that one can then overlay the stellar image with a neat software program called ‘AlsCollimationAid’.  When opened, this produces an overlay that can be placed over the image of the stellar disk having two circles; one to surround the disk and the other the shadow of the secondary mirror.  Their sizes can be adjusted as the overlay is centred on the stellar image. This makes it very easy to see when the telescope is perfectly collimated. 

To get this nice piece of software, go to this website and click on the download link which is at the very bottom.

http://sweiller.free.fr/collimation.html