Collimating a Schmidt-Cassegrain using an imaging camera and ‘AlsCollimationAid’

[This is just one of many articles in the author’s Astronomy Digest.]

These telescopes are not the easiest to collimate – particularly one as large as my 9.25 inch Celestron.  They can really only be done accurately by centring the shadow of the secondary mirror within the out of focus image of a star.  The problem is that, as a slight adjustment is made to the three screws controlling the position of the secondary mirror, the position of stellar disk moves.  Manually altering  the telescope pointing to bring the disk back into the centre of the field of view  is not easy and it would be ideal if this could be achieved by using the slew commands of the mount supporting the telescope.  The problem with using an artificial star is that the mount will be siderealy tracking and so the artificial star will move out of the field of view.   So a real star makes this easier, but the atmosphere will degrade the stellar image unless the seeing is very good.

The optimum strategy is to move the stellar disk to the edgeof the field of view in the direction where the secondary shadow is closest tothe edge of the disk.  One then adjustsone of the three adjustment screws to bring the disk back towards the centre ofthe field.  The offset of the shadowshould then be less.  This procedure isrepeated until the secondary shadow is centred within the disk.

Having had to remove the corrector plate of my 9.25 to cleanits interior surface I found that, not surprisingly, when it was replaced thetelescope was way out of collimation. I made a couple of attempts to visuallyobserve the out of focus image of an artificial star with telescope positionedon a fixed mount moving each time to the front of the telescope to adjust thecollimation screw.  This really did notwork for me.

I realised that if I were tracking a ‘real’ star, I could use the telescope’s fine slew commands to adjust the position of the stellar image and that the ease at which the collimation process can be done would be greatly helped if a camera is used to image the disk with the screen of the laptop controlling the camera located where it could easily seen from the front of the telescope. Thus, adjustments can be made whilst observing the stellar image and mount adjustments could be made using the fine slew commands. 

During the evening of September 16th 2021, Altair was at asuitable elevation – high enough so that the atmosphere would not affect theimage too much but low enough so that the collimation screws could be easilyreached.  Obviously one cannot use a dewshield and so, as it was a dewy night, after a while I had to use a hair drierto remove the dew from the corrector plate. An Altair Astro 294C colour camera having Micro 4/3 sensor was used toimage the star.  The relatively largesensor meant that fine adjustments to the collimation screws did not move theimage out of the field of view.

This process made collimating the SCT far easier and thefinal adjustments were made using a neat software program called‘AlsCollimationAid’.  When opened, thisproduces an overlay that can be placed over the image of the stellar disk withtwo circles; one to surround the disk and the other the shadow of the secondarymirror.  Their sizes can be adjusted asthe overlay is centred on the stellar image. This makes it very easy to seewhen the telescope is perfectly collimated.

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

The image below shows the overlay when the collimation process was nearly complete.

© Ian Morison