Collimating by star testWhen a Newtonian telescope is permanently mounted in an observatory, with an imaging camera in place,
it is very tedious to have to remove the camera and check the collimation with a laser collimator, Cheshire eyepiece and so on.
The careful alignment of the camera will be lost, and new flat frames will have to be taken.
A useful method is to aim at a reasonably bright star, and de-focus the camera by a considerable amount.
The resulting image will be something like this:
The three collimating screws (bolts, nuts) behind the primary mirror, can now be adjusted to
bring the hole to the centre.
On my scope I labelled the wingnuts 'A', 'B' and 'C'.
I found that with my telescope, wingnut C moved the hole to the right when rotated clockwise.
(This was with the focuser moved outwards. If moved inwards, I suspect the movement might be opposite).
B and C moved the hole as shown below:
When a nut is turned, the image of the star will shift dramatically. A quarter-turn moved it right out of the field of view
so only a 20 degree turn was made each time.
Maxim's 'Point telescope here' function was used to return the star to the centre of the screen.
This is very important - the star must be dead centre!
Keeping the star in the centre is quite easy once you realise that a shift of a nut will move the whole star image
in the same direction in which you are trying to nudge the central hole.
In the above image it looks as if a small anticlockwise turn of nut B is required.
When I had finished this operation I took an image and examined it with CCDInspector.
The collimation appeared to be perfect!
This was just a bit of luck.
I took another 5 images and CCDInspector gave this report on the group:
A collimation reading of only 1.0 arcseconds is most acceptable!
Some Newtonian telescopes have an offset secondary mirror. The effect of this is to produce
an off-centre central hole, even when the telescope is perfectly collimated.
I checked this using Niels Noordhoek's Maskulator programme. The image below shows a simulated diffraction pattern
produced by a perfectly centred obstruction, and one that is displaced.
The solution to this problem is to make a circular mask which will fit over the central obstruction.
The mask is made sufficiently large to completely cover the offset secondary mirror.
I made a simple one from a piece of cardboard.
Once the collimation is completed, the mask can be removed.
The cardboard mask