The precise collimation

  • Posted on: 29 January 2021
  • By: tihomiry

When we have a new newton telescope we shroud avoid touch the collimation of the secondary mirror as we may get into troubles.
Good collimation is very important if we would like to have perfect high resolution images. But sometimes brand new telescopes may come misaligned.
So this is a short guide how to do a precise collimation. Unfortunately the best way to collimate is to disassemble the whole telescope.
You best do this when cleaning up the mirrors. Use optical collimator eyepiece. Laser collimator is not enough to ensure correct collimation as there is a
position in witch the secondary can be tilted and still beam compensated by the primary return back at the centre. You will best find that, when observe a
diffraction image of a bright star with high magnification 200x+ and it will show a bit comma. You can use laser in addition for more precise alignment after
using a collimator eyepiece. Best way to check if the telescope is collimated is to observe a star just in-front and after the focus with high magnification.
You should see good ring under good atmospheric seeing. But if you have a secondary mirror tilt, you will see it only into the diffraction image of a star.

So here is the procedure. Consider you had removed the primary mirror for washing.
1. Print a sheet with concentric circles and put the tube vertically on it so you can precise align the secondary first.

target1

2. Check for secondary axis rotation with the collimator eyepiece! You should see the secondary mirror well concentric to the focuser tube and the printed circles.
This is the key for the alignment as if there any rotation or tilt, you will not able to correct it after putting back the primary mirror.
If you need to do correction first loosen the rest of the screws then do rotate the centre bolt. Be careful with the alignment screws do not over tighten them,
as they are getting easily cracked. To tighten one you need to loose a bit the rest 2.
3. Make sure that the the collimator eyepiece cross or the laser beam hits the centre of the sheet with the concentric circles.

target2

4. Put the primary mirror. Look again trough the collimator eyepiece. You have again 3 screws that define the position of the mirror and 3 that tighten it.
5. Make sure that the centre of the primary mirror, the cross of the collimator eyepiece and the shadow of the hole you look trough, exactly match. Do not
touch the secondary mirror as we had already verified in step 3 that it is well aligned.
6. Now you can put the laser collimator to adjust this even more accurate and make yourself more conformable as for the larger newtons you are not able to look and reach the screws of the primary at the same time.

Laser complicators do not work with cassegrain telescopes, because of the hole in the centre :) The beam never touch the primary. Put a flash light into the eyepiece holder and project the light. By mesaringsaring the shadow you can do the collimation. Then you can use a real star for more precise adjustment.

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Witch is better. Stack or long exposure? It depends…

NGC 7000, South America nebula in Cygnus, taken by me from NAO Rozhen. 2 shots composed by 45 frames @ 30s, ISO 51200 with SW 200pds and Pentax k-5. Same processing with DeepSky Stacker and LightRoom.

To check the polar alignment of your telescope look trough the polar scope. When turning the Ra axis the polar star should move in the exact circle in the eyepiece without any deviation.

Removing the DSLR build in IR-cut filter will give you at least 50% more sensitivity for hydrogen alpha emissions of the nebulas. Also the camera will be more light sensitive at dark allowing you to shoot more faint objects.

We can check what is like to process day shot with similar to astrophotography techniques. It is cloudy in Bulgaria for the next week or so and I took shots of the nearby mountain trough the fog with 200mm lens.

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