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My method for focuser collimation

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    Viking 1

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Posted 16 September 2020 - 01:51 PM

Hi everyone smile.png
Why the topic in this section?
Because this is a key issue for many astrophotographers.

With a truss telescope, we are in a difficult position when it comes to setting the focuser at right angles to the optical axis of the telescope. In fact, the top box of the structure can be in any position relative to the rest of the telescope. The presented description will be especially useful for owners of this type of telescope.

By collimating the telescope with the Autocollimator, we will obtain the perfect angle of the focuser optical axis and the primary mirror, but Pan Focuser is the Lord and Ruler, it is His position that determines the setting of the rest. So when the focuser is set incorrectly, the secondary mirror and primary mirror will align under its dictation curves in the collimation process. Therefore, there is a need to adjust the focuser as precisely as possible in relation to the axis of the telescope tube and the base mirror.
The Right Angle Axis collimator is something long, perfectly straight, fitted to the dimensions of our telescope, extends from the center of the cell without the primary mirror, to the center of the spider without the secondary mirror. To this perfectly simple element, we glue the mirror strip, the longer the better, because there is less chance that it will undergo a temporary micro-inequality of the base. I got the mirror from an old copier, but they are also available in other multifunction devices.
The collimation process consists in inserting a laser collimator into the focuser and positioning the secondary mirror in such a way that the laser dot hits the mark of the primary mirror perfectly. Then we take out the primary mirror and mark the place where the laser hits the cell. In this way, we determined the place where the Collimator would be supported in the cell. Then we mount our Collimator in the telescope tube, and through the Autocollimator mounted in the focusser, we observe the mutual reflections in the mirror strip.
A crooked focuser will give us reflections that move up or down.
A perfectly positioned focuser will give us an image of symmetrical reflections.

If you have an adjustable focuser, you can do everything with collimation screws, if you don't have one, you have to use washers placed under the focuser's corners.


Right Angle Collimator



before collimation


after collimation


Edited by GA-HAMAL, 16 September 2020 - 02:05 PM.

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Posted 16 September 2020 - 03:10 PM

Very Good! That will set 90o which is baseline nominal. One would then expect the nom focuser to be found ~square~ to the "tube". Folds other than 90o are of course also perfectly fine, provided one tilts the focuser to be coaxial with the PM optical axis, as seen via the folding flat. Either/any member of that extended family will perform the same.    Tom

Edited by TOMDEY, 16 September 2020 - 03:10 PM.

#3 calypsob


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Posted 18 September 2020 - 06:33 AM

Sounds awesome, i think a video demonstration would be very helpful! After re-reading this makes more sense.
Since the secondary is removed you could also possibly
Mount an optical flat on a block drilled to mount the hole where the secondary is placed.

Edited by calypsob, 18 September 2020 - 08:06 AM.

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