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Mirror Stability Modifications & Collimation Retention


laser_jock99

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Fed up of the ever shifting primary mirror in my 12" F4 imaging Newtonian I have now done a series of mods starting with the 'six spring mirror mod' previously described here:

which effectively controls any excess movement in the z-axis (up & down the tube). However- examine the average Chinese mirror mount and you'll find there plenty of scope for the mirror to shift around in the x-y axis. Chief culprit is the poor design and fit of the mirror adjuster screws- just way too much slop for a weighty mirror.

The 'slop' room around a standard adjuster screw.

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Now these screws are 'meant' to move somewhat to allow for angle changes when adjusting the mirror. But I think the loose fit bushes are just as likely to slide under the weight of the mirror so collimation is lost. The engineering answer is a conical engagement surface so there is no possibility of 'mirror slide' and yet still some angular play.

So I made a new adjuster screw with conical engagement face (arrowed) pictured beside a standard screw.

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The new screw sits nicely into a conical recess made with a countersink tool in the mirror cell.

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So now the mirror can't slide very much in x-y on the adjuster screws- but since these screws are longer than standard after moving the mirror up the tube by 10mm I decided to add some extra x-y stability. This was done by adding 6x M3 nylon screws from the tube wall to the mirror cell.

First stage is to drill six holes for the Rivnuts in the main tube. The yellow tape protects the tube paint during drilling and foam block is to support the tube.

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The Rivnuts are set using a tool (borrowed from work!)

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nice mods. could you have also drilled a tapped the side face of the mirror clips and added some nylon grubs to retain the lateral position? or even blobs of silicone on the mirror sides adhering to the clips?

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One of the M3 Rivnuts in place inside the main tube.

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How it looks with an M3x25 nylon screw holding the mirror cell (six in total around the tube)

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Finally the mirror cell with the finished 'six spring mod' and cone engaged adjuster screws in place (extra springs replacing the 'locking screws' arrowed).

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Now all I need is some clear nights to test the theory!!

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nice mods. could you have also drilled a tapped the side face of the mirror clips and added some nylon grubs to retain the lateral position? or even blobs of silicone on the mirror sides adhering to the clips?

There's probably several ways of doing this- but the tube-to-cell gap is quite large around 23mm.

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The only quick stability test so far:

Collimated scope and imaged M8 in Sagitarius low on Southern horizon:

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Slewed scope through 90 degrees of altitude and 3h of RA to IC1396 in Cephius

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There doesn't appear to have been too much collimation shift in this limited test, bearing in mind I'm working at F2.9? Certainly less than I normally get - but I think more testing is needed.

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