Collimating the 8 inch RC

After discovering that my 8 inch RC was out of collimation ( read previous post ) I thought I’d run through the steps needed to re-align the two mirrors. The first thing to do is to dismantle the focuser end of the telescope to gain access to the primary mirror, this was because when I washed the mirror I had to remove the centre light baffle tube – this has a thin rubber O ring that needs to be seated correctly on the baffle, if not then the mirror will not be secured in place properly and will have a certain amount of play.

Remove the top and bottom Losmandy plates and the Radius blocks from the focuser end only, undo the two remaining cross head screws and gently lift of the mirror housing, do not drop and place on a flat surface. Unscrew the centre baffle tube counter clockwise and check the rubber O ring ( do not touch the mirror ), it has to be sat in the groove and not on the threaded portion of the tube. Re-assemble the telescope in reverse order.

The Howie Glatter laser collimator I use is quite expensive costing about £200 with attachments, but it is the only reliable method to collimate these RC scopes because of the two Hyperbolic mirrors, Newtonian telescopes on the other hand have a concave primary and a flat secondary. Now the safety bit, Never look directly at the laser beam or eye damage and blindness may result.

Insert the collimator into the focuser and only just slightly tighten the focuser thumbscrews so that it turns freely in the focuser without being loose. using the tight beam attachment that screws into the end of the laser, get the red laser dot of light onto the secondary mirror centre spot ( it marks the centre of the secondary mirror and is essentialy a paper disc with a hole in the middle just like a ring binder reinforcing ring. do this by adjusting the primary mirror collimation screws ( there are three sets of two screws and work in a push/pull fasion ) only make small adjustments to get the laser on the spot, this can be seen from the front of the telescope and looking at the reflection of the secondary in the primary.

When you have the laser in the centre of the spot or ring, you then look at the reflected laser spot and see where it falls on the white disc at the end of the laser, you will probably see two red dots this indicates the misalignment of the secondary to the primary, move the outer red dot to cover the centre red dot by making small adjustments to the secondary collimation screws arranged in a triangle around a larger centre screw.

When this is done it’s time to change the laser attachment to the Holographic ring projector, this as the name suggests projects a series of concentric rings down the telescope tube to the secondary mirror, these are then reflected back onto the primary and then out of the end of the scope tube and onto a white surface ( a wall or large piece of white card, or in my case the side of my fridge ) this is for fine adjustment to get the rings concentric with the shadow of the secondary in the middle of the rings, the actual proccess will take two or three iterations of the above procedure until finally no adjustment is needed and you can swap between the dot and the circle attachments and the laser spot will be on the secondary centre spot and projected circles will be concentric.

Only fine adjustments are needed and the proccess can be time consuming and also frustrating as these telescopes seem to be on the tipping point of going out of alignment, especialy if you forget where you were in the procedure. Lets hope I got it right, you can only know when you take an image, any remaining misalignment will be instantly obvious as the stars in each corner will be different shapes.

4 thoughts on “Collimating the 8 inch RC”

  1. I’m glad I’ll never have to collimate a RC scope. Anyway, after all that, I hope you get to check it out soon though the forecast doesn’t look too promising at the moment. What a surprise.

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    1. No problem, one small point I should of mentioned, the focuser assembly and primary mirror move as one and cannot be adjusted seperatly, the only way to do this is by adding a second tilt ring between the focuser and the back of the telescope. This can cause problems by blocking access to the primary mirror adjustment screws and by reducing your inward focus travel, when my asi1600mm is focused at the native f8 of the telescope, I only have about 3mm of inwards focuser travel left.

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      1. I´m on the way to build a concrete pillar for my telescopes. If the pillar is ready for use, I want to recollimate my 10 inch RC (using your instructions). I hope, the focuser needs no calibration (Feather Touch)

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