Barlowed laser collimation behavior

[coenie777]

I use the above method to collimate the primary of my MN190.

 

I noticed today that if I put the tube horizontally pointing say left, and align the laser donut and then proceed to turn the tube through 90 degrees, the just now collimated donut "jumps" slightly off the mark. If I move the tube horizontally pointing right now, and re-do the collimation, it would shift again as I move it through 90 degrees to point the other way (this is with the scope on my EQ mount).

 

Would this behaviour be caused by a loose primary mirror cell or what would you say should be the first thing to fault find to try and cure this issue?

[ronin]

Since it is or at least appears to be normal that a mirror is not gripped then it could be the mirror shifting a little. What happens is if that the mirror gripping bits get to make contact then that leads to slight mirror distortion and triangular shaped stars.

Sounds as if yours may just be a little too loose therefore allowing enough movement to be able to see the change. Exactly what the method would be to set it all up I cannot say from experience - own 5 refractors. At a guess it will be the get it all flat (tube vertical, mirror horizontal) then slide into each of the 3 mirror holding screws (?) the thinnest feelar gauge you can get, literally baking foil thickness, then adjust until the feeler is experiencing very slight resistance. Leave it there and move on to the next. After that recenter the mirror via whatever movement there is in the actual cell - I assume there is some method to move the cell+mirror as a single unit.

As said what often occurs is that people adjust until they feel the mirror being gripped a little, but that is too much. The situation is you do not want it to move but you want nothing touching/gripping the mirror. So you allow say a 1 thou movement, 0.025mm. Likely you cannot get anything that thin, cannot recall what the feelers I had used to go down to.

The other and probably first to check option is that the laser collimator is held firmly in the focuser. If that is not held then movement there then it appears greater across the mirror itself.

[coenie777]

Thanks for the reply Ronin

I am pretty sure that the movement is not coming from the laser or focuser. I also notice that when putting the scope on the mount or taking it off; when you turn it through 90 degrees as you position it for carry/placement on the mount, there is a small "thud" sound, like something moved ever so slightly. Sounds like I will need to remove the mirror cell after all to now for sure.

[coenie777]

I removed the primary cell and indeed this was the cause of the "thumb" when rotating the tube.

The rubber holders were very loose, so much so that I could move the mirror around the cell with some side pressure. I tightened these down in line with Ronin's suggestions and the movement were gone. Took some time to re-do the focuser alignment and for the first time I have a good picture through the Cheshire (or maybe it is psychological : - ) ).

Now for the long wait for a clear night but I suspect I cleared up a major issue on this scope.

[John]

Don't overtighten the mirror clips - you should be able to slide a thin piece of card between them and the mirror surface.