Star test: the donut is triangular. What to do? :-S

[blusky]

Hi, a cooled f6 8" Newt exhibits a fairly pronounced triangular-ish doughnut when slightly out of focus on a bright star overhead at 240x. Diffraction rings hardly discernible. The beam of the laser collimator is squarely in the middle of the doughnut on the primary even when rotated around its axis.

Is this a case of cell clamps being too tight?

Thanks.

[John]

At a guess it sounds like the mirror clips could be too tight, slightly distorting the mirror. At most the mirror clips should be lightly brushing the surface of the mirror. Some say you should be able to slide a business card (ie: thin card) between the clip and the mirror surface.

[steppenwolf]

I'm with John, this does sound rather like a pinched mirror - it is a very fine balance between 'retaining' and 'gripping' the primary.

[blusky]

Ok, the first time the primary goes out

[perfrej]

Primary pinched or, as an alternate and not completely unlikely cause, slight rotated secondary. I had the latter problem with my 190MN. I solved it by doing the following:

1. Use a tube and look down the focuser. Highlight the wall behind the secondary with a flashlight and use the collimation screws to center the secondary horizontally (scope in vertical position)

2. Put your laser into the draw tube and turn the secondary until the dot is somewhere in an imaginary diagonal on the primary that is in the focuser's line of sight.

3. Finish collimation as usual

It got my secondary perfectly rotated and collimation appears excellent. Please note that I do not rule out the pinching

/per

[Astrobits]

Yes, check the mirror clips ( both primary and secondary ) are not tight. I think that is the most likely cause. If it still occurs then one of the mirrors is probably to blame and they need checking.

Nigel