Newtonian Collimation

[Roy@Aldermaston]

This is just a quick question about collimating my 200 PDS. I've followed a number of illustrated guides, most notably Astrobaby's.

The secondary mirror is centred to the eyepiece and the secondary aligned to the primary so that the three mirror clips are equally visible, and finally the primary has been centred using a Cheshire collimator. Checking the collimation on an out-of-focus star isn't easy because the poor seeing creates a lot of turbulence, but it looks as though the rings are concentric.

So far so good, but why do I see three slight indentations around the periphery of the star image, which correspond to the primary mirror clips? So far as I know, none of the guides illustrate this as a possible collimation error. But if it is an error, I'm not aware that it affects the image when viewing, or perhaps it does - I'm not sure since this is my first Newtonian. I did wonder if I need to adjust the central screw on the secondary to move the mirror closer to the primary, or is it further away from the primary. What do you think?

[andrew s]

Roy - What you see is quite normal for the out of focus image. As you move out of focus the shadow of the secondary, mirror clips and anything else in the way start to show up in the star image. Andrew

[Telrad]

Roy, have a look at Astrobaby's Guide to Collimation, it's excellent:

Astro Babys Guide to Collimation

[Roy@Aldermaston]

Andrew,

Thanks for that reassurance. I tend to be a bit fussy about these things and try to get everything spot-on. I think that part of the problems is down to poor seeing conditions of late, which means that my out-of-focus star is rather ragged around the edges.

Terad, I've been through the tutorial, but I couldn't see any reference to the condition that I described, probably because as I've just discovered it doesn't seem to be a problem at all.

Roy

[Telrad]

Good news Roy. I often do a star test to ensure that my optics are correct. Worth a try..

Good luck !

[Staypuft]

Thanks for the Astro Baby link, very useful!

[Jason D]

Unfortunately, a large number of reflector owners perform star collimation incorrectly. If you could see the diffraction of the spider vanes and the primary mirror clips then you have defocused too much. You need to be a high magnification and defocus by a small amount – enough to see only few diffraction rings.

Jason

[RikM]

It is different to the 'pinched mirror / optics' image in A-B's tutorial?