Star testing collimating - perfect circles?

[Aenima]

After attempting to collimate my 130p, and even with a cheshire it still seems like there's a huge error margin, I try a star test. Apart from bad seeing, the image always looks like there is one part of the circle that has a straight 'edge' like a little bit is sliced off. Otherwise it looks almost like the dark spot is central and spaces between the circles makes them seem concentric, but . . . the whole image often fails to stay perfectly circular, sometimes the outer edge blurs or pulls away.

Does anyone have actual images of stars showing in/out of collimation? I've seen the usual images and read the descriptions, but what i actually see through the eyepeice is always more complicated. I really want to avoid touching my secondary as its a nightmare to adjust and seems fine (Its also a newish scope) and have the primary pretty close to right, but the star test always fazes me, and I would really appreciate a good shot of real stars being tested for collimation. Also any advice on this matter, though you all probably heard it a million times, would be gratefully recieved.

Thanks and thanks again, ppl. Clear skies and all the rest.

Regards.

[Merlin66]

Obviously something is cutting into the light beam to give the straight edge....

you may be able to wave your hand around the edge of the aperture to determine which edge and where it's being blocked. This may make the problem solving easier.

(My star test gives the images in the book...)

[Jason D]

the image always looks like there is one part of the circle that has a straight 'edge' like a little bit is sliced off.

The bottom of the focuser drawtube could be intruding into the light path.

I would really appreciate a good shot of real stars being tested for collimation.

I took the attached photos few weeks ago using my iphone.

The first shows a star collimation image

I am including the 2nd one for fun. Seeing was good that night.

Jason

[cs1cjc]

If there is one bit of the diffraction pattern missing, pulling away, then it will likely be due to tube currents, warm air distorting the image. The mirror needs to be almost at ambient temperature, within a degree or two. My 8 inch scope takes well over an hour to cool and once recently had not cooled fully after two hours when I was driven in by the mist, though the outside temperature that evening had fallen quickly. A fan helps and I shall be fitting one.

[Brent]

The straight edge could also be one of the mirror clips - I have this problem sometimes.

Inside focus the star test looks perfect, outside focus I mostly get a flat edge, or even two flat edges. If you bring the focus right out until the rings disappear and you can see the mirror & spiders, it will be very obvious if it's a clip causing it.

I think on my scope that it's down to the focuser (it's an old blue 150p which doesn't have the 2 inch adapter), so I've stopped worrying about it until I can afford a new focuser!

[John]

If you have the focuser racked right in when you are star testing the flat end of the focuser can intrude onto the light path and that chops the edge of the defocused star image.

[cs1cjc]

For a star test you should be using a high power eyepiece and should only be moving a little from the true focus point. In this condition, the secondary mirror, mirror clips and any draw tube obstructions will not be visible. There is a good short text here:

TMB Website

or if you really want to know the full story, the Suiter book can be recommended:

Star Testing Astronomical Telescopes, A Manual for Optical Evaluation and Adjustment, Second Edition by Harold Richard Suiter

[Aenima]

"The first shows a star collimation image"

Thanks ppl.

BTW jason, is the pic of a star taken with a collimated scope or uncollimated?

Ta for the pics as well.

[Jason D]

BTW jason, is the pic of a star taken with a collimated scope or uncollimated?

Ta for the pics as well.

Yes, that is for a collimated scope.

[Aenima]

wow, just been reading various stuff on accurate collimation of reflectors and the fine points of adjusting them, and how the slightest turn of a screw can ruin the telescope's optical performance. It really does make a big difference when your scope is not quite in line.

Still scary stuff when you dont know what your doing, even when everyone says it'll be easy the sheer amount of room for mistakes is daunting.

I'm still trying to get it right.

Cheers for the input people.

[Tribal-Wolf]

Be careful not to get Collimation OCD or you'll spend half your night just messing with the scope instead of looking through it lol

[scuffer]

Not collimated mine yet , only done star test and all looks as described , sharp edge is the focuses - had me worried for a couple of minutes !

[Aenima]

Yeah good point. Those pictures that show you a blurry saturn next to a crisp one that is only a turn of a screw different is the worst, gets me every time. At least with the cheshire you can see something actually line up however accurate it may be.

Appreciate the replies ppl,

Jay.