What is a Star Test? And how to do it?

[lulaz]

Hello!

It has been some time that I hear this term and I know it has to do with collimation, but I don't know what it is, how to do it and what is the point of doing it?

Can you guys help me?

Thanks in advance!

[NGC 1502]

Hi lulaz. Star testing is using an out of focus star at medium to high magnification

to check, and if necessary adjust, the collimation. The out of focus star should

look concentric ( the secondary shadow should appear central within the out of

focus image ) It is important to keep the out of focus image central in the field

of view when doing this check.

Best regards, Ed.

Edit : the star test can also be used to check the quality of the optics,

but thats a whole new subject.

[Astro_Baby]

YOU can see what a star test should look like here Astro Babys Guide to Collimation

Scroll to the bottom of the page for star test diagnosis. Bear in mind the ideal/perfect pattern almost never shows that clear due to atmospheric turbulence in the UK.

[lulaz]

Hi lulaz. Star testing is using an out of focus star at medium to high magnification

to check, and if necessary adjust, the collimation. The out of focus star should

look concentric ( the secondary shadow should appear central within the out of

focus image ) It is important to keep the out of focus image central in the field

of view when doing this check.

Best regards, Ed.

Edit : the star test can also be used to check the quality of the optics,

but thats a whole new subject.

with only a collimation tool, can I reach a very good collimation? or is needed a startest?

[brantuk]

A star test is a way of checking the collimation after you have collimated the scope. If you see concentric circles just out of focus both ways then you are pretty well bang on with the collimation

[Jason D]

with only a collimation tool, can I reach a very good collimation? or is needed a startest?

With quality tools and good knowledge you can achieve better collimation compared to star test collimation.

Bear in mind that the star test only reveals one piece of the collimation puzzle. It only tells you if the focal point of the primary mirror is too far from the eyepiece focal point. Star test will not tell you if your secondary is positioned optimally and it will not tell you if the focal plane of the primary mirror is tilted with respect to the eyepiece focal plane.

Bottom line: If the star test is performed correctly, you will achieve good collimation which should give you pleasant views but it is not more accurate than using quality collimation tools. Not to mention you need good seeing to perform star test collimation at high magnification.

Jason

[sixela]

( the secondary shadow should appear central within the out of focus image )

If you can see that shadow you're much too far from focus (and on some scopes, the secondary shadow is also visibly slightly offset, not centred). Star testing is done just outside of focus where you see just a diffraction pattern with three to five rings.