Is a star test a true sign your scope is collimated?

[Madhatter]

If a startest comes back good, does that mean your scope is in good collimation? I read a very informative post here that says otherwise but i cannot find it!!

my startest looks good, however looking through the focuser at the primary mirror i cannot see all the mirror holders from the mirror cell-confused here:icon_confused:

[John]

If the star test is good and the scope seems to show nice contrasty, sharp views then you are home and dry.

I tend to trust what my eyes tell me more than what gadgets seem to show

[Madhatter]

Im not so sure how pinpoint the stars are tbh-in the 25mm they are pinpoint..but in the 10mm not so much so?

When i observed jupiter and 4 of its moons the moons were far from pinpoint but this may be down too a few issues-no cooldown time, no collimation (although fine the night before) low hight in the sky of jupiter and starting to get light?

Can the secondary mirror spider vanes (i think) be a good test of collimation? the donut on the primary is a little out from the center of these in the secondary. i hope im making scence here lol:)

[brantuk]

For a star test you need to find the focus point then take it slightly out of focus in both directions (called "in-focus" and "out-focus"). If you see perfectly concentric circles on both sides of the focus point, then you're good to go

[todd8137]

hi ,i did not touch ny mirrors for the first six months to scared but when a did get round to it the dohmut wa a bout a inch away lol i still do not no how a saw a thing but a seen plenty but once done it did make for better viewing

[FraserClarke]

If a startest comes back good, does that mean your scope is in good collimation?

Yes -- but only if your test set-up is sensitive enough and you're interpreting the images right. You also need high magnification and very good seeing to make the final measurements/adjustments using a star-test (ideally you want to be seeing the diffraction rings and making them nice and even). It doesn't happen that often

From what you say about the out-of-focus doughnuts not being concentric suggests that you're slightly out of collimation. Make sure the out of focus images are correct before worrying about the in (or very near as brantuk says) focus images.

Doesn't sound like you're very far away though.

[Madhatter]

Thanks guys:) honestly i seem to have no problems performing a startest-everytime i goto vega i always see the circles and there pretty much perfect.

Could you elaborate what you mean by in and out focus? when you say in focus for the perpouse of the star test do you mean when the star just starts showing the circles?

Sorry just to clarify i mean the donut on the primary mirror does not seem central on the secondary when looking through the focuser,in star tests all circles are evenly space and central.

[Bazzaar]

I should think the ultimate test involves laser interferometry in an optical lab.

Failing that the star test produces excellent results. The only down side being bad seeing can destroy the off focus circles you are trying to see.

Who ever started the collimation procedure that involves worrying about the mirror holders wants shooting! They are completely secondary and incidental, nothing more than clues to being on the right track.

Regards

Barry

[Moonshane]

if your scope is not a tracking scope then you need to do the star test on Polaris not Vega for a couple of reasons. Vega is a bit bright and also Polaris does not move. The latter point is vital as it is crucial to have the star in the exact centre of the field for a star test. You need to re-centre the star each time you tweak the collimation too.

but in short, yes, the in and out of focus circles are literally a small tweak on the focuser either side of sharp and at as high a power as the seeing will allow (preferably about 30-50x the aperture in inches) so for a 10" scope between 300-500x.

[Madhatter]

Thanks for your help everyone:) ahh using polaris does seem like the better option as i noticed as soon as vega moves out the center view-the circles are not evenly spaced anymore, but when central they look very good.

I will try polaris next time:)