How important is getting the secondary aligned?

[pipnina]

With the Cheshire eyepiece, it's really easy to align the primary mirror, but even after reading the astrobaby's collimation guide, the secondary mirror is a little confusing.

I understand that, when using a Cheshire, I should be aiming for the centre spot on the primary to be under the middle of the Cheshire's cross hairs? But what I don't get is how to make sure it's central?

It mentions making sure the secondary is in the middle of the focuser, and that it should be circularly shaped, but in fast scopes under f/5 (like my f/4.8) it should form an elliptical shape with the primary reflection in the middle, how close do I need to be? I can get the primary almost bang on (maybe within 1/2mm tops) but even if I get the centre spot under the Cheshire cross hairs, can it still be misaligned?

 

    ~pip

[jetstream]

Yes... but..

Just make sure the sec looks round and is centered under the focuser. You center it using the " back and forth" middle screw on the holder. I just loosen the screws, make it round, snug the 3 screws up and then place it under the focuser using the back and forth one.

After this line the crosshairs up on the primary dot.

[Moonshane]

The other guides are great and more than one reference is often important but my guide takes a more simplistic approach with less (I hope) complex explanations.

 

 

[Moonshane]

The main thing to consider and in answer to your question; unless you are imaging the secondary unless wildly out (like facing the wrong way completely) will be unlikely to have a perceptible effect upon the views. The primary is far more critical. Get both as good as you can though naturally.

[pipnina]

Ok, I won't fret over it too much then.

Thank you!

    ~pip

[bomberbaz]

Funnily enough my collimation cap arrived today, natty little thing off astroboot. I checked my secondary which I though was collimated and it showed to be a tiny bit out, the laser then confirmed this. I then tried the cheshire which said the same.

Few tweeks and bingo, tested with all the three above and it was shown to be bang on.

Some might call it overkill i=using all three but at F4.7 I have found the mirrors soon show the views to be degraded if its not near perfect. Still thinking abiut a paracorr though.

[bomberbaz]

Sorry just re-read this post of mine above and then realised I never got my point accross.

Recently a few of us had a mini star party and during the night I tried doing a couple of doubles. Whilst viewing I noticed a distinct slur in the stars. I got a mate to check and he said check your collimation even though I checked when we first got there. I do not know how or why it happened but my secondary had go around an inch out of alignment and the stars certainly showed it in the eyepiece.

So in my opinion it pays to get all your bits collimated and if things start to look awry, check again.