Collimation quandary

[Policharizard]

Hello,

I'm new to using telescopes and having some trouble collimating my reflector telescope. I have a Celestron Astromaster 130, F5 telescope. i'm using an unpowered collimating eye piece, and can't quite seem to get the secondary mirror to align.

This is a picture of the closest I feel i've gotten it:

I have really tried for a solid 25 minutes to get the mirror to be a nice even circle through the collimating eye piece, but can't quite get it into a perfect circle. I feel like I need to adjust the center screw on the secondary mirror, but the manual clearly states in the collimating section "DO NOT loosen or tighten the center screw on the secondary mirror support, because it maintains proper mirror position."

Does anyone have this scope? It kinda seems like the secondary is actually larger than can fit in the collimating eye piece.

Thanks in advance.

[Policharizard]

Sorry, this post might be more appropriate in General Help and Advice.

[ronin]

"DO NOT loosen or tighten the center screw on the secondary mirror support, because it maintains proper mirror position."

Could this simply mean do not loosen or tighten IF the mirror is in the proper position as you will then cause it to move.

If the mirror is therefore not in the correct position then you would seem to need to loosen it, or something, to adjust, and so return the mirror to the proper mirror position.

It sort of reads "Don't touch it!" without the bit saying "Unless you have to".

Whatever you do make sure the primary is protected if you do loosen it - just in case gravity takes total control.

As they are not an uncommon scope here I suspect a number of others will give a better answer.

[Ally8446]

I used to have one of these scope and it's perfectly ok to adjust the secondary by using the central screw.

The secondary definitely needs to be moved up or down dependent on where the mirror holder is in relation to your photo. The dark patch to the right of the mirror.... If it's on the side of your primary then loosen the central screw or vice versa. Make sure the ota is horizontal when you do this.

Loosen all the secondary adjustment screws then holding the secondary with one hand tighten or loosen the central screw to get it perfectly central then tighten all outer adjustment screws. Then look through your collimation eyepiece and readjust the secondary collimation screws as required. Hope this helps.

Ally

[cuivenion]

Try this guide:

http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

Helped me a lot. You need to get the secondary centred under the focuser and in a circle using a Collimation Cap. Make sure the scope is horizontal so if the secondary comes loose it won't fall on the primary, like what happened to me.

[acey]

To be honest, it was probably well enough collimated to begin with, and is probably well enough collimated now. If you want it to be "perfect" then yes, you will need to adjust the secondary. Just be careful when you start unscrewing things that the secondary doesn't fall out, as stated in the preceding post. Either keep the scope horizontal, or be prepared to catch the secondary with your free hand if something goes wrong.

Once you're happy with it, leave well alone. Unless you're regularly going bunjee jumping with it, a 130mm Newtonian will probably need recollimating once every year or two. The mirrors are small and light, and aren't going to move a lot. The alignment only needs to be good enough, not perfect.

[Jason D]

 The dark patch to the right of the mirror.... If it's on the side of your primary then loosen the central screw or vice versa. Make sure the ota is horizontal when you do this.

That dark patch is more indicative of "focuser axial" misalignment. That is the step needed to ensure the sight-tube's cross hairs are aligned with the primary center reflection. With little tweak using the secondary mirror 3 set screws should eliminate the dark patch.

As far as the need to use the central bolt, it is unclear from the photo since the secondary mirror edge is not visible. Well, since the edge is not visible, that tells me the secondary mirror placement can't be way off.

Jason

[Moonshane]

if you pull out the cheshire a little you may be able to get the secondary edges more visible and then you can centralise it with the bottom of the cheshire tube, and by definition the centre of the drawtube.

other than that you seem to understand collimation pretty well. the main thing with loosening the central bolt is ensuring that you have enough thread left to avoid it dropping off which it can potentially do. unlikely but move it a little at a time. I often undo it completely (while holding it) to gauge how many turns are left on the bolt; there's usually plenty. 

[Policharizard]

Thanks for the replies. I just got back from a few days away and took another attempt at collimating it. Pulling the cheshire further out while still looking down revealed that the dark spot is actually a shadow on the wall of the focuser tube, not wall of the optical tube. I had been using the cheshire all the way racked in. Hopefully I can take it out and try to test the collimation with a star test and see how I did.

[Jason D]

The inner dark disc is the reflection of the area around inner OTA surface around the focuser plus whatever is inside the focuser including the collimation cap. The crescent dark area around the primary mirror as shown in the first photo in this thread is the reflection of the area around the primary mirror.

Jason