Collimation and the secondary mirror

[EdgesofTwilight (Brendan)]

I am trying to collimate my small reflector using this guide: http://www.galacticfool.com/collimat...ian-telescope/ and when it comes to the step of aligning the secondary mirror it says I should be able to see the clips on my primary mirror but all I can see is roughly 70% of the centre on the primary mirror. Is this just a sign of a poor scope which this is (I got it for free) or is this something that can be fixed with proper set up and adjustment?

thanks

EDIT* the telescope is 114mm x 500mm

[DavidU (Dave)]

It sounds like it's wrong. What scope is it?

[EdgesofTwilight (Brendan)]

thanks for the quick reply. the scope is just a tasco 303911

[DavidU (Dave)]

See if you can move the primary mirror away from the secondary by tightening the collimation screws. There may be just enough travel to help.
Does the eye piece focus about mid way of the focuser travel?

[Blue Skies (Jacquie)]

Hang on, hang on, its not 'wrong' that he can't see the clips. This will happen if the secondary is on the small side. "Well, why would anyone do that?" you might ask. If you are trying to minimise the impact of the secondary on your visual observing so that you can maximise the contrast - something you might want to aim for if you were a dedicated observer of the planets and were only using the centre of the field of view and not the outside.

The only fix that I know about is to get a bigger secondary, but with a small scope like that its probably not worth it. Moving it up the tube will only stuff up the collimation. You only need to move the secondary up if its not centralised under the focuser.

This is an interesting case as usually the makers will err on the larger side. Something I've noticed with modern short focal length scopes is the secondary is oversize. I'm wondering if this is an older scope?

[EdgesofTwilight (Brendan)]

Dave yes it does focus around the midway point on focuser when I was observing Saturn.

Jacquie what you are saying make perfect sense to me. so for aligning this secondary mirror I should just try and centre it on the Primary mirror?

[DavidU (Dave)]

Mmmm, it's a Chinese made Tasco. If it focuses mid travel the primary is in the correct place. They have just used a smaller secondary.
Some of these type scopes have a barlow lens on the focuser itself.
Sorry to say this but they are not of the highest quality.An f4.3 reflector needs quite a sizable secondary and would suffer light losses.

Originally Posted by EdgesofTwilight

Dave yes it does focus around the midway point on focuser when I was observing Saturn.

Jacquie what you are saying make perfect sense to me. so for aligning this secondary mirror I should just try and centre it on the Primary mirror?

[EdgesofTwilight (Brendan)]

No worries about it being a cheapo I was just given it by someone who was going to trash it. I've just been using it before I decide to get a better telescope. Im thinking about getting an 8" DOB, that seems like entry level for good scopes.

[DavidU (Dave)]

A very nice choice !

Originally Posted by EdgesofTwilight

No worries about it being a cheapo I was just given it by someone who was going to trash it. I've just been using it before I decide to get a better telescope. Im thinking about getting an 8" DOB, that seems like entry level for good scopes.

[astro_nutt]

I agree with Dave...an 8" dob is an excellent learning scope, very practical size and easy to set up, very good indeed!

[Blue Skies (Jacquie)]

Originally Posted by EdgesofTwilight

Jacquie what you are saying make perfect sense to me. so for aligning this secondary mirror I should just try and centre it on the Primary mirror?

Yes, you just have to do your best. I'd wiggle it a bit side to side to see where the clips are (wiggle as in push to one side gently until you can see them, then let it go again) and from that get it as close as you can. With a small scope like this as long as you're getting something out of it its ok. You wont be able to see much detail on Saturn anyway and no one is getting anything from Mars so you're not missing out there. Use it as a learning tool - you'll get used to pointing it and this exercise in collimation is useful even if you don't get it perfect, you're at least learning the basics.

If you're really keen after all this the 8" dob is a good way to go as a starter. Mine has done me good service over the years.