Collimating fast newtonian???

Ok i am completely lost here...i am using collimation cap (home made) but cant get it right, at least i think so.went thru i think all the tutorials and everything looks right but for some reason i cant do star tests?When defocused i see spider vanes and secondary rather then airey disks???any tips what i am doing wrong?? or i am defocusing a star too much?Plus when i tried to image it was awfull....

Hi it could be down to the seeing conditions,which have been really bad the last few nights.
You wont always see an airy disc if the seeing isnt good,in fact it is very rare.
So i would wait till you have a night of good seeing before you can really do a good star test.
Hope this helps.

Paul

Thanks Paul , that actually makes sense but i dont think weather would impact that much???when defocusing i can see secondary almost straight away on 15mm EP?Didnt even tried higher magnifications.When i had 6" SCT i dont remember such problems.Yes you are right that the better the seeing the easier to spot those disks but even then i could make out something no matter the conditions...

Hi Nerro.

I've read a lot on this topic so hopefully some of this may be of use to you:

The aim of a collimation cap is to ensure the gross alignment of the optics. You're attempting to visually verify that the edge of the secondary is concentric with the focuser and that the primary is concentric within the secondary. If the collimation cap hole does not have sharp edges or if the hole is not exactly in the centre you may not get good results. Irrespective, a well collimated laser (cheap ones can be really terrible) or a cheshire can be a good investment (the latter is critical in collimation of schmidt newts).

You should consider if there is slop in the focuser - rack and pinion focusers can perform poorly here - and if the collimation cap is actually centered in the focuser. Assuming neither is an issue you should then check if the primary is centered in the tube, this is not critical but can make overall gross alignment easier.

When you are confident that the gross alignment is sound put the scope outside and let it cool. Once it cools retest the gross alignment.

On to star testing: I'm by no means an expert here but what you need is a star near zenith, dark skies and good seeing. When your dark adaptation kicks in you should notice an airy disc when the star is very slightly defocused. As you move further out of focus on either side of optimal focus you should see an image of the primary with the shadow of the secondary and vanes - the secondary should be dead-centre but you're not going to be able to do more than estimate visually. e.g.: galacticfool.com/uploads/galacticfool.com/2009/05/img_0045.jpg

As you move close to either side of focus you should see something similar to this:
www.cdcc.usp.br/cda/telescopios/hp-mbartels/kolli/Kolli_8.jpg

Note that you cannot see the vanes and secondary - what you're looking at now are the diffraction rings (which can tell you if there are any issues with the primary mirror, I'll not go in to that here). Note that the diffraction rings are concentric and remain so from front to back focusing. Compare to the following image which shows simulations of optimal (leftmost) to badly uncollimated (rightmost):
www.backyardvoyager.com/Tutorials/Focus/comacomp.jpg


Finally, if you're attaching a camera to the telescope and it's heavy or requires extensions to reach focus you may be getting some amount of misalignment due to the weight. Make sure everything's well attached.

If you want more information you should ask on cloudy nights or perhaps one of the guys here can better advise you.

That makes sense.So much for DIY...thought i would get away only with a cap , but apparently not in fairnes i could only get as much as the look in the first link and after that the star just comes to focus...anyways going to try and rob a cheshire from someone to try it out as i do not trust the lasers because there is a lot of bad words said about them...but thanks that helped!

If you don't know anyone nearby I can lend you a cheshire if you send me an SAE.

As for lasers, I invested in a Hotech self-centring, cross-hair laser and it's an excellent tool. It is expensive though but the speed at which one can do gross collimation with it makes it easily worth it. If you're paying £25 for one of those Antares lasers or a no-name brand you're likely to get a non-centred beam - centring it can be straightforward but it's not something you really want to worry about (IMHO).

There is another option though which is the barlowed laser. You can take a cheap laser and place it in a barlow, you then put a paper disc over the end of the barlow with a hole punched dead-centre. It's simple to make and is very well tested. Here's a how-to:
www.cameraconcepts.com/barlowed%20laser%20collimation.pdf

It won't self-centre but that's going to happen with a cheshire as well if the focus-tube is poorly constructed.

Food for thought.