Collimation (Secondary Mirror)

[littlebeagle]

I'm having issue's with the collimation, the secondary mirror isn't aligned with the focuser, like the whole thing needs to move up, but I don't know how.

Flextube 200p

[NGC 1502]

The secondary can be adjusted towards or away from the primary mirror using all three secondary adjusters. You must slacken the centre lock screw first.

To shift the secondary away from the primary you will need to back off the three screws by the same amount. Once the secondary is directly under the focuser you then use the three screws independantly to collimate it.

Hope you sort it, Ed.

[Tinker1947]

Then come back here for lesson No.2

[littlebeagle]

Do the secondary adjustors need to be locked tight after fiddling or does it not matter where they are left as long as it's aligned?

[Tinker1947]

The 3 screws need to take up any slack them just nip them so there not loose, check collimation after turning each screw re-ajusts as necessary.

[umadog]

Hope this helps: Telescope Reviews: Useful info about secondary mirror alignment

[littlebeagle]

A bit better I think?

Would you say it's still a little bit out?

[umadog]

Much better. It's still shifted towards the south west corner of the image, though. You want the gap between the mirror and sight tube to be equal all the way around. The best way of seeing this is to rack out the sight tube until its inner edge appears to be the same size as the edge of the secondary. This way it's easiest to judge when secondary is centered because the gap between the secondary and edge of the sight-tube is very small. Right now the gap is large so it's hard to tell. Either rack out with the focuser or pull the sightube outward. Don't pull back so far that the tube isn't well supported and starts to alter its angle with respect to the focuser. For the primary adjustment, place the pinhole roughly where the eye lens of your eyepieces would be when you're focused at infinity.

[littlebeagle]

Yay getting there .

Thank you all for your help so far, god knows where I'd be without the help of this forum.

[littlebeagle]

Just been out for a couple of hours as the forecast was pretty good tonight. I was able to separate Almaak with the 25mm (Of course I went for higher power after).

As well got some of my best views of Jupiter despite their being quite a lot of cloud around.

Can only put this down to the collimation being much improved, I left it as it was in the last photo as I wanted to get my scope outside, will get it much closer to perfect for my next session.

Once again thank you for your help .

[umadog]

Good stuff!

There are basically three adjustments which you're trying to optimise. The first is to centre the secondary in the focuser, make sure it appears round, and to ensure that you can see all of the primary mirror in the secondary. This is what we've been talking about here. If you get this wrong then you will potentially be throwing away light. Generally speaking, getting this alignment wrong will not result in a more blurry (less sharp) image.

The remaining two things you need to do we have discussed relatively less but are more important for getting a sharp image. These are known as axial alignments because you are trying align the optics and the focuser (and therefore the eyepiece) along their axes. You seek to ensure that the focuser points straight at the centre of the primary mirror and that the focal plane isn't tilted. You achieve this by altering the tilt of the secondary mirror and the tilt of the primary mirror.

The secondary is adjusted so that the cross hairs point at the primary mirror centre spot. The primary is adjusted so that the centre spot doughnut surrounds the pinhole of the Cheshire. The latter alignment is more critical than the former because the tollerences are much tighter. If you purposefully misalign the primary, stars will readily appear asymmetrical and not focus sharply. If you star-test (Google it) your scope you will readily be able to tell if your axial alignment is good.