100% improvement in laser collimating Newtonians?????

[Merlin66]

One of the biggest problems when using a laser collimator is making sure the secondary mirror is in the correct position.

I've come up with a very simple method of ensuring the secondary mirror is centred in both the focuser and aligned with the main mirror....

A piece of card cut to a rectangle the width the same as the secondary and the length the same as the face length of the seconadry ie it fully covers the secondary mirror. Mark the centre ( use the diagonals) and punch or cut a small hole ( about 4mm diameter)

Gently stick it over the secondary using masking tape tabs at the edge ( watch you don't touch the secondary reflecting surface). This will give you a 4mm central aperture on the secondary mirror.

Insert the laser and align the secondary such that the laser spot sits inside the 4mm hole.

Look at the reflected spot on the main mirror, re-align the secondary until this spot is central to the main mirror.

At this stage the reflected laser spot should be directed back to the secondary by adjusting the main mirror, the spot should once again be in/ on the 4mm hole. When this is achieved, the final reflection from the secondary should illuminate the circular scale in the laser itself.

The secondary mask can then be removed....

You should find this gives you very good collimation and you can be assured the secondary is correctly positioned relative to the focuser and the main mirror optical axis.

NB This " simple method" works for smaller mirrors ( around 250mm and F ratios of f6 and above.).

If you have a large fast newtonian ie F4 or so, you have to make allowance for the off-set of the diagonal. In these "fast" systems the diagonal is not placed on the optical axis but displaced AWAY from the focuser by an amount = T/4F*F

where T is the distance from the centre of the tube to the focus and F is the focal ratio.

The hole in the secondary mask, because it's positioned along the surface of the secondary mirror should be punched 1.7 x off-set on the centre-line of the mask towards the open end of the tube.

This formulae, for a 400mm f4 mirror, based on T= 320mm, gives an off-set of 5mm, and an off-set to the mask alignment hole of 8.5mm ( ie 5mm x 1.7).

When the hole is off-set by this amount (ie 8.5mm) it will appear central to a Cheshire eyepiece etc.

Give it a go, and give me some feedback on this "Magic Merlin Mask"..............

Edited March 23, 2009 by Merlin66
Corrections added

[Psychobilly]

Sounds good but "Merlins Magic Mask" sounds better

[Andrew*]

Great idea, Merlin!

But with a closed-tube newtonian where you can't see the face of the secondary I would imagine it would be pretty fiddly getting the beam to fall in the hole. You'd have to watch the primary until you saw the beam reflected...

Definitely going to give it a go though - sounds very useful.

Andrew

[Merlin66]

I'm one of the lucky ones, I made my own laser collimator by putting a small laser penlight into a 1.25" extension tube; centred it with six nylon screws ( three at the front and three at the back) so you can actually look through it from the back and see where its pointing!

Helped me when I built the Littrow spectroscope.

http://www.iceinspace.com.au/index.php?id=63,500,0,0,1,0

[Kaptain Klevtsov]

Would work ace if the laser was supposed to hit the centre of the secondary. Its supposed to hit one focus of the elipse shape actually. To find this, take a pair of compasses and set them to half the long side of the rectangle you have made. Then put the pointy leg in the centre of one long edge and mark where the arc crosses the long axis of the rectangle. (Or do it from both sides and where the arcs cross are the two foci).

The focus nearest the eyepiece is the one to use.

Kaptain Klevtsov

[daz]

KK

Is this what this secondary offset business is all about?

[Merlin66]

Not true about the ellipse focus, to should actually hit the point on the secondary off set by the conical section... There's a very good diagram of the problem in Suiter's "Star testing astronomical telescopes", p112

" The diagonal should appear as a perfect circle neatly centred at the bottom of the focuser tube. The offset is caused by perspective foreshortening of the far side of the diagonal."

In most amateur telescopes > f4 the offset is less than 1 or 2 mm, and I'm not sure that the secondary is ACTUALLY offset ( by design and construction) in commercial scopes.... A 12" f5 should have an offset of 1.25mm, in a 8" f6 the offset is 0.6mm

BTW the actual elliptical diagonal ( a conic section) in every telescope I've seen is NOT an elliptical diagonal mirror!!!!

These secondaries are cut from a sheet of glass using a "biscuit cutter" which cuts as a cylinder, therefore the section produced is a 45 degree cut from a cylinder NOT a 45 degree cut from a cone. If you compare the two "outlines" you'll find you loose about 5% of the light reflected from the main mirror, due to the mis-fit to the conical focussing rays. - Just another useless piece of information you pick up when designing and building telescopes for 45 years.

[Kaptain Klevtsov]

Daz, yes thats what thats about.

Merlin, I stand corrected because of the method of manufacturing the shape. I didn't know that, and don't know enough to say if a slice through a cylinder would be an ellipse.

Kaptain Klevtsov

[Merlin66]

By definition a slice at an angle through a cone is an ellipse. The similar slice through a cylinder has a slightly different shape, sort of has shoulders at the top and narrow hips at the bottom relative to a similar ellipse, hence the 5% difference. Economies of manufacturing!!!

[Merlin66]

Looking at the the issues raised, I've rechecked my "secondary mask" notes and I must admit that the one that I made and used ( on a 17 1/2" f4.5) a couple of years ago WAS NOT set -up as I explained.

I actually had made the hole in the mask 1.7 x offset towards the end of the tube thereby "pushing" the secondary down the tube to compensate for the off-set. This was also in a telescope where the centre of the secondary had also been off-set in the tube AWAY from the focusser.

Unfortunately I don't have a fast newtonian available to re-check and verify. I do believe that if the system is a fast f4 etc then an off-set of:

Offset = T/4F*F

where T is the distance from the centre of the tube to the focus and F is the focal ratio, should be applied.

This formulae, for a 400mm f4 mirror, based on T= 320mm, gives an offset of 5mm, and an off-set to the mask alignment hole of 8.5mm.

When the hole is off-set by this amount (ie 8.5mm) it will appear central to a Cheshire eyepiece etc.

I apologise for the confusion; I should have checked my original notes earlier..

[G4YVM]

Merlin, does your last post mean that your first post is now wrong? When it comes to setting up my 16" I was going to use your MMM method of centering the mirrors prior to final collimation...I'm lost now!

david

[Merlin66]

David,

I've just "corrected and updated" the first post to reflect my error of simplification.

The issue of applying the secondary off-set really comes into play when you have large Dobbie optics ie 400mm f4 type systems.

I apologise the anyone who has read this thread and been confused.

[johninderby]

One point to keep in mind if dealing with a modern commercially made dob/newt such as a SkyWatcher, is that the offset is already built in. They fasten/glue the secondary onto the holder offset by the correct amount, so if you try to add offset by adjusting the position of the secondary holder you've going to end up with double the correct offset.

To make sure you'd have to remove the secondary and check to see if the mirror is already offset.

John

[G4YVM]

David,

The issue of applying the secondary off-set really comes into play when you have large Dobbie optics ie 400mm f4 type systems.

.

Merlin, many thanks. I think though this is whaat I have...a 16" primary at f4.5. I'll re-read.

As an aside, what will be the effect if I do NOT offset? K and B, in their book, suggest, if I haave read it correctly, that offset is a wonderful tweek but not essential. is this right?

EDIT In your formula Merlin, what point defines the "focus"? is it the outer edge of the eyepiece glass (i.e. effectively where the light leaves the instrument before it hits the eyeball)?

david

Edited March 23, 2009 by G4YVM

[Merlin66]

John,

It doesn't matter if the construction is 100% correct or not.

The mask off-set should still be applied to fast systems. In an ideal world the secondary on a fast newtonian would be mounted with the calculated off-set away from the focuser and towards the mirror.

By using the mask with the off-set hole you at least compensate for one of the off-sets.

David,

The distance is measured from the centre of the tube to the point on the focuser where the "prime focus" would be. Doesn't have to be 100%, just measure to the end of the focuser tube when the eyepiece is focused on the moon etc.

The only problem, if the secondary is not off-set in fast systems is the loss of light when some of the reflected beam "misses" the secondary....