Newtonian Reflector Collimation

•Goals and Assumptions

•Collimation procedure and issues

•Construction of a collimation target

•New primary mirror alignment procedure

•Results

•Other ideas Presenter: Howard Page

Goals

•Obtain a sharper image

•Measure misalignment prior to collimation.

Assumptions •All expectations of a purchasing a cheap telescope

would be realised.

•Astronomy as a hobby can be kept within the budget of

a hobby.

•Supplied collimation procedure inadequate.

Collimation Procedure

Looking down the focussing tube …

1. make secondary mirror concentric with bottom edge

the focussing tube.

2. In the secondary mirror get the image of the primary

mirror concentric.

3. In the primary mirror get the image of the secondary

mirror concentric.

Right or wrong, good or bad, this is the procedure I use.

Collimation Procedure: Issues

1. make secondary mirror concentric with bottom edge

the focussing tube.

• Can’t tell how far I’m out when aligning a small circle

within a big circle.

• When using crosshairs … Difficult to focus on

crosshairs close to the eye and a small mirror at twice

the distance at the same time.

Solution = use concentric circles

instead of crosshairs, but

still have focussing problem.

Collimation Procedure: Issues

2. In the secondary mirror get the image of the

primary mirror concentric.

• Can’t tell how far I’m out when aligning a small

circle within a big circle, but no focussing problem.

Solution = Collimation target.

Collimation target: Construction

Concentric circles printed onto transparency foil (as

per OHP) and attached to front of tube

Measruements taken using precision sliding calipers:

• tube aperture 129mm

• secondary mirror obstruction 32mm

Measurements assumed as per specification:

• Primary mirror at 114mm

108mm Dia

110mm Dia

34mm Dia

32mm Dia

28mm Dia

114mm Dia

112mm Dia

30mm Dia

Mark 1

Too much

light ….

dazzling

Collimation target: Procedure in 4 steps

1. Fix target to secondary mirror mount with Blu-tack™

2. Looking through the eyepiece (using the film

canister), move target around so that it is perfectly

concentric with the secondary mirror

Effectively you now have circular measurements

outwards from the secondary mirror mount every

1mm, simulating a secondary mirror mount at the

same size as the primary mirror which make it so

very easy to align

3. Align the primary mirror to the large outer circles on

the target (very easy to do for first timers like me)

Mark 2

108mm Dia

110mm Dia

34mm Dia

32mm Dia

28mm Dia

114mm Dia

112mm Dia

30mm Dia

Collimation target: Procedure in 4 steps

4. Repeat steps 1 through to 3

• Collimation is an iterative procedure.

• More adjustments (with one more iteration) usually

required but only very fine ones

Mark 3

Final

Prototype?

108

11034

32

114

112

Primary Mirror

Collimation Target

for 114mm reflectorby Howard Page

1) Stick on secondary

mirror mount (with

bluetack) and centre to it

2) align primary mirror to

centre on target

Mark 3a

Final

Results

Observed results against collimation target:

• Primary mirror dia.106mm inside mirror clamps.

• Primary mirror dia.108mm at the mirror edge.

• Secondary mirror obstruction 33mm (32mm spec)

• Primary mirror misaligned by more than 5mm.

Observed results to a hilltop trig station at 10Km

1. Writing/scratching visible on Trigs’ skirt.

2. Two birds sitting on top of trig preening themselves

Also

1. Focus was sharper, easier and faster to achieve

2. Edge of the FOV out of focus

Other ideas

1. Hartmann mask. Cost = $3.95

- Great for focussing

- Outperforms a moon filter

2. Motorised focussing.

- Finer focus without bumping the scope

- Can use secondhand tracking motor

3. Finderscope with crosshairs and concentric circles

that doubles as a collimation eyepiece

Newtonion Reflector Collimation

Questions

Presenter: Howard Page