Australian Sky & Telescope - April 2018

64 AUSTRALIAN SKY & TELESCOPE April 2018

SCOPE MAINTENANCE

TOP: ROD MOLLISE; BOTTOM:

SKY & TELESCOPE

Youcanbegincollimationwithanyof

thescrews.Justpickoneand,usingthe

propertool,tightenitexperimentally.In

this stage, you may need to turn screws

afairamount;butevenso,makeonly^1 / 8

turnatatime.Ifthereflectionsmoved

intheproperdirection,continueturning

thescrewinsmallamountsuntilthe

reflections in the telescope’s primary

mirror are concentric. If not, pick another

screw,tightenit,andchecktoseeifyou

are going in the correct direction. When

allthereflectionslineup,it’stimeto

move to rough collimation.

Inallstagesofadjustment,tryto

avoid loosening the screws; that will

ensurethesecondaryissnugandthe

telescopewillremainincollimationfor

months or even years. If you still need

tomovethesecondaryinaparticular

direction but the corresponding screw is

alreadyhand-tight,youcanstillmake

theadjustmentinthesamedirectionby

loosening the other two screws slightly.

Rough collimation

Start with the star centred in the field

and defocus untilitlookslikeadonut

withadarkcentre.Isthatdonuthole

(which is act ually t he shadow of t he

secondary mirror) in the middle, or is it

off to the side? If it’s not centred, adjust

thescrewsasdescribedaboveuntilitis.

Note that turning a screw will move

thestartotheedgeofthefield,so

besuretore-centreitfollowingeach

adjustment. When the donut looks

good,movethefocusintomakethe

donuthalfthesizeasitwaspreviously,

andthenseeifthedarkholeisstill

centred. If it is, you can move on to

fine collimation. If not, follow the

same steps until it’s as well centred

as you can get it. Some people stop at

this ‘donut stage,’ but that is rarely

good enough for demanding tasks like

planetary observing.

Fine collimation

Re-focus the star until it is almost

sharp, such that diffraction rings

appear around the star. If the bullseye

of these rings isn’t concentric, then

adjust the secondary screws until it is.

There are various methods that can be

used to determine which screw needs

to be adjusted, but I like to keep things

simple, so I use a variation of my low-

tech ‘pick one’ strategy.

Look at the bullseye. Which side is

squished? Move the telescope until that

side is against the edge of the eyepiece

field. Now, move the screws by small

amountsuntilyoufindtheonesthat

movethestartowardthecentre.Use

thatscreworscrewstoputthestarback

inthecentreofthefield.Keeprepeating

this,movingthestartothefieldedge

closesttothecompressedsideofthe

bullseye and re-centring it with the

screws,untiltheringsareconcentric.

In-focus collimation

If you have excellent seeing conditions,

you can attempt the ultimate in SCT

collimation — in-focus collimation. This

is made possible by the fact that, thanks

tothelawsofphysics,whenastar

S SMALL TURNSWhen performing the fine collimation steps, limit the amount you turn your

tool to no more than^1 / 8 of a turn to avoid pushing the test star out of the eyepiece field.

S FINE COLLIMATION Left: While aimed at the star, rack the focus out slightly so that its

diffraction rings are visible. Imperfect collimation will make the rings appear skewed in one

direction. Right: Adjust the collimation screws until the rings are perfectly concentric, so that the

star looks like a perfect bullseye target.