2019-07-01_Australian_Sky_&_Telescope

64 AUSTRALIAN SKY & TELESCOPE July 2019

a camera’ssensorisn’ttippedwithinthefocalplaneof

theimagingtelescope.Largesensorsaremoresensitiveto

thesetinymisalignmentsthatmimictheeffectsofpoor

collimationbutaren’tcorrectedbyadjustingtheoptics

orthefocuser.Imagesaffectedbysensortiltappearthe

sameregardlessofwherethetelescopeis pointed.Some

astronomicalcamerasequippedwithlargedetectorsinclude

a push-pulladjustmentplateonthefrontofthecamerathat

permitsyoutotweakthealignmentofthesensor.

MostrefractorsandMaksutov-Cassegrainsare

collimatedat thefactoryanddon’trequirecollimationby

theuser(althoughthecameraandfocusermayrequire

adjustmentasnotedabove).Seearticlesoncollimating

a Schmidt-Cassegrain(AS&T:April2018,p. 62)and

Newtonianreflectors(AS&T:May/June2019,p. 62).

SpecialisedcollimatingtoolssuchasthoseofferedbyHotech

(hotechusa.com) permitcollimationofNewtonianand

Cassegrainopticsduringtheday.

Polaralignment

Eventhefinestequatorialmountwon’tproduceimageswith

roundstarsif it isn’tproperlyaligned.Polaralignmentis

whenthepolaraxisofthetelescopeis madeparallelwith

therotationalaxisofEarth.Ourplanetrotatesaround

thisaxis,makingstarsappeartospinaroundthenorthor

southcelestialpole.Asanobjectmovesthroughthesky

fromeasttowest,a properlyalignedmountwillcancelout

thismovement,keepingtheobjectcentredinthetelescope.

Misalignedequatorialmounts(andalt-azimuthGoTo

mounts)cankeepanobjectcentredforvisualobservation

butwillslowlyintroducefieldrotation,whichmakessuch

setupsunsuitableforlong-exposure

astrophotography.

Sohowaccuratedoespolar

alignmentneedtobefortracked

imagestohaveroundstars?The

answerdependsonseveralfactors,

suchasthelengthoftheexposure,

thetarget’slocationinthesky,

thetelescope’sfocallengthand

pixelscale,andeventhedifference

inpointinganglebetweenthe

guidestarandtheimagingtarget.

RichardHookpublishedequations

concerningpolaralignmentin

theFebruary 1989 issueofThe

Journalof theBritishAstronomical

Association. Oneoftheseequationstellsushow well-aligned

a mountneedstobefora givensetup:

Intheequation,Eis themaximumpermitted polar alignment

errorinarcminutes,andSrepresentsthetolerance for field

rotationinmicrons.Dis thedeclinationofthe target, while

Tis theexposuredurationinminutes.Focallength (in

millimetres)is representedbyF, andAis theangle in degrees

betweentheguidestarandtheoppositeedgeof the field.

Thisequationconfirmsthatshortexposures and short

focallengthscombinedwithlargerpixelsaremore tolerant of

imperfectionsinpolaralignment.Inotherwords, the higher

yourpixel-per-arcsecondratiois,thelongeryou can expose

withless-than-perfectpolaralignment.

If youknowhowaccuratelypolaralignedyour mount

is,theequationcanberearrangedtoanswerthe question

“Givenmycurrentpolarmisalignmentandequipment,

what’sthelongestexposureI canachievebefore stars appear

elongated?”likethis:

Whenshootingwithanimagescaleof1 to 4 arcseconds

perpixel,polarmisalignmentbyasmuchas3 arcminutes is

adequateforexposuresuptoabout 15 minutes long.

Sometelescope-controlsoftwareandautoguiding software

programsincludetoolsthatmake precise polar alignment

relativelysimple.Hardwaretoolssuch as the QHYCCD

PoleMasterarealsoavailable.The tried-and-true manual

methodof drift alignment takes

longerbut works well even if you

don’thave a clear view of the

celestial pole. Numerous drift

alignment tutorials are available

online.

Autoguiding

Whilegood polar alignment,

tracking, and collimation can

improve the roundness of the stars

inourimages, many telescope

mountsrequire a little help staying

ontarget. This is because most

mountsuse gears that produce a

repeating error known as periodic

error(PE), which causes a slight

oscillation of the field. This often

is represented in the technical

specifications of your mount as, for

example, a PE of +/- 3 arcseconds.

Somehigh-end mounts reduce this

errortonear-imperceptible levels

uAUTOGUIDINGMostastrophotography

rigsrequireautoguidingtokeepthe

telescopepointingexactlyatthetarget

throughouttheentireexposure.Thissetup

includesa 25-cmASAf/3.8Newtonian

astrographthathasa piggybacked 80 - mm

refractor for autoguiding.

E= 45,000×S×cosD

T×F×A

T= 45,000×S×cosD

E×F×A

.

.

IMAGING TIPS