Australian Sky & Telescope - May 2018

http://www.skyandtelescope.com.au 67

addition to having 2¼ times more light
grasp than the 30-cm, the Stargate’s
longer focal length (1,900 mm versus
1,524 mm) offered noticeably more
magnification for a given set of
eyepieces. By itself, this longer focal
length would have yielded 25% more
magnification. But because the Stargate
is an f/4.1 Newtonian and suffers more
from coma than the 30-cm f/5, I did
most of my Stargate observing with a
Tele Vue Paracorr coma corrector that
increased the Stargate’s effective focal
length by a factor of 1.15×. And this
meant a 43% increase in magnification
for the same eyepiece used with the
Stargate compared to my 30-cm Dob.
TheStargateshipswith2-inch
28-mmand1¼-inch10-mmeyepieces
of decent quality. They yield 68×with
a49-arcminutefieldofviewand190×
with a 15-arcminute field, respectively.
ThereisalsoaCheshireeyepiece
collimationtool,whichisverygoodfor
checkingthescope’sopticalalignment.
Duetothelengthoftheinstrument,
however, collimating the optics with
theincludedtoolisbestdonewith
twopeople—onelookingthrough
theCheshireeyepiecewhiletheother
adjusts the collimation screws on
theprimary-mirrorcell.Ialigned
theopticswithalasercollimator
thatallowedmetotweakeverything
withoutadditionalhelp.Andthegood

news is that once collimated, the scope

held its alignment well.

IalsousedmyownTeleVue

eyepiecesthatIkeephandywithmy

30-cmscope.Theserangedfroma

35-mmPanopticthatyielded(withthe

Paracorr) 62×andafieldslightlymore

than 1ºacross,toa7-mmDeLitegiving

308 ×andan11-arcminutefield.Ialso

had some exceptionally memorable

viewswitha21-mmEthos,whichgave

almostthesamefieldofviewasthe

Panoptic, but at an impressive 103×.

First night out

Myfirstnightunderthestarswithany

telescopeisusuallyreservedforjust

gettingafeelforwhatit’sliketoset

upandusetheequipment—serious

observingisn’ttypicallyontheagenda.

AssuchIpickedaspotonthewalkway

outsidemygaragewheretheskyis,

unfortunately,heavilyobscuredbythe

house and surrounding trees.

WhileIsetupthescopebymyself,

itwouldbefareasierwithtwopeople.

Thebasewithitsoptionalmotordrives

S The Stargate comes with a 9×50 straight-through inder and a 2-inch, two-speed focuser with

extension tubes/adapters for 2- and 1¼-inch eyepieces. As explained in the accompanying text,

the author typically observed with the Tele Vue Paracorr coma corrector pictured here.

X The Stargate Dobs have noteworthy primary
mirrors made from fused glass. The 18-inch has
a front plate (about 20 mm thick) and back plate
separated by a dozen ribs surrounding a central
hollow core. As such the 8.9-cm-thick mirror
weighs only about 13.5 kg and acclimates to
ambient temperatures much faster than a solid
disk of glass would.

XX In the workshop, the author’s Foucault test of
the primary mirror shows no sign of ‘print through’
from the supporting rib structure. There is a slight
hint of a central depression due to the hollow core,
but it falls completely within the shadow of the
secondary mirror and is of no consequence. The
mirror is an overall well-corrected paraboloid with
subtle artifacts typical of machine-polished optics.
It delivered excellent performance at the eyepiece.