Vixen SD103S apoc­hro­matic re­frac­tor

A por­ta­ble scope that hap­pily mul­ti­tasks for both ob­serv­ing and imag­ing du­ties

Sky at Night Magazine - - CONTENTS - WORDS: PETE LAWRENCE

Smaller aper­ture re­frac­tors, such as the Vixen SD103S apoc­hro­matic re­frac­tor we’re re­view­ing here, are a pop­u­lar choice for both vis­ual and imag­ing tasks. The un­ob­structed aper­ture of these purely lens-based in­stru­ments de­liv­ers higher-con­trast views that are su­pe­rior to those achieved with re­flec­tors or SCTs of sim­i­lar size. With those types of tele­scope, it’s the use of a sec­ondary mir­ror that dif­fracts in­com­ing light and ul­ti­mately re­duces the view’s con­trast.

Where re­frac­tors do fail, though, is in terms of price per mil­lime­tre of aper­ture. A large aper­ture re­flec­tor or SCT will typ­i­cally cost sig­nif­i­cantly less than a sim­i­larly sized re­frac­tor. In ad­di­tion, while a large SCT is rel­a­tively easy to han­dle, a large re­frac­tor will typ­i­cally be long, heavy and un­wieldy.

The Vixen SD103S has a 103mm ob­jec­tive lens with a fo­cal length of 795mm, mak­ing it an f/7.7 mid-speed in­stru­ment. This means that as well as be­ing a fine in­stru­ment for vis­ual work, the SD103S should make a su­perb imag­ing tele­scope. The SD103S’s in­ter­nal baf­fles have been re­designed from pre­vi­ous mod­els to al­low large sen­sors, such as those found in full-frame DSLRs, to be fully il­lu­mi­nated. In our tests we found that al­though this was the case, an amount of vi­gnetting still oc­curs in the ex­treme frame cor­ners, an is­sue eas­ily fixed by flat-field cal­i­bra­tion.

The fo­cuser draw tube has a 60mm thread and is sup­plied pre-fit­ted with a 2-inch adap­tor ring. The fo­cuser is of the Cray­ford-style. Ad­just­ments are ba­sic with just a fo­cuser stiff­ness ten­sion knob. Tight­ened, this locks the fo­cuser in po­si­tion. We were im­pressed with the strength of the lock – even a heavy cam­era with a bit of man­ual as­sis­tance wouldn’t budge.

Bal­anc­ing act

The SD103S weighs in at 4.7kg and is ex­tremely por­ta­ble. A carry han­dle makes it very easy to trans­port and lift onto a mount. The sup­plied mount­ing plate is of the clas­sic Vixen type rather than a rail and re­quires a smaller Vixen mount­ing clamp for a se­cure con­nec­tion. Once mounted, the tube rings are eas­ily loos­ened and tight­ened to al­low the tele­scope to slide back and forth for bal­anc­ing. A flip-mir­ror di­ag­o­nal is sup­plied. This slides into the 2-inch fo­cuser adap­tor ring and presents two

SKY SAYS… The high-spec op­tics sim­ply ooze qual­ity, pro­duc­ing im­ages of stun­ning clar­ity and crisp­ness

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