Scope Doc­tor

Our equip­ment spe­cial­ist cures your op­ti­cal ail­ments and tech­ni­cal mal­adies

Sky at Night Magazine - - CONTENTS - With Steve Richards

I have a Wil­liam Op­tics Zenith­star 61 telea­cope. I'm us­ing it with a 0.6x flat­tener-re­ducer. What is this do­ing, and what mag­ni­fi­ca­tion would I get with a 25mm eyepiece?

PHIL CREMMEN

The Wil­liam Op­tics Zenith­star 61 is a por­ta­ble, well-made re­frac­tor with a short fo­cal length of 360mm mak­ing it ideal for wide-field ob­ser­va­tions. The multi-coated, dou­blet op­tics use lowdis­per­sion FPL-53 glass so the scope is also suit­able for imag­ing deep-sky ob­jects. As with all re­frac­tors, a field flat­tener or field flat­tener-fo­cal re­ducer is rec­om­mended for imag­ing pur­poses but it is un­usual to use such an adap­tor for ob­ser­va­tional use. There’s no rea­son why you shouldn’t use one for this pur­pose as it will al­low you to ob­serve even larger swathes of the night sky than the in­stru­ment al­ready dis­plays.

The mag­ni­fi­ca­tion of a tele­scopeeye­piece setup is cal­cu­lated by di­vid­ing the fo­cal length of the tele­scope by the fo­cal length of the eyepiece, which in the case of your Zenith­star gives 360÷25 = 14.4x mag­ni­fi­ca­tion. This type of mag­ni­fi­ca­tion is in the binoc­u­lar realm and will de­liver great views of large as­ter­isms, con­stel­la­tions and many deep-sky ob­jects, but plan­ets and the Moon will be very small in­deed. The field of view will be 4.7° across, or roughly the equiv­a­lent of nine Moon widths.

In­sert­ing a 0.6x fo­cal re­ducer pro­duces an ef­fec­tive fo­cal length of 360x0.6, which equals 216mm, yield­ing a mag­ni­fi­ca­tion of 8.64x, which equates to a field of view of ap­prox­i­mately 15 Moon widths.

When I look at Jupiter with my Ce­le­stron CPC I see a huge black spot in the cen­tre. What could be caus­ing this?

SUSY EDYVEAN

The Ce­le­stron CPC range of tele­scopes are of the Sch­midt-Cassegrain de­sign, with a large con­cave pri­mary mir­ror at the base, a cor­rec­tor plate at the front and a con­vex sec­ondary mir­ror at­tached to the cor­rec­tor plate on the in­side of the tube.

The light they collect fol­lows a com­plex path: it trav­els through the cor­rec­tor plate to the pri­mary mir­ror; back up the tele­scope’s tube in a con­verg­ing beam to the sec­ondary mir­ror; back down the tube again and through a hole in the mid­dle of the pri­mary mir­ror; and fi­nally through the fo­cuser and star di­ag­o­nal to reach the eyepiece. All in the name of an ef­fec­tively long fo­cal length.

Tha black spot you are see­ing is the shadow of the sec­ondary mir­ror, in­di­cat­ing that you have not achieved cor­rect fo­cus.

The Zenith­star 61 is ideal for wide-field ob­serv­ing and you can make its view even wider

A pho­ton needs a GPS to nav­i­gate through an SCT Pri­mary mir­ror Starlight Eyepiece Cor­rec­tor plate Sec­ondary mir­ror Starlight

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