Scope Doc­tor

Our equip­ment spe­cial­ist cures your op­ti­cal ail­ments and tech­ni­cal mal­adies

Sky at Night Magazine - - CONTENTS - PETER GOLDSTEIN With Steve Richards STEVE RICHARDS is a keen astro imager and an as­tron­omy equip­ment ex­pert

I like view­ing dou­ble stars. I cur­rently have a 6-inch re­flec­tor but would a re­frac­tor work best? What would you rec­om­mend for a bud­get of £300?

Ob­serv­ing dou­ble stars is a very pop­u­lar as­pect of as­tron­omy and an area where am­a­teurs can con­trib­ute greatly as very few pro­fes­sional ob­ser­va­to­ries make such ob­ser­va­tions th­ese days. Ob­ser­va­tional data is vi­tal for in­creas­ing our un­der­stand­ing of stel­lar evo­lu­tion, so am­a­teurs can carry out real science as well as en­joy­ing the won­der­ful sights.

A 6-inch re­flec­tor will pro­duce some great views of dou­ble stars but a good re­frac­tor is likely to give a bet­ter ob­serv­ing ex­pe­ri­ence as there is no spider vane to cre­ate dif­frac­tion spikes which can cause is­sues with very close dou­bles. A long fo­cal length re­frac­tor will give ex­cel­lent views and make fo­cus eas­ier to achieve but this needs to be com­bined with as large an aper­ture as you can af­ford. Larger aper­tures will re­veal fainter com­pan­ions but, just as im­por­tantly, they will have a higher res­o­lu­tion al­low­ing you to split closer dou­bles. Long fo­cal length re­frac­tors re­quire a sub­stan­tial mount to counter the ef­fects of wind shear in par­tic­u­lar.

Un­for­tu­nately, large aper­ture re­frac­tors can be quite costly, so a very pop­u­lar, lower-cost al­ter­na­tive in­stru­ment for dou­ble star ob­serv­ing is the 5-inch Mak­su­tov-Cassegrain. This in­stru­ment has a wide aper­ture and a long fo­cal length of around 1,500mm in a short phys­i­cal length and would be an ex­cel­lent choice within your bud­get. The Sky-Watcher Sky­max-127 Mak­su­tovCassegr­ain or Orion Apex 5-inch Mak­su­tov-Cassegrain tele­scopes would be ex­cel­lent choices. It is un­likely that the ‘clip­ping’ you’re ob­serv­ing is caused by the positionin­g of the eyepiece in the eyepiece holder. How­ever, eye­pieces in­cor­po­rate a field stop which con­sists of a metal or plas­tic ring that de­fines the edge of the field of view as part of their de­sign. If cor­rectly placed at the fo­cal plane within the eyepiece, a field stop pro­duces a wellde­fined cir­cle which avoids a grad­ual drop-off in the fi­delity of the view. The field stop lim­its the ap­par­ent field of view of the eyepiece which in turn lim­its the true field of view ob­served through the tele­scope.

If you know the di­am­e­ter of the field stop, you can cal­cu­late the true field of view through the tele­scope in de­grees by di­vid­ing the eyepiece field stop di­am­e­ter by the fo­cal length of the tele­scope and then mul­ti­ply­ing the re­sult by 57.3.

I have trou­ble see­ing un­clipped images through high mag­ini­fi­ca­tion eye­places Is the po­si­tion of the holder in­cor­rect? TINA COX

The Sky-Watcher Sky­max-127 Mak­su­tovCassegr­ain is a good, low-cost choice if you want to see dou­ble

Field stop A field stop gives a crisp edge to the field of view

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