logo

<< - Archive - >>

Southern Cross - July 2005

COTM: Corona Borealis (The Northern Crown)

John Morland

This is an "ancient" constellation (as in dating back to antiquity – and Ptolemy lists it) which represents the jewelled crown worn by Ariadne when she married Bacchus. Bacchus cast her crown into the sky to mark the happy event and lo behold we know have an arc of seven stars that just fit in the field of view of a pair of 7x50 binoculars.

To the naked eye, Corona Borealis (CrB) is made up of six 4th magnitude stars and one 2nd magnitude star named Gemma - and as the name suggests it really does appear as a sort of central gem. Gemma (alpha CrB, Alphecca is the Arabic name) is a blue white star, 75 light years distant and an Algol type eclipsing binary of 17.4 days but only a 0.1 magnitude drop.

Zeta CrB is a pair of blue-white stars with magnitude 5 and 6 that is easily visible in my 6" telescope that I use to easily wheel around my yard to dodge trees. This is a good double for medium magnification.

Nu CrB is an apparent double star (both being orange) of magnitude 5.2 and 5.4. They are 490 and 590 light years away and easily resolved in binoculars.

Sigma CrB is another nice double for small telescopes, both yellow at 5.6 and 6.6 magnitudes and have an estimate period of over 1000 years.

Eta CrB is beyond a 6-inch except when widest, and at present very close, about 0.6". A 42 year binary, it won't be much wider than this until the 2020s.

R CrB is a yellow supergiant (halfway between Gemma and Iota CrB) at around 7000 light years distant. Usually it is 6th magnitude but then can drop to 15th within weeks!! It will then take several months to slowly crank back up to 6th. This happened in 1917, 1938, 1949, 1962, 1972 and 1977. Spectroscopic analysis shows high levels of carbon – apparently ejected from the star forming a puff of soot, which, if it happens to be in our line of sight, dims the star.

T CrB or the Blaze Star (16h 00m, 26 deg) is a recurrent nova - an almost opposite to R CrB. It usually hangs around 11th Magnitude then suddenly will rise to 2nd magnitude! It brightened in 1866 and 1946 without warning.

Corona Borealis also contains a cluster of about 400 galaxies that are more than a 1000 million light years away, and no brighter than 16th magnitude. I was raring to go to see if I could let the photons flow in my 6" telescope that I was using at the time but the clouds and haze rolled in late evening - Aw Shucks!

Review: Celestron 9.25-inch SCT Part II

Ross Gould

I had intended a detailed look at Celestron’s Advanced Series Go-To Mounting as part 2 of my review of Celestron’s 9.25-inch SCT. But while I’ve been tardily putting together notes, Sky & Telescope (International Edition, August 2005) has produced a more comprehensive review of the mounting than I’d planned. This item, by Alan Dyer, has also appeared also in Australian Sky & Telescope. I’ll therefore limit myself to a short commentary on the mounting, and refer the reader to Dyer’s excellent article for more information.

When the C9¼ first appeared it was on a flimsy tripod that handicapped its excellent optical quality. This was in the days when Celestron was owned by Tasco, and some engineering decisions were obviously being made by bean-counters. Since then, the C 9 ¼ has been remounted, along with other SCTs in the Celestron line, and the improved equatorial head is now on a very stable steel tripod with 2-inch diameter tubular legs of adjustable length, a considerable improvement. The legs also have a centre brace. Previously it made sense to buy the optical tube by itself, and mount it on a Losmandy or similar quality mounting. This is no longer essential.

The equatorial head is a re-worked Synta EQ-4 (itself a copy of Vixen’s Polaris mount) and seems better made than the EQ-4s I’ve seen previously. The head and tripod together are more than adequate for a short tube-assembly of modest weight (c.9 kg) such as the C 9 ¼. For visual observing focusing is no longer a delicate procedure to avoid shake.

As I’ve mentioned in the earlier review, one benefit of this mounting is portability. It can be dismantled easily – head and legs together are quite easily carried, and the counterweights can then be put on, followed by the optical tube. For those of us needing portability, this combination gives a reasonable aperture telescope with easy mobility.

The go-to system works nicely. Celestron require three alignment stars instead of two. The intention is to eliminate the effect of the telescope’s polar axis not being perfectly lined up with its optical axis. Stars can be selected for you by the software. And, in Hibernate mode, the scope can bypass alignment stars (though not time and date inputs) when woken up again. The Nexstar system retains its location information, but not date and time. You can get the (extra-cost) GPS accessory which eliminates this, though I don’t have it. I find that the go-to pointing accuracy is good, with objects consistently found within the low-power eyepiece field of about 40 arcminutes. And this despite not having a super-accurate polar alignment (I do a rough setting only).

The mount also allows usage without go-to for those who simply want to set it up and use the finder to get to Jupiter, the Moon, or other bright objects. Slewing noise is fairly obvious at high speed, though not at the level of some Meades I’ve heard. When tracking, nothing is audible. For power, I use a compact 12v battery, rechargeable, of a type obtainable from various sources (mine was a Dick Smith item). It works well, though I haven’t tested its endurance by doing dusk to dawn observing runs with frequent big slews across the sky.

I don’t have imaging equipment for astro-photography at present, so I can’t comment on the mount’s suitability for this purpose. Alan Dyer thinks it not very good for astro-photography, mainly due to some backlash in the dec drive – which makes it unsuitable for auto-guiders. The fairly high periodic error (with no periodic-error correction facility) could be dealt with by an autoguider. His conclusion is that for manual guiding or for short exposure track and accumulate it might be okay. I’m not surprised by this given the modest price of the mounting if bought with the telescope.

So, in conclusion. A good mount for visual observing, but not of high standard for astro-photography. For those wanting to join the astro-photography club, buying the optical tube with a Losmandy or other precision mounting would be preferable – less frustration, and better quality pictures. Go and read Dyer’s detailed account if you want to know more.

Started in November 2004
©2005 Canberra Astronomical Society Inc.


Last updated: 2005-08-14

Click here, or on the CAS Logo at the top of the page to return to the CAS Home Page