Binocular observing in Lichfield 27/10/2017 @ 22:00-23:30: Vixen SG 2.1×42 Widefield Binoculars vs. Canon 10×30 IS binoculars

When we went to the International Astronomy Show, Damian was not going to buy anything……but then got tempted by a discounted pair of Vixen SG 2.1×42 Widefield Binoculars at 76% normal price. These premium binoculars are quite different to your usual astronomy binos – they are only 2x magnification with a massive 25 degree field of view.

The following is an extract on a review on these unusual binoculars from https://urldefense.proofpoint.com/v2/url?u=http-3A__alpha-2Dlyrae.co.uk_2014_10_25_vixen-2Dsg-2D2-2D1x42-2Dbinoculars-2Dreview_&d=DwIFAg&c=MI93KOczqiyhXk-AHGoDDUkr93TIMXWlgkPNdzxg9FA&r=4QbXFqVFP3-iwXXmdJhGrxCWcMPIAE70hQGiF6C2bW0&m=554ps7561Y2QFKTN9e8AF_TUnM9Am1RivTHFCxSjC5U&s=qQFqLy91c8jfq_MLjFBDJK6nJ_9dP5P7SLzMNerDhRY&e=:

“So why use 42mm lenses? In a Galilean system, the actual field of view scales linearly with objective diameter.  As such, for any given power, a doubling of the objective diameter results in the real field doubling.  As the exit pupil is virtual and located inside the optical system there is no fixed eye point and hence to maximise the field seen, the eye must be placed as close as possible to the eye lens, with the obvious implication for spectacle wearers.  However, the Vixen SG have fairly generous sized eye lenses which provides a more comfortable “eye relief” (stated as 8.4mm) without cutting off parts of the achievable field of view.

The question of light grasp and magnitude gain is not approached in quite the same way as a Keplerian telescope. The magnitude gain of a telescope (ignoring the benefits of darkened sky background) can be calculated as 5 x Log10 (D1/D0) where D1 is the diameter of telescope’s primary objective and D0 is the diameter of the eye’s dark adapted pupil.  In a low power Galilean system, the light grasp is dependent on the magnification and light transmission.  For the sake of simplicity, if we assume that the transmission efficiency is 100%, the Vixen with their magnification of 2.1x results in the effective increase in your pupil diameter by 2.1x, and as such the increase is 2.12 which is 4.41x more light.  The magnitude gain is given as Log10 (4.41) x 2.512 = 1.62.  If we assume 95% transmission, that factor is applied to the light grasp gain and results in magnitude improvement of 1.56.”

So, how do they measure up in practice? First light for us occurred last night – Damian bought the pair he bought around to my house and we compared it to my Canon 10×30 image stabilised binoculars. This is a pair I bought years ago from Astronomy Buy and Sell and I am particularly found of them – small, portable, great contrast – and they often go with me on holiday, when I have been banned from taking anything larger for observing….I even bought a “portable” 80mm Equinox and have been banned taking that!

Saturday night was predicted to be clear all night on BBC Weather website – which in practice meant rolling banks of cloud with intermittent 20-30 minute periods of clearness. It was not the best sky out although the Milky Way was visible overhead and the Double Cluster visible to the naked eye, although we debated whether we could see M31 with the naked eye.

Neither of us could see > 1 or 2 stars at best in the Great Square of Pegasus by naked eye alone tonight.

….Then we used the Vixen binos. Wow!! Wow!! They fit snuggly over your eyes and you move your head around as if you had no optical aid. The vast field of view makes it an experience similar to naked eye observing but the increase in number of visible stars was dramatic. I could see 11 stars in the Great Square of Pegasus. M31 was obvious. We could not see M33 last night with them but clusters were often obvious, especially in Cassiopeia and the Double Cluster was impressive. We each saw meteors x several that the other could not see with then naked eye – and they appeared bright in the Vixen binos. I can’t wait to try these out at the next meteor shower.

The view is quite different to that in the Canon IS binos. Not surprisingly, with the Canon IS 10×30 binoculars, the large light grasp and higher magnification meant that more could be seen – M31 was magnificent and big and bright and M33 just visible by averted vision and the Double Cluster incredible and smaller open clusters became visible including at least 2 of the main clusters in Auriga. However, the Canon did not allow you to move your head around the sky in the way you do with the naked eye or with the Vixen binos. The higher magnification means you need to know what to look for and where and aim for it.

Our conclusion is that the Vixen binos do have a unique role. They open up a whole new vista and have a solid well-made feel. It is worth buying eye cups to prevent stray light to go with them. Damian mentioned that after their initial introduction into the marketplace, Vixen did start to supply hard plastic cups that just fit over the metal eyepieces. These appear to push the eye just slightly further away from the eye lens – thereby reducing the field of view (FOV). He then tried some spare Televue eyecups he’d received when buying extenders for his 40 and 32mm TV plossls that he uses for solar observing. Even though they practically only extend a mm or two past the eye lens on the binoculars (once secured so they don’t fall off), they did start to cut stray light from your periphery. After some investigating on the web, he found a review of the binoculars on US forum Cloudy Nights. One post mentioned the use of Baader Hyperion ‘winged’ eyecups. These are around £10-14…. each! The ‘best’ place to buy Baader gear in the U.K. appears to be MicroGlobe – although you often have to wait for them to order the stock themselves!

The new winged eyecups arrived on Friday morning and do indeed cut stray light when observing, helpful both in the daytime and under darkness.

They are quite expensive and are not a total observing solution so this needs to be considered if you are thinking of buying them.

Andy & Damian

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