Strathspey 100 mm - 45 dg (BT-100-45) Binoculars.

[Laurie61]

I have had a few pm’s asking for my thoughts regarding the BT-100-45’s and their performance so for anybody interested in buying these Big bino’s, or just wondering what they are like to use, I have put together this quick review.  

Originally, my brother and I had considered building a bino from a pair of 80<100 mm achro refractors but the more we looked into it the more problems and costs emerged so we decided on going down the more conventional route.    It was only a few weeks later that a pair of BT-100’s appeared on ab&s and we arranged to go and see them, although a daytime visit the moon was available and we were both impressed by the view. There was a small discrepancy between the performance of the individual optics, the right hand side being a little less sharp then the left but at the lower magnifications we were intending to use we decided it would not be a problem. They came with three pairs of eyepieces the lowest magnification being 30X provided by a pair of 20 mm Plossls. This was a higher magnification than we really wanted, instead looking for a usefully large exit pupil and wide field of view. As we already owned a set of TV plossls it was decided to pair up a 32 and 25 mm for use in the bino’s along with a pair of Baader neodymium filters. With the 32’s fitted the bino gives a 2.5 dg field; generate a 5.3-ish mm exit pupil and 18 X magnification, the 25’s giving 24 X and a slightly smaller field of view.

Before going on to optical performance it’s worth mentioning mounting, these bino’s came with a yoke mount, on a sturdy tripod, and do need something substantial to use them on as they are heavy.

When using the BT-100’s for the first time the obvious difference, compared to a standard bino, are the 45 degree angled eyepieces. This makes using them very comfortable and allows you to view objects for long periods without getting neck ache.

Light throughput seems good considering all the glass in the optical path, the Bino’s use a triplet achro objective along with the necessary prisms to give a ‘Real’ view. The image is sharp across 90+ percent of the field of view (F.O.V.) and although there is some false colour on bright objects it is by no means objectionable.

Targets like the Orion Nebula show a lot of detail and the BT 100’s stereo view provide a lovely immersive quality. The double cluster and Beehive look impressive, showing the different coloured stars nicely whilst fitting comfortably within the F.O.V. Recently I used them to view the Supernova in M82; at 24 X they showed both M81 and M82 in the same F.O.V whilst the SN was visible without averted vision, the SN was then at around magnitude 10.5.

One party trick the bino’s have is to use different eyepieces at the same time, working as two 100mm refractors side by side. If I don’t feel like setting up a scope, just for half an hour, I can use the bino’s in this configuration and view the planets, at up to 100X, and DSO’s at low power. The view, even without significant cool down time, is usually fine. Another use is white light solar observing; fitted with the Baader solar film they show good detail around sunspots and the stereo view also helps reveal granulation in the photosphere. With a bit of experimentation I have also managed to take some images of the Sun’s disc using a 40 mm ortho and a point and shoot camera.

Having now used these for three years I feel the BT-100-45’s do make a strong case for themselves. The build quality is bomb proof and although the optics won’t quite compete with a good refractor/diagonal the ability to view comfortably, in stereo, at a range of magnifications provides a very versatile package. 

Laurie.

A few photo's, an afocal shot using the 32 mm TV plossl and a canon g9. The mobile phone tower is 1.5 miles away. 

The Bino's in solar mode.

The business end 

White light solar shot with the bino's