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Sweet spot for binocular observing?

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mdrapp's picture
Sweet spot for binocular observing?

I just finished up Mike's excellent CHOICE course on visual observing.  One of the things I found useful about the course is that it systematically makes one think about one's skies and one's equipment and opportunities for observing.

The course helped me realize that the vast majority of my VSO'ing will probably be done on weeknights with relatively short sessions, and most likely with binoculars.  So I am now thinking about optimizing my equipment for that.

It is very easy to go crazy with binoculars.  Before you know it, you've talked yourself into a pair of 25x100s on a mount heavier than one of your telescope's mounts!  I want to be careful not to do this.

My current binoculars are a pair of Pentax 10x50s.  My NELM is 4.5 or so, a little better in the winter, a little less in the hazy summer.  That plus the streetlights in the vicinity allow me to get down to mag 7.5 or so in the 10x50s.  The two things I would like to improve on are going a little deeper and also a steadier view (which should also help me go deeper).

I'm thinking about a pair of 15x70s.  I looked at larger binoculars, but they get really heavy, really fast and the field of view starts getting quite small.  I really don't want to go smaller than four degrees, which is the typical field of view for 15x70s.  This size would also keep the mount from getting too unwieldy.  Peterson Engineering has a pipe mount that is relatively inexpensive plus it can be left outside (  It's main disadvantage is its large footprint, nearly seven feet.  

The one thing that is nagging me is that the comparison stars for the variables in the AAVSO Binocular Program are often more than 5 degrees away from the variable.  Handheld, even though the view isn't steady, I can go back and forth between the variable and the comparison star fairly quickly.  With a mounted pair of binoculars, this probably will be curtailed.  I am uncertain how much of a problem this will be.

Thoughts, guidance?

--Michael in Houston (RMW)

jbirriel's picture
Summer 2016 offering of this?


This sounds like a great course.  I would be interested in taking it if offered again in the summer of 2016.  As a full time faculty member, I could not find the time to take the course this time around! 


williampme's picture
Binocular observing

I have been doing the binocular boogie since 1988.  I will tell you what I have experienced.  Over 80% of my variable star observations have been via binoculars.  I only use telescopes when the binoculars won't do.  I use mostly 10x50 and 15x70 binoculars, but I do have 25/40x100 binoculars and 7x35 as well and I have pretty much owned and used everything in between.   

I guess the first thing I should admit is that I do not use the binocular charts.  The lack of comparison stars is a major factor in that decision.  I use the regular charts and they have served me well.  I have found that for 15x70 binoculars a "B" scale chart is ideal.  It covers 3 degrees and goes to a little under 10th mag usually.  As an example, I can follow Z UMa through the entire cycle with a B chart and 15x70 binoculars.  Smaller binoculars work well with an "AB" chart.  That is generally for stars brighter than mag 5 or so.  A "C" chart works for the 25x100 giant binoculars. 

The 15x70 binocular is a very good astronomy instrument - large enough to show many things, portable, and does not require too large a mount.  But it DOES need a mount.  I also mount my 10x50 glasses.  They have a surprising reach when steady and are invaluable for the brighter stars when the comparison gets difficult in the 70mm glasses.

There is no real "one size fits all" approach to this.  You will find yourself modifying your methods as you get more familiar with your stars, adding to and deleting from the list as you find what works for you in particular.

BRJ's picture
Mike, given your described

Mike, given your described sky conditions I, like others responding in this thread, concur that your best choice in binoculars would be a pair of 15x70's. I've split my observing time between binocular and telescope VSO for better than half a century now, intitially employing a pair of WA 10x50's and in more recent years 15x70's. With a little practice you'll find that hand-holding 15x70's pretty steady while working your VS program becomes no problem at all...especially if you can be seated in a lawn chair, or some such, while doing so.

Although at one time or another I've owned a pair of just about every size binoculars available (3x20 to 20x120) my workhorse instrument has always been either the 10x50 or 15x70 binoculars for working my "bright" variables, or comets, no real need for any larger examples.

J.Bortle   (BRJ)



mdrapp's picture
Jennifer - yes, it was a

Jennifer - yes, it was a great course.  The weekly exercises force you to write out your thoughts on aspects related to your observing and conditions.  It is also great seeing everyone else's responses and how that illuminates your own.   It was exactly what I needed, someone to walk me though the process of finding which stars would be best for me to observe.  

It also showed me VSX.  I thought I knew what VSX was...turns out I had no idea what VSX was!   It is so cool we have a program that I can essentially ask, "Dear computer, what variables in the AAVSO Binocular Program are visible in Cygnus whose minimia are brighter than magnitude seven?"  :)

William & John -- thanks for the advice!  I was out with my 10x50s last night, which have a 5 degree FOV and confirmed I probably don't want to go any smaller than 4 degrees in FOV.  So if I do decide to go larger, at 15 or 16x70 seems definitely to be the answer.  The other nice thing with the mount is that it removes the constant up-down-up-down-up motions to look at a chart.  

It's interesting that one of the things that the CHOICE course helped me realize is that the vast majority of my observing opportunities will be short, weeknight sessions, in an increasingly light polluted enviornment and I need to find the best instrument for that.  

William, I'm also going to experiment with using some traditional charts with some of the variables.  Most of the binoculars charts work well for me, at least for the variables I've done, but for a few something seems not optimal and a customized chart might work better for me.

pox's picture

I used to use 10*50 and then 20*70 for the (former) Bunocular Sky Society. Both were mounted on a conventional camera tripod, no problem. You can secure both axes on your star while you consult charts/go in for a coffee etc, and the tripod had no difficulty in supporting the large bins. Because all 3 axes are usually changeable (alt, az, and tilt) you could even rig up an equatorial binocular mount if you wanted! Another advantage is the fact that you can stow both bins and tripod in a backpack and travel to a less light-polluted area. I know that some folks don't like camera tripods (fair enough) but I have always found them perfectly useful.

Charts... never used an AAVSO binocular chart but the classic (b) charts will nearly always contain suitable comp stars.

LKR's picture
my binoc experiences

I have used 10x50s and 20x70 (the latter mounted on a binocular tripod). I have had good luck with the binocular charts so far.

TYS's picture
Binocular Observing

I've been using 12x63's or 10x50's for VSOing for many years and I also use the regular (a, ab) charts with excellent results.

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