Binocular recommends and exit pupil

[murgen]

I'm looking to get something light (hand-held) by way of optical equipment to take with me on holiday, seeing as how it might be the only chance of some clear skies i get this summer.

Of course there is a limit to what a) the airline, and (more crucially) the good lady would consider as reasonable for packing.

This leads me to think that medium sized binoculars are the way to go, and are likely to give some good use back at home too.

So, we're talking in the region of £100 i suppose, and there seem to be many choices and as many opinions as to what might be suitable.

As well as any recommends, can someone explain binocular exit pupil and whether this is as big an issue as it is with scope eyepieces for a slightly "senior" participant in the hobby?

They say an exit pupil (night-wise) of around 5 or 6 is right once you have passed the first flushes of youth.

But presumably during the day (when the bins will also get a lot of use) your exit pupil will be a lot smaller.

They say 10x50 are OK weight-wise, but are these OK from an exit-pupil perspective?

cheers

M

[glowjet]

Your exit pupil is a function of the main objective divided by the mag, eg, 10x50 has an exit pupil of 5mm, which is the minimum recommended, it represents the cone of light coming through the eye piece, and as you have said it also related to your age as you can see far better when you were young then when your pupil size diminishes as you get older. large magnification with a small exit pupil could leave your view looking like peering down a drainpipe. Having said that good quality bins with an exit pupil between 4and 5 should not be discounted, for the average sighted person they should, in the most part, be fine for Astro work. the other thing to take into account is weight, if they are much larger than 10X50 then holding by hand is going to become an issue, unless you want to invest in a larger pair, then a tripod or some form support will be the order of the day

John.

[glowjet]

I forgot to add, amongst others Strathspey Marine 10X50 are well regarded, they came top in a review by the Sky@Night mag some time ago, although I am sure there will many other makes suggested to you before long, enjoy your holiday and the dark skies, Oh and the dry weather

John.

[umadog]

large magnification with a small exit pupil could leave your view looking like peering down a drainpipe.

If I understand what you mean, I think the "drainpipe effect" you mention isn't related to the exit pupil diameter. It's related to apparent field of view of the eyepiece (which is fixed). Looking down an eyepiece with a 30 degree apparent field feels like looking down a drain pipe irrespective of the magnification or exit pupil. On the other hand, the 3.7mm Ethos has a 110 degree apparent field of view and that will feel wide even if the exit pupil is under 1 mm. The exit pupil is the diameter of the light beam entering the iris. The apparent field of view is the range of angles over which that light beam can arrive at the iris. You can see this diagrammatically here: http://www.telescope-optics.net/eyepiece1.htm Apparent field of view is the angle epsilon. You can see how it is possible to alter epsilon whilst keeping the exit pupil the same diameter.

Exit pupil diameter decreases as magnification increases, which is why higher powers produce a dimmer image for the same objective. Exit pupils approaching 1mm will show a lot of floaters.

[glowjet]

You are so right Umadog, but just trying to give our poster a general overview relating to some of the cheap end of the market bins with high powered eye pieces, narrow FOV, and in some cases inadequate objectives. Also I might add, those with brightly coloured optics

John.

[cantab]

Indeed in bright daylight the eye pupil will be smaller than the exit pupil of a 10x50 or 7x50 binocular. But in bright daylight it doesn't really matter that some of the light misses your eyes, there's plenty of it anyway. The only problem with using a 10x50 in the day is that a smaller aperture binocular could give the same views for less weight and bulk. And the "oversized" exit pupil for daylight use does bring the advantage of allow leeway in eye positioning, which is I believe why 7x50s are popular for marine use.

[E621Keith]

I highly recommend the Pentax PCF WP2 8x40 which can be purchase from Microglobe for £99 (or you can get FLO to price match it)

If you want to use the binocular handheld, the view through a 8x will be more stable than a 10x. The 40mm objective might sounds a bit small, but unless you have very dark sky, your pupil won't dilate more than 5mm anyway so there are nothing to gain from a larger aperture for a 8x binocular. The Pentax's click lock twist up eye cup is also one of the best I've used.

Alternative: Optricon Aspheric WA ZWCF GA 8x40

[E621Keith]

Indeed in bright daylight the eye pupil will be smaller than the exit pupil of a 10x50 or 7x50 binocular. But in bright daylight it doesn't really matter that some of the light misses your eyes, there's plenty of it anyway. The only problem with using a 10x50 in the day is that a smaller aperture binocular could give the same views for less weight and bulk. And the "oversized" exit pupil for daylight use does bring the advantage of allow leeway in eye positioning, which is I believe why 7x50s are popular for marine use.

I think the other reason is it's very hard to keep the binocular steady on a rocking boat for any magnification over 7x.

[umadog]

...trying to give our poster a general overview relating to some of the cheap end of the market bins with high powered eye pieces, narrow FOV, and in some cases inadequate objectives. Also I might add, those with brightly coloured optics

Indeed, I see. I have a set of Meade Travelview which are about 10x50 and were pretty good for under 100 quid. Wide FOV and little chromatic aberration. Maybe they are still being sold?

One good thing about binoculars is that they're easy to test if you go down to a shop and have a look at a few sets.

[murgen]

thanks for your replies (even if i didn't quite follow some - my fault for raising a technical issue past my pay grade ).

so an 8x40 might out-perform a 10x50 by the fact that it is a bit easier to hold?

Nice choices Keith, i have been looking at the Opticron at FLO, SKyAN test winner and they are quite a bit under budget.

The Pentax are £136 at FLO, i suspect they would baulk a bit at price reducing to £99.

cheers

M

[umadog]

so an 8x40 might out-perform a 10x50 by the fact that it is a bit easier to hold?

Quite possibly.

Sorry, what I wrote above was a little over-complicated and not really relevant.

[E621Keith]

Yes, an 8x40 can potentially out perform a 10x50 simply because it's easier to hold steady. I found 10x magnification to be the upper limit of what can be used handheld, an 8x will leave more leeway. It's harder to see details in an image that's shaking than one that's stable. The stability of a binocular depends on your own hands, the weight and how well the binocular sits in your hands. The latter depends on the ergonomic of that binocular design and it's highly subjective, so it's best to try it out in a shop if you have a chance. The Pentax PCF is shaped in such way that it fits my hands nicely, unlike my cheap 10x50 which feels less stable in my hand.

Of course generally a 10x50 of equivalent quality will be better because it is able to gathered more light and give a slightly more magnified view, but don't discount a good 8x40 if the binocular is to be used handheld only.

[LukeSkywatcher]

IMHO,the biggest you can go on bins that are handheld is 10X50. Anything over that really needs a tripod to be mounted on. Strathspey bins are simply amazing. I have the 20x90 version. The 10X50 marine version is considered one of the best bins for their price.

[murgen]

some nice choices.

i have had a look at the 10x50 strathspey marine on their website. They seem to be well thought of, although they are 1200g which sounds a tad heavy, especially with a 20Kg case limit on the plane.

They also do an 8x42 that are half that weight, however they are roof, not porro prism.

Are this type not regarded for astro use?

i think I'm edging towards the opticron 8x40.

thanks for your views.

M

[E621Keith]

Usually you need to spend twice as much on a roof to get the same optical quality as a porro. Roof prisms also loose a bit of light, so a porro are often brighter for the same aperture.

Since you have a limited budget, I'd say go for the Opticron.

[murgen]

Thanks Keith,

i suppose the budget is only limited by what i want to part with for this use. I still need a raci finder to replace the straight one that came with my scope and that's a similar price.

I'd love to take my scope abroad, but that would take up two lots of weight allowance and struggle to fit in the cases.

the 8x40's sound like a light-ish item to take to add a bit of casual observing to the holiday.

Hears a thought, maybe the 9x50 raci would do the holiday job - what do you think to that?

Or even the 9x50 straight - upside-down, but still a clear view.

M

[E621Keith]

A large finderscope will make a decent rich field refractor (and vice versa), but you will need a tripod to mount it, so the overall system weight could be higher. In the end, it's up to you how much 'quality' you want. For casual travelling a binocular could be more useful because it can be used for terrestrial viewing as well.

[glowjet]

Just a thought, if you invest in your Raci finder, could one of those short desktop tripods from a photo shop be adapted with a universal camera holder and the finder mounting bracket, you could then use it at a nice dark country park site with it sat on a pick nick table with a nice brandy, or whatever is your tipple, you are bound to see stars and many nice doubles

John.

[murgen]

That is a very good point in favour of not spending too much.

However keen one is to do a spot of casual continental clear sky viewing, on ones jollies, one will almost certainly be at least partially oiled by the time it goes dark .

M

[Alan_B]

Hears a thought, maybe the 9x50 raci would do the holiday job - what do you think to that?

I think i'd find the cross hairs in the finder annoying

[Alan_B]

As maginification reduces brightness would a 7x50 give brighter views than an 8x40 despite the exit pupil being (an oversized for an ancient astronomer like me) 7mm

[glowjet]

With the extra light gathering of the 50mm optics I should think the answer will be yes, 7X50 is often quoted as the best alternative to the 10X50 as it easier to hold steady with the slight reduction in mag. With my own bins the 10X50 Adlerblicks, in the reviews the 7X50 model was regarded as the best, giving almost perfect star shapes right at the edge of the field .

John.

[murgen]

course you would.

i'm ordering the bins as we speak.

M