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Telescope light transmission figures (1 Viewer)

William Lewis

Wishing birdwatching paid the bills.
United Kingdom
I've noticed again that even the latest and greatest scopes have much less transmission than binoculars of a similar quality. As a case in point the new ATC scope from Swarovski has a transmission figure around the 85% range according to the recent review on house of outdoor where as their binoculars are in the low to mid 90's with the old school porro's in the mid 90's. Your paying more money than an SLC for example with 2 barrels of the same size as an ATC with 7/8% less light coming through its one barrel. Also my ctc draw tube if memory serves had a transmission around 86% the last time it was published by swaro.

I would assume this figure varies depending on whether a zoom eye piece is fitted (more glass) but would be interested to hear if anyone has tested this? With a fixed mag eye piece I'm not sure what the causes are?

I do a bit of astronomy (more in winter, there's only so much owl watching you can do!) and reading around it seems that astro refractors have much higher transmission values than our spotting scopes so is all that light lost in turning the image the right way up? I know in astro to a degree aperture is king and portability and size are less of a concern so I can understand transmission being less of an issue but for birders portability is very important and the ATC is very portable but also comparably dim in terms of transmission when for its small objectives this should be even more important.

Is it caused by the higher magnification of scopes? I doubt this as when compared to binoculars the higher magnifications seem to have no effect on transmission.

Just curious.
Coated opera glasses and coated astro refractors with inverted images may have transmission figures above 95%.

However, Newtonian reflectors with ordinary mirrors may have transmissions of 70% to 80%.
The same with compound telescopes.

One can get enhanced mirror coatings but for 200 years or more low transmissions did little to stop discoveries being made.
Transmission was probably 60%.

I used uncoated refractors and uncoated eyepieces for decades.

I think that birdwatchers may over emphasise the importance of high transmission, unless the lost light is bouncing around inside the scope and causing a lot of glare.

I am not sure why spotting scopes may have less transmission than some binoculars.
Maybe the light path goes through thicker glass elements.

I have no problem using binoculars with transmissions of 80%, maybe less.

I know what you mean, transmission is less of a thing with astro, most makes don't even quote it, want more light - get a bigger scope! I'd suppose in theory straight scopes would have a slightly higher transmission than an angled one as they don't need the additional prism to turn the light up by 45°
Good point on the glass though, focal length of spotters is way lower than there equivalent aperture astro refractors in general.
Taking into account refractor secondary spectrum and transmission loss in glass.

Refractor uncoated objective, uncoated eyepiece transmission 62%.
Coated objective and coated eyepiece 79%.

Cemented objective, coated objective, coated eyepiece 81%.

Newtonian reflector including losses due to central obstruction.
Aluminium coated mirrors and uncoated eyepiece. 57%.
Aluminium coated mirrors and coated eyepiece 63%.

The above coatings are single layer coatings.

If multicoated surfaces and in Newtonians enhanced silver or multilayer coatings, then modern instruments do much better.

William Herschel's Newtonians had speculum mirrors that he repeatedly repolished and refigured.
He made hundreds of telescopes which he sold.
The transmission figures must be very low, yet he made hundreds of discoveries. and with his sister discovered many comets.

With modern multicoated scopes there are still glass transmission losses, although some glass types may be high transmission.
There are still secondary spectrum losses in refractors.
I don't know how much eyepieces lose.

In actual use it is likely most scopes are not completely clean, with some haze or dirt build up both on external surfaces and maybe internally.
So actual transmission is probably less than when brand new.

I have had a brand-new waterproof Chinese binocular with fungus inside.
William Lewis post 1,
If you look at the transmission values of other telescopes, see quite few on the WEB-sitee of House of Outdoor, you can notice than the top telesopes I have investigated (Leica, Swarovski, Zeiss, Nikon) also show transmission values of 80-86% so nothing new here.
Gijs van Ginkel
William Lewis post 1,
If you look at the transmission values of other telescopes, see quite few on the WEB-sitee of House of Outdoor, you can notice than the top telesopes I have investigated (Leica, Swarovski, Zeiss, Nikon) also show transmission values of 80-86% so nothing new here.
Gijs van Ginkel
Hi Gijs. Yes I noticed that - thanks for all your work! I'm just curious as to why this may be.
It's a good question.

I think your intuition may be sound: transmission ends up lower for zoom eyepieces due to their complexity -- and I'd suggest also budget models like a CTC drawtube due to simpler coatings. Someone could measure it for top-quality scopes with prime eyepieces to confirm that it's higher, as expected.

Usefully, Meopta give separate transmission figures for their S2 scope (95% in daylight) and 20-70x eyepiece (92%), giving an overall result around 87% with the zoom looking like the weak link. They don't make a prime anymore, but the 30x for the discontinued S1 had 96%, which would give closer to 91% overall, on par with many high-end binoculars.
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