light loss in telescope?

[Ags]

Starlight passes through five glass-air surfaces in my Mak before it gets to my eyepiece. Given that my Mak features Celestron's "Starbright XLT" coatings, how much light is lost in the process?

I've heard that some diagonals lose 10% of the light - and my Mak reflects the light three times! (plus two more surfaces for the corrector plate).

[Ags]

I've now found out that XLT coatings have 96%-plus transmission. Four five surfaces that means at least 15% light loss! Ulp!

[haitch]

Don't worry too much - a reflector with standard coatings will lose around 15% for each surface. That's around a quarter of the light (the second calc is 85% of 85%) but don't forget how many times bigger the surface area of the aperture of your scope is compared to the pupil of your eye.

Some EPs will have 6 or 7 elements, think about that!

Don't forget even when you look through a double glazed window you lose around 5% for each pane - can you tell?

[FraserClarke]

Some EPs will have 6 or 7 elements, think about that!

Eyepieces *should* have decent anti-reflection coatings on them, which will make the loss ~1% at each surface.

Decent mirror coatings are >95% reflectivity.

Multiply it all up and most telescopes are >70% efficient.

What you really want to worry about your eye, which is only detecting 1-2% of the photons which get to it anyway...

[haitch]

Eyepieces *should* have decent anti-reflection coatings on them, which will make the loss ~1% at each surface.

Decent mirror coatings are >95% reflectivity.

Multiply it all up and most telescopes are >70% efficient.

What you really want to worry about your eye, which is only detecting 1-2% of the photons which get to it anyway...

I'm off to the doctors to see if I can get mine transplanted with CCDs. A high resolution long exposure monochrome in the left and a philips webcam in the right. What do you think I should have fitted in the way of filters?

[steveangel]

I'm off to the doctors to see if I can get mine transplanted with CCDs. A high resolution long exposure monochrome in the left and a philips webcam in the right

if only!!!

[FraserClarke]

What do you think I should have fitted in the way of filters?

That's what colour contact lenses are for

[Ags]

According to Celestron a catadiatropic telescope with XLT coatings has 83.5% transmission.

[haitch]

According to Celestron a catadiatropic telescope with XLT coatings has 83.5% transmission.

I wasn't far off then but as has been pointed out it's your eye that's more of a limiting factor.

If you are still worried about losing light you could always splash out on Hilux...

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