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Light gathering question


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Does a 6" Newtonian gather as much light as a 6" refractor?

I ask because when I mentioned I was thinking of buying a 4.75" refractor to a member of my local astronomy society, he said that it would be equivalent to an 8" dobsonian. I think it has something to do with the central obstruction(?) in a reflector telescope, but to be honest I'm more interested in the comparable performances of both telescopes.

Everyone says that an 8" reflector is a good size for serious observing. If a 4.75" refractor gathers as much light, would the same be true?

PS: The telescope I'm looking at is the Skywatcher Evostar 120 (EQ3-2)

Edited by John P
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Size for size, a refractor will transmit more light than a reflecor will reflect. Modern coatings on refractors permit a very high percentage transmission. If a reflectors mirror reflects, say 90%, the secondary reflects 90% of that, resulting in a toal of 81%, much less than the refractor. However, the much greater area of an 8" reflector should give it an advantage over the 120mm refractor as far as brightness is concerned as well as theoretical resolution.

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I've heard of this 'double' rule regarding refracting equivalent of mirrors. To be honest I would argue that it's more like 1.5 rather than 2, so a 4" refractor is equivalent to a 6" mirror rather than an 8" one.

James

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I have owned a 120mm f8.3 refractor (achromat) and own a 6" f11 reflector. I would say on the whole the refractor is the better buy in this case for general observing. The views of star fields and double stars are certainly sharper and with better colours etc. BUT in my experience (and I know having sold my refractor to someone who knows their stuff and said it was a very good one) for planetary and lunar use, the 6" f11 really wiped the floor with the 120mm achro when I compared them together.

I may possibly buy another refractor in the future but this would be solely for double star work and possibly where I want a slightly wider field of view. for now though my two dobs serve quite different purposes very well and supplemented by my 15x70s I have the wide field covered too.

the above may be an unfair comparison though as my 6" is atypical. the view through a standard 6" f5 reflector (I have compared them) was probably similar to that through the refractor (subject to the above beneficial aspects of the refractor mentioned above) but of those two I'd definitely choose the refractor.

in a fight between an 8" f5 reflector and a 120mm f8.3 refractor, I'd definitely go for the 8" f5.

hope this helps but I fear you'll do much soul searching before you reach a decision and even then.......

we are all in the same boat even when we have a scope already! :)

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Many reflective coating reach far more than 90% (95% for aluminium, 99% for dielectric). Do not forget that glass may absorb 5 to 10% per cm of thickness. The main issue when it comes to light gathering is aperture: doubling aperture quadruples light gathering power. Even a 20% central obstruction does little to change that. The main impact of central obstruction is on image contrast.

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Many reflective coating reach far more than 90% (95% for aluminium, 99% for dielectric). Do not forget that glass may absorb 5 to 10% per cm of thickness. The main issue when it comes to light gathering is aperture: doubling aperture quadruples light gathering power. Even a 20% central obstruction does little to change that. The main impact of central obstruction is on image contrast.

very true Michael I think an obstruction of less than 20% has little / no impact on light gathering or contrast?

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Read that the eye is more sensative to contrast then to brightness. A refractor has the advantage of contrast hence the eye "sees" more in the image even if it a bit dimmer overall.

The other aspect being unless you collect sufficent light then you won't see much no matter what the contrast is.

Wouldn't let it worry you, just get the type that you want for the reason you want.

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