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Jon_Doh
sage
Reged: 09/16/11
Loc: Area 51
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Re: Aperture Comparison
[Re: simpleisbetter]
#4891631 - 11/01/11 09:30 AM
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There are other factors that come into play besides simply the size of the glass such as light loss due to obstruction or reflection or simply the difference in the quality of glass and its coatings. I have read tests that actually measured the light that was gathered and transmitted to the eyepiece and that was what I was referring to - the actual amount of light that gets to the eyepiece relative to aperture and each type of telescope design.
I've looked through comparable apertures at star gazings and I tend to agree with the study I cited. But others may see differently and have a different opinion. Plus, there are all sorts of math formulas one can quote that will keep the debate raging. Anyway, it's just something to chew on. Meantime, I'm off to try and catch a comet.
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simpleisbetter
Carpal Tunnel
Reged: 04/18/11
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Re: Aperture Comparison
[Re: Jon_Doh]
#4891688 - 11/01/11 10:18 AM
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Curious, would you care to share the links to those tests? Even though I don't accept your argument, based on the fact my experiences over the last 40+ yrs I've been observing have shown the opposite, I'd like to read of actual throughput tests, especially WRT both intensity and wavelength response. I've not seen or heard about those tests. Otherwise without the data, and I'm just speaking for myself, I can only accept these statements as hyperbole.
If you don't have links, can you reference the articles, publications, and dates so that I might be able to read them?
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Mr Onions
Two Time International Photographical Competition Winner
Reged: 04/14/07
Loc: Newcastle upon Tyne.
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Re: Aperture Comparison
[Re: simpleisbetter]
#4894000 - 11/02/11 02:50 PM
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Charts mean NOTHING!
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simpleisbetter
Carpal Tunnel
Reged: 04/18/11
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Re: Aperture Comparison
[Re: Mr Onions]
#4894139 - 11/02/11 03:56 PM
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I'll agree with you there Mr Onions, a chart is nothing, especially when it's incomplete - and I find the chart in the original article lacks enough information to be incomplete; information like aperture area, secondary obstruction size, coating efficiency, and it does NOT take into account the effect magnification and the resulting contrast from it has on a scopes magnitude limit. I know I've seen stars to mag 13-14 with my 4" ED and transparent skies & have observed Stephen's Quintet with it; and more than once my old 8" Coulter showed me the central star of M57 and neighboring IC1296. Neither of those situations should be possible according to that chart, yet they did/do happen. I find SkyTools models the magnification-vs-magnitude limit pretty well, much better than the static tables do.
As for the claims above though of a 6" beating an 8"; my eyes have repeatedly shown me that no 6" refractor I've looked through yet can match an 8" newtonian or SCT in image brightness or resolution when all other factors are equal(clean optics, cooldown, collimation, magnification). Because of that I'm curious to read those test and OBJECTIVE comparisons, if they exist.
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SeattleScott
scholastic sledgehammer
Reged: 10/14/11
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Re: Aperture Comparison
[Re: simpleisbetter]
#4894837 - 11/02/11 10:36 PM
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A newtonian with high reflectivity coatings will trump an SCT of the same aperture due to the SCT having a larger central obstruction, a corrector plate and diagonal mirror. Just more obstacles for the light to get through. The 83.5% reflectivity for the SCT is system transmission of the corrector, primary and secondary as I understand. And it is also overall transmission, not peak reflectivity which is closer to 90%. Based on Celestron's system percentages, I estimated that their SCT's have 95% reflectivity coatings. The corrector plate is pretty minimial, maybe a 1-2% light loss and the diagonal could be as high as 99% reflectivity. So the main differences are the CO and the reflectivity of the mirrors, which can vary considerably. One advantage of the SCT is that the mirrors stay cleaner so they do not fade as quickly.
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simpleisbetter
Carpal Tunnel
Reged: 04/18/11
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Re: Aperture Comparison
[Re: SeattleScott]
#4894914 - 11/02/11 11:22 PM
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Exactly Scott. Of course, the biggest advantage of the SCT is its versatility and flexibility of uses, and its size is pretty good too WRT the size of mount needed vs same aperture newtonian.
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Mr Onions
Two Time International Photographical Competition Winner
Reged: 04/14/07
Loc: Newcastle upon Tyne.
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Re: Aperture Comparison
[Re: simpleisbetter]
#4895137 - 11/03/11 04:39 AM
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There' one way and one way only to test a telescope and thats eye up to the eyepiece.
All of these charts and graphs and maths leave me cold.
Astronomy books are the biggest culprits in this regard.
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simpleisbetter
Carpal Tunnel
Reged: 04/18/11
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Re: Aperture Comparison
[Re: Mr Onions]
#4895339 - 11/03/11 09:26 AM
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But subjective testing by itself is full of bias and a predisposed desire of what we want the outcome to be, mostly to justify what we have. We must get our bias and predisposition out of the way so that our evaluation is as honest as possible. The only way to do that is backup observations with objective testing and measurement. Subjective testing by itself is meaningless and unscientific. They must go hand in hand.
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Lee Jay
Pooh-Bah
Reged: 02/27/08
Loc: Westminster, CO
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Re: Aperture Comparison
[Re: Mr Onions]
#4896148 - 11/03/11 06:34 PM
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Quote:
There' one way and one way only to test a telescope and thats eye up to the eyepiece.
All of these charts and graphs and maths leave me cold.
Astronomy books are the biggest culprits in this regard.
Sadly, humans are the poorest instruments ever constructed. They are inaccurate, inconsistent, subject to personal bias, subject to professional bias, and subject to peer pressure. That's why scientists dismiss them and why they are given the least weight in a legal proceeding.
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