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Aperture Comparison

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#1 Tom T

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Posted 16 May 2009 - 09:25 AM

By Jared Willson

#2 Samir Kharusi

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Posted 16 May 2009 - 10:36 PM

A very good article! By the way, that question of a premium 5" APO outperforming an average 8" SCT is unlikely to fade away as long as a 5" premium APO costs 3 to 7x an 8" SCT...

Here are examples of actual planetary images taken at the same rooftop (so very likely similar seeing conditions but unfortunately different years), same person (so same level of ability in post processing), same webcam (ToUcam Pro):
Posted Image
Left to right: TV140 (5.5" premium APO), C8 and C14

#3 Zad

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Posted 17 May 2009 - 08:18 AM

Great article! Thanks for all the hard work. I did notice one error though. The Intes Micro MN65 is listed as having a 150mm apeture, but it has a 165mm apeture. The MN66 is a 152mm according to specs. Not sure where the 22% CO came from, but the MN65 is 26% and the MN66 is 20%. Sorry to nitpick, but this was confusing me for a bit.

Anyway, thanks again for the great post. This is certainly an area of interest that gains a lot of attention in the forums. It will come in handy!

Joe

#4 Midnight Dan

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Posted 17 May 2009 - 01:36 PM

Excellent article! Thanks, Jared!
-Dan

#5 jrbarnett

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Posted 18 May 2009 - 07:59 PM

Nice report Jared.

The chart is very handy too.

Regards,

Jim

#6 Psyire

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Posted 20 May 2009 - 07:42 PM

Very nice chart, thanks a bunch for the write up!

#7 kfred

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Posted 22 May 2009 - 07:29 AM

Great info and chart.

Fred

#8 astrodon

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Posted 30 May 2009 - 11:58 PM

Is the "Meade" 8" Mak-Newt perhaps an Intes?

#9 tomstep

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Posted 08 September 2009 - 06:59 PM

Great article Jared. I am just catching up.

Your article considers elements affecting through put and contrast but these comparisons also need consideration of aberration, e.g., the chromatic aberration in refractors and coma in Newtonian reflectors. Do you have some guiding thoughts on how these affecting seeing details, especially associated with CCD photography?

#10 Juan Irias

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Posted 16 September 2011 - 12:37 PM

Good information for "choose".
A question: the vixen VMC200L is "open tube", what implies this vs a closed tube like a LX200??

#11 Mr Onions

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Posted 22 September 2011 - 02:51 PM

How come an observers eye is never brought into these equations?
I bet years of observing experience is worth more than a few inches step up in aperture.

#12 Daniel Mounsey

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Posted 22 September 2011 - 10:27 PM

A very good article! By the way, that question of a premium 5" APO outperforming an average 8" SCT is unlikely to fade away as long as a 5" premium APO costs 3 to 7x an 8" SCT...

Here are examples of actual planetary images taken at the same rooftop (so very likely similar seeing conditions but unfortunately different years), same person (so same level of ability in post processing), same webcam (ToUcam Pro):
Posted Image
Left to right: TV140 (5.5" premium APO), C8 and C14


Ironic you say that considering your images say otherwise. :question:

#13 xession

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Posted 21 October 2011 - 09:34 AM

He is saying that most people will never have the chance to easily compare the throughput of a 5" apo to a C14 but if they did have the chance, it would be clear that the latter outperforms by a lot.

#14 David Knisely

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Posted 21 October 2011 - 01:10 PM

I think the limiting magnitudes given by the formula used in the article may be somewhat conservative. In my "cheap" 80mm f/5 "short tube" refractor, I have seen the faint star that sits next to the Ring nebula, which is visual magnitude 13.0 (the article's calculated value for an 80mm is 12.2). In my 10 inch Newtonian (22% central obstruction, standard coatings), at high power, I have reached past 15th magnitude (low to mid 15's), and even in my 9.25 inch SCT, I have gone to 15th or just past that point on occasion. Exact magnitude limits are dependent on seeing, the magnification used, and the visual sensitivity of the observer, so a simple formula often cannot accurately calculate a magnitude limit. Even comparisons between apertures are often difficult to do in a way that yield real-world observational figures, so comparisons done in tabular form like this, while interesting to contemplate, may not reflect the "in the field" experiences. Clear skies to you.

#15 Daniel Mounsey

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Posted 25 October 2011 - 05:11 PM

He is saying that most people will never have the chance to easily compare the throughput of a 5" apo to a C14 but if they did have the chance, it would be clear that the latter outperforms by a lot.


Huh? Relative to what? These images are not even remotely close to reality at the eyepiece.

#16 heavyhitter

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Posted 27 October 2011 - 08:37 AM

How can a 6" Newt with a 20% CO have LESS effective throughput and less effective aperture than the 6" SCT that has a 36% CO. This does not make sense to me. I would think it to be the opposite.

Same with the 8"new vs the 8" LX200.

#17 jrcrilly

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Posted 27 October 2011 - 08:56 AM

How can a 6" Newt with a 20% CO have LESS effective throughput and less effective aperture than the 6" SCT that has a 36% CO. This does not make sense to me. I would it to be the opposite.

Same with the 8"new vs the 8" LX200.


The effect of the central obstruction on light gathering is not necessarily the most significant factor. A 20% obstruction reduces throughput by about 4%. A 36% obstruction reduces throughput by about 13%. If enhanced coatings are employed on the SCT optics the increase in throughput from that could overwhelm the 9% obstruction difference.

#18 heavyhitter

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Posted 27 October 2011 - 09:14 AM

Assuming that a newtonian has a low 88% reflective mirror, some can be quite higher, however once you factor in a corrector plate even Celestron's Starbright technology only comes to 83.5% reflectivity. I would think a newt with a 96% coatings will still trump the SCT.

I am not optics expert and with simple math I feel like the standard newts would pull ahead but like I said, I am no expert so that is why I am asking. Thanks.

#19 Jon_Doh

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Posted 31 October 2011 - 02:33 PM

The overused euphemism of "aperture is king" us difficult to justify among various types of scopes. It certainly is true among any given class or type of scope, but you can't compare an SCT with a refractor at the same aperture. A 6" refractor will pull in as much light as an 8" SCT and a little more than an 8" reflector as tests have shown. Thus the generalization that you always here bandied about on the forums that aperture is king, ergo, buy the 8" SCT or dob over the 6" refractor is simply not a true statement. And I think that is what the chart is trying to convey.

#20 simpleisbetter

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Posted 31 October 2011 - 03:32 PM

Forgive me if I don't follow, but I can't agree because I don't see it Jon. If you're considering aperture area for determining light gathering power, then the math doesn't add up in your favor. The figures I come up with are:

6" Refractor - 28 cu in
8" SCT - 45 cu in
8" Newtonian - 47 cu in

Based on those figures, the 6" refractor is behind both, and there is no contest between them WRT light gathering power. Even if you consider efficiency, which the SCT in theory would lose to the newtonian due to the corrector and larger secondary, either 8" scope is still going to beat the 6" in light gathering, and my eyes have witnessed that. And when it comes to resolution, the 6" loses.

I've not seen much or any difference between 8" SCT or newtonian as far as light gathering ability goes. The only differences I've seen between the two, when set side by side and used at the same magnification, are usually due to contrast, which can be mitigated with flocking and a dew shield to prevent scatter in the SCT. And I've still to look through a 6" refractor that beats either.

#21 Jon_Doh

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Posted 01 November 2011 - 08:30 AM

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.

#22 simpleisbetter

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Posted 01 November 2011 - 09:18 AM

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?

#23 Mr Onions

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Posted 02 November 2011 - 01:50 PM

Charts mean NOTHING!

#24 simpleisbetter

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Posted 02 November 2011 - 02:56 PM

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.

#25 SeattleScott

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Posted 02 November 2011 - 09:36 PM

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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