Tom Trusock
Reged: 02/26/02
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Aperture Comparison
#3108279 - 05/16/09 10:25 AM
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By Jared Willson
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Samir Kharusi
Carpal Tunnel
Reged: 06/14/05
Loc: Oman
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Re: Aperture Comparison
[Re: Tom Trusock]
#3109627 - 05/16/09 11:36 PM
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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):
Left to right: TV140 (5.5" premium APO), C8 and C14
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Zad
super member
Reged: 01/05/08
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Re: Aperture Comparison
[Re: Tom Trusock]
#3110131 - 05/17/09 09:18 AM
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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
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Midnight Dan
Postmaster
Reged: 01/23/08
Loc: Hilton, NY, Yellow Zone (Bortl...
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Re: Aperture Comparison
[Re: Zad]
#3110659 - 05/17/09 02:36 PM
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Excellent article! Thanks, Jared!
-Dan
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jrbarnett
Eyepiece Hooligan
Reged: 02/28/06
Loc: Petaluma, CA
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Re: Aperture Comparison
[Re: Tom Trusock]
#3113287 - 05/18/09 08:59 PM
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Nice report Jared.
The chart is very handy too.
Regards,
Jim
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Psyire
Pooh-Bah
Reged: 06/24/07
Loc: 55* North
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Re: Aperture Comparison
[Re: jrbarnett]
#3117548 - 05/20/09 08:42 PM
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Very nice chart, thanks a bunch for the write up!
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kfred
Carpal Tunnel
Reged: 11/11/03
Loc: Dayton, Ohio
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Re: Aperture Comparison
[Re: Psyire]
#3120399 - 05/22/09 08:29 AM
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Great info and chart.
Fred
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astrodon
Carpal Tunnel
Reged: 04/07/05
Loc: Portland, OR, USA
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Re: Aperture Comparison
[Re: kfred]
#3136810 - 05/31/09 12:58 AM
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Is the "Meade" 8" Mak-Newt perhaps an Intes?
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tomstep
member
Reged: 08/25/09
Loc: Indianapolis, In
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Re: Aperture Comparison
[Re: astrodon]
#3322826 - 09/08/09 07:59 PM
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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?
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Juan Irias
newbie
Reged: 02/08/11
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Re: Aperture Comparison
[Re: tomstep]
#4811069 - 09/16/11 01:37 PM
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Good information for "choose".
A question: the vixen VMC200L is "open tube", what implies this vs a closed tube like a LX200??
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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: Juan Irias]
#4822195 - 09/22/11 03:51 PM
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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.
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Daniel Mounsey
Vendor - Celestron
Reged: 06/12/02
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Re: Aperture Comparison
[Re: Samir Kharusi]
#4822882 - 09/22/11 11:27 PM
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Quote:
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):
Left to right: TV140 (5.5" premium APO), C8 and C14
Ironic you say that considering your images say otherwise. 
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xession
member
Reged: 10/02/10
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Re: Aperture Comparison
[Re: Daniel Mounsey]
#4872394 - 10/21/11 10:34 AM
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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.
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David Knisely
Postmaster
Reged: 04/19/04
Loc: southeastern Nebraska
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Re: Aperture Comparison
[Re: Tom Trusock]
#4872803 - 10/21/11 02:10 PM
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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.
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Daniel Mounsey
Vendor - Celestron
Reged: 06/12/02
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Re: Aperture Comparison
[Re: xession]
#4879999 - 10/25/11 06:11 PM
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Quote:
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.
Edited by Daniel Mounsey (10/25/11 08:26 PM)
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heavyhitter
member
Reged: 06/18/07
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Re: Aperture Comparison
[Re: Daniel Mounsey]
#4883006 - 10/27/11 09:37 AM
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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.
Edited by heavyhitter (10/27/11 09:58 AM)
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jrcrilly
Refractor wienie no more
Reged: 04/30/03
Loc: NE Ohio
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Re: Aperture Comparison
[Re: heavyhitter]
#4883040 - 10/27/11 09:56 AM
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Quote:
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.
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heavyhitter
member
Reged: 06/18/07
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Re: Aperture Comparison
[Re: jrcrilly]
#4883068 - 10/27/11 10:14 AM
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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.
Edited by heavyhitter (10/27/11 10:15 AM)
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Jon_Doh
sage
Reged: 09/16/11
Loc: Area 51
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Re: Aperture Comparison
[Re: heavyhitter]
#4890552 - 10/31/11 03:33 PM
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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.
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simpleisbetter
Carpal Tunnel
Reged: 04/18/11
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Re: Aperture Comparison
[Re: Jon_Doh]
#4890640 - 10/31/11 04:32 PM
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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.
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