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Equipment Discussions >> Refractors

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Eddgie
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Re: Reflector/Refractor equivalence formula new [Re: timps]
      #5766509 - 03/30/13 11:00 AM

Quote:

So should I buy the 152mm Explore Scientific Apo or a 14" Meade/Celestron?




Two very different instruments.

I own both a 6" APO and a C14.

I use the 6" APO mostly for wide field observing during the summer and winter Milky Way, and for doubles during this time of the year.

It works well for planets and if it is already set up, I will use it for planets, but if there is no scope set up and I want to look at planets, I bring out the C14.

But I get the appeal of the big refractor. For wide field work, the view is the best I have ever had. At all powers, stars are sharp right to the edge of the field stop.

And I get the appeal there. I love the pinpoint stars offered by the 6" APO.

But for must general use, if it fits into the field of the C14, I use that.

If it doesn't fit into the field of the C14, but fits into the field of the EdgeHD 8", I use that.

Only if it doesn't fit into the field of the EdgeHD 8" am I inclined to pull out the big refractor.

So, to me, they are different scopes for different uses.

What do you want to do most?

If it is deep sky, the C14 is clearly a better choice even with the off axis curvature and coma. For most deep sky targets, you don't notice the off axis abberations of the C14.

So, I cannot advise you really on what you should get. Think about what you want to observer and how important off axis performacne is to you.

Good luck.


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GlennLeDrew
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Re: Reflector/Refractor equivalence formula new [Re: Eddgie]
      #5767092 - 03/30/13 04:01 PM

Eddie,
One must be careful of treating the greater aperture as delivering a brighter image, this being the main reason for seeing more detail. Aperture, image brightness and resolving are inextricably intertwined. The larger instrument delivers better resolution because of the diameter of the entrance pupil, extra light is part of the equation.

To assess performance differences most objectively, and to minimize the variables, comparisons must be conducted at identical exit pupil. The exit pupil is the arbiter of the visual appearance of diffraction effects. A 1" and 100" aperture, both working at the same exit pupil, will provide identical appearances for a star (the stars chosen being of suitable brightness for the aperture, of course.) And for extended objects, like planets, at the same exit pupil the object surface brightness is the same, and the degree of resolution *as perceived on the retina* will be the same. It's just that the larger scope delivers a commensurately larger image and a concomitantly more detailed view. But on the retina, the *apparent* angular resolving power is the same.


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Eddgie
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Re: Reflector/Refractor equivalence formula new [Re: GlennLeDrew]
      #5767178 - 03/30/13 05:06 PM

This does not differ at all from what I have said.

I have said that a larger aperture will enjoy a better illuminated image for a given magnification.

And of course that means that at the same exit pupil, the image will be larger, but just as bright.

And the consequence of that is that you can magnify the image more in the larger aperture which makes it easier to resolve the detail.

And I have repeatedly stated here that these dialogs usually only talk about what happens at the focal plane and ignore the working of the human eye.

It would appear that we are not in dis-agreement here.

But I disagree that at the retina there will be no difference.

If the larger aperture is allowed by seeing to transfer more contrast, that contrast will show in more detail at all powers in the scope with the larger clear aperture. Angular resolution is quite a bit different than contrast ttransfer. You can resolve 192 line pair per millimeter in even a very poor f/10 telescope.

One observer might see a particular detail rendered with 6% contrast (right at the edge of perception for the dark adapted eye) while the other might see it with 15% contrast. The one that sees the higher contrast will say that the view is "Sharper". After all, that is what contrast transfer is. It is image sharpness. But the loss of contrast for mid-frequency detail does indeed make it less crisp in the eyepiece for that same instrument,

The MTF of the instrument allows the instrument to simply transfer more of the low contrast detail to the focal plane.

If there is more on the focal plane to see, then the observer will see it.

But once again, in my very first post on this topic, I said it was very complex. We are far more in agreement than we differ I think,

Edited by Eddgie (03/30/13 05:13 PM)


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t.r.
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Re: Reflector/Refractor equivalence formula new [Re: CollinofAlabama]
      #5767360 - 03/30/13 06:28 PM

Quote:

First, Jon, I'm pretty sure you recently wrote this, but I'm on an iPhone right now, not the tool to find your posts on reflectors' performance (you have quite a few in the regard, mind you!)

Ed, first, thanks for doing the graph. This kind of information is enjoyable to look at. Something tangible, and yet ...

Sean made the point about "in the field". And here's where your graphs run afoul of reality. Joe Bergeron wrote this review in 2007, comparing his 92mm apo, 150 XLT and AP 155. In his estimation they lined up as one would expect, with the AP 155 best, the 92mm the worst, and the 150 XLT performing in between. Of course, this sheds little light on where and to what degree the 150 XLT would fall -- above a 102mm? above a 110mm? above a 120mm? -- since all those certainly fall between a 155mm and a 92mm scope. But Bergeron specifically notes it performing better than the 92mm, thus implying the graph you supply, Ed, may not apply in the field. "In overall performance, the $400 Omni was midway between these two fabled refractors, and closer to the big one than the little one." You may say this proves nothing, and you may be right. But you could be wrong, too.

I'd just like to hear from someone who has used the Omni XLT and one (or more) of the 4", 110mm or 120mm refractors.

I have always liked refractors, but can't forget the way I saw an 8" LightBridge put an Orion 120 ED to shame on Saturn one evening.




I can't speak about a 150XLT...but I can assure you that my 1/6-1/7th wave C-6XLT handily beat out my old Tak Sky 90, Megrez 90 and TMB 92L! Take it to the bank. So I have concluded that 6" of sct aperture trumps 3.5" of unobstructed aperture consistently. In addition, for a season I compared a 4" apo to a C5, and the C5 fell short. Bracketing would put a 3.5" apo in line with a C5.


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timps
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Re: Reflector/Refractor equivalence formula new [Re: Eddgie]
      #5767419 - 03/30/13 07:18 PM

I would like to use the telescope for most things. (Nebula, star clusters, galaxy, planetary and lunar).Visual and imaging. I believe that a 14" SCT, be it Meade or Celestron, is the best telescope for this. I think they are the best "all rounder". They are good value for money too when you consider the price of even a "cheap" 6" Apo.
However, I would still like a 6" Apo. Maybe I should start off with the SCT & get the Apo further down the track.


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GlennLeDrew
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Re: Reflector/Refractor equivalence formula new [Re: timps]
      #5767492 - 03/30/13 08:06 PM

This originally was the first portion of the complete post following below. How and why it got sent while I was composing it (with a short distraction taking me away for a bit) is a mystery...

Edited by GlennLeDrew (03/30/13 09:06 PM)


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Fomalhaut
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Re: Reflector/Refractor equivalence formula new [Re: junomike]
      #5767540 - 03/30/13 08:21 PM

Quote:

My main issue with all of this (formula's and graph's) is It's only in "theory"! In the field the main contributor to how my C11 is gonna compare to my Apo (in this case an AT111EDT ~ 4.37") is.........SEEING! IME the Central Obstruction in a Reflector is much more harmful in regards to Contrast in poor seeing.

Last year I had both in the field and on a few nights the much smaller Apo brought in more detail than the larger scopes (my C11 and an excellent 12" Dob).

When seeing did permit the use of higher magnification, the smaller Apo was easily bested by the larger scopes. This occurred less than 50% of the time however!

This is the main reason I believe there is so much of the
"this scope beat that much larger scope" and Refractor vs. Reflector controversy.

It's all about the four rights: The right scope of the right size used on the right target in the right Seeing Conditions.

This is also why one scope is not usually sufficient for everything.

Mike




Mike,
According to Anton Kutter, the inventor of the original TCT, the diameter of the first diffraction ring being ~1.8 times the one of the airy disk, together with the percentage of energy in the first diffraction ring increasing with increasing size of obstruction, causes the light in the airy disk to merge with the one in the 1st diffraction ring. This effect increases with decreasing quality of seeing (or optics!!!) and causes an ~1.8times "enlarged virtual airy disk" in moments of poor seeing.

According to these thoughts (and independant of what we can derive and quantify more exactly by means of comparing contrast transfer graphs), in poor seeing an unobstructed telescope shows an ~1.8 times smaller "airy disk" compared to a substantially obstructed telescope in bad seeing:

=> Refractor ~ as good as an 1.8 times bigger, substantially obstructed reflector during moments of insufficient seeing. (The susceptibility to this effect increases with size of obstruction.)

Kutter said these considerations showed him the way to the TCT... (Anton Kutter, "Mein Weg zum Schiefspiegler", lecture given at the Swiss Astronomical Society in 1962.)

Chris


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GlennLeDrew
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Re: Reflector/Refractor equivalence formula new [Re: timps]
      #5767611 - 03/30/13 08:59 PM

Eddie,
Indeed, we agree upon much more than we differ.

Perhaps I was not so clear. What I was stressing is this. Other things being equal (e.g., quality, transmission efficiency, etc.), any two telescopes working at the same exit pupil deliver images having identical characteristics as regards the scale of diffraction on the retina.

A larger aperture does not deliver higher contrast; it merely delivers a larger image. In essence, the contrast transfer scales as the exit pupil diameter. The following should illustrate...

Consider two otherwise equal and equally good telescopes, one being simply scaled up so as to be 10X larger. We have the Jupiter we all know and love in the field of the smaller scope, and a hypothetical *identical* Jupiter 2.0 which happens to be located 10X farther away (and identically illuminated, so that it has identical surface brightness) and in view with the 10X bigger scope.

With the same eyepiece installed, the exit pupil would be the same, and so the views through each scope, aimed at its respective planet, would be identical (assuming perfect seeing, naturally.) One would have no idea whatsoever which scope one is peering through. Image size, surface brightness, degree of resolution and contrast transfer would be precisely the same. The only difference is that the 10X larger scope reveals 10X smaller details. But to the eye, at given exit pupil diffraction effects and hence perceived image quality are the same.

The MTF chart for each of these same f/ratio and same optical quality scopes is identical when considered in terms of linear scaling at the focus (e.g., line pairs per millimeter), which is of direct import for imaging. And when used afocally (with an eyepiece), linear resolving power and contrast transfer are directly related to the exit pupil diameter.

I stress these points so as to disabuse anyone of the false notion that a larger aperture has better contrast transfer.

If one considers performance differences on any one particular target whose angular dimensions are fixed, then certainly will the larger aperture afford better performance. But in terms of the *perceived* effects introduced by diffraction, aperture by itself has no impact whatsoever. Exit pupil diameter is the prime arbiter here.

For those who tend to skim longish posts like this, I can already hear the protests generated by the previous paragraph (and much else preceding). Just to be clear, this concerns not *absolute* angular resolving power, but the perception of image quality and resolution *on the retina*, which is entirely exit pupil dependant (again, assuming perfect seeing and otherwise similar optical configuration and quality.)


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CollinofAlabama
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Re: Reflector/Refractor equivalence formula new [Re: Fomalhaut]
      #5768980 - 03/31/13 02:54 PM

Well, Chris and other theorists, you now have the written 'in the field' testimony of two CNers that a 6" F/5 reflector and a 6" SCT consistently performed better than 90 (and 92mm) clear aperture scopes. Tim writes you can take it "to the bank". Pretty definitive, I'd say. Now, perhaps you think Tim and Joe Bergeron don't know what they're doing and are not to be trusted compared to charts and "definitive publications from optical experts". I don't think this way and, bumpkin that I am, trust them and their analysis as somehow more valid.

All this leads me to believe that, in the field, "aperture illumination" Ed writes of is more valuable than charts and essays when it comes to what you'll actually see at the eyepiece comparing scopes. Of course, these antecedotes still don't tell me where the XLT 150 would fall compared to a 102mm/110mm/120mm refractor, but I do believe it would outperform any 90mm (or 92mm) apo. I already figured that was true (Joe wrote his article in 2007, btw), and wasn't very swayed by theories, any way. But my fundamental comparison scheme is still a mystery. Anyone else with more experience in these matters is more than welcome to join in.


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t.r.
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Re: Reflector/Refractor equivalence formula new [Re: CollinofAlabama]
      #5769077 - 03/31/13 03:54 PM

There are NO absolutes Collin...I still prefer the stellar images in the 90 apos!

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sg6
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Re: Reflector/Refractor equivalence formula new [Re: t.r.]
      #5769389 - 03/31/13 06:03 PM

People don't seem to buy scopes on the basis of what is equivalent to one another, they buy because they like one or another. You have posted this in the refractor section, so people here will generally prefer refractors immaterial of the diameter. If they wanted large apertures then they wouldn't be here.

I didn't buy the Megrez 90 because it was equivalent to anything, I bought it because I liked it.

I have read several of these "equivalences" and you seem to be able to choose 1:1 or as mentioned here 1:1.8 and so any ratio in between. That really is if you care. Will say I for one don't care.

If I had a 10" reflector and a 80mm refractor which would I use, easy, both, and on the same things, DSO's, planets, doubles. Would I throw away one because I had the other? Not a chance in hell.

Is a 102mm refractor equivalent to a 150mm reflector? I don't know, and in honesty the whole question is of no interest. They are different instruments and the one thing I would expect is they give different results and by that they cannot be equivalent.


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Fomalhaut
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Re: Reflector/Refractor equivalence formula new [Re: sg6]
      #5769449 - 03/31/13 06:31 PM

Living in a region close to the northern Alps in Europe with often quite unsteady seeing, 20 years ago I "upgraded" (sorry, but that's how I felt and still feel) from an 8"-SCT to a perfect (!) 4"-apo performing most of times better or then at least more beautifully. I've never regretted this, but supplemented the apo with a 7"-DK later on in order to have "the best of both worlds".

I know (from own experience) that a good 7 to 8 inch SCT or DK CAN sometimes do a little bit better on the planets, but most of times doesn't in my climate. As compared to a 33 to 34% obstructed 6"-scope, the same apo virtually always produces better (more crisp and contrasty) planetary images with at least as much detail.

By the way, I have the utmost respect for good Newtonians with reduced obstructions (not more than 20%): Such a 6-incher definitely can do what an SCT or DK with its bigger obstr. cannot, i.e. outperforming a 4-inch apo even on planets (good seeing provided).
And this all isn't theory but practice first of all (having been more or less in this pastime for over 50 years now).

Chris


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Sean Puett
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Re: Reflector/Refractor equivalence formula new [Re: Eddgie]
      #5772064 - 04/02/13 01:23 AM

Quote:

Quote:

Sean made the point about "in the field". And here's where the graph runs afoul of reality.
Quote:



Sean did not make this point. Mike made this point. I happen to agree with Eddie. He is far more knowledgeable about this hobby and the gear used than I am. I have never experienced the "smaller apo showed more than the larger mirrored telescope" phenomena. Some people just prefer refractors, and that is fine but, I don't want to be converted (in the religious use).

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csrlice12
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Re: Reflector/Refractor equivalence formula new [Re: Sean Puett]
      #5772889 - 04/02/13 01:02 PM

In my endeavour to find the "one"scope; I've discovered it is has green lettering and rectangular shaped, and comes in 1, 5, 10, 20, 50,&100 sizes. There's also the rare sought after 2. If you go to the scope store they'll trade you all kinds of nice stuff for it! Anybody know where I can get more of these?

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Ziggy943
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Re: Reflector/Refractor equivalence formula new [Re: csrlice12]
      #5773114 - 04/02/13 03:06 PM

There is no reflector equivalent to a 9" Clark refractor period.

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Eddgie
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Re: Reflector/Refractor equivalence formula new [Re: GlennLeDrew]
      #5773233 - 04/02/13 04:05 PM

Quote:

during moments of insufficient seeing.




This is why I always say that the most important factor in doing good planetary observing is patience.

I never really use the 6" APO for planets anymore. I just found that under my own local conditions, I just about always see more in the C14.

And I like seeing more. Resolving features on Ganymede, or seeing spokes in the rings of Saturn, or seeing lots more detail on Mars are all things that make me prefer the C14.

And now, I will add "Binoviewers" to that.

While it is true that larger apertures suffer more from seeing, I would say that there is when using both scopes side by side, there was rarely a night that I did not see more detail in my C14 than in my 6" APO.

This has always been the case with the most serious planetary observers I think. They had the patience to endure maybe an hour of viewing to get maybe 10 minutes of seeing that is sufficient for getting the most detail that conditions will permit.

And the reason I am so in favor of binoviewers is because going from even the best "Planetary" eyepieces to binoviewers made a consistently larger difference in my ability to be patient and catch those moments of good seeing. I don't know if I can resolve more detail using binoviewers or not, but I know that when I use them, at the end of the session, I always feel like I have seen more than before using them under typical conditions.

So for me, it is simple. I would rater be patient and see more detail than I can see with a 6" APO rather than look through he 6" APO and be limited to what that scope will show.

Even on the best nights, I have not seen detail using the 6" APO that I routinely see in the C14.

And when seeing is so poor that I can't see more than in the 6" APO, then it just is not worth doing, because even the 6" APO will suffer on such nights.

So the choice is to be content with never seeing more than 6" can show, or seeing more, but investing more time to do it.


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buddyjesus
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Re: Reflector/Refractor equivalence formula new [Re: Ziggy943]
      #5773241 - 04/02/13 04:10 PM

respectfully ziggy, your post needs a pic of such a glorious instrument!

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Eddgie
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Re: Reflector/Refractor equivalence formula [Re: GlennLeDrew]
      #5773346 - 04/02/13 04:59 PM

Quote:

A larger aperture does not deliver higher contrast; it merely delivers a larger image. In essence, the contrast transfer scales as the exit pupil diameter. The following should illustrate...




I hope that you take the time to read this complete post.

I think that your statement is not really correct.

Here is the way it works. All of this is detailed in two sources.. The book "Telecope Optics" and the Book "Star testing.."

Linear resolving power in line pair per millimeter at the focal plane is a function of focal ratio and only focal ratio.

Any two f/10 telescopes will have a linear resolution at the focal plane of 192 line pair per millimeter in green light, regardless of aperture or obstruction.

This is a function of the focal ratio and the focal ratio alone.

The maximum spatial frequency (width of the smallest line pair that can be resolved) is related to the aperture though, and not the focal ratio.

Here is where many people get confused. They believe that since any two f/10 telescopes can resolve 192 line pair per millimeter at the focal plane that they both have the same contrast transfer.

That is not the case. Not at all.


Suppose you have a 6" f/10 scope and a 12" f/10 scope.

Also, suppose that you have a special eyepiece with a 1mm field stop.

Suppose you start with a chart with and MTF chart and you start moving it away from each scope.

At a point that is some distance from the focal plane, the 6" scope will start to show the lines as blurring together. This happens when the chart is far enough away that 192mm line pairs now occupy the space between the edges of your 1mm field stop.

Ah, but if you now place the chart a the same exact distance from the 12" scope, you are seeing at only half of that scopes maximum spatial frequency.

So, this means that in this scope, rather than seeing 192 line pair, you are only seeing 96 line pair in your one millimeter field stop when the chart is at the same distance.

Same chart, same lines, same distance, but now, your scope is seeing the same exact detail at only .5 of its maximum spatial frequency.

At this point, the lines in the 5" scope have lost all contrast. The chart shows as a gray chart with no lines resolved at all.

You have reached the 6" scopes maximum spatical frequency and run out of contrast.

But the 12 inch scope easily shows 96 lines.

So, the image on the focal plane is rendered at twice the size because the aperture is twice as large, so every detail is rendered in the larger scope at .5 of the maximum spatial frequency of the larger scope when it is rendered at 100% of the maximum frequency of the smaller scope.

All telescopes loose contrast, and that contrast loss is a function of diffraction. The smaller the aperture, or any obstruction lowers the contrast transfer, but all things being equal, the bigger instrument will always transfer more contrast because there is less diffraction caused by the aperture.

Here is another way to look at it.

The formula for spatial frequency is s' max = 1/f x wavelength [cycles/length]. I am sorry, I don't know how to do formulas on a computer.

Anyway, suppose you have a 152mm aperture that is f/8.12
If you do the formula (and I am taking this from Suiter's book), the linear resolving power would be 220 cycles per line pair at the focal plane.

But.. And this is important... The S' Max (max spatial frequency) would be 1.32 cycles per arc second. This represents a line pair that where the lines are alternating from black to white with 1.32 crests for each arc second of true field.

At this point, the 6" scope is out of gas. It cannot present linear detail smaller than this size.

The 12" f/8.12 scope though has a S' max of twice this (2.64 cycles per arc second)

Another way to look at this is that the line width for a single line would be 1/2 of the S' max.

In the 6" aperture, all contrast would be lost when each line was .66 arc seconds in width.

In the 12" aperture, the line would be .33 arc seconds before all contrast was lost.
.

A line on the target that was .66 arc seconds wide would still have 42% contrast in the 12" aperture when all contrast was lost in the 6" aperture.

I needed to add that for MTF, sinusoidal lines are used.

For true black and white lines, the S' max will be somewhat off the chart and represented by the fact that often the Sparrow Criterion is referenced at a bit higher than S' max.

But for Sinusoidal lines on a distant target, the 6" scope will loose contrast when the lines 1.32 arc seconds peak to peak, but the 12" will not loose contrast until the lines are .66 arc seconds peak to peak.

Edited by Eddgie (04/02/13 06:03 PM)

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Eddgie
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Re: Reflector/Refractor equivalence formula [Re: Eddgie]
      #5773492 - 04/02/13 05:36 PM Attachment (14 downloads)

Here is an example from above.

In this chart, we see that a 12" f/8.12 scope will resolve 2.64 lines per arc second and this is represented by the 1 on the X axis.

The 6" f/8.12 scope will resolve 1.32 line pairs per arc second and this is represented but the green line ending a .5 on the X axis.

At 1.32 lines per arc second on the target, the 6" aperture is loosing all contrast.

In the larger scope though, these lines are half the S' max of the 12" scope. If 2.64 lines per arc second is the S' max, then 1.32 lines per arc second is .5 of the scopes S' max.

This means that the lines that have lost contrast in the 6" scope are still being shown with 42% contrast in the 12" scope.

so, any detail that is smaller than about 1.32 arc seconds in the small scope is almost out of contrast.

But for the larger scope (assuming a perfect aperture), the detail is shown with 42% contrast. You can see this by following the line up from .5 on the X axis to where it intersects the contrast loss line on the Y axis.


A perfect 12" aperture can resolve 2.64 line pair per arc second and will loose 42% contrast on 1.32 line pair per arc second.

The 6" aperture will loose 100% contrast at 1.32 line pair per arc second.


Now of course a central obstruction can (and does) lower that contrast transfer, but even with an obstruction, the 12" scope is still transferring contrast while the 6" aperture is completely done and cannot render detail smaller than this.

Contrast transfer is about the light that is taken from a geometric point (star or point on an extended target) and transferred away from the geometric center of this point into the area around it.

It should be common sense that if the aperture has a much larger Airy disk to start with, then light is deposited much further from the center of that geometric point in a smaller scope than in a larger scope.

And that is what reduces contrast as the aperture gets smaller. The light from the center of that geometric point is spread further away from it.

If that geometric point is a point on the edge of a white line against a black background, then that light will spread further away from the edge of the line in a 6" scope than in a 12" scope because the Airy Disk in a 12" scope is smaller (half the size in a telescope twice as large).

And this is why all things being equal, a larger aperture preserves more contrast than a smaller aperture.


And by the way, the C14 has a specified resolving power (according to Celestron's literature) of .39 arc seconds.

Guess what you get when you divide 1 arc second by 2.64 which is the S' max for a 14" aperture? You get .37 arc seconds. But that is for a Dawes split where there is no separation. A Raleigh Spit is listed at .39 by Celstron for the C14, so the MTF plot is right between these two at .38 arc seconds

For a C6, Celestron lists the resolving power of the C6 as .77 arc seconds.

Guess what you get when you divide 1 arc second by 1.32. About .76 arc seconds.

The difference of course is that these are obstructed apetures which very slightly changes the size of the Airy Disk, which in turn changes the contrast transfer very slightly.

MTF shows how much contrast loss will occur for a detail of a given linear size. The bigger the detail in terms of S' max, the less contrast that will be lost.

But all things to do with design and quality being equal, a bigger aperture starts with a smaller limit on detail size, and for any given size detail up to 1 line pair per millimeter at the focal plane, the larger instrument will always show that detail with more contrast.

Edited by Eddgie (04/02/13 07:44 PM)


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Fomalhaut
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Reged: 08/16/08

Loc: Switzerland
Re: Reflector/Refractor equivalence formula [Re: Eddgie]
      #5773655 - 04/02/13 06:38 PM

Quote:

While it is true that larger apertures suffer more from seeing, I would say that there is when using both scopes side by side, there was rarely a night that I did not see more detail in my C14 than in my 6" APO.

Even on the best nights, I have not seen detail using the 6" APO that I routinely see in the C14.

And when seeing is so poor that I can't see more than in the 6" APO, then it just is not worth doing, because even the 6" APO will suffer on such nights.





Yours really must be a fantastic C14!
And I envy you your obviously paramount seeing conditions.

Chris


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