The trouble with a rule of thumb is that people's thumbs vary greatly in length.....
Dave nailed it with his analogy. A vast majority of the arguments summarized by the thread title are pure bunk.
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Posted 02 April 2013 - 11:09 PM
The trouble with a rule of thumb is that people's thumbs vary greatly in length.....
Posted 02 April 2013 - 11:22 PM
Posted 02 April 2013 - 11:38 PM
When they refer to the percentage of the central obstruction, Is that a percentage by area or diameter?
Posted 02 April 2013 - 11:43 PM
I was recently given Suiter's 1994 edition by a friend. In it, the MTF chart is always normalized to the *fraction* of the maximum spatial frequency. This is appropriate when considering a telescope in the afocal configuration, where diffraction effects scale as the exit pupil.
My emphasis in this discussion is on the appearance of the image at the eyepiece and at given exit pupil. The better quality instrument, irrespective of aperture, will deliver the better quality view.
The eye sees an image, and knows not what is the aperture delivering it. All it 'knows' is whether the image is good or not so good as regards such things as diffraction and aberrations. If at some particular exit pupil one rates the view less afflicted by diffraction and aberrations for a smaller aperture, then that smaller aperture, in some respect at least, delivers by definition better contrast transfer.
Any two telescopes, no matter how much they may differ in aperture but which have identical MTF charts, will deliver identical contrast at the same exit pupil.
Posted 03 April 2013 - 02:42 AM
Posted 03 April 2013 - 04:22 AM
Posted 03 April 2013 - 05:31 AM
Posted 03 April 2013 - 06:52 AM
Concentrating on the *absolute* differences in resolving power resulting naturally from differences in aperture is all well and good. But that this scales linearly with aperture is so easily appreciated it hardly bears more than the briefest thought.
Posted 03 April 2013 - 08:41 AM
Posted 03 April 2013 - 09:48 AM
The section continues, exploring various aspects of the central obstruction, equivalence formula, etc.
It appears that (from that section, and his formula on page 163), that all other things being equal (including the crucial element of seeing...) that the unobstructed equivalent of a 14" with a 32% would be 9.52", according to Suiter. On that score, the 34% CO equivalent of a 6" unobstructed aperture would be a tad more than 9 inches?
Posted 03 April 2013 - 09:59 AM
Posted 03 April 2013 - 10:20 AM
I live in an area with bad seeing as the norm. I have set up both my 4" refractor and 12" reflector together numerous times. When seeing is bad, both scopes are always affected. Occasionally, low mag images in the refractor are slightly better. Most of the time this is not the case. Seeing affects magnification limits, from my experience, and has little to do with aperture or telescope design. The only reason refractors sometimes seem to perform better in bad seeing is that they can operate at a much lower minimum magnification.
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.[/color]
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.
Posted 03 April 2013 - 12:14 PM
Posted 03 April 2013 - 03:11 PM
Posted 04 April 2013 - 12:47 AM
Now it gets interesting.
Here, I have modeled the same scope as appears above.. A 9.25" 36% obstructed apeture.
This one though, is less than perfect. I have modeled in a bit of spherical abberation and a bit of astigmatism, both of which are not at all uncommon in mass produced scopes.
By compraison, just about any 5" APO from a specialty provider you can buy these days will have optics that consistently border on perfect.
Notice now that the sag has incresed a bit. While it does not seem like much, suddenly, at the important visual frequenceis, the 9.25" 36% obstructed aperure is not transferring contrast any better than perhaps a perfect 5" instrument.
And this is perhaps why accounts when comparisons are made differ.
The "Rule of tumb" formula is only really good for visual observing. For imaging, obstruction is not usualy an issue.
All defects or design quailites (secondary obstruction, and optical defects) add to the sag of the MTF line.
When the scope is unobstructd, even a little quality error does not affect the contrast enought to be a concern.
But when the obstrucion is large, the quaity becomes much more critical.
Any meaningful amount of spehrical abberation or astigmatism can quickly lower the MTF performance so that it is reduce to contrast transfer no better than an apeture half its size.
So now, you have one person that says thier C9.25 is better than a 5" APO and one that says theirs is not as good.
Sample to sample quality varations could easlily account for that.
The purple line shows that with a little sperhcial abberation and a little astigmatism, the 9.25" apeture is now performing with less contrast transfer at the lower (visually important) frequencies than a perfect 5" instrument!!!!
And that is the beauty of MTF.
Posted 04 April 2013 - 02:08 AM
Posted 04 April 2013 - 04:14 AM
I take the example of the Celestron Omni XLT 150, which has a 150mm objective with a 46.5mm CO. Now according to Jarad's formula, that should make the Omni XLT equivalent to a 103.5mm refractor for planetary performance. But in my proposed reworked formula, the "Refractor Equivalence" of the Omni XLT should be a 126.75mm scope. Now, people may say this is wishful thinking, and I cannot say with any certainty that it isn't, but it would be nice to see how an Omni XLT 150 compares with 4" ED, 110mm ED, and 120mm ED (as well as the equivalent achromatic) scopes. I own neither the 110 nor 120mm ED scopes, nor any achromat, though there's a STRONG temptation for me to buy the XLT and compare it to my 102mm ED scope. According to Jarad's formula, it should still surpass it, but really, just barely, and the planetary performance should really be about the same. Stay tuned, but anyone else wishing to evaluate my recalculation of that old Reflector ~ Refractor Equivalence formula is more than welcome to educate me and the rest of the CN Brotherhood.
Posted 04 April 2013 - 06:32 AM
5) Finally, while it is completely valid to analyze the MTF performance of the telescope objective, you guys haven’t included the eyepiece in the discussion. Remember, MTF can only be cascaded for incoherently connected components. An eyepiece is coherently coupled to the objective and must be considered a part of the system to correctly analyze full system performance. That means that you can completely destroy the performance of even a perfect objective with a poor eyepiece.
Posted 04 April 2013 - 08:47 AM
Posted 04 April 2013 - 10:59 AM
So, a 9.25" 36% obstructed aperture, when used visually, will have conrast transfer that is fairly close to a 6" perfect apeture.
This is a crucial qualification though. Again, this is only if the obstucted scope is perfect.
Finding 6" APOs with perfect optics is not difficult. No leanding manufacturer is going to sell you an expensive APO that is less than near perfect.
Near perfect C9s though are not the norm. And when you add the typeical inperfections, it can cause the MTF line to further sag.
And it doesn't take much for the line to sag enough so that the contrast falls to a 10% difference.
And when it does, the difference starts to show at the eyepeice.
If you know the amount of error though, you can model it.
Posted 04 April 2013 - 12:37 PM
Posted 04 April 2013 - 12:43 PM
Posted 04 April 2013 - 12:58 PM
I keep seeing the comparison of mirrored optics that are perfect used in the mtf and formula comparisons. I would love to see an objective way to compare not perfect mirrored optics whether newtonian or sct which is much more realistic, to a high quality apo refractor.
And I like the approach I see by some who are taking into consideration all of the factors including the human eye, seeing, cool down, etc. Somewhere I remember reading that our eyes can resolve the most detail with a 2mm exit pupil which should also be a factor
Posted 04 April 2013 - 01:14 PM
Posted 04 April 2013 - 02:48 PM
Maybe I missed it but is anyone considering the difference in the amount of light lost due to the two reflective surfaces required for reflectors?
Wouldn't this significantly reduce light gathering whereas a refractor loses much less light throughput?
I have no idea how much or how this is calculated by the way
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