Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
Get a Cloudy Nights T-Shirt Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green GuÖ uh, User

Equipment Discussions >> Refractors

Pages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | (show all)
Eddgie
Postmaster
*****

Reged: 02/01/06

Re: Reflector/Refractor equivalence formula new [Re: Eddgie]
      #5775244 - 04/03/13 10:59 AM Attachment (17 downloads)

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.


Edited by Eddgie (04/03/13 11:24 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Sean Puett
Carpal Tunnel
*****

Reged: 09/06/10

Loc: always cloudy, washington
Re: Reflector/Refractor equivalence formula new [Re: Fomalhaut]
      #5775316 - 04/03/13 11:20 AM

Quote:

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



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.

These contrast debates seem to ignore the fact that you need great contrast with a small scope to see what is easily seen in a larger scope. My 4" refractor shows the belts on Jupiter as a light pinkish to brownish tone that is low contrast. My 12" reflector shows them as dark reddish brown and it shows that there are more than 2 of them. Detail can be seen in the bands as well and again, it is easily seen. The downside is that it is very bright and you don't want to be fully dark adapted when looking at Jupiter.
I do get the contrast side of the debate in that on the moon, in my refractor, I can see more detail in the bright area than with my reflector. Maybe because I am completely blinded by looking at the moon with a 12" scope, maybe not. But, it is a more pleasing view in my refractor.

I really enjoy refractors. That being said, if my wife threatened to divorce me unless I cut down to one telescope, I would choose my 12"newt. It shows more detail on everything. There are things that do not fit in the fov though. I really hope she never says that to me...


Post Extras: Print Post   Remind Me!   Notify Moderator  
GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Reflector/Refractor equivalence formula new [Re: Sean Puett]
      #5775537 - 04/03/13 01:14 PM

In the context of finding the equivalence between an unobstructed and obstructed telescope, one is naturally concentrating on the image representation in the range out to about 5 times the resolution limit (0.2 on the MTF's ordinate). It is in this region, after all, where the most significant differences lie. And so the comparative representation of image contrast is of paramount importance.

What inspired my more generalized approach was the greatly unfair juxtaposition of a C14 and a 6" refractor. Unless that big Cat has horrid optics, it naturally will present better contrast over the full range of resolution provided by the much smaller refractor.

I digressed from the specific requirements of the thread and presented a more 'holistic' way to appreciate the images as presented by more widely disparate apertures. That is, to consider contrast transfer of a system as normalized to the exit pupil.

In a fashion, this approach still has validity here. Each telescope has the same exit pupil limit as regards the realization of its theoretical resolving power. That is, if for observer X some telescope realizes its resolution limit at an exit pupil of 1.1mm, so will all others. If aberrations are bad this limit might be reached at a somewhat larger exit pupil. But at given exit pupil, the image more afflicted by diffraction and aberrations belongs to the poorer telescope as regards optical quality. But then, this is so obvious, ain't it?

Getting back on track... A poorer but larger telescope often bests a smaller, "perfect" scope, by virtue of its greater resolving power. But then, this is so obvious, ain't it?

The $64,000 question: Can a general equivalence formula be derived? If we restrict to the limit of several times the resolution limit, and consider only a limited variance on quality, I shouldn't see why not. That is, if we discard the poorer specimens from consideration, and consider only the small scale regime of the image, we should be able to predict with some confidence.

As long as it's always borne in mind that the problem is restricted to contrast transfer as regards the rendering of small details. In terms of image brightness, the larger obstructed aperture (nominally) is the better...

...which may have some small contribution to make, partially mitigating against its reduction in contrast.

Outside the small scale regime of several times the maximal resolving power, the larger obstructed scope is the better, for the contrast reduction becomes of little importance, with the brighter image handily making up for this.


Post Extras: Print Post   Remind Me!   Notify Moderator  
KaStern
scholastic sledgehammer
*****

Reged: 04/18/06

Loc: InTheDark
Re: Reflector/Refractor equivalence formula new [Re: CollinofAlabama]
      #5775901 - 04/03/13 04:11 PM

Hi,

to answer your question right one has to know
what type of refractor and what type of reflector
you are talking about?

I can tell you that my 8"f/6 newt was able to give equally good views
of Jupiter and M13 when I compared it to an 7"f/6 TMB
during the german ITT astrofest. Secondary sice is 39mm.

An f/8 achromat falls short due to it`s colour aberration
that lowers contrast transfer.

But there are some unobstructed reflectors out there an the best of these
can rival an equally apertured apochromat.

And there are some reflector types wich are very much compromised.

In addition one can find very differing optical qualities in real world telescopes.

And not at least there are very many miscollimated reflectors
amd some miscollimated refractors too.

So in the end you can only do the following:
Get a scope of sufficiently good quality, collimate it,
let it cool down and observe the objects the scope is suited for.

Cheers, Karsten


Post Extras: Print Post   Remind Me!   Notify Moderator  
jhayes_tucson
professor emeritus
*****

Reged: 08/26/12

Loc: Bend, OR
Re: Reflector/Refractor equivalence formula new [Re: Eddgie]
      #5776927 - 04/04/13 01:47 AM

Quote:

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.






One of the things that impresses me about this group is how much many of you guys know about optics; however, some of this discussion has veered completely into the weeds so I want to clarify a few points:

1) ALL optical systems have wavefront errors. Over many years, I've tested hundreds of components and systems with state-of-the-art PSI and dynamic interferometers and it is extremely rare to see any component with PV errors less than about 1/25 wave (and even that is a difficult level to measure on an absolute accuracy scale--but that is another subject.) Any contention that refractors have fewer errors than reflectors is a generalization that just ain't so. There are high quality reflecting, refracting, and catadioptric systems. It all depends on the design, manufacturing tolerances, and alignment of the system. Keep in mind that on-axis performance isn't all that counts either. Few (if any) of these systems are shift invariant and the size of the aplanatic patch is generally small, which means that spatial frequency response will vary significantly with field angle.

2) A theoretically perfect reflector with a 32.5% obscuration meets the Rayleigh (as well as the Marechal) criteria for diffraction limited performance in both Strehl and MTF. In the real world, it is possible to balance the degradation in performance due to wavefront errors against the obscuration ratio such that the performance remains diffraction limited but that will require a smaller obscuration.

3) You guys are getting way too hung up on individual frequency response components and forgetting that actual image detail is composed of the Fourier sum of the transmitted frequency components (assuming a zero PTF.) A larger aperture always allows a wider range of frequency components to exist in the details of an extended image. That produces sharper edge response making small details easier to see. Furthermore, an obscured aperture actually has slightly better high frequency response (meaning higher contrast at higher spatial frequencies) than an unobscured aperture. Small losses in contrast in the middle frequencies generally have little effect on the perceived "sharpness" of an extended object. It is the high frequency response that drives the "clarity" of small details in an extended object. Obviously, if the mid-frequency transmission falls "too-much", image quality can begin to suffer, which is why we use MTF as a tool to evaluate system performance in the first place.

Johnson's law correlates the cycles per dimension required for observing features on a target (with incoherent illumination.)

Detect = 1 cyc/dimension
Orient = 1.4 cyc/dimension
Recognize = 4.0 cyc/dimension
Identify = 6-8 cyc/dimension
Recognize (50% accuracy) = 7.5 cyc/dimension
Recognize (90% accuracy) = 12 cyc/dimension

The more cycles/dimension that you can produce is what allows better imaging and that always requires bigger aperture.

BTW, if you want to get hung up on the shape of the MTF curve, you might as well start building telescopes with square apertures. A square aperture has a (non-diagonal) linear decrease in MTF. Get over it, keep the MTF curve within the diffraction limit, and youíll be fine.

4) I not sure that I understand what it means to "normalize the MTF to an exit pupil." MTF is commonly normalized to spatial frequency with the maximum at 1/lambda*F...where F= F/#. That means that all F/5 telescopes will have contrast fall to zero at 0.4 cyc/micron at a wavelength of 0.5 microns--regardless of aperture. That .4 cyc/micron projects onto the sky from the focal plane (not the exit pupil) at different angles which depends on the focal length of the system. For a 6", F/5 telescope that translates to 1.48 cyc/arc-sec and for a 12", F/5 telescope, 2.96 cyc/arc-sec.

5) Finally, while it is completely valid to analyze the MTF performance of the telescope objective, you guys havenít 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.

John

Edited by jhayes_tucson (04/04/13 01:50 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Reflector/Refractor equivalence formula new [Re: jhayes_tucson]
      #5776976 - 04/04/13 03:08 AM

John,
That was an excellent contribution to the discussion.

Regarding your point #4. The phrase "normalize the MTF to the exit pupil" is a clumsy way of mine to try to get across the following point. Any equally good telescope (as a system, with eyepiece of course) working at a given exit pupil will present to the eye an identical degree/scale of visible diffraction on an image point. If the Fresnel pattern subtends some angular scale in a small scope at some particular exit pupil, it will be the same for a larger scope at the same exit pupil.

In other words, the resolving power as a fraction of the maximum is a function of the exit pupil.

Again, a clumsy way to place a basic principle of the afocal system into some perspective, so as to provide something of a more complete picture.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Sasa
professor emeritus


Reged: 11/03/10

Loc: Ricany, Czech Republic
Re: Reflector/Refractor equivalence formula new [Re: CollinofAlabama]
      #5777013 - 04/04/13 05:14 AM

Quote:


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.





I had Orion Optics N150/750 with about 33% central obstruction with lambda/8 optics and Hilux coating. Quite often it was outperformed on planets by my 80mm apochromat (Lomo 80/480 triplet). There is no comparison with my ED100. This 4" refractor showed me much more on Mars and Jupiter than 150mm Newton ever did (but in this case it was not side-by-side comparison as was in case with 80mm lens).

I think the obvious difference has nothing to do with theoretical expectations. In my case, I'm storing telescopes at home at room temperature. And I was often travelling for dark skies with former 150mm Newton. I think that thermalization and constant need of colimation were the main reasons why even the small 80mm refractor was outperforming much bigger brother on planets. This is the reason why I sold 150mm Newton at the end and bough ED100. I still consider it, after few years of using ED100, as one of the best moves that I made. On the other hands, I have no doubts that a wisely used 150mm Newton with properly designed OTA would outperform 100mm refractor on planets (not necessary at f/5, it would probably require something longer, like f/8).

Edited by Sasa (04/04/13 05:40 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Reflector/Refractor equivalence formula new [Re: jhayes_tucson]
      #5777123 - 04/04/13 07:32 AM

Quote:

5) Finally, while it is completely valid to analyze the MTF performance of the telescope objective, you guys havenít 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.




Indeed.. but these days, there are very few "poor eyepieces" and very good eyepieces are not expensive... The differences between eyepieces is subtle, the difference between apertures is not.

Jon


Post Extras: Print Post   Remind Me!   Notify Moderator  
GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Reflector/Refractor equivalence formula new [Re: Jon Isaacs]
      #5777244 - 04/04/13 09:47 AM

The difference between observers' eyes is greater than found across the bulk of available modern (and not so modern) eyepieces. On axis, at any rate.

Post Extras: Print Post   Remind Me!   Notify Moderator  
olivdeso
super member


Reged: 02/20/11

Loc: Paris FR
Re: Reflector/Refractor equivalence formula new [Re: Eddgie]
      #5777513 - 04/04/13 11:59 AM

Quote:



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.




having compared the TEC160ED to a C9 and a C11, I can confirm this.

I would add, that the C9 should be perfectly colimated and at thermal equilibriuum. I mean collimated on the Airy pattern, which already requires a excelent seeing for these diameter.
So on the field, the TEC160ED will often perform at its maximum while the C9 will require much more care and good seeing condition to deliver similar visual performances.


Post Extras: Print Post   Remind Me!   Notify Moderator  
timmbottoni
Carpal Tunnel


Reged: 08/25/05

Loc: W Chicago suburbs, IL USA
Re: Reflector/Refractor equivalence formula new [Re: olivdeso]
      #5777756 - 04/04/13 01:37 PM

Fascinating topic!

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

Timm


Post Extras: Print Post   Remind Me!   Notify Moderator  
timmbottoni
Carpal Tunnel


Reged: 08/25/05

Loc: W Chicago suburbs, IL USA
Re: Reflector/Refractor equivalence formula new [Re: timmbottoni]
      #5777765 - 04/04/13 01:43 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

Timm


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Reflector/Refractor equivalence formula new [Re: timmbottoni]
      #5777804 - 04/04/13 01:58 PM

Quote:

Fascinating topic!

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

Timm




I think the same assumption is being made about the Newtonian optics as are being made for the refractor optics. High quality apo optics are about 1/8th-1/10th wave. This is possible with a Newtonian...

As far as maximum resolution of the eye occurring at a 2mm exit pupil, I think that is a number mentioned in terms of seeing faint DSOs. Consider resolving a double star. The 2.3 arc-second pairs of the double-double are easily resolved in an 80mm but not at a 2mm exit pupil. A 2mm exit pupil corresponds to 40x, in my experience, the cleanest splits of these pairs are over 100x in an 80mm and if I am pushing the limits of an 80mm, a 0.5mm exit pupil or smaller is desirable.

The eye works better with more light...

Seeing, that's important but variable... some folks live where seeing is rarely excellent, then a small, fast cooling scope might be good. Some folks live where the seeing is good and often excellent. Then a bigger scope that can take advantage of the seeing is useful.

Last night, I got a nice clean split of 52 Orionis, "wide enough to drive a car through"... Skytools 3 tells me it's 1.0 arc-seconds... that didn't happen with a 4 inch or a 6 inch...

Jon


Post Extras: Print Post   Remind Me!   Notify Moderator  
olivdeso
super member


Reged: 02/20/11

Loc: Paris FR
Re: Reflector/Refractor equivalence formula new [Re: Jon Isaacs]
      #5777834 - 04/04/13 02:14 PM

There is another factor which may count: the vitreous floaters.

They are much more annoying at x2D on a refractor than at x1D on a reflector


Post Extras: Print Post   Remind Me!   Notify Moderator  
jhayes_tucson
professor emeritus
*****

Reged: 08/26/12

Loc: Bend, OR
Re: Reflector/Refractor equivalence formula new [Re: timmbottoni]
      #5777983 - 04/04/13 03:48 PM

Quote:

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

Timm




Throughput due to transmission issues is generally not a big issue--it depends on the quality of A/R coatings for the refractor and the reflective coatings for the reflector. Modern, multi-layer reflective coatings can easily achieve 96-99% reflectivity values. To find the total throughput simply multiply the reflectivity of each mirror together. So, for a two-mirror system, the throughput will be in the range of 92-98%. The biggest light loss in the reflector is due to the shadow of the secondary. Just remember that throughput is not what determines peak brightness in the Airy pattern. That is determined by the Strehl ratio. For circular apertures with zero wavefront errors it is easy to show (using a little Fourier optics) that the Strehl relative to an unobscured aperture is given by the square of the throughput (due to the obscuration.) So for an obscuration ratio of 32.5%, the throughput will be just shy of 90% and the relative Strehl will be 0.80 (all relative to a refractor.)

For a refractor system, the quality of the A/R coatings and the absorptivity of the glass are what matters. For small systems (<10"), the absorptivity of the glass is generally negligible so we assume the transmission to be 1.0. Broadband A/R coatings range in efficiency from about 2% to 0.25% (very difficult even at one wavelength!). If we assume that the broadband transmission at each surface is about 99% (probably a bit high), then we can compute the total transmission. For an air-spaced, double element achromat or apochromat, there are four surfaces and the total transmisision will be about 95%. Three element apochromatic objectives may be oiled together or cemented so they will likely have a little higher transmission value; but, thatís harder to compute on the back of an envelope. In either case, if the coatings are good, the light loss due to the coatings is roughly equivalent so itís not worth worrying about.

John


Post Extras: Print Post   Remind Me!   Notify Moderator  
GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Reflector/Refractor equivalence formula new [Re: jhayes_tucson]
      #5778203 - 04/04/13 05:43 PM

The referring to the ~2mm exit pupil as about the 'optimum' for the eye goes to the point I've been making earlier. For example. In all telescopes (of reasonable quality), the observer will find an exit pupil where for his eye the effects of diffraction *just* becomes apparent. The exit pupil just marginally larger than this might be considered the 'best' compromise between image resolution and a clean view not visibly marred by diffraction.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Reflector/Refractor equivalence formula new [Re: GlennLeDrew]
      #5778538 - 04/04/13 08:53 PM

Quote:

The referring to the ~2mm exit pupil as about the 'optimum' for the eye goes to the point I've been making earlier. For example. In all telescopes (of reasonable quality), the observer will find an exit pupil where for his eye the effects of diffraction *just* becomes apparent. The exit pupil just marginally larger than this might be considered the 'best' compromise between image resolution and a clean view not visibly marred by diffraction.




I am not quite following... Are you suggesting giving up resolution to have a prettier image, operating at a lower magnification than will allow the observer to resolve the maximum detail or most closely separate binary?

Jon


Post Extras: Print Post   Remind Me!   Notify Moderator  
timmbottoni
Carpal Tunnel


Reged: 08/25/05

Loc: W Chicago suburbs, IL USA
Re: Reflector/Refractor equivalence formula [Re: Jon Isaacs]
      #5778575 - 04/04/13 09:10 PM

The 2mm exit pupil comes from the book, "The Backyard Astronomer" but it basically means that your eyepiece that you are likely to find looks the best to your eye, and you are likely to use the most is one that produces a 2mm exit pupil. Just double your focal ratio to pick the eyepiece in that sweet spot. For example, an F/10 SCT a 20mm eyepiece is likely to look the best. I found a complex link that explains the biology behind it here - http://www.telescope-optics.net/eye.htm so for planets it makes sense sort of. I'll let you decipher this at your own leisure.

Timm

Edited by timmbottoni (04/04/13 09:11 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
jhayes_tucson
professor emeritus
*****

Reged: 08/26/12

Loc: Bend, OR
Re: Reflector/Refractor equivalence formula [Re: GlennLeDrew]
      #5778577 - 04/04/13 09:11 PM

Quote:


Regarding your point #4. The phrase "normalize the MTF to the exit pupil" is a clumsy way of mine to try to get across the following point. Any equally good telescope (as a system, with eyepiece of course) working at a given exit pupil will present to the eye an identical degree/scale of visible diffraction on an image point. If the Fresnel pattern subtends some angular scale in a small scope at some particular exit pupil, it will be the same for a larger scope at the same exit pupil.

In other words, the resolving power as a fraction of the maximum is a function of the exit pupil.

Again, a clumsy way to place a basic principle of the afocal system into some perspective, so as to provide something of a more complete picture.






Glen,
I had to sit down and work it out, but you are correct. The angular size of the Airy disk in the exit pupil will always be the same for equal size exit pupils regardless of the diameter of the primary aperture. I find it a bit easier to think of it as follows: The Airy disk will appear to subtend the same angle in two telescopes when the ratio of the magnifications equals the ratio of the diameters.
John

Edited by jhayes_tucson (04/04/13 09:12 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Reflector/Refractor equivalence formula [Re: timmbottoni]
      #5778595 - 04/04/13 09:21 PM

Quote:

The 2mm exit pupil comes from the book, "The Backyard Astronomer" but it basically means that your eyepiece that you are likely to find looks the best to your eye, and you are likely to use the most is one that produces a 2mm exit pupil. Just double your focal ratio to pick the eyepiece in that sweet spot. For example, an F/10 SCT a 20mm eyepiece is likely to look the best. I found a complex link that explains the biology behind it here - http://www.telescope-optics.net/eye.htm so for planets. I'll let you decipher this at your own leisure.

Timm




Timm:

Certainly a 2mm exit pupil does not provide the most detail for bright extended objects nor resolve the closest binaries. At 2mm it is unlikely one can actually see the Airy disk.

Jon Isaacs


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | (show all)


Extra information
16 registered and 28 anonymous users are browsing this forum.

Moderator:  FirstSight, panhard, star drop, dr.who 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 6108

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics