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 Gu… uh, User

Equipment Discussions >> Refractors

Pages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | (show all)
timmbottoni
Carpal Tunnel


Reged: 08/25/05

Loc: W Chicago suburbs, IL USA
Re: Reflector/Refractor equivalence formula new [Re: Jon Isaacs]
      #5778698 - 04/04/13 10:15 PM

Hi Jon,
It's been a long time since I got my degree in Biology so I'm not going to try to explain it but the link below has a more readable explanation in the section titled "Optimum Magnification"

http://starizona.com/acb/basics/observing_theory.aspx

It's complex but this attempts to explain what you perceive and why. It's just not as simple as the way you stated it, but no worries, let your eyes tell you what is best for viewing based on your scopes aperture and focal ratio when viewing different types of objects.

I only brought it up in the discussion because it has to be considered if you are trying to compare reflectors of different sizes to refractors of different sizes because the human eye has to be taken into consideration, as some of the posts have pointed out here.

Theory is great, but what you actually see, and how you see it, is often a much more personal and practical experience. That is why some people will make comments about how a smaller high quality APO will appear better than a larger reflector or SCT. Their eye is the biggest factor that the theories aren't considering.

Timm


Post Extras: Print Post   Remind Me!   Notify Moderator  
mgwhittle
Pooh-Bah
*****

Reged: 08/24/11

Loc: Chattanooga, TN
Re: Reflector/Refractor equivalence formula new [Re: timmbottoni]
      #5778706 - 04/04/13 10:19 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 it makes sense sort of. I'll let you decipher this at your own leisure.

Timm




I believe you are confusing exit pupil of eyepieces with actual pupil size of the eye and the effects on resolution.


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: mgwhittle]
      #5778721 - 04/04/13 10:36 PM

Quote:

I believe you are confusing exit pupil of eyepieces with actual pupil size of the eye and the effects on resolution.




The effect of the pupil diameter and the exit pupil diameter are one and the same. The eye cares not a whit what it is which is stopping it down; if the pupil feeding light to the retina is 2mm in diameter, whether it be its own natural one or that produced by a telescope eyepiece, the perceived diffraction (and image brightness) is identical.

This is why I have belaboured, to the annoyance of some, I suspect , this whole business of 'normalizing to the exit pupil.'

Even one as obviously knowledgeable as John "had to work it out." From a whole lot of discussions and conversations I've witnessed over the years, I often get a feeling that a focus has been brought to bear on the finer details before having fully absorbed the fundamentals.

My mission, insofar as my meagre knowledge permits, is to ensure the basics are understood.

Jon,
Indeed, the notion of the so-called 'sweet' ~2mm exit pupil is predicated upon a willingness to forgo ultimate resolution in the quest for a view which has not *very* much less detail, but is more aesthetically pleasing.


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: mgwhittle]
      #5778723 - 04/04/13 10:36 PM

Mark,
If your eye is dilated to 5mm and the exit pupil of the eyepiece that your eye is pressed up against letting in light is at a 2mm exit pupil, do you think your retina sees 5mm of light or 2mm of light?

Timm

Edited: Glenn beat me to the reply with a much more detailed explanation but the point is the same - the exit pupil of the eyepiece is the limiting factor unless it is larger than your pupil dilation.

Edited by timmbottoni (04/04/13 10:39 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
mgwhittle
Pooh-Bah
*****

Reged: 08/24/11

Loc: Chattanooga, TN
Re: Reflector/Refractor equivalence formula new [Re: timmbottoni]
      #5778738 - 04/04/13 10:46 PM

Timm,

You mentioned planets. Please correct me if I am wrong, but a 1mm exit pupil has always been touted as giving the most resolution on planetary detail without being affected by the aberrations in the eye. My apologies, I should have been more specific in what I was questioning.



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: mgwhittle]
      #5778780 - 04/04/13 11:17 PM

Quote:

Timm,

You mentioned planets. Please correct me if I am wrong, but a 1mm exit pupil has always been touted as giving the most resolution on planetary detail without being affected by the aberrations in the eye. My apologies, I should have been more specific in what I was questioning.






Could be, if you find 1mm to be better for looking at planets, then that is the right exit pupil for you. It could even be smaller! If you read the article below, where it talks about Maximum Magnification and it refers to planets, you might find it useful. It really depends on the scope, the seeing conditions, and your eyes...

http://starizona.com/acb/basics/observing_theory.aspx

I don't honestly worry about what exit pupil I'm at, I just posed it as a consideration in the discussion. I just use trial and error on any given night and change eyepieces until I find the magnification that seems best for the scope and viewing conditions. Basically, who cares about theory, what looks best to you is what matters, right? There are reasons that things look better at different magnifications and exit pupils, which is why in my C8 I like to use the WO Zoom eyepiece. It allows you to control it easily without having to fumble with switching out eyepieces.

Timm


Post Extras: Print Post   Remind Me!   Notify Moderator  
mgwhittle
Pooh-Bah
*****

Reged: 08/24/11

Loc: Chattanooga, TN
Re: Reflector/Refractor equivalence formula new [Re: timmbottoni]
      #5778809 - 04/04/13 11:37 PM

Timm,

While my knowledge on the subject comes from the writings of more intelligent people like you and Glenn, I am still getting confused regarding the general assumption about a 1mm exit pupil, planetary detail and where the 2mm exit pupil falls into all this. I have read the link you provided in the past and I will peruse it again. I actually do worry about exit pupils and try to match eyepieces to individual scopes in order to have at least one at 2mm and one at 1mm exit pupils when observing. I do this because of the points you and Glenn have been talking about regarding a 2mm exit pupil, which I have found in practice to match theory. But I have also found the practice of a 1mm exit pupil and planetary detail to be valid.......now how do I make that correspond to the theory regarding 2mm? Insert exasperated sigh at this point......no need to respond directly, just clarifying my thoughts....and do some more reading over my head.


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

Reged: 10/05/08

Loc: Palo alto, CA.
Re: Reflector/Refractor equivalence formula new [Re: mgwhittle]
      #5778861 - 04/05/13 12:43 AM

So... ae we after a rational function here? Straying from the idealism for a moment, as apeture increases - each also has potential for errors from figuring/finishing, how do you porpose to quantify them(I think they are all rational to begin with), I don't see them as scalar values that somehow stand still. I can understand using the Airy function as a component, or the Fresnel-Huygens, but what exactly is the function you are using for non point source? I think most if not all the formula/functions are based on point source? If looking for something as a 'predictor' it should have some reasonable error function tandem to the specific design, IOW I'd expect a +/- from some average and not ideal perfect which I think useless. It's all that needs to be put in a harness that I think difficult, you'd need for each type and sub type.

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: CounterWeight]
      #5778936 - 04/05/13 02:38 AM

Mark,
I wouldn't overly fret over quantizing to exact and precise values for the exit pupil. Between the useful range (0.5-0.7mm up to about 7mm), it's a continuum. For example, the approximately 2mm moderately high power 'sweet spot' will not be quite the same from one person to another. And even for an individual, going to a different scope, and certainly seeing conditions, will slightly modify this.

Getting a better handle on what works best for you will come with experience.


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

Reged: 02/21/05

Loc: Ex NYCer, Now in Denver CO!
Re: Reflector/Refractor equivalence formula new [Re: GlennLeDrew]
      #5779028 - 04/05/13 06:53 AM

Quote:


For example, the approximately 2mm moderately high power 'sweet spot' will not be quite the same from one person to another.



Absolutely!
As a physician: As exit pupil decreases, a smaller and smaller axial sector of the physiologic pupil is being utilized. A such, individual irregularities and defects within that sector of imperfect living tissue come to occupy a greater percentage of the entrance pupil of the eye, now being defined by the exit pupil of the EP. This reintroduces and exacerbates the whole issue of imperfect and obstructed aperture (and it and it impact on the MTF), but now at the level of the eye as a an optical instrument. Floaters are but one physiologic example but there are others as well. While generalization across a population regarding optimal small exit pupil can and should be made, these must be distinguished from statements about such optimization for a given individual.
Joe

Edited by jpcannavo (04/05/13 07:06 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: Reflector/Refractor equivalence formula new [Re: jpcannavo]
      #5779051 - 04/05/13 07:30 AM Attachment (10 downloads)

Here it is:

Post Extras: Print Post   Remind Me!   Notify Moderator  
t.r.
Post Laureate
*****

Reged: 02/14/08

Loc: Upstate NY
Re: Reflector/Refractor equivalence formula new [Re: orion69]
      #5779071 - 04/05/13 07:55 AM



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

Reged: 02/21/05

Loc: Ex NYCer, Now in Denver CO!
Re: Reflector/Refractor equivalence formula new [Re: orion69]
      #5779094 - 04/05/13 08:16 AM

Quote:

Here it is:



You left something out!
Completing the formula!


Post Extras: Print Post   Remind Me!   Notify Moderator  
t.r.
Post Laureate
*****

Reged: 02/14/08

Loc: Upstate NY
Re: Reflector/Refractor equivalence formula new [Re: t.r.]
      #5779101 - 04/05/13 08:21 AM

Quote:

Indeed, the notion of the so-called 'sweet' ~2mm exit pupil is predicated upon a willingness to forgo ultimate resolution in the quest for a view which has not *very* much less detail, but is more aesthetically pleasing.



Indeed! All one has to do is look through the myriad of observer reports out there and note where the claimed "best" image is attained with any given scope. THe vast majority conclude without realizing it, that it happens at a magnification that corresponds to a 2mm EP! For planetary on occasion, personally I profitably use .4 to 1mm to see obscure details in small apertures more easily. Yes, I know that no additional detail is resolved, but if the bigger image even if stretched out abit and dimmer, helps me see a small feature more easily , I do it! Many, many times I ran a TV Genesis (with CA to boot!) up to 260x with the Televue marketed, TV 2.5x barlow and 4.8 nagler combination, which was advertised as this scopes highest power capability. I had many memoriable views of planets and doubles at that overkill .39mm exit pupil, rules of thumb be damned!


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]
      #5779118 - 04/05/13 08:34 AM

Quote:

Hi Jon,
It's been a long time since I got my degree in Biology so I'm not going to try to explain it but the link below has a more readable explanation in the section titled "Optimum Magnification"




Tim:

I am reasonably well versed in this subject, this something I have spent some time studying, both in literature, here on the web and at the eyepiece. The article makes my point very nicely:

" Empirical evidence suggests that an exit pupil of 2mm produces very detailed views of most deep-sky objects."

2mm is an empirical number for deep sky objects, not for double stars or planetary viewing where more magnification will clearly reveal more detail. I believe this was what I pointed in in my first post. For example, in an 100mm scope, a 2mm exit pupil is only 50x, hardly an optimal magnification for planetary observation and the article supports that. In my experience, in a 4 inch, something more than 100x is best for showing planetary detail.

There are some things in the article I disagree with, they suggest that for small low surface brightness objects, one should use between 150x and 250x in a 6 inch, 80x-150x in a 12 inch. Since one can see smaller, low surface brightness objects with a 12 inch than a 6 inch, working at the limit, the same exit pupil is appropriate in both scopes.

But, as you say, in the final analysis, your eye should tell you, the rest of this stuff is just guidelines. If you are getting the best view of Jupiter at 50x in a 4 inch apo, go for it... doesn't work for me though.

Jon


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]
      #5779136 - 04/05/13 08:52 AM

Quote:


Jon,
Indeed, the notion of the so-called 'sweet' ~2mm exit pupil is predicated upon a willingness to forgo ultimate resolution in the quest for a view which has not *very* much less detail, but is more aesthetically pleasing.




Glenn:

I find two or more closely separated central disks surrounded by the interacting diffraction ring structure to be about the most aesthetically pleasing view possible. At a 2mm exit pupil, a Dawes limit double star is certainly not resolved and may not even be detectable.

Planets are a different story, at least in a larger scopes because one is rarely working at the limit of resolution, it's more a combination of optimizing the magnification in terms of brightness and seeing. Since it is generally seeing limited, Jupiter most often at it's best somewhere between 160x and 240x pretty much regardless of aperture. The eye will definitely see more contrast and detail in the larger scope at the same magnification. For viewing the planets exit pupil is not so useful because the limit is the seeing.

How does this all apply to an equivalence formula: Optical quality is most important at higher magnifications, smaller exit pupils where one can actually resolve the differences. Wandering around the Milky Way with a 6mm exit pupil is mostly about off-axis aberrations in the scope and the eyepiece. Even at a 2mm exit pupil, the eye (at least my eye) will not resolve the Airy disk structure of a star, to see the difference in on-axis performance requires pushing the magnification.

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 new [Re: Jon Isaacs]
      #5779391 - 04/05/13 11:01 AM

Quote:

Quote:

Hi Jon,
It's been a long time since I got my degree in Biology so I'm not going to try to explain it but the link below has a more readable explanation in the section titled "Optimum Magnification"




Tim:

I am reasonably well versed in this subject, this something I have spent some time studying, both in literature, here on the web and at the eyepiece. The article makes my point very nicely:

" Empirical evidence suggests that an exit pupil of 2mm produces very detailed views of most deep-sky objects."

2mm is an empirical number for deep sky objects, not for double stars or planetary viewing where more magnification will clearly reveal more detail. I believe this was what I pointed in in my first post. For example, in an 100mm scope, a 2mm exit pupil is only 50x, hardly an optimal magnification for planetary observation and the article supports that. In my experience, in a 4 inch, something more than 100x is best for showing planetary detail.

There are some things in the article I disagree with, they suggest that for small low surface brightness objects, one should use between 150x and 250x in a 6 inch, 80x-150x in a 12 inch. Since one can see smaller, low surface brightness objects with a 12 inch than a 6 inch, working at the limit, the same exit pupil is appropriate in both scopes.

But, as you say, in the final analysis, your eye should tell you, the rest of this stuff is just guidelines. If you are getting the best view of Jupiter at 50x in a 4 inch apo, go for it... doesn't work for me though.

Jon




Yes Jon,

I would agree with you. My initial use of the planet was probably not well thought out because if you don't have enough magnification to see the details then 2mm is not the best exit pupil.

Timm


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

Reged: 07/07/10

Loc: Davison, Michigan
Re: Reflector/Refractor equivalence formula [Re: orion69]
      #5779661 - 04/05/13 01:48 PM

Quote:

Here it is:




thanks for clearing that up. haha


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

Reged: 10/05/08

Loc: Palo alto, CA.
Re: Reflector/Refractor equivalence formula [Re: buddyjesus]
      #5779674 - 04/05/13 01:53 PM

Going back to comparing the two and thinking out loud, to be useful formula (though i think you need to derive one for each optic and it becomes more a function or matrix or ratio(see below))a requirement IMO is the usefulness would be bound somewhat by initial conditions as marketed. Though i see no way to avoid so called 'magic numbers, you'd need to plug in some values based on optics theory. Holding apeture and or focal ratio constant for the two scopes I see as an unnecessary burden, but should be useful for that. Should give a realistic expectation for where the person will observe. Though expressed algebraic, should be expressed arithmatic with real values as plug-ins, with capability to graph variance if desired suited to obsevers location, look for tipping points and diminishing returns.


qualities tables-

refractor, doublet refractor>| apeture | corrected to what end or not, poly strehl 'efficiency | focal ratio | quality of figure, polish, coatings | overall build quality | any added correction

refractor, triplet or quad refractor>| apeture| | poly strehl 'efficiency' | focal ratio | quality of figure and polish and coatings | overall build quality | any added correction


reflector Newtonian>| apeture | size of central obstruction (as percent of apeture area) | poly strehl 'efficiency' |quality of figure, polish,coatings | overall build quality | any added correction

Reflector 'other' | anything more to add for an SCT or Maksutov or DK/CDK/??...


Values/units of measure-

Poly strehl efficiency would be 98% of ideal "for the design as measured" corrected for materials/methods used then 'quality' could be a number .2 to .98 and used as a decimal multiplier. Higher=better.

Apeture a positive real number, just keep both measurements same units.

Focal length a positive real number, units same.

Focal ratio a positive real number.

Quality of figure and polish and coatings - a decimal multiplier 0.2 to 0.98, positive real number. Higher = better.

Overall build quality, non optical, lens cell or mirror support, focuser, baffle/shroud, fans? - a decimal multiplier 0.2 to 0.98 positive real number. Higher=better.

Many folks use a corrector on a faster Newtonian, a four lens fast refractor has it built in. Some sort of bounding box necessary here.

Magnitude/size of reference object. Here use Luna, Jupiter, a point source reference star, and something very low magnitude 'distributed' (ex nebula or galaxy disk) for real world upper lower bounding. This unfortunately would be multi valued?, but I feel important to traverse utility for object types - planets, doubles and multi star, globulars, emission and reflection nebula, galaxies.

Sky conditions of where used (LP, atmosphere stability and clarity), again I'd use a decimal multiplier that would decrease the final value for the optic - but I'd keep it the same for both scopes. Again something like 0.2 and 0.98. Higher number=better.

No small amount of fudge factor implied by the efficiency and quality measures and probably best to go on past performance as measured or 'rule of the club'? Example being if Zambuto/Lockwood/... or A-P/TEC/... vs others.

Expressed as a matrix, rational, or as a ratio I'd expect to represent known observed and be valid without ever using the term 'ideally'...



poor quality vs. poor quality different type.
poor quality vs. best quality of same type.
poor quality vs. best quality different type.
best quality vs. best quality different type.

Areas of difficulty.

-Says nothing about observers visual acuity, eyepiece, personal preference or aesthetics.
-Allows for overly optimistic values where subjective to location and quality / efficieny.
-Using 0.2 to 0.98 possibly too granular, and instead keep open the option to use integer divisors instead?
-need to include ground effect?





.


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: CounterWeight]
      #5779994 - 04/05/13 04:04 PM

Quote:

Expressed as a matrix, rational, or as a ratio I'd expect to represent known observed and be valid without ever using the term 'ideally'...




I think the reality of the matter is that such a matrix would be a lot of work and not much use in comparing reflectors to refractors because there is very little overlap. The difficulties with reflectors disappear with increasing aperture, the difficulties with refractors increase with increasing aperture.

Such a matrix might be more useful in comparing refractors to refractors and reflectors to reflectors but in the long run, by the time one knows enough about ones own preferences and observing habits, one has looked at and through enough telescopes to already know...

Jon


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


Extra information
33 registered and 41 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: 6111

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics