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Are refractors really mainly solar system & imaging instruments

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#151 russell23

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Posted 02 December 2019 - 07:40 AM

Stepping back a second:  If conditions are good enough that 320, 360, or 525x are even tolerable, then I would far prefer the view through the 16" or a 10", 12" or my 20", or my 8" SCT.  The sort of conditions that favor the best seeing are mild ones where cooling the optics has essentially been a non-factor--the sort I didn't even bother with a fan on the 20".  That has been my experience in different locations.  The commonality has been when the conditions are showing that last bit of detail available to my 110ED, the others show more planetary detail every time (talking about the 8, 10, and 20" that I have) and are what I will end up looking through.  The difference with a 120 to 110 is trivial...about 20x. 

 

And remember, Dave is suggesting 525x or even 850x as an "aesthetic" choice.  Looks like an example of Fonzie not only jumping a shark, but following it up with an orca and a blue whale.  But this is the refractor forum, where it always seems to be "Jump the Shark Week".

 

 

At 320x it would already be on the wrong side of optimum for detail or aesthetics to my eye in perfect seeing, so 525x offers only negatives.  I guess after having tried these sorts of things as a novice and for years after in 10/10 seeing, and since then in various conditions, my impression remains the same:  other than novelty I have not seen value in it.  The novelty wears off quickly.

 

And back to the point gwlee made which I responded to and you disagreed with...which is why we are still having this discussion.  Refractors run out of exit pupil too soon because of their aperture.  The point has already been effectively surrendered by admitting that there is nothing being gained as regards to image detail and that if anything the quality of the image is not as crisp as the image scale increases.  You and others can go on about how huge image scale is worth it, but it is providing nothing other than making the image bigger.  Diffraction always wins...because its the law.

And there you go again - claiming I said things I never said.   I never said refractors do not face issues due to smaller exit pupils at lower magnifications.  I said in my area, where seeing conditions most of the time limit maximum magnifications to ~140x but often less, the refractor does not run out of exit pupil on most nights because the seeing conditions do not allow it.

 

The reason this discussion is still going on is not because of any shark jumping, but because you have showed a lack of ability to conceive of experiences beyond your own personal preferences.   I've lived my entire life observing in upstate NY - where nights that allow high magnifications are exceedingly rare, and many times when those conditions do arise I probably have missed them due to needing to get enough sleep for my job. 

 

So as I said originally, when I actually had a chance to see the Moon at 525x on that night a couple years ago, I liked the image scale provided and found the view sharp enough to be an enjoyable viewing experience.    We are still discussing this because you are rigidly demonstrating a severe lack of ability to see this from another person's point of view.   

 

Think about it - you are actually trying to insist that I am a fool for having enjoyed viewing the image scale of the Moon at 525x with a 120mm APO.  This is very reminiscent of another person that has written angry rants about people choosing to own APO's because of the cost-aperture difference vs a dob.   

 

I've tried to explain this to you from a personal point of view - you know that whole thing where this is a hobby and people have things they like and don't like.   As you get older you would benefit from developing more flexibility in understanding that varying personal circumstances have a big impact on circle 3 and therefore what constitutes an acceptable observing experience.


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#152 russell23

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Posted 02 December 2019 - 07:49 AM

BTW: apparently I'm not the only one that understands this:

 

https://www.cloudyni...at102-ed/page-5

 

posts #'d 114 and 115


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#153 jag767

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Posted 02 December 2019 - 08:00 AM

Most posts here are about the Moon, planets and imaging.

Are refractors with their small apertures but excellent optics really only good for detailed views of solar systen objects, and for imaging?

I suppose as higher magnification monoculars, taking over from binoculars, the grab & go hassle free side ipof smaller sizes is good. But beyond that, refractors' apertures are limiting.

So refractors seem more specialised than average sized Newtonians & SCTs, and less good for general viewing of deep sky.


Answered simply, no.

In reality, the answer is to use the right tool for the job, combined with personal preference. To me, lihht pollution plays more of a role on what I observe regularly, rather than what telescopes I own at the time. Were I fortunate enough to live where there's wonderfully clear, dark, skies, I'm confident what I observe would be significantly expanded from what I look at in my heavily light polluted backyard. My 4" f11ed would be fun on all kinds of targets, while even my new 50mm apo little guy with an 8.5° fov would be enjoyable to sweep around with. No other design will come close to giving a wide field like that.
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#154 Jon Isaacs

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Posted 02 December 2019 - 10:06 AM

Stepping back a second:  If conditions are good enough that 320, 360, or 525x are even tolerable, then I would far prefer the view through the 16" or a 10", 12" or my 20", or my 8" SCT.

 

 

My point was that Dave does not have an 8 inch, Dave has a 120mm refractor.

 

And then there's always that little leprechaun called thermal equilibrium. My 120mm ED does a very good job the moment I set it outside, not it's best but close.  That isn't the case with reflectors, even in San Diego and it certainly isn't the case in rural New York.. 

 

In racing parlance... "You run what you brung."

 

Jon


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#155 Jared

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Posted 02 December 2019 - 01:33 PM

mikeDknight's point on exit pupils & floaters is a good one. For my refractors, especially the Taks, I use extenders that give an effective longer FL, meaning easier viewing with longer FL eyepieces, and prevent minute exit pupils.


One minor correction... Focal length extenders let you reach a given magnification with a longer focal length eyepiece which may be more comfortable since eye relief is generally better, but they have no effect on exit pupil for a given magnification. 200x with a 4” telescope will result in a 0.5 mm exit pupil whether it is achieved with an f/10 scope, an f/5 scope with a focal length extender, a Barlow and eyepiece, or just a short focal length eyepiece. The focal length extenders are essentially barlows. That’s not a bad thing, but they don’t help with exit pupil or floaters.
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#156 Redbetter

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Posted 02 December 2019 - 02:24 PM

My point was that Dave does not have an 8 inch, Dave has a 120mm refractor.

 

And then there's always that little leprechaun called thermal equilibrium. My 120mm ED does a very good job the moment I set it outside, not it's best but close.  That isn't the case with reflectors, even in San Diego and it certainly isn't the case in rural New York.. 

 

 

And 120mm is still running out of exit pupil quite quickly, whether one is in New York, the southeast, Texas, Midwest, California, etc.  You added the part about various apertures and I responded to the observed flaw in that logic.   Even with the 10" coming out later than the 110ED, I have watched the Dob catch up in a few minutes on the more stable nights, then keep on going well past what the 110 was showing at the same time--with the 110 effectively topped out by resolution/brightness = exit pupil/aperture. 

 

The point you seemed to miss is that the "little leprechaun" of thermal equilibrium doesn't apply to the stated conditions (by Dave) of excellent seeing.  That is something I have noticed repeatedly, when the seeing reaches such levels, the conditions are mild enough that cooling is not a significant factor.  I have seen that with the 8" SCT and 20" Dob.  In fact I rarely even used the cooling fan until I moved out here where the seeing is far worse (yes, in the refractors too) and the temp swings are far wider.  The most stable nights don't have those rapid and wide temp swings.  That is what I have found everywhere I have lived.



#157 Jon Isaacs

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Posted 02 December 2019 - 03:06 PM

And 120mm is still running out of exit pupil quite quickly, whether one is in New York, the southeast, Texas, Midwest, California, etc.  You added the part about various apertures and I responded to the observed flaw in that logic.   Even with the 10" coming out later than the 110ED, I have watched the Dob catch up in a few minutes on the more stable nights, then keep on going well past what the 110 was showing at the same time--with the 110 effectively topped out by resolution/brightness = exit pupil/aperture. 

 

The point you seemed to miss is that the "little leprechaun" of thermal equilibrium doesn't apply to the stated conditions (by Dave) of excellent seeing.  That is something I have noticed repeatedly, when the seeing reaches such levels, the conditions are mild enough that cooling is not a significant factor.  I have seen that with the 8" SCT and 20" Dob.  In fact I rarely even used the cooling fan until I moved out here where the seeing is far worse (yes, in the refractors too) and the temp swings are far wider.  The most stable nights don't have those rapid and wide temp swings.  That is what I have found everywhere I have lived.

 

Red:

 

This is not about whether a 10 inch can provide better views than a 120mm ED or that a 120ED is running out of exit pupil at 320x, it's simply that the views in the 120ED at small exit pupils can be enjoyable, they can provide detailed views.  

 

"Do not let what you can't do interfere with what you can do."  (John Wooden)  

 

As far as excellent seeing and mild conditions not requiring cooling.. That has not been my experience.  It is my experience that if I want to take advantage of the excellent seeing, (which is relatively common here 5 miles from the Pacific Ocean in the very southwest corner of the CONUS) then I have to make sure that the scope fully cooled.  There are few places with milder climates and milder conditions.  Certainly New York where Dave lives has far more severe conditions. 

 

 Bryan Greer says that the scope/mirror should be within 1 degree C of ambient..  You can use the program mirror cooling to see how fast the mirror will reach thermal equilibrium.. Without a fan, it is not fast at all. 

 

The bottom line is that refractors are not just imaging and solar system scopes.  They're capable of viewing objects of all classes... 

 

I very much appreciate the virtues of large apertures but I also appreciate and understand the virtues of smaller scopes.  There is probably no one who has posted more in the refractor forum about the virtues of large aperture scopes.  But I also get it that there is more to this than just large scopes...

 

Dave is a skilled observer and gets the most out of his equipment with the time he has to observe.  

 

Jon 


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#158 Redbetter

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Posted 02 December 2019 - 04:09 PM

And there you go again - claiming I said things I never said.   I never said refractors do not face issues due to smaller exit pupils at lower magnifications.  I said in my area, where seeing conditions most of the time limit maximum magnifications to ~140x but often less, the refractor does not run out of exit pupil on most nights because the seeing conditions do not allow it.

 

The reason this discussion is still going on is not because of any shark jumping, but because you have showed a lack of ability to conceive of experiences beyond your own personal preferences.   I've lived my entire life observing in upstate NY - where nights that allow high magnifications are exceedingly rare, and many times when those conditions do arise I probably have missed them due to needing to get enough sleep for my job. 

No.  That appears to be your MO in this thread, not mine, which is what makes this so incredibly ironic.  I will remind you now that the post your responded to by gwlee said this:  "That’s the reason, I don’t consider any 4 inch scope an entirely satisfactory planetary scope: for me, a 4 inch scope starts to run out of usable exit pupil at about 100x."  Last time I checked, 100x was considerably less than 140x as you said above...yet it was being noted as limiting for another observer's eye that you responded to.  In a later post gwlee stated it again: "To clarify, the seeing here usually supports no more than about 150-200x, so that’s about the magnification range that I typically want to use for planets here. Achieving 150-200x without reducing exit pupil below 1mm requires at least a 6 to 8 inch scope."

 

Your response to his initial post was:  "On many nights ~130x seems to top out the seeing conditions.  <snip> So the minimum exit pupil of a 4-5" refractor does not max out the seeing conditions on most nights."  I am not going by things you "never said".  I am responding to what you have said.  (As the discussion continued I noted the major logical fallacy in your three circles diagram with respect to aesthetics vs. image scale.)

 

100x for a 4" refractor is 1mm exit pupil, 25x/inch.  Not 0.5mm or 0.25mm for 50x/inch or 100x/inch.  My current area is similar to gwlee's with respect to seeing most nights, which is similar to yours.  It is constant twinkle in the back yard here.  I didn't have any steady nights this year in the backyard that I can recall, zero.  Seeing at elevation was more limited than prior years as well.   

 

Yet you claim that others are failing to appreciate what you are saying.  Which part of the fact that small scopes are running out of exit pupil at 25 to 50x/inch for the vast majority is lost on you?  There are very good reasons that the historically empirically determined, and physics/physiology calculated guides arrived at ~0.5mm and ~50/x inch type limits for skilled observers.  It was skilled observers who developed the empirical rules of thumb.

 

The amusing thing with this being the refractor forum is that there are frequent debates about whether one is really gaining anything from 1mm to 0.5mm exit pupil, since in theory all of the detail is already there at 1mm.  Our eyes usually do benefit from scale, to a point.   Past that point the image is degraded both by the physiology of our eyes, as well as reduced surface brightness, and of course diffraction.  And diffraction is there for everyone, regardless of any differences in eyes.

 

Where do we differ on this?  Others have pointed out that for them 1mm. 0.5mm, etc. exit pupil have proven limiting for satisfactory planetary views and thereby make refractors exit pupil limited for planetary.  You disputed that based on your personal preferences for large image scale regardless of the other known impacts.  While that is fine for you, it doesn't change the general case that applies to the majority. 

 

And FWIW, although I have not focused on lunar observing in this thread, I have found detail and aesthetics on the Moon to be governed by the same considerations as the planets.  This is true whether I am looking for Plato craterlets with a 60mm or observing Jovian features and moons transiting with the same aperture.  To me reaching the same exit pupil detail/aesthetic limit for planets of and lunar across a surface brightness range seems to be the result of diffraction limitations.  If others were seeing more detail with a given aperture, then I would question my visual acuity/floaters/physical features of the eye that become more noticeable at 0.5mm and below on bright objects.  But from what I can tell from discussions is that I see as much as others and more than most...which leaves diffraction as dictated by the aperture. 



#159 Redbetter

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Posted 02 December 2019 - 05:00 PM

 

As far as excellent seeing and mild conditions not requiring cooling.. That has not been my experience.  It is my experience that if I want to take advantage of the excellent seeing, (which is relatively common here 5 miles from the Pacific Ocean in the very southwest corner of the CONUS) then I have to make sure that the scope fully cooled.  There are few places with milder climates and milder conditions.  Certainly New York where Dave lives has far more severe conditions. 

 

 Bryan Greer says that the scope/mirror should be within 1 degree C of ambient..  You can use the program mirror cooling to see how fast the mirror will reach thermal equilibrium.. Without a fan, it is not fast at all. 

 

 

And yet I never saw it as an issue for 8" or 20" in very stable seeing when I had such available, which is why I was not really using my fan much.  From what I recall the best views I ever had of Mars were through that 20" without even using the fan.  Whatever cooling was needed was achieved rapidly enough that thermals weren't an issue.  This wasn't an exception, it was the norm.

 

This has come up before.  Some others haven't found that much need for cooling fans on large apertures, and I suspect a lot of that has to do with their local conditions.  I had experiences similar to theirs in some other locations.  When I moved here I found I needed to take more active measures, particularly if I wanted to try to catch the brief window of better seeing early in the evening, before it turned to mush as regular evening breezes picked up.

 

The interesting thing about it is that even with a fan on the 10" and 20" I notice a plateau to image improvement fairly rapidly when the seeing is good...even on the rare nights that the air remains still and breezeless throughout.  These are also the mildest conditions--as indicated both by the refractors and various other types.  Ironically, on poor nights the effect is similar even though I am cooling the mirror by 30 or even 40 F.  Much of the differential is shed quickly.  In that case the image stabilizes after perhaps an hour and remains limited by seeing hour after hour for the remainder.  The plateau is reached early because the seeing is so poor.  The seeing will fluctuate up and down across those hours to some extent, but the ambient temp typically has leveled out for hours, so it isn't like the scope has fallen behind. 

 

It makes an interesting conundrum:  while one would expect that good seeing would require more effective cooling for the scope to take advantage of it compared to poor seeing, what I have actually seen in various locations is little difference.  When the seeing is good, conditions are mild.  When the seeing is poor and temp differentials are high, one doesn't need to be as effective with the cooling to max out the seeing. 

 

This is why I shelved plans to add cross flow fans for the front surface of the 20".  After examining this and watching the seeing patterns I concluded that the benefit would be so marginal that it wasn't worth the hassle.  It probably would be worthwhile if I ever moved somewhere that regularly had good/great seeing in less than mild conditions.  But I have yet to come across that.



#160 JMW

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Posted 02 December 2019 - 05:35 PM

We use our TEC 140 as a general purpose scope. It is big enough to enjoy on it's own at dark sites. I still use smaller refractors for extreme wide fields of view. I also have larger scopes for looking at stuff that is much dimmer. I use our club's 20 and 24 inch Dobsionian scopes at star parties because I don't want own a scope that large anymore.


Edited by JMW, 02 December 2019 - 05:35 PM.


#161 russell23

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Posted 02 December 2019 - 08:22 PM

Redbetter,

 

You are continuing to mischaracterize what I was saying.  It is a subtle thing you are doing, but I do not believe it is intentional.  It is partly my fault because in the post #101 you are pitching a fit about I was perhaps not clear enough.  Nonetheless if you would try to actually piece together my additional explanations as our exchanges have progressed and try to understand that I am providing a good faith effort here to be clear for you, it would go a long way toward resolving your concerns about what I have stated. 

 

So going back to post #101 and then your latest response to me, I will once again break this down for you.Gwlee is quoted for the following statement:

 

To clarify, the seeing here usually supports no more than about 150-200x, so that’s about the magnification range that I typically want to use for planets here. Achieving 150-200x without reducing exit pupil below 1mm requires at least a 6 to 8 inch scope.
That’s the reason, I don’t consider any 4 inch scope an entirely satisfactory planetary scope: for me, a 4 inch scope starts to run out of usable exit pupil at about 100x.  

Now there are two aspects to this quote that are most relevant.  First is gwlee’s seeing conditions and the second is gwlee’s definition of “run out of usable exit pupil”. 

 

So here is how I responded.  And as I stated in one of my first responses to you.  I was not making any effort to argue with gwlee about anything.  I was noting a point of commonality in seeing conditions.  Here is what I said:

My area is similar.  On many nights ~130x seems to top out the seeing conditions.  Once in a while I can get to 200x and on rare occasions I can manage 300x on the planets.   So the minimum exit pupil of a 4-5" refractor does not max out the seeing conditions on most nights.    When seeing has allowed 300x or more the 120mm ED has been excellent on the Moon and planets.

So I noted that we have similar issues with seeing conditions to what gwlee is describing – probably worse actually.    This was the point of commonality between what gwlee wrote and what I wrote.   Notice I said “My area is similar.” 

Then I stated that the minimum exit pupil of a 4-5” refractor does not max out the seeing conditions on most nights.  My meaning here was possibly not as clear to you as it was to me.  So let me try to clarify (again – because I’ve already done this for you).  

I do not agree with gwlee that a 1mm exit pupil is the point where a refractor runs out of light.  That is too big IME.  With my 120mm APO, I tend to find the optimal maximum magnification is between 160x and 200x.  That is 0.6mm to 0.75mm exit pupils.   Note that is still larger than the typically quoted 0.5mm exit pupil.    So I did not specify this as the point of my post was not to argue with gwlee over exit pupil size.  My point was that for my observing circumstances the seeing conditions are rarely so good that I can get magnification large enough to make the exit pupil smaller than the optimal range with my 120mm APO.

 

Well what about that I mentioned observing at ~300x when seeing allows?  You’re making way too big a deal about this.  Who says a person must always observe at the optimal exit pupil?  And what is your big concern if I choose to do that once in a while?   Your overreaction is mystifying. 

 

100x for a 4" refractor is 1mm exit pupil, 25x/inch.  Not 0.5mm or 0.25mm for 50x/inch or 100x/inch.  My current area is similar to gwlee's with respect to seeing most nights, which is similar to yours.  It is constant twinkle in the back yard here.  I didn't have any steady nights this year in the backyard that I can recall, zero.  Seeing at elevation was more limited than prior years as well.  

Yet you claim that others are failing to appreciate what you are saying.  Which part of the fact that small scopes are running out of exit pupil at 25 to 50x/inch for the vast majority is lost on you?  There are very good reasons that the historically empirically determined, and physics/physiology calculated guides arrived at ~0.5mm and ~50/x inch type limits for skilled observers.  It was skilled observers who developed the empirical rules of thumb.

The amusing thing with this being the refractor forum is that there are frequent debates about whether one is really gaining anything from 1mm to 0.5mm exit pupil, since in theory all of the detail is already there at 1mm.  Our eyes usually do benefit from scale, to a point.   Past that point the image is degraded both by the physiology of our eyes, as well as reduced surface brightness, and of course diffraction.  And diffraction is there for everyone, regardless of any differences in eyes.

Where do we differ on this?  Others have pointed out that for them 1mm. 0.5mm, etc. exit pupil have proven limiting for satisfactory planetary views and thereby make refractors exit pupil limited for planetary.  You disputed that based on your personal preferences for large image scale regardless of the other known impacts.  While that is fine for you, it doesn't change the general case that applies to the majority. 

 

Now to the above in bold.  You have made zero effort to understand what I had to say about observing Jupiter at 320x or the Moon at 525x with my 120mm APO.  Here was my first effort to explain to you what I was saying:

If we get conditions that allow higher magnifications the scope's optics can handle very high magnification on the Moon and planets.  I've used it to 320x  (0.37mm exit pupil) on Jupiter and it was just fine.   I've used it to 525x (0.23mm exit pupil) on one very rare night on the Moon and the image was fine.  Is it as crisp at 525x as it is at 138x?  Of course not, but when the seeing allows the 120mm APO allows much higher magnification than you would think on the Moon. 

 

Notice I specifically said that it is not as crisp at those higher magnifications as it is at lower magnifications.   So I clearly explained I do understand that it is past optimal for the aperture.  However, as I explained in later posts – I enjoyed looking at the Moon in particular at the larger image scale despite the loss of crispness.   Again I will quote myself since it was clear enough that you should be able to grasp the concept if you will actual engage your brain in a good faith effort to understand what I am and am not saying:

 

In this hobby each person's "eye test" is the most important test.  If 525x in a 120mm APO offers something of value to the observer, then what is the problem?  In my case I felt that the greatly increased image scale vs. what is normally possible in my location made for a very different and mesmerizing viewing experience with the Moon.   The minutiae of optics theory was irrelevant to that night's observing session.

 

I also made this point:

 

I'm not saying that optical theory is not relevant to the real world observing experience.  I'm saying it’s not the only thing relevant to the observing experience.    The VPOE also incorporates the individuals aesthetic observing preferences and those need not be focused on "seeing the most details" or "seeing the sharpest possible view" or "seeing objects at the limit of detection."

 

This is a hobby where individuals will have a range of tolerance for what are acceptable observational results.  I noted in this post that 160x-200x is what I consider the optimal range for what I consider the best views of the Moon & Planets with my 120mm APO.   Regarding the higher magnifications all I have said is that when I had the opportunity to go to 525x I found that my scope’s optics were good enough that I found it enjoyable to look at the Moon at that image scale – especially considering how rarely I have had seeing conditions that allow that sort of magnification. 

 

It is astonishing to me that you have been completely incapable of grasping that a person might make a personal choice to once in a while observe at magnifications that are above optimal for the aperture.  Further, given the assurances I’ve provided you that I did not find the view to be as sharp at that magnification as the lower magnification, I also find it astonishing that you have been unable to grasp that this is hardly a big deal and falls squarely into the YMMV aspect of discussion on this forum.

 

Dave



#162 Kunama

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Posted 02 December 2019 - 09:38 PM

Most posts here are about the Moon, planets and imaging. 

 

Are refractors with their small apertures but excellent optics really only good for detailed views of solar systen objects, and for imaging?

 

I suppose as higher magnification monoculars, taking over from binoculars, the grab & go hassle free side ipof smaller sizes is good. But beyond that, refractors' apertures are limiting.

 

So refractors seem more specialised than average sized Newtonians & SCTs, and less good for general viewing of deep sky.

I use my refractors for every kind of target. I especially like them for double stars, open clusters, Jupiter & Saturn, bright DSOs.  On the bright stuff like the Moon I like to use binoviewers.

I regularly use exit pupils from 0.35mm to about 4mm.  I don't rely on optical theory, I just use what works on the night....


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#163 Redbetter

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Posted 03 December 2019 - 12:18 AM

Dave,

 

No.  It is very simple.  There is no mischaracterization by me.  The quoted exchange illustrates the problem, as does your repeating the same bit again a few posts later:  "Gwlee's point is fine.  I wasn't attempting to contradict his point.  I was pointing out that in my area - where seeing conditions most of the time limit magnification to about 130-140x, the exit pupil of a 4-5" refractor is not yet too small."  Yet what you responded to was a post that said even 100x was too small for gwlee with the aperture.  So you were directly contradicting his experience...and claiming you weren't.   What???  He was noting that the resolving power of the aperture and resultant exit pupil hit that limit for his eye.  I agreed with gwlee while noting that my eye can handle smaller exit pupil, as well as there being some effective minimum limits.

 

As gwlee noted, and as I have noted, and as you are effectively admitting to/then not/then saying you do/whatever:  a person can go smaller, but (other than blowing it up to match the dept. store refractor ad copy) it is just bigger, and at some point it actually damages the image to various levels.  The same applies to Maks, SCT's, and reflectors.

 

You have claimed that I have ignored the viewpoints of others and you have been so accepting.  Yet the opposite is the case.   You disputed gwlee's limit/point twice (and then said you didn't, including when you directly contradicted yourself in the next sentence.)  I recognized gwlee's limit as being a personal one that didn't specifically apply to me but represents part of the continuum of image degradation as exit pupil shrinks.  I also recognize there is some additional difference in where folks hit their optimum with respect to detail and aesthetic--and that tends toward larger, not smaller.  And yes, one's demonstrated optimum matters because beyond it the image does suffer.   It doesn't help when something appears more as an emotional appeal, not a rational one, e.g. statements like "minutiae of optics theory was irrelevant to that night's observing session."  

 

Of course you mischaracterized what I said, which was not "a notion that the limitations of optical performance imposed by optical theory necessarily provide the limits of usable magnification in a scope."  Actually what I have said is that the combination of it and our eyes as well as aesthetics pose an effective limit to useful magnifications.  Optical theory suggests larger exit pupils are sufficient to show all the detail, but in practice this is not really so, hence the empirical and personal limits.  However, the numbers you were touting are generally classified as empty magnification since as you note they are past optimum.

 

So while you and some other refractor folks can go on about magical refractor properties that make it aesthetically pleasing to operate at miniscule exit pupils, I will continue to note that this is out in the range of empty magnification and generally less pleasant to use for physical reasons.  Whether or not one finds some sort of aesthetic pleasure in empty magnification is not relevant to the core problem:  the instrument is out of exit pupil for providing additional detail, and well past the point at which diffraction effects, and those of the eye itself become increasingly noticeable for bright objects.

 

Notice that I wouldn't call this empty magnification if the extra scale was providing some additional detail in great conditions.  Instead, what you have confirmed ("So I clearly explained I do understand that it is past optimal for the aperture.  However, as I explained in later posts – I enjoyed looking at the Moon in particular at the larger image scale despite the loss of crispness") is that it is about the scale.  To borrow a term, "embiggening" it makes you happy.  Great, but that doesn't change the fact that the scope is out of exit pupil.   Neither seeing, nor optical quality, nor personal preferences change that.

 

Anyway, this is why I ignore most of the fluff about magnifications used on an instrument for planetary/lunar.  It tells me nothing about the quality of the instrument or even the observer.  What informs me is the detail seen.  I can sometimes "calibrate" another observer by noting what minimum aperture and magnification it takes for them to resolve some planetary feature in good conditions--from that I can apply a multiplier factor relative to my own.  Those optimums are valuable as reference points, aesthetics are not.



#164 Jon Isaacs

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Posted 03 December 2019 - 05:30 AM

It makes an interesting conundrum:  while one would expect that good seeing would require more effective cooling for the scope to take advantage of it compared to poor seeing, what I have actually seen in various locations is little difference.  When the seeing is good, conditions are mild.  When the seeing is poor and temp differentials are high, one doesn't need to be as effective with the cooling to max out the seeing.

 

 

As i have said, that is contrary to my experience. When the seeing is excellent, that's when the scope is not seeing limited and the little details like thermal equilibrium matter most.  It's rare that a 20 inch scope is not seeing limited.  But I think if you want to split that Dawes limit double, it better be.  

 

In my San Diego backyard, conditions are, by any normal standards, always mild. It's a different world.  I can only say, when the seeing is excellent, thermal equilibrium is critical.  When the seeing is poor it's because the high pressure system in the desert is results in high winds at high altitudes, the conditions are still mild.  

 

In any event, this is the refractors forum and not the reflectors forum.  This discussion of Newtonians is pretty much off-topic and this will be my last post on this topic and I won't be reading anything more.  If you want to continue on, it really needs to be in reflectors where other reflector people can contribute.  

 

Jon

 

 

 

Jon



#165 russell23

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Posted 03 December 2019 - 06:29 AM

Dave,

 

No.  It is very simple.  There is no mischaracterization by me.  The quoted exchange illustrates the problem, as does your repeating the same bit again a few posts later:  "Gwlee's point is fine.  I wasn't attempting to contradict his point.  I was pointing out that in my area - where seeing conditions most of the time limit magnification to about 130-140x, the exit pupil of a 4-5" refractor is not yet too small."  Yet what you responded to was a post that said even 100x was too small for gwlee with the aperture.  So you were directly contradicting his experience...and claiming you weren't.   What???  He was noting that the resolving power of the aperture and resultant exit pupil hit that limit for his eye.  I agreed with gwlee while noting that my eye can handle smaller exit pupil, as well as there being some effective minimum limits.

 

As gwlee noted, and as I have noted, and as you are effectively admitting to/then not/then saying you do/whatever:  a person can go smaller, but (other than blowing it up to match the dept. store refractor ad copy) it is just bigger, and at some point it actually damages the image to various levels.  The same applies to Maks, SCT's, and reflectors.

 

You have claimed that I have ignored the viewpoints of others and you have been so accepting.  Yet the opposite is the case.   You disputed gwlee's limit/point twice (and then said you didn't, including when you directly contradicted yourself in the next sentence.)  I recognized gwlee's limit as being a personal one that didn't specifically apply to me but represents part of the continuum of image degradation as exit pupil shrinks.  I also recognize there is some additional difference in where folks hit their optimum with respect to detail and aesthetic--and that tends toward larger, not smaller.  And yes, one's demonstrated optimum matters because beyond it the image does suffer.   It doesn't help when something appears more as an emotional appeal, not a rational one, e.g. statements like "minutiae of optics theory was irrelevant to that night's observing session."  

 

Of course you mischaracterized what I said, which was not "a notion that the limitations of optical performance imposed by optical theory necessarily provide the limits of usable magnification in a scope."  Actually what I have said is that the combination of it and our eyes as well as aesthetics pose an effective limit to useful magnifications.  Optical theory suggests larger exit pupils are sufficient to show all the detail, but in practice this is not really so, hence the empirical and personal limits.  However, the numbers you were touting are generally classified as empty magnification since as you note they are past optimum.

 

So while you and some other refractor folks can go on about magical refractor properties that make it aesthetically pleasing to operate at miniscule exit pupils, I will continue to note that this is out in the range of empty magnification and generally less pleasant to use for physical reasons.  Whether or not one finds some sort of aesthetic pleasure in empty magnification is not relevant to the core problem:  the instrument is out of exit pupil for providing additional detail, and well past the point at which diffraction effects, and those of the eye itself become increasingly noticeable for bright objects.

 

Notice that I wouldn't call this empty magnification if the extra scale was providing some additional detail in great conditions.  Instead, what you have confirmed ("So I clearly explained I do understand that it is past optimal for the aperture.  However, as I explained in later posts – I enjoyed looking at the Moon in particular at the larger image scale despite the loss of crispness") is that it is about the scale.  To borrow a term, "embiggening" it makes you happy.  Great, but that doesn't change the fact that the scope is out of exit pupil.   Neither seeing, nor optical quality, nor personal preferences change that.

 

I'm just going to pick one thing here because it illustrates that you really aren't trying to have a discussion in which you make an effort to understand where the other person is coming from.  It is highlighted in blue above.

 

When I said "Glwee's point is fine.I wasn't attempting to contradict his point."  in post #116 what I was saying to you is that gwlee has his opinion about exit pupil and I was not going to engage in debate with him about it because exit pupil is one of those things that varies with the individual.  I was explaining to you why I find that a 120mm APO does not "run out of exit pupil" very often at my observing location.  This was all well explained in my last post.

 

The rest also has been clarified for you repeatedly.  I will no longer continue to make those attempts because my last post was more than clear enough for a person who's actually trying to engage in a dialog with another person to understand.  At some point I find you just let a person continue in their attempts to make mountains out of molehills.


Edited by russell23, 03 December 2019 - 06:34 AM.


#166 fred1871

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Posted 03 December 2019 - 09:12 AM

I use my refractors for every kind of target. I especially like them for double stars, open clusters, Jupiter & Saturn, bright DSOs.  On the bright stuff like the Moon I like to use binoviewers.

I regularly use exit pupils from 0.35mm to about 4mm.  I don't rely on optical theory, I just use what works on the night....

Sums it up pretty well, in my experience also (50 years or more). Refractors I've used range from 60mm to 9-inch (no, wasn't mine), and I like refractors as general purpose scopes that settle quickly and rarely need collimation.

 

Small ones are fine for grab'n'go. But these days I prefer a bit more aperture, and I've seen some good planetary and Lunar detail with the 140mm to 180mm refractors I've used. The 140mm I've owned for 11 years takes magnification very nicely and as a double star scope has been excellent. Rayleigh and Dawes are no problem with near equal doubles, with elongation down to a bit closer than 0.5-Rayleigh. Where it really shines is close unequal pairs - no extra light in the diffraction rings, so it matches larger reflectors, Newts and SCTs, for that kind of double.

 

For the tightest equal pairs I've used magnifications to 570x, just over 100x per inch, or about 0.25mm exit pupil. This is to get extra scale and confirm what I thought I saw at 400x, a 0.35mm exit pupil. 400x does show everything the optics can deliver, but scale helps for high contrast as in doubles.

 

Star clusters come up nicely, with lower power for the wider ones, set in their context. On moonless nights the magnitude limit - medium power - is around 13.2, from a suburban location. Quite a few globulars show partial resolution; it helps that more of the bright ones can be seen from South of the equator.

 

Jupiter, because the scope is an achromat, is better viewed with filters once the exit pupil gets below 1.0mm. Mars is fine unfiltered, and a few oppositions ago I could follow some details as the diameter shrank with distance longer than another observer with a Meade 20cm SCT.

 

I've now acquired a 140mm ED, the APM version, and if this dratted weather improves I'll see how much better an ED can do than an achromat.

 

I have reflectors as well, currently a C9.25 is the largest, and provides benefits for DSOs. But I've never been a faint galaxy hunter, so I've never really wanted a big Dob. The C9.25 does nicely on nebulae, brighter galaxies (including spiral arms on a few, better from out of town of course), and rambling through the Magellanic Clouds or the Winter Milky Way. Yes, there are benefits to bigger apertures, but I've long thought that if I had to make do with only one scope it would be a refractor though not less than a 130mm. 

 

My first real telescope, a 6-inch Newtonian, had a permanent pier in the yard with the equatorial head left on it, under a cover. So only the OTA had to be carried out to obsrve, and the mount was already aligned. That's practical for refractors as well if they're not too big and heavy. A 140mm can be used that way, OTA around 9 or 10kg (say 20-22 lbs).

 

I like refractors for the aesthetics of their star images as well. I've seen some reflectors that with very good optics, cooled and collimated, can give nearly that effect  - but not quite. The secondary mirror still has some effect  on the star image. Conclusion? - for me, at least -  if you need big aperture, go Newtonian. If you're happy with moderate sizes, a refractor can be very satisfying.


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#167 mikeDnight

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Posted 03 December 2019 - 07:24 PM

Takahashi proudly claimed their FS refractors would deliver 100X per inch when conditions permit, as can all Takahashi refractors. But its important to recognize the variables. Seeing and transparency as well as the biggest variable, the visual acuity and observing skill of the person using the scope. Ive never given the slightest consideration to exit pupil, and don't concern myself with whatever the experts say. From experience I'm well aware that my FS128 has proved its worth while observing Uranus at over 800X, and my FC100DC at 474X while observing albedo features on Mars at sub 5 arc seconds. I've already been told that that's impossible, but what do I know, I'm just a guy who gets on with it. 

 

I'm equally uninterested about light pollution scales. If the stars are reasonably steady and the sky reasonably transparent,  I simply throw a blackout hood over my head and eyepiece and enjoy the fabulous deep sky views my 100mm refractor offers. But getting those wonderful views takes time and patience. I'd merrily observe an object from 30 minutes upto an hour before sketching it. And if I observe just one object in one night, I'm quite satisfied.

 

John Mallas proved refractors are worthy deep sky instruments when he sketched the Messier catalogue using his 4" F15 Unitron. Walter Scott Houston used a 4" refractor as a deep sky scope, And of course, Steven J Omeara has more than proved how powerful a 4" refractor can be as a very capable deep sky telescope. (And you don't need to live on Hawaii to get spectacular views)!

Then there's TeleVue, a company that's survived over 4 decades with the purpose of delivering the "Space Walk Experience."

And its flag ship for most of that time was a specialist 4" refractor, designed not really for high power planetary but as the ultimate flat rich field deep sky scope.


Edited by mikeDnight, 03 December 2019 - 07:27 PM.

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#168 barbie

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Posted 03 December 2019 - 08:21 PM

VERY well stated and my thoughts exactly!!


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#169 Jeff Morgan

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Posted 03 December 2019 - 10:59 PM

It's rare that a 20 inch scope is not seeing limited.  

 

Indeed it is:

 

https://www.telescop...nd_aperture.htm

 

And that would be an aperture thing, not a Refractor vs. Reflector thing.

 

Well, if there is one good thing that has come out of the last five or six pages of posts: I thought I was the only one struggling with seeing and rarely working above 200x. Seems most folks here are fighting the same battle.

 

The grass isn't always greener on the other side of the fence!


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#170 daquad

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Posted 04 December 2019 - 10:00 AM

Not really.

 

Consider the Messier Catalog.  With an exception or two (misidentifications) all are DSOs.  Most were discovered with small, low quality refractors.  Today we have small and medium aperture high quality refractors available are historically low prices. 

 

Aperture has a role but so does efficiency.  Smaller aperture gathers less light but less efficiency means less of the collected light lands where it belongs in the final image.  For DSOs in particular where it's more about getting the light to the eyepiece than it is resolution, a smaller refractor is a match for  larger reflector since it scatters less by design and has no obstruction and fewer surface-coating related transmission losses.

 

My rule of thumb is that a refractor will deliver the DSO performance of a reflecting scope (Newton, SCT, whatever) of 1.4x the aperture.

 

I use refractors often for DSO observing despite also having other scope of other designs available.

 

Best,

 

Jim   

For DSO observing, we are concerned with light gathering power.  With today's enhanced coatings, I'm inclined to believe the factor is more like 1.2 or less.

 

As an example, take an 8" Newtonian with standard aluminum coatings (reflectivity ~0.88) on both the primary and secondary. And assume a diagonal 25% of the primary diameter, i.e, 6.25% by area.  The net efficiency is (0.88X0.88X0.93) = 0.726.  The equivalent perfect aperture is SQRT(0.726)X8 = 6.81.  

 

So an 8" Newtonian with standard coatings is equivalent to a 6.8" refractor.  8/6.8 = 1.17.  Round to 1.2.  Note that 1/SQRT(0.726) = 1.17.

 

With 94% coatings the transmission efficiency is 0.82 and the equivalent aperture is 0.90.  So with enhanced coatings the 8" reflector is equivalent to a 7.2" refractor.  8/7.2 = 1.11.  1/SQRT(0.82) = 1.11.

 

In order for the factor to be 1.4 a Newtonian would have to have a transmission efficiency of 51%.  I suppose that is possible if the mirror coatings are very old or very dirty or both, but hey that would not be a fair comparison.

 

Dom Q.


Edited by daquad, 04 December 2019 - 10:41 AM.


#171 SonnyE

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Posted 04 December 2019 - 05:07 PM

When my itch was about the size of a mosquito bite, I started at Celestron's 6" x 51" refractor.

But the more I studied, the smaller the telescopes got for what I was into it for, Nebulae. (Curse you Orion!)

 

So my expectations took me to the best refractor I could afford, with DSO astrophotography in mind.

And I asked several places and became reassured that my "Pea-Shooter" would take me where I wanted to be.

So I got an Orion ED80T CF. Basically, I guess I'm Galileo-ian. lol.gif

 

I did not think I wanted to fiddle with a reflectors needs and Collimating one. And to my way of thinking, a refractor had less to go wrong with it.

A bonus I saw was I could also use my ED80T as a telephoto lens here on Earth with my DSLR, or as a visual telescope as well.

It just seems there are many multi-use things about a refractor telescope.

But my main interest was/is for deep space imaging. To that end, the advice I got here was spot on!

 

And right now while my mount is Out Of Order, I'm using my telescope visually on my tripod. (When a hole appears in the clouds, that is. undecided.gif

 

So my answer is NO, refractors are very adaptable to a lot of things.


Edited by SonnyE, 04 December 2019 - 05:12 PM.

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