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Thoughts On The Refractor?

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#176 Daniel Mounsey

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Posted 17 August 2014 - 12:19 AM

Shien,

I agree with your comments. There's nothing like the view of a nice globular in a big reflector IMO. Jon Isaacs has a few large aperture dobs. They really have their place sometimes. To this day though when it comes to planets, specialized reflectors are the best when the execution is done right. The other day I spoke with Ed Grissom and he was working on a smaller reflector and he was complaining about how there wasn't enough clearance around the primary because the tube was too small, so he put a smaller optic in it and got better performance. He's so particular about tube clearance but the end results are second to none when aimed at the Moon and planets.



#177 Sarkikos

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Posted 17 August 2014 - 11:27 AM

Aperture counts for nothing, if you're too tired to drag it outside and use it.

 

 

Clear skies!
Thomas, Denmark

 

But aperture will count for a lot on those nights when you do have the energy to take the bigger scope out.  Since when do we rate telescope performance by the energy level of the observer?

 

Mike



#178 Sarkikos

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Posted 17 August 2014 - 11:39 AM

I understand what you're saying however, I'm not at all debating the resolving limits of telescopes with Mike as that wasn't the point. When I mentioned superior stellar points, I meant something like a tailored suit or a telescope's potential or capacity to work to the best of its ability at any given moment in good seeing or bad. When the seeing is good, stars will still always look cleaner and nicer in a refractor compared to a 10" reflector, especially some random off the shelf model as long as both telescopes being compared within their theoretical limits. I can afford any telescope I like, yet I have no problem using a 4" apo at all. Why would Mike think doubles look nicer in a 10" even if the seeing is good? How does that make any sense?

 

 

I didn't say anything about which telescope doubles look "nicer" in. What I did say is that "I don't see the advantage to smaller aperture for double stars unless perhaps the seeing is very poor - in which case you really should be viewing something else ... or you don't want to bother with the weight and bulk of the larger scope."  You assumed that the advantage is the aesthetics of the view, and that your critique of the aesthetics is the proper one. My assumption is that the advantage is in getting the job done, that is, splitting the stars. Which telescope double stars look "nicer" in is a matter of opinion. Which telescope can split closer doubles is a matter of fact.

 

Mike


Edited by Sarkikos, 17 August 2014 - 11:45 AM.


#179 Sarkikos

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Posted 17 August 2014 - 11:41 AM

 

Aperture counts for nothing, if you're too tired to drag it outside and use it.

 

 

Clear skies!
Thomas, Denmark

 

But aperture will count for a lot on those nights when you do have the energy to take the bigger scope out.  Since when do we rate telescope performance by the energy level of the observer?

 

I take out the C80ED because it is easier than my 10" Dob, not because it performs better.

 

Mike


Edited by Sarkikos, 17 August 2014 - 11:43 AM.


#180 Astrojensen

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Posted 17 August 2014 - 12:43 PM

 

 

But aperture will count for a lot on those nights when you do have the energy to take the bigger scope out.  Since when do we rate telescope performance by the energy level of the observer?

I take out the C80ED because it is easier than my 10" Dob, not because it performs better.

Precisely my point. This is also the reason, why it's useful to own more than one scope. I never said you could only own ONE scope. I do tend to give the advice, that if you *can only* own one scope, it should not be too big to be useful on a weekday night, when you're tired from work. For the same reason, I also tend to recommend a fairly small scope as your first. 

 

Telescope performance in relation to the energy level of the observer is VERY important and has always been. A 20" dob would be useless for me on most nights and I would quickly lose all my observing skills, because I would only get to use it five to ten times a year. A 20" in combination with a 4" apochromat, for example, would be MUCH more powerful, since I would keep my skills sharpened with the smaller scope, when I couldn't use the 20". 

 

 

Clear skies!
Thomas, Denmark



#181 BillP

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Posted 17 August 2014 - 01:41 PM

the "challenge" of splitting a 1" binary in a 140 refractor is not any more meritorious than the "challenge" of splitting a 0.5" binary in a 250" reflector. ...  in the larger scope the "challenge" is punching through the seeing made worse by the larger aperture. the "challenge" is mostly in overcoming the handicaps that you impose on your observing -- or in "predicting" that the larger aperture will always do a better job.

 

 

Excellent post!



#182 BillP

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Posted 17 August 2014 - 02:06 PM

 

Aperture counts for nothing, if you're too tired to drag it outside and use it.

 

 

Clear skies!
Thomas, Denmark

 

But aperture will count for a lot on those nights when you do have the energy to take the bigger scope out.  ...

 

 

So there we have it....aperture counts a lot when the circumstances are one way, and aperture counts for nothing when the circumstances are another way.  So really one cannot count on aperture without specific qualifications as by itself it is insufficient for any assessment...why the adage "aperture rules" is incorrect as it stands.  It's just one of a number of factors that may be advantageous or disadvantageous. 



#183 Sarkikos

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Posted 17 August 2014 - 03:23 PM

 

 

 

But aperture will count for a lot on those nights when you do have the energy to take the bigger scope out.  Since when do we rate telescope performance by the energy level of the observer?

I take out the C80ED because it is easier than my 10" Dob, not because it performs better.

Precisely my point. This is also the reason, why it's useful to own more than one scope. I never said you could only own ONE scope. I do tend to give the advice, that if you *can only* own one scope, it should not be too big to be useful on a weekday night, when you're tired from work. For the same reason, I also tend to recommend a fairly small scope as your first. 

 

Telescope performance in relation to the energy level of the observer is VERY important and has always been. A 20" dob would be useless for me on most nights and I would quickly lose all my observing skills, because I would only get to use it five to ten times a year. A 20" in combination with a 4" apochromat, for example, would be MUCH more powerful, since I would keep my skills sharpened with the smaller scope, when I couldn't use the 20". 

 

 

Clear skies!
Thomas, Denmark

 

Yes, I agree. At the extreme end of the continuum, there would be no sense in anyone owning a telescope that would be so cumbersome or difficult to set up that it is NEVER used.   For myself, I already know that I would never own a telescope so large that I would need to climb a ladder to look in the eyepiece.  I would also never own a telescope so heavy and cumbersome that I would need help - or at least a safety spotter - to mount the OTA.  But these are self-imposed limitations that have nothing to do with the performance of the telescope.

 

Mike



#184 Astrojensen

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Posted 17 August 2014 - 03:40 PM

 

But these are self-imposed limitations that have nothing to do with the performance of the telescope.

Of course, but these limits are very real in our choice of telescope and rightly so, and so must always be taken into consideration. Those who only choose a scope based on raw performance are very often getting very unhappy with it and sell it shortly afterwards. 

 

The best telescope is still the one that gets used. Now, which telescope, of my small collection, is the best for me on any given night, will depend on a number of variables.

 

 

Clear skies!
Thomas, Denmark 



#185 karstenkoch

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Posted 17 August 2014 - 06:14 PM

It is like a complicated Venn Diagram with at least the following circles:

economics/finances
time available for observing
seeing and other atmospheric effects
physical ability/lifting ability
aesthetic preferences/desired targets
storage space/transport space
aperture

I'm sure you could add more. Some of the above are interrelated, too. In any case, aperture DOES rule if you hold all the other variables fixed, or better yet do away with them. However, if you don't, then I propose that the aperture solution for each person will not be the same, and for some it will be smaller than the advice to "go big" recommends.

#186 beanerds

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Posted 18 August 2014 - 08:25 AM

Hell yes , here is my latest and its a beauty , all of 60mm or 2.4 inches of sweet little telescope , the new generation of small scopes .
Here she is side by side in testing with my sky90 , and she did herself proud this night , all 2.4 inches of her but that's another story .

Brian.

Why are people so enamored with small refractors when they can't get that detail (visual)?[/size]

Because we get other things in return. 
 
 
Clear skies!
Thomas, Denmark

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Edited by beanerds, 18 August 2014 - 08:27 AM.


#187 -Starfighter-

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Posted 18 August 2014 - 10:40 AM

"The best telescope is the one you use" 

 

I think this quote (used over & over again) may have it's limits. I would be interested to see how many people actually spend large dollars on scopes + equipment and "do not use them" If you have one scope, obviously it will be the one used the most. For people with many scopes, there probably is a scope they use more than the others but the other scopes are valued just as much. So...the meaning of the above catch phrase does not escape me, however is weak at best...

 

To the posts above about people trying to be right. I don't see that at all. This thread is not about being right. My OP was not about being right. This is also not a "refractor vs dob" thread. This was about shedding the light on preferences and discussing the limitations of each instrument. I don't see a discussion on right and wrong as there is no premise to be right or wrong about.  



#188 Derek Wong

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Posted 18 August 2014 - 11:40 AM

 

I take out the C80ED because it is easier than my 10" Dob, not because it performs better.

 

Mike

 

 

Agreed, except there are things like the conjunction this morning that an small refractor is better suited for.  The seeing was fuzzy enough at the low altitudes that I used an 80mm ED refractor that gave very nice views.

 

Derek


Edited by Derek Wong, 18 August 2014 - 11:41 AM.


#189 Astrojensen

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Posted 18 August 2014 - 01:00 PM

 

I would be interested to see how many people actually spend large dollars on scopes + equipment and "do not use them"

A lot, based on how often I see ads for big scopes that are, in the words of the seller, "very little used". 

 

 

Clear skies!
Thomas, Denmark 



#190 WebFoot

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Posted 18 August 2014 - 03:59 PM

 

 

I would be interested to see how many people actually spend large dollars on scopes + equipment and "do not use them"

A lot, based on how often I see ads for big scopes that are, in the words of the seller, "very little used". 

 

As is the case with many very fine refractors for sale here and on Astromart.  People buying telescopes and then not using them is not in any way limited to a particular type of scope.  But if there's an inclination, I'll bet it's the cheap department store refractor, that gets used once and then never again.



#191 tomjones

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Posted 18 August 2014 - 04:11 PM

Go way out in the middle of nowhere and suddenly a 4" refractor makes sense.



#192 WebFoot

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Posted 18 August 2014 - 04:41 PM

Go way out in the middle of nowhere and suddenly a 4" refractor makes sense.

Unless you're backpacking, I could say the same thing about a 20" reflector....

 

And I say that as the owner of a fabulous 4" refractor.



#193 Arizona-Ken

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Posted 18 August 2014 - 07:30 PM

Go way out in the middle of nowhere and suddenly a 4" refractor makes sense.

 

Go way out in the middle of nowhere and I would hardly ever take out my five inch APO, I would take out my 11 inch SCT.

 

Arizona Ken



#194 Jon Isaacs

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Posted 19 August 2014 - 02:55 AM

 

 

 

 

 

 

 

 

 

Neither of you acknowledged the last clause of my sentence: "I don't see the advantage to smaller aperture for double stars unless perhaps the seeing is very poor - in which case you really should be viewing something else ... or you don't want to bother with the weight and bulk of the larger scope." Many nights my 10" Dob sits in the house while I choose a smaller scope to take out for an observing session. That's not because the smaller scope performs better, but because it's, well ... smaller. :thinking:

 

:lol: 
Mike

 

 

Then let's acknowledge your first comment first. I'm more interested in getting the facts straight.

 

So, your increased aperture 10" reflector produces superior stellar points than a smaller 4" apo when the seeing is good? Why would a 10" do that while observing doubles if both telescopes are working within their limits? Why? 

 

 

The ability of a telescope to work within its limits is not the same thing as ability to do the job, i.e., split double stars.  When the seeing is good, a decent 10" reflector will split closer doubles than a 4" APO.   How could it not?

 

Mike

 

 

If the 4" apo is already doing the job then what's the difference?

 

 

Daniel;

 

I think I addressed this once before:

 

There is a difference even if the 4 inch is "doing the job",  if the seeing is sufficient, the disks will be smaller, brighter in the 10 inch.  But the real issue is that there are a good number of jobs within reach of the 10 inch that are beyond the limits of the 4 inch..

 

I agree with Thomas, a telescope in the house does no good.. that's one reason why I own a variety of small refractors.  When I do have the energy, when I have the time, then it is nice to enjoy the views a larger scope provides.  That's why I own a variety of reflectors.. For me, this is not a this or that thing, it's a this and that..When the skies are dark and clear, I'm out there with both refractors and reflectors, they make great companions.  

 

Refractors are good, reflectors are good. 

 

Jon 

 

Jon

 

 

Jon,

I understand what you're saying however, I'm not at all debating the resolving limits of telescopes with Mike as that wasn't the point. When I mentioned superior stellar points, I meant something like a tailored suit or a telescope's potential or capacity to work to the best of its ability at any given moment in good seeing or bad. When the seeing is good, stars will still always look cleaner and nicer in a refractor compared to a 10" reflector, especially some random off the shelf model as long as both telescopes being compared within their theoretical limits. I can afford any telescope I like, yet I have no problem using a 4" apo at all. Why would Mike think doubles look nicer in a 10" even if the seeing is good? How does that make any sense?

 

 

Daniel:

 

It is all in how the comparison is made, what the targets are, what the interests of the observer might be. 

 

Mike made the statement that his 10 inch Dob provided sharper,  more point like stellar images. Given that angular diameter of the central disk is inversely proportional to the aperture, the 10 inch provides Airy disks that are 40% of the diameter the 4 inch if the magnifications are equal. My 10 inch is a GSO that a dozen years old.  When cooled and collimated and seeing sufficient, it performs as expected, the stars are tight and round.  

 

Splitting lambda oph at 1.43 arc seconds, it's quite apparent that the 10 inch provides those tighter, brighter more point like stellar images than possible with the 4 inch. I think that is consistent with what Mike wrote.  The double is widely split at the same magnification that are required in the 4 inch to see the separation. 

 

This a direct comparison, no handicap for the larger aperture. Direct comparisons are a bit tricky at lower magnifications because of the human eye's inability to resolve the Airy disk art larger exit pupils.  I keep seeing mention of compared within their theoretical limits.  This implies possibly some handicap is given the larger scope.  I simply point out that any double that is witfhin the limit is also within the limit of a 10 inch and comparisons are best made without a handicap factor.

 

I will say this.  Getting tight, round star stars from a Taiwanese 10 Dobsonian does require more effort and attention to detail than with a 4 inch apochromat.  My own scope had a pinched secondary, often the mirror clips are too tight.  And then there is collimation, thermal equilibrium and body heat management.  Even with one of Floyd's fans cooling the scope, even in coastal San Diego's mild climate, reaching rock solid, Dawes limit thermal equilibrium takes between one and two hours for my beater 10 inch. 

 

Jon

 

 

 

Jon, I've had these scopes at my disposal numerous times and I have a blast of a time using the 4" refractor. Why would anyone in their right mind use a 4" refractor on a great night of seeing? The answer is simply because the 4" produced perfect images of certain double stars, there was no need to use the other scopes at the time. I just made a list of some double stars I liked for the 4" and the 4" delivered wonderful images. If the doubles required more aperture, then I had more scopes to choose from. In the OP, Starfighter was clearly inexperienced and was comparing apples and oranges on the wrong targets. 

 

 

Daniel:

 

It makes perfect sense to me, I spend a lot of time with my 4 inch refractor in my backyard when the seeing is most excellent. It requires no justification. It just is.  But at the same, I know that some doubles would be more pleasing, more perfect, in a larger aperture scope if I had taken the time and made the effort to set one up.  

 

One really never knows when that excellent seeing will happen, one can only guess. Myself, I am an opportunistic observer, I take what the sky gives me... if the seeing is excellent, then I look at doubles that require excellent seeing. In that sense, it seems you and I differ, you choose the stars and then the telescope.. I choose the telescope based on my energy level, on my gut feeling about the conditions.  And then, the game is on... 

 

Jon



#195 Astrojensen

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Posted 19 August 2014 - 03:44 AM

 

you choose the stars and then the telescope.. I choose the telescope based on my energy level, on my gut feeling about the conditions.  And then, the game is on...

 

And I am in both camps. Sometimes, I choose the targets and then the scope, at others, I choose the scope and then the targets. I am mostly in the latter category, I think. In any case, there's often more targets to see in any telescope, than you can physically manage in a single night, so you never run out of new things to see, no matter what scope you bring outside. 

 

 

Clear skies!
Thomas, Denmark



#196 BillP

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Posted 28 August 2014 - 08:52 PM

Sometimes, I choose the targets and then the scope, at others, I choose the scope and then the targets. I am mostly in the latter category, I think. In any case, there's often more targets to see in any telescope, than you can physically manage in a single night, so you never run out of new things to see, no matter what scope you bring outside. 

 

 

 

I always choose the scope first.  Doesn't matter what the seeing is...irrelevant as far as I'm concerned.  Better seeing just means more fun with whatever scope is coming out the door.  The scope is like a companion in the adventure with the stars, and every aperture is capable of bringing one more to see than they can even come close to exhausting.  So with that, it's usually the more personable scope that joins in the journey, not the one with the biggest mouth :lol:



#197 BigC

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Posted 28 August 2014 - 09:16 PM

 

 

 

I would be interested to see how many people actually spend large dollars on scopes + equipment and "do not use them"

A lot, based on how often I see ads for big scopes that are, in the words of the seller, "very little used". 

 

As is the case with many very fine refractors for sale here and on Astromart.  People buying telescopes and then not using them is not in any way limited to a particular type of scope.  But if there's an inclination, I'll bet it's the cheap department store refractor, that gets used once and then never again.

 

Unless they are ALL  prevaricators ,based on their ads  it would seem many people buy or gift some very nice  $1000+  scopes that get used once or twice !

 

As for the department store refractors I daresay most are set aside out of laziness,impatience, and simple ignorance of how to use a scope.Ask any customer service representative how many people bother to read the instructions for anything.


Edited by BigC, 28 August 2014 - 09:17 PM.


#198 Glen A W

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Posted 28 August 2014 - 11:28 PM

I set up my SW 100ED at a star party this past weekend.  People loved using it.  I had it at low power with a 2" 32mm Plossl and on a steady alt-az mount.  I would show folks a few things and then if they were interested, show them how to use the red dot finder - and then I turned them loose.  The low power, wide field views under a dark sky are awesome, and the only way you can get that is with a short focal length.  I got no complaints from anyone about the views - lots of stars, so many stars, I had them tell me over and over.

 

Glen



#199 Bomber Bob

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Posted 29 August 2014 - 02:51 AM

Small refractors with their limited but high quality views are like astronomical poets describing what they see in a few perfect words.

 

Lord Byron couldn't top that sentence!

 

Lots of good points in this thread, both pros & cons, and I agree:  If you have the money, space, and observing conditions, a variety of scopes is the way to go.  If not, get the best made scope you can afford that suits your observing preferences & location.  I moved frequently during my military career, so an 80mm F15 EQ rig that fit into a single wooden case made the most sense.  I'm settled now, but live in the city, so a large reflector or SCT would go unused most of the time, since it's far easier to tote a 4" RFT and mount out to the country; and for in town observing, I'm perfectly happy with my 3" F15 Classic achro on the Moon & planets, while my very well made 60mm F15 excels at double stars and open clusters. Plenty of targets for the 3-5 hours per night I get to spend outside when the weather cooperates.

 

IMO, a 60mm refractor is a very useful tool.  With its neglible set up time, you can quickly assess the seeing; however, once you're outside with it, you may decide hauling out the bigger guns is simply superfluous.

 

As for imaging, how about this cheap shot (literally - a $35 Orion SSII!), made with my 1962 76x1200 achro:

 

 

Attached Files


Edited by Bomber Bob, 29 August 2014 - 05:09 AM.


#200 BillP

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Posted 29 August 2014 - 07:31 AM

Small refractors with their limited but high quality views are like astronomical poets describing what they see in a few perfect words.

 

Lord Byron couldn't top that sentence!

 

That was CNer karstenkoch that said that btw.  And yes, we should have a thread of memorable quotes of members because that one sure belongs there!  :bow:








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