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Why are refractors considered to be sharper than reflectors if resolution is a function of the aperture?

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#76 caveman_astronomer

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Posted 18 March 2017 - 07:17 AM

 

When is the last time a major observatory built a refractor? As far as I know, the largest refractor still in use is the 36-incher on Mt Hamilton, built in the 1880s (according to Wikipedia)! It's still a fine telescope, but there's a reason observatories are building reflectors. Perhaps they scale better. That wouldn't really be a problem with everyday astronomers.

Yes, reflectors scale vastly better, for several different reasons. To be precise: false color scales linearly with aperture, large lenses are hard to support, and the glass for a lens has to be perfect throughout its thickness rather than just at the surface. And this is indeed an issue for everyday backyard astronomers.

 

Refractors pretty much rule supreme in apertures smaller than 90 mm. There are some pretty good 76-mm Newtonians on the market, but they're only marginally cheaper than equivalent reflectors, and they have a number of disadvantages. So they appeal mainly to people who are really hard-up for money. There are also a handful of Mak-Cas scopes in apertures of 60 or 70 mm, but since the main benefit of that design is small physical size, and 60- or 70-mm refractors are already quite small, the tiny Mak-Cas's aren't very popular.

 

Refractors are also quite competitive in apertures from 90 to 125 mm. But toward the top of that range, the disadvantages of the design are beginning to kick in big-time. At 125 mm, either you end up with a short-focus achromat with tons of false color, or a long-focus achromat that's really unwieldy and hard to mount, or an apochromat that costs a minor fortune.

 

At 150 mm, refractors are really a stretch. Very few people can afford apochromats in this size, and with achromats you typically end up with both lots of false color and an unwieldy size. There are nonetheless some people who love 150-mm achromats because of their low light scatter, but that's truly the end of the line. Refractors bigger than 150 mm (6 inches) are rare indeed in the amateur world.

 

With reflective designs, by contrast, you're just getting started at 150 mm. That's considered quite small for a Newtonian, and not quite there for an SCT. Eight-inch Newts are really cheap and effective, especially on Dobsonian mounts, and eight inches is the standard size for SCTs.

 

In the modern world of amateur astronomy, where deep-sky objects are the most popular targets, even 8 inches isn't much. That's barely enough to resolve most globular clusters or see the spiral arms of the biggest and brightest galaxies. So while refractors certainly have their place for viewing wide fields, for viewing the planets in less-than-perfect seeing, and above all for photography, the fact that they scale up poorly definitely limits their popularity among amateur astronomers.

 

Newtonians, provide a natural, simple viewing position for the eyepiece at all apertures.  Refractors and Cassegrains require tall tripods and star diagonals.  We're not going to make the artificial distinction and comparison between 90mm refractors and 90mm reflectors or between any other refractors and reflectors that happen to have nominally matching apertures.


Edited by caveman_astronomer, 18 March 2017 - 07:45 AM.


#77 Redbetter

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Posted 20 March 2017 - 04:30 AM

 

Newtonians, provide a natural, simple viewing position for the eyepiece at all apertures. 

 

Do they?  Newtonian refers to the optical design, not the mount.  An equatorial Newtonian appears to have some rather unnatural eyepiece positions depending on the declination of the target and the position on relative to the meridian.  The position of a stool or ladder could be rather challenging.   I have never had one, so I don't speak from experience in this matter, but large EQ mounted Newtonians have mostly disappeared.

 

A Dobsonian is more natural for eyepiece placement as are most alt-az systems.  Unfortunately they don't conform well to the rotational nature/coordinate system of the night sky.   They are easy to us near the meridian and more challenging as one moves away from it.

 

No system is without its drawbacks.  I don't find alt-az or GEM refractors hard to use although each is different and should be matched to the OTA/focuser.   I have a chair that makes both easy enough to use with standard tripods.  Dobs are also easy enough, including the 20" with platform ladder, or 10" with chair.  Each system has its obstacles, so it is more a matter of personal preference/circumstance/cost rather than clear superiority in all categories.


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#78 Tony Flanders

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Posted 20 March 2017 - 05:32 AM

 

 

Newtonians, provide a natural, simple viewing position for the eyepiece at all apertures. 

 

An equatorial Newtonian appears to have some rather unnatural eyepiece positions depending on the declination of the target and the position on relative to the meridian.

No, I'd say that if an equatorial-mounted Newt has rotating rings, it's always easy to find some comfortable viewing position regardless of where the scope is pointing.

 

However, I don't really agree that Newts provide the best viewing position regardless of aperture. I do agree that alt-az mounted Newts (including Dobs) have the best ergonomics of all designs up to a focal length of around 1,500 mm, maybe even to 2,000 mm. But beyond that, they start to require increasingly tall ladders, which begin to get genuinely dangerous and/or scary around 3,000 mm. In those focal lengths, I think that Cassegrain designs are quite clearly superior, due to the fact that you're observing from the bottom of the tube and the fulcrum is closer to the back than the front.

 

Refractors certainly have the worst ergonomics, at least in focal lengths above 1,000 mm. They really have the worst of all possible worlds: bottom viewing, long tube, fulcrum far from the eyepiece, viewing angle exacerbates variation in head height rather than counteracting it as with a Newtonian.



#79 caveman_astronomer

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Posted 20 March 2017 - 06:59 AM

 

 

Newtonians, provide a natural, simple viewing position for the eyepiece at all apertures. 

 

Do they?  Newtonian refers to the optical design, not the mount.  An equatorial Newtonian appears to have some rather unnatural eyepiece positions depending on the declination of the target and the position on relative to the meridian.  The position of a stool or ladder could be rather challenging.   I have never had one, so I don't speak from experience in this matter, but large EQ mounted Newtonians have mostly disappeared.

 

A Dobsonian is more natural for eyepiece placement as are most alt-az systems.  Unfortunately they don't conform well to the rotational nature/coordinate system of the night sky.   They are easy to us near the meridian and more challenging as one moves away from it.

 

....

Newtonians can easily be designed with rotating tubes on equatorial mounts. 

 

Large equatorially mounted Newts haven't disappeared so much as they are now outnumbered by similar-sized Dobs, which are more affordable. 


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#80 caveman_astronomer

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Posted 20 March 2017 - 07:10 AM

 

 

 

Newtonians, provide a natural, simple viewing position for the eyepiece at all apertures. 

 

An equatorial Newtonian appears to have some rather unnatural eyepiece positions depending on the declination of the target and the position on relative to the meridian.

No, I'd say that if an equatorial-mounted Newt has rotating rings, it's always easy to find some comfortable viewing position regardless of where the scope is pointing.

 

However, I don't really agree that Newts provide the best viewing position regardless of aperture. I do agree that alt-az mounted Newts (including Dobs) have the best ergonomics of all designs up to a focal length of around 1,500 mm, maybe even to 2,000 mm. But beyond that, they start to require increasingly tall ladders, which begin to get genuinely dangerous and/or scary around 3,000 mm. In those focal lengths, I think that Cassegrain designs are quite clearly superior, due to the fact that you're observing from the bottom of the tube and the fulcrum is closer to the back than the front.

 

Refractors certainly have the worst ergonomics, at least in focal lengths above 1,000 mm. They really have the worst of all possible worlds: bottom viewing, long tube, fulcrum far from the eyepiece, viewing angle exacerbates variation in head height rather than counteracting it as with a Newtonian.

 

Cassegrain-style telescopes have an ergonomic sweet spot starting at about five-inch aperture and ending at about 10-inch aperture.  Not everyone really needs to have a 4-inch Cass and above 10-inches you are faced with lifting and supporting an increasingly heavy and bulky OTA and mount well above ground level.

 

That the eyepiece on a large Dob can get a little high off the ground is beside the point; there aren't very many practical telescopes with the eyepiece at the "bottom of the tube" in sizes larger than 16-inches.  Those are in the province of observatory instruments, not a practical arrangement for most amateurs.



#81 Mitrovarr

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Posted 20 March 2017 - 09:35 AM

Cassegrain-style telescopes have an ergonomic sweet spot starting at about five-inch aperture and ending at about 10-inch aperture.  Not everyone really needs to have a 4-inch Cass and above 10-inches you are faced with lifting and supporting an increasingly heavy and bulky OTA and mount well above ground level.

 

That the eyepiece on a large Dob can get a little high off the ground is beside the point; there aren't very many practical telescopes with the eyepiece at the "bottom of the tube" in sizes larger than 16-inches.  Those are in the province of observatory instruments, not a practical arrangement for most amateurs.

 

To be fair, I always thought the intended niche of large SCTs was as observatory instruments, either for particularly ambitious amateurs or small universities and such.



#82 jrbarnett

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Posted 20 March 2017 - 11:01 AM

Also resolving power and "sharpness" are NOT the same things.  A big mirror has resolving power, but if it is rough or the system it is in scatters light, admits stray light from other sources, isn't aligned such that all of the light collected goes where it is supposed to, etc., the result is a mushy, unsharp image.  Sharpness has more to do with actual in-field-delivered contrast than it does on-paper resolving power.

 

- Jim


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#83 jrbarnett

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Posted 20 March 2017 - 11:08 AM

 

 

Newtonians, provide a natural, simple viewing position for the eyepiece at all apertures. 

 

Do they?  Newtonian refers to the optical design, not the mount.  An equatorial Newtonian appears to have some rather unnatural eyepiece positions depending on the declination of the target and the position on relative to the meridian.  The position of a stool or ladder could be rather challenging.   I have never had one, so I don't speak from experience in this matter, but large EQ mounted Newtonians have mostly disappeared.

 

A Dobsonian is more natural for eyepiece placement as are most alt-az systems.  Unfortunately they don't conform well to the rotational nature/coordinate system of the night sky.   They are easy to us near the meridian and more challenging as one moves away from it.

 

No system is without its drawbacks.  I don't find alt-az or GEM refractors hard to use although each is different and should be matched to the OTA/focuser.   I have a chair that makes both easy enough to use with standard tripods.  Dobs are also easy enough, including the 20" with platform ladder, or 10" with chair.  Each system has its obstacles, so it is more a matter of personal preference/circumstance/cost rather than clear superiority in all categories.

 

Apparently he's never been up a ladder for a peek through a 32" Dob.

 

Fork mounted SCTs provide the most ergonomic viewing, irrespective of aperture.  The eyepiece position from zenith to horizon, even on a ginormous SCT, remains within a relatively small range.  No so for anything but a small-ish Dob, not so for anything but a smallish eq mounted Newt, and not so for eq mounted refractors.

 

- Jim


Edited by jrbarnett, 20 March 2017 - 02:26 PM.

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#84 caveman_astronomer

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Posted 20 March 2017 - 01:41 PM

 

 

 

Newtonians, provide a natural, simple viewing position for the eyepiece at all apertures. 

 

Do they?  Newtonian refers to the optical design, not the mount.  An equatorial Newtonian appears to have some rather unnatural eyepiece positions depending on the declination of the target and the position on relative to the meridian.  The position of a stool or ladder could be rather challenging.   I have never had one, so I don't speak from experience in this matter, but large EQ mounted Newtonians have mostly disappeared.

 

A Dobsonian is more natural for eyepiece placement as are most alt-az systems.  Unfortunately they don't conform well to the rotational nature/coordinate system of the night sky.   They are easy to us near the meridian and more challenging as one moves away from it.

 

No system is without its drawbacks.  I don't find alt-az or GEM refractors hard to use although each is different and should be matched to the OTA/focuser.   I have a chair that makes both easy enough to use with standard tripods.  Dobs are also easy enough, including the 20" with platform ladder, or 10" with chair.  Each system has its obstacles, so it is more a matter of personal preference/circumstance/cost rather than clear superiority in all categories.

 

Apparently he's never been up a ladder for a peek through a 32" Dob.

 

The 32-inch refractor and the 32-inch Cassegrain are not likely to be easily portable and therefore are likely to reside in observatories that have flat, solid floors and rolling ladders, stairs or platforms.  A 32-inch Dob, given the same advantages, would not be so objectionable with regard to its eyepiece height?



#85 Phil Cowell

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Posted 20 March 2017 - 01:51 PM

 

Refractors are great.  Too bad they are all so small in aperture.

 

I love my refractors..  I love my reflectors.. 

 

To directly answer the original question:

 

I think of high quality refractors as providing sharp views because within the limits of their aperture,  they are able to work closer to that limit more often than other designs. I also think of them as sharp because the only off-axis aberration they typically exhibit is field curvature so the views can be very clean. 

 

But that sharpness is not resolving power,  it's relative resolving power..  Relative to the aperture..  If you want to resolve M80 to the core,  that takes horsepower, cubic inches.. (Actually inches of diameter, square inches of area.) 

 

Jon

 

I think it's always good to have a few diverse scope in the stable. That way you have the right one for the occasion to hand.



#86 Redbetter

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Posted 20 March 2017 - 05:02 PM

 

 

 

Newtonians, provide a natural, simple viewing position for the eyepiece at all apertures. 

 

An equatorial Newtonian appears to have some rather unnatural eyepiece positions depending on the declination of the target and the position on relative to the meridian.

No, I'd say that if an equatorial-mounted Newt has rotating rings, it's always easy to find some comfortable viewing position regardless of where the scope is pointing.

 

 

That is a big if which is why I stated it as I did.  It is the same as assuming rotating focusers, etc. for a refractor with a GEM. 

 

The point is that as the aperture increases these equatorials are likely to lead to increasing unnatural or precarious viewing positions, and massive mounts.


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#87 caveman_astronomer

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Posted 20 March 2017 - 05:32 PM

 

 

 

 

Newtonians, provide a natural, simple viewing position for the eyepiece at all apertures. 

 

An equatorial Newtonian appears to have some rather unnatural eyepiece positions depending on the declination of the target and the position on relative to the meridian.

No, I'd say that if an equatorial-mounted Newt has rotating rings, it's always easy to find some comfortable viewing position regardless of where the scope is pointing.

 

 

That is a big if which is why I stated it as I did.  It is the same as assuming rotating focusers, etc. for a refractor with a GEM. 

 

The point is that as the aperture increases these equatorials are likely to lead to increasing unnatural or precarious viewing positions, and massive mounts.

 

A rotating tube arrangement has been a part of most of the German equatorial Newts I have ever seen, except for a few very small, cheap ones.



#88 precaud

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Posted 22 March 2017 - 09:12 AM

The point is that as the aperture increases these equatorials are likely to lead to increasing unnatural or precarious viewing positions, and massive mounts.
 

 

AND have inadequately-supported primaries... that, in my experience, is the most serious limitation of larger Newts.



#89 Olle Eriksson

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Posted 10 July 2019 - 09:56 AM

I've just recently got myself my first refractor (a 120mm f5 achro) after having used an 8" f6 dob my whole life. I was actually quite surprised to find the ergonomics much worse and I have had to constantly adjust the height of the tripod to find a good position. Despite this, observing close to the horizon for long periods of time seems quite awkward for the neck.

 

But besides that, I've had great views in the refractor, alas at a great dark sky. But as people have already said here, I think it has most to do with magnification. Yes, the larger aperture can resolve smaller details, but if you look at larger objects they appear just as bright as a smaller object would appear with a larger aperture. Basically, as long as the f-ratio is the same, the views will be similar (all other things equal, and ignoring the central obstruction etc) only the magnification is lower in the smaller aperture (refractor). Some objects fit the small refractor better (large open clusters, Milky Way star clouds, dark nebulae, large bright nebulae) while the large dob works better for faint fuzzies, planetary nebulae and such things. I suppose, after having seen most large objects, the higher magnification you can go the more objects are out there.


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#90 Dave Mitsky

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Posted 10 July 2019 - 12:38 PM

That is a big if which is why I stated it as I did.  It is the same as assuming rotating focusers, etc. for a refractor with a GEM. 

 

The point is that as the aperture increases these equatorials are likely to lead to increasing unnatural or precarious viewing positions, and massive mounts.

I realize that this is a rather old discussion but can attest that even a moderately large Newtonian on a GEM can present some rather awkward viewing positions.

 

Here's a shot of the ASH 12.5" f/6.5 Cave Astrola Newtonian at the Naylor Observatory.

Dave Mitsky

 

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#91 Jon Isaacs

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Posted 10 July 2019 - 04:13 PM

I realize that this is a rather old discussion but can attest that even a moderately large Newtonian on a GEM can present some rather awkward viewing positions.

 

Here's a shot of the ASH 12.5" f/6.5 Cave Astrola Newtonian at the Naylor Observatory.

Dave Mitsky

 

 

 

Rotating rings are a big help.  This is a 12.5 inch F/6.  

 

6054223-Meade Winter in San Diego.jpg
 
Jon


#92 csrlice12

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Posted 11 July 2019 - 09:56 AM

Bare feet on a metal ladder at night.....not where I live. wink.gif



#93 csa/montana

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Posted 11 July 2019 - 10:21 AM

Folks, this is a two year old thread; so will let it go.  However, please know that Gen. Astronomy is not for equipment discussions.  There are specialized forums for those.


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#94 REC

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Posted 11 July 2019 - 11:06 AM

For all the above reasons, that is why I have all 3 designs. Reflector, refractor and a SCT.  I choose the scope to meet the objects that I'm going to observe.


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#95 GeneT

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Posted 11 July 2019 - 06:18 PM

A few years ago, the  Scobee Planetarium in San Antonio had finished installing in its observatory a new 10 inch refractor made by D & G optical. Several of us had set up our telescopes outside the observatory for outresearch. Jupiter was up nicely in the sky. I had set up my 12.5 inch, F5 Portaball. Viewing Jupiter, I would spend about 15 minutes at the eyepiece at my Portaball, then do the same at the 10 inch refractor. The D&G refractor is a beautiful instrument. It is also a huge mama. So is the mount necessary to keep the telescope steady. The views of Jupiter that the D&G refractor produced were excellent--tack sharp. So were the images in my 12.5 inch Portaball. In fact, as far as amount of detail viewable, it was basically a tie between the two telescopes. I was surprised. I have read postings on CN that imply that a four inch refractor will outperform an 8 or 10 inch reflector (assuming good optics, and proper collimation in the reflector.) Don't buy into that myth. Refractors and reflectors do produce different flavors of views and many find the views produced by refractors to be more aesthetically pleasing. That accounts for the many who would rather view with a three or four inch refractor than a larger reflector. Where I come out on all this is that I would rather have the larger optics of a reflector which will yield more planetary detail and brighter deep sky objects.  


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#96 bobhen

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Posted 12 July 2019 - 03:02 PM

Seeing limits the resolution of all telescopes. If the seeing in your backyard is greater or equal to one arc second, as it is in most backyards on most nights, then on many nights no telescope larger than around 5-inches will show you any more lunar/planetary detail

 

So on many nights and  in most backyards that 10-inch reflector actually acts like a 5-inch when it comes to resolving power.

And if we compare typical 5-inch refractors and 5-inch reflectors, then the refractor’s advantages of higher contrast, no CO and no spider veins, etc. will come into play and the smaller refractor will/can produce a better defined image in the eyepiece.

 

Move to a location like Florida with Florida’s better seeing and that 10-inch reflector will have many more nights to reach it true sub arc second resolving potential and best smaller refractors.  Here in the northeast those nights are a lot more rare.

 

From Sky & Telescope...

 

“A 5-inch refractor is ideal for high resolution visual use on the sun, moon, and planets. Its resolving power of 0.9 arcseconds is perfect for sampling the typical seeing of 2 to 3 arcseconds common at most amateur observing locations….”
 

"…and they (5-inch refractors) aren’t as susceptible to poor seeing as large scopes."

 

Bob



#97 Jon Isaacs

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Posted 12 July 2019 - 09:50 PM

Seeing limits the resolution of all telescopes. If the seeing in your backyard is greater or equal to one arc second, as it is in most backyards on most nights, then on many nights no telescope larger than around 5-inches will show you any more lunar/planetary detail

 

So on many nights and  in most backyards that 10-inch reflector actually acts like a 5-inch when it comes to resolving power.

And if we compare typical 5-inch refractors and 5-inch reflectors, then the refractor’s advantages of higher contrast, no CO and no spider veins, etc. will come into play and the smaller refractor will/can produce a better defined image in the eyepiece.

 

Move to a location like Florida with Florida’s better seeing and that 10-inch reflector will have many more nights to reach it true sub arc second resolving potential and best smaller refractors.  Here in the northeast those nights are a lot more rare.

 

From Sky & Telescope...

 

“A 5-inch refractor is ideal for high resolution visual use on the sun, moon, and planets. Its resolving power of 0.9 arcseconds is perfect for sampling the typical seeing of 2 to 3 arcseconds common at most amateur observing locations….”
 

"…and they (5-inch refractors) aren’t as susceptible to poor seeing as large scopes."

 

Bob

It doesn't take sub arc-second seeing for a 10 inch to excel.  

 

The Dawes limit for a 5 inch refractor is 0.91 arc-seconds.  The Dawes limit involves splitting two equal magnitude stars and the disks are overlapping to the point where there is only a slight minima between them.  The Rayleigh Criteria for a 5 inch is 1.09 arc-seconds and the disk are still overlapping, the first minima passes of one star passes through the center of the second star.   In 1 arc-second seeing, this is blurred by 1 arc-second.  

 

A 10 inch has a much smaller Airy disk half the size of the 5 inch. In one arc-second seeing, it can have superior resolution because it is drawing the image with much smaller pixels.  In 1 arc-second seeing, a 5 inch refractor will have a difficult time of it with a near Dawes limit split, it'll be clean and wide in a 10 inch.  

 

Jon

 

Jon


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#98 Illinois

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Posted 13 July 2019 - 06:14 AM

I remember one night that I cant see E and F star in M42 and cant see double double in Lyra in my 10 inch dobsonian and I can see all at the same night in my 5 inch APO refractor. Refractor for me is sharper pinpoint stars than Dobsonian at high power. I like my 16 inch for low power up to around 120X for faint deep sky objects that my 5 inch APO cant!

Edited by Illinois, 13 July 2019 - 06:15 AM.


#99 Jon Isaacs

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Posted 13 July 2019 - 07:46 AM

I remember one night that I cant see E and F star in M42 and cant see double double in Lyra in my 10 inch dobsonian and I can see all at the same night in my 5 inch APO refractor. Refractor for me is sharper pinpoint stars than Dobsonian at high power. I like my 16 inch for low power up to around 120X for faint deep sky objects that my 5 inch APO cant!

 

Are you actively cooling your 10 inch and 16 inch Dobs? How are you collimating them?

 

Most nights I use my 16 inch from around 70x to upwards of 600x to show me deep space objects, doubles, the planets. Doubles and the planet's are about seeing and scope prep. At 120 x in a 16 inch the exit pupil is 3.4 mm, that's far too low for the eye to resolve the airy disk..

 

The double-double is doable in a 70 mm, I was able to split it a couple of nights ago in as 70 mm F/4.5 achromatic finder that doesn't have a focuser. It was a difficult split. In my 10 inch It was a clean split and easy. 

 

Refractors are nice because they give you their best nearly all the time with very little effort on the part of the owner/observer.  Reflectors require more effort and attention, attention to thermal equilibrium is critical, collimation is critical. But the payoff can be big. 

 

Sharp stars, lots of em = globular clusters.. nothing like a large aperture scope to resolve globulars. It doesn't take super seeing to show this.. Omega Centauri culminates at 10.0° from my dark site, at 10°, the seeing is never good but Omega Centauri is always much better resolved in any of my Dobs than in any of my refractors.

 

And then there are those nights of rock solid seeing, probably more common in San Diego than in Illinois, where a larger scope is not seeing limited, doubles way under an arc- second, Jupiter and Saturn looking like photos..

 

Jon


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#100 Illinois

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  • Loc: near Dixon, Illinois USA

Posted 14 July 2019 - 05:53 AM

Of course I do cooling my 10 and 16 inch at least 1 or 2 hours and always check on collimation before look at stars. My 10 and 16 inch is better for deep sky objects than refractors. Not as sharper pinpoint stars at 3 mag or brighter stars. I don't use Paracorr or Coma Corrector. Low to medium power look great on deep sky objects. I can see NGC6207 by M13 easy in my 16 inch than my 5 inch refractor. I sold 10 inch Dobsonian and 5 inch refractor. For now I am happy with my 80ED, 100ED, 150ED, 180 MakCass and 16 inch Dobsonian.  Yes Illinois night might not like Calfornia. Perfect clear and no hazy night is rare in my area. Darker sky in upper Michigan but not long night because of tempeture drop so fast and need heat dew! Maybe I need Paracorr for my 16 inch and someday I might buy it!


Edited by Illinois, 14 July 2019 - 05:54 AM.



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