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Dob vs Apo

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#151 bandazar

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Posted 20 October 2020 - 06:20 AM

Yep, from my experience, CHAS is right.    Best views of jupiter were from a 10" zambuto mirror telescope that I used to have.  Voyager like, but image scale was much smaller, and colors more muted - when the seeing allowed for it of course. 

I have viewed planets from many sct's and only remembered once where the image of jupiter came close to that of an apo refractor from someone who knew what they were doing.  Maybe the image was 80-90% as good as an apo refractor.   But that was from a person who knew what they were doing.  The majority of people I've met personally who had sct's either did not colliminate them correctly, or probably did not have them cooled down.  or had bad diagonals, etc.



#152 bobhen

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Posted 20 October 2020 - 07:23 AM

Which one gives you the best views in your opinion: a good 8" dobsonian with premium optics or a 4" apo refractor? I never owned a newtonian so, to me, the answer is not so obvious.

Thanks 

pao

Without any caveats, the 8” Newtonian with a high quality (Zambato-like) mirror will give the better views –period. So if all you are concerned about are the views, then the Newtonian is for you.

 

However, there are some other considerations…

 

1. The refractor, especially a high quality doublet like the Tak 100DZ, will acclimate much quicker and can be more stable as temperaturs fall during the night.

 

2. The refractor is more conducive to solar Ha observing, with the right filters of course.

 

3. The refractor on a GEM will be more conducive to imaging.

 

4. The refractor can also be used on an easy to setup alt/az mount and is the better grab-and-go telescope, especially considering acclimation.

 

5. The refractor is the better daytime spotter or daytime imaging scope or better for homes with a view

 

6. The refractor will not need collimation.

 

7. Depending on your location: the further north you are the worse the seeing will be and the colder the nights will be and the closer to the jet stream you will be and the lower in the sky the planets will be. These things can all conspire to compromise the 8” Newtonian’s performance more so than the 4” refractor.

 

Telescopes are more than just something you look through you have to: live with them and store them and set them up and take care of them and use them.

 

For the astronomer willing to deal with the ergonomics and care needed for the Newtonian and who lives in a location that will allow the Newtonian to perform to it fullest on most nights, the Newtonian will definitely deliver more and is the better tool for the job.

 

For the astronomer who wants a telescope that does more things and that takes less care to perform and who lives where conditions are more challenging, the refractor will be the more versatile and the more user-friendly and the more location-friendly tool.

 

Bob


Edited by bobhen, 20 October 2020 - 07:26 AM.

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#153 Jon Isaacs

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Posted 20 October 2020 - 07:38 AM

For the astronomer willing to deal with the ergonomics and care needed for the Newtonian and who lives in a location that will allow the Newtonian to perform to it fullest on most nights, the Newtonian will definitely deliver more and is the better tool for the job.

 

For the astronomer who wants a telescope that does more things and that takes less care to perform and who lives where conditions are more challenging, the refractor will be the more versatile and the more user-friendly and the more location-friendly tool.

 

 

A good post but I do want to make a couple of comments:

 

-  An 8 inch Newtonian does not need to perform to it's fullest to provide superior performance to a 4 inch.  

 

-  These discussions always seem to get hung up discussing planetary viewing.  Planetary viewing is primarily about seeing.  But there is far more to amateur astronomy than viewing the planets and here in a few months, the planets will be gone.

 

Most objects are not as sensitive to seeing so the greater resolution and light gathering of the 8 inch can be put to good use even in less than optimal seeing.  DSOs, an 8 inch goes deeper, galaxies, globulars, planetary nebulae, open clusters, these are all better seen in the 8 inch.  

 

- I think that ideally one owns both a 3-4 inch refractor and a larger reflector of some sort.  Some 4 inch refractors make decent daytime spotting scopes but many are rather clumsy, I think an 80mm is a better choice. 

 

Jon


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#154 bobhen

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Posted 20 October 2020 - 08:16 AM

A good post but I do want to make a couple of comments:

 

-  An 8 inch Newtonian does not need to perform to it's fullest to provide superior performance to a 4 inch.  

 

-  These discussions always seem to get hung up discussing planetary viewing.  Planetary viewing is primarily about seeing.  But there is far more to amateur astronomy than viewing the planets and here in a few months, the planets will be gone.

 

Most objects are not as sensitive to seeing so the greater resolution and light gathering of the 8 inch can be put to good use even in less than optimal seeing.  DSOs, an 8 inch goes deeper, galaxies, globulars, planetary nebulae, open clusters, these are all better seen in the 8 inch.  

 

- I think that ideally one owns both a 3-4 inch refractor and a larger reflector of some sort.  Some 4 inch refractors make decent daytime spotting scopes but many are rather clumsy, I think an 80mm is a better choice. 

 

Jon

 

An 8 inch Newtonian does not need to perform to it's fullest to provide superior performance to a 4 inch.

 

True. But as I said, it will “compromise” the potential "full performance" of the larger scope. How much the larger scope will be compromised depends on seeing, etc.

 

These discussions always seem to get hung up discussing planetary viewing.  Planetary viewing is primarily about seeing.  But there is far more to amateur astronomy than viewing the planets and here in a few months, the planets will be gone.

 

Yes there are a lot more deep sky objects. BUT solar Ha observing and imaging is less condition dependent (light pollution, dark sky) and can add many hours (possibly doubling) the time spent using one’s telescope. And its fun and thrilling as well. The sun is the most dynamic object that one can observer. And, of course, the sun is far more dynamic and dramatic than the much more numerous but never-changing/static deep sky objects.  

 

Most (deep sky) objects are not as sensitive to seeing so the greater resolution and light gathering of the 8 inch can be put to good use even in less than optimal seeing.  DSOs, an 8 inch goes deeper, galaxies, globulars, planetary nebulae, open clusters, these are all better seen in the 8 inch. 

 

True, of course. And even more so if the Newtonian has a high quality mirror that delivers excellent contrast.

 

Some 4 inch refractors make decent daytime spotting scopes but many are rather clumsy, I think an 80mm is a better choice.

 

Maybe and maybe not, it depends on the scope. In any event, an 80mm was not in the comparison. So between the 8” Newtonian and the 4” refractor in the comparison, the refractor makes the much better daytime spotting and imaging scope.

 

Having both types of scopes is, of course, the answer, as one can take advantage of the strengths of each.

 

Bob


Edited by bobhen, 20 October 2020 - 08:18 AM.

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#155 KBHornblower

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Posted 20 October 2020 - 09:36 AM

I agree with this.  My cheap Orion 203mm F/5 ($249) out performs my premium 4" refractor on Jupiter as far as resolution goes.  The only thing I really notice with the refractor is it shows better color and contrast.  Jupiter features are darker and more colorful.  I think the cheaper newt mirror may attribute to this as the star test is okay but not great. Possibly some zones and/or TE?

For imaging though it seems to be okay or at least for the price I paid. I can see where if you were partial to color/contrast you may say the refractor is better than the newt. No doubt though that the newt out-resolves the refractor.

That's interesting.  The reflector should not be suppressing color on Jupiter.  Let's control the variables by using an 8" mirror that is 1/10 wave or better, having both scopes at the same magnification and using a 25% ND filter on the reflector to equalize the brightness.  Hold out for great seeing.  If the refractor still shows more color, my educated guess is residual chromatic aberration.  If not, I would praise the maker of the refractor for great color correction and attribute the previous view to an illusion of some sort.



#156 daquad

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Posted 20 October 2020 - 10:28 AM

Without any caveats, the 8” Newtonian with a high quality (Zambato-like) mirror will give the better views –period. So if all you are concerned about are the views, then the Newtonian is for you.

 

However, there are some other considerations…

 

1. The refractor, especially a high quality doublet like the Tak 100DZ, will acclimate much quicker and can be more stable as temperaturs fall during the night.

 

2. The refractor is more conducive to solar Ha observing, with the right filters of course.

 

3. The refractor on a GEM will be more conducive to imaging.

 

4. The refractor can also be used on an easy to setup alt/az mount and is the better grab-and-go telescope, especially considering acclimation.

 

5. The refractor is the better daytime spotter or daytime imaging scope or better for homes with a view

 

6. The refractor will not need collimation.

 

7. Depending on your location: the further north you are the worse the seeing will be and the colder the nights will be and the closer to the jet stream you will be and the lower in the sky the planets will be. These things can all conspire to compromise the 8” Newtonian’s performance more so than the 4” refractor.

 

Telescopes are more than just something you look through you have to: live with them and store them and set them up and take care of them and use them.

 

For the astronomer willing to deal with the ergonomics and care needed for the Newtonian and who lives in a location that will allow the Newtonian to perform to it fullest on most nights, the Newtonian will definitely deliver more and is the better tool for the job.

 

For the astronomer who wants a telescope that does more things and that takes less care to perform and who lives where conditions are more challenging, the refractor will be the more versatile and the more user-friendly and the more location-friendly tool.

 

Bob

Good post Bob.  My conclusion:  Best to get both.

 

Dom Q.



#157 Ihtegla Sar

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Posted 20 October 2020 - 02:36 PM

A good post but I do want to make a couple of comments:

 

-  An 8 inch Newtonian does not need to perform to it's fullest to provide superior performance to a 4 inch.  

 

These discussions always seem to get hung up discussing planetary viewing.  Planetary viewing is primarily about seeing.  But there is far more to amateur astronomy than viewing the planets and here in a few months, the planets will be gone.

 

Most objects are not as sensitive to seeing so the greater resolution and light gathering of the 8 inch can be put to good use even in less than optimal seeing.  DSOs, an 8 inch goes deeper, galaxies, globulars, planetary nebulae, open clusters, these are all better seen in the 8 inch.  

 

- I think that ideally one owns both a 3-4 inch refractor and a larger reflector of some sort.  Some 4 inch refractors make decent daytime spotting scopes but many are rather clumsy, I think an 80mm is a better choice. 

 

Jon

I think these discussions get hung up on planetary viewing because that is where problems with cooling and seeing really start making small refractors seem like a better choice than a larger reflector for people who live under the jet stream and/or in places where night time temperatures steadily drop most of the night.  

 

For deep sky objects, the benefits of larger aperture are obvious.  For planets the difference between a four and an eight inch aperture is somewhat less obvious, since planets are bright enough to be seen quite well in a four inch refractor.  Add in seeing and cooling problems due to location and the smaller refractor might give a better viewing experience than a larger reflector under the circumstances.  

 

For example, in the past week I have seen not less than four separate threads scattered across various subforums in CN where people with reflectors ranging in sizes from 5 inches to 12 inches were complaining that Mars just looked like a bright blurry disc of uniform color and they couldn't even make out the south polar ice cap, much less any albedo features.  Many of these threads talk of using neutral density or UHC filters to dim the image in an attempt to solve what sound to me like cooling and seeing issues that these owners of largish reflectors are experiencing.  

 

My 4 inch refractor has no such problems with seeing or cooling.  Any night that the clouds part, I can see the south polar cap quite clearly with no filter and a great many albedo features as well in my 4" refractor, regardless of how bad the seeing and local temperature fluctuations are (and they can be quite bad where I live).  Hence, my preference for small refractors to dobs for planetary.

 

I also prefer small refractors for double stars.  Sure, the Raleigh/Dawes limit is better for a larger aperture, but there are plenty of stars in the range of a four inch refractor to keep me busy for a long time, and double stars, like planets are more affected by seeing and cooling than deep sky objects.  There is also a whole thread about the aesthetics of small aperture being better for double stars.  Having compared epsilon Lyrae many times in my dobs to my small 4" refractor, I do prefer the aesthetics of it in my smaller scope.

 

I definitely agree that the answer to the question of a four inch refractor or an eight inch reflector is both because they both do different things better than the other.  When looking for Messier objects in my 20" I often happen across small galaxies that would be invisible in my four inch.  Except for field of view, there is no question a larger reflector beats a small refractor for deep sky.


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#158 Mitrovarr

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Posted 20 October 2020 - 03:24 PM

I wonder if coating loss is any factor here. My dobs have coatings that are starting to go.

#159 Bomber Bob

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Posted 20 October 2020 - 05:35 PM

I definitely agree that the answer to the question of a four inch refractor or an eight inch reflector is both because they both do different things better than the other.

 

And, by buying used, I have an even more specialized tool kit -- lots of scopes...

 

For multiple stars with one or more colorful members, I'll use my Tak 50mm F8 fluorite.  For casual Milky Way sweeping, it's tough to beat my Edmund 6" F4 Newt.  For an all-rounder that's light enough to tote around the yard, my 1971 RV-6 on a Mizar AR-1 + short tripod is a great choice.  My Meade 826 on a StarFinder EQ is less mobile, but it can pull in all sorts of DSOs, and its planetary views are comparable to the APM 152ED that I recently sold.  My weird 5" F5 non-ED triplet is a cross-over scope -- ultra-sharp wide fields at 15x, yet can zoom in at 150x as needed.   Or, my new AT102ED, which can show more planetary detail on nights when the weather / seeing limit my 6" / 8" reflectors...

 

But if I had to, I could cut my collection down to just my Meade 826 and my Dakin 4" F10 achromatic, and I wouldn't miss a thing that I want to see...


Edited by Bomber Bob, 20 October 2020 - 05:37 PM.

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#160 CHASLX200

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Posted 20 October 2020 - 06:12 PM

I definitely agree that the answer to the question of a four inch refractor or an eight inch reflector is both because they both do different things better than the other.

 

And, by buying used, I have an even more specialized tool kit -- lots of scopes...

 

For multiple stars with one or more colorful members, I'll use my Tak 50mm F8 fluorite.  For casual Milky Way sweeping, it's tough to beat my Edmund 6" F4 Newt.  For an all-rounder that's light enough to tote around the yard, my 1971 RV-6 on a Mizar AR-1 + short tripod is a great choice.  My Meade 826 on a StarFinder EQ is less mobile, but it can pull in all sorts of DSOs, and its planetary views are comparable to the APM 152ED that I recently sold.  My weird 5" F5 non-ED triplet is a cross-over scope -- ultra-sharp wide fields at 15x, yet can zoom in at 150x as needed.   Or, my new AT102ED, which can show more planetary detail on nights when the weather / seeing limit my 6" / 8" reflectors...

 

But if I had to, I could cut my collection down to just my Meade 826 and my Dakin 4" F10 achromatic, and I wouldn't miss a thing that I want to see...

My 826 that you have now was another killer 8" Newt.  I guess if i could have two scopes it would be a FS78 and a 8" slower Newt.  If i could have three then throw in a 16" to 20" F/6 Newt.


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#161 Thomas_M44

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Posted 21 October 2020 - 01:55 PM

Color rendition on planets:  I feel this is a HUGE positive in favor of Newtonian reflecting telescopes as compared to even very high-quality APO refractors.

 

I recently acquired a TV85 refractor, and I very much enjoy the lovely wide-field views it provides. This is a fantastic visual/aesthetic perspective which is not obtainable with reflecting telescopes. I'm hooked. I also like the presentation of larger nebulae. The moon looks pretty nice as well in the small 3-inch aperture, however....

 

I quite miss the vivid Newtonian color presentation.

 

The planets look markedly "bleached" and lacking in color as compared to my memories of the reflectors I have used.

 

Even the little inexpensive 3" 1970's era Japanese-made Newt I played with years ago, I recall offered  a more  colorfully engaging planetary view. Same goes with my recollection of my stepfather's ancient 6-inch ( f/9 or f/10 ?) Newt which was a $15 yard sale find.

 

Compared to the views I remember  from 1993 to 2006 with an 8-inch f/7 Coulter Dob, the lacking planetary color richness with the TV85 refractor (both with diagonal and also straight-through) is quite apparent.

 

I plan to keep the versatile and fun little TV85 refractor, primarily for its lovely rich-field capabilities below 50X, but... I *must* also get another f/7 or slower Newtonian of 130mm or more (likely another 200mm f/7) for planetary viewing and also to brighten dimmer objects.

 

 

In my *personal opinion*  between 75mm to 102mm and from f/6.5 to f/8  is a "sweet zone"  for APO refractors when used for visual observation.  You get the portability, versatility and rich-field capability, at a premium but relatively affordable price.  Above 102mm, however, the price-per-aperture curve gets very steep, the mounting costs rise precipitously, and one starts losing the rich-field capabilities as compared with the smaller aperture refractors in addition to having to compete for light grasp against 150mm or larger Newtonians which are *much* less expensive.


Edited by Thomas_M44, 21 October 2020 - 02:30 PM.

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#162 Bomber Bob

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Posted 21 October 2020 - 05:41 PM

Color rendition on planets:  I feel this is a HUGE positive in favor of Newtonian reflecting telescopes as compared to even very high-quality APO refractors.

 

Yep.  But not just Newtonians.  My Tinsley 6" F20 Cass presents Saturn with at least 3 colorful belts in 8/10 or better seeing.  But it's my (formerly CHAS's) Meade 8" F6 Newt that breaks out the fine details and the wonderful colors of Mars / Jupiter / Saturn.



#163 CHASLX200

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Posted 21 October 2020 - 06:07 PM

Color rendition on planets:  I feel this is a HUGE positive in favor of Newtonian reflecting telescopes as compared to even very high-quality APO refractors.

 

Yep.  But not just Newtonians.  My Tinsley 6" F20 Cass presents Saturn with at least 3 colorful belts in 8/10 or better seeing.  But it's my (formerly CHAS's) Meade 8" F6 Newt that breaks out the fine details and the wonderful colors of Mars / Jupiter / Saturn.

The mirror in that 826 was above avg i would say.


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#164 kfrederick

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Posted 22 October 2020 - 08:04 AM

You can have  a APO Dob Unobstructed reflector of any size for about the same cost as a newt . Get rid of the biggest defect in reflectors .Can show any clear night  a unobstructed 20 inch reflector  .Deep sky is inky black 



#165 Jon Isaacs

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Posted 22 October 2020 - 09:47 AM

You can have  a APO Dob Unobstructed reflector of any size for about the same cost as a newt . Get rid of the biggest defect in reflectors .Can show any clear night  a unobstructed 20 inch reflector  .Deep sky is inky black 

 

The central obstruction has nothing to do with the sky brightness.  If the sky is "inky black", either your eye is not dark adapted or the exit pupil is tiny, 

 

On earth, the sky is never black to the dark adapted eye.  

 

Jon


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#166 Jon Isaacs

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Posted 22 October 2020 - 09:58 AM

An 8 inch Newtonian does not need to perform to it's fullest to provide superior performance to a 4 inch.

 

True. But as I said, it will “compromise” the potential "full performance" of the larger scope. How much the larger scope will be compromised depends on seeing, etc.

 

These discussions always seem to get hung up discussing planetary viewing.  Planetary viewing is primarily about seeing.  But there is far more to amateur astronomy than viewing the planets and here in a few months, the planets will be gone.

 

Yes there are a lot more deep sky objects. BUT solar Ha observing and imaging is less condition dependent (light pollution, dark sky) and can add many hours (possibly doubling) the time spent using one’s telescope. And its fun and thrilling as well. The sun is the most dynamic object that one can observer. And, of course, the sun is far more dynamic and dramatic than the much more numerous but never-changing/static deep sky objects.  

 

Most (deep sky) objects are not as sensitive to seeing so the greater resolution and light gathering of the 8 inch can be put to good use even in less than optimal seeing.  DSOs, an 8 inch goes deeper, galaxies, globulars, planetary nebulae, open clusters, these are all better seen in the 8 inch. 

 

True, of course. And even more so if the Newtonian has a high quality mirror that delivers excellent contrast.

 

Some 4 inch refractors make decent daytime spotting scopes but many are rather clumsy, I think an 80mm is a better choice.

 

Maybe and maybe not, it depends on the scope. In any event, an 80mm was not in the comparison. So between the 8” Newtonian and the 4” refractor in the comparison, the refractor makes the much better daytime spotting and imaging scope.

 

Having both types of scopes is, of course, the answer, as one can take advantage of the strengths of each.

 

Bob

A few comments:

 

- I just wanted to make it clear that a "compromised" view in an 8 inch can still be easily better than what is seen in a perfect 4 inch.

 

- Again.. the planets will be gone soon enough.  Too much focus on the planetary side in these discussions.  As far Ha, that is an expensive proposition, for a 4 inch refractor, an Ha rig will likely cost more than the refractor.

 

-  A 4 inch refractor can be a much better spotting scope that an 8 inch Newtonian and still be a poor spotting scope.  A true spotting scope is easily transported.  This photo required a half mile hike each way, I don't think I would have carried a 4 inch F/9 down that particular path.

 

6051962-Canyon de Chelley Francis.jpg
 
I think the takeaway is that one is best served by good quality refractor and a good quality reflector.  Exactly what sizes those might be....
 
Jon

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#167 Jon Isaacs

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Posted 22 October 2020 - 10:24 AM

I think these discussions get hung up on planetary viewing because that is where problems with cooling and seeing really start making small refractors seem like a better choice than a larger reflector for people who live under the jet stream and/or in places where night time temperatures steadily drop most of the night.  

 

For deep sky objects, the benefits of larger aperture are obvious.  For planets the difference between a four and an eight inch aperture is somewhat less obvious, since planets are bright enough to be seen quite well in a four inch refractor.  Add in seeing and cooling problems due to location and the smaller refractor might give a better viewing experience than a larger reflector under the circumstances.  

 

For example, in the past week I have seen not less than four separate threads scattered across various subforums in CN where people with reflectors ranging in sizes from 5 inches to 12 inches were complaining that Mars just looked like a bright blurry disc of uniform color and they couldn't even make out the south polar ice cap, much less any albedo features.  Many of these threads talk of using neutral density or UHC filters to dim the image in an attempt to solve what sound to me like cooling and seeing issues that these owners of largish reflectors are experiencing.  

 

My 4 inch refractor has no such problems with seeing or cooling.  Any night that the clouds part, I can see the south polar cap quite clearly with no filter and a great many albedo features as well in my 4" refractor, regardless of how bad the seeing and local temperature fluctuations are (and they can be quite bad where I live).  Hence, my preference for small refractors to dobs for planetary.

 

I also prefer small refractors for double stars.  Sure, the Raleigh/Dawes limit is better for a larger aperture, but there are plenty of stars in the range of a four inch refractor to keep me busy for a long time, and double stars, like planets are more affected by seeing and cooling than deep sky objects.  There is also a whole thread about the aesthetics of small aperture being better for double stars.  Having compared epsilon Lyrae many times in my dobs to my small 4" refractor, I do prefer the aesthetics of it in my smaller scope.

 

I definitely agree that the answer to the question of a four inch refractor or an eight inch reflector is both because they both do different things better than the other.  When looking for Messier objects in my 20" I often happen across small galaxies that would be invisible in my four inch.  Except for field of view, there is no question a larger reflector beats a small refractor for deep sky.

 

Sure, a 4 inch refractor can approach an 8 inch when the seeing is crappy when viewing the planets.  And it's not just people with reflectors complaining about Mars..  In my experience, if a planet is crappy in my 10 inch, it's crappy in my 4 inch.  When it's amazing in a 10 inch, (or 8 inch) it won't be as amazing in a 4 inch.  

 

I should note that I suspect that many of those having difficulty with Mars were not skilled observers and were trying to observe Mars in the early evening when it was low on the horizon.  This is rarely productive.  The fact that newbies are having difficulty viewing the planets is not about the right telescope, it's about understanding planetary observing.  Sure, you get good views with your 4 inch because you know what you're doing but that's not what those threads were about.

 

In any event, my point is a little bigger than that. The planets are transitory, they come and go.  Pretty soon, there will be no planets to observe.  That's a game changer.

 

I spend a lot of time viewing doubles from my urban backyard.  Refractors are nice for the easy ones, the double double is doable in a 60mm. But challenging doubles are like challenging DSOs, is M13 OK as a blob or would I rather resolve it?  Is Lambda Cygni OK as a slightly elongated disk or would I rather resolve it clean and wide?

 

The focus of these discussions is generally about the one thing a 4 inch might do that's competitive with an 8 inch and in the process, the things an 8 inch does far better get lost in shuffle.  

 

That's my point, something to think about. 

 

If I had to decide between an 4 inch refractor and an an 8 inch Dob, which one would I choose??  

 

That would be a difficult choice, I would really want to have both. The refractor is handy, an easy scope to deal with.  But I would probably go with the 8 inch Dob simply because it has the capability of providing better views of most objects.  It may not deliver all the time but as an all around performer, it'll show me more.

 

When Clyde Crewey, a long time CN member was just getting started, he called up Televue.  He got Al Nagler on the line.  Clyde asked Al about the TV-85.  Al told Clyde that for anything that fit in the field of view, a decent 6 inch Dob would show more.  

 

Jon


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#168 Mitrovarr

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Posted 22 October 2020 - 10:29 AM

I wish there were more midrange options for newtonians and dobs available. In particular reflectors despertately need good active cooling but its hard to find a commercial option with anything but a fan pulling air across the mirror. Having a couple boundary layer fans plus a couple creating air movement inside the tube and maybe you could start to have decent planetary performance.
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#169 kfrederick

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Posted 22 October 2020 - 11:04 AM

The central obstruction has nothing to do with the sky brightness.  If the sky is "inky black", either your eye is not dark adapted or the exit pupil is tiny, 

 

On earth, the sky is never black to the dark adapted eye.  

 

Jon

Hi Jon  Where is the light scattered from the central obstruction and spikes end up  ?  It brightens the background . 


Edited by kfrederick, 22 October 2020 - 11:05 AM.


#170 Bomber Bob

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Posted 22 October 2020 - 11:43 AM

I wish there were more midrange options for newtonians and dobs available. In particular reflectors despertately need good active cooling but its hard to find a commercial option with anything but a fan pulling air across the mirror. Having a couple boundary layer fans plus a couple creating air movement inside the tube and maybe you could start to have decent planetary performance.

Bad as The Swamp is, I don't have to worry about dropping temperatures during a session.  I have the reverse problem most of the year, taking a scope from the air-conditioned indoors to the steaming hot outdoors.  And, all year, I like to set up the pedestal EQs on concrete decks / porches.  These soak up a lot of heat on sunny days, and I can feel it percolate up at night -- at about mid-mirror height on my Newts, which is why I'll mount my 6" F4 or 6" F8 Newts on the Mizar ShortPod EQ -- I set that up on the lawn.

 

On Topic:  IMO, a good quality 3" frac & 6" reflector are a good pair - especially to start out with.  4" frac & 8" Newt work best for me as I rapidly approach age 60, but in my 30s & 40s a 5" frac & 10" Newt would've been reasonable.  (I had a 5" frac back then, but I also had a poor opinion of reflectors.  Live & Learn!)


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#171 Jon Isaacs

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Posted 22 October 2020 - 12:05 PM

Hi Jon  Where is the light scattered from the central obstruction and spikes end up  ?  It brightens the background . 

 

A central obstruction transfers light from the the central "spurious disk" to the diffraction rings, this light remains within a few airy disk diameters of the object.

 

This is an easy experiment.  

 

Start with a refractor and a reasonably large exit pupil.

 

While observing the sky glow, place your hand in front of the objective. Or rig up a CO.

 

Does the sky get brighter or darker?

 

It doesn't get brighter.

 

Jon



#172 Ihtegla Sar

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Posted 22 October 2020 - 12:32 PM

Sure, a 4 inch refractor can approach an 8 inch when the seeing is crappy when viewing the planets.  And it's not just people with reflectors complaining about Mars..  In my experience, if a planet is crappy in my 10 inch, it's crappy in my 4 inch.  When it's amazing in a 10 inch, (or 8 inch) it won't be as amazing in a 4 inch.  

 

I should note that I suspect that many of those having difficulty with Mars were not skilled observers and were trying to observe Mars in the early evening when it was low on the horizon.  This is rarely productive.  The fact that newbies are having difficulty viewing the planets is not about the right telescope, it's about understanding planetary observing.  Sure, you get good views with your 4 inch because you know what you're doing but that's not what those threads were about.

 

In any event, my point is a little bigger than that. The planets are transitory, they come and go.  Pretty soon, there will be no planets to observe.  That's a game changer.

 

I spend a lot of time viewing doubles from my urban backyard.  Refractors are nice for the easy ones, the double double is doable in a 60mm. But challenging doubles are like challenging DSOs, is M13 OK as a blob or would I rather resolve it?  Is Lambda Cygni OK as a slightly elongated disk or would I rather resolve it clean and wide?

 

The focus of these discussions is generally about the one thing a 4 inch might do that's competitive with an 8 inch and in the process, the things an 8 inch does far better get lost in shuffle.  

 

That's my point, something to think about. 

 

If I had to decide between an 4 inch refractor and an an 8 inch Dob, which one would I choose??  

 

That would be a difficult choice, I would really want to have both. The refractor is handy, an easy scope to deal with.  But I would probably go with the 8 inch Dob simply because it has the capability of providing better views of most objects.  It may not deliver all the time but as an all around performer, it'll show me more.

 

When Clyde Crewey, a long time CN member was just getting started, he called up Televue.  He got Al Nagler on the line.  Clyde asked Al about the TV-85.  Al told Clyde that for anything that fit in the field of view, a decent 6 inch Dob would show more.  

 

Jon

On your first point about 4" apo vs larger reflector in crappy seeing, my experience is just a little different, although unlike you I live in an area under the jet stream when declining temperatures during the night are the norm.  Yes, a larger reflector can show more if seeing and cooling cooperate, so we agree on that but where we differ is in crappy seeing, which I have to contend with regularly.  In my experience if seeing is crappy in a 4" apo it is a whole lot crappier in a larger reflector.  

 

Some people in this thread have said that if seeing is crappy they will just come back another night but under the jet stream, the seeing is kind of crappy almost every night, and in the pacific northwest there are also a lot of clouds to contend with, so coming back another night is not really a good option.  For people who have to deal with crappy seeing on top of cooling issues on a regular basis a 4" refractor is a better choice for planetary since it will give consistently less crappy views than a larger reflector of SCT under those conditions.  

 

On your point that newbie planetary observers may have been looking at the planets when they are too low, I would respond by pointing out that there may also have been collimation issues and that a small refractor doesn't need collimation.  Since a refractor is easier to use in a variety of conditions and refractors "just work," I think a 4" apo refractor is a better beginner telescope than an 8" reflector or SCT.

 

I've compared my 4" apo to my 8" SCT side by side on Mars when Mars was above 45 degrees and the SCT had been cooling for over four hours.  The seeing was somewhat crappy but the 4" gave acceptable views and some good details could be seen during brief periods of stability.  The 8" gave views that were somewhat less acceptable than the 4" so after an hour of going back and forth between the two, I put the 8" in the garage and spent another couple hours with the 4".

 

Further, it is going to be years before Jupiter or Saturn higher in the sky for northern observers.  So looking at planets when they are low on the horizon isn't just a newbie issue.  Sure, I can wait until midnight for Mars to rise above 45 degrees (well, actually last night I couldn't because I started falling asleep at 10:00 and had to pack it in, but was still able to see the southern polar cap and Mare Sirenum and Cimmerium in my 4" when it was lower than optimal for viewing).  But I would have to wait years before Saturn or Jupiter climb that high.

 

 

Turning to your main point that pretty soon, there will be no more planets to observe, we probably have another couple months to observe Mars -- that's pretty much the only planet I am looking at these days, since Mars won't be this close again for another 15 years.  

 

But in a way I am looking forward to the planets being gone since I will then be able to turn my attention to double stars again.  I only started becoming interested in double stars last year but have found that I prefer the aesthetics of double stars in the refractor, both because of no diffraction spikes and also because of the aesthetics of viewing stars in smaller apertures as discussed in this thread.

 

Also, the same issues of seeing and cooling also affect larger reflectors more than smaller refractors when viewing double stars, just like viewing planets.

 

Sure there are double stars that cannot be fully resolved in a 4" that can be in an 8", but there are thousands of double stars that are within the range of being fully resolved in a 4".  For instance, last April, I tracked down STF 1830-1831 in my 4" and was able to resolve all 7 components -- it is a fantastic multiple star system.   I haven't tracked this particular double star down in any of my reflectors, but based on my experience viewing other double stars in my other scopes, I cannot imagine a double star like this would look any "better" in a larger reflector.  It is all about picking a target to match your scope, and there are hundreds if not thousands of double stars that are well matched to a 4" scope.


Edited by Ihtegla Sar, 22 October 2020 - 12:38 PM.

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#173 Mitrovarr

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Posted 22 October 2020 - 02:19 PM

Personally I think double stars look the best in a 4" refractor. In larger scopes, you will usually end up seeing the "seeing", so to speak, and not get nice clean textbook Airy disks. Even a larger refractor will not look as nice (although larger scopes in general will indisputably split closer or fainter doubles).
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#174 CHASLX200

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Posted 22 October 2020 - 06:18 PM

The central obstruction has nothing to do with the sky brightness.  If the sky is "inky black", either your eye is not dark adapted or the exit pupil is tiny, 

 

On earth, the sky is never black to the dark adapted eye.  

 

Jon

I sure don't get inky back grounds in my Mag 4.8 skies on a avg nite. But i will tell ya my 1960 4" M152 Unitron was my most inky scope in a dark sky, M13 had a look to it like no other scope.



#175 CHASLX200

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Posted 22 October 2020 - 06:22 PM

Bad as The Swamp is, I don't have to worry about dropping temperatures during a session.  I have the reverse problem most of the year, taking a scope from the air-conditioned indoors to the steaming hot outdoors.  And, all year, I like to set up the pedestal EQs on concrete decks / porches.  These soak up a lot of heat on sunny days, and I can feel it percolate up at night -- at about mid-mirror height on my Newts, which is why I'll mount my 6" F4 or 6" F8 Newts on the Mizar ShortPod EQ -- I set that up on the lawn.

 

On Topic:  IMO, a good quality 3" frac & 6" reflector are a good pair - especially to start out with.  4" frac & 8" Newt work best for me as I rapidly approach age 60, but in my 30s & 40s a 5" frac & 10" Newt would've been reasonable.  (I had a 5" frac back then, but I also had a poor opinion of reflectors.  Live & Learn!)

Same for me as my scopes need to heat up and not cool. Many nites are 87f at 10pm in the summer and the seeing is never that good in the summer in my area. While a super humid and warm nite in Feb has always given me my best dead still nites.  On nites with bigger temp drops i just sweep or do low power deep sky as planets never look good on colder nites with temp drops.  They are fuzzy at best.


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