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5" Celestron NexStar is better than 8" ?

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#1 JP-Astro

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Posted 17 May 2019 - 11:25 AM

A wise man reviews his Celestron NexStar 5"SE and derives a conclusion that it's much better than the same 8" version.

Where did he go wrong?

Here's his review on YT:

https://www.youtube....h?v=f7uURzPRiuI

 

While I agree with him on some points like portability... ehhh (???)... I don't buy his idea that the views of planets will be the same through 5" and 8". Won't 8" provide more detail at the same magnification and won't it provide brighter images for visual observations than 5" ? Won't 8" allow to use higher magnifications on a rare occasion when the sky is clear and steady and isn't it worth it?



#2 SeattleScott

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Posted 17 May 2019 - 11:31 AM

Correct, assuming similar optical quality, collimation, and cooling. Given they are both Celestron it seems reasonable to assume quality would be about the same. Certainly if one compared a poorly collimated C8 with a precisely collimated C5, the 5” would show more detail. That’s why you don’t want to review on single data points when making purchase decisions. Or if you do it is because the reviewer really knows his stuff and controls for variables.

Scott
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#3 ngc7319_20

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Posted 17 May 2019 - 02:26 PM

 

While I agree with him on some points like portability... ehhh (???)... I don't buy his idea that the views of planets will be the same through 5" and 8". Won't 8" provide more detail at the same magnification and won't it provide brighter images for visual observations than 5" ? Won't 8" allow to use higher magnifications on a rare occasion when the sky is clear and steady and isn't it worth it?

Hes mostly discussing portability and cost, at least in the first couple minutes I could stand to watch.  And to some extent it is click-bait (working eh?).

 

Yes under good seeing and appropriate cool-down, the 8" will show much more. 

 

But he may have a point -- for someone that wants to put the scope out and be viewing 2 minutes later, even in poor conditions, the 5" might be preferred. 



#4 Stephen Kennedy

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Posted 17 May 2019 - 10:11 PM

The C8 has 8/5 = 1.6 times the resolving power and (8/5)^2 = 2.56 times the light gathering capacity of the C5.  Unless the only things you care about are portability and cost it is not even close. 



#5 junomike

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Posted 18 May 2019 - 06:54 AM

I agree with Stephen, unless there's something terribly wrong with the 8", it will show you things the 5" can only dream about.


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#6 bogg

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Posted 18 May 2019 - 07:15 AM

I agree that to compare on just one data point is not a fair comparison. It brings to mind at a star party I was at. I had brought a C6 and was looking at the veil Nebula. I compared the view in the C6 to that in a neighbours 10 inch Newtonian. The viewing that night did not favour probably anything above a 8 inch scope. As a result if I only took that data point I would conclude that the C6 was just as good as the 10 inch. This can happen but I know that under good skies that the 10 is far superior. I have read in a article  that if you live close to sea level  that due to varying conditions that a scope under 8 inch will give just as good a view as the larger scopes most of the time. But of course  as they were indicating the few days a year where a larger scope would shine and opportunity to use, it might be better to buy the smaller scope rather than spend more for little gain in the area they were talking about. .  



#7 Stargazer3236

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Posted 21 May 2019 - 11:01 AM

You all have to realize that increasing aperture also increases the atmospheric distortions unless you have adaptive optics. A 5" will show better detail when the seeing is bad. An 8" will show good detail too, but with more aperture, the details magnified will degrade more than the 5" , as will an 11" and so on and so forth.


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#8 Jon Isaacs

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Posted 21 May 2019 - 11:39 AM

You all have to realize that increasing aperture also increases the atmospheric distortions unless you have adaptive optics. A 5" will show better detail when the seeing is bad. An 8" will show good detail too, but with more aperture, the details magnified will degrade more than the 5" , as will an 11" and so on and so forth.

 

That has not been my experience.  If the larger scope is cooled and properly collimated, I see more in my larger scopes...  The seeing related aberrations maybe be more apparent in the larger scope but so it are the details of the planets.

 

I often setup a 4 inch apo alongside a larger Dob in the high desert where the seeing is generally average or below average. If the views of the planets are poor in the Dob, they are never any better in the refractor. 

 

Jon


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#9 Don W

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Posted 21 May 2019 - 11:42 AM

If it's on YouTube, it must be true!!


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#10 CharlesC

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Posted 21 May 2019 - 12:06 PM

On that video he didn't do well with that judgement:

700 thumbs up

800 thumbs down

 

With eyes alone, a C8 is generally the smallest recommended aperture for DSOs.

However, when used with an EAA camera the C5 can see a large majority of DSOs in reasonable detail.

 

Visually, the M13 globular cluster will look like a fuzzy unresolved ball in C5, but you can resolve stars with C8.

With a low resolution EAA camera on a C5 (rig in my sig) and in seconds you'll see this.  A C5 can be adequate, if you use a camera.

 

M13noFR_Stack_241_3.jpg

 

 



#11 Kon Dealer

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Posted 21 May 2019 - 12:17 PM

I have a C6 and. C8. The former gets a lot more use. It is a rare occasion when the C8 can demonstrate its superiority.

if I had to part with one it would be the C8.



#12 RogeZ

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Posted 21 May 2019 - 08:56 PM

All things equal the bigger scope will always show more. There is no magic to smaller scopes.

#13 gezak22

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Posted 21 May 2019 - 10:16 PM

Perhaps it's a variation of this argument?



#14 dakinemaui

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Posted 22 May 2019 - 05:42 PM

The C8 has 8/5 = 1.6 times the resolving power and (8/5)^2 = 2.56 times the light gathering capacity of the C5.  Unless the only things you care about are portability and cost it is not even close. 

Unfortunately, the seeing will not typically allow that full 60% better resolution. The eye's response is also quite nonlinear, so while the light grasp is indeed 2.6x, it will only appear to be about 37% brighter when looking through the eyepiece (at the same magnification).

 

So yes, there are advantages to be had from the larger scope, but it's often easy to overstate the real-world impact of those advantages.


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#15 RogeZ

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Posted 22 May 2019 - 06:52 PM

The issue that I have with this type of argument is that it almost reduces extra aperture to a waste of money and effort while the reality is that increase aperture will always show more. If the seeing is so bad, turn the scope to a DSO where the seeing wont impact the view as much and rock on. Now, when the skies are steady the extra aperture will make for unforgettable views imposible in smaller scopes.
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#16 Jond105

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Posted 22 May 2019 - 07:06 PM

He mentioned at the end, you would have to buy a case for the telescope at $300??? 


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#17 Matt78

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Posted 28 May 2019 - 03:02 PM

On that video he didn't do well with that judgement:

700 thumbs up

800 thumbs down

 

With eyes alone, a C8 is generally the smallest recommended aperture for DSOs.

However, when used with an EAA camera the C5 can see a large majority of DSOs in reasonable detail.

 

Visually, the M13 globular cluster will look like a fuzzy unresolved ball in C5, but you can resolve stars with C8.

With a low resolution EAA camera on a C5 (rig in my sig) and in seconds you'll see this.  A C5 can be adequate, if you use a camera.

 

attachicon.gif M13noFR_Stack_241_3.jpg

I've found I can resolve M13 and M3 with a C5 in fairly dark skies if I give myself time to dark adapt, and pump the magnification (15 mm minimum, usually more like my 9.7 mm). Dark skies probably help me, though. Not saying I wouldn't rather look at it through a C8, but I definitely get more than a gray blob out of those two globs if I'm patient and look after my night vision. 



#18 Procyon

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Posted 28 May 2019 - 10:00 PM

A wise man reviews his Celestron NexStar 5"SE and derives a conclusion that it's much better than the same 8" version.

Where did he go wrong?

Here's his review on YT:

https://www.youtube....h?v=f7uURzPRiuI

 

While I agree with him on some points like portability... ehhh (???)... I don't buy his idea that the views of planets will be the same through 5" and 8". Won't 8" provide more detail at the same magnification and won't it provide brighter images for visual observations than 5" ? Won't 8" allow to use higher magnifications on a rare occasion when the sky is clear and steady and isn't it worth it?

It depends also, if I'm say 70-90 and live in the deep country side, having owned a 6SE, I think the 5SE can possibly be picked up with one hand. Depends on age, circumstances, etc.

 

From a nice dark location one can see a lot with a 5".

 

Now if I click on the video and he's talking about it being a Mak design, I may laugh. 


Edited by Procyon, 28 May 2019 - 10:33 PM.


#19 BravoFoxtrot

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Posted 28 May 2019 - 10:18 PM

My 8” Edge on an Evo mount is a one trip outside affair.  I’m not sure you’re gaining much in portability over a C5.  All things being equal, the 8” is pretty decent step up in performance, IME.



#20 Jon Isaacs

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Posted 30 June 2019 - 07:23 AM

I've found I can resolve M13 and M3 with a C5 in fairly dark skies if I give myself time to dark adapt, and pump the magnification (15 mm minimum, usually more like my 9.7 mm). Dark skies probably help me, though. Not saying I wouldn't rather look at it through a C8, but I definitely get more than a gray blob out of those two globs if I'm patient and look after my night vision. 

 

:waytogo:

 

Being best obsever you can be with a smaller scope is the best way to be the best observer you can be with a larger scope. It's easy to be lazy with a larger scope, smaller scopes keep you honest.

 

M13 and M3 are two big bright globular.  By my standards, under dark skies a 5 inch will resolve some stars in M13 and M3, a C-8 will resolve more.

 

In my experience, good seeing or poor seeing aperture is always a help with globulars. A larger scope will show more globulars, will resolve more globulars and will resolve more stars in a globulars. Even in very large scopes, most globulars are poorly resolved.  What a large scope does is increase the number of M-13-like globulars visible.

 

Unfortunately, the seeing will not typically allow that full 60% better resolution. The eye's response is also quite nonlinear, so while the light grasp is indeed 2.6x, it will only appear to be about 37% brighter when looking through the eyepiece (at the same magnification).

 

So yes, there are advantages to be had from the larger scope, but it's often easy to overstate the real-world impact of those advantages.

 

I think it's easy to understate the advantages of larger apertures. Make your comparisons at equal exit pupils and the numbers are very different..   with deep sky seeing is much less important.

 

Look at M13 in the C5 at 125x, the equivalent exit pupil in a C-8 is 200x. 

 

Once I was selling a fairly large scope. The prospective buyer brought along a scope for comparison. The scope I was selling was about 2.3x larger in aperture. The buyer asked if I minded if he set up his scope.. I said no but that one look through the larger scope would be enough for him to realize a comparison was unnecessary.

 

Indeed, that turned out to be the case. He commented later that M-79 in Lepus in the bigger scope looked like M13 in his scope... 

 

Jon


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#21 Eddgie

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Posted 30 June 2019 - 08:02 AM

Owned several of both.  

 

On a night of bad seeing, I would not look at planets with either one of them.  What is the point?

 

On the average night, it might be true that the C8 is not working at its fully potential most of the session, but there are almost always brief periods where seeing improves, and in either scope, these brief moments is where the thrill comes in.

 

I can see a couple of belts on Jupiter using a cheap refractor, but even on night of perfect seeing, is all I can see is a couple of belts on Jupiter. 

 

We don't buy bigger and bigger telescopes for planetary observing on nights of poor seeing.  We use them for the thrill we get on nights of good seeing.  

 

Anyway, on nights where I though conditions were decent enough for planetary viewing, the C8 always did better than the C5.  Not that the C5 was bad at all, it just lacked the contrast transfer of the C8, and the bigger exit pupil meant that the view in the C8 was always more colorful.


Edited by Eddgie, 30 June 2019 - 08:23 AM.


#22 whizbang

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Posted 30 June 2019 - 12:04 PM

I owed both and compared them side-by-side.  No contest.  Aperture wins.



#23 Ed D

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Posted 30 June 2019 - 05:28 PM

popcorn.gif

 

Ed D


Edited by Ed D, 30 June 2019 - 05:36 PM.


#24 Peter Besenbruch

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Posted 30 June 2019 - 07:18 PM

I'm going to argue for the Nexstar 5 over the 8, but not for optical reasons. The mount will handle the 5" better. For the C8, I would recommend the Evo, if you want a single arm mount.


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#25 Traveler

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Posted 30 June 2019 - 11:11 PM

Funny, basicly we are speaking here about mounts and not about the telescopes although the threadtitle suggests otherwise,...just saying.

 

There is no way a 5" SCT performs better than a 8" SCT as Eddgie explains as well.




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