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Telescope size, magnification, and seeing conditions?

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#1 Dave1066

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Posted 26 March 2020 - 12:19 PM

So guys I need some help?

 

I'm opening what could be a can of worms here, but I'm trying to get to the bottom of this, so that I can make my next astronomy equipment investment decision wisely.

 

I'm thinking of getting an 8" Newtonian telescope with 1/10 PTV mirrors. But I'm worried about 8" telescope not being supported by UK seeing conditions and having a bright image with little detail? Which is why I am tempted to keep my Skylight 4" F15. 

 

Am I missing the point? Is it more a case of the chosen magnification being supported by the seeing conditions regardless of aperture?

 

For instance if the seeing conditions will only support 150x, will it matter if I'm observing with a 80mm at 150x vs 8" at 150x, will the amount of detail appear the same in both? Or will the bigger aperture be bright washed out, less sharp, and less details? Because of the bigger aperture not being supported by the seeing conditions? ( smaller air cell size than 8" )

 

 

David



#2 sg6

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Posted 26 March 2020 - 12:40 PM

Am I missing the point? Is it more a case of the chosen magnification being supported by the seeing conditions regardless of aperture?

 

For instance if the seeing conditions will only support 150x, will it matter if I'm observing with a 80mm at 150x vs 8" at 150x, will the amount of detail appear the same in both? Or will the bigger aperture be bright washed out, less sharp, and less details? Because of the bigger aperture not being supported by the seeing conditions? ( smaller air cell size than 8" )

First one it will be both, how the split goes is the big problem.

 

Second, the 8" should out perform the 80mm.

 

Now I will be one of the first to say an 8" Zambuto is likely to out perform a 12" Skywatcher, certainly a 10" Zambuto would. Some will come down to the quality of the mirror.

 

Quality costs and one thing I see often is astronomers do not want to spend money on something like "quality" that in general cannot be seen. A 12" mirror can, all it takes is a tape measure.

 

Unless you spend a lot of money as a mirror gets bigger the edges get worse. Mirrors start out spherical, then get parabolized. The center is the bit that gets parabolized, so the edges are very much spherical, and there is a lot of edge on a big mirror.

 

Also when you think it means most parabolic mirrors are not parabolic. Getting the whole mirror right takes time, and work. The result being higher cost. It is the classic:

You can choose any 2 of 3 - Size, Quality, Cost - the third is out of your control and a function of the chosen 2.

 

Is it a standard Skywatcher mirror or something like an OOUK one?


Edited by sg6, 26 March 2020 - 12:48 PM.

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#3 Dave1066

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Posted 26 March 2020 - 04:54 PM

If I go down the Newtonian route, it will be OO UK 1/10 PTV mirror. And the OO UK will either be a 6" F11 or 8" F8. With a 18% CO. 



#4 maxiecat2016

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Posted 26 March 2020 - 06:27 PM

I live in the Southern United States near the the North Georgia Mountains so good seeing is a problem,compared a 6 inch newt at f/8 to a 8 inch newt at f/6 so I could use the same  eyepieces to have the same power,so over a period of time the 8 inch newt would do better than the 6 inch when the seeing was good, when the seeing not so good the 6 inch was better,if the seeing was horrible pack it up!!!!!! Buy a good 8 inch telescope and make some mirror masks for 6 inch and 4 inch your f/l will change from the 6 inch masks  to the 4 inch. Mike



#5 Tony Flanders

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Posted 26 March 2020 - 07:53 PM

It's a bad night indeed when the seeing brings an 8-inch telescope down to the level of a 4-incher.

 

It's not just a matter of magnification, mind you. Although you can use both a 4-inch and an 8-inch scope at 150X, the 8-incher will show details much more clearly at that magnification.


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#6 Mike Spooner

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Posted 26 March 2020 - 08:37 PM

You basically need to break the decision in two parts. The first is seeing conditions. It's true there are places the stability is abysmal - factors you can't control would be like the jet stream. Terrain is another but possibly that can be overcome by a bit of mobility. No doubt you have seen lots of agonizing on CN about scopes that fit in autos for transport. Timing, local heat sinks may or may not be problems. Here in the American SW we can have spectacular seeing for 15 - 30 minutes at sunset before convection cooling becomes a hindrance. I rarely observe if the jet stream has dropped down this far. East side of mountains are bad here as prevailing winds cause turbulent air. Anyway that's the idea I'm presenting of seeing. If your 4" shows stable high power images then you likely have solid seeing at least some of the time.

The second factor has been dealt with in above posts concerning quality. We all probably wonder about the quality of our scopes. Removing that concern is a quest in itself and not the easiest thing to determine. But getting top notch optics removes one limitation that will not get better if conditions otherwise improve. It's almost impossible to convey what a truly great 8" scope will show when conditions allow - I've had some decent glass in the past 30 or so years and the result is stunning when it all comes together. I've done and still do a lot of observing with less than perfect scopes but the closer the optics get to design, the less I notice them and the more I concentrate on viewing.

Mike Spooner

Edited by Mike Spooner, 26 March 2020 - 08:38 PM.

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#7 Dave1066

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Posted 27 March 2020 - 08:26 AM

Thanks for the replies guys.

Mike, that confirms the age old theory on apatite and seeing. I had forgotten about the mask option.

Tony, you've just confirmed that that a 4" and 8" will only see the same detail in poor seeing conditions, just the 8" makes it easier to see.

Mr Spooner, I live in the UK a little island stuck in the middle of the ocean, and generally under the jet stream most of the time. Which is why I agonise over aperture decisions. The only real difference mobility makes is to darker skies. My position is in one of the best locations for planets as it is, but can be better, it is possible to go further south.

Agreed on quality of optics being important. Hence why I will choose OO UK 1/10 PTV mirrors with 97% reflectivity. Usually when you see comparisons between refractors Vs reflectors, they usually put a premium refractor against a mass produced reflector. Unfair test really.

David

#8 Jon Isaacs

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Posted 27 March 2020 - 08:34 AM

Tony, you've just confirmed that that a 4" and 8" will only see the same detail in poor seeing conditions, just the 8" makes it easier to see.

 

 

It may well be better resolved.  The thing of it is that if an 8 inch is showing no more than a 4 inch, it's not a night for viewing the planets because neither will be at all good.  

 

The advantage the 4 inch has is that it cools much more quickly so you don't have to worry about the scope cooling the way you do with a Newtonian. 

 

Jon



#9 jaraxx

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Posted 27 March 2020 - 08:39 AM

Get the eight inch. Making an 8 inch scope into a six or four inch scope is easy and free, so you've got it covered no matter the seeing. I don't think that will happen very often (if ever).

Making a six inch scope into a eight inch scope requires money and effort.



#10 Dave1066

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Posted 27 March 2020 - 11:17 AM

It may well be better resolved. The thing of it is that if an 8 inch is showing no more than a 4 inch, it's not a night for viewing the planets because neither will be at all good.

The advantage the 4 inch has is that it cools much more quickly so you don't have to worry about the scope cooling the way you do with a Newtonian.

Jon


Thing is Jon, that would mean hardly ever viewing the planets in the UK. 😄

OO UK telescopes come with a fan, so atleast they will cool and keep tube currents at bay.

David

#11 charlesgeiger

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Posted 27 March 2020 - 05:03 PM

If you have an 8" you will have 4 times the light gathering power of the 4".  That really only means that you will have better contrast (with decent optics in both sizes).  So although you may not have better planetary images through the 8" (which is only the worst conditions) you will have so much more light grasp in the 8" that all other observing will be noticeably much better.  You will see fainter galaxies, see more detail in galaxies that can be seen in both scopes, and everything deep sky will be much better.  Again, this is taking into account the rough atmospheric conditions you describe.  Now you must know that an 4" f/6 will have a larger visual field than an 8" f/6 so that the largest clusters...using the same eyepieces (M45, M44, double cluster) will see a larger cluster per unit area of field.  But all those large clusters will be readily seen in the field of an 8" f/6 using an eyepiece with an apparent field of view of 70 degrees and with a focal length of 30 mm or higher.  Anyhow, an 8" will give you so much more than a 4" that you will greatly appreciate the difference immediately.  If you look at the magnitude depth between the two you will see that the 8" will give you nearly two magnitudes fainter which is huge.  

Also, without an double pass interferometer test report, any quotes of optical quality really are without merit.  As long as the optics are diffraction limited (which is about 1/4 wave surface accuracy) which most Chinese optics meet, you will have good views.  With no disrespect intended regarding OO UK optics, at least some of the earlier units had poor optics as the contracted opticians were not doing the best work.  You can look up test reports by Rohr.  I see you are thinking about a longer focal length Newtonian but you should still have a reasonably large FOV if you choose the right wider field eyepiece.

Anyhow, best of luck in choosing your optics.

Charlie 



#12 bobhen

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Posted 27 March 2020 - 05:31 PM

Just something to consider...

 

“How often do nights of excellent seeing occur?

 

At the William Herschel Telescope site in the Canary Islands, even this superb viewing location  (second best in the northern Hemisphere) has many nights of relatively poor seeing: the distribution is positively skewed, and at this excellent site, a 10 inch telescope will be seeing limited on 9 out of 10 nights.”

If a 10 inch scope is seeing limited 9 out of 10 nights at La Palma, which has some of the best seeing in the world, what are the odds in your backyard?  2 nights out of 100?  0 nights out of 100? Those are the real odds for a lot of backyards.

 

The question for the planetary observer is not that a larger scope doesn’t have the “potential on paper” to show more, the question is: do you want to set up a telescope that will be seeing limited on many nights without delivering any more planetary detail than you would see in a smaller scope that is easier to manage, cools quicker and delivers “what can be seen” sooner after being setup on moments notice?

 

Even with the above. I think a very high quality 8" F8 is your best bet. It's just that you might be surprised how close your 4" F15 comes on many nights, especially considering thermals and acclimation and low planet position.

Bob


Edited by bobhen, 28 March 2020 - 06:35 AM.


#13 Keith Rivich

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Posted 27 March 2020 - 09:20 PM

All things being equal it is very rare for a smaller scope to out perform a larger scope. IMO so rare as not to be a factor when deciding what scope to get. The exception to that would be if you are strictly observing the planets or close double stars. Then a case could be made to match your scope with your average seeing conditions. 

 

When the seeing here gets real bad I just switch targets...galaxies and nebulae are rarely affected by bad seeing. 



#14 astro744

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Posted 28 March 2020 - 01:05 AM

If I had a Skylight 4” f15, I would keep it regardless of what else I wanted to buy.  It is an heirloom telescope that will appreciate in value and be appreciated by those whom you pass it down to.

 

 


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#15 Dave1066

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Posted 28 March 2020 - 09:52 AM

Just something to consider...

“How often do nights of excellent seeing occur?

At the William Herschel Telescope site in the Canary Islands, even this superb viewing location (second best in the northern Hemisphere) has many nights of relatively poor seeing: the distribution is positively skewed, and at this excellent site, a 10 inch telescope will be seeing limited on 9 out of 10 nights.”
If a 10 inch scope is seeing limited 9 out of 10 nights at La Palma, which has some of the best seeing in the world, what are the odds in your backyard? 2 nights out of 100? 0 nights out of 100? Those are the real odds for a lot of backyards.

The question for the planetary observer is not that a larger scope doesn’t have the “potential on paper” to show more, the question is: do you want to set up a telescope that will be seeing limited on many nights without delivering any more planetary detail than you would see in a smaller scope that is easier to manage, cools quicker and delivers “what can be seen” sooner after being setup on moments notice?

Even with the above. I think a very high quality 8" F8 is your best bet. It's just that you might be surprised how close your 4" F15 comes on many nights, especially considering thermals and acclimation and low planet position.
Bob


Hi Bob,

Yes that's how I generally view it, as you've written. That's why I'm airing on the side of caution. Before diving in to buying an 8". Or even 6". And why I am considering every parameter. That's why I'm considering even getting a better mount for my Skylight 4" F15 which is the sample the astronomy press reviewed all those years ago. The experts that works for those magazines put the optics in my 4" at 1/8 wave optics.

Thinking to get a Skywatcher AZ EQ 6 mount.

Dave

#16 Dave1066

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Posted 28 March 2020 - 09:56 AM

If I had a Skylight 4” f15, I would keep it regardless of what else I wanted to buy. It is an heirloom telescope that will appreciate in value and be appreciated by those whom you pass it down to.


Thanks Keith, yes it is collectable. My Skylight 4" is the prototype the astronomy press reviewed. So somewhat even more special. I have the review which I keep with the telescope. And the wooden box Richard built for the telescope is signed by him.

David

#17 Tony Flanders

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Posted 28 March 2020 - 11:38 AM

Mr Spooner, I live in the UK a little island stuck in the middle of the ocean, and generally under the jet stream most of the time. Which is why I agonise over aperture decisions.


William Herschel did pretty well observing with an 18-inch scope from southwest England. It's not the best location in the world, but it's very far from the worst. In general, being on a west coast in the temperate zones is a good thing. Onshore breezes tend to bring good seeing, because air over water is thermally uniform.


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