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How does seeing and light pollution affect which telescope I should use?

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#1 entrpy53

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Posted 01 June 2020 - 12:25 AM

Hello, I am a young and new ametuer astronomer and I live in Singapore, a small island city. It is Bortle 8-9 here and the seeing is around 4-5/10 (sometimes 3) on the Pickering scale. Quite bad conditions. I heard that the seeing limits the useful aperture I can get for a telescope but then the bad light pollution makes me want to go a little bigger. Right now, I am considering an upgrade to a 127mm Skywatcher az5 Mak f/11.8 and the 130mm Skywatcher az5 Newtonian f/5 for visual, maybe a few snap photos using my phone. I currently own a Celestron astromaster 70az. I do not know if those two skywatchers above will give me much better views of the moon, planets, double stars, open star clusters than the Celestron astromaster 70az because of the seeing. But I also heard of something called a Mak to sct adapter and a focal reducer to make the Mak into a Low f/ratio scope to replace the Newtonian. So I have a few questions: Will I be able to make full use of the two skywatchers in my bad seeing and light pollution? And if so, should I get the Mak and Newtonian with one quality altaz mount or should I just consider the Mak and get a sct adapter and a focal reducer. Thanks!

                                                                                                                                    -Entrpy53


Edited by entrpy53, 01 June 2020 - 12:31 AM.


#2 SeattleScott

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Posted 01 June 2020 - 12:42 AM

I am in Bortle 6-7 skies and consider 4” aperture the minimum for deep space. So at least 5” for your skies.

The seeing is only really an issue for the Moon, planets and double stars. Not really a problem for DSO like open clusters. I doubt the seeing conditions would make either of those scopes worse on planets than the 70mm.

I have a Mak with SCT adapter and focal reducer. It does give a wider view, although it vignettes a little. However it can induce glare from internal reflections when looking at bright stuff so best to take it off for lunar/planetary. You would also need a 1.25” visual back or SCT diagonal.

Scott
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#3 philinbris

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Posted 01 June 2020 - 03:18 AM

Have you checked this guy out? You may even know of him.

https://www.cloudyni...-my-home-window

 

His website is n the link above - well worth checking out.

Cheers

Phil


Edited by philinbris, 01 June 2020 - 03:19 AM.


#4 MellonLake

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Posted 01 June 2020 - 06:48 AM

Entrpy;

  Bortle 8-9 is very hard to view in for sure (I am Bortle 8 at home) with poor seeing it is a tough mixture for any telescope aperture.  The added aperture in Bortle 8-9 will give you a little more on open clusters and open up more double stars but is not really going to help on other DSOs; nebula, galaxies, and globular clusters are not really going to be visible regardless of the aperture. The light pollution is simply going to wash these DSO objects out entirely or if there are couple you can find (Core of M31 and heart of M42) the aperture will only improve them very slightly.  Like Scott said, I think you will get a little more on planets with more aperture but it will be subtle, especially since your seeing will not likely let you go up much in magnification. 

 

Is there a star party you can attend and try larger apertures to see if they help much in you conditions? This may help with your decision.  

 

In my experience the large aperture of my 10" is much more beneficial at dark sky sites than at home in the heavy light pollution.  I realize that finding dark skies from Singapore is not easy frown.gif.  While not being great for astronomy, Singapore is a beautiful city, I loved my visit there.  



#5 Tony Flanders

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Posted 01 June 2020 - 06:49 AM

Hello, I am a young and new ametuer astronomer and I live in Singapore, a small island city. It is Bortle 8-9 here and the seeing is around 4-5/10 (sometimes 3) on the Pickering scale. Quite bad conditions. I heard that the seeing limits the useful aperture I can get for a telescope but then the bad light pollution makes me want to go a little bigger. Right now, I am considering an upgrade to a 127mm Skywatcher az5 Mak f/11.8 and the 130mm Skywatcher az5 Newtonian f/5 for visual, maybe a few snap photos using my phone. I currently own a Celestron astromaster 70az. I do not know if those two skywatchers above will give me much better views of the moon, planets, double stars, open star clusters than the Celestron astromaster 70az because of the seeing. But I also heard of something called a Mak to sct adapter and a focal reducer to make the Mak into a Low f/ratio scope to replace the Newtonian. So I have a few questions: Will I be able to make full use of the two skywatchers in my bad seeing and light pollution? And if so, should I get the Mak and Newtonian with one quality altaz mount or should I just consider the Mak and get a sct adapter and a focal reducer. Thanks!

                                                                                                                                    -Entrpy53

Singapore may have poor seeing right now, but I know that at some times of year the seeing is excellent. In general, equatorial regions tend to have poor transparency but good seeing. Moreover, the planets always transit high in the sky, making them easier to view than they are from the temperate zones. However, the light pollution in Singapore is going to severely limit what you can see in terms of nebulae and galaxies.

 

Regardless, a 130-mm telescope is vastly more capable than a 70-mm scope across the board, on every kind of target. Even in bad seeing, the larger aperture will reveal a lot more detail on the Moon and planets. However I don't see any point in buying two different 130-mm scopes; that would truly be a waste of money. If you care almost exclusively about the Moon and planets, pick the Mak. If you also want to view large star clusters like the Pleiades, pick the Newtonian. Both scopes should deliver very similar planetary views, but you will need to spend a bit more on eyepieces to achieve those views through the Newtonian.


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#6 entrpy53

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Posted 01 June 2020 - 10:00 AM

I am in Bortle 6-7 skies and consider 4” aperture the minimum for deep space. So at least 5” for your skies.

The seeing is only really an issue for the Moon, planets and double stars. Not really a problem for DSO like open clusters. I doubt the seeing conditions would make either of those scopes worse on planets than the 70mm.

I have a Mak with SCT adapter and focal reducer. It does give a wider view, although it vignettes a little. However it can induce glare from internal reflections when looking at bright stuff so best to take it off for lunar/planetary. You would also need a 1.25” visual back or SCT diagonal.

Scott

 

 

Have you checked this guy out? You may even know of him.

https://www.cloudyni...-my-home-window

 

His website is n the link above - well worth checking out.

Cheers

Phil

 

 

Entrpy;

  Bortle 8-9 is very hard to view in for sure (I am Bortle 8 at home) with poor seeing it is a tough mixture for any telescope aperture.  The added aperture in Bortle 8-9 will give you a little more on open clusters and open up more double stars but is not really going to help on other DSOs; nebula, galaxies, and globular clusters are not really going to be visible regardless of the aperture. The light pollution is simply going to wash these DSO objects out entirely or if there are couple you can find (Core of M31 and heart of M42) the aperture will only improve them very slightly.  Like Scott said, I think you will get a little more on planets with more aperture but it will be subtle, especially since your seeing will not likely let you go up much in magnification. 

 

Is there a star party you can attend and try larger apertures to see if they help much in you conditions? This may help with your decision.  

 

In my experience the large aperture of my 10" is much more beneficial at dark sky sites than at home in the heavy light pollution.  I realize that finding dark skies from Singapore is not easy frown.gif.  While not being great for astronomy, Singapore is a beautiful city, I loved my visit there.  

 

 

Singapore may have poor seeing right now, but I know that at some times of year the seeing is excellent. In general, equatorial regions tend to have poor transparency but good seeing. Moreover, the planets always transit high in the sky, making them easier to view than they are from the temperate zones. However, the light pollution in Singapore is going to severely limit what you can see in terms of nebulae and galaxies.

 

Regardless, a 130-mm telescope is vastly more capable than a 70-mm scope across the board, on every kind of target. Even in bad seeing, the larger aperture will reveal a lot more detail on the Moon and planets. However I don't see any point in buying two different 130-mm scopes; that would truly be a waste of money. If you care almost exclusively about the Moon and planets, pick the Mak. If you also want to view large star clusters like the Pleiades, pick the Newtonian. Both scopes should deliver very similar planetary views, but you will need to spend a bit more on eyepieces to achieve those views through the Newtonian.

Hello, thank you all for your replies. After thinking for awhile, I feel that the Sky-Watcher skymax 127 az5 is a better option to view, like Tony and MellonLake said planets and moon. However, I also want to see open star clusters like the Pleiades, Ptolemy cluster and want to cruise along the Milky Way just looking at patches of stars. Like Seattlescott said, he used something called a Mak to sct adapter and visual back with a focal reducer to lower the f/ratio...sorry but I don’t really know what those are...I hope you can explain if all those would be needed or I only need a focal reducer for the az5 skymax 127, it comes with a 1.25” 90 degrees prism which I intend to replace with a dielectric if I purchase it.
And yeah philin I saw that guy on my local news haha! he has great pictures!



#7 PalomarJack

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Posted 01 June 2020 - 10:19 AM

First my qualifications, I have been at this since my teen years in the 1970s. I have owned one store bought scope, the rest, and there is a lot, I built.

 

There are different kinds of "seeing", some you can do something about and others you can't. You will hear from people that will say that this or that kind of telescope is more immune. Forget it. I am not going to "re-invent the wheel", so just go here and come back when you are done reading.

 

https://skyandtelesc...ing-the-seeing/

 

Alright, now that the "seeing controversy" is cleared up, let's get down to it. I see you have a 70mm, a decent size for a beginner 'scope and later a "travel" 'scope. Consider other things more important that seeing to decide what to get. As a starting point, I would say the next size up from any given size is at least 4X wider aperture. For your 70mm that would be about 280mm. 130mm is just not going to be satisfactory. Yes, it is four times the light gathering power, but it is not even two magnitudes and it is only twice the resolution, it will barely resolve M13. I would say an 8" (203mm) Newtonian or 9" (230mm) SCT or Maksutov is the minimum. Now that you know each types advantages and disadvantages regarding local seeing from reading the above, you can concentrate on cost and portability. Yes, the bigger 'scope will show more, but if it is a 16" Dob mounted beast you are going to need to assemble and re-align every time you want to use it. Will it really show more than an 8 to 10 inch 'scope because of that? Your 70mm will get more use over the long run.

 

If money is the problem, download some plans from on line for a an 8" Dobsonian mounted reflector, buy a cheap saber/jigsaw and the optics and build your own. Trust me, it can be extremely rewarding. Stay with at least f/6. The eyepieces needed to perform well with say a 13" f/4 get very expensive, the same length. If you use a good tube for an 8 or 10 inch you can upgrade the mount to an equatorial made from pipe and bearings or a commercial one and you will have more to do than you thought possible. You will probably never exhaust the capabilities of 8 - 10 inches, especially after if you get into imaging and it is properly mounted for it. Many store bought 8 or 10 inch Dobsonian mounted reflectors qualify, too.

 

Finally, a larger aperture has to be used at higher and higher magnifications. You will hear about 30" 'scopes, but their minimum magnifications are about 160x. You will need an eyepiece with a 100 degree apparent field just to get a .63 degree true field. There is no such thing as rich field with something that big. But a 40mm eyepiece with 72 degree apparent field on an 8" f/6 for 2.5 degree true field is something to behold. I know you were not considering a huge 'scope, I am just pointing out the extremes and how someone can be disappointed with them if they do not know what they are getting into.

 

What do I use? My main 'scope is an 8" f/6 Newtonian on a clock driven equatorial mount. Except optics and the drive it is all home made. With a UHC or skyglow filter almost none of the NGC and many of the IC objects are not out of my reach. And there is a lot of them. Speaking of which, UHC, skyglow and especially OIII filters do not work well visually on much less than 6 to 8", you just won't have the reserve light power from anything less. So forget about 130mm with them.

 

I know that was a lot, I hope it was not redundant and confusing. Cheers...



#8 MellonLake

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Posted 01 June 2020 - 10:19 AM

The issue with Maksutov Cassegrain Telescopes (MCTs) like the Skymax 127 is that they have a relatively narrow field of view.  This means you may not be able to see the entirety of the Pleiades or Beehive Cluster for example.  This due to the both the long focal length (1500mm) and high focal ratio f/12 of the telescope and the MCT design. The SCT and focal reducer for the MCT are a way to try and get a wider field of view with the MCT (reduce the focal ratio).  I.e. make the telescope operate at something like 1000mm and f/8 (maybe Scott can estimate his focal length and focal ratio)  

 

The 130mm f/5 Newtonian reflector has a focal length of 650mm and focal ration of f/5 which is kind of the opposite of the MCT as it is a wider field instrument that will give great broad views of clusters and stars.   This telescope is great for broad field views and will perform adequately on planets especially if the seeing is not generally good.  



#9 Tony Flanders

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Posted 01 June 2020 - 10:35 AM

The 130mm f/5 Newtonian reflector has a focal length of 650mm and focal ration of f/5 which is kind of the opposite of the MCT as it is a wider field instrument that will give great broad views of clusters and stars.   This telescope is great for broad field views and will perform adequately on planets especially if the seeing is not generally good.


Given identical optical quality, a 130-mm f/5 Newtonian will actually deliver better planetary images than a 130-mm f/13 Mak-Cas due to the Newtonian's smaller secondary mirror. The only real advantage of the Mak-Cas is that the tube is a good deal shorter.
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#10 MellonLake

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Posted 01 June 2020 - 11:41 AM

Tony is correct.   I was concerned about difficultly with a really small exit pupil size on the Newtonian reflector limiting its magnification on nights of good viewing.  However, I redid the math.  Both telescopes have the same maximum usable magnification of about 240X. The MCT should allow for 240X with a  2mm exit pupil while the same magnification on the Newtonian the exit pupil is 0.5mm (the lower limit of exit pupil size).  

 

Sorry about that.

 

Rob  



#11 SeattleScott

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Posted 01 June 2020 - 12:59 PM

Personally I have scopes at 2” intervals, 4”, 6”, 8” and 10”. So you don’t need to quadruple aperture to get a meaningful improvement.

I have the iOptron 6” Mak and it came with a SCT thread adapter. Some new Maks have SCT threads. Just have to look at specs and see. Of course the 2” visual back that came with it had Mak threads so I had to get a 2” visual back, in addition to the Antares 0.63 focal reducer. This brings it down from F12 to about F7, yielding about 1.8-1.9 degrees FOV with my 24mm ES 82. The baffle tube restricts going any wider.

Assuming the Skywatcher has Mak threads, you would want https://agenaastro.c...-telescope.html

And This: https://agenaastro.c...-back-e-15.html

Along with a 0.63 reducer.

Scott

Edited by SeattleScott, 01 June 2020 - 01:41 PM.


#12 Andrekp

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Posted 01 June 2020 - 02:01 PM

You should get 4x the aperture for an upgrade!?

 

so someone with an 8 inch scope is just doing it wrong until the settle on at least 32 inches for their next upgrade?  That’s an absurd standard. 


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#13 zleonis

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Posted 01 June 2020 - 03:29 PM

Tony is correct. I was concerned about difficultly with a really small exit pupil size on the Newtonian reflector limiting its magnification on nights of good viewing. However, I redid the math. Both telescopes have the same maximum usable magnification of about 240X. The MCT should allow for 240X with a 2mm exit pupil while the same magnification on the Newtonian the exit pupil is 0.5mm (the lower limit of exit pupil size).

Sorry about that.

Rob


I think the scopes in question have essentially the same aperture, so both would have the same exit pupil when operating at equal magnifications.

#14 MellonLake

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Posted 01 June 2020 - 04:15 PM

Yep divided wrong again. 6mm eyepiece in the MCT at F12 is 0.5 not 2.

#15 Jon Isaacs

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Posted 02 June 2020 - 02:27 AM

Given identical optical quality, a 130-mm f/5 Newtonian will actually deliver better planetary images than a 130-mm f/13 Mak-Cas due to the Newtonian's smaller secondary mirror. The only real advantage of the Mak-Cas is that the tube is a good deal shorter.

 

One night I was going to do a comparison of my 120 mm apo/Ed refractor with a 127 mm Orion Starmax Mak Cassegrain and decided to add a 130 mm F/5 Newtonian to the mix. The targets were Saturn and some double stars, sets Herculis was one, splittable in refractor, not in the Mak.

 

The Newtonian got left out early on simple due to thermal issues.  I'm not a fan of Maks and am a big Newtonian guy but for planetary, I'd probably choose the Mak based on it's thermal behavior. I wasn't aware of it at the time, but recently it's come to light that insulating a closed tube scope like a Mak or SCT is very effective in managing the thermal effects of a warm scope so they're quite effective without waiting for the scope to cool.

 

Jon



#16 philinbris

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Posted 02 June 2020 - 03:06 AM

And yeah philin I saw that guy on my local news haha! he has great pictures!

I hope Singapore recovers form Covid quickly. I am itching to go back (been there 6 times).

I always buy a serious souvenir when I go and this time I know its going to be Astro related so I will be looking out for a good Astro shop or two.

Hope you succeed with the Mak - it will probably require some Astro Photo approach though.

Cheers, good luck and clear skys.

Phil




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