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Maximum magnification reachable with 6 inch telescope

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#1 Sorana

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Posted 09 July 2020 - 02:39 AM

Hello everyone!

 

I have a question... What is the highest magnification you can go on a usual basis with a 6 inch telescope? I know that they are marketed as having useful maximum magnification 300x but due to atmospheric turbulence etc. you can't go that high.

 

I'm planing on buying a 6 inch f/8 telescope and an eyepiece to complement the 25mm SPL eyepiece that already comes with the telescope and I want an eyepiece that I can use most of the time for higher magnification. 

 

Thanks in advance!

Clear skies!



#2 Freezout

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Posted 09 July 2020 - 05:06 AM

300x is useless unless exceptional circumstances. You will just see a big blurry stuff. If you live in a place with a perfect seeing, it might be worth it, but for now be aware that your 25mm eyepiece will already provide you a magnification of 48x. It's a lot, but not too much, perfect for many DSOs.

Maybe try to go for x100 or little bit above (nice for planet, moon, globular clusters), it's a good start (eyepiece 9-12mm), and if you see that your sky is allowing more, buy smaller FL.

 

Personally in my bad seeing I hardly use x200.


Edited by Freezout, 09 July 2020 - 05:06 AM.

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#3 Tony Flanders

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Posted 09 July 2020 - 07:20 AM

Realistically, expect to do the lion's share of your observing at 150X or lower. It's hard to say exactly what that means in terms of buying eyepieces. Two different magnifications is better than one, but to even come close to exploiting your telescope's full capabilities, you need at least three eyepieces, or two eyepieces plus a Barlow lens. So you might want to plan your purchases with that in mind.


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#4 sixela

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Posted 09 July 2020 - 07:37 AM

300x is useful on some targets but not all.

 

First of all, for bright objects (e.g. planets) the seeing indeed does not always support it. Secondly, targets like Jupiter may not have a surface brightness that is high enough to warrant such magnification in a 150mm scope.

 

For less bright objects (seen with night vision but comparately bright for such objects, like e.g. bright planetary nebulae) 300x can also be useful.

 

If you have one eyepiece to pick for higher magnifications on an f/8 scope, it's, it's a 6mm. It's close to the optimum, I find, for Jupiter on an f/8 scope (magnifying more dims Jupiter so much that you start seeing less rather than more, even with good seeing), and on many nights it's also what you'd use on Mars and Saturn. And it's not so small to make it completely ridiculous to barlow it (with a 2x barlow) for Saturn and Mars, which are relatively bright as far as surface brightness is concerned.

 

On an f/8 scope, a 25mm will work well for nebulae when using OIII or UHC filters, but you need a 14-16mm  for deep sky objects like moderately sized galaxies too.


Edited by sixela, 09 July 2020 - 12:24 PM.

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#5 Star Geezer

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Posted 09 July 2020 - 07:56 AM

A telescopes theoretically maximum magnification is that magnification that yields a 0.5mm exit pupil. For a scope with a 150mm aperture that equates to 300x, twice the aperture in millimeters. For Planetary viewing on average I can go 75% of TMaxM. For DSO's you will want the exit pupil no lower than 1.0mm max.


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#6 Echolight

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Posted 09 July 2020 - 09:24 AM

I can see a lot on the moon at high mags in my 6 inch f8. So far always less than 300x gives the cleanest view. Everything else a little lower so far.

 

But I have heard from other members here who have gone well over 300x with a 6 inch f8 on the best nights.


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#7 Andrekp

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Posted 09 July 2020 - 09:42 AM

If you don’t actually HAVE a 6” then why answer the question posed?  This isn't a math problem.

 

I live in South a Florida, Where the seeing varies from lousy to excellent.  6” f6.  On the excellent end, I have no problem working at high magnifications.  And no, it does not wash out Jupiter.  When I use high magnifications, A 6mm Is my usual max, but I can get away with a 4mm at times.  But realistically, 9 or 10mm is the norm high that I use, and a 6mm is about as high as I bother.

 

if you think you have really good seeing, I’d start with getting a 9 or 10mm.  If that works out well for you.  Try a Barlow on it, which will make it equivalent to 4.5-5mm.  If that doesn’t completely ruin the image, then you might be able to get away with a 6 or 7mm.  I would not actually buy anything smaller unless you KNOW you can use it.  It is really more seeing related than scope related, so you will need to do the practical tests for yourself in your actual location.


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#8 Waddensky

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Posted 09 July 2020 - 09:46 AM

The resolution of a 6" telescope is about 0.78 arcsec (Dawes' limit). If you want to magnify that to the resolution of the human eye, you'll need a magnification of about 150x. Magnifying more is possible (and sometimes necessary), but you won't resolve more details by doing that. The atmosphere usually limits the magnification to around 200x, in my experience.


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#9 kathyastro

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Posted 09 July 2020 - 09:47 AM

A rule of thumb for max magnification is that it is equal to the aperture in millimetres.  6" = 150mm, so 150x is your maximum in most conditions.  In excellent seeing, you can double that, which is where the manufacturers get their 300x figure from.


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#10 Tony Flanders

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Posted 09 July 2020 - 10:13 AM

If you have one eyepiece to pick for higher magnifications, it's a 4mm. It's close to the optimum, I find, for Jupiter on an f/5 scope ...


The Original Poster, however, is planning to buy an f/8 telescope. The equivalent of a 4-mm eyepiece in an f/5 scope would be 4/5 * 8 = 6.4 mm. And I agree that this is, in fact, just about ideal for Jupiter in excellent conditions in an f/8 scope.

 

But since conditions are rarely excellent in most locations, I would on the whole recommend something a bit longer. Less magnification, in other words.


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#11 Echolight

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Posted 09 July 2020 - 10:25 AM

I chose an 8-24 zoom and a 2x barlow for the 6 inch f8. Gives 50x to 300x magnification.

 

I've had clear high contrast views on Saturn at 190x with this combo. And well over 200x on the moon.

 

I'll eventually add a 38 SWA for widefield. But have to get a 2 inch diagonal first.


Edited by Echolight, 09 July 2020 - 10:34 AM.

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#12 dmgriff

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Posted 09 July 2020 - 11:06 AM

Some of the 6in f/8 have 25mm and 9mm or 10mm plossl as included eyepieces. If the scope comes with only 25mm, I would suggest a additional 9 or 10mm eyepiece and a 2x barlow, yielding 25mm, 12.5mm, 9mm/10mm, 4.5mm/5mm.

 

I found a 9mm with 2x barlow (4.5mm) at 266x a nice planetary high power with my Hardin 6in f/8 dob.

 

Good viewing,

 

Dave


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#13 stubeeef

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Posted 09 July 2020 - 12:15 PM

With a 5x Barlow and 3mm eyepiece I’m betting you can get a very high magnification. The view will be worthless but you might get well past 1000x depending on your scope. Be a waste of time and money. Posts above are dead on. I’m rarely over 100x
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#14 sixela

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Posted 09 July 2020 - 12:25 PM

The Original Poster, however, is planning to buy an f/8 telescope. 

You're right -- edited. That's what you get when you own too many f/5 Newts ;-).


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#15 Chesterguy1

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Posted 09 July 2020 - 01:28 PM

I have been tooling around with a Celestron C6 to complement my other scopes. I've added the focal reducer to make it f/6.3. I am rarely blessed with good seeing in my location, it's almost always below avg to avg. With those caveats, the magnification typically tops out at between 125x to 157.5x with the EPs in my arsenal. I was able to split Izar on a night with slightly better than avg seeing at 201x. I wouldn't call it the cleanest of splits, but you take what you can get.

 

Chesterguy


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#16 Sketcher

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Posted 09 July 2020 - 04:17 PM

I have a question... What is the highest magnification you can go on a usual basis with a 6 inch telescope? I know that they are marketed as having useful maximum magnification 300x but due to atmospheric turbulence etc. you can't go that high.

 

I'm planing on buying a 6 inch f/8 telescope and an eyepiece to complement the 25mm SPL eyepiece that already comes with the telescope and I want an eyepiece that I can use most of the time for higher magnification. 

As others have suggested, for someone wanting a single high magnification eyepiece to go with a 6-inch f/8 telescope that comes with a single 25mm (about 50x) eyepiece; 150x seems to be about right.

 

But ideally, one ought to have a wide variety of available magnifications.  Different "targets" react differently to being magnified.  Sky conditions vary.  Different 6-inch telescopes are made to different quality levels.  And yes, even different observers (with all other things being equal) may prefer using different magnifications

 

With that being said, a decent-quality 6-inch telescope, under good enough conditions, can be usefully used at 300x or even a bit higher; but such magnifications aren't likely to be of use very often. -- thus the 150x suggestion.

 

When I go out with a 6-inch telescope (mine is f/6.5, not that it matters all that much), I may take out with me a single eyepiece when I know in advance what I'll be observing, etc.  But when I go out without a specific observation in mind, I take out with me a variety of eyepieces   The variety isn't always the same, but it's always a variety.  Based on what's currently in my "take-out-box":

 

I'll often choose one 2-inch wide-field eyepiece that provides the widest true field of view I can achieve with the specific telescope.  For me, this ends up being a 26x eyepiece.  Then I'll likely choose a more "reasonable" (for me) wide-field, 2-inch eyepiece of 31x.  Next come the 1.25-inch eyepieces.  Again, I start with an eyepiece that provides the widest true field possible in the 1.25-inch format.  This time, it's a specific 40x eyepiece.  Then I go with a 'medium' magnification eyepiece of 110x.  Next up are the higher magnifications -- 165x, 200x, and 330x.  That last one is just in case the object, conditions, etc. all work out in such a manner that 330x might be practical and useful.  Most of the time it won't be.  200x is often useful for me and my telescope; but I still think that 150x is the better choice if you can only get one additional eyepiece for high magnification use.  150x is a more "flexible" high magnification eyepiece -- useful for a wider variety of objects.

 

But we all have to start out somewhere.  A 150x eyepiece provides a useful, reasonably high magnification for a 6-inch telescope that ought to be useful quite often.  Higher magnifications (depending on the telescope's quality, etc.) may be useful at times; but much of the time they may not be.  Circumstances (especially with atmospheric contidions) are not always the same and will not always support the use of higher magnifications.


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#17 vtornado

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Posted 09 July 2020 - 07:28 PM

Hello as others have said above.

 

For Planets in a 6 inch f/8 I use a 8mm (150x) on most nights, and if seeing is good I use a 6 (200x).

Some planetary detail is low contrast, like banding on Jupiter and Saturn, and shading on Mars.

By increasing the magnification you dim the image.   Then your eyes may not have enough light to

pick out subtle details.

 

For moon with good seeing, I sometimes use a 4mm, but that is a rare night.

Another use for  300x is trying to split tight double stars.


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#18 izar187

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Posted 09 July 2020 - 09:48 PM

For a 6" another vote for 150x.

And a vote for up to 200x, when cooled, collimated and with man-made heat sinks out of the line of sight to target.

250x or so, very rarely a significant gain for me. 

No vote for 300x, not without a tracking mount and exceptional seeing both locally on site and regionally. IMHO 


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#19 rhetfield

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Posted 10 July 2020 - 07:59 AM

If the scope is not yet bought, I would make sure it has a 2" focuser to maximize low magnification field of view.

I would buy a 2x Barlow that the lens element can be removed to make 1.5x.

I would get either an 8mm or a zoom that goes at least as low as 8mm.

Assuming perfect atmosphere, the moon will look good up to 300x. You also want to star collimate at 300x. The bright planets would be good to 225x. DSOs would rarely be over 150x. The Barlow with a 25mm will give you low and mid range magnification.
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#20 Sorana

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Posted 10 July 2020 - 09:06 AM

If the scope is not yet bought, I would make sure it has a 2" focuser to maximize low magnification field of view.

Hello! What difference does the 2" focuser make? I was about to buy the SkyQuest XT6 which has 1.25"...



#21 rhetfield

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Posted 10 July 2020 - 09:21 AM

Hello! What difference does the 2" focuser make? I was about to buy the SkyQuest XT6 which has 1.25"...

2" eyepieces have wider field of view. Not important for high magnification planet work, but very useful for bigger DSOs like Pleiades and Andromeda at lower magnifications. You would get one 2" eyepiece in the 20-35mm range.
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#22 Tony Flanders

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Posted 10 July 2020 - 10:33 AM

2" eyepieces have wider field of view. Not important for high magnification planet work, but very useful for bigger DSOs like Pleiades and Andromeda at lower magnifications. You would get one 2" eyepiece in the 20-35mm range.

Actually, in an f/8 scope I would likely buy a 40-mm wide-field eyepiece.

 

To amplify on this, one of the problems with telescopes that have relatively long focal ratios, like f/8, is the inability to achieve wide fields of view. In the SkyQuest 6, with its 1200-mm focal length and 1.25-inch focuser, the widest field of view you could achieve would be roughly 1.3 degrees, using a 32-mm Plossl to achieve 37.5X. Here's the arithmetic:

 

1200/32 = 37.5X, Plossl 50-degree FOV / 37.5X = 1.33... degrees

 

That will fit all the main stars of the Pleiades, but it will miss many of the cluster's outliers. More to the point, to get a reasonable sense of the cluster, you really need a bunch of background sky all around to frame it. Personally, I find 2 degrees the minimum for a pleasing view.

 

With a 2-inch focuser, by contrast, you could use a 40-mm eyepiece with an apparent field of view in the 60-70 degree range. That would give you at the very least a 2-degree field of view at 30X. Much better for the handful of star clusters that match the Pleiades in size, and also better for browsing the sky and star-hopping.

 

Having said that, a 6-inch f/8 with a 1.25-inch focuser is still a great telescope.


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#23 Mike T.

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Posted 10 July 2020 - 12:17 PM

I have a 6" f/8 scope. I used the 25 and 9 mm plossls it came with for a year. Upgraded to 12 and 8 mm astro tech ED. On planets I first start with the 12 mm at 100x which always looks good. I then bump up to 150x with the 8 mm. Usually looks good but sometimes seeing conditions don't support it so will go back down to 100x. I also have a GSO 2x shorty barlow and will go to 200x and 300x on the moon. The image is rippling a ton but it's just fun to see what the scope can do sometimes. Also went up to 300x on Venus this spring.There is no detail to see on Venus so the image just got bigger and bigger as I went up in mag. It was great. I have never been above 300x.


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#24 sg6

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Posted 10 July 2020 - 12:30 PM

At 6" so 150mm I would say good up to 150x, should get 180x, likely 200x but not always, any more is chance, luck and smiling astronomy gods.

 

Ball park max therefore 150x-180x.


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#25 Dave Mitsky

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Posted 10 July 2020 - 03:37 PM

I own two 6" telescopes, one an f/8 Newtonian Dob and the other an f/10 SCT, and a number of larger ones.  Depending upon the object and the quality of the seeing, I sometimes use a high magnification of 200x, which means a 6mm focal length eyepiece, with the Dob.  A magnification of 150x (an exit pupil* of 1 millimeter) works better more often than not.

 

https://explorescien...sure-exit-pupil

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