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Orion ST80 magnitude limit

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#1 tswoodshop

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Posted 10 October 2019 - 08:21 PM

What is the practical magnitude limit of the 80mm Orion ST80? For example, assuming I could somehow get it aimed at the right spot in the sky without benefit of go-to, would I be able to see Neptune (approx mag 8)?


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#2 StarBurger

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Posted 10 October 2019 - 08:33 PM

Neptune will be easy in this scope, no problem.

The ultimate limiting magnitude will depend on sky conditions but expect 12th mag on an average night.


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#3 Shorty Barlow

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Posted 10 October 2019 - 08:38 PM

Yes, it should be possible. 

 

http://skywatcher.co...duct/bk-804az3/

 

Magnitude limit is 12.2 according to Synta,



#4 tswoodshop

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Posted 10 October 2019 - 08:55 PM

Thanks guys... I didn't think this little ST80 would be able to pick out 12th magnitude. Appreciate the info.



#5 Shorty Barlow

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Posted 10 October 2019 - 09:07 PM

med_gallery_249298_10284_22312.jpg

 

You're welcome. Never underestimate the ST80! lol

 

med_gallery_249298_10284_206061.jpg


Edited by Shorty Barlow, 10 October 2019 - 09:08 PM.

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#6 vtornado

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Posted 10 October 2019 - 09:35 PM

In some of my bigger scopes, neptune is a little blue disk. 

Would it appear as a disk in the ST80?

That might help the OP pick it out.

 

======== P.S. =======

forget I said that old guy brain cramp.

Uranus appears as a little blue disk.

 

Ever since there is no Pies any more in Just Served Us Nine Pies,  

Us and Nine keep getting flipped in my mind.


Edited by vtornado, 11 October 2019 - 01:39 PM.


#7 cookjaiii

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Posted 10 October 2019 - 10:29 PM

I don't think it will look like a disk.  In my 100mm refractor, Neptune looked like a bluish star that I couldn't quite bring to focus even though all the surrounding stars were pinpoint.


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#8 astro744

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Posted 10 October 2019 - 11:02 PM

See http://www.cruxis.co...ngmagnitude.htm

 



#9 HellsKitchen

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Posted 11 October 2019 - 02:58 AM

You'll see Neptune in the ST80, but you won't see it as a disk. It should however appear bluish. 


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#10 Shorty Barlow

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Posted 11 October 2019 - 04:52 AM

I've seen it with a 90mm Mak. It isn't quite a disc IIRC, but the blue colour can be seen and it definitely isn't a star. A 5x Barlow can be useful with fast short tube achromats for planetary viewing.



#11 Tony Flanders

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Posted 11 October 2019 - 04:55 AM

Thanks guys... I didn't think this little ST80 would be able to pick out 12th magnitude.

Not from Philadelphia, you won't. Figure that it will add perhaps 6 or 7 magnitudes to your naked-eye stellar magnitude. In Cambridge, MA, I can see stars roughly down to magnitude 4.5 with my unaided eyes, and down to perhaps 11 through my 70-mm refractor.



#12 Starman1

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Posted 11 October 2019 - 12:07 PM

What is the practical magnitude limit of the 80mm Orion ST80? For example, assuming I could somehow get it aimed at the right spot in the sky without benefit of go-to, would I be able to see Neptune (approx mag 8)?

Tony is dead on.  However, bear in mind the limit of a telescope is a moving target.

It goes lower when you use higher powers.  You will see fainter stars at 80x than at 20x.

It goes lower in darker skies.  A lot of lights turn out between 11pm and 2am.  The 2am to dawn period is darker in most cities.

It goes lower when the air is clearer.  Haze is the enemy of seeing deep.

It goes lower when you are completely dark adapted, after, say, being outside for 45 minutes away from lights.

It goes lower, much lower, with averted vision, especially if you accept a star that isn't visible 100% of the time with averted vision, but winks in and out.

When I use a limiting magnitude calculator on an 80mm scope, I see a limit of 14.7 for absolutely ideal conditions, and 10.5 for a newbie under poor conditions.

The likelihood is that it will be somewhere in between.  

But Uranus and Neptune will be no challenge.  I see both in my 50mm finder here in Los Angeles, where my night sky is orange.

 

Edit: by "lower", I mean of fainter magnitude.


Edited by Starman1, 11 October 2019 - 04:11 PM.


#13 Redbetter

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Posted 11 October 2019 - 03:51 PM

By "lower" above I believe Don means dimmer since the a magnitude limit will be higher instead.

 

What I have measured on calibrated stars with an 80ED in dark skies is ~14.3 magnitude at about the minimal/optimal exit pupil to provide the highest magnification without overly bloating the star into an extended object.  This was threshold detection in averted vision only seen at moments, but consistently enough that I could be certain it was real.  With less experience sorting the real from the unreal for threshold, I wouldn't be able to break 14 with the 80ED; those last few tenths are difficult.  How difficult?  I have tried for Pluto with it in Bortle 1 conditions but poorer seeing, and could not reliably break 14 for stars in the surrounding field. 

 

The ST80 will not perform to the same level as the 80ED because the 80ED is a good apo, while the ST80 is a short achro that spreads much more of the spectrum over a wider area.  This reduces the central intensity of the spurious disk and reduces contrast at high power since it becomes less of a point source and becomes an extended object somewhat sooner (at lower magnification than the 80ED.)  The impact is perhaps a few tenths of a magnitude--not huge, but enough to notice if one is "keeping score."

 

Seeing Neptune is easy.  It is slightly brighter than 8th magnitude and I can easily pick it out in 2.3x Galilean binocs in the suburbs (where the aperture is 2.3x times my pupil diameter...so probably about 14 to 16mm for me) but it is just a point that way. 

 

Seeing Neptune as a disk is another matter and requires two things:  resolution and magnification.  Resolution comes from the aperture itself, the larger the aperture the smaller it can resolve before the wave nature of light smears/blurs the edges of a disk.  Magnification is required to provide enough scale for our eyes to see the resolution limit of the aperture.  An ST80 is hard pressed for both.  The aperture is somewhat less than the theoretical limit for Neptune, and the magnification required is not easily/comfortably achieved in a scope with only 400mm focal length unless one has the eyepiece/Barlow combinations for it. 

 

Neptune is about 2.4" in diameter.  The airy disk diameter is about 3.46", although the central intensity's dimming edge (the spurious disk of the airy pattern) is somewhat narrower than that, perhaps 70 to 85%.  We can resolve somewhat better than that, particularly with two point sources:  1.73" for Rayleigh or 1.45 for Dawes limit.  But Neptune is an extended source, not a point source like a star light years away; so putting it all together the result is a somewhat bloated disk with very soft/indistinct edges rather than a bright stellar airy disk pattern.  With enough magnification (scale) one can see the effective size difference compared to 7 to 8 mag star. (This assumes decent seeing and normal vision.) 

 

Generally, about 150x is often recommended to see Neptune clearly as a disk rather than a point...assuming one has sufficient aperture to resolve it as disk in the first place.  This minimum varies some based on individual visual acuity and experience.  150x for a 2.4 arc second object is 62.5x per arc second--call it 60x per arc second.  This is essentially 20/20 resolution on an eye chart as I understand it.  Perfect vision can resolve to about 30x per arc second, 20/10 around the bottom line on a typical eye exam chart.  Had that until I went off to college or a year or two after my degree when I began to notice that I couldn't always read things at a distance anymore. 



#14 Redbetter

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Posted 13 October 2019 - 04:47 AM

Gave Neptune a try tonight in Meade AS80 (same as Orion ST80), backyard viewing in town with full Moon.  Seeing was poor and transparency was mediocre.  While Neptune was found easily enough and the color visible, there was little to distinguish it from a 7 to 8 mag star.  With the skies as bright as they were and the seeing rough, I could not really see the airy disk pattern at high power on stars dimmer than about 5th magnitude with this scope.  I was probably only able to see down to about 11th magnitude on the stars near Neptune in the scope tonight, but I didn't do a telescopic limiting magnitude check. 

 

As I noted in the prior post above. the resolution limit of the 80 is similar to the angular diameter of Neptune.  Even at 160x with a 2.5 Nagler it was not really possible to discern disk that was clearly larger in size than the spurious disk of the blue-white 7th mag star to the north.  This is rather high magnification for the 80 f/5, beyond where the image is sharp for planetary.  I didn't make a comparison with an 80ED.  Hard to say if the conditions were to blame or if 7 to 8 mag is just too dim to discern the difference between a planet this size and a similar mag star and its diffraction pattern in the AS80.

 

When the Moon is a few days further east I might try to examine Neptune and Uranus with both the AS80 and 80ED to get a better feel for the appearance relative to stars. 



#15 Redbetter

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Posted Yesterday, 03:41 AM

I did a comparison tonight in the backyard with the AS80 f/5 achro and 80ED which is visually apochromatic.    The Moon was bright, one day removed from being closest to Uranus so I started with that planet, then switched to Neptune.  Seeing was somewhat better rising to borderline mediocre. 

 

Uranus:

  • Uranus, at 3.7 arc seconds was more obviously a planet rather than a star even in the short achro.  I could tell it was different at only 67x, while ratcheting up to 80, 100, 133, and 160x made it more clearly a disk.  However, the edges were soft in the AS80.  Most noticeable was that the central intensity was muted compared to stars of similar magnitude, while the disk was clearly wider and more uniform in intensity. 
  • Another interesting effect for the achro was that at all powers Uranus disk had a warmer tone than I expected.  It did not look green, blue, or even gray.  Instead it had some yellow/coffee stained hue.  I attribute this to chromatic aberration since racking the planet in and out of focus rendered it more of a muted green/gray on both sides of focus. 
  • Uranus' color was more of a subtle greenish gray with the 80ED (although warmer than Neptune in this scope.)   I observed it at 100, 120, 150 and 200x.   It was clearly a planet at 100x with a very small but somewhat crisper disk than in the AS80.  It was still sharp at 150x but becoming somewhat less defined at 200x, although this was still somewhat better than the AS80 at 160 or even 133x.

Neptune:

  • The planetary disk itself remains a tough target for the AS80 f/5 achro due to chromatic aberration.  The color is more of a slate gray in this scope than Uranus was, but the smaller size of the disk is even more ill-defined, apparently as result of the general blurring/lower resolution resulting from substantial chromatic aberration. The size difference it less apparent as is the central intensity difference.  Still, with the better seeing there was some subtle difference with comparison stars--the most obvious being that it was steadier.
  • In the 80ED the resolution was clearly superior.  The non-stellar nature was somewhat apparent at 100x and more so at 120, 150 and 200x.  Panning to the 7th mag star to the north revealed a very faint 1st diffraction ring and greater central spurious disk intensity.  Neptune appeared similar in diameter to the first ring of that star, and more uniform in surface brightness although soft at the edge at 200x.  I was unsure if I was seeing an extremely faint and broader ring around at 200x (darker skies would have helped.)   

Conclusion:  An ST80/AS80 is at best marginal at revealing Neptune's disk  The high level of chromatic aberration has some deleterious impacts to both the color and resolution of Neptune's image when compared to an 80mm apochromatic scope. 

 

EDIT:  I also did a quick comparison with the 127mm Mak targeting Neptune.  It more readily revealed the difference between star and planet due to size and steadiness as well as the more obvious diffraction pattern around the star.  The central intensity was also greater for the star vs. the planet and the color contrast was more readily seen with sharper edges to the planet.


Edited by Redbetter, Yesterday, 03:51 AM.

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#16 Redbetter

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Posted Yesterday, 03:56 AM

Generally, about 150x is often recommended to see Neptune clearly as a disk rather than a point...assuming one has sufficient aperture to resolve it as disk in the first place.  This minimum varies some based on individual visual acuity and experience.  150x for a 2.4 arc second object is 62.5x per arc second--call it 60x per arc second.  This is essentially 20/20 resolution on an eye chart as I understand it.  Perfect vision can resolve to about 30x per arc second, 20/10 around the bottom line on a typical eye exam chart.  Had that until I went off to college or a year or two after my degree when I began to notice that I couldn't always read things at a distance anymore. 

If I could edit this old post I would strike out the part about it being ~60x per arc second and delete the rest following it.  That should actually be 150x2.4 = 360 arc second size at 150x.   foreheadslap.gif  The central problem is that at 150x the airy disk is 150x3.46 = 519"...but the actual visually illuminated (spurious disk) is closer to the 360" value at 150x.




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