Formula to calculate what Magnitude objects you should be able to see (visual astronomy)?

[Scoobyroo]

Hi all.  Very new to astronomy and have a question I can't seem to find a definite answer on relating to 'magnitude'.

When suggesting objects to view 'Best Tonight', the Go-To software I currently use (ScanSyn) will order potential objects by magnitude... my working assume is that this means how bright the object will appear?  

My question is, would that be doing so based on the Scope it knows I'm using (aperture, focal length, etc)... or is their literally a set 'magnitude for objects, that either doesn't change... or only changes based on the objects position relative to the earth?  I'm also struggling to understand if the location you are viewing from (i.e weather / Bortle) has an impact, other than the obvious (a Bortle 4 trumps a Bortle 6 / if it's behind a cloud, you aren't seeing it! 😆).  Also what about measuring the Eyepiece impact (as I understand the lower the focal length, the dimmer the object will appear - though I'm sure EP quality plays a part here as well).

I note the Stellarium (which I've literally just downloaded and spent a few hours with), will display 'Magnitude readings for any object selected.  Same question really, what is it using to calculate this - or is it a pre-set.  It seems to have a greater ability to let me drop info in about my Scope / EP's / Bortle rating, so am wondering if this then becomes more accurate?

If not, is there a calculation I can use to figure out the relative 'magnitudes' for my set-up under given conditions?

Thanks all... 

Edited August 30, 2022 by Scoobyroo

[Zermelo]

Synscan will be sorting according to the apparent magnitudes, which depend on both the absolute (intrinsic) magnitude of each object and its distance from Earth.

Then, as you say, there will be various factors that determine the magnitude of each object as you see it (or don't), whether by naked eye or with assistance. But that won't change the sequence of the objects.

If you're interested in the limiting magnitude, i.e. the faintest object you can see, then it's not easy to be precise as there are quite a few things to take into account, but there are web sites that give you approximate answers, e.g.

http://astronomy.tools/calculators/telescope_capabilities

 

[EDIT:  I was really thinking about single stars in what I wrote. Things work differently for extended objects, see Stu's comprehensive answer below]

Edited August 30, 2022 by Zermelo

[Stu]

Firstly welcome to the forum 😀.

You’ve asked quite a few excellent questions in your post, I’ll do my best to give a few helpful hints, not necessarily in the same order as you asked!

Firstly there are two main sorts of objects. Stars, which are considered point sources from our perspective ie they never appear as anything other than points of light, and extended objects like galaxies and nebulae. Why is this relevant? Well, the magnitudes quoted for a star represent all the light concentrated into a point, whereas a galaxy of the same magnitude will have this light spread out over the area it covers, meaning that it actually appears dimmer. It’s like having a torch beam that you can turn from being a bright spot (star) to covering a large area (galaxy) when it appears dimmer per unit area. The measure is actually called surface brightness and nebulae and galaxies tend to have lower surface brightness so they get washed out by light pollution much more easily than stars.

In terms of visibility, skies can be classified by something called Naked Eye Limiting Magnitude (NELM) which means the dimmest star you can see with the naked eye, normally at the zenith (straight up!) Anything from Mag 5 is ok and above Mag 6 gets much more interesting.

A scope will show fainter stars not because it makes them brighter, but actually because magnification dims the background brightness whilst maintaining the stars brightness, so you get better contrast and the fainter stars become visible. In this case, the bigger the scope, the dimmer the stars you can see but the limit will still be reduced by the brightness of the background sky (Bortle or NELM)

There are some useful calculators on this link:

https://www.bbastrodesigns.com/NewtDesigner.html

Just be wary of many of the calculators which are based around field of view and magnification. They show you how big things will appear in the eyepiece but not how bright or visible they are. It tends to be the larger objects are, the fainter they appear against the sky background as their light is more spread out. In very rough order, stars are point sources so appear brightest relative to their quoted magnitude, then come globular clusters, planetary nebulae, then galaxies and large nebulae such as the Veil or North America nebula.

Finally....it is counter intuitive but larger scopes cannot actually make objects appear brighter on a per unit squared basis vs the naked eye. What they can do is maintain that tightness whilst magnifying the object, and the larger the scope, the more it will be able to magnify these small faint objects whilst maintaining their brightness. That’s galaxy hunters, who tend to go after small faint galaxies use the largest dobsonian they can. Making objects larger makes it easier for you eye to perceive the contrast changes along the boundaries, and so that makes them easier to see.

I hope some of my rambling makes sense. It is a complex subject and I kind of fumble around with a basic knowledge of it. Others will be able to add more I’m sure.

[Scoobyroo]

55 minutes ago, Zermelo said:

Synscan will be sorting according to the apparent magnitudes, which depend on both the absolute (intrinsic) magnitude of each object and its distance from Earth.

If you're interested in the limiting magnitude, i.e. the faintest object you can see, then it's not easy to be precise as there are quite a few things to take into account, but there are web sites that give you approximate answers, e.g.

http://astronomy.tools/calculators/telescope_capabilities

That makes great sense, thanks.  Let me check out the link and have a play with those calculators 

53 minutes ago, Stu said:

Well, the magnitudes quoted for a star represent all the light concentrated into a point, whereas a galaxy of the same magnitude will have this light spread out over the area it covers, meaning that it actually appears dimmer.

In terms of visibility, skies can be classified by something called Naked Eye Limiting Magnitude (NELM) which means the dimmest star you can see with the naked eye, normally at the zenith (straight up!) Anything from Mag 5 is ok and above Mag 6 gets much more interesting.

A scope will show fainter stars not because it makes them brighter, but actually because magnification dims the background brightness whilst maintaining the stars brightness, so you get better contrast and the fainter stars become visible. There are some useful calculators on this link: https://www.bbastrodesigns.com/NewtDesigner.html

stars are point sources so appear brightest relative to their quoted magnitude, then come globular clusters, planetary nebulae, then galaxies and large nebulae such as the Veil or North America nebula.

That's the best description (and torch analogy) I've come across...  even I understood!  it

Some absolute gems of info there... thank you so much.  Helps me understand much better what to target (also helped with my understanding of 'Bortle')  and that link gives me some great follow up info.  

Thanks both.! 🙌

 

Bonus question (if I may).  How come planets can go into -(negative) magnitudes (for example, Venus is showing as -3.9 tonight)?  Is it because 0 is supposed to represent the brightest point light (i.e. star) and everything else is calibrated from there? i.e. Venus is able to reflect  localised star-light (i.e. the Sun), over a much winder area (relative to us... i.e. in out solar system)?  Or do planet have a separate scale (I don't think this is the case from what I'm reading)?

[Zermelo]

23 minutes ago, Scoobyroo said:

Bonus question (if I may).  How come planets can go into -(negative) magnitudes (for example, Venus is showing as -3.9 tonight)?  Is it because 0 is supposed to represent the brightest point light (i.e. star) and everything else is calibrated from there? i.e. Venus is able to reflect  localised star-light (i.e. the Sun), over a much winder area (relative to us... i.e. in out solar system)?  Or do planet have a separate scale (I don't think this is the case from what I'm reading)?

I'll have a stab, then @Stu can improve on it!

The magnitude system is logarithmic, so each gap of 1 magnitude corresponds to a multiplication in brightness of a factor of about 2.5.  I believe that when astronomers were setting up the scale, they tried to make it compatible with the observers in the classical world, who talked about "stars of the first magnitude" etc.  So the brighter the star, the lower the number, but they couldn't make it fit exactly, so the really brightest have to go to zero and then negative. Planets (and the moon, and sun) use the same scale, so they are even more negative. As you say, Venus appears very bright because it can reflect a lot of sunlight towards us.

 

[Stu]

Nicely done sir. Can’t improve on that, other than to cheat and post a Wiki link which has plenty of detail in it, supporting what you said 👍

https://en.m.wikipedia.org/wiki/Apparent_magnitude

[Scoobyroo]

Lovely...   the logarithmic nature of the scale never occurred to me, was an eye opener... and actually explains why such a small numerical value was having such a large impact on my ability to see certain DSO's.  This should have been obvious - I clearly still have a lot to learn, so appreciate your time / guidance both!

[Zermelo]

Although ... the reason for a logarithmic scale is that the eye itself perceives brightness in that way. It evolved that way so that it can respond to a much wider range of brightness.

The ear is similar, which is why the decibel scale is also logarithmic.

[Ags]

I think the original ancient magnitude scale wasn't a measure of brightness, it was a measure of percieved "size".

[Stu]

8 hours ago, Ags said:

I think the original ancient magnitude scale wasn't a measure of brightness, it was a measure of percieved "size".

There is a line here from Wiki saying that Hipparchus used the terms Big and Small to describe stars.