Bortle scale, object magnitude, and scope size

[jm3]

Is there a chart or some way to figure out which objects you should be able to see given the Bortle number of your location and the aperture of your telescope?

 

For instance, I'm at Bortle 8-9 and have an 8" dob.  I *think* I located and saw M10 the other night (hunting around with an angle gauge on the OTA), but it looked like a barely visible circular fuzzy dust smudge. I wasn't able to see it at all in the finder scope (8x50 RACI). I was very excited thinking I had found it after 10-15 minutes of hunting, but also surprised at how difficult it was to see.

 

Thanks!

[Ags]

I don't think there is a strict mathematical rule. Every DSO is different and reacts to light pollution differently. Then there is the difference between (1) being dark adapted and the sky being bright from area light pollution sources and (2) not being fully dark adapted because of local light pollution. And everyone's eyes are different and we see differently.

Also sky transparency is very important, when the sky is more transparent the DSO is brighter and the sky is darker...

Edited August 1, 2022 by Ags

[Stu]

There are a number of visibility calculators out there which you may find of benefit. As Ags says, there are no hard and fast rules, but there are ways of getting good indications as to what is possible.

This is one for stellar visibility

https://www.cruxis.com/scope/limitingmagnitude.htm

This is one from Mel Bartels for DSO visibility.

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

It is a very complex area with different factors affecting it, almost more art than science but these calculators give a good way of starting to try to understand what will be visible and what won’t.

 

[AstroMuni]

13 hours ago, jm3 said:

Is there a chart or some way to figure out which objects you should be able to see given the Bortle number of your location and the aperture of your telescope?

Stellarium has an option to set the Bortle of your location and adjusts the star visibility to show the effect.

[Louis D]

17 hours ago, jm3 said:

For instance, I'm at Bortle 8-9 and have an 8" dob.  I *think* I located and saw M10 the other night (hunting around with an angle gauge on the OTA), but it looked like a barely visible circular fuzzy dust smudge.

At what power were you observing it?  If you push the power up to at least 200x, you might start to see some outer straggler stars and brighter core stars resolving as twinkling pinpoints of light.  Globular clusters take power and aperture really well because they start to resolve at high power in large scopes, which cuts through light pollution.

M10 from Hubble:

Do you see how some stars are intrinsically brighter?  Those are the ones that start to stand out from the fuzzy background collection of dimmer stars at higher powers.

[PeterStudz]

On 02/08/2022 at 13:47, Louis D said:

At what power were you observing it?  If you push the power up to at least 200x, you might start to see some outer straggler stars and brighter core stars resolving as twinkling pinpoints of light.  Globular clusters take power and aperture really well because they start to resolve at high power in large scopes, which cuts through light pollution.

I’m in Bortle 7 and I do exactly this with my 8” Dob. Eg on M13 at 200x I can see dozens of stars as twinkling pinpoints of light. It can look stunning, although some nights are far better than others. At high magnification like this I’ll put the globular just outside the FOV and watch it drift through the eyepiece with stars twinkling away. 

[astronymonkey]

I'd say darker skies can add a magnitude or so to what you can see especially with DSOs when you are trying to spot contast against the background sky.

In a lot if cases the scope is seeing the object but the sky is just as bright so drowns it out, that's why even a big scope will struggle in light polluted skies.

So it's hard to truly say what you can see with a given scope but if you are struggling to see something and it's on the limit of visibility, get the scope to a dark site and you'll definitely see it.

Cheers