Views on aperture for combatting light pollution

[Mr Spock]

37 minutes ago, Mr Spock said:

Even with LP though, globular clusters are startling with the 12" compared to a 4" apo.

I should qualify that by saying the higher the magnification you use, the darker the background. So items like globulars can be most satisfying. 

[Ratlet]

9 minutes ago, Mr Spock said:

No. The brightness depends on the exit pupil. So the 10" would be 32mm/5 = 6.4mm and the 5" 16mm/5 = 3.2mm. The brightness of extended objects, and therefore the background, is proportional to the square of the exit pupil. 

Thank you very much.  I can feel the clutch in my brain slipping every time I think to hard about this sort of thing without adult supervision , but this makes sense.

[Stu]

1 hour ago, Mr Spock said:

I should qualify that by saying the higher the magnification you use, the darker the background. So items like globulars can be most satisfying. 

My assumption has always been that globulars behave differently because they are stellar in nature, so the stars do not dim as mag increases but the background does. Nebulae dim at the same rate as the sky background so don’t benefit in the same way. Does that sound correct?

[jetstream]

9 minutes ago, Stu said:

Does that sound correct?

Yes

[Mr Spock]

1 hour ago, Stu said:

. Does that sound correct?

Yes.  It’s easier to see fainter stars with higher magnification as the background gets darker. 

[pipnina]

1 hour ago, Mr Spock said:

Yes.  It’s easier to see fainter stars with higher magnification as the background gets darker. 

Sometimes I struggle to get my head around this.

My best guestimate is that as stars are pinpoint, we capture 4x of their light with a 2x as big mirror, but because they also shrink with mirror size they appear brighter?

I.e. if we had a 1 meter scope and viewed way above the seeing limit, we'd reach the end of returns for brighter stars as the smudge gets larger with magnification?

I think this can also mean imagers see fewer stars with a 50mm frac than a 130mm? I have seen some photos that show really tight narrowband images where the stars almost seem to have disappeared but they still seem quite strong in NB in my 130mm frac and in my 200mm newt

[Zermelo]

I've seen this argument in previous threads, and I think I buy it, however:

with star groupings, there must be some transition between the two behaviours? A nearby open cluster behaves, under increasing magnification, as a bunch of (essentially point-like) stars, but a distant galaxy will behave more like an extended diffuse nebula, at least in amateur kit?
Do globulars (those belonging to our galaxy) behave more like open clusters, or more like distant galaxies? Or perhaps both - is it possible that an unresolved core behaves like an extended source, but the resolved outer regions like individual stars?

[Stu]

9 hours ago, Zermelo said:

I've seen this argument in previous threads, and I think I buy it, however:

with star groupings, there must be some transition between the two behaviours? A nearby open cluster behaves, under increasing magnification, as a bunch of (essentially point-like) stars, but a distant galaxy will behave more like an extended diffuse nebula, at least in amateur kit?
Do globulars (those belonging to our galaxy) behave more like open clusters, or more like distant galaxies? Or perhaps both - is it possible that an unresolved core behaves like an extended source, but the resolved outer regions like individual stars?

I’m not sure I must say though I suspect there may be something in what you say; take the extreme of M31, made up of billions of stars but behaves in the same way a nebula does so why shouldn’t unresolved light in the centre of a glob behave the same?

@pipnina, my understanding is that because stars are point sources, they do not increase in size as magnification increases which means that their surface brightness remains the same. At the same time, the surface brightness of the background decreases with increasing magnification so as mag increases, the contrast between stars and background increases so you see fainter stars. Hope that makes sense.

Nebulae behave in the same way as the sky background, so they decrease in brightness at the same rate.

Mel Bartels has some useful calculators and discussion pieces on visual contrast.

https://www.bbastrodesigns.com/ObjectContrastCalculator.htm

The visual detection option here is useful too.
https://www.bbastrodesigns.com/NewtDesigner.html#visual

[Mr Spock]

1 hour ago, Stu said:

my understanding is that because stars are point sources, they do not increase in size as magnification increases which means that their surface brightness remains the same. At the same time, the surface brightness of the background decreases with increasing magnification so as mag increases, the contrast between stars and background increases so you see fainter stars. Hope that makes sense.

Nebulae behave in the same way as the sky background, so they decrease in brightness at the same rate.

Exactly this 👍

[Mr Spock]

11 hours ago, pipnina said:

My best guestimate is that as stars are pinpoint, we capture 4x of their light with a 2x as big mirror, but because they also shrink with mirror size they appear brighter?

Not quite (see above) but you are on to something else - the definition of pinpoint. When you use higher magnification you see airy discs rather than pinpoints. This decreases with aperture so the airy disc of a 12" is a third the size of a 4" - which is where the extra resolving power comes from - 0.38" v 1.14". As it is a finite size it will increase size with magnification, but wont be so large as to reduce brightness, unless you use stupid magnification.

I have seen a formula for calculating the actual size of an airy disc v aperture but can't locate it at the moment.

I think there should also be a relationship between apparent size and exit pupil - I'm sure there'll be a formula for that somewhere too. I'm on the edge of my knowledge here though  I'm an observer not a technical expert 👁️

[Guy65]

17 hours ago, RobertI said:

Good advice above. My only addition is that a zoom eyepiece can often help with finding the best magnification to make a DSO stand out - there’s usually a point at which you get maximum contrast between the object and the sky background, sometimes it’s a higher magnification than you’d expect. The downside of zooms is the limited FOV at the longer end of the focal length range (typically 40-50 degrees at 24mm). 

Thanks - sounds like something else to add to the Wishlist!

[Guy65]

17 hours ago, mikeDnight said:

If it's the Messier objects you're hoping to see, you might find that shielding your peripheral vision from even slight light intrusion, will help you get maximum dark adaption for your site. Doing this and patiently studying the object will give you the chance to reveal detail you'd otherwise miss. If you can't see the object but you know you're looking at the correct area of sky, maintaining your dark adaption using a dark blanket or hood over your head and eyepiece, then moving the scope just a little while using averted vision, can help you to eventually detect it. You may not need nebula filters at all for the Messiers as they are quite bright in the main, and your 127mm Mak should handle them all.

Thanks - that's really helpful!
There's certainly a lot of local light sources from streetlights down the hill and next door's security light...
I am probably being too optimistic about being able to spot objects immediately and just need to take my time under a hood to maximise my chances.

[Guy65]

15 hours ago, ScouseSpaceCadet said:

I observe in light polluted Bottle 7/8 skies. Starting with a 130mm newtonian to a 127mm refractor, 150mm & 204mm newtonians and 102mm refractor with a couple of small Maksutovs thrown in.

I found the 8" dob didn't help me observe significantly more objects however extra details were observed in the objects I can see. Globular clusters for instance resolved more peripheral stars. Open clusters looked brighter and more dense due to resolving some of the fainter stars & and the extra aperture gathering more light overall.

In the end I decided to downsize to a 102mm refractor as my main scope because I discovered I prefer observing with an easily managed mounted telescope while also managing my own expectations!

Aperture is king but personal preference and circumstances trump the generic ideal.

Hopefully that makes sense. I'm slightly distracted watching Liverpool stuff Manchester Utd.🥳🤣

 

Thanks - very helpful and totally makes sense. The consensus I'm getting here is that aperture isn't going to be that helpful for me.

[PeterStudz]

I’ve only been intro this hobby for just over two years. I have a small 4.5” reflector and an 8” Dob. I’m in Southampton which is Bortle 7. Generally and as far as DSO go, there’s only a small difference between the two in the number that I can see from my back garden. Although the 8” does resolve more details. And it does blow the 4.5” away as far as doubles, the planets and moon go but thats a different matter. An example are the galaxies M81 & M82. In the 4.5” I can only JUST make these out. In the 8” they are both obvious. 

And globular clusters are far better in the 8”. If I crank up the magnification on these, so that they fill the whole FOV, then I can resolve far more stars and many will “twinkle” - I assume due to the atmosphere. I also have an EQ platform and this allows me to view things like this at high magnification without constant nudging. Allowing me to relax at the eyepiece and just take in the view. On a good night they can be magnificent. In the 4.5” globular clusters are no more than a fuzzy blobs.

A few other points. Not all nights are the same. Even though I’m in Bortle 7 there have been a few times when I could easily make out the Beehive Cluster naked eye. These are the nights that you want to get out. Generally my sky is at its darkest around 2am. Of course this does make for late nights. And the darker parts are straight up (obviously) and to the E-SE. If possible I’ll try and look at DSO at this sort of time and when they are at these places in the sky. 

Personally I do get pleasure out of what I can see from my back garden. But I’ve actually seen more/better DSO in the small 4.5”. And that was because I was able to pack that up in a small case, take it on holiday via an aircraft and use it at a true dark site. There, it was Bortle 1-2, the sky was amazing and I can well remember being able to see the Swan Nebula naked eye. The views in the little 4.5” were, for me, stunning.

As far as star hopping goes I do use an app on my phone called PSAlign Pro that includes a PushTo feature. The phone being mounted to the OTA. Just sync to a known star or two, select your target from a list and push to it. And there it is in the eyepiece. For me it works every time.

[scarp15]

17 hours ago, ScouseSpaceCadet said:

I observe in light polluted Bottle 7/8 skies. Starting with a 130mm newtonian to a 127mm refractor, 150mm & 204mm newtonians and 102mm refractor with a couple of small Maksutovs thrown in.

I found the 8" dob didn't help me observe significantly more objects however extra details were observed in the objects I can see. Globular clusters for instance resolved more peripheral stars. Open clusters looked brighter and more dense due to resolving some of the fainter stars & and the extra aperture gathering more light overall.

In the end I decided to downsize to a 102mm refractor as my main scope because I discovered I prefer observing with an easily managed mounted telescope while also managing my own expectations!

Aperture is king but personal preference and circumstances trump the generic ideal.

Hopefully that makes sense. I'm slightly distracted watching Liverpool stuff Manchester Utd.🥳🤣

 

They thrashed Newcastle last wk-end in the Carabao cup, comeuppance, no tears shed on Tyneside. 

Dark transparent skies and dark adaptation are king, aperture likely will facilitate some gain in these circumstances, yet a portable set-up within a non light polluted environment can be everything. Getting to such places may require some determination and motivation, stepping out a bit of your comfort zone, yet when in such a circumstance any size aperture telescope will behave way above its anticipated expectation. Then yes regardless, convenience of smaller, manageable portable equipment does by definition gain more use certainly and managing / tailoring expectations, makes each opportunity a rewarding potential.  

 

2 hours ago, Guy65 said:

Thanks - very helpful and totally makes sense. The consensus I'm getting here is that aperture isn't going to be that helpful for me.

From the perspective of my backyard, when I haul out my 14" dob, the difference is quite significant on familiar objects, compared to using my more frequently used 8" dob. Therefore on familiar brighter objects, aperture gain in a light polluted circumstance will enhance observations. However I do not haul the 14" dob outside very often (primary use has been for dark sky trips), the 8" or refractor, more nimble and easier for nipping outside, thus gaining a lot more user time. It is perhaps about making the most regarding where you are and what you have, time out under the stars trying to locate things is still engaging and certainly, an 8" or 10" dob is indeed a versatile option for consideration.  

[Carbon Brush]

Unless I have missed it.....

No mention has been made of blackening a scope tube.
The light pollution enters the tube from all over the sky and bounces off the dark gey tube inner. Don't think so?
Take almost any reflector straight from the shop, shine a dim red torch across the front so some light hits the tube inside
Look in the eyepiece and you see red.
Tube flocking, better black paint, blackening a focus tube, etc. all have their part to play.
A similar blackening argument applies to refactors and compound scopes.
I mention a reflector as it is easy to carry out the torch test.

My uneducated opinion on aperture is that is gives you more light at the eyepiece end of things.
A dim obect stands a better chance of being bright enough for your not-dark-adapted eyes.
Secondly, you can afford to lose some of the 'object of interest' light when filtering out pollution.
Though this does not work very well for full spectrum pollution.