Sky Vs Optics

[lawsio]

This question is born out of curiosity rather than influencing any upcoming purchases, but it's kind of prompted by the wide field scopes discussion happening elsewhere on the forum. 

Basically, is there a way of determining the benefit of taking smaller scopes to darker skies? Can this be quantified and linked to magnitudes of visible objects?

For example, I live in a Bortle 5 area and my main telescope is an 8" Dobs. I live about a 15 minute drive from parks in Bortle 4 areas and an easy overnight camping stay away from Bortle 2 areas. I wouldn't want to have to lug the Skyliner with me everywhere I go so a smaller scope would be needed, but I assume there comes a point where if you go too small you might as well just stay at home with the bigger scope. 

I'm guessing it's not as simple as Bortle 5 + 8" = Bortle 4 + 6" = Bortle 3 + 4" etc, but how to people determine the kit they have is worth travelling to darker skies to use?

 

[DirkSteele]

I cannot provide a quantitative method to determine the benefit, only anecdotal experience. I have taken my 2.4”, 3” and 4” scopes to Bortle 1 and 2 skies around the world and under the best skies(which I call Bortle 1+)  I have seen my 4” out perform my 11” in suburban UK Bortle 6/7 type skies).

 

The impact can be quite startling.

 

[The60mmKid]

It's also worth noting that it depends on what you're looking at. Different objects are impacted differently by light pollution.

[Stu]

I think small but decent quality ie apo scopes can be useful under a variety of skies. They can be surprisingly capable for the Moon, planets and doubles if skies are brighter, and on those larger DSOs if you have better skies. I tend to think that things start to get properly interesting for deep sky at mag 20.5 ish or I guess NELM 5.5 or more. I’ve certainly had some fun with small scopes under these sorts of skies.

[John]

I can't really quantify the impact of darker skies but my personal experience has included taking a 6 inch scope to the SGL star party and observing under dark skies in Herefordshire which, at that time were probably generally around bortle 3 compared to my skies at home which are around bortle 5. Under the darker star party skies, the 6 inch seemed to show deep sky objects at least as well as my 10 inch could from home. It was a noticeable difference lets put it that way 🙂

 

Edited August 30, 2023 by John

[bosun21]

Your 8” will outshine smaller aperture scopes in a bortle 1 sky. When I last was observing in bortle 1 skies in Scotland I had a 4” refractor and 8” dobsonian with me and it was no contest. The 8” was brighter and went far deeper.

[Stu]

2 minutes ago, bosun21 said:

Your 8” will outshine smaller aperture scopes in a bortle 1 sky. When I last was observing in bortle 1 skies in Scotland I had a 4” refractor and 8” dobsonian with me and it was no contest. The 8” was brighter and went far deeper.

That’s always going to be the case, but I guess the point is if you can only take a small scope then is it worth it. My answer would always be yes if it’s a reasonably dark site. When I went to Tanzania on Safari which I guess was Bortle 1, my 66mm refractor seemed to perform like an 8” at home!

[Carbon Brush]

You have to think about the effect of off axis light pollution reducing scope contrast.

Take an average low cost newt reflector, look at the sky. Maybe a dark grey sky background?
Now shine your dimmest red torch across the top of the scope, just catching a bit of the black tube inside.
The sky will appear red.

Now repeat the test with an up-market flocked newt. Or Mak Newt with baffles or a quality baffled refractor.
The sky won't be as grey or red as in the low cost newt.

At the Bortle 1-2 dark site is a 4" properly blackened reflector better than a cheap (dark grey painted) 8" reflector?
I would say no. Because there is negligible off axis light.
Here you are looking for the best optics, coupled with scope portability.

Another factor is your eye dark adaptation.
At your light polluted home, you do not fully dark adapt.
You therefore need a big scope to produce a bright image at the eyepeice.
Go to the dark site and adapt fully. Then a small scope will produce an image that is bright enough for you to use.

No easy answers!

[bosun21]

9 minutes ago, Stu said:

That’s always going to be the case, but I guess the point is if you can only take a small scope then is it worth it. My answer would always be yes if it’s a reasonably dark site. When I went to Tanzania on Safari which I guess was Bortle 1, my 66mm refractor seemed to perform like an 8” at home!

I totally agree with you. On my next excursion to Scotland I’ll only be taking my 4” frac as that’s what I ended up using for a lot of the time I was last there. Plus it’s a lot easier to transport.

[lawsio]

5 hours ago, Carbon Brush said:

You have to think about the effect of off axis light pollution reducing scope contrast.

Take an average low cost newt reflector, look at the sky. Maybe a dark grey sky background?
Now shine your dimmest red torch across the top of the scope, just catching a bit of the black tube inside.
The sky will appear red.

Now repeat the test with an up-market flocked newt. Or Mak Newt with baffles or a quality baffled refractor.
The sky won't be as grey or red as in the low cost newt.

At the Bortle 1-2 dark site is a 4" properly blackened reflector better than a cheap (dark grey painted) 8" reflector?
I would say no. Because there is negligible off axis light.
Here you are looking for the best optics, coupled with scope portability.

Another factor is your eye dark adaptation.
At your light polluted home, you do not fully dark adapt.
You therefore need a big scope to produce a bright image at the eyepeice.
Go to the dark site and adapt fully. Then a small scope will produce an image that is bright enough for you to use.

No easy answers!

Interesting stuff, thanks!

The dark eye adaptation is a good point actually, in my garden I get decent enough skies but there is a street light that you can see from certain angles and every now and then when I've been outside for a while carefully using a red light to sort through my EPs it catches my eye and blinds me for a good few minutes!

 

 

[Guest]

It comes down to what are you willing to transport.  My eight inch easily fits in the back seat of my truck and the base fits in the bed.  I can break down my 16 inch if i wanted to and take it somewhere, but i am too lazy to do that.  It won't be soon but i will be buying a frac in the 4 to 5 inch area as my travel scope.  

[cajen2]

8", Mike? I thought it was 10? Maybe you're just being modest.....😂

[Captain Scarlet]

I used to live in a Bortle 7/8 location, and generally used a 4”-class scope (127 Mak and 105mm frac). It was nice enough but the skies were orange soup.

I live now in a much darker place, Bortle 2/3, and often the only scope I have available at short notice has a mere 5mm aperture. My naked eye.

I vastly prefer the latter over the former.

Cheers, Magnus

Edited August 30, 2023 by Captain Scarlet