10-inch Dob vs 4-inch refractor for galaxies — settled?

I know we do this every year. I am doing it again because I now have numbers instead of opinions, and I would like somebody to tell me why the numbers are wrong.

Setup: my 10-inch f/5 Dobsonian, side by side with a friend's 4-inch apochromatic refractor, same night, same targets, in my backyard in western Sydney. On paper the Dob gathers over six times the light.

On double stars and planets the Dob wins every time, easily, and I am not disputing that.

But on faint galaxies — NGC objects, nothing famous — the 4-inch refractor showed things I flatly could not see in the 10-inch. Not "slightly better." Genuinely there versus genuinely not there. I counted nine galaxies visible in the little refractor against four in mine.

So my question is genuinely a question: if the Dob gathers more light, why do I see fewer galaxies through it?

-- dob_or_bust
western Sydney · 10" f/5 Dobsonian · backyard observing only

Your refractor is not showing you more. Your Dobsonian is showing you your own backyard, which is a different achievement and a smaller one.

Thirty-eight years of this and the answer has not changed: more aperture always wins. You want as much glass as your back and your car will hold. A four-inch lens intercepts a fraction of the light a ten-inch mirror does and no amount of enthusiasm alters the physics of that.

What you have actually demonstrated is that your site is bad. Buy more aperture and stop fiddling with little refractors.

-- old_dob
Newcastle · observing since 1961 · a Dobsonian older than you

You are both right and you are arguing because you are quoting different quantities at each other. This happens every year and it will keep happening until we all stop treating "aperture" as one number that answers every question.

> On double stars and planets the Dob wins every time,
> easily.

Of course it does. A star or a planet is a point source, or close enough — more aperture means more photons collected from that point, full stop, and the sky background around it barely matters because the target is concentrated and bright against it.

Here is the bit that resolves the argument. A galaxy is not a point source — it is faint light spread over an area, and what you are actually detecting is the contrast between that light and the sky glow behind it. In a light-polluted backyard the sky itself is bright. More aperture gathers more light from the galaxy, but it also gathers more light from the glowing sky behind it, in the same proportion. The contrast — the thing your eye actually needs to notice something is there — does not improve nearly as much as the raw light-gathering number suggests, and past a point it barely improves at all.

A narrowband filter does not rescue this for galaxies either. OIII and UHC filters work by passing only the specific wavelengths an emission nebula puts out and blocking the broadband sky glow around them — brilliant for planetary nebulae, useless for a galaxy, which shines across a continuous spectrum with no narrow band to isolate. There is no filter trick here. There is only a darker sky.

Which is what the 4-inch actually had going for it, if it was used somewhere darker, or even just held to a smaller exit pupil that suppresses skyglow more per unit of aperture. Either way: it was never about which telescope has more glass.

-- seeing_hermit
southern Tasmania · 12" Newtonian · keeps a logbook and an argument in equal measure

That is the answer, and it is slightly humbling because I have been posting the nine-versus-four figure for two years as though it proved refractors were better on galaxies. It does not. It proves my backyard is a bright sky.

I borrowed a trailer and took the same 10-inch Dobsonian, the same telescope, out to a friend's paddock two hours west, well away from any town glow. Same galaxies I could not find at home were obvious. Not subtle — obvious. Same aperture, same mirror, same eyepieces. Nothing about the telescope had changed.

Still not sure the refractor would have beaten the Dob out there too, but the backyard-versus-paddock gap alone accounts for most of what I was blaming on the telescope.

-- dob_or_bust
western Sydney · 10" f/5 Dobsonian · backyard observing only

May I point out that a proper report has been trying to tell you this for fifty years and everybody keeps leaving out the middle of it.

"I could see it" or "I couldn't see it" is not a report. At minimum you need the instrument, the sky darkness at the time — naked-eye limiting magnitude is the honest, measurable number — and the seeing conditions. If you report only the eyepiece result you have thrown away the two numbers that would have settled this thread in February.

dob_or_bust's own observation, written properly, is not a contradiction at all. Backyard: NELM 4.2, ten-inch, four galaxies seen. Dark paddock: NELM 6.3, same ten-inch, most of the target list visible. There is no paradox in those two lines and there never was — it only looked like one because the telescope was being blamed on its own. See the reporting sticky.

-- chart_stickler
Perth · 8" Schmidt-Cassegrain · a very large notebook

I have been telling people to buy bigger telescopes since before some of you were born, and I have been telling them for incomplete reasons.

seeing_hermit's paragraph about contrast against sky background is correct and I knew the pieces of it without ever putting them together against my own advice. "More aperture" was excellent guidance in 1961, from a genuinely dark rural sky, with no light-polluted backyards to compare it against. That condition does not hold for dob_or_bust's street in 1999, and I did not stop to check whether the ground had moved under the advice.

I withdraw the blanket "buy more aperture" line. It still holds for double stars, planets, and for anyone actually observing from somewhere dark. It stops being useful advice the moment the limiting factor is what the sky itself is doing.

-- old_dob
Newcastle · observing since 1961 · a Dobsonian older than you

This thread is the most useful thing I have read about telescopes, so let me try to write down what a beginner should take from it, and somebody can correct me.

1. Judge an instrument by what you can actually see, not by aperture alone. Log the number of targets you could actually identify, both ways, on the same evening.
2. Point sources (stars, planets, double stars) reward aperture regardless of sky background. Extended faint targets (galaxies, faint nebulae) reward a dark sky far more than they reward extra glass. Work out which kind of object you are chasing first — that is free.
3. A trip to a genuinely dark site with the telescope you already own is cheaper than a bigger telescope, and it stopped dob_or_bust's Dobsonian from being drowned out by his own backyard's glow.
4. A narrowband filter is not a fix for a light-polluted sky on galaxies. It only helps emission-line objects, and it is worth knowing the difference before buying one.
5. Report your sky darkness (NELM) and seeing, not just what you saw, so the next person does not have to reconstruct your evening from one eyepiece impression.

I am under a reasonably dark suburban sky myself, so on that reading I should save for aperture rather than a trip out of town. Thank you all — including for the part where two people changed their minds in public, which I did not expect from a thread with this title.

-- nightfader
Adelaide · 8" Newtonian · binoculars for the wide stuff