August 11, 2018: Don’t Buy A 10-inch Dob Over An 8 . . . Yet! Magnitudes Explained

No, no, this isn’t yet another anti-dob rant; longtime readers will know that I actually do like dobs.  No, this is more of a heads-up as to the differences between these two popular sizes of dobs, the 8 and the 10, and why, unless you’re a gym rat, you might not want to buy a 10-inch dob instead of an 8.  (Note the word “might” there.)  There are two main differences between a 10-inch and an 8.  Obviously, because of the increased aperture, you’re getting two things:  more weight and more light.

Let’s first take a look at the weight differences between two popular lines of 8 and 10-inch dobs.  Orion’s XT8, probably the most popular 8-inch dob out there, is 41 lbs., which breaks down almost exactly evenly between the tube and the base at about 20 1/2 lbs. each.  This relatively light weight is just another reason why the 8-inch dob is right at that sweet spot for beginners of practically any age.  The XT10 is a total of 53.4 lbs., with the tube being 50% heavier than the XT8 at about 31 lbs., and the remaining 22 1/2 lbs. in the base.  Orion’s Intelliscope versions of each of these apertures weigh basically the same, as do Skywatcher’s dobs.

The Apertura AD8, sold by High Point Scientific (which is the same model as the Orion Skyline 8″, and both of which are built in the same factory in Taiwan called GSO), is probably the best of the mass-marketed line of dobs out there, in my opinion.  The AD8 comes in at 52.2 lbs. total, with 24 1/2 in the tube and 27 1/2 in the base.  The AD10 bumps that up to 66.2 lbs.; about 35 in the tube, 31 in the base.  Being your standard weakling nerd, which most of us in astronomy tend to be, and speaking on behalf of all my fellow weakling nerds, I find that things start to get really heavy once you get over 30 lbs.  Heavy to the point of not actually using the scope as much as you would otherwise want to.

Picture this.  You’ve just come home from a long, hard day at work or school.  The sky is finally clear for the first time in weeks, and the moon isn’t out to light pollute your view and wash everything out.  You’re absolutely itching to get the scope out.  But instead, in your exhausted state, you look over at that huge, heavy, water heater-sized dob sitting in the corner, and just sigh.  Instead of going out and doing some observing like you really want to, you crack open a beer, ease back into the couch, and watch some more Seinfeld reruns.  Again.  Ah, well; another night of observing lost.

The best scope is the one you use.  The 8-inch dob provides the perfect balance between large aperture with portability.  The 10-inch?  Not quite as much, depending on whether 30 pounds is a lot for you to carry out to your observing spot.  Before buying the scope, if you want to see how heavy that tube is, and use the same muscles to carry it, try this test.  Get a nice big sturdy shipping box, a box from something you ordered online.  Get 4 one-gallon milk jugs (save ’em up as you drink ’em), and a 1/2 gallon jug, too.  Fill up all the jugs with water.  A gallon of water weighs just over 8 lbs.

For the XT8, put 2 gallon jugs and the 1/2 gallon jug in the box; for the AD8, put 3 gallons in there.  Then carry it by cradling it – arms underneath the box, box at your chest.  Now repeat the test and put all 4 gallon jugs (and the 1/2 gallon jug if you’re considering the AD10) into the box – that’s the weight of the 10-inch tube, and that’s how you’d be carrying it, too.  Carry that box from your house out to your observing site.  Do this one cloudy night after getting off a long day at work.

Even if you lift, bro, and you’re okay with the extra weight, there’s still the matter of aperture itself.  There isn’t a whole lotta difference in light gathering capacity between the 8 and the 10, even though it seems like there should be, especially given the approximately two hundred dollar difference in price between 8-inch dobs and 10-inchers.  To explain this, let’s discuss magnitude.

The brightness of everything in the sky is measured in magnitudes.  It’s a reverse scale, so lower numbers – even negative numbers – are brighter.  The brightest stars are 1st magnitude, even zeroth (0th) magnitude, like Vega, which is defined as being magnitude 0.0 (it’s actually 0.03).  The brightest stars are Sirius (-1.47) and Canopus (-0.74) at negative 1st magnitude.  (The system wasn’t completely well thought out when it was introduced by the ancient Greeks a couple thousand years ago.)

The magnitude system is defined so that a difference of five magnitudes is precisely a 100 times difference in brightness.  Brightness is measured logarithmically, just like sound is. So, if you cover up one ear, you’re hearing 50% less; if you close one eye, you’re seeing 50% less light.  Does it sound that way?  Does it look that way?  No.

This is because the brain doesn’t interpret sound or light linearly.  For example, 10% more light simply is not detectable by the human eye/brain combination; well, not unless you’re like a trained pro doing a side-by-side test, or something.  At 15% more (or less) light, you can see the difference, but only if you’re really trying.

The dimmest stars you can see with the naked eye are around 6th magnitude – and that’s under perfectly dark skies (far, far away from any light pollution), with good, sharp eyesight (if your vision isn’t sharp, the dim light from a 6th magnitude star will blur into the background), good night vision (eat your carrots), good dark adaptation (up to half an hour without looking at any white light sources), as well as good, steady, stable, still, and dry and clear atmospheric conditions (no dust, no wildfires, and relatively low humidity).

When you take a photometer = a light meter, and you actually measure the light given off by a 1st magnitude star – like Antares or Spica, both at 1.0 – and you compare that to the light given off by a 6th magnitude star, the difference is 100 times more light.  A difference between stars of 1st magnitude and 2nd magnitude, like Polaris, is 2.512 times more brightness.  Because the brightness increases logarithmically, 2 magnitudes difference, which is the difference between Vega (0.0) and Polaris (2.0), is 2.512 squared = Vega is 6.3 times brighter.

When you look up at the summer night sky, you can see Vega and Polaris at the same time.  Vega is obviously brighter, but does it look over 6 times as bright?  In decently dark skies, 5th magnitude stars are not too hard to see.  Does Vega really seem to be 100 times brighter than the dimmest stars you can see?  No; again, because our eye/brain combination just doesn’t detect light linearly that way.

3 magnitudes is 2.512 cubed = 15.8 times brighter.  That’s the difference between Sirius at -1.46 and Adhara, the bright star in Canis Major forming the dog’s rear leg, at 1.50.

Obviously, that’s Sirius up top.  If you imagine Canis Major as a balloon dog, Adhara is the dog’s rear leg.  The camera exaggerates the brightness difference; take a look this winter and compare these two with your eyes.  Copyright:  ©2008 Akira Fujii/David Malin Images.

4 magnitudes difference = 39.8x brigher; that’s the difference between Vega and Alcor (4.0).  Alcor is the rider of the “horse and rider” pair double star, Mizar and Alcor, in the handle of the Big Dipper.  Finally, as I mentioned, 5 magnitudes difference is 100 times brighter.

So, all this talk about magnitudes is very interesting, but what does that have to do with the difference between 8-inch and 10-inch dobs?  Telescopes do two things: they magnify what is invisible to the naked eye to allow us to see more detail; and they collect more light than the human eye can because of their greater aperture.  An 8-inch scope, under those nice dark skies I described above, with an observer who’s got good eyesight and all, will let you see down to 13.5 magnitude.

A 10-inch scope will let you see down to 14.0.

That’s just half a magnitude difference.  Regardless of how much light pollution there is in the skies you observe under, a 10-inch scope will always go half a magnitude deeper than an 8-inch in terms of the dimmest stars it’ll show you.  This isn’t quite as true in terms of nebula and galaxies, where light pollution washes them out and the additional aperture won’t help you.

To understand the difference in brightness that half a magnitude is, think about the constellation Gemini. You’ve got Castor and Pollux right next to each other as the heads of the twins.  Does one look hugely brighter to you than the other?

The Constellation Gemini.  Castor (1.6) is more centered and on the top than Pollux (1.1), which is a bit lower and to the left.  That’s open cluster M35 at their feet.

That’s why, when you’re comparing one scope to another, you’ve gotta go at least 50% larger in aperture. You’ve gotta compare an 8-inch to a 12-inch. The 12-inch will let you get down to 14.4, and that’s almost an entire magnitude deeper. That really is a significant difference.

A one magnitude difference at the eyepiece will let you see a ton more dimmer objects that the smaller aperture won’t let you see, as I’ll discuss below.  More importantly, with an additional magnitude, the brighter objects you could already see with the smaller aperture will look better, more filled in.

Which is not to say that I’m recommending you get a 12-inch scope, either, because that thing is heavy.  The AD12 weighs 86 lbs., with 48 of that in the tube alone.  That’s almost 6 gallons of milk in that box.  Or, think of it another way – those big blue jugs that sit on the top of water coolers are 5 gallons; about 42 pounds.  Ever lift one of those?  Remember how heavy it was?

You get that same “almost one magnitude deeper” (it’s actually about nine-tenths of a magnitude) whenever you move up in aperture by 50%, not just when you’re going from 8 to 12.  So, going from a 4-inch (102mm) to a 6-inch (154mm) scope will also get you that jump up.  Going from a 5-inch (130mm) to an 8 (203mm) will get you a full magnitude deeper because it’s just a shade more than a 50% increase.

Doubling your aperture gets you 1.5 additional magnitudes, and that’s a very big jump.  That’s what I did when I went from my 5-inch Mak, which operates at 120mm because of an unfortunately undersized primary mirror, to my C9.25 at 235mm.  You get this same effect of 1.5 magnitudes going from a 4 to an 8, a 5 to a 10, etc.

In any case, going deeper like this has an exponential effect on what you’re able to see.  Going one magnitude deeper allows you to see almost THREE HUNDRED PERCENT more stars.  Yes, you read that correctly.  Let’s walk through that.

In Manhattan, the light-pollution limited me to just 3rd magnitude, and that was only after midnight, after people stopped driving, turned off their house lights, and went to sleep.  That’s just 283 stars total.  Of course, half of those stars haven’t risen yet.  Also, some percentage of that 283 you can never see because they’re too low down in the southern hemisphere and never rise above your southern horizon.  So, at any one time, when I was observing from my rooftop in Manhattan, there were less than 100 stars in the sky.  But let’s ignore that diminishment for now and stick with the total numbers.

If you are observing from a light-polluted suburban environment where you can only see stars visually down to 4th magnitude, there are about 900 stars total that you can see.  (Magnitudes are measured from the half magnitude above to the half magnitude below, so 4th mag is actually mag 3.50 to mag 4.49.  Fourth mag skies really means 4.49 mag skies, not 4.00 skies.)

If you drive to a dark site a few miles out into the boonies that lets you get to 5th magnitude visually (actually 5.49), there are now 2800 stars total you can see.  If you drive way, waaaaaay out of town, to the darkest of dark sites, where 6th magnitude (6.49) is possible using just your eyes, that number increases to an incredible 8800 stars.  This represents just about the absolute limits of human vision from dark sites here on earth, although some have claimed to see 7th mag visually from waaaaay out in the middle of nowhere.  I, of course, hate those people.

Theoretically, using very small 8×20 binoculars under 5th mag skies, you’ll get to 7th mag and 26,500 stars; 7x35s under those same 5th mag skies gets you a little past 8th mag and to 77,600 stars; and 10x50s under 5th mag skies gets you to 9th mag and 217,700 stars.  Under pristine 6th mag skies, a 5-inch (130mm) scope will theoretically allow you to see down to 12th mag and 5.3 million stars.  An 8-inch goes down another full magnitude past the 5-incher and theoretically lets you see 15.4 million stars at 13th mag.

Note that word “theoretically“, though.  The magnitudes provided are under optically perfect conditions all the way around.  So, this means:

  1.  You’re observing from a truly dark site that lets you get all the way down to 6th magnitude, no humidity or water vapor, and no dust or other particulates in the air (smog, wildfires) to diminish that;
  2.  Your optics are collimated perfectly, so that all the light is getting to your eyeball, and there’s no stray light entering the tube from off to the side of what you’re observing to diminish contrast;
  3.  There are no scratches or dust on any of your optical surfaces that would diminish or scatter light; in other words your optics are practically pristine;
  4.  Your eyes are in excellent shape – no astigmatism or other visual aberrations (other than near- or far-sightedness; the scope acts as your glasses in those two circumstances), you don’t have cataracts or yellowing of your eye lens, your pupils dilate nice and wide; you have all the rods you’re supposed to have on your retina to detect light under low-light conditions, meaning, you’ve eaten all your carrots so that your night vision is good;
  5.  Your eyes are fully dark-adapted – which can take up to a full half hour, or more, depending on whether there is any ambient white light near you that keeps you from getting all the way there.

In my experience, even if you are observing from a 6th mag dark site, you should reduce the maximum theoretical magnitude as to the dimmest objects that you can see with binocs or a scope by anywhere between a half and a full magnitude.  This is based on the severity of the other four factors beyond the first one:  maybe your optics are collimated well, but not perfectly; your optics aren’t as clean as when they left the factory, or maybe you scratched ’em up a bit the last time you cleaned ’em (oops); your aging eyes just ain’t what they used to be, etc.  Others have disagreed with me that the reduction is as severe as this.

The biggest diminishment comes from light pollution.  If you start off observing from light-polluted skies, the telescope will only add the same amount onto what you’re able to see as it would add above 6th magnitude.  In other words, if you’ve got an 8-inch scope under perfectly dark, 6th mag skies (like waaaay out in the desert or deep into farm country), the 8-inch scope gives you an extra 7 mags to get you to 13.5 mag.  But if you’re observing from a decent dark site that’s only 5th mag visually (remember, that’s actually 5.49), that extra 7 mags only gets you to 12.5.  If you’re observing from the suburbs where you’re under 4th mag skies, then the 8-inch will only let you see down to 11.5.  And then, you need to subtract an extra something based on those other factors, so you’re down to around 11 or perhaps even 10.5.

It’s important to note that all of this discussion so far has been about what mag stars you’ll be able to see with various apertures.  This is because stars are a point source of light:  no matter how much or little magnification you use on a star, a star will always be a point source, a dot through your eyepiece.  The extra 7 magnitudes difference between what your eyes see visually and what an 8-inch dob will show you applies only to point sources.

Point sources are stars, double stars, open clusters, and to some extent, globular clusters.  The first three are obviously made up of just stars, so increasing your aperture or getting out to darker skies will let you see more of them.  Globs are also made up of stars, but many of the stars are so dim, 12th, 13th, 14th magnitude, that they form a sort of haze, a combined glow.  And it is exactly that kind of glow that gets swallowed up by the skyglow of light pollution.

And this is why an increase in aperture won’t always let you see more in the way of galaxies and nebula if you’re observing from light-polluted skies.  This is because of the concept of integrated magnitude, which I discussed at length in this blog post.  The tl;dr version is that integrated magnitude is the magnitude an object would be if you shrank all the light that an extended object is emitting down to a single point source – exactly like a star.  “Extended object” means anything that isn’t a star – anything that’s larger than a single point source of light, which includes, of course, nebulae, galaxies, and globular clusters.

The magnitude that’s given for these extended objects is “misleading” precisely because the object is extended – it’s not a point source, like a star.  My 5-inch Mak should be able to get down to 12th magnitude at the DAS dark site.  Even subtracting a bit for the reasons I’ve indicated above, 11th magnitude should be absolutely no problem.  Yet even something as bright as the Triangulum Galaxy (M33), which has an integrated magnitude of 5.7 is a difficult catch – even out there – because that magnitude is spread out from a point source to almost a full square degree.

Triangulum has a notoriously low surface brightness.  Under light-polluted skies, something like Triangulum just fades into the background glow and becomes invisible; additional aperture doesn’t always help to pull an object like this out.

The aphorism we astronomers like to roll out is, “The best scope is the one you use.” As discussed above, an 8-inch is significantly lighter, more portable than a 10-inch.  The other aphorism is, “Get the largest scope you can afford.”  You will hear people saying to get the 10 over the 8.  I can’t exactly argue with that – but there’s a fine balance between the two aphorisms.  The aphorism is really, “Get the largest scope you can both afford and move around easily.” I don’t think the heavier 10-inch falls into that category for a lot of people.

The 8 can be a lifetime scope for many people. Even if (when) people move on from an 8-inch scope because they catch aperture fever and move onto 12, 14, or 16-inch scopes, they keep their 8-inch because the 8-inch is the Goldilocks size – it’s not too big, not too small; it’s just right.

It’s not for me because I’m already deep into middle-age, I’ve always been a deskbound nerd, I never work out, I’m outta shape, and as weak as I am now, I want a scope to last me for the rest of my life, not something I’ll use as a gym substitute.  That’s why I bought my C9.25 at 21 lbs.  That’s about the largest aperture I can comfortably move around.  The C11 comes in at 28 lbs., which I figured was just a bit too much for me – especially to lift up to chest height and maneuver around to get it up on the mount.  It was also close to the payload limit of my Orion Sirius Pro AZ/EQ-G mount.

Plus, and this was really the key consideration for a cheapo like me, the C11 OTA is about $600 more than the C9.25, but only goes just over a third of a magnitude deeper (0.37, to be exact).  That’s about the difference in brightness between Rigel and Betelgeuse.  It didn’t seem to be nearly worth it for me to pay so much and get so little in return.

“Orion Head to Toe”, my absolute all-time favorite astrophoto ever.  Of course, Betelgeuse is the bright orange star at bottom left; Rigel is the bright bluish-white one at top right.  Credit:  Rogelio Bernal Andreo

But, but, but . . .

Maybe you’re young, or maybe you’re strong, or maybe you go to the gym and work out . . . or are about to start.  If so, great, the 10 could be for you.  I don’t want to dissuade anyone from getting a 10-inch scope if it’s the right fit for them.  Just here to edumacate, people!


8 thoughts on “August 11, 2018: Don’t Buy A 10-inch Dob Over An 8 . . . Yet! Magnitudes Explained

  1. Jon..informative writing ..thx

    For sure , as we age, our eyes change chemically and that detracts from our ability to decern contrast.

    I probably did 350 of 400 HI list with my 8” Dob. Probably could have done almost all but I happened to be where a bigger Dob was available to,use.

    As we have talked …the way to increase your scope performance is with EAA. As 8” performs like a 20”..even with a modest video camera and the newest sensors. For.the price of a high end eye piece you are taken into the really faint fuzzy world

    C U at Okie Tex

    Liked by 2 people

  2. I have found increasing magnification can also increase contrast. I was looking at ngc 7662 with a 12 inch dob, focal length 1500 mm and a 5.5mm Meade Ultrawide 82° , only to find a multi armed spiral just behind the field of view.


    1. Yup, this is true. My understanding is that the magnification spreads out the background light pollution so that the contrast differences become more apparent.


  3. >>That’s the difference between Sirius at -1.46 and Shaula, the bright star in Canis Major forming the dog’s rear leg, at 1.50.

    Great article. Not to nitpick, but Shaula is one of the two stars in the stinger of Scorpius. Adhara is in Canis Major.


    1. Whoops! I got it right in the photo caption, but not in the body of the article. Fixed, and thanks for pointing out my errors, I appreciate it. Where the hell did I get the name Shaula from? I wasn’t even looking at Scorpius.


  4. I have a buddy down in Texas who recently graduated from an 8″ to a 12″ Dob, both from Zhumell, both simple basic Newtonian tubes…no fancy telescoping truss poles. He couldn’t be happier, loves the increased aperture. His solution to moving the big stovepipe is a wheeled platform. His best viewing spot is in his driveway, he just rolls it out of the garage and plugs in an eyepiece.

    Liked by 1 person

    1. See? This is why they invented the wheel. Just to move scopes around. 🙂

      Yeah, this is part of the solution for bigger scopes like that. But, but, but . . .

      Obviously, it only really works if the place you keep the scope and the place you’ve gotta get the scope to, to observe, are both relatively on the same level. If not, well . . .

      But heck yeah, the wheelbarrow method is the way to go.


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