Archive for the ‘Aperture’ Category


From sub-aperture mask to replacement dust cap

February 23, 2017


Here’s something dumb. The Bresser AR102S Comet Edition is optimized for two things: widefield, low-power scanning, and portability. At 20″ for the OTA it’s just within the bounds of airline carry-on-ability, but you can unscrew the dewshield and shave off another 4″, at which point the options for storage and transport expand wildly.

BUT the stock dust cap for the objective is dome-shaped, for no good or obvious reason, which means it sticks out about a full centimeter longer than necessary. When you’re thinking about flying with a scope, that is one centimeter more stupidity than you should have to put up with.

There’s another problem with the stock dust cap: when the scope gets cold, it gets loose and falls out easily. Nothing unique to this scope about that – I’ve had to shim the majority of my scopes’ dust caps for the same problem, including the C80ED and XT10. One cheap package of sticky-back green felt has kept me going since 2010. I think I’ve used almost a third of it.

Now, I already have a nice 60mm sub-aperture mask for this scope (construction details here). If I could plug the central hole securely, I’d have a replacement dust cap that would be shorter, would get tighter rather than looser if it shrunk in the cold, and would serve double-duty as both a dust cap and a sub-aperture mask. The problem was finding a plug the right size, with a good lip on it to keep dust out, that would grab the edges of the mask hole securely.


And it’s the dollar store to the rescue again, with this container of Tootsie Rolls that is intended to double as a coin bank. The hard plastic lid snaps down into the cardboard tube very securely, and the plug bit is just a shade over 60mm in diameter.


I used the Dremel and some sandpaper to enlarge the hole in the sub-aperture mask ever so slightly, and voila. There’s a small lip that runs around the top edge, and even a little recess in which to hook a finger and pull out the plug.


Here you can see the ridges on the plug. By sanding in short increments, I was able to fine-tune the hole diameter until the plug snapped in very securely, without stressing either piece. I need to put some tape or a little epoxy or something over the perforated slot, which is intended to be punched out so the candy container can become a coin bank. Or cut out the center and replace it with another, smaller plug, so I’d have a dust cap and two aperture masks in one package…


Boom. Now the scope is a centimeter shorter for travel, and I don’t have to keep the sub-aperture mask in my eyepiece case.

What I really want is for someone with even rudimentary 3D modeling skills to create a series of nested aperture masks, like Russian dolls, in 10 or 20mm increments, which could be 3D printed on demand in whatever combinations people needed. Most of them could be standard sizes, with only the outermost adapter for each telescope model needing to be custom. Then you could order the adapter for your scope and whatever set of nested masks you wanted, or maybe all of them to simplify, so your 100mm scope could also be an 80mm, a 60mm, a 40mm, and even a 20mm (the “Galileo model”) if you liked, just by taking out the relevant bits from the dust cap. Sure, it would be gross overkill for most people, but for those of us who like playing “what if” (“what if my C80ED was a C40ED?”) it would be a godsend. And with 3D printing no-one would be stuck with a bunch of useless stock when the idea inevitably bombed.

Anyway, if someone would to that, it would save me the trouble of building my own “Mask-ryoshka” dust cap out of junk from the dollar store. But if I’m being totally honest, avoiding building my own stuff out of junk from the dollar store was never the point of the exercise, was it?*

* With apologies to Adam Savage.

Why and how to make a sub-aperture mask for a refractor

February 11, 2017


Here’s the Bresser Messier AR102S Comet Edition with a homemade aperture mask. I just converted the scope from a 102mm f/4.5 to a 60mm f/7.7.

“WAT!? You took a refractor, the most aperture-challenged of the three basic telescope designs, and made it even smaller?”

Yup. For several reasons.

The first and most obvious is to control chromatic aberration (CA), also known as false color. Despite the name ‘achromat’, which literally means ‘no color’, doublet refractors without extra-low dispersion (ED) glass do show some false color, because their lenses do not bring all of the colors of light to the same focus point (they’re still a LOT better than scopes with a singlet objective lens, like those used by Galileo). For dim objects like galaxies, nebulae, and most field stars, the effect is not noticeable, even in large and optically fast scopes like the AR102S Comet Edition (nickname needed). But bright objects like the moon, planets, and first magnitude stars will be surrounded by purplish halos, and may have yellowish margins. In effect, the purple and yellow-orange parts of the spectrum are forming out-of-focus images that are superimposed on the main in-focus image.

The problem is that CA gets bad fast as refractors get bigger. There are a couple of standards that are commonly used to describe the focal ratio necessary to minimize CA to acceptable levels, the Conrady standard and the Sidgwick standard. By the Conrady standard, the focal ratio must be 5 times the aperture in inches; by the less stringent Sidgwick standard, 3 times the aperture in inches is good enough. Note that the standards describe focal ratios, not focal lengths, so they go up fast with increasing aperture. Here are some apertures, focal ratios, and focal lengths required to meet the Sidgwick standard:

  • 50mm (2″) : f/6 : 300mm
  • 76mm (3″) : f/9 : 684mm
  • 102mm (4″) : f/12 : 1224mm
  • 127mm (5″) : f/15 : 1905mm
  • 152mm (6″) : f/18 : 2736mm

This, along with mounting considerations, explains why reflectors and catadioptric scopes are progressively more common past 4″ in aperture. A 6″, f/8 Newtonian will be free of false color (as are all reflectors) and has such a gently converging light cone that it is easy to collimate and to focus – it’s easy for such scopes to achieve ‘planet-killer’ status if the mirror is good. A 6″, f/8 achromat will be a beast to mount and it will show lurid false color on bright objects.

But people still make, buy, and use such scopes! Why? Horses for courses: big, fast achromats can be superb deep-sky scopes, where chromatic aberration is typically not a problem. With the fixed sizes of standard eyepieces, achieving wide true fields requires short focal lengths (not just short focal ratios), and bright images require aperture, which drives the development of large but optically fast scopes like the AR102S Comet Edition. At f/4.5, it is well into ghastly CA territory on bright targets. The other night I stayed up late to catch Jupiter, and in the AR102S the planet wouldn’t even come to a clean focus. It was just a bright ball of light inside a sea of purple. I switched over to London’s 60mm f/11 Meade refractor and Jupiter snapped into a sharp and essentially color-free focus. There was a moon emerging from behind the limb of planet, already one moon-diameter out into black space, that was completely invisible in the CA-smudged view of the AR102S.

I’m okay with that – as I noted in a previous post, observing bright solar system targets with the AR102S is deliberate misuse of the scope. When I want good planetary views, I have a 5″ Mak and a 10″ Dob that can both be pushed to 500x (assuming the atmosphere is steady enough). But their max fields of view are pathetic compared to the AR102S – about 1.1 degrees for the Mak, and a shade over 2 degrees for the Dob, versus 3.6 degrees for the refractor, which is enough to take in all of Orion’s sword at once, with space left over on either side.

Still, I’m not going to take all of my scopes out with me every time I go observing, and chances are good that at some point I’ll want to look at something bright even if my main goal for the evening was low-power sweeping with the AR102S. Under those circumstances, it’s easier to have an aperture mask shoved in my eyepiece case than to pack a second scope. Hence this project and this post.

But I’m getting ahead of myself. There are other reasons to stop down a scope besides reducing CA:

  • To reduce glare from bright objects. Mostly applies to the moon when it’s full or very gibbous.
  • To give a more aesthetically pleasing image when the seeing is bad. Opinions differ on this point. Some folks prefer to look through a larger aperture despite the increased susceptibility to bad seeing, on the grounds that in the moments when the atmosphere does settle down a bit, you’ll see more detail. I suppose it depends on whether one is in exploration mode or aesthetic observation mode.
  • To make it easier to focus. F/4.5 is a steep light cone, and it’s easy to overshoot the point of best focus. Stopping down the scope makes a shallower light cone, so it’s easier to watch the image transition from out of focus, to near focus, to in focus. I’m going to test this method of finding best focus on some close double stars.

I had done some calculations in advance to figure out what sizes of aperture masks I’d want to try out. Given that the AR102S has a fixed focal length of 459mm, here are the focal ratios at full aperture and at 10mm decrements:

  • 102mm gives 459/102 = f/4.5
  • 90mm gives 459/90 = f/5.1
  • 80mm gives 459/80 = f/5.7
  • 70mm gives 459/70 = f/6.5
  • 60mm gives 459/60 = f/7.7
  • 50mm gives 459/50 = f/9.2
  • 40mm gives 459/40 = f/11.5


I didn’t want to trade away too much resolving power, so I tested the scope on the moon using cardboard masks of 76mm and 60mm, made from the light cardboard spacers from a box of wet cat food. The 76mm is shown above. Perhaps unsurprisingly, at this aperture and focal ratio (f/6) the view was still unappealingly soft. But 60mm looked good, with minimal CA. This makes sense – the working focal ratio of f/7.7 is a healthy step beyond the f/7.2 that the Sidgwick standard suggests for a 60mm aperture. Going any smaller would be trading away valuable resolution, without significantly improving the image.


The light cardboard aperture masks were fast and easy to make, but they weren’t very sturdy. To make a more permanent mask, I needed plastic, heavier cardboard, or foam-core board. So I unscrewed the dewshield from the scope and walked down to the dollar store, where I looked for food packages and storage containers that might fit. Finally on the last aisle I found this 1-gallon plastic jar. The lid slip-fit over the dewshield with just a bit of extra room, which I knew I could shim out with some sticky-back felt.


I wanted to make sure the lid would fit before I did the hard work of cutting, so I put the felt on first. This was very familiar – it seems like every other scope I get has a loose dust cover that has to be shimmed to fit correctly. I’ve been slowly chipping away at the same package of sticky-back felt since 2010. I didn’t have a compass handy, so I used a small paper ruler to make a ring of marks around concentric 60mm circle inside the lid. Then found a lid to a jar of vitamins that was exactly 60mm in diameter and used that to trace the circle neatly.


I was going to cut out the aperture using hobby knife, but the plastic was too tough. So I moved up to a box knife, and then a linoleum knife. Then I said heck with it and got the Dremel. The hole I cut wasn’t perfectly circular and had rough edges to boot, so I wrapped some sandpaper around a pill bottle to make a tool for rounding out the aperture.


Here’s the scope before…


…and after.

Even with the aperture mask, the AR102S is not a champion scope on solar system targets. The C80ED blows it away, which makes sense – it has a 33% resolution advantage over the stopped-down AR102S, and frankly just better glass. But at least the view now is clean and not appallingly degraded. A dramatic way to see the difference is to get a good tight focus on the moon with the mask on, then quickly take it off without removing one’s eye from the eyepiece, and watch the view get a lot brighter and a lot softer at the same time.

I have a few more things I want to do. The 60mm aperture mask fits over the end of the scope so securely that it could work as a dust cover, if only I can find or make something to plug the central hole. Also, I think I am going to play with making aperture masks in other sizes, just to see what happens.

And finally, I have another 4″ scope that will be fun to make an aperture mask for. But that will be a subject for another post.


Some noodling about my dream scope

September 1, 2012

Fair warning: this post is just me thinking out loud about my dream scope. If you’d rather read about the stars, good on ya–there are plenty of other posts here about that stuff, that will be more interesting than this extended episode of navel-gazing about gear. Feel free to skip backward or–hopefully soon–forward.

As I’ve mentioned here a few times before, eventually I want to have a bigger scope, something in the 14″-18″ range. There are four boundaries that define it:

1. It has to be enough of a gain over my current scope to be worth the expense. Some people say that small gains in aperture are not worth it, that you won’t notice enough of an improvement to make it worthwhile. I have looked through 8″ and 10″ scopes in quick succession, and 10″ and 12″ scopes in quick succession, and in both cases the gain in light gathering and resolution was immediately noticeable at the eyepiece. But it’s the “to make it worthwhile” part that’s the kicker. As we saw in What Aperture Costs, above 10″ prices increase sharply. As long as I’m saving up for my dream scope, I might as well save a little longer and get the wow factor instead of settling for a scope I’ll want to trade up from before long. To really get the wow factor from a bigger scope, most people recommend doubling your light grasp–which for me means going from 10″ to 14″ (78.5 to 154 in^2)–or going one magnitude deeper, a factor of 2.5, which for me means going to 16″ (200 in^2).

2. It has to be small enough to fit in my current vehicle or any foreseeable future vehicle not specifically purchased for hauling around big telescopes. That means it has to be collapsible. But it has to be collapsible anyway, because I’ve had a solid-tube 12″ and didn’t keep it. In that case, the gain over the XT10 was noticeable but not worth it. And it still has to fit in a regular car. My friend Ron has a minivan that he bought specifically for hauling around his 22″, but I will probably never be in the position to base my vehicular purchases around my telescopes.

3. The pieces have to be light enough that I can set it up by myself. My friend Jeff has a collapsible 16″–it’s the scope we took on the LCROSS impact watch–but the mirror box is so damn heavy it takes both of us to get it up into the back of his pickup. I’d prefer a max weight for each piece under 75 lbs, and under 50 would be better still. If those sound like light loads for a healthy 6’2″ dude, go move big scopes around for a while. It’s like moving furniture–awkward weight with a center of gravity far from your body that makes the load on your back a lot worse than when you’re pumping iron. My XT10 weighs 55 lbs assembled. I can move it around in one piece if I have to, but I usually feel it in the morning. A 55-lb chunk of a bigger scope would probably be smaller, less awkward, and hurt my back less.

4. I have to be able to afford it. More specifically, I’d like to be able to afford it with no more than a year or three of saving up. Maybe someday I’ll save for a decade or two and get a custom-made ultralight 25″ that packs into the back of a compact car (such things do exist), but that would be my last scope, not my next scope. For me, right now, given the disposable income I can afford to dedicate to astronomy, the one-to-a-few year saving duration means a one-to-a-few thousand dollar budget. (If that sounds low, hey, congrats, feel free to buy me a scope. I promise to use it for outreach! If it sounds high, go price ATVs or boats or campers or any of the really high-end grown-up toys.)

Those conditions give me a range of options to think about while I’m saving up.

For a long time, my dream-scope ideal was a T-Scope, a custom 14″ truss-tube dob with a low rocker box, starting at $3195. Pros: light, 65 lbs total and heaviest single component is 35 lbs; very high quality, very compact when disassembled. Cons: among the pricier options I’m looking at, cost does not include shipping from New York state (not a jab against T-Scopes, almost no-one has free shipping on scopes like these, it’s just one more thing I have to think about). UPDATE: another con is this negative customer experience reported on CN. I’m going to try to find out more about it–stay tuned.

These days I’m thinking more and more about DobStuff. Dennis Steele makes big scopes that  look awesome, weigh next to nothing, and cost surprisingly little for ultralight custom scopes. A 14″ weighs 70 lbs assembled, heaviest single component is 30 lbs, and goes for $2195. A 16″ would weigh about 90 lbs assembled, heaviest single component 45 lbs, for $3495. Apparently there is a price jump from 14″ to 16″ optics, which explains why the 16″ costs so much more than the 14″. Anyway, super-cool scopes that are pretty much exactly what I’m looking for. One CN member says his 16″ DobStuff has a footprint of 24″x24″ and sits in the back seat of his car when collapsed for travel. I need something like that.

Turning to mass-produced scopes, there’s the Orion XX14i, a 14″ semi-truss dob, starting at $1899. That’s a lot of scope for not a lot of dough, especially considering it comes with digital setting circles (i.e., non-motorized “push-to” object locator). And I could drive to someplace that actually has them in stock and save on shipping. Downsides: compared to the other scopes I’m considering, it’s a pig. I call it a semi-truss dob because although it has trusses connecting the ends of the tube, and they do allow it to break down into smaller pieces, they don’t actually lighten the scope. Orion’s 12″ truss dob weighs just as much as the solid-tube version. The assembled weight is 120 lbs, and the heaviest single component is 55 lbs, as much as my XT10 and almost as much as an entire T-Scope. Also, there’s no way to get the scope without the digital setting circles, and it irks me to know that I’d be paying a few hundred more for a feature I’d happily do without. Finally, there have been some quality control issues; at least one Cloudy Nights user got an optical dud and Orion did not replace it, which is the first time I’ve ever heard of that happening. In fairness, Orion has apparently improved the quality of the optics shipping with the newer XX14s, so maybe–hopefully–the optical disappointments are all in the past.

I discovered as I was writing this post that Orion has just introduced a 16″ semi-truss scope, the XX16g. Apparently it’s just like the XX14i but more so: more weight (195 lbs assembled), more cost ($3599), and more paying through the nose for stuff I don’t need–unlike the XX12 and XX14, which can be ordered in the “i” push-to versions or the “g” go-to versions, the XX16 is so far only available with go-to. So I’d be paying even more money for even more stuff I’d happily do without. I’m sure go-to is nice and if I had it I’d get addicted. My objections to go-to basically fall into three categories: (1) I spend at least half of each day working at a computer. I go stargazing to get away from all that. (2) At any given cost, adding electronics means taking away aperture. Given my limited budget, I prefer to buy aperture, for which there is no substitute, rather than electronics, which don’t do anything I can’t do myself with a star atlas and some elbow grease.* And (3) like all electronics, all go-to systems eventually fail. This is why Uncle Rod recommends buying CATs on equatorial mounts instead of fork mounts–EQ-mounted tubes are a lot easier to remount when the motorized mount craps out. I should say that these are my personal reasons for not wanting go-to for myself. If you have, love, or want go-to, that’s cool–may a thousand gardens grow. No need to sell me on it; it’s just not my scene, man.

* John Dobson says that dobsonian telescopes are held together by gravity and powered by yogurt (you eat the yogurt, and push the scope around with your muscles). Preach it, Brother Dobson.**

** That’s funny, see, because Dobson actually was a monk (before he got kicked out for doing too much sidewalk astronomy).

Meade’s 16″ LightBridge is a contender. At about $2000 it costs only a shade more than the XX14, but delivers a third again as much light. Another way to look at it is that it delivers the same light grasp as the XX16g for a little over half as much dough. The weight is high, but no worse than the XX14i: 128 lbs total, and heaviest single component is 58 lbs. Downsides? From everything I’ve read, the LightBridge series do not  quite match the comparable Orion scopes on build quality. They seem to be “work in progress” scopes. This is especially true of the 16″–I’ve heard of lots of people who have rebuilt the base, which is apparently too wobbly for such a heavy scope, and Dennis Steele at DobStuff offers replacement base kits for just this purpose. When there’s a thriving aftermarket to fix the problems with a telescope as delivered, that’s a problem. I’m saving up for my dream scope, not a project scope. But it’s a lot of scope for two grand; even budgeting an addition $395 for a replacement base, it’s a solid deal.

And it’s an affordable way to lay one’s hands on a set of 16″ optics. I realized something odd the other night. A 16″ DobStuff is $3495, but a DobStuff makeover, where you supply the optics, is only $895 for a 16″, plus another $150 for the Easy Transport Telescope option. So it’s actually about $400 cheaper to buy a 16″ LightBridge ($2000) and have it made over ($1045, for $3040 total) than to buy a 16″ DobStuff straight up. About the only risk I can see is the possibility of variable quality control in the LightBridge optics, although from what I’ve read the baseline quality is quite good and I haven’t read any horror stories.

Two options I haven’t discussed are building my own and buying used. Regarding building my own, see comments above about dream scope vs project scope. And although I am normally a big fan of buying and selling used telescopes, I am a little leery in the case of my dream scope. If this is either going to be my last scope or my last scope for many years, I want to get exactly the right thing. I’ve already taken one poorly-considered leap into larger aperture and regretted it. I don’t want to make that mistake again.

And yet…a used scope could just be a delivery mechanism for big optics. The other day someone on CN was selling a used 16″ LightBridge for $1500, and I have seen them go for even less. That plus a DobStuff makeover could be a faster, cheaper track to my dream scope than going all-new. It’s something to think about, anyway–I’m sure I’ll have plenty of time to consider, and reconsider,  and rereconsider, etc., over the next few months and probably years.


A quick observation about aperture

August 22, 2012

I had an epiphany last night. The busier I am, the less often I get out to observe, the more likely I am to use the XT10. The rationale: if I’m only getting out once or twice a month, I want to enjoy myself as much as possible, and that means using the biggest available scope. The only exceptions are (1) truly quick peeks, under half an hour, where I’ll usually grab the 90mm Mak, (2) sidewalk astronomy, where I use the 90mm Mak just because it’s the most portable, and (3) double star work, where I prefer the slightly cleaner, sharper views provided by the Maks. Any other small-scope observing is a luxury, for stretches when I’m already getting plenty of dark-sky time with a bigger scope–not my current situation, unfortunately. But fall is coming, and that’s traditionally my season for getting out to the desert on dark weekends. I can’t wait.

Maybe this would not even be worthy of comment for most observers, but I’ve always had a thing for small scopes. I still do, and probably always will. But now that I’m sort of settling into my current lineup and not especially wanting or needing another (small) scope, I’m spending more time thinking about the scopes I have and their strengths and weaknesses. Although every one of my current scopes fills a need, I am finding that the overall enjoyment they provide correlates with aperture more than anything else. It’s one thing to hear that aperture rules, another thing to really understand why, and still another thing to discover that for yourself, out of your own experience.


Why aperture matters

August 15, 2012

Most people who have gotten as far as even doing research on buying a telescope can probably rattle off the two benefits of aperture: greater diameter allows increased angular resolution–the ability to resolve fine details–and greater collecting area simply gathers more light. But what do these advantages really mean? Just in the past month I’ve read a couple of explanations that gave me new ways to think about this, and that I think need to be more widely read.

The first comes from Richard Panek’s fine little book, Seeing and Believing: How the Telescope Opened Our Eyes and Minds to the Heavens (page 111):

Double the diameter of the aperture, and its light-gathering capacity increases fourfold; triple it, and the capacity goes up ninefold. At the same time the brightness of the object under observation depends on the square of the distance. Double the distance of a light source, and its brightness decreases fourfold; triple it, and the brightness drops off ninefold. The implication was clear: Double the diameter of the aperture, and you double the distance the mirror can see. Herschel understood that for the purpose of investigating the starry depths the major advantage of a reflecting telescope wasn’t in greater magnification, but in gathering more light–wasn’t in seeing more detail, but in seeing farther.

Okay, cool stuff. I knew that light grasp increases proportional to the square of the diameter, and that brightness drops off proportional to the square of the distance, but I’d never thought to put those two thoughts together.

Still, deep sky observers usually start with the bright stuff, like the Messier objects, and with sufficiently dark skies even the Herschel 400 are within reach of a 2-inch scope. Given that you’re already looking out more than 60 million light years to see the most distant Messiers, does seeing father really help?

There’s another piece to this, and it’s one I’d never considered until I read a very clear explanation of it by Ed Moreno (CN username Eddgie) on Cloudy Nights. Here’s the link to the CN post, but it’s so good I’m just going to copy and paste the whole thing here:

Make no mistake here. If you want to see more structure or detail in any kind of extended target such as nebula, galaxies, or planets, there is absolutely no substitute for clear aperture.

Almost all of the design discussions totally omit the function of image brightness/image scale that is offered by a larger aperture. We tend to discuss equipment like we don’t have eyeballs.

But we do have eyeballs… Or Cameras.

The image brightness and the ability to trade it for image scale is a huge part of the value of a larger aperture.

Keep in mind that a 16″ telescope and a 4″ telescope can both show the Orion Nebula with the exact same brigntness. Is all one has to do is use an eyepeice in each scope that has the same exit pupil.

For example, if I use a 16″ telescope at 100x, this will give about a 4mm exit pupil for the observer. In a 4″ scope, to get the same exit pupil (same image brightness), you will have to use 25x.

Now in the 4″ scope, the image is exactly as bright as in the 16″ scope, but the scale is only 1/4th as large.

Now is where it gets interesting. The structure inside the nebula is often very low contrast and very fine.

It is often stated on these forums that the human eye can resolve down to one arc minuted. This actually greatly overstates the eye performance of the scotopic eye for low contrast detail. This figure (one arc minute) is only achievable when the target is well illuminated and the contrast is 100%.

If the eye is in scotopic mode, and the contrast is very low, the eye may struggle to resolve detail that is smaller than about 3 or 4 arc minutes of apparent field unless the contrast is very high or the target is well illuminated.

There are two ways to increase the likelihood of the human eye (or camera) being able to resolve this very low contrast detail. They are to A: Make it bigger by magnifying it more, or B: make it brighter. Of course if you can make it slightly bigger and slightly brighter, this greatly enhances your ability to resolve fine, low contrast structure present in the extended target.

Now, lets take our 16″ scope and our 4″ scope and use the 16″ scope with our 4mm exit pupil. Remember, the illumination (determined by the exit pupil) is the same, but the angular size of any detail in the target will be four times as large. Detail within the target that does not have the angular magnification to be resolved in the smaller scope is now presented as four times as large. This means that there is now a lot more detail that has been magnified enough to be seen in the much bigger scope. Of course I can make the detail larger in the smaller telescope but this comes at the expense of loosing illumination (smaller exit pupil) which makes it harder for the eye to see the detail.

The other option is to make the image bigger and brighter in the bigger scope. Suppose I use the bigger scope with a 6mm exit pupil. Now, the exit pupil being larger gives me a brighter image. Once again, the eye likes illumination as one of the two ways to improve the ability to detect detail. When I make the image brighter in the larger scope, I see the extended target as extending over more area. The added brightness expands the extent of a nebula or galaxy. And guess what…. With the 6mm exit pupil in the 400mm scope, the magnification is 66x, so every detail present is still presented at almost 2.5 times the image scale as it would be in the 4″ aperture!!!!!! So now I have an image that is both bigger AND brighter in the larger aperture. I don’t care what kind of scope this is… It can be a refractor, a reflector, or a compound scope. If I make the image bigger and brighter, even if the contrast of the detail is exactly the same, my eye will have a better chance of seeing that detail if I make it bigger or brighter, or both at the same time.

Again, make no mistake about it… For seeing the most detail in extended targets of all types, clear aperture is king. You cannot isolate the telescope from the detector, and when the human eye is the detector, with a larger aperture you usually get both increased scale AND better illumination of the target and all of the structure present in the target.

I have owned perhaps 40 telescopes in the last 10 years, and for all classes of extended targets I would rank them in terms of performance in almost 100% agreement with the amount of clear aperture the scopes provided. I personally have never compared two telescope (even of different types) that the scope with the most clear aperture didn’t equal or exceed the performance of the smaller clear aperture.

There is no substitute for aperture. That is why the Hubble is a 2.4 meter instrument and not a 4″ refractor.

Aperture gives you image scale and illumination. It doesn’t even matter how you get that aperture (design). It is only important to have a lot of it.

Big telescopes simply show more. At the end of the day, if you want to see more structure in all class of extended targets, or if you want to see these extended targets at their maximum size, the best way to do this is to use as much aperture as you can manage.

The problem with refractors is that to get any meaningful amount of clear aperure (I recommend 8″ to 10″ for seeing a lot of structure in most deep sky extended targets) the expense becomes impractical as does the physical size of the instrument.

Everyone gets to use what they like, but the physics of image formation give clear aperture the best image formation, and lots of aperture gives the human eye the best chance of seeing any existing detail that might be present in the target.

There really isn’t any substitute for clear aperture. The more you apply to almost any target in the sky, the more structure or detail you will be able to see.

Now, I had experienced this, but I hadn’t thought about it–in fact, hadn’t known about it–in terms of brightness and image scale. And now that I have,  I agree with Ed completely.

I like small scopes. A lot. Maybe more than is healthy. But I will readily admit that my XT10 blows away all of my smaller scopes, on almost every target. Maks are often described as being fine planetary scopes, and they are. My 5″ Mak rocks on planets. But the XT10 just annihilates it on max detail. Even on the moon–I will put one of the smaller scopes on the moon and think I’m getting a good view, and then I put the XT10 on the moon and BAM! It’s the same field of view, but not the same view–suddenly everything is just bursting with details that I couldn’t see before. And now I know why–it’s not just angular resolution and not just light grasp, but rather the ability to get an image that is bigger, brighter, and more detailed, all at the same time.

Fortunately, I didn’t buy the Mak under the illusion that it would outperform the dob–I bought it because it fits in a space only a little bigger than a gallon milk carton, for those times when I don’t have room for anything bigger. And there is in fact one class of targets where I strongly prefer the Maks, and that is double and multiple stars. Oh, there’s no doubt that the XT10 will split closer doubles, period, and split wider doubles more easily, but I have not (so far) been going after doubles that are at the limit of resolution of a 10″ scope. And for things the Maks can split, they provide a more pleasing image than the dob, with no diffraction spikes. There is something entrancing about seeing a just-barely-split star as two clean little balls of light separated by the thinnest of black lines, surrounded by nice concentric diffraction rings but with no other visual noise to clutter the view.

So, dobs are cheap, but people end up buying other scopes for other reasons, and probably the two most important of those reasons are portability and aesthetic “cleanness” of view. Those are, in fact, the very reason that I own scopes other than dobs. BUT if you are interested in the deep sky–or pretty much anything else up there–and you’re on a budget, then clearly it is in your best interest to get as much aperture as you can afford. So this post is a deliberate follow-up to the last one; that one showed which scopes provide the most bang for the buck, and this one explains why you want as much bang as possible.

All of this is just part of an extended run-up to the Suburban Messier Project. I’m not ready to get started on that, not yet. I’m too busy with teaching, and my skies are lousy right now anyway. But I am thinking about the SMP, and when and how to best get started, and what information you may want or need if you’d like to tackle the project with me.

Until next time, clear skies!


What aperture costs

August 14, 2012

Just for the heck of it, I decided to find out which telescopes are the best deals in terms of light grasp. My comparison group consists of widely available commercial dobsonian reflectors ranging from 3″ to 16″ in aperture. Sometimes I included two models at the same aperture to show the effect of differing features, like the finder scope and better eyepieces on the Orion Funscope versus the Celestron Firstscope, or the collapsing truss-tube design on the Meade Lightbridge scopes. Prices are street, not list, as of this writing. Each entry follows this layout:

Brand Model – mirror diameter (mm) – mirror area (in^2) – price – cost per in^2

Celestron Firstscope – 76mm – 7.065 in^2 – $38 – $5.38/in^2

Orion Funscope – 76mm – 7.065 in^2 – $50 – $7.08/in^2

Orion SkyScanner – 100mm – 12.56 in^2 – $100 – $7.96/in^2

Orion StarBlast 4.5 – 114mm – 15.89 in^2 – $200 – $12.59/in^2

Bushnell Ares 5 – 127mm – 19.63 in^2 – $165 – $8.41/in^2

Orion StarBlast 6 – 150mm – 28.26 in^2 – $280 – $9.91/in^2

Orion XT6 – 150mm – 28.26 in^2 – $280 – $9.91/in^2

Orion XT8 – 200mm – 50.24 in^2 – $350 – $6.97/in^2

Orion XT10 – 250mm – 78.5 in^2 – $580 – $7.39/in^2

Meade 10″ Lightbridge – 250mm – 78.5 in^2 – $700 – $8.92/in^2

Meade 12″ Lightbridge – 300mm – 113 in^2 – $1000 – $8.85/in^2

Orion XT12i – 300mm – 113 in^2 – $1100 – $9.73/in^2

Orion XX14i – 350mm – 154 in^2 – $1700 – $11.04/in^2

Meade 16″ Lightbridge – 400mm – 200 in^2 – $2000 – $10.00/in^2

Interestingly, there are two low points where the price dips below eight bucks per square inch: at the low end, with the 3″ scopes, and in the middle, with the 8″ and 10″ solid-tube dobs. It’s also interesting to note that the StarBlast 4.5, which is an extremely popular scope, is the most expensive in this group in terms of cost per square inch.

I only included dobs because everything else is more expensive–tripod-mounted Newtonians, catadioptrics, and refractors alike. Here are some comparative costs for beginner instruments of those other designs; for fairness, I only picked models with mounts included.

Orion SpaceProbe 3 Alt-Az (reflector) – 76mm – 7.065 in^2 – $100 – $14.15/in^2

Orion GoScope 80 (refractor) – 80mm – 7.74 in^2 – $100 – $12.92/in^2

Orion StarMax 90 Tabletop (catadioptric) – 90mm – 9.85 in^2 – $200 – $20.30/in^2

The difference here is that there is no mid-aperture dip as there is for the dobs, or if there is, it doesn’t bring the price per square inch near the $10 mark, let alone under it. Indeed, one would be hard-pressed to find a new, high quality 4″ achromatic refractor for less than $300-400, which is up in the neighborhood of $30/in^2. Similar prices obtain for mounted 5″ Maks and 8″ SCTs. That’s why I didn’t bother to account for the effect of the central obstructions in calculating the costs of the reflectors and catadioptrics; even what seem to be large secondary obstructions are usually less than 10% of the total collecting area, and dobs cost anywhere from a half to a fifth as much as other popular, “everyman’s” scopes in terms of collecting area.

Now,  I’m not saying that dobs are objectively superior to other scope designs. They’re cheaper. We all knew that, I’ve just quantified it, snapshot style, using currently available models and prices. I did it because I had a gut feeling that 8″ and 10″ solid-tube dobs were in sort of a sweet spot, price-wise, and that actual costs (per square inch of collecting area) rose a bit on either side. And they do.

Finally, these numbers put some classic deals into perspective. When the SkyWatcher 130N was on sale for $100, its cost was just a hair over five bucks per square inch–better than any of the models listed above by a considerable margin. The SkyWatcher 70AR was at one point selling for $36 shipped, or just a hair rover six bucks–a bit more expensive, per square inch, than the Celestron Firstscope, but for a much more capable instrument. I think the  only refractor deal in history that has ever equalled that was the Galileoscope. With 50mm of aperture and an introductory price of $15, it was originally selling for $4.78/in^2, but that was without a mount. Similarly, the value of buying used is now apparent–when I got my XT10 for $350, that was a cost per square inch of $4.46, much less than any of the new scopes, even the cheapies.

What should you buy? That’s a more complicated question, and it can only be answered by reference to your situation and your observing goals. I own a mid-sized Mak because sometimes it’s nice to have 5″ of light grasp in a package a foot long, and I bought it new because sometimes it’s nice to have something fresh off the line. But if you’re interested in deep-sky work, where every photon counts, and you’re on a budget, then you might find the numbers above useful in considering which scope will give you the most bang for your buck, or for evaluating future scopes.

Why you want as much aperture as you can get is the subject of the next post–and for a contrary view, see this post.