January 10, 2019: Telescopes Are Like Shoes – One Scope Can’t Do It All

“Okay, now WTH is he talking about?”  It’s an analogy I use to explain the differences between different scopes, and why there’s no one perfect scope that does it all.  You have different shoes for different purposes:  sneakers, dress shoes, boots, slippers.  Yes, even Crocs.  They each serve their purpose, and you don’t wear one where another one is called for.

Now, you can go play basketball in boots, and you can go to a dance in your sneakers.  But that’s not really ideal, is it?  It is the same with scopes:  different scopes have different purposes.  While you can definitely use a scope for a purpose for which it isn’t designed, it’s not quite going to do the job for you as another scope that actually was designed for that particular purpose.  And more importantly, generally speaking, it’s not going to the job nearly as well.  There is no one scope that truly does it all, although some come close.

This is why many of us have two, three, or even more telescopes.  Like different kinds of shoes, they perform different functions.  By the way, this also makes an excellent analogy when trying to explain/convince your spouse that you “need” a second (or third!) telescope.  Let’s look at a few of these different functions.


The Wide-Field Scope

This is a scope with a short focal length, generally under about 700mm, usually with a correspondingly short focal ratio as well – about f/5.  (My 700mm cutoff is just that – mine, which is relatively arbitrary.)  Such a short focal length gives you lovely wide-field views of the sky.  That means the patch of sky you can see is huge, on the order of 2 1/2 to 4 degrees.  And remember, the area of a circle is pi times the radius squared, so a 4-degree patch is enormous (especially when compared to a more normal 1 to 1 1/2 degree field of view you get with longer focal length scopes).  Four degrees is approaching handheld binocular territory, except that it’s not handheld – it’s rock steady.

When I was just getting back into the hobby 5 years ago (after a 30-year layoff), people were describing to me how viewing the sky at this size is just an entirely different experience from the scale at which we normally see it through a telescope – that 1 to 1 1/2 degree field of view that you usually have when you’re looking at an object.  Well, they sure were right.  When you see objects fully in context with their surroundings, with a nice strip of black surrounding them, it is a paradigm shift in understanding their place in the sky, and it is much more beautiful, to boot.

For me, this is why my Orion ST-80 is such a joy to use.  A simple 32mm Plossl – about $30-40 – gives me almost 4 degrees in it at 12.5x.  My Explore Scientific 68-degree 24mm gives me just a bit more than 4 degrees in it at a rock steady 17x.  It is an absolutely glorious way to view the sky.

However, there are other scopes in this category as well.  There are the 5-inch tabletop dobs, which include the Astronomers Without Borders One Sky scope and the Meade Lightbridge 130.  Both of these are f/5 5.1-inch scopes with a 650mm focal length, which is plenty short enough to show you a nice wide field of view.  A standard 32mm Plossl will get you a very wide 2.4 degrees at 20x in these scopes.

If the idea of a tabletop scope doesn’t appeal to you, the Celestron NexStar 130SLT is a computerized scope in this same category, as it is optically the same.  The 130SLT goes one better than the tabletop dobs because it has a focuser that lets you use two-inch eyepieces for a really wide field.  With an Explore Scientific 82-degree 30mm, you’re back to almost 4 degrees again.

This is why I’ve been drooling over the ES AR 152, a six-inch refractor, for years now.  Yes, yes, at f/6.5, the chromatic aberration will make it seem like I’m doing acid back in the Sixties with Jimi Hendrix – there’ll be purple haze, all through my brain, all right.  I’ve found that for me personally, it’s easy for me to ignore the CA that comes with my ST-80; others have a big problem with it.  If the CA through the AR 152 proves to be overwhelming, well, that’s what semi-apo and minus-violet filters are made for.

But using the AR 152 paired with that ES 30mm 82-degree eyepiece will give me 2 1/2 degrees with six full inches of clear aperture!  Holy cats!  Sure sounds like fun to me!  Imagine moving all around and observing the Milky Way with that rig!


Handheld Binoculars

Going the same way as the wide-field telescope, but even further, are handheld binoculars.  Yes, even though I have three telescopes, I still use my binoculars plenty often.

Obviously, as between a 4-degree field of view with the ST-80 or 130SLT and 2-inch eyepiece, and a 6 or 7-degree field of view with binocs, there will be a significant difference in scale.  Remember that to calculate the area of the sky you’ll be able to see, you use the formula pi-r-squared.  So you’re going from 4 squared = 16 versus 6 or 7 squared = 36 or 49, and that is certainly a big jump.

So again, like the wide-field scope, binocs are for even wider field sky scanning, like going up and down the Milky Way, for picking out open clusters and some globular clusters as well.  Because of its enormous size, M31 (Andromeda) looks great in binoculars.

Beyond the increase in the size of patch of sky you can see, the main difference between a wide-field scope and binocs is their immediacy:  you want to look at something, so you just put the binocs up to your eyes, and three seconds later, you’re looking at it.  Of course, while the great benefit of binocs are that they’re portable, they’re easy to just take out and start using immediately, their drawback is that their magnification is both fixed and limited.  You can’t swap out the eyepieces to change the magnification with binocs like you can with a widefield scope.

Because of this, you’re not going to be seeing that much increased detail on anything with handheld binocs.  (Tripod-mounted binocs are a different story.)  7x, 8x, or 10x just isn’t that big of a magnification jump over the 1x of your own eyes that will enable you to see lots more, and the natural shakiness of your hands (and body) will limit the increase in detail you can see.  You’ll certainly see more detail on the moon, you’ll see Jupiter’s four Galilean moons, you just might be able to make out the thinnest crescent phases of Venus (when Venus is largest).  But you won’t be able to see the rings of Saturn, and all the other planets will look like brightly colored dots.

If you decide to move up to 12x, 15x, even 20x binoculars, you’re going to lose their one main advantage – immediacy.  Any binoculars over 10x have to be mounted on a tripod, which takes away from your being able to quickly grab a pair in your hands, take them outside, and start looking around.  And if you’re mounting them on a tripod, it’s not quite as fast to move around the sky as it is with handheld binocs.  But you do get a deeper view with 15x70s, 20x80s, and 25x100s, and on a tripod or parallelogram mount, that view is going to be nice and steady, so there is a huge increase in the amount of detail you’ll see.

Now, some people absolutely love those big views they get using both eyes with these giant binocs on a tripod or a parallelogram mount.  To each his own.  For me, I’d rather just have my ST-80 to accomplish the same thing.  Although I’ll admit that when my buddy Bill is nearby with his big binocs on a giant mount, it’s not as if I’m gonna say no, y’know?


The Planetary Specialist

At the completely opposite end of the field of view spectrum are scopes with very long focal lengths, and usually high focal ratios to accompany them.  I’m talking especially about Maks here, but also SCTs. Maks start out at 1250mm focal length for an f/13.9 90mm Mak, go up to 1500mm for an f/12 127mm one.  (At least the Synta version of the 5-inch Mak, anyway.  The Meade and ES 127mm Maks are at f/15 and 1900mm.)

They get even longer from there – the 7-inch (180mm) Mak has a whopping 2700mm focal length – whoa!  SCTs also fall into this same category, with an f/10 5″ SCT starting out at 1250mm focal length, the f/10 6″ going to 1500mm, and the f/10 8″ checking in at a cool 2032mm.  My C9.25 has a focal length of 2350mm.

Maks are particularly known as “planet-killers”, both because of their very long native focal lengths and because of their superb optics.  All of this native focal length translates directly into two things:  high magnification, and a relatively smaller – but not necessarily small – field of view.

When viewing planets, which are less than one arc-minute across, field of view becomes relatively unimportant.  The five-inch Mak and the six-inch SCT, both at about 1500mm focal length, have maximum fields of view of just a smidge over 1 degree.  These scopes will get you so close to the moon with enough detail that you’ll feel like you’re in lunar orbit inside Apollo 8.

Viewing the entire moon through one of these planetary specialists is fine, because the moon is about half a degree across.  Even at relatively higher magnifications, you can still fit all, or at least most, of the moon in the same field of view.  (A 10mm 82-degree eyepiece, like the 10mm Luminos, will give you a hair over half a degree at 150x in either the 5-inch Mak or the 6-inch SCT.  This is just enough to see the entire moon all at once, except when it’s at perigee.)

Contrary to popular belief, you don’t actually lose too much with such a “narrow” field of view that a Mak or SCT gives you.  Note the scare quotes there.  Basically, 98% of the brighter DSOs up there will still fit in a field of view of one degree.  Yes, the exceptions are a handful of the most glorious objects up there:  the Andromeda Galaxy, the Pleiades, M6 and M7 in Scorpius, M44 – the Beehive Cluster.  These are better viewed through binoculars anyway . . . or through the widefield scopes I described above.


The Light Bucket

This is where your 10-inch, 12-inch, or even larger dob comes in.  The light bucket’s sole purpose is to collect as much light as possible to let you go as deep as possible.  And if you factor expense into the equation, that means that you’re talking about a giant dobsonian.

You need as much aperture as you can both afford and can effectively carry around to be able to see the faint fuzzies – galaxies, especially, but also nebula and some of the dimmer globular clusters.  You use the light bucket to increase the magnitude that you can see to its maximum, which I explain here.  In my case, my C9.25 serves this purpose as I was unwilling to go back to starhopping with a manually operated dob, and I liked the smaller footprint of the SCT.

Because it is so big – and that’s the entire point of the light bucket – this is not the first choice of telescope to take out when you’re just going to do some casual observing, say for a half-hour or an hour or so.  This is the scope that you might want to put on a wheeled base to make it easier to take in and out.  This is the scope that you might want to build an observatory for because taking it in and out is such a big deal.  This is the scope that you take out, and take out in your car with you, to drive to dark sites, to do some serious observing for a couple/three hours or more.

And by serious observing, I mean just that – dobs are ill-suited for doing astrophotography.  As a general matter, any dob that you buy will be on a regular, unmotorized mount – a lazy susan base.  That means that they can’t track objects in the sky so as to permit you to take exposures of more than a couple of seconds.  For an additional price (natch), you can get buy tracking dobs.  This will permit some limited astrophotography, but the altitude-azimuth mount will limit your exposure times before field rotation creeps in.


The Grab-and-Go

On the opposite end of the spectrum from the light bucket is the grab-and-go scope.  This could very well be the exact same scope as the wide-field scope discussed above, as any small, widefield scope is going to be light and easy to take out.

The grab-and-go, as its name explicitly states, is a scope that’s small enough to not be a hassle to take in and out, but still has enough aperture to show you some good stuff.  Again, as opposed to the light bucket, the grab-and-go is the scope you take out for a quick peek at the moon or planets, and maybe a couple bright clusters or such, before bringing it back in half an hour or an hour later.

Ideal in this department is a four-inch refractor.  The Meade Infinity 102 is my scope of choice here.  However, that’s the one I recommend, as I don’t own one.  On the other hand, I have gained a lot of experience using it at the scope store.  It’s light, it’s compact, it has almost no cooldown time.  Just plop it down outside and you’re ready to start observing.  A four-inch refractor is just enough aperture to let you both collect enough light and see enough detail to keep you satisfied for your quick looks, when you need that astro fix.

However, for me personally, my little 127 Mak serves this grab-and-go purpose very well.  Yes, there is a significant cooldown time, but I avoid this by keeping the tube and tripod all set up together in my outdoor, unheated closet off of my balcony, which, if you must know, is where I took that pic with the shoes.  (Nosey.)  And since you’re generally going to be looking at the moon and planets in a quick observing session like this, why not use a planet-killer to do so?


The Spotting Scope

A spotting scope sure looks like an astronomical telescope, doesn’t it?  It’s usually a high quality, compact refractor, with a 60mm, 80mm, or 100mm aperture.  However, while spotting scopes are ideally suited for terrestrial observing – birds, wildlife, trees and flowers and such – they do not work well on the sky.

The most important reason is because they come equipped with a 45-degree diagonal.  This angle is perfect for viewing things on earth, within a few degrees of the horizon.  However, as you raise a spotting scope up towards the sky, that 45-degree angle makes viewing more and more uncomfortable the higher you go.  This is why astronomical refractors come equipped with 90-degree diagonals – so, no matter how far up you point the scope, you’re still looking down, which is much, much easier on your neck.

Of course, you can still try to use a spotting scope on the sky.  Unfortunately, with very few exceptions, both the diagonal and the zoom eyepiece on a spotting scope are fixed in place and cannot be removed.  The result is that you cannot change either that 45-degree angle or the maximum amount of magnification the spotting scope gives you.


The Astrophotography Scope

Basically, none of the scopes I’ve discussed are very much good for doing AP with.  Yes, you can do AP with them, but this is like wearing high heels to play a dodgeball game.  Using a non-AP rig to do AP will make it harder on yourself than it has to be, and will prove frustrating.

As many of my 9 regular readers already know, I famously don’t do AP.  For me, it’s all about having those photons travelling all those light years and hitting my eyeball directly.  But let’s say you want to do AP – how should you start?

Well, the easiest way to get started is to just hold your smartphone up to the eyepiece and take a few snaps that way.  This can get a little annoying as you have to hold your phone in just the right position so that its camera lens is centered right above the eyepiece.  One small level up from that is to get a smartphone adapter, like this one.  You attach the adapter to your phone and then clamp it onto the eyepiece to make it easier on yourself, so you don’t have to fiddle with the proper alignment of the camera lens on your phone to the eyepiece each time you want to take a photo.

Either way, using a smartphone on a manually operated mount, like a dob or a refractor, you’ll be limiting yourself to some quick pics of the moon and planets.  Not much more than that.  If you’re really a masochist, you can try to go for a longer exposure to get something bright, like the Pleiades or the Orion Nebula.  But again, when you do that, you’re using a sledgehammer in place of a flyswatter.  Not the right tool for the job.

(I have been proven “wrong” on this:  there was a guy on one of the Facebook astro groups who used to post some pretty nice pics of the Orion Nebula that he took through his unguided dob.  However, even he admitted that this was not the right way to go about doing this.)

On an alt-az tracking mount, with either your smartphone, or perhaps with a DSLR with a connector ring and a T-adapter, you’ll be able to take short exposures of around 30 seconds or so, before you start getting field rotation in your exposure.  This is because the alt-az mount is tracking in short stutter steps, up and to the right, up and to the right.  Unfortunately, the stars don’t move across the sky like that.

If you’ve ever seen star trails, you know that instead of moving up and to the right, stars move in great big circles around the North Celestial Pole (or South, in the southern hemisphere, natch), roughly denoted by the position of Polaris in the sky.

startrails_gemini
Star Trails Above Mauna Kea 
Credit & CopyrightPeter Michaud

To understand field rotation, think of Saturn rising in the east with its rings perpendicular to the horizon.  As Saturn moves across the sky, the rings flatten out.  When it reaches its peak altitude in the sky, due south, the rings are now parallel to the horizon.  Then they start tilting the other direction as Saturn sets in the west.  As you take an exposure of a DSO on an alt-az mount, you can only go for so long before the field rotation starts blurring that individual exposure.

You counter field rotation by doing AP on a relatively expensive (a minimum of around $1000) equatorial mount.  Or on a dob with something called a barn door tracker.  Or on an alt-az mount with a wedge.  Any of these will let you take longer exposures than a regular alt-az mount will permit.  Again, even here, the expensive EQ mount is ideal; the barn door and wedge, less so.


Getting a Little More Seriously Into AP

As a general matter, I had thought that to dip your toe into the very pricey pond of AP, you needed to spend that $1000 just to get yourself a decent EQ mount, like the Celestron AVX mount ($899 for the mount alone) – never mind the additional cost of the small scope you’d be putting onto that mount.  However, in working at the scope store, I have learned of an interesting introductory AP alternative to that $1000-minimum EQ rig.  As I mentioned up above, this is the wedge.

se wedge
The Celestron wedge for use with 6/8SE and Evolution mounts, about $300.

Some alt-az telescopes already come equipped with a wedge built in to the mount to raise the telescope to your observing location’s latitude here on earth.  With this wedge comes the ability to track the sky properly on just one axis, exactly like full-blown EQ mounts do.  The Celestron 4SE ($499) and the 5SE ($699) both have this wedge built into their mounts, as does the Meade ETX125 ($699).  Beyond this, the 4SE and ETX 125 also have flip mirrors so that you can have both an eyepiece and your DSLR attached to the scope simultaneously.

5se wedge
The Celestron 5SE with its wedge in action.

So, for less than that initial $1000 expenditure, you can start doing some AP.  Now, it won’t be world-class stuff – your ability to polar align these scopes to a high degree of accuracy is compromised by the fact that these scopes do not come with polar scopes to permit perfect polar alignment.

And again, all three of these scopes have fairly long focal lengths and focal ratios:  1350mm-f/12, 1250mm-f/10, and 1900mm-f/15, respectively.  This makes doing beginning AP with these scopes a bit more challenging than it otherwise would be, in both the scale of objects you’ll be able to capture, as well as the exposure times necessary to capture dimmer objects, objects other than the moon and planets.  But it’s certainly a way to start doing AP before going down the road of buying a hefty – and more expensive – EQ mount.


In summation, always try to use the right tool for the job.  As with shoes, there are plenty of different types of scopes out there, each designed with a particular purpose in mind.  Make sure the one you’re buying will do what you want it to do.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s