December 4, 2014: History of My Astronomizing, Part 2 – Getting the Mak

The Mak has a very different light path than a Newtonian.  In a Newtonian, light enters from the top of the tube, reflects off of a primary parabolic mirror at the bottom of the tube, concentrating the light to a single point.  Before it gets to that single point, the light reflects off of a flat secondary mirror at the top of the tube that’s placed at a 45-degree angle to reflect the light 90 degrees.  This second reflection directs the light towards an eyepiece that’s also at the top of the tube, on the side, where you can observe the object.  Focusing is done by some kind of rack and pinion mechanism that holds and moves the eyepiece in and out.  Typically, most tubes for Newtonian scopes between 6 and 12 inches in aperture are almost 4 1/2 feet long, which makes them both a bit unwieldy to move around, as well as taking up a lot of room in a Manhattan apartment.

Enter the Maksutov.  Light again enters the top of the tube, but there at the top is a thick spherical glass meniscus.  As the light passes through, the meniscus spreads it out somewhat as it travel down to the bottom of the tube, where it reflects off of the primary spherical concave mirror there.  Just as in the Newt, the primary mirror again concentrates the light to a single point at the top of the tube, but again, before reaching that singularity point, the light encounters another spherical mirror.  This is actually an aluminized convex spot on the bottom of the meniscus, and not a separate structure at all.

This is a description of the Gregory-Maksutov design, which is used in the NexStar 127SLT; there is another type of Maksutov design called a Rumak-Maksutov where instead of using an aluminized spot, there actually is a separate spherical mirror at that point that is attached to the bottom of the meniscus.  Because the secondary mirror is separate from the meniscus in the Rumak design, the convexity of the secondary can be adjusted to be a different shape from the meniscus, giving the telescope designer an additional degree of freedom in constructing the light path.  Because the focal length of the Mak is very long, they generally use this freedom to change the shape of the secondary to shorten up the focal length a bit.

In both designs, the convex shape of the mirror spreads the light out so that it does not immediately come to a focus, but only does come to a focus at the bottom of the tube.  At that point there is a diagonal mirror that reflects the light 90 degrees into the eyepiece, where you can view the object. Focusing in a Maksutov is done by actually moving the primary mirror up and down on a track inside the tube; this is controlled by a focus knob outside of the tube.

One of the benefits to this Maksutov design is that because the eyepiece is at the bottom of the tube, the place where you view the object doesn’t vary in height all that much whether the object is close to the horizon or at the zenith – just a few inches.  This is because the back of the tube doesn’t move very far, no matter how high or low it’s pointed, because the short Mak tube is mounted on the side in the middle.  Since the NexStar 127SLT tube is only about 15 inches long, this means that the back of the tube is only eight inches from the pivot point.

Conversely, in a Newtonian, this height difference can be a couple of feet because the tube is over 4 feet long; being further away from the pivot point, the front end of the tube moves much more.  The result of this is that with the Mak that you can easily sit down and observe, which makes a long observing session very comfortable indeed.  And being comfortable means you stay at the eyepiece longer and see more.   This is important because most observing isn’t limited by anything inherent in the scope itself (the aperture, the magnification, the quality of the optics), but by the atmosphere.

Stars twinkle because the atmosphere is roiling with air that’s at different temperatures at different heights.  This is the same principle that causes you to be able to see shimmering heat waves rising off of a hot roadway in summer.  Unfortunately, this turbulence in the atmosphere is always present, and this affects the “seeing” – the ability to observe an object clearly so as to be able to discern detail.  This is why you have to observe an object, as opposed to merely looking at it.  As you observe it for a few minutes, you will eventually get moments of perfect seeing through the atmosphere where additional details will pop out at you.  And this is where the Maksutov design is great, because with the nice short tube, you can just sit at the back and observe away.

Another benefit of the Maksutov design is that for whatever reason – which I have not been able to fully divine – it does not generally need to be collimated, and if it does, it can be sent back to the factory to have it done there, at least during the warranty period.  This has partially to do with the fact that the meniscus is mounted to the front of the scope, the same way a refractor’s objective lenses would be, as well as because the secondary mirror is fixed to the bottom of the meniscus, and therefore, cannot be adjusted.  It’s also because the Mak is built solid, like a little tank.  But when it comes down to it, I really don’t know why the primary mirror stays in such excellent alignment with the meniscus, especially when Maksutovs are focused by moving the primary mirror back and forth along a track on the inside of the tube.

Finally, one of the great benefits to a Maksutov is that because all the optical surfaces are spherical, they can be made excellently easier than the parabolic mirror surface of a Newtonian can.  In other words, it’s easier to make a great Mak than a great Newt.  Or at least, so I’ve read.  This doesn’t quite make sense to me, because there are four optical surfaces in a Mak: the front and back of the meniscus, and the primary and secondary mirrors.  Since the secondary silvered spot on the back of the meniscus takes whatever shape the meniscus is, this really means that there are really only three optical surfaces, versus the two in a Newtonian:  the primary and secondary mirrors.

In any case, this excellent optical quality, combined with its long focal length, gives the Maksutov the ability to really go to high powers without “stressing” the optical design.  I routinely use a 4mm Plossl in the scope on the moon to get to 385x.  Although the focus does definitely starts to go soft at this high of a power, I can still see detail on the moon’s surface with great clarity.

This is particularly amazing when the rule of thumb is that the theoretical maximum magnification a scope should achieve is two times the aperture of the scope in millimeters, or 127 x 2 = 254x.  So getting a usable view at 385x is just insane!  I’ve never looked through any telescope with magnification that high and seen anything at all.  Even though the theoretical maximum means that no additional detail will be seen once you go beyond that level of magnification, it is still useful to magnify beyond this because the tiny details become larger, and therefore easier to see and examine.

On the other hand, one of the very distinct disadvantages to a Mak design, shared by the design of a Schmidt-Cassegrain telescope (SCT) is that because they have such a long focal length, they have a somewhat limited field of view.  This means that a few of the larger astronomical objects cannot be seen all at once in one view through the eyepiece.  For example, the Apertura AD8 I discussed in yesterday’s post, has an aperture of 8 inches and a focal length of 1200mm.  Because you can use 2-inch eyepieces in the AD8, it has a nice-sized field of view of almost two-and-a-half degrees.  The 5-inch Mak, with an even longer focal length of 1540mm, has a tight field of view of just over one degree.

This smallish field of view isn’t very important for two reasons.  First, because only about a half-dozen or so of the very largest objects will not be able to be seen in one view:  the Andromeda Galaxy, the Pleiades, the Veil Nebula.  The list is fairly small.  And more this isn’t very important to me because I’ll never see most of these big galaxies and nebula anyway from the light-polluted city.  I have never truly seen any of these objects very well due to light pollution; they all look the same to me, and once you’ve seen one grayish dim fuzzball, you’ve seen them all.  The only nebula that I have seen with any detail in it is the astounding Great Orion Nebula, which has tendrils and filaments extending from it.  It is a sight to behold.

The second reason the small field of view doesn’t matter is because of the motorized computer goto system.  Without such a system, having such a small field of view would be a distinct disadvantage, because the way to find something in the sky without such an aid is by starhopping.  This is where you start at a known point, such as a bright star, and then move the scope in little hops along the way to known waypoints – dimmer stars – until you get in the area of the object that you want to see.  You then scan the area by moving the scope and scanning the area.  With a one-degree field of view, this would be extraordinarily difficult.  However, the goto system puts you on target almost all the time, so that this limitation just isn’t necessary.  And since almost all of the other objects in the sky are significantly smaller than one degree, this narrow field of view problem becomes meaningless.

Another supposed short-coming of a Mak is an inordinately longer cool-down period, due to the closed tube design and the thickness of the meniscus.  This is more fable than anything else.  All telescopes require cool-down periods to become acclimated to the ambient temperature of the outside air.  A five-inch Mak requires no longer to cooldown than an 8-inch or 10-inch dobsonian does, about 30 to 45 minutes depending on the temperature differential.  Larger Maks take longer, as do large dobs.  But in both cases, this period can be greatly diminished with the use of cooling fans.

After reading up extensively about the Mak on websites like Astronomy Forum and Cloudy Nights, I decided the Mak was the scope for me.  No collimation, portable, high magnifications, excellent optics – described as “Apo-like,” referring to the very top-of-the-line refractor possible.  Although I am unemployed, I had a $500 gift card sitting at Amazon, so the scope would kinda sorta be free!  I ordered it from Amazon and received it two days later.

Next up:  upgrading the Mak!

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