by Will Sager
It’s that time of year again. The Halloween candy has been consumed, the turkey is gone, and the mall is playing Christmas carols (endlessly). Once again many people will think that purchasing a telescope as a Christmas gift for a budding astronomer would be the perfect thing. If you are in this group of well-meaning people, we are going to have a tough conversation here and my goal is to strip away the gauzy rose-colored filter from your blurred vision. There are myriad ways to mess this up and plenty of retailers who would love to take your money. But do not despair, I know many amateur astronomers in this very organization whose love of astronomy started in just this way (myself included)
An example of a high power telescope ad from 1952. Criterion actually made some good telescopes in the 1960s and 1970s, but this was probably not one of them.
I have started and restarted this article several times because it rapidly bogs down in telescope details. What makes a good telescope? How do you recognize one that is not? Let me start by repurposing an old saying: there are good telescopes and there are cheap telescopes, but there are no good cheap telescopes. Of course, this depends on your definition of “good” and “cheap”. Start this debate at a star party and it will provide amusement for hours. The motivation here is that nobody (except cheap telescope sellers) wants you to buy a “hobby killer”: a telescope so frustrating that it kills your neophyte’s interest before it can bloom. There are so many variables in this quest that it is difficult to give foolproof suggestions. I will try to give some perspectives based on many years of looking through telescopes, but your mileage may vary.
A Primer on Telescope Types
Before we can discuss telescopes, we need an understanding of telescope types. There are three broad categories: refractor, reflector, and catadioptric. All telescopes use an objective lens (or mirror) and an eyepiece lens. The objective gathers light and focuses it into an image. The eyepiece is a secondary lens (usually several lenses) that takes the objective image, enlarges it, and projects it into your eye. A refractor has an objective lens in the front that collects and focuses light rearward. A reflector telescope uses an objective mirror, located at the back of the scope, and focuses the light forward. In a Newtonian design, a secondary mirror redirects the light out the side where the eyepiece is located. A catadioptric telescope (“cat”) also uses an objective mirror, but usually has a figured secondary mirror and perhaps a “corrector plate” at the front. Often the light beam is sent to an eyepiece on the rear of the scope.
Simplified diagrams of telescope types. From the Abrams Planetarium web site: https://www.abramsplanetarium.org/telescopes/
All objectives are defined by two measurements: diameter and focal length. The diameter controls how much light is gathered. By doubling the objective diameter, say 100 mm (3.9 in) to 200 mm (7.9 in), the amount of light gathered increases by 4 times. Focal length is the distance from the objective to the point at which the light is focused. If you take a lens or mirror and train it on a light bulb then put a sheet of paper at the focus, you will see a little image of the light bulb. The eyepiece also has a focal length, which is the distance from the center of the lenses to the point at which it is in focus.
Two more important numbers are f-ratio and magnification. The f-ratio is the focal length divided by the objective diameter. For example, a 152 mm (6 in.) objective with a 1219 mm (48 in.) focal length is f/8 (48/6 = 8). The lower the f-ratio, the brighter the image. Magnification (aka power) is the enlargement of the image. For example, 50x enlarges the image by 50 times. Magnification is calculated by dividing the focal length of the objective by the focal length of the eyepiece. On that 48 inch focal length scope, a 25 mm (1 in) focal length eyepiece produces 48x (=48/1). Obviously, the higher the magnification, the larger the image appears in your eye. But magnification also makes the image dimmer.
Since we are talking here about Christmas scopes and probably looking for the cheaper options, look again at the scope diagrams. The reflector has only one glass surface (the mirror) that must be precisely figured. The secondary is a flat mirror. This makes this type the least expensive to manufacture. The refractor objective lens is really not one lens as pictured because glass does not refract all wavelengths of light the same. Usually, the simplest objective is an achromat, which has two paired lenses together. With an achromat, there are four glass surfaces to be figured and they must be assembled into one paired unit. Better refractors have 3 lenses in the assembly. This makes refractors the most expensive of the telescopes. Cats have a primary, objective mirror and a secondary mirror and sometimes a corrector plate. They are more expensive than simple reflectors, but not as much as refractors.
One final point about telescopes. The image is usually upside-down and sometimes also backwards. If you see this, your telescope is not broken. It is just optics. Look at the telescope diagrams and follow the straight lines that indicate the incoming light. Notice how on all telescope designs, the light that comes in on one side ends up coming out the eyepiece on the other side. To get correct images, as in binoculars, you need special prisms to twist the image back to normal.
A Bit About Mounts
All telescopes need a mount that allows the observer to move it easily to objects in the sky. Although there are many variations, most fall into two categories: alt-az and equatorial. Alt-az is short for altitude-azimuth, which describes the motions of this particular mount. It has two perpendicular axes, one that turns horizontally (azimuth) and another that turns the scope vertically (altitude). The equatorial mount also has two perpendicular axes. One (right ascension or RA) is pointed at the North Star (Polaris) to coincide with the Earth’s rotation axis. When aligned, the mount can follow the stars simply by turning on the RA axis. The other axis is declination and moves the scope either towards or away from the celestial equator. Because the alt-az mount is simpler (and therefore cheaper to make), it is the one most often found on inexpensive scopes. Although the equatorial mount makes tracking the moving stars easier, it must be aligned to Polaris (at least approximately) to work properly.
Caveat Emptor
A big problem with the Christmas telescope is that inexperienced observers are prone to common misconceptions that all telescope manufacturers will exploit, but none worse than the unscrupulous cheap telescope makers. Do you see that fantastic photo of a nebula on the box? You won’t see that, even with a light bucket (a large reflector telescope). Human eyes can’t integrate light for extended periods like cameras can. You must adjust your expectations. That nebula may be well be visible in your scope, but you have to know how, when and where to look. If you live in the city, you will need to take your scope away from the light pollution to see anything dimmer than the Sun (with a proper filter!!!), Moon, or bright planets. Even in a dark place, you will have to be satisfied with seeing a ghostly smudge that has traveled hundreds or thousands of light years to your eyeball. What is more, a cheap telescope will probably be the least likely to give you a good image. So don’t let unscrupulous sellers distort your expectations. Those same sellers will also try to catch you with POWER! They make unreasonable claims about how powerful their telescope is (see the section “Power Mad”).
Where to Start?
There are so many pitfalls that it is hard to give you foolproof instructions. I was going to say to stick with established manufacturers (Celestron, Meade, Explore Scientific, Skywatcher) or quality telescope sellers (Orion, OPT, Highland Scientific) and you will probably be OK. But even these folks market some cheap clunkers. I highly recommend a trip to our local telescope shop, Land Sea & Sky on Richmond. (I have no connection to this establishment nor am I compensated to mention them here.) You can actually see the telescopes and you can pose questions to people who know their stuff. They sell a lot of high end gear but they also have some beginner scopes. In fact, they have a glossy brochure “How to Buy a First Telescope” that they will give you.
So what are the most important aspects of a good telescope? I will highlight two. Good optics and a solid mount. Good optics should be self-evident – the better the optics, the better the image that you see. Good is relative and many an astronomer with a good telescope is still lusting after some better glass. There is a basic level of performance that once you get beyond that level, the improvement in image quality is probably not appreciated by a beginner. So stay away from the cheap scope and you are probably OK. No matter how good the optics, an unsteady mount will ruin your view. The telescope will vibrate, shake, wiggle, and shimmy and whatever object you are viewing will do the same. Make sure whatever scope you get has a good, steady mount. It is worth it.
Making a Cheap Scope Cheap
Makers of cheap scopes have to cut corners. So what do they cut? They use cheap optics that can be manufactured easily but probably are not figured (shaped) well to give an optimal image. Some scopes don’t even use glass for lenses (they use plastic). The optics will be the simplest possible that will give you an image, so you don’t benefit from designs that are more expensive and better corrected for sharper images. Cheap scope makers cut the mount cost by using less parts and smaller parts. A good mount allows the observer to move the scope fluidly and easily stop on a target. A bad mount makes that difficult and the undersized parts make it rickety. Remember, you want solid. Another place to cut costs is metal. Cheap scopes often have plastic pieces for the focuser, eyepieces, finder scope, and some mount parts. Metal is better and more durable. Well-machined metal parts fit together better and work better. But they add to the cost.
Let’s take a look at the offerings from a well-known big box retailer specializing in low prices. These scopes are a sample of dozens that they offer online. A check of the Evil Empire of online commerce shows similar offerings.
These telescopes are on the website of a famous big box retailer. None are recommended.
The scope on the left is a Newtonian reflector and other three are refractor scopes. Where have these manufacturers gone cheap? The first obvious thing is the mount. The three refractors appear to be on cheap camera tripods. The reflector mount may be a bit more robust, (it is not clear from the photo) but it looks undersized. A camera tripod makes a poor mount because it is meant to be shifted and clamped to hold a camera steady – usually in one go. When you train your scope on an object, the object will move across the field because of the Earth’s rotation. So you have to keep moving the scope to keep up. This will not be easy with a camera tripod. Furthermore, the telescopes are much larger than a camera, so they will be unbalanced and hard to keep steady. The tripods also look undersized (and for this price, they are probably cheaply made). A rickety mount that is hard to adjust will be highly frustrating to use and nigh impossible to use with much magnification because the image will do a dizzying dance. It is hard to know about the optical performance of these scopes, but at this price point, the optics must be simple and cheap. My bet is that they are substandard. Refractor lenses must have two to three elements to bend all wavelengths of light to a sharp focus point. Cheap refractor lenses are probably poorly corrected, so you may see fringes on bright objects and sharp lines. This may not bother a neophyte, but will make experienced observers weep. In all cases the objective lens or mirror is small. The refractors all have 70 mm (2.8 in) objective lenses and the reflector has a 114 mm (4.5 in) mirror. The focal lengths are short at 400 mm (15.7 in) for the refractors and 500 m for the reflector, so it will be hard to coax high magnification. To their credit the ads don’t scream about high power (see the section Power Mad below).
All ads all show a smartphone on the eyepiece. Astrophotography is difficult at best and will be extra difficult with these telescopes. The smartphone must be aligned with the eyepiece and held still. You can purchase a good smart phone adapter for about $50, but I wonder what they are giving you at this price point. At a minimum, this will be difficult because the phone will throw the scope out of balance and it will be hard to keep it in place. And with no tracking mount to follow the motion of the stars, the Moon is really the only target that will have enough light for a short exposure.
And one more thing. I checked the reviews because I was curious. These scopes have something like 400 excellent reviews and only a few that are less glowing. Call me skeptical. There should be a “bell curve” of reviews (i.e., some reviews that are less than glowing). The fact that this distribution is so skewed makes me suspicious.
Power Mad
Power is a common misconception about telescopes. You will almost never use high magnifications like 500x, even if your telescope could deliver it. Physics says that even good optics cannot give more than about 50x per inch of aperture. For example, let’s say that you get a telescope with an 80 mm diameter front objective (3.2 in.). This means that you can get about 160x from that telescope. You can push it beyond that, but you will not see more detail and the image will be dim and fuzzy. An even bigger limitation is something called “seeing”, i.e., the steadiness of the atmosphere. The atmosphere is like looking up through a swimming pool. If seeing is poor, high power just magnifies the squirming caused by air currents. To get detail at high power requires the atmosphere to be very still. Ask any experienced observer and they will tell you that they use their low power eyepieces a lot more than their high power eyepieces. Furthermore, the low-power view is brighter, clearer, and seems less wiggly. And another thing. When looking for an object in the sky, you always start with your low power eyepiece. As power increases, the width of your field of view decreases. High power is like looking through a narrow straw.
A Few Good Scopes?
Next I will take a look at some recommendations from respectable retailers that I have seen on various lists. There are dozens of possibilities and here I am exploring the lowest end of the price spectrum. I have not actually laid eyes or fingers on most of these scopes, so I can only comment on the specifications and photographs based on my experience. Other well meaning astronomers may disagree with some (if not all) points. The telescope photos come from web advertisements.
On the left is a small, tabletop reflector from Meade Instruments that retails for about $250. The scope has an aperture of 114 mm (4.5 in) and the focal length is 450 mm (17.7 in). Celestron makes something similar (FirstScope 76) with a 76 mm (3 in) mirror and a 300 mm focal length that sells for about $75. Zhumell sells the Z100, which has a 100 mm mirror with a 400 mm focal length and sells for $160. You probably detected a pattern here: the price depends strongly on mirror aperture – bigger is more expensive. These are small mirrors, so they will not collect much light. Remember also that reflector telescopes have a secondary mirror in the light path that blocks some of the light. For low f-ratio scopes like these, that blockage must be a little larger than for higher f-ratio scopes. This obstruction means that a 3-inch reflector does not collect as much light as a 3-inch refractor, which has no obstruction. The focal lengths of these scopes are short. This means that you will not be able to coax high magnification. The alt-az mounts all look similar with one upright that has the altitude axis connected to one side of the tube, so it is inherently unbalanced. But the tube will be light, so maybe that will work OK. It looks like a lot of plastic is used in the construction. There is only a unit-power finder, so you can only find bright objects. I doubt that this type of scope will be very inspiring. However, the main reason I can’t recommend this scope is the table-top mount. Unless you want to lie on the ground, you need a table to put the scope on. Are you going to drag a table everywhere you observe? Moreover, the table adds instability. Although the scope looks cute on a table top, I say no to this one in particular and to any table-top scope in general.
On the right is a little refractor from Celestron, the PowerSeeker EQ60 that sells for about $130 at a local big box electronics store. It has a 60 mm (2.4 in) objective lens with a focal length of 900 mm (35.4 in), making it f/15. It comes with two eyepieces, 20 mm and 4 mm, that give magnifications of 45x and 225x. You can also get a PowerSeeker EQ80, selling for about $200. That one has an 80 mm (3.2 in) objective and a 965 mm (38 in) focal length (f/12). The same two eyepieces give 48x and 241x. The mount is an equatorial mount and the knobs are slow motion controls (you turn the knob and it causes the scope to move slowly on that axis). This little scope does not appear outlandishly bad, but a lot depends on the details. The objective lens is on the small side, so it will do OK for the Moon, bright planets, and the Sun (remember the solar filter!!!), but does not have much light-gathering ability. If you can afford the 80 mm scope, you get 70% more light. Because of the long focal length, the objective lens does not need to be well corrected to produce a decent image. The 225x claim is aspirational. Remember from the Power Mad section, you will find that a 60 mm scope can’t deliver that much power. Neither will the 80 mm scope give you 241x. Both are achieved using a 4-mm focal length eyepiece. This is very short focal length and hard for the beginner to see through. If you get this scope, pick up another eyepiece with a longer focal length, for example 10 mm (90x). The mount looks good, but a lot depends on how badly Celestron cut corners. It is probably somewhat undersized. Beware though, long refractors are very difficult to hold steady on a light mount because their length means the scope will shake when you focus it (this is true even for good mounts, but will be especially troublesome on a cheap mount). Here the devil is in the details - how good are the optics and mount?
Here are a couple of small reflector telescopes from Orion telescopes that are on mounts that look reasonably robust. The one on the left is the SpaceProbe II and has a 76 mm (3 in) mirror with a 700 mm (28 in) focal length. It normally sells for about $99. The mount is an alt-az with the telescope in a fork. It comes with two eyepieces, 25 mm and 10 mm, which give 28x and 70x, respectively. It has a unit-power (red dot) finder. This scope has a rather small mirror. Because part of the field is blocked by the secondary mirror, it probably gathers about the same amount of light as the 60 mm refractor, mentioned above. The positive thing about this scope is that the mount looks solid. I don’t like the little rod on the mount fork – that is a locking mechanism, which means that the fork by itself won’t hold the scope tube in a fixed position. This is probably necessary with a small altitude bearing with no slip clutch. Another good thing about this scope is the f-ratio, which is f/9.2. At this ratio the mirror can have a simple, spherical curve that is easier to manufacture well. In addition, the 700 m focal length gives higher power for the same eyepiece.
The reflector on the right is the Orion Observer 114. It has a 114 mm (4.4 in) mirror with a 500 mm (19.7 in) focal length. It comes with the same 25 mm and 10 mm eyepieces, which give magnifications of 25x and 50x. Because of the shorter focal length and wider aperture, the f-ratio is f/4.4. This gives a brighter image than the smaller reflector, but at this f-ratio, the mirror must have a parabolic curve, which is harder to manufacture cheaply. With the low f/ratio, it will probably suffer from coma at the edges of the field of view that cause stars not to be pinpoints (this happens on all low-f-number reflectors). It is on a decent-looking equatorial mount with slow motion knobs that will allow the observer to move the telescope more easily. This bundle, which costs $280, includes a solar filter (larger circular object in the photo), which is necessary to look at the Sun. It also comes with a Moon filter, which fits on the eyepiece and cuts down the glare of the bright Moon. This seems like a reasonably nice small telescope, depending on how well the manufacturer did on the scope optics, fittings, and mount. I suspect that many people would be disappointed with the low magnification. You will probably want to purchase a barlow lens, which multiplies (most commonly doubles) the power of a given eyepiece.
Next, let’s look at a couple of “computerized” scopes. The scope on the left is the Celestron StarSense Explorer LT 80AZ. It is a small refractor with an 80 mm objective lens with a focal length of 900 mm (f/11.2). The mount is alt-az with a stabilization rod to lock the altitude axis. A typical “bundle” comes with 25 mm and 10 mm eyepieces, which give 36x and 90x and a 2x barlow lens that multiplies the magnification by two (i.e., 72x and 180x). The thing that makes this scope “computerized” is the contraption on top, which is a cell phone mount with a mirror so the phone camera can see the sky. The phone is placed in the cradle on top and a Celestron StarSense app captures a picture of the stars and calculates the telescope location and orientation. StarSense shows a sky map and when a user wants to go to an object, the app tells the user how much to push the scope in azimuth and altitude until the object is in the field of view. This app sounds cool, but it is new and I have not heard from anyone who has used it. If it works well, it could be a boon for inexperienced observers. I have concerns, though. Most people today live in places with severe light pollution that hides most stars. I don’t know how many stars StarSense needs to get a lock or whether nearby sources of light (e.g., streetlights) will throw it off. This particular scope sells for about $240. Celestron also sells a 114 mm reflector with a StarSense phone holder on a similar mount for about the same price. I was able to examine one of these at Land Sea & Sky. It seems like a budget telescope with lots of plastic parts and the mount does not seem very rigid. Nonetheless, it will probably be serviceable for a first scope.
The scope on the right, the Celestron 4SE, is truly computerized. The scope itself is a small cat with a 102 mm (4 in) objective and a focal length of 1325 mm (f/13). It comes with one eyepiece only, 25 mm, which gives 53x. The mount is alt-az. The computer is in the hand controller (the thing with buttons), which is shown mounted on the side of the mount (its stowed position). The controller contains a small computer that controls motors on the azimuth and altitude axes. The user must align the scope with some stars and the computer figures out the orientation. From there, the user can choose objects from a large database and the scope will move itself to put the object in the field of view. You can get optional modules that make the alignment process automatic (basically, the computer checks the stars in a camera). The tripod and mount on this scope are very solid and with the short scope tube, there is little image shake. But you pay a price to go computerized. This scope is about $600. Frankly, a 4-inch f/13 cat doesn’t gather a lot of light. You can get larger aperture cats on similar mounts for more. One of the most popular scopes around is an 8-in computerized cat, sold by both Celestron and Meade, which sells for about $2,000. The 8-incher has kept many an amateur observer happy for years.
Recommendations
That was a lot of scope stuff, but what recommendations can I leave you with?
1. The best telescope is the one you will use. Of all considerations, the telescope has to be one that gets pulled out of the closet and put to use. You have to decide where the scope will be stored (best in a climate controlled space) and how it will get to the observing site. If you live in the country, maybe all you have to do is put it out in the yard. But if you live in the city, you are probably transporting it to a location where the sky is darker. And this also brings up an important point – telescopes don’t use themselves. If you are giving the telescope to a child, you will want to encourage learning about the sky. Get a planisphere (a time-adjustable star map) to learn the constellations. Get a simple set of star maps like the Sky and Telescope Pocket Sky Atlas. Since this is the HAS newsletter, you already have a good start because you joined a club with a lot of in-house knowledge and resources. Take the telescope someplace dark (like the club dark site) and make an adventure of it with your kids. Talk to other amateurs – they are usually quite chatty.
2. Stay out of the bargain basement. The most suspect telescopes are the ones that are ultra-cheap online from manufacturers having no obvious track records (use Google). You could get lucky, but probably not.
3. Go simple. Simple scopes are cheaper
4. Buy as much aperture and as sturdy a mount as you can afford. On the low-price end of things, manufacturers have cut corners with cheap optics, small optics, and rickety mounts. Of the two, I recommend the solid mount over the bigger aperture. No matter how good your optics, you will be frustrated if your image dances around too much.
5. Refractor or reflector? Refractors are more expensive per inch of aperture, so you can get a bigger scope for less money with a reflector. Reflectors require occasional collimation. This means adjusting the optics into alignment. To collimate a reflector, the user turns screws on the mirror mount that move it until the mirror optical axis goes straight into the eyepiece. This is not very hard, but a scope out of collimation gives poor images (and may not focus properly).
6. Phone or not? Yes, I know, you probably want your kid to get off the phone. This is the 21st century and that is unlikely. Suppose the phone could be something that gets your kid interested in astronomy? Well, OK then. As I said above, I have no idea how well the StarSense app works, but I think Celestron may have come up with something to interest youngsters in observing the sky. On the other hand, a bright cell phone screen at a dark site (and specifically at the HAS dark site) is about as welcome as car headlights. It will blind you and everyone else around. At minimum, you will need to set the screen to be dim and colored red (get a red transparent cover), so it doesn’t mess up your dark vision.
Which scopes to buy?
You already saw that I find plenty to dislike with low cost scopes. If I were trying to stay around $100, the Celestron 60 mm refractor and Orion 76 mm reflector would be decent bets. Jumping up to just above $200, consider the Orion 114 mm reflector or the 80 mm refractor. The Orion scopes look like they have good mounts. I’m concerned about the StarSense telescopes because the mounts are not very sturdy – and I don’t know whether the StarSense app is good or gimmick.
If you want to jump up to the next level and prices near $500 don’t deter you (maybe you have a giftee who has a demonstrated interest in astronomy), consider the scopes in the picture below. These are SkyWatcher Dobsonian reflectors. Dobsonian has become a term for reflector telescopes on inexpensive alt-az mounts. The scope on the left has a 6 inch mirror with a focal length of 1200 mm (47.2 in), which gives f/7.8. This is enough aperture to start seeing faint fuzzy things (in a dark sky). The focal length will give decent magnification. In the middle and on the left are Dobsonians with 8-inch mirrors. This is a scope that will satisfy for many years. The one on the right is “collapsible”, meaning that the ring with the focuser can be pushed down to make the tube shorter for storage. The prices are about $450 for the 6 incher, $550 for the 8 incher, and $750 for the collapsible version. These scopes have real finder scopes (rather than red-dot finders) and have mostly metal parts and reasonably good focusers. Orion also makes similar Dobsonians at similar prices (in fact, I think they are made in the same factory in China).
Three Skywatcher Dobsonian reflectors at Land Sea & Sky. Left, 6 inch; middle, 8 inch, and right, 8 inch collapsible. (author photo)
Online resources
“Telescopes Explained”, Cosmic Pursuits
https://cosmicpursuits.com/943/telescopes-explained/
“Choosing a telescope”, Sky and Telescope Magazine
https://skyandtelescope.org/astronomy-equipment/how-to-choose-a-telescope/
“Buying a Telescope”, OPT Corp.
https://optcorp.com/blogs/telescopes-101/telescope-buying-guide
“Best Telescopes Under $500 for 2022”, Popular Science
https://www.popsci.com/gear/best-telescopes-under-500/
“Best Beginner Telescopes”, Space.com
https://www.space.com/31229-best-beginner-telescopes.html
“17 Best Scopes for Astronomy Beginners”, Sky at Night Magazine
https://www.skyatnightmagazine.com/top-astronomy-kit/best-telescopes-beginners/
“Best Telescopes for Beginners”, New York Times
https://www.nytimes.com/wirecutter/reviews/best-telescopes-for-beginners/
Orion Telescopes
www.telescope.com
OPT Corp (Ocean Pacific Telescope)
Optcorp.com
High Point Scientific
Highpointscientific.com
Anacortes Telescopes
Buytelescopes.com
Astromart
This is an online selling forum mostly inhabited by amateurs turning over gear. To be useful, you need to pay a $15 subscription. If you know what you are looking for, you may be able to find it at a discount here. But of course, you are at the mercy of the seller and may have no recourse if unhappy with your purchase.
www.astromart.com
Layla Ryan
This article is distributed by NASA’s Night Sky Network (NSN). The NSN program supports astronomy clubs across the USA dedicated to astronomy outreach. Visit 
After a brief hiatus (life seems to intrude on my astronomy) this installment of Astrophotography Corner concerns one of the two important mechanical requirements for getting good image data, polar alignment. Next to autoguiding (which we cover in the next installment), getting your mount well polar aligned is essential.
Autoguiding is also not a reliable substitute for an accurate polar alignment. If you are autoguiding, you can be well out from polar aligned and still keep a star on the imaging chip for exposures of 5mins or longer. You can still, however, get oval stars and blurred images due to field rotation.
The Garnet Star
