Purchasing a Telescope
by Jeff Polston (October 16, 1997)
I've had quite a few people ask me for advice on purchasing their first telescope. Since I was their "resident" astronomer, they naturally came to me. I thought I would try to assemble all my advice into one article. This article is mainly geared toward helping beginners choose their first telescope. A lot of people have no idea what to buy or have never even looked through an astronomical instrument. Like any new hobby, you need to do a little research before buying the "tools".
If you don't want to read the article, or just plan on glancing at it, my general recommendation for a beginning telescope used primarily for astronomy is a 6" to 8" Newtonian on a Dobsonian mount (i.e. a Dobsonian telescope).
This is an example of a very simple 8" Dobsonian telescope.
If you want to know a little more about the reasons for choosing a particular style of telescope, hopefully this article will help you along.
If you plan on making astronomy a lifelong hobby then you will definitely want a telescope. But you don't absolutely need a telescope to enjoy the night sky. A lot of "experts" recommend that you start out with a pair of binoculars. Binoculars generally cost less and are much more portable. There's no setup time unless you plan to mount them on a tripod. And you would be amazed at what you can see with binoculars.
You can see lots of nebulas and star clusters. You can even pick out a few galaxies here and there. Our own Moon will offer up some tantalizing details. Most people don't realize you can see craters on the Moon with ordinary binoculars. You can even see the four main moons around Jupiter. And this is just gravy to all the colorful stars you can observe. Plus, binoculars can be used during the day for terrestrial viewing or bird watching. So starting with binoculars can be very rewarding. I still devote some nights entirely to binocular observing.
If you decide to get a pair of binoculars, a good all around size is 7x50, which is 7x power coupled with 50mm size objectives. Some people take a small step up and get a 10x50 size. I actually prefer the extra 10x power, but it can be kind of hard to hold it steady. It's a personal judgment call. Binoculars vary immensely. For example, my current pair is a 9x63. You can get bigger or smaller objectives, and higher or lower power. It's up to you.
I will say that a smaller size higher quality binocular will be better than a larger, less quality binocular. Though the larger one might bring in more light and give you more power, if it is of poor quality, the image will not be as sharp, nor as uniformly focused across the entire field. But as quality goes up, so does price, so you eventually have to make a compromise, unless of course you're rich. Look for a pair that has fully multi-coated lenses and BAK-4 (versus BAK-7) glass prisms.
Okay, we'll make the assumption that you definitely want to buy a telescope. When you start looking you'll quickly realize that telescopes come in all shapes, sizes, and prices. And they also have all sorts of mounts and accessories. The common saying use to be "to buy the biggest telescope you can afford, but not so big and cumbersome that you will never use it". To some extent that saying is still very true. But with advances in technology, now people have a choice of getting a smaller telescope, but one that is equipped with electronics that aid in finding astronomical objects. It can present quite a dilemma. And some telescopes work better than others. Some are designed to give high power views of the planets, while others are designed to give lower power views of deepsky objects (galaxies, nebulas, and star clusters). Let's briefly discuss the three main types of telescopes you'll encounter.
When most beginners think of a telescope, the image of the classical refractor comes to mind. These scopes look like long tubes. You point one end toward the sky and you look through the other end. This is the type that Galileo used to explore the night sky. The heart of a refractor is a lens, that collects and bends (or refracts) the light. Take a look at the simple drawing below:
The light path is in yellow and the lens is in light blue. The arrows show the direction the light is traveling. As you can see, light travels through the front of the telescope, where it is refracted by the lens. It then travels out the back and through an eyepiece. While this is a simple design, a quality refractor can be very expensive.
This is an image of my Vixen 80mm refractor, on an alt-azimuth tripod mount.
The next telescope to come on the scene is called a reflector. The basic reflector is also called a Newtonian (so named because it was invented by Isaac Newton). It uses a parabola shaped mirror to gather the light. These telescopes look like fat stovepipes. Take a look at the simple drawing below:
The light comes in the front and strikes the parabola shaped mirror at the bottom. It is bounced back up the tube to a small, flat mirror mounted at 45 degrees. This small mirror bounces the light out the side of the tube and through the eyepiece. It is easier and cheaper to make a high quality reflector than it is a high quality refractor.
This is an image of my Meade 10" Newtonian reflector, on a German equatorial mount.
The third type of telescope is called a catadioptic. It's also a reflector telescope, but with a compound design. They come in a variety of designs. The most common are Schmidt-Cassegrain and Maksutov-Cassegrains. It is called a compound telescope because using mirrors, and sometimes special glass, it "folds" the light, making the telescope much shorter than its focal length (the distance from the main lens/mirror to the point where the image is focused). The Schmidt-Cassegrain style uses a doughnut shaped spherical mirror. Take a look at the simple drawing below:
The light comes through the front, passing through a glass corrector plate (shown in dark blue). Since the mirror is spherical, this corrector plate "corrects" the different wavelengths of light so they come together at the same point. The light then bounces off the main mirror at the back of the telescope and is reflected back up the tube. It then strikes a small secondary mirror at the front of the telescope. But, unlike the 45 degree flat mirror in a Newtonian, this mirror is shaped so that it also magnifies. The light then bounces back down the tube, through the hole in the center of the primary, and out the back of the telescope and through an eyepiece. What you get is a compact telescope with a focal length much longer than the actual tube.
This is an image of my Meade 8" Schmidt-Cassegrain telescope on an equatorial wedge.
When astronomers talk about the size of a telescope, they are generally referring to the diameter size of the primary optics. An 8" reflector has a primary mirror that is 8" in diameter. An 80mm refractor has a primary lens that is 80mm (or 3.1") in diameter. The larger the primary optics, the better. This is what gathers the light. The bigger mirror or lens, the brighter an image you will see. Also, the bigger the mirror or lens, the more resolution you will get. So, with telescopes, bigger is better. Just remember that if it gets too big, you will never use it.
You will also hear about a telescope's focal length, which is simply the distance (usually expressed in millimeters) that the image will focus from the primary optics. This is important in that this figure determines the power of a telescope given a certain eyepiece. You vary the magnification power of a telescope by using eyepieces with various focal lengths. The magnification power is simply the focal length of the telescope divided by the focal length of the eyepiece. So, a telescope with a 910mm focal length, used with a 25mm eyepiece, would give a magnification power of about 36x (i.e. 910 / 25 = 36.4). Beware of telescopes that advertise by power. A telescope advertised as 400x power isn't necessarily better than a telescope advertised as 200x power, because you can vary the power of any telescope with different eyepieces. Generally, the most useful high magnification that can be used on a telescope, is the diameter of its primary expressed in inches, multiplied by 50. So, the highest useful magnification of a scope with a lens of 3" would be about 150x (i.e. 3 x 50 = 150).. Oh, you can definitely go higher than this. It's just that the images will get fuzzy and more fuzzy (unless you have a night with very stable skies). So, the longer a focal length a telescope has, the easier it is to get the higher powers with a certain eyepiece. For example, the same 25mm eyepiece on a 2000mm focal length telescope (your typical Schmidt-Cassegrain), gives a magnification of 80x (i.e 2000 / 25 = 80). That's over twice as much as 910mm focal length telescope. But power isn't everything. For most deepsky objects, you'll use low to medium powers. The high power views are usually saved for the Moon, planets, and double stars.
Another term you will hear is the f/ratio. The f/ratio is simply the focal length of the telescope, divided by the diameter of the primary mirror/lens. For example, an 8" (203mm) Schmidt-Cassegrain, with a focal length of 2000mm, has an f/ratio of about 10 (i.e. 2000 / 203 = 9.85). This is expressed as f/10. Higher f/ratios are called "slow" telescopes while lower f/ratios are called "fast" telescopes. These are basically photography terms. A fast scope will record an image on film faster than a slower scope. A lower f/ratio telescope, such as f/4.5, will generally give you wider fields of view. These are great for low power views of deepsky objects. Also, when considering photography, the faster f/ratios mean shorter exposure times. The higher f/ratios, such as f/10 or f/12, will generally give you more narrow fields of view. Consequently, you'll also have more power with a given eyepiece, needed for detailed observations of the planets. This also means longer exposure times for photography. Generally, I don't think beginners should concern themselves too much with the f/ratios of telescopes. The exception is if you're considering photography or if you really want a wider field of view (which I favor).
There are a variety of telescopes available today. Which type of optical design you should get is sometimes dependent on the type of a mount. There are basically two types of mounts; an alt-azimuth and an equatorial. And of course, to really confuse you even more, each type comes in various styles. An alt-azimuth mount is one that moves up/down, and left/right. It is the simplest to use and understand. An equatorial mount is more complex. It allows you to point an axis toward the north celestial pole. The telescope will then rotate around this axis. This type of mount will allow you to find objects by their coordinates and if powered by a clock-drive, will keep an object centered in your eyepiece (if properly aligned). Remember that the Earth is turning so anything you look at will slowly drift out of your field of view unless you move the telescope to follow it. Equatorial mounts are more complicated and heavier, but needed if you want equatorial tracking and the ability to take long exposure pictures.
It should be noted that there are now many automated and computerized versions
of alt-azimuth mounts that will find and track any object you want. And
their prices can be quite low. The thing you have to worry about is
whether or not this technology is coupled with a telescope that has poor
Okay...NOW Which One Should You Get?
It's still not quite that simple. It's still a double question. Not only do you have to decide on what type of mount, alt-azimuth versus equatorial, but you also have to think about whether or not you want it computerized. The main questions to ask are; How much do you want to spend?, and, What are you going to use it for?, with emphasis placed on what you are going to do with it. Let me say that you can use any of these telescopes on any object, in the sky or on the ground, but some people like to specialize. Normally, when people buy a telescope, they buy it so that can explore the night sky. However, some people might have a mountain or ocean view and might want to use the scope during the day. It's still a complicated discussion, but hopefully I can help narrow down some choices and help you from heading down a totally wrong path.
If you plan on using the telescope only during the day, say for landscape, nature, or bird watching, then Newtonian reflectors are generally out. They are not well suited for daytime use because they are generally low to the ground, and they will rotate or invert the image. You could still use a Newtonian if you wanted to, but a refractor or Cassegrain design is probably your best bet. With these two scopes, you can use corrective lenses to give an upright and correct image. I have an 80mm (diameter of the lens) refractor and it's a great daytime scope. There are also a number of smaller Maksutov-Cassegrains on the market. I suggest getting a telescope that's on an alt-azimuth mount that has slow motion controls on both axis. Slow motion controls are knobs or better yet, cables, that you turn to slowly move the telescope to track an object or move to another object. Some of the cheaper mounts require you to lock everything down when you center your object. Slow motion controls are much better and easier to use. With mine, I just twist the knobs on the end of stiff cables to move my telescope slowly up or down and left or right. A Schmidt-Cassegrain or Maksutov-Cassegrain will generally be a little bit more expensive than the lower end refractors. But there are a number of smaller, but higher quality refractors to choose from (with higher price tags).
Another option is to get a small refractor or Cassegrain spotting scope and use it on a camera tripod. While this is certainly a usable daytime configuration, it will be a little bit more cumbersome if you decide to use it at night. It's still able to be used at night, it's just that it's not as smooth to move when trying to keep up with the moon or planets that are drifting out of your field of view.
As I said before, the Newtonian is generally out if the telescope is to be used during the day. I'm not saying that you can't use it, it's just not as well suited for it. Again, a refractor or small Cassegrain style is a good choice. But, if you also want to use the telescope at night, you might seek a little larger aperture. An 80mm to 102mm telescope is what I would recommend. I'd also consider a Maksutov-Cassegrain or a Schmidt-Cassegrain in the 90mm to 102mm range. You could also consider getting one on an equatorial mount for use at night. For beginners, I still recommend an alt-azimuth mount with slow motion controls on both axis.
If you only plan on using the telescope at night, the market really opens up. Now you can choose any telescope design. But, before you make your decision, you have to decide whether or not you are going to do astrophotography with it. I will say that most beginners probably shouldn't factor photography high on their list of requirements when purchasing a first time telescope. Photography adds a much higher level of complexity.
Again, my general recommendation for a beginning telescope used primarily for astronomy is a 6" to 8" Newtonian on a Dobsonian mount (i.e. a Dobsonian telescope).
However, if you are just going to look at the brighter objects like the Moon and planets, then an 80-90mm refractor or smaller Cassegrain style (Schmidt or Maksutov) might suit you fine. These can also provide pleasing views of the brighter deepsky objects like nebulas and star clusters. And you can also do snapshot pictures of the moon without the need for tracking.
If you want to look at all objects, from planets to galaxies and nebulas, then consider getting a larger telescope in the 6" to 8" and bigger range. Generally you get the most scope for your money if you go with a Newtonian design (refractors are exponentially more expensive when you get into this size range). An alt-azimuth mount called a Dobsonian will give you the most aperture for your money. These mounts are made of wood and are very low tech. Most of the money goes toward the optics. There are no locking screws or anything. They are balanced on Teflon bearings and you just push or nudge the scope to the position you want. They are easy to use and setup. There are two pieces; the telescope tube and the base (called the rocker box). You just plop the tube down on the rocker box and you're ready to observe. This is probably the most recommended design, if you don't consider the technology side of additional electronics. As a side note, Dobsonians are now offered with digital setting circles that will help you find objects.
If you definitely want to do some deepsky astrophotography, then you'll have to have a telescope on an equatorial mount that has a clock-drive for tracking. Astrophotography, or rather long exposure deepsky astrophotography, places high demands on the mount. This is the most expensive route and it varies between expensive and outrageously expensive. There are still a variety of choices to make but generally I recommend a Schmidt-Cassegrain telescope , in the 8" to 10" size, on an equatorial fork mount. I find these easier to setup and use, and there are tons of accessories available. Some argue that the Schmidt-Cassegrain doesn't perform as well as its Newtonian counterpart when it comes to optics. Yes, it should have a little less resolution due to a larger secondary mirror, but beginners and even more advanced observers might not notice a difference. These telescopes are great all around scopes, whether you are just observing or doing photography. They excel at photography because there are lots of available accessories and they have a larger focusing range. Their main drawback for beginners, other than a little more complexity, is their cost.
As I've alluded to earlier, technology has come a long way in the telescope market. Not so much as it pertains to optics, but more toward the electronics that are available for telescopes. There are now telescopes that have computers and automated drive systems. Usually referred to as GOTO, these telescopes and mounts allow you to find objects with just the push of a button. Once aligned, the telescopes will slew themselves to the requested object. All an observer needs to do is look through the eyepiece. You can even interface them with a personal computer. The user clicks on an object in an astronomy software package and the telescope moves to that object. While these GOTO telescopes use to have a high price tag (the higher quality ones still do), now a lot of them are quite affordable, even to the beginner.
Finding objects can be very frustrating for beginners. Some may want to give up quality in the telescope and optics in order to have a better way of finding those celestial delights in the sky. Personally I still think the plain and simple telescope is the best way for beginners to go but it is something to think about. I have seen a lot of people disappointed because they could not find the objects they wanted to observe. And I've also seen people disappointed with their GOTO telescope because they couldn't figure out how to use it, or get it to work reliably. The problem is that the more affordable GOTO ones might also have poor optics and poor mounts. Generally what you might see in the local department stores are of poor quality, but there are exceptions. And to confuse the matter, the leaders in the market, such as Meade, makes both high end scopes and low end (trash) scopes. So if you decide to go the GOTO route with the lower priced scopes, be careful with your choices.
If you just want to get a plain telescope for looking at the sky, there is also an alternative route with I refer to as the "poor man's GOTO telescope". This route is a Dobsonian style telescope coupled with a device called called digital setting circles. They work similar to the GOTO mount, but require that the user manually move the telescope instead. They just telll you which way to move it (by arrows or numbers). They too can interface with personal computers. With these units, the user selects an object and the hand control (or computer monitor) displays which way to push the telescope. You can either purchase a telescope that comes with digital setting circles, or you can buy them as an aftermarket item to add to a telescope that doesn't already have them.
When people tell me they want a telescope, I assume it is for looking at the night sky. I've already said this twice, but for beginners, I recommend getting a 6" or 8" Newtonian on a Dobsonian mount. If you can afford it, get the 8" over the 6". It's not much bulkier and that extra 2" of aperture will be apparent on those faint galaxies. A Dobsonian scope is where you get the most bang, in regards to aperture, for your buck. Dobsonians are also easy to setup and use. And, if for some reason you decide this is not a hobby you like, then you haven't made a gigantic investment. If you know for sure that you are also going to be doing some serious astrophotography, and you are comfortable with making the investment, then get an 8" Schmidt-Cassegrain. Even if you don't use the scope for photography, Schmidt-Cassegrains are wonderful telescopes to use. I find them easier to polar align for tracking and the built in clock-drive motor and setting circles are a plus.
If you're concerned that you won't be able to find anything, then consider a GOTO computerized telescope. Just be careful because the lower end ones might also have poor optics. If you decide to go with a Cassegrain type telescope, it might already come with GOTO. And remember that you can also add digital setting circles to a variety of scopes. I understand that some find the hunt as rewarding as viewing the object itself. There are plenty of books and devices that can help you find objects relatively easy. While I might prefer this route, I realize that some people just want to observe without having to find the object, and for those the GOTO technology fits the bill.
Again, here is the general word of wisdom when buying a telescope: "Buy the largest telescope you can afford, but not so big that you will never haul it outside to use". The larger the scope, the more you can see and the more resolution you will have. But if you buy one that is so large and heavy that you are never motivated to use it, you would have been better off getting a smaller telescope. The best telescope is the one that you will use. Other than my decided nights for observing, I still find that I use my small 80mm refractor the most because I can just grab it and go.
Please remember that these are only my suggestions and opinions. It doesn't mean that a particular telescope will not suit you. Just take the time to think about what you want and need. Read the advertisements carefully so you will know what you are paying for. You can enjoy the night sky with a simple Dobsonian or you can cruise the heavens with a fully automatic Schmidt-Cassegrain. It all depends on your needs, tastes, and budget.
Okay, so you think you might have an idea of what kind of telescope you want but you're still not quite sure about what's available and all the techno-gizmos that some scopes have. I'll see if I can steer you in the right direction.
The Cheap Stuff
If you go to the department stores, you'll see telescopes made by Tasco, Bushnell, Simmons, Meade (who also makes high end scopes), Jason, and others. These scopes (small refractors and reflectors) will range from slightly below $100 on up to $200, or $300, or $400. Most advance observers consider these scopes as trash scopes. Their optical components and construction are frequently poor and they are generally on shaky mounts. Meade, unfortunately, is also dominant in this market. And unfortunately, they do produce some very poor quality telescopes at this end of the spectrum.
But, they are priced low. And for someone on a very, very tight budget, they may be worth the price. They will show you tons of details on the Moon (craters, mountains, canyons, etc.). They will show the cloud belts and moons of Jupiter as well as the rings and moons of Saturn. So, even though I very rarely if ever recommend these types of telescopes, they may indeed serve a purpose for those truly wanting to spend the least amount of money. Just remember that your small investment might be a big investment in frustration. Personally, I'd stay away from them.
The Cheap GOTO Stuff
At these department stores and specialty shops, like the Nature type stores found in malls, you'll also find the cheap GOTO telescopes. The technology has come a long way in recent years so now you can get inexpensive telescopes that will automatically point to and track objects. I'm not sure on their prices, but I think they start around $200 or $300.
I'm am cautious about the quality on these scopes, but they may be okay for budget minded consumers. I have heard a couple of good things about them, despite looking like they are the cheap models. These are probably the cheapest GOTO telescopes you'll find in regards to price. Just be careful of the optics and general quality. Consider these if you absolutely must have GOTO and it has to be the lowest price. Overall, they are probably okay for beginners.
The Better semi-Cheap GOTO Stuff
If you up your price somewhat, but still just have to have GOTO electronics, there are quite a few options.
Meade's ETX line is generally quite good. I think the smallest model is a refractor and the others are Maksutov-Cassegrains. The size range is from 90mm on up to about 5". They are on GOTO fork mounts. The quality of the optics are usually pretty good. If you want GOTO and good optics, the ETX line might be a good choice. I only have two concerns with this line. First of all, while the optics are good on these Maksutov-Cassegrain scopes, they generally have a more narrow field of view. A beginner could get frustrated with it. But since it's GOTO, that may be a moot point. Secondly, factor in buying a good tripod to put these things on if they don't come with one. They use to come with table-top legs (might still do), but they were pretty much useless because you had to lay down to look through the thing.
Celestron's Nexstar series has a 5" Schmidt-Cassegrain scope.
The better, more high end GOTO Stuff
Both Meade and Celestron make good quality, larger and more high end GOTO telescopes. Meade has their LX line of scopes. If you are definitely considering photography, the LX200 series is great. Celestron continues their Nexstar line, and they ahve a CPC series. They also make even bigger and more advanced scopes/mounts, but they are expensive and not really beginner type scopes.
The NON-GOTO telescopes
These are the type of scopes I generally steer beginners toward. These scopes are for people who want a telescope but not necessarily the techno-gadgets. With these scopes, people will hunt down the objects on their own. On the smaller size, day or night use, consider an 80mm or 90mm refractor or Maksutov-Cassegrain on a good, solid mount. But for night only use, I still generally recommend a 6" or 8" reflector on a Dobsonian mount. These have large enough apertures for some serious observing, are easy to use, and won't destroy the bank account. And you could even consider a larger scope if you have the budget. Just keep in mind how you're going to move it around. If you are more serious, you might also consider getting one on an equatorial mount. This will give you tracking and also some manual setting circles for use with object coordinates.
Which one would I buy if I was the beginner?
Well, I'd take my advice and probably go with an 8" reflector on a Dobsonian mount. This is a good size scope that you will never outgrow. Not sure which name brand to get, as the market has changed a bit, and quality goes up and down.
Where do you get these scopes?
Well, unless you happen to live in an area that has telescope stores, you'll most likely have to get it via mail order or Internet. Just start searching. Even Amazon seems to sell them all. I've got some companies listed on my Astro Links page.
For the telescope, you'll definitely want another eyepiece or two. The first time you take a look at the Moon or Jupiter, you are going to immediately want more power. A telescope with only one eyepiece is like a car that only has one gear. If you only have the lower power eyepiece usually supplied with the telescope, you'll be stuck there, wishing you could see just a little bit more. So, when you order your scope, go ahead and get another eyepiece or two to go with it if it doesn't already come with them.
How will you point the telescope (if it's not a GOTO)? Most telescopes come with a small finder scope (which is just a tiny refractor). Beginners sometimes find these awkward to use. If you are not use to looking through one, the small, inverted view can be confusing. For beginners and veteran observers, I recommend getting some type of reflex sight. With a reflex finder, you see the sky non-magnified and right side up. The reflex sight will project a red dot or set of rings onto the sky. By looking with both eye's open, you know exactly where your telescope is pointed. So, if you are going for a nebula between this star and that star, you use your reflex sight to point your telescope between this star and that star. It's that easy. There are several of these sights on the market. I have one called the Telrad. It usually gets the best ratings. The others work fine too. A competing finder is the Rigel Quikfinder, which works in a similar fashion as the Telrad. I even modified one built for Daisy BB guns so I could use it on my telescopes (see my other articles). Again, I highly recommend you getting a reflex finder. It will make finding things a lot easier. I use mine in conjunction with my regular finder.
Finally, since you'll be exploring new territories, you'll need a good "map" to find your way around. If you get one of the astronomical magazines, they have a monthly star chart in the middle. I recommend getting a good and simple star chart to star out with. You don't need the gigantic ones, just a simple one that shows stars down to about 6 magnitude and plots the brighter objects. The beginner first needs to learn how to find the brighter constellations and stars before they can start finding the galaxies and nebulas and other things that are scattered about. Constellations and stars are used as stepping stones or pointers. You could also consider getting one of the beginner books. One I've heard recommended is Turn Left At Orion. This book tells you how to find objects by "star hopping" from one star to another. I think it lists about 100 objects or so. Consider a book on the Messier objects. These objects tend to be the brighter of the so called deepsky objects and is generally the first list of objects that beginners try to go after. Most if not all of the objects in the Turn Left At Orion book are probably Messier objects. Another handy device for beginners is a planisphere. A planisphere is basically a round star chart. They are generally designed for a particular range of latitudes. They are marked with the local horizons. You just "dial" in your date and time and it will let you know which constellations are up. Likewise, you can use them to forecast when a particular constellation is rising or setting. A good beginners star chart and book are essential to finding your way among the stars.
There you have it. I hope I haven't confused you more. With so many products and styles to choice from, it can be quite a dilemma. Generally, if you avoid the trash scopes you see in the department stores, you'll do just fine with any choice you make, be it Meade, Celestron, Orion, etc. Even the smallest telescope can be a joy to use. Please do consider your future plans in the hobby while making your decision. If you'd like to take a look at the telescopes I own, visit my Equipment page. If you have any specific questions or concerns, feel free to email me. My work and home email addresses are located on my home page.
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