How To Use a Dobsonian Telescope

hey everyone I'm astronomer John Reed host of learn the stargaze and author of the things to see with the telescope series we're here at St Mary's University at the Berke G The Observatory and in this video we're going to teach you everything you need to know about how to use a dobsonian style telescope the easiest type of telescope to use if you're a beginner these telescopes are free to point in any direction with a simple push of the hand to find Targets in the sky you get them centered in the finder and then view them through the eyepiece these are very powerful instruments and to get the most out of these telescopes there are a few things you'll need to learn in this video we'll talk about the different types of dobsonians how dub zonian telescopes work what they're good at and what they're not we'll talk about colation that's how to align the mirrors and I'll show you how to align the finder we'll talk about choosing an eyepiece and finally how to choose which targets to observe let's talk about the different types of dobsonians there are dobsonians for pretty much every budget and in terms of how much you're able to see these telescopes typically offer the best bang for your buck now there's the first scope series for little kids which run around $50 these aren't much more powerful than binoculars and are really only good for looking at the moon then there's the Orion Skys scanner and the Z100 which are found for around $100 my kids really like these ones because they're so easy to use next there are the tabletop style dobsonians like the skywatcher Heritage 130 which are typically in the $200 range there are larger tabletop versions like the star sense 150 and these tend to be around $500 then there are what are considered the classic dobsonians these come in apertures of 4.5 in 6 in 8 in and 10 in there's even a 12in version as well after that dobsonians tend to get really large in fact you typically need a stool or a ladder to view things that are high in the sky these telescopes are for those who really want epic views of deep space if you're attending a star party with lots of seasoned astronomers you'll often see two other types of dobsonians first you'll see the homebuilt dobsonians just like John Dobson the inventor of the dubs sonian made them back in San Francisco in the 1960s and 1970s finally there are the obsession dubson these are typically very large dobsonian telescopes with frames made of wood and metal tubing these tend to be motor driven as well let's take a moment to talk about how dobsonian telescopes work what makes the dobsonian so popular is its Simplicity these telescopes typically sit on a lazy Susan which is a simple rotating base the tube is typically supported by a few pieces of plywood or particle board in some cases the tube is held on by Springs but that's not actually that common most dobsonians simply use gravity to stay on the frame dobsonians are newtonan telescopes with the primary mirror located at the base of the telescope the primary mirror collects light from space it reflects off a secondary mirror and into an eyepiece you might wonder why you can't see the secondary mirror in the image the simple answer is that it's just so out of focus you can't see it the more complicated answer is that any light from space that hits the primary mirror will form a full image in the eyepiece in other words if you're looking at the moon let's say and someone blocks off half the light coming in through the telescope the image of the moon will get dimmer but you won't see the obstruction it's actually common when viewing bright objects like the Moon Venus and Jupiter with a dobsonian to block half or more of the light I found that this often increases the amount of detail you can see these days most dobsonians come with a red dot finder instead of a finder scope for me I actually have a t rad fitted to my telescope in fact most every telescope here at the university is fitted with a t rad as well T RADS project a bullseye onto the sky which can easily be lined up to match the image in a star map that's why every page in my book 110 things to see with a telescope includes Tad rings on the skyms binder telescopes included with dobsonians are often pretty big often with apertures of 50 mm or more the reason for the large aperture finder Scopes is so that you can actually see the Deep Sky objects you're looking for in the finderscope now let's talk about what dobsonians are good at and what they're not dobsonian telescopes are the ultimate tool for visual observing they're large apertures make them great for planets and deep Sky objects alike as you've seen they're extremely easy to point and you can easily track objects as they move across the sky with a light touch on the optical tube what they're not good at is taking photos it's a extremely challenging to try to take an image with a dub zonian assuming you're not using a motorized tracker the focal length is long enough that even a one second exposure will smear the image and even if you have a fancy devonian with a motor it will still not account for what's called field rotation that's the fact that objects appear to rotate in your field of view when you're not using an equatorially mounted telescope that said there are groups of people who are up to the challenge there are Facebook communities of people who have modified their Dov Stone on equatorial platforms so that they can be used for deep Sky Imaging that said I consider stargazing and astrophotography to be very different Hobbies alog together so that's all I'm going to say about that today every time you set up your dubon and Telescope it's generally a good idea to check the colation colation is the alignment of the mirrors if the mirrors are out of alignment the images may look distorted the easiest way to check the colation is to pop off the eyepiece and look into the focuser if the primary mirror is centered in the field of view then the secondary mirror is in alignment to check that the mirror is centered I usually watch the mirror clips which should appear to rest on the side of my field of view on the focusing tube to check the alignment of the primary mirror you want to look at the reflection of the spider arms if they're all the same length then you're good to go but what if you're out of colation how do you fix it in this video I'm going to show you how to do it with a colation laser since this is the quickest and easiest way to go about it these lasers can be purchased on Amazon for about $20 all post post the link in the description to cumate the telescope you pop the colation laser into the ipce tube and secure it snugly in place you can check that the laser beam is coming straight out by rotating the laser within the focuser if the beam is not straight then the point will spin around on the primary mirror if this is the case you can adjust the beam Direction with an allen wrench but that's really a topic for another video Let's assume for the purpose of this video that our laser is pointing straight the first thing we'll need to do is align the secondary mirror to align the secondary mirror we need to get the laser dot in the center of the primary mirror we're going to do this with a screwdriver adjusting these screws here ever so slightly until that laser beam is centered on the primary mirror make sure the optical tube is horizontal so that you don't drop the screwdriver onto the primary mirror that would be terrible now we need to move on to the primary mirror you can see here that the cator has a screen on its side now the goal here is to get the return beam centered in the bullseye hopefully the laser beam is somewhere on the screen if not you may have to do a little bit of guess and check but for the most part it should probably be there okay the first thing we need to do is to loosen the mirror and in this case we're going to loosen it by loosening these white knobs here on this scope but on most dobsonians it's going to be the smaller knobs in this case this one this one and this one the next step is to adjust the direction of the mirror with the other knobs in this case the black knobs or in this case the larger knobs and what we're going to do is adjust the mirror and until the laser is centered in the bullseye on the screen once the beam is centered you want to tighten the smaller knobs to lock the mirror in place again but you'll need to do this slowly and carefully because these will also adjust the position of the mirror colation takes a bit of practice but after a few attempts you'll have it down in a matter of seconds now that the telescope is cated it's time to align the finder this is something you should also check every time you set up your telescope and it's far easier to do during the day I generally use a distance object like a chimney or a flag at night you'll want to use a bright star just make sure that you're aligning the telescope to the same bright star now even if you're using star sense or other technology you'll still need to do this because you'll need to align star sense to the telescope as well you want to make sure that star sense and the telescope are pointed at exactly the same star and this is far easier to do if your telescope and finder are correctly aligned to align the finder attach your highest focal length eyepiece put that in the focuser and get an object of interest centered in the telescope you might take a moment to get the telescope in Focus here as well while keeping the telescope in the same place move over to the finder use the knobs on the finder to move the position of the dot Bullseye or crosshairs until they're centered on the target then move back to the telescope to confirm that both the finder and the telescope are pointed at exactly the same spot now let's talk about choosing an ipce this is probably the most popular question I get on this channel some variation of what IP should I use or what IP should I get from my telescope well in general if you're new to stargazing with a telescope it's best to get proficient at using the ipes that came with your telescope they came with your telescope for a reason dobsonian telescopes and telescopes in general tend to come with two eyepieces one eyepiece is generally larger and usually has a focal length of between 20 and 25 mm this is the eyepiece you'll use most of the time the other eyepiece has a lower focal length offering higher magnifications this eyepiece is only used after you found your target with this eyepiece and you want to zoom in for a closer look note that zooming in makes objects look bigger but it doesn't necessarily make them look better it's kind of like reading a book you can move the book closer to your face the words may look better but it won't improve the story if you have several eyepieces to choose from or if you're planning to upgrade your eyepieces here are some tips to help you with that if you want an eyepiece for up close views of the planets you can choose an eyepiece up the telescope's maximum useful magnification this is approximated by multiplying the aperture of the telescope by two for example this telescope has an aperture of 200 mm so the maximum useful magnification is 400 divide your telescope's focal length by this number and that will give you the eyepiece focal length that will give you that magnification for example if the focal length is 1200 mm then a 3mm focal length eyepiece will provide that magnification now 3 millimeters is extremely small you'd probably just choose your closest popular focal length which is probably 5 millimeters which would give you a magnification of 240x note that this is still a lot of magnification and that this power objects will be difficult to track and difficult to get back into frame if you lose them but learning to deal with these challenges can be part of the fun that said I have a nice 10 mm eyepiece and that's enough magnification for me now for deep sky objects there's a trick to choosing the ideal eyepiece this will make a big difference because dobsonians are designed to be used to observe deep Sky objects the trick is to match the size of the beam of light coming out of your telescope to the diameter of your pupil exit pupil is the term for the size of the beam of light coming out of your eyepiece and it's calculated by dividing the aperture of the telescope by the magnification you can rearrange this formula to solve for the eyepiece as follows I piece equals pupil size times telescope focal length divided by aperture let's say your pupil size is 4 mm your telescope has an apature of 200 mm and the focal length is 1200 mm then the eyepiece you would need is 24 mm and this is why 24 mm is a very popular size for a telescope ipce if we estimate that our pupil size is only 3 mm then 18 mm is the eyepiece that would solve this formula need to take just a moment to talk about about the importance of Dark Skies it's really important to note that your success in observing deep Sky objects like galaxies and nebuli is determined far more by the darkness of your Sky than the quality of your telescope for most deep Sky objects a moonless night far from City Light is required for optimal views you'll also need to adapt your eyes to the dark by abstaining from looking at artificial lights including your cell phone for 20 minutes or more leading up to your observing session to find the Dark Skies nearest you you can check an online light pollution map I'll post a link in the description now that you know which eyepiece you're going to use it's time to choose your targets if you're new to telescopes or you're having trouble pointing or focusing it's best to practice on the Moon make sure the craters are crisp and sharp in your field of view and that you're able to successfully track the moon as it moves across the sky that said the Moon is one of the most impressive Targets in the sky which is partly why our book 50 things to see on the moon is so popular this book won the RC's Simon Nukem award in 2020 if you're looking for planets you'll need to use astronomy software like stellarium since planets change position over time planets can be found near the ecliptic which is the path the sun takes across the sky but they can easily be confused with bright stars so it's best to check the software to confirm where and when the planets will be visible for finding deep Sky objects like galaxies and nebula and star clusters it's best to use a stargazing guidebook as astronomy software tends to oversell what you're realistically able to see many of the Targets in astronomy software are photography targets and can only be seen with long exposures the human eye is just not that sensitive the most popular list of targets for stargazers is called the Messier list the Messier list was developed by French comet hunter Charles Messier about 200 years ago in our book 110 things to see with a telescope we provide a custom star map for every Target and a telescope view image showing you what you can expect to see under ideal seeing conditions if you see all 110 Messier tar organizations like the Royal Astronomical Society of Canada or the astronomical league in the United States will provide you with a certificate for your observations I've provided application instructions in the appendix of the book I hope you enjoy this tutorial on how to use a dobsonian telescope subscribe to learn to stargaze to take your stargazing experience to the next level if you'd like to support us you can find us on patreon and remember the future is looking up and a special thank you to St Mary's University and the Burke Gaffney Observatory for letting us film here today if you're interested in taking astrophysics here at St Mary's University check it out at www.m.com

Channel: LearnToStargaze

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Length: 14min 46sec (886 seconds)

Published: Fri Jun 21 2024