# Finding your way on the sky with a telescope **URL:** \ **Category:** Factual Questions\ **Created:** [April 28, 2020, 7:15am UTC](https://boards.straightdope.com/t/finding-your-way-on-the-sky-with-a-telescope/852712 "2020-04-28T07:15:40Z")\ **Posts on this page:** 15\ **Page:** 1
**Author:** ![Schnitte](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/schnitte/32/9033_2.png) [@Schnitte](https://boards.straightdope.com/u/Schnitte)\ **Post date:** [April 28, 2020, 7:15am UTC](https://boards.straightdope.com/t/finding-your-way-on-the-sky-with-a-telescope/852712/1 "2020-04-28T07:15:40Z")
I recently bought a telescope on an equatorial mount. I’m having a lot of fun with it, but so far I’ve been observing mostly things that can be seen with the naked eye and therefore be found very easily with an optical finder or a telrad. Now I want to move towards fainter objects which can only be seen in the main telescope. That means I have to find my way on the sky more systematically in the tiny field of vision of the eyepiece, so I read up on the coordinate system and the handling of the equatorial mount. Am I right in understanding that, with respect to stars, the cardinal directions north etc. have nothing to do with the respective directions on the ground, but rather only with the directions relative to Polaris? So suppose I have instructions saying a particular object lies so-and-so many degrees northwest of star X. Here, “north” means “from star X on a straight line towards Polaris”, and “west” means “from star X counterclockwise on a circle centered on Polaris”, right? And east and south are the exact opposites? So any cardinal direction can be up, down, left, or right, depending on how star X currently stands relative to Polaris? Building on that: Am I right in assuming that (provided I have set up the telescope properly) any sweep of the telescope along the right ascension axis is exactly in east-west direction, and any sweep along the declination axis exactly along the north-south direction? ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [April 28, 2020, 8:06am UTC](https://boards.straightdope.com/t/finding-your-way-on-the-sky-with-a-telescope/852712/2 "2020-04-28T08:06:59Z")
The cardinal directions in the sky are the same as those on the ground: Polaris is north, the Sun rises in the east. But, depending on the type of telescope and eyepiece, when you look through it north might be down and east to the right, or north might be up but the image flipped left-right. I would start by figuring it out for your particular kit. Now, star catalogues and almanacs that I have seen give the coordinates of an object in terms of right ascension and declination. But Jupiter, the Moon, etc most certainly have a north pole and a south pole. The Earth’s axis is somewhat tilted, but unless I am horribly mistaken the north pole of planets (except ones like Venus and Uranus) should point towards the north. ---
**Author:** ![Schnitte](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/schnitte/32/9033_2.png) [@Schnitte](https://boards.straightdope.com/u/Schnitte)\ **Post date:** [April 28, 2020, 8:16am UTC](https://boards.straightdope.com/t/finding-your-way-on-the-sky-with-a-telescope/852712/3 "2020-04-28T08:16:08Z")
> [@DPRK](#): > > The cardinal directions in the sky are the same as those on the ground: Polaris is north, the Sun rises in the east. But, depending on the type of telescope and eyepiece, when you look through it north might be down and east to the right, or north might be up but the image flipped left-right. I would start by figuring it out for your particular kit. This for sure - in my case it’s a Newton, which - to my understanding - has an even number of mirrors and therefore does not produce mirror images in the eyepiece, but it’s still flipped upside down. But are you sure about the rest of your reply? It sounds to me like you’re saying that for any given eyepiece, the cardinal directions are constant within the eyepiece (so that once you find out north is up, it will always be up when you look in the same direction), and that’s not how I understand it. Suppose you have a star that is, at one moment, exactly above (when seen from my position) Polaris. Obviously the star is south of Polaris, everything is. I would imagine that twelve hours later the same star would be below Polaris, but it would still be south? ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [April 28, 2020, 8:22am UTC](https://boards.straightdope.com/t/finding-your-way-on-the-sky-with-a-telescope/852712/4 "2020-04-28T08:22:48Z")
> [@Schnitte](#): > > Am I right in understanding that, with respect to stars, the cardinal directions north etc. have nothing to do with the respective directions on the ground, but rather only with the directions relative to Polaris? So suppose I have instructions saying a particular object lies so-and-so many degrees northwest of star X. Here, “north” means “from star X on a straight line towards Polaris”, and “west” means “from star X counterclockwise on a circle centered on Polaris”, right? And east and south are the exact opposites? So any cardinal direction can be up, down, left, or right, depending on how star X currently stands relative to Polaris? An extreme case would be standing at one of the poles. Then the stars are clearly whirling in a circle in a certain direction, so will appear at any angle around you. Anyway, if I understood your description it sounds right, just imagine the usual equatorial coordinates you would find on a map projected onto the sky. That is what your right ascension and declination basically represent, except right ascension is measured in hours instead of degrees like longitude. To convert to ecliptic or horizontal coordinates you have to do some trigonometry. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [April 28, 2020, 8:40am UTC](https://boards.straightdope.com/t/finding-your-way-on-the-sky-with-a-telescope/852712/5 "2020-04-28T08:40:11Z")
> [@Schnitte](#): > > It sounds to me like you’re saying that for any given eyepiece, the cardinal directions are constant within the eyepiece (so that once you find out north is up, it will always be up when you look in the same direction), and that’s not how I understand it. Suppose you have a star that is, at one moment, exactly above (when seen from my position) Polaris. Obviously the star is south of Polaris, everything is. I would imagine that twelve hours later the same star would be below Polaris, but it would still be south? I think I understand your question. Once you set up your telescope on a equatorial mount, it will stay pointed at the same point in the sky. If the axes are fixed, the stars will indeed rotate around Polaris counterclockwise, but using a clock drive will compensate for that rotation and the stars will stop moving around. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [April 28, 2020, 9:08am UTC](https://boards.straightdope.com/t/finding-your-way-on-the-sky-with-a-telescope/852712/6 "2020-04-28T09:08:41Z")
Of course, with the motor drive the telescope itself is moving relative to the ground, so, if you think about it, leaving it pointed at an object that rises and sets will result in the tube pointing towards the earth some of the time. But then you would not be able to see it anyway, so no need to contemplate where you would have to stand. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [April 28, 2020, 12:47pm UTC](https://boards.straightdope.com/t/finding-your-way-on-the-sky-with-a-telescope/852712/7 "2020-04-28T12:47:09Z")
> [@Schnitte](#): > > Building on that: Am I right in assuming that (provided I have set up the telescope properly) any sweep of the telescope along the right ascension axis is exactly in east-west direction, and any sweep along the declination axis exactly along the north-south direction? Yes, that is correct. But in practice, it’s difficult to find objects using coordinates even if you have [setting circles](https://en.wikipedia.org/wiki/Setting_circles). You really should learn to [star hop](https://en.wikipedia.org/wiki/Star_hopping). I generally look at a star chart and look for distinct star patterns that guide me towards the object I’m looking for. It could be as simple as “take these two bright stars, extend the line by 2x the distance between those stars” - as in the case of the [double cluster](https://earthsky.org/clusters-nebulae-galaxies/double-cluster-clusters-of-supergiant-suns). Make sure your finderscope is aligned, and get a feel for how big the field of view of the finderscope is. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 28, 2020, 1:02pm UTC](https://boards.straightdope.com/t/finding-your-way-on-the-sky-with-a-telescope/852712/8 "2020-04-28T13:02:03Z")
Yes, you’re correct so far. One point that trips up some newbies, though: For a map of the Earth, the directions will be labeled ```auto N W E S ``` while for a map of the sky, the directions will be labeled ```auto N E W S ``` Also, don’t give up on your finder scope and Telrad just yet. There are some sky map books that show the visible stars in the vicinity of an object of interest, often with the Telrad reticle shown on the map. So if you line up the visible stars in the Telrad so they match the diagram, and then look through the main scope at the middle of the spot, you’ll see what you’re looking for (you may need to do an intermediate step with the finder scope, but I’ve found that’s usually not necessary). Or you can just aim at some random spot in the general vicinity of Sagittarius. That works a lot oftener than you’d think, too. ---
**Author:** ![Schnitte](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/schnitte/32/9033_2.png) [@Schnitte](https://boards.straightdope.com/u/Schnitte)\ **Post date:** [April 28, 2020, 2:41pm UTC](https://boards.straightdope.com/t/finding-your-way-on-the-sky-with-a-telescope/852712/9 "2020-04-28T14:41:09Z")
Thanks everyone. Things are becoming clearer for me, and I think I’ve grasped the theory and geometry behind it. Putting it into practice is a different thing, though, so I’ll need to practice it next time I’m in the field. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [April 29, 2020, 1:55pm UTC](https://boards.straightdope.com/t/finding-your-way-on-the-sky-with-a-telescope/852712/10 "2020-04-29T13:55:53Z")
Just to add, the purpose of the Telrad isn’t to just point at objects you can see with your naked eye. The true value is that the Telrad allows you to point the telescope at a specific point in the sky, relative to the surrounding stars. Look at a star chart, figure out where the object is relative to a few of the closest stars, then look up in the sky and find that spot. Then point the Telrad at that spot. Then look in the telescope using the lowest magnification available. ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [May 5, 2020, 6:29am UTC](https://boards.straightdope.com/t/finding-your-way-on-the-sky-with-a-telescope/852712/11 "2020-05-05T06:29:10Z")
Slight tangent: Do those telescopes with auto-aim work well? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 5, 2020, 1:50pm UTC](https://boards.straightdope.com/t/finding-your-way-on-the-sky-with-a-telescope/852712/12 "2020-05-05T13:50:35Z")
They’ll always get you into the vicinity. But unless you too great care to set your telescope up just right (probably requiring a permanent installation, not just a tripod out in a field), you’ll still have to do some wandering around the sky to zero in on your target. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [May 5, 2020, 9:43pm UTC](https://boards.straightdope.com/t/finding-your-way-on-the-sky-with-a-telescope/852712/13 "2020-05-05T21:43:44Z")
> [@Reply](#): > > Slight tangent: Do those telescopes with auto-aim work well? From reports in astronomy magazines, they usually work very well. But only if they’ve been properly aligned. The alignment procedure will vary depending on the telescope. It’ll be part of the setup you have to do whenever you place it in a new location. BTW, if you want to google for them, use the term “goto telescope”. ETA: you may also have to go through the alignment again if the scope gets bumped. ---
**Author:** ![outlierrn](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/outlierrn/32/3005_2.png) [@outlierrn](https://boards.straightdope.com/u/outlierrn)\ **Post date:** [May 5, 2020, 10:59pm UTC](https://boards.straightdope.com/t/finding-your-way-on-the-sky-with-a-telescope/852712/14 "2020-05-05T22:59:05Z")
I sometimes use a wide field eyepiece to find my way, something with a field of view of 2-3 deg can makes it easier to see where you are. I use a reticle finder more than a finder scope, but keep in mind that the flips of your telescope may be different than the flips of your finder scope. I have skysafari on my tablet for star charts, it lets me scroll in to the same field of view I’m seeing in the eyepiece. It also lets you adjust the minimum magnitude of stars it displays, so you can approximate what you’re seeing without your chart display being cluttered with a bunch of things your can’t see, or missing useful stars that you\* can\* see. While there’s nothing intrinsically wrong with looking willy nilly, it can be more efficient to plan ahead, have 2 or 3 targets in mind, not too far apart form each other, reasonably well placed for when you’re going to be out (at least, that’s what I’ve heard). ---
**Author:** ![outlierrn](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/outlierrn/32/3005_2.png) [@outlierrn](https://boards.straightdope.com/u/outlierrn)\ **Post date:** [May 5, 2020, 11:11pm UTC](https://boards.straightdope.com/t/finding-your-way-on-the-sky-with-a-telescope/852712/15 "2020-05-05T23:11:50Z")
> [@scr4](#): > > Just to add, the purpose of the Telrad isn’t to just point at objects you can see with your naked eye. The true value is that the Telrad allows you to point the telescope at a specific point in the sky, relative to the surrounding stars. Look at a star chart, figure out where the object is relative to a few of the closest stars, then look up in the sky and find that spot. Then point the Telrad at that spot. Then look in the telescope using the lowest magnification available. For example, M101 is not visible to the naked eye, but it does form the 3rd point of an equilateral triangle with Mizar and Alkaid, the last 2 stars in the handle of the big dipper, which makes it easy to find with a reflex dot finder.