Podcaster:  Hens Zimmerman

Title: My experiences as an amateur astronomer with a computer controlled telescope

Organization: Adventures in astronomy blog


Description: In this episode of the 365 days of astronomy podcast amateur astronomer Hens Zimmerman talks about what it’s like to own a computer controlled “goto mount” telescope. There are hints and tips for those who consider getting such a telescope themselves. Or maybe already own one, but have been struggling to get adequate results.

Bio: Hens Zimmerman used to have a DIY telescope when he was a kid. Now, as a busy adult with a family and a day job, he has a computer controlled “goto mount” telescope. He lives in the light polluted and often bad weathered Netherlands. Hens is married, has a daughter and is a post production audio engineer during the day.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. We still need sponsors for many days in 2014, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at


Hi, my name is Hens. I’m from the Netherlands, one of the most light polluted areas in the world. Nevertheless, I’m the owner of a moderately sized telescope with a computer controlled goto mount and that’s what I’m going to talk about in this episode of the 365 days of astronomy podcast. I got my telescope from my wife as a birthday present a few years ago. I’m 47 now, but when I was 10 I looked at the sky with a small home made telescope. It had a wooden mount and a primitive finder scope my father had constructed out of an old binocular. Even with such a modest little telescope I learned the basics of finding objects in the night sky. The great Orion nebula, also known as Messier 42 was one of my favorite objects in the winter sky. I also looked at double stars like Albireo, planets like Jupiter and Saturn and I could gaze at the moon for a long time, imagining what it would’ve been like for the Apollo astronauts who had traveled all the way to the moon and back in that same decade. Finding anything else but the brightest objects was pretty much impossible. Now, 37 years later, I own a Celestron NexStar 5SE, a telescope with a computer controlled goto mount. This means it has a dedicated little computer with a hand controller attached to it, which can automatically point the telescope to astronomical objects that I can select from a built in database. The computer of such a telescope is a clever little machine which knows exactly where stars, planets and galaxies are located now and in the future. You only need to tell it where you are standing, what time it is and how it should align itself to the night sky. You do this once every time you power up the telescope. After that, you only choose objects from the hand controller and the motors that turn both axes will point the telescope to the desired object. When you are looking at an object, the motors slowly compensate for the Earth’s rotation so the object stays centered in your eyepiece.

Although my telescope is from the Celestron brand, much of what I will talk about today is applicable to any kind of amateur telescope. The NexStar 5 has a five inch aperture. The aperture of a telescope limits the amount of detail you can see. It doesn’t matter what eyepiece you use, the amount of detail stays the same. For instance, if I look at Jupiter with this telescope, I can see *some* surface features, but not the exquisite detail you see on some pictures that were made with an 8 inch telescope. It’s always impressive to look at Jupiter, though. It’s one of those objects you can show your neighbors, and when you tell them those 4 little dots lined up next to Jupiter are actually its 4 biggest Moons, you can tell by their silence that they are very much impressed.

When you read an advertisement for a computer controlled telescope, it looks like things couldn’t be easier. “Get your telescope aligned and ready for observing in minutes!”. Well…. It took me at least a year to get comfortable with this telescope. Like many of us, I have a day job and a family. I also live in a crowded light polluted country where the weather is grey and rainy lots of days. So I can go outside to watch the stars with my telescope only on those rare occasions when I’m not tired, I don’t have to work the next morning *and* the weather is good enough. Living in a city also means that I can only look at bright objects that are not obscured by buildings, trees or light pollution. On my first nights out with the telescope, I got very frustrated with the alignment process. After all, I should be able to align it with three random objects in the night sky in minutes, right? Running the telescope on batteries turned out to be a joke. Even with brand new batteries, I quickly ran out of power. Aligning on three random objects often resulted in a failed alignment, according to the computer. So this was all very disappointing. The first thing I did to improve this situation was connect the telescope to a DC power supply. This was a real improvement over the batteries. I eventually exchanged the DC power supply for a rechargeable power tank, which is more portable. I also bought a GPS device, because it was too much of a hassle to input the exact time, date and location. The Celestron GPS i got is quite different from what I’m used to on my mobile phone. This particular device easily takes several minutes to link to the GPS satellites and adjust the time and location of the telescope’s computer. But because it’s a good practice to let the telescope adjust to the outside temperature before you use it anyway, these are two things you can combine. I put my telescope outside and power it on. Half an hour later when I go outside, it has adjusted itself to the outside temperature and the GPS has linked to several GPS satellites. So you can see, observing already starts to become a thing where you need a lot of patience. With the GPS and the external power supply, I found it easier to experiment with my telescope.

Quickly and precisely aligning a telescope is crucial when you want to actually enjoy your precious time outside with your telescope. I abandoned the three star align method. You see, I can easily identify a few bright stars in the night sky, so there’s no reason to use three random objects when I can find two bright stars myself. Right now, in July, it’s easy to locate stars like Altair, Deneb and Arcturus if you live in the Northern hemisphere. With just two known stars, I can align the NexStar computer with the night sky through a procedure called TWO STAR ALIGN. It’s a simple matter of aligning the alignment star with a little red dot in the finder scope and then centering it in your eyepiece. You repeat this procedure with the other alignment star and then the telescope know the rest. After a successful alignment, the NexStar computer knows which planets, stars and galaxies are above the horizon for your location at that precise moment in time.

If you spend some time outside with your computer controlled telescope, you will quickly notice that sometimes the telescope is aligned pretty good, but most of the time it’s a little off. The NexStar computer has all kinds of options to re­align itself or temporarily align the telescope on certain parts of the sky. For me it took at least a year to get really comfortable with the whole alignment process. So my advise would be to experiment as often as you can. You will find that it’s best to align on stars that are far apart and also not exactly above your head.

Maybe it sounds a bit like lazy astronomy: having a computer controlled telescope find objects. Where’s the romance in telling a machine to find an object instead of hunting it down yourself? But finding faint objects in light polluted skies just becomes a new kind of activity, with lots of trial and error. Only there’s a handy little computer to help you with it.

One of the best features of my NexStar telescope is called PRECISE GOTO. Here’s how it works. At first you align your telescope as good as you can, most likely with two bright stars as I described earlier. Even with the aid of a GPS and a lot of patience, you will notice that the alignment is not always spot on. For faint objects like comets or galaxies it can be quite frustrating when you don’t see anything except for a few stars in your eyepiece when the motors stop turning and your desired object should be visible. PRECISE GOTO remedies this situation: you enter an object from the catalog of the hand controller or the exact coordinates for something like a comet, and then the computer first suggests a bright nearby star. It asks you to center this nearby star as good as you can, and when you hit the align button, only then does it point the telescope at the desired location where that faint comet or galaxy is supposed to be. In effect, the telescope temporarily becomes a lot more precise at a small region of the night sky. This is how I’ve been able to look at comets like C/2011 L4, which were barely visible through my eyepiece.

So how do you find the exact coordinates of a comet? Well, this brings me to another important issue. It’s very beneficial to plan your observing sessions before you go outside. If I don’t have a plan, I usually look at the 1 or 2 planets that are currently dominating the night sky and maybe the moon, but that’s it. On the other hand, when I plan my observing session in advance, I may be able to see a comet or a very faint galaxy that I’ve never seen before from my backyard. I plan my observing sessions using Stellarium, a free open source program which runs on all the major operating systems. Stellarium has an option called the Solar System Editor, which lets you find the exact coordinates for comets at a given time for your location. For instance when I’m writing this, comet C/2012 K1 is still very well visible in the constellation Leo. At magnitude 7.8 this comet should just be visible with my 5 inch telescope at my location. On a clear night, I enter C/2012 K1 in Stellarium’s search window. I can advance the time to something like 5 minutes into the future and note down the right ascension and declanation of C/2012 K1 at that exact moment in time. Outside I enter these coordinates in the precise goto menu of the telescope’s hand controller. It then searches for a bright nearby star. With that one centered I hit align and there it is, C/2012 K1 centered in my eyepiece. Comets can move pretty quickly against the background stars, so it’s important to be as precise as you can. I have described the complete procedure of finding a comet using Stellarium and PRECISE GOTO on my blog, so if you need to look it up, it’s there with screenshots and all.

Planning in advance becomes even more important when you start to take pictures of celestial objects. A 5 inch telescope is of questionable quality for astrophotography. If you are really serious about taking photos of planets and deep sky objects, invest in a larger aperture telescope. One fun aspect of astronomy is that you can upgrade parts of your equipment while keeping everything else. For instance, right now I have a Meade DSI II PRO ccd camera and if I ever get myself a bigger telescope, I can keep the ccd and use it with my new telescope. It’s the same for eye pieces and filters. It’s easy to spend a lot of money on equipment, so do consider buying some of it second hand. There are specialized websites where people sell used equipment.

After having spent a lot of time with my 5 inch telescope I can now set it up pretty quickly. With a little planning ahead I know what to expect. I know which part of the sky is unobscured by buildings or trees and I can try to find an interesting object in that particular patch of the sky. I enjoy gazing at Venus, Mars, Jupiter and Saturn. The moon, comets, globular clusters such as Messier 3 and deep sky objects such as the great Orion Nebula. And when I align my telescope on a star like Arcturus, I smile, knowing that those photons were ejected from that particular star almost 37 years ago, when I was 10 years old, struggling with my little DIY telescope.

In conclusion:

  • It’s great to have a computer controlled telescope, but make sure you address the following issues:
  • Get an external power supply
  • Buy a GPS device that your telescope can talk to directly
  • Plan your observing sessions in advance behind your computer with a program like Stellarium ­ Experiment a lot, don’t believe the advertisements and be very patient with yourself and your equipment.
  • Try out some things during daytime if you can.
  • ­ Explore the PRECISE GOTO feature if your telescope has it.
  • Read the manual.
  • Take notes of what you have done, because months may go by where you have little or no spare time to gaze at the stars.
  • Don’t expect to see Hubble space telescope quality images
  • ­ Push yourself a little further every time and try to enjoy the process.

Thanks for your attention and happy stargazing!

End of podcast:

365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by Astrosphere New Media. Audio post-production by Richard Drumm. Bandwidth donated by and wizzard media. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at or email us at In the new year the 365 Days of Astronomy project will be something different than before….Until then…goodbye