In this article, I’ll teach you how to quickly frame whatever target you have on your wishlist with the Star Adventurer Pro, regardless if you are using a wide-angle or telephoto lens or your target being large, small, bright or faint.
On paper, star trackers are meant to be for wide-field astrophotography or starry landscapes. And the Skywatcher Star Adventurer Pro is no exception. On paper…
Show of hands! Who here, after taking a few Milky Way shots and a couple of star fields, didn’t want more. Nobody? 🙂
When you find yourself ditching your wide-angle lens for a 70-200mm zoom or 300mm telephoto lens, this is where the trouble begins.
Shows of hands! Who here doesn’t know the struggle and the frustration trying to frame this faint nebula or that small galaxy? Who didn’t wish for a GoTo version of the Star Adventurer*?
I know, I know, I have the SkyWatcher AZ-GTI GOTO mount too, and I know you can turn it into a Star Adventurer with GoTo, but if you think GoTo is the solution to every problem, you haven’t tried it yet 😉
What Is Star Hopping?
The term star hopping refers to the practice of finding a particular target by hopping to it from nearby stars that are readily visible.
Say you are trying to spot the Andromeda Galaxy. First, you will look for the Andromeda constellation.
Then, you will look for the star Mirach. With the Andromeda constellation oriented as in the image below, you will go right of Mirach by two stars and the hop a bit down and to the right. You should see a puffy grayish cloud.
Congrats, you have just found the Andromeda Galaxy.
Star Charts For Astronomy
Star charts allow astronomers to orient themselves when observing the night sky. Today you can download for free star charts like the one below.
They are great tools for getting you to know the night sky, to make observations with binoculars and telescopes, and for wide star field astrophotography.
Since the stars magnitude is also indicated, you can use them to estimate the brightness of the sky using the Naked-Eye Limiting Magnitude method.
Apps For Astronomy
Nowadays, we all walk around with a small computer in our pocket and there are many apps for our smartphones that are appealing to astronomers and astrophotographers.
An atlas such as Sky Guide (iOS), Stellarium (Desktop, iOS, Android), etc., are great and interactive alternatives to star charts.
Those apps give you tons of info about the night sky and can read the orientation of your phone: simply point it at the sky, and you will see on the screen what there is in that particular direction.
You can use them to plan the shot, combining info like the size of the target and its visibility.
And augmented reality lets you see the sky map as a semi-transparent layer over the scene seen by your camera phone: a welcome feature for starry landscapes.
Measure Distances In Degrees: Your Hand As A Measuring Tool
Because it is not practical to measure distances between stars using a measuring tape, we turn to angular separation.
Looking at the star chart shown before, the Andromeda Galaxy and Mirach are separated by about 5º.
But did you know you can use your hand as a measuring tool to calculate angular distances between stars?
Stretch your arm in front of you:
- The width of your pinky covers about 1º. Since the size of a full Moon and of the Sun is about half a degree, your pinky will cover them entirely.
- The width of your first three fingers side-by-side covers a separation of about 5º.
- The width of your close fist is about 10º.
- If you spread apart your pinky and your first finger, you will cover a separation of about 15º.
- Finally, if you spread apart your pinky and your thumb, you will cover a separation of about 25º.
This method is great if you want to guide someone towards a particular part of the sky and you don’t have a green laser, but it is way too coarse for framing targets in astrophotography.
How To Manually Frame A Target For Astrophotography
In astrophotography we can use wide-angle lenses, offering a wide field of view, or long telephoto ones and telescopes, the latter having a much narrower field of view of a few degrees or less.
Here is what you need to remember:
- The longer the focal length, the narrower the field of view;
- The smaller the camera sensor (or the shorter the eyepiece), the narrower the field of view;
- The narrower the field of view of your setup, the more difficult it is to find your target.
The comparison below shows the field of view around Deneb.
With a 170mm equivalent focal length (top), both Deneb and the North America & Pelican Nebulae are in the frame. This makes framing the scene real easy.
With a 600mm lens (bottom), though, things get more difficult as Deneb is not in the field of view of the instrument.
We have three ways of manually find and frame a target:
- We can use a red dot star finder, or …
- We can plate solve an image, or …
- And this is the best way, we can use celestial coordinates to hop from a star to our target.
Before hopping (pun intended) straight at the star hopping section, let’s briefly see what red dot star finders and plate solving are.
Use A Red Dot Star Finder To Frame Visible Targets Or Regions of The Sky
A red dot star finder is a great tool to let you frame a particular part of the sky.
Basically, the star finder shoots a red laser on a transparent screen, forming a visible red dot. Simply superpose the red dot to the intended target to bring it inside the field of view of the instrument.
This works wonderfully to frame the Moon, the planets, stars, and bright deep-sky objects like M42.
For most of the wide-field astrophotography you can do on a star tracker, you do not need more than this.
Take Test Shots and Plate Solve Them: Hi-Tech, But Slow
This is a very time consuming way to find a target. You have to take a test shot and plate solve it by comparing the star field in the image with that of a star chart or an app.
Since I am not a backyard astrophotographer, I don’t usually carry my laptop with me and I was used to plate solving “by-eye”.
I do this by comparing the star field in the photo with that shown by Sky Guide, my favorite astronomic app. Sometimes it could take me more than 10 minutes to find my target.
The Best Way: Using Celestial Coordinates For Precise Star Hopping
This is by far the best way to frame with ease any kind of target with your Star Adventurer.
Once you get a grasp on the method, you will realize how the often overlooked graduation circles at the back of the Star Adventurer are worthy of half of a GoTo.
And they are also what, in my opinion, set this tracker above the other competitors.
Those graduation scales at the back of your tracker are there to let you polar align the Star Adventurer without the use of an app or a computer.
But a less-known feature is that they can also be used to measure changes in Right Ascension.
To complete this “Manual GoTo”, you need a graduation circle for measuring changes in declination (more on this later).
Right Ascension And Declination: A Global Coordinate System For Stars
Right Ascension (RA) and Declination (DEC) are defined with respect to the Earth rotational axis and its equator.
The direction of the Sun over the equator, the Vernal Equinox (March 31st), sets the 0h for the RA.
This image below shows how these coordinates are defined.
For these reasons, those sets of coordinates do not depend on your latitude. And because RA and Declination change very slowly in time, we can consider them to be constant in a period of 20 years or so.
Find And Use Right Ascension And Declination Coordinates
Any astronomy app can show the RA and DEC coordinates of a celestial body.
All you need to do in order to star hop is to compute the differences between the RA and DEC coordinates of your intended target and those for the star you want to hop from.
As usual, an example can clarify. Say you want to photograph the Ring Nebula, M57, in the Lyra constellation.
This planetary nebula is very small, and you will probably use at least a 400mm lens to image it, meaning you have to deal with a rather narrow field of view.
But the nebula is also faint, and you will not probably see it in your live view.
So, how do we frame it without going crazy?
Easy peasy: nearby, there is Vega, one of the brightest stars in the Northern Hemisphere. You cannot miss it, and that makes it a good star to hop from.
Sky Guide, Stellarium, and all similar programs tell me that:
- Vega has RA = 18h 37m and DEC = 38º 48’.
- M57 has RA = 18h 54m and DEC = 33º 03’.
All we need to do is to compute the difference between those coordinates to know how much to move the camera in RA and DEC after having framed Vega.
These differences are: ΔRA = + 17m (with “+” meaning we move eastwards) and ΔDEC = -5º 45’ (with “-” meaning we have to rotate the camera downwards). Just remember you have 60m in 1h and 60’ in 1º.
The beauty of this method is that you can make your own database of ΔRA and ΔDEC computed in the comfort of your home, and use it in the field for the next 20 years or so.
But how should you implement those movements with your Star Adventurer?
Prepare your Star Adventurer For Star Hopping
In this post on my personal blog, I explain to you how to build a graduation circle for the declination using spare parts you may find lying around in your home or at your local Home Depot.
If you are not into DIY, you can buy 3D printed kits, such as this one from Astrokraken.
The Hopping Procedure In 6 Easy Steps
Here is where things appear to be complicated (but really, they are easier done than said), so bear with me.
Step 1: Polar Align The Mount.
At least a coarse polar alignment is needed for a precise star hopping.
Whatever is the method you use to polar align your mount, note the position of a mark on the date scale that lines up with one on the time scale: we will use this info to refine the PA at the end of the procedure.
Let’s call the mark on the date scale datePA and call timePA the mark on the time scale the datePA is aligned to.
Step 2: Frame The Visual Target You Want To Hop From
Go to frame the object you want to hop from.
You can quickly do this by using a red dot star finder. Choose an object to hop from that is close to your intended target, to make the whole process easier and more precise.
Step 3: Hop In RA
Check the amount of rotation you have to perform in RA in order to hop on your intended target. In our example, to hop from Vega to M57, you would have to rotate the camera by +17m in RA.
Since M57 has higher RA, we rotate the payload of +17m eastwards, i.e., clockwise. I assume here you are on the side of the polar scope and the mount is pointing to North.
In the southern hemisphere, you would rotate in the opposite way.
If you can, try hopping from a star close enough to your target, so as to use the motor of the Star Adventurer instead of manipulating the clutch. This will reduce the chances of losing the initial polar alignment.
Beware though: to cover 1h RA rotation, you will have to keep pushing the button for 5 min.
Step 4: Hop In DEC
Time to hop in declination.
For this, look at your camera orientation and use the fine adjustment knob to raise or lower your camera orientation by the proper amount.
In our example, we have a ΔDEC = -5º 45’, and so we have to lower the camera of 5º and 45’.
The graduation scales usually can rotate so that it is easy to line up the 0 with the 0 on the fix scale we taped on the declination deck: this way, it is easy to rotate the camera of the proper amount.
Don’t go crazy because of the 45’: the precision of these circles is about 1-2º.
Try to do your best eyeballing the proper amount of rotation: all we want is to have the target in the field of view of our instrument. We can always refine its position once we see it.
Step 5: Take A Test Shot
Take a test shot: you should now have your intended target visible in the frame.
If you need or want to, you can use the motor controls and the declination fine adjustment knob to refine the position of your target in the frame.
Step 6: Refine The PA
After all these steps, it is essential to check and, eventually, refine your polar alignment.
To do this, we have to know the final rotation with did in RA, so that we can find the position of Polaris in the reticle inside the polar scope.
Begin by checking to which mark in the Time scale your datePA is now aligned to and compute the difference in RA with respect timePA.
This way, you have the total amount of rotation in RA since you did the initial polar alignment.
If you use an app such as PS Align Pro, you can simply input this rotation in the app, and it will show you the position of Polaris on the reticle that is now oriented as inside your polar scope.
If you were using the graduation circles to PA, you should consider that every rotation of 10m in RA corresponds to a rotation of 2.5º of the reticle.
For example, let’s say we did a total rotation in RA of +2h 20’ since the initial polar alignment.
When you look at the reticle, Polaris must be on the mark that is 8 x 2.5º = 20º before the “6” of the reticle (as we have rotated it clockwise).
This is because the 6, where you put Polaris for the initial polar alignment, as now moved clockwise by 20º.
How Accurate Is Star Hopping?
This method of star hopping is limited by the precision of the graduation scales you use.
With telescopes having very long focal lengths, you may struggle to frame the target even when you are doing star hopping, as the field of view of the instrument is very narrow.
But in practice, considering the limitation of a tracker, for anything you can throw on the Star Adventurer for astrophotography, the star hopping method is accurate enough that you will not miss a GoTo functionality.
A Video Tutorial
In this video, I will explain in more detail what star hopping is, what stellar coordinates are, how to prepare the star adventurer for star hooping, and I will give a practical demonstration about star hopping with the star adventurer.
In this article, we have discovered how to use the graduation circles at the back of the Star Adventurer to frame with ease any deep-sky target in the sky, no matter what gear you throw on the mount or how small and faint your intended target is.
You need a graduation scale to measure differences in declination and a bit of practice, but, honestly, it is easier and faster to do it than it is explaining how you must proceed.
Give it a try, and I assure you will not regret anymore not having a GoTo mount.