The popularity of Dobsonian telescopes is increasing exponentially day by day. As a result, new astrophotographers often want to start the astrophotography career with a Dobsonian telescope. However, they often get confused about if a Dobsonian telescope can be used for astrophotography or not. I am going to tackle that confusion in this article.
Can a Dobsonian telescope be used for astrophotography? Yes, you can. However, Dobsonian telescopes are not ideal for Deep Space Object (DSO) photography. There are mainly 3 methods to do astrophotography using a Dobsonian telescope: Drift method, Driven method, and equatorial approach method.
Drift method is the simplest way to do astrophotography. It doesn’t require any driven mount. All you need to do is record multiple shots of video as the target object drifts through the field of view and then combine these shots into one image.
It has been found that a planet drifts around 15 arcseconds as each second passes. So, if you set the exposure to 30 milliseconds, you’ll get a drift smearing around 0.5 arcseconds. The image will have similar details that we normally see at night.
Here is a list that shows the time limit of capturing image until rotational smearing causes a drift about 1 arcsecond:
- Saturn: 10 minutes
- Jupiter: 5 minute
- Mars: 5 minutes
- Venus: no real-time limit
If you need to increase the magnification, then using a Barlow lens would be the best way. However, be careful when selecting magnification.
If the magnification is too low, there will be a lack of details in the image. On the other hand, too much magnification will require you to frequently change the position of the telescope which may prove to be tiresome.
A good starting point will be using a magnification that gives F/10 focal ratio. Starting from this setting, experiment until you get the desired image quality. Also, use cameras with bigger and higher-quality chips for the best resolution images.
Here are the steps to capture images with the drift method:
Step 1: Setting Up Your Telescope
The first thing you need to do is set up your telescope. After setting up, allow your telescope a few minutes to cool off. It is necessary to reduce the thermal current. After that, collimate the telescope as you would normally do.
Now, connect a digital camera (preferably a DSLR) to the telescope. Use your laptop to focus clearly on the north start. North Star or Polaris don’t change position with time in the night sky. That’s why this is the best starting point.
Step 2: Adjusting Finderscope
First, adjust your finderscope so that the Polaris is at the center of the frame. Now, align on the target object. Have a look at the exposure settings.
The brightness level of the planet or target object should be at 70% to 80% of the saturation level. The gain should be between 50% to 75% of the maximum.
Step 3: Start Recording
Now, reposition the Finderscope until the planet is within the crosshair of the Finderscope. When the planet enters the frame, start recording. Keep on recording until the planet or target object drifts out of the frame.
Repeat again. Keep on doing until you are outside the time gap for your target object.
Step 4: Joining Videos
Finally, you need to join your video shots together. For this, you can use your preferred software or PIPP using the mode ‘JOIN’. There are also many functions such as object detection, center, crop, etc. which are geared specifically for making your astrophotography stand out!
Though drift method is the easiest to capture planetary images, if you want higher quality and better resolution, driven method should be your choice of preference. With driven method, there is no need to worry about the drift smearing.
As a result, you can increase the magnification as much as you need for a good image. Also, as there is no need to repeatedly set the telescope, you can capture more images in the meantime to tackle the noise later on.
There are 2 popular methods for tracking a planet using a Dobsonian telescope:
- Using a go-to drive system
- Using an equatorial platform
Though these methods are good for planetary images, they are not suitable for taking deep-sky long exposure photos.
Go-to drive systems use friction drives in the altazimuth mount. The setting can be engaged or disengaged if you want to switch from go-to to push-to. This allows the telescope to take more detailed picture.
Equatorial Approach Method
As the name suggests, the equatorial approach method uses an equatorial mount. It gives you about 1 hour to image the planets after which you may have to setup your telescope again. The equatorial mount makes it really easy to track the planetary object.
There are many advantages of using an equatorial mount. You can interchange scopes on the mount. Also, you can set your DSLR tripod on the mount to turn your DSLR camera into a fully equipped track camera.
We don’t need absolute accuracy of tracking for planetary imaging. However, you may need to have control of the drive speed so that you can adjust the speed to keep the object longer in the field of view.
One of the major enemies to a high-resolution astrophotograph is vibration. Vibration makes an image blurry and less detailed. Equatorial mounts are prone to vibration as they use stepper motor which functions like the main drive. This is a major drawback of equatorial approach method.
Why Dobsonian Telescopes Are Not Suitable for Astrophotography?
We all know earth rotates around the sun. Consequently, all the objects seen from the earth rotates too. The drawback of Dobsonian telescope is, it cannot compensate for that movement automatically.
Dobsonian telescopes use altazimuth mount, which means, you need to manually adjust the Finderscope to track any object. As there is no automatic way of tracking the object like an equatorial mount, astrophotography with a Dobsonian telescope gets challenging.
The problem gets worse when you have to use higher magnification. With Dobsonian telescope, we can’t do long exposure photographs. In long exposure setting, the shutter is kept open for a longer period of time so that it can gather more light from the dim target object.
The problem with that is, the target object is always moving. So, when gathering light, the motion of the target object makes the image blurry.
But if you photograph considerably brighter objects such as the moon or the planets, then it won’t be that much of a problem. In fact, Dobsonian telescopes are good for planetary images for a number of reasons:
- As there is no refraction mechanism, you won’t have to deal with color fringing. Also, the central obstruction is comparatively smaller than other type of reflector telescopes, such as the Cassegrain.
- Dobsonian telescopes have big aperture sizes. This is ideal for planetary images. The bright aperture surface can make the image brighter, more contrasted and offers extra resolving power which can make the image super-detailed!
Dobsonian telescopes can be used for astrophotography. However, they are not suitable for photographing faint and dim objects in the night sky. If you want to take images of the bright planets or the Moon, then, by all means, use a Dobsonian telescope.