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Astrophotography itself is a huge topic. It is challenging, expensive and frustrating but also the most rewarding kind of photography. It’s combining the art with a deep scientific understanding of the optics, skies, and image processing. In this article, I will try to explain what is the best telescope for astrophotography and make your way into this hobby as easy as I can. And don’t be disappointed in the beginning, people spend their entire life to improve their astrophotography.
It can take you months or years until you get astrophotos as professionals do. So if you don’t get the same results as professionals do at the beginning, don’t worry just keep going and keep trying. Sooner or later you will get stunning pictures of the celestial objects. It is the most satisfying feeling, and you can experience it even with the cheapest setup for astrophotography. Let’s get started!
What Is OTA
You will hear and read this a lot so let me explain what OTA means. OTA means “Optical Tube Assembly”. It’s just the telescope tube usually with accessories but without the mount. In astrophotography, you have to build your telescope setup. That means buying OTA, mount and other stuff you need separately. It is because everyone has different preferences and also specialized scopes for astrophotography are sold independently as OTA without the mount. Sometimes you find the complete setup, but I prefer to get everything separately.
We have two types of OTA. Refractors and reflectors. In short, refractors are using lenses to capture light and reflectors are using mirrors. You can read more about them in my article refractors vs. reflectors. Here, I will focus only on explaining the differences related to astrophotography.
Prime Focus Astrophotography vs. Digiscoping
Let’s start with digiscoping so we can better understand what the prime focus is. You can do digiscoping with any optical instrument like binoculars, spotting scopes or refracting telescopes and reflecting telescopes. All you need to do is focus the image in your optical instrument and then attach the camera or mobile phone to your eyepiece with a simple adapter. There is no issue with the focus in digiscoping. But this is not how astrophotography is done.
Most telescopes are made for visual astronomy- with the eyepiece. Prime focus astrophotography is when you remove the eyepiece from your telescope and attach modified DLSR camera or CCD astro camera into the eyepiece holder. Then move the focuser until the camera sensor reaches the prime focus point in the focuser, and the image is clear. Here comes the issue. You can’t do that on every telescope. There is almost no problem with refractors and Cassegrain, their focusers are at the back of the tube and have a long range so you can reach the prime focus with the camera sensor.
But reflectors have focuser on the front side of the tube and can’t do long-range movements because they will start to block a lot of light going inside the tube. That’s why the prime focus is deeper in the focuser so you will not be able to focus the image with your camera. It is very common for people to buy the Newtonian reflector, add a camera and then they can’t reach the focus, what a bummer (I made the same mistake)! That’s why many people are using refractors for astrophotography. Anyway, it is not a rule because we already have hacks and specialized Newton reflectors that can be used for astrophotography, but I will talk about that later in the article.
Best Refractor For Astrophotography
So, we know why refractors are so common among the astrophotographers. However, it is not as simple as it might look. Just buying any refractor hook it up on the mount and start shooting award-winning images doesn’t work. Refracting telescopes have one big problem, the chromatic aberration. It is because they are the only ones using convex lenses.
The light is bending in the lens and anytime you bend the light the short blue wavelength bends more than the long red wavelength, so you end up with color fringes around bright objects, called the chromatic aberration. Every photography enthusiast knows this happens even with small camera lenses. It is very annoying and for astrophotography unacceptable.
But the optics came a long way and nowadays we have more advanced optics and lenses in refractors. Simple refractors are still using one element convex lens, and they are not suitable for use in astrophotography because they suffer from horrible chromatic aberration. You need to look for something more expensive with a complex lens system.
Achromatic Refractors “AR”
The refractors with more advanced optics are called Achromatic(AR) refractors. These scopes are using the achromatic lens – two lenses put right next to each other, one convex and one concave. This type of lens is reducing but not eliminating the chromatic aberration. It is still not enough to perform decent astrophotography. But they are more than enough for visual astronomy. You can find them in every astro shop.
Apochromatic refractors “APO”
The next step up is the Apochromatic(APO) refractor. The objective is using three lens element along with extra-low dispersion glass(ED) to reduce the chromatic aberration almost completely. You can’t really see it anymore. Some chromatic refractors also have elements made from extra-low dispersion glass, they are called “near APO,” but they are not even close to APO objective. Chromatic aberration in a good ED-APO refractor is very very low. These scopes are the best choice for astrophotography. Now, we know that if we are looking for a good telescope, we have to look for the ED-APO in its name or specifications. Apochromatic refractors produce the sharpest, highest quality image obtainable from any telescope optical design. So let’s take a look at one example of refracting telescope that is commonly used by astrophotography beginners.
Sky-Watcher Pro ED 80mm Doublet APO Refractor Telescope
The entry-level APO from Skywatcher with ED Schott glass is the best budget choice for starting with astrophotography. You don’t want to spend a lot of money in the beginning. You need to fell for the hobby first, try it and learn the basics. You may think that the 80mm aperture is small, but it is enough for astrophotography because you will do long exposures photos and then stack them together in specialized software. It is a fast scope f/7.5 thanks to the short focal length of 600mm. Because of this, you can do stunning wide field photographs of the sky. Many deep sky objects are huge in the sky even when you can’t see them with the naked eye. For example, the closest spiral galaxy M31 Andromeda is 10x the size of the full moon on the night sky. So, having a short focal length is good to do wide field deep sky objects. If you need more magnification, you can help yourself with a Barlow lens to double or triple the magnification for smaller galaxies, the moon craters or planets. But beautiful pictures of the moon and the planets can be done with a simple telescope (not dedicated to astrophotography) and basic webcam.
The whole package contains the OTA, 8×50 finderscope, 2″ dielectric diagonal and Dual-speed 2″ Crayford-type focuser with 1.25″ adaptor. Tube-ring attachment hardware is included, and you can use two 20 mm and 5 mm eyepieces from the box for visual observing. But my favorite thing that comes with this telescope is the aluminum carry case; convenient for traveling with the scope because of the small size and portability. It is a no-brainer if you are looking for good astrophotography refractor to start with the hobby.
Best Reflector For Astrophotography
Buying a reflector of the same or even bigger aperture compared to refractor can be a much cheaper option for astrophotography. But most hardcore astrophotographers will tell you that it is a bad idea. I don’t agree with them though!
I was talking about the issue of reaching the prime focus with the camera in the focuser with reflecting telescopes for astrophotography at the beginning of the article. There is a solution for that and not only one actually.
1. Barlow Lens Trick
The cheapest and easiest solution is to use the Barlow lens that will push the focus point higher so you can reach it with the camera. However, it’s not recommended because the whole setup is too long and unstable which can destroy your long exposure photos. The Barlow lens is also doubling or tripling the magnification. So, it’s hard to get a wide field of view with the camera.
2. Moving The Primary Mirror
The second solution is to move the primary mirror deeper into the tube. By doing that you will shorten the focal length of your scope and push the prime focus little bit out of the scope. Now you can reach the focus with the camera with ease. Problem solved, yeah? Well, yes but after this modification, you can’t use the telescope for visual astronomy with the eyepiece, only for astrophotography. Another issue is the actual modification. It is not an easy task, and on some reflectors, it’s impossible to do. I don’t recommend to perform this kind of operation unless you are a skilled engineer and you know what you are doing.
3. Reflectors Made For Astrophotography
The best solution is to buy a reflector build specifically for astrophotography. They have a special focal ratio compared to regular reflectors to get the focus point up from the tube. These scopes are made explicitly for astrophotography, but you can also use them for visual observing of the night sky. The best part is that reflectors made for astrophotography are much cheaper than their refractor equivalents so they should be the first choice for a beginner. And not only beginners, but many experienced astrophotographers are now exploring the possibilities with big reflectors. Big aperture refractors are simply too expensive and too heavy.
The advantage of the big aperture in astrophotography is that you can do shorter exposures with the same result as longer exposure with a small aperture. And the smaller exposure also means that you don’t need a very expensive high-end mount with auto-guiding.
Orion 6 Inch f/4 Newtonian Astrograph Reflector Telescope
The astrograph series from Orion manufacturer are made for astrophotography. The word “astrograph” in the name already indicates that. They come in different aperture sizes starting with 6inch very fast f/4 reflector. As you can see in the picture above it looks different from standard reflectors. The secondary mirror with the focuser is moved towards the middle to move the prime focus up from the scope making it perfect for astrophotography and also to block stray light and boost contrast. The focal length of this scope is 608mm.
- precise 2″ dual-speed (10:1 ratio) linear-bearing Crayford focuser
- two heavy-duty hinged tube rings
- 8×50 finder scope
- dovetail mounting bar
- Orion-exclusive steel reinforcing plate underneath the focuser that eliminates flexure, even with heavy cameras
And how much you have to pay for the same refractor aperture for astrophotography? For example, this Meade Series 6000 130mm f/7 ED Triplet APO Refractor Telescope is around $3000! It has even smaller aperture than Orion 6inch we are talking about. And you can also have 8-inch and 10-inch Astrograph Reflector much cheaper. So, I hope that you understand why I recommend reflectors made for astrophotography for people starting with this hobby. The ED-APO refractors are a different level, and it should be your next option after some time spent with a cheaper reflector. Maybe it never happens, and you will keep using your reflector for astrophoto forever or just keep upgrading it to a bigger one. Friendly reminder: don’t forget to do mirror collimation with every reflector before every session.
You can see for yourself what kind of results are possible with astrophotography reflectors in this thread on Stargazers Lounge forum.
Comatic Aberrations in Reflectors
As chromatic aberration with refractors, there is a similar issue with reflectors called a comatic aberration. But don’t be scared because comatic aberration is not as annoying and can be fixed for much less money unlike in refractors where you need expensive ED-APO optics. And the comatic aberration is only present in specific fast reflectors.
What Is Comatic Aberration?
Parabolic mirror in a reflector can provide a perfectly clear image only in the small section in the center of the field of view(sweet spot). You can also hear definitions “on axis” and “off axis”. On axis is the exact center of the view and off axis is the edge area of the view. The more you go from the center to the edges of the view the more apparent coma you see. To explain coma more straightforward, imagine that stars will start to look like ovals or small comets towards the edge of the view. So, on what kind of reflectors can you experience coma more?
Usually, on fast scopes with the short focal ratio from f/5 down. The faster the scope is, the smaller “coma-free” area is in the field of view. So with the slower scopes like f/6 and up the coma is not a big issue. But what to do if you have fast reflecting telescope for astrophotography? How to correct this error? Very simple, you only need a coma corrector lens. I can assure you that it is not necessary for visual observations but for astrophotography with fast reflector, it is kind of must-have accessory.
It doesn’t matter if you choose reflecting or refracting OTA for astrophotography. I understand that your choice will mostly depend on your budget and with the smaller budget, the reflector telescope is the best choice in my opinion. If you are looking for the best mount for your OTA, check my next article the best telescope mount for astrophotography. Remember that it is only a hobby so don’t stress or be sad if it will be hard in the beginning. Enjoy it and relax. Join astronomy forums online or visit your local astronomy club to ask questions and learn from experts. People in the astrophotography community are very kind and helpful. Clear skies!