Astrophotography Equipment
Choosing Your First Telescope
Choosing your first telescope can be a daunting task, so let’s break it down. There is no one size fits all, at least not to get the results I’m sure you’re looking for. This is where making a choice as to what interests you most makes this task easier. The two choices are simple, and that’s planetary imaging and deep sky imaging (galaxies and nebulae).
Planetary imaging and deep sky imaging require different abilities from a telescope, hence there is no one size fits all. To understand the differences, we need to cover three specifications of a telescope: aperture, focal length and focal ratio.
Aperture is the width of the objective lens of a refractor (lens at the front of the telescope) or the width of the primary mirror of a reflector telescope (mirror at the back of the telescope). This is usually displayed in inches or millimeters such as 8 inches (200mm) or 6 inches (150mm). Aperture determines how much light can enter the telescope.
Focal length is how long the telescope’s optical train is and is usually displayed in millimeters. For example, the Skywatcher 200P has a focal length of 800mm. The longer the focal length, the more magnification (zoom) the telescope has. There is a give and take, though. The more magnification (zoom) your telescope has, the smaller the field of view (less area covered in the image) it has. On the flip side, a shorter focal length will have less magnification (zoom), but has a larger field of view (more area covered in the image).
A good way to picture field of view is take a shorter focal length telescope which has a wider field of view and point it at the moon. For this example, the entire moon is visible. Now, lengthen the focal length of that telescope and you’ll find you can’t fit the entire moon in view anymore like you are zooming in and that’s because you are, in fact, zooming in!
Focal ratio is how fast the telescope can resolve the light that enters it. This is usually displayed by a number with the letter F in front. This statistic of a telescope is calculated by dividing the focal length of the telescope by the aperture. For example, for a telescope with 800mm of focal length and 200mm of aperture, the focal ratio is 4 which would be displayed as F4. On the flip side, having 2500mm of focal length with 250mm of aperture gives you a focal ratio of 10 displayed as F10. This is the important piece that determines what your telescope is capable of as far as planetary imaging or deep sky imaging because the lower the focal ratio, the faster the telescope can process the light entering it and the higher the focal ratio, the slower the telescope processes the light entering it.
Think about which imaging interests you most, because that will matter with what you get, and everything needs to be matched. While you are deciding, here are a couple of things to keep in mind.
To image deep sky such as nebulae and galaxies, the more aperture you can get, the better off you are. These objects are dim, so larger aperture allows more of the light to enter, resulting in better detail and contrast in the final image. These objects also require a fast telescope, meaning lower focal ratio so the little bit of light from these objects that enters gets resolved by the telescope fast. As you can probably already tell, a Schmidt Cassegrain is probably not the best choice for this as they carry long focal lengths which mean higher focal ratios. For deep sky, my recommendation is staying with a focal ratio of F6 or less. A reflector or refractor are best suited for this type of imaging but remember the following pros and cons. There is always a give and take!
A refractor is relatively lightweight which means you don’t need such a powerful, expensive mount, however, requires corrective lenses for items such as chromatic aberrations which can yield hefty price tags. Also, getting this type of OTA in a big aperture is also very expensive in itself.
A reflector is also a very powerful instrument and getting a large aperture reflector style OTA is relatively inexpensive compared to the refractor of similar aperture, however these get heavy fast as you get up in size and require a more powerful mount to be able to carry the payload, which can carry a decent price tag depending on the mount you choose.
For planetary imaging, you will need large aperture to get the fine details. The larger the aperture, the better, and another thing to consider is these objects are very bright, so you’re going to want a higher focal ratio such as F10 and higher. Also, planetary imaging requires high zoom capability which means long focal length. You are probably noticing by now that the best suited OTA for this is a Schmidt Cassegrain!
Everything in your setup needs to be matched together! Once you decide which type of imaging you want to do, simply pick your OTA. Your mount needs to be able to carry the payload so you will need a mount that can handle the weight. Make sure you add the weight of all the equipment you are going to be attaching to your OTA such as guider, rotator, auto focuser etc to the weight of your OTA. Once you have that weight, make it sure it not more than 50%-60% of the mount’s payload capacity as this will result in poor tracking and guiding performance. For example, if all of your gear weighs 30lbs, you want a mount with a payload capacity of at least 60lbs. Remember, we need to track these objects across the sky to within the tolerance of a human hair, so if you overload your mount and it cannot track or guide properly, your final image will be harmed due to the variances produced by inability to stay on target.
Now that you have chosen which type of imaging you want to do, all you need to do now is choose what’s in your budget! It's ok to start small and build your skills. If your budget allows, I always recommend trying your best to future proof yourself. Future proofing yourself means to think of where you plan to be and get items that will work with your future plans. For example, I started with a Celestron AVX mount and eventually bought a Skywatcher EQ6-R Pro. If I could do this all over again, I would have spent the little extra money on the Skywatcher EQ6-Pro mount in the beginning. This is not a fault of the AVX and it has treated me very well, however, the payload capacity of the AVX mount is not enough for my current Skywatcher 200P OTA.
The last thing to consider is a camera that matches your OTA. This is important so you get the full resolution capability of your telescope as well as not over sample or under sample your images! There is a very simple way to do this and I explain exactly how to do it in one of my YouTube tutorials. Please click on the link below "Matching Your OTA to Your Camera" to view exactly how to do this. I have also included a link to an index of all of my equipment video tutorials.
