I’ve seen this question posed too many times to count, but it seems like even still information is a little scattered and it can be difficult to differentiate between the different options. This guide can be taken as a beginner-intermediate level buying guide for all types of telescopes and what to consider when looking at them.
The most common and the go to scope type for most people and for good reason. Refractors have long been the favorite of many amateur astrophotographers for their ease of use, good performance and lighter weight than their mirror driven counterparts. The big thing that drew me to them is the maintenance. There isn’t much of any with these. They have fixed lenses which generally do not need to be touched, are a bit more durable than mirror scopes (I can personally vouch for this through some very traumatic drops) and are not as demanding for filters since they are a bit slower in focal ratio and therefore don’t vignette as much on smaller filters.
The biggest downside is their performance versus reflectors and this can be overcome through a few different ways. They suffer chromatic aberration whereas reflectors have very little in most cases (of course there are the very low quality reflectors that do also suffer from this). The result of this chromatic aberration is not just color shift that shows itself as halos of usually blue around your stars, but also issues focusing. The reason this aberration happens is because one of the colors does not focus at the same spot as the other two, so when shooting if you want to correct for this you could shoot R, G, and B all separately, focusing each time and combining them later. Even then though, the bloat from that defocused color channel on stars will make correcting it in post a massive headache though it can be done. Lastly on the downsides, refractors are also generally slower than reflectors which means they collect less light each second than other scopes. This means your exposure times need to be longer on a slower refractor to match a faster reflector which can be a big deal for those of you who have short windows for imaging usually.
Refractors also need to be paired with a field flattener basically every time. A field flattener is an attachment for your scope that curves the image frame to flatten it out because as you’ll see with an unflattened image out of a refractor, the stars look very oblong as you move out to the corners. A field flattener will make these stars look round again. It’s a necessary attachment and will run you between $100 and $150 generally speaking.
Reflectors are taking a bit of an upswing in popularity lately with the advent of the hugely popular RASA line of scopes as well as the introduction of hyperstar. RASA is an ultra-fast imaging line of Schmidt-Cassegrain reflectors that shoot at F/2, which means they collect many times more light than your average high quality refractor. A five minute image on a RASA may be comparable to a 10 or 20 minute image on a much slower refractor. This isn’t an article about the RASA though, so why are people seemingly rediscovering reflectors?
Well I can speak from personal experience using a few different reflectors. I’ve owned a 10 inch dobsonian, a 6 inch imaging reflector and a 5 inch Schmidt-Cassegrain and they all share a few of the same downsides. The biggest for me was maintenance. You have to make sure first of all that your mirrors are aligned. This requires, to get the best results, a well collimated laser collimator and usually 5-10 minutes of fiddling with your scope before beginning imaging. This time may be extended if you travel with the scope in which case the mirror can get misaligned from the car moving. You also have to be very careful with them as the mirror is extremely fragile (my 5 inch mirror broke). I’ve also found that reflectors can take some time to get balanced because the camera does not sit directly in front of the mirror (except on RASA or similar scopes) so there can be a weird balancing axis on them.
These also need a corrector just like the refractors though. Most reflectors will need a coma corrector. Coma in an image is when your stars look sharp instead of round, almost like triangles in extreme cases. Coma correctors remove this. They generally cost the same as field flatteners, but some scopes like the RASA look very good right out of the box and may not require anything like this.
The upsides however, are that they collect in general a lot more light each second than a comparable refractor, can be more compact depending on your model, produce very little in terms of chromatic aberration and can be very low cost for good image quality. My 6 inch reflector only cost me $175 on the used market and performed worlds better than my $300 achromatic refractor that I started with. Of course you have to factor in a collimator to the cost, but my point still stands.
Things to look out for
While these are the two mainly looked at types of telescopes, there are subgroups of scopes that you should look out for when shopping for a scope. First is the reflector web, the biggest of which is the dobsonian. You’ll see these all the time and tons of people will recommend you get one as your first scope. If you are interested in astrophotography, do not get a dobsonian. They are mounted on very basic mounts usually with no level of tracking, but more importantly they usually cannot focus on a camera sensor. This is because the distance from the secondary mirror where light focuses to a point is further back than your DSLR sensor will sit and many people get confused when they aren’t able to take pictures properly with a DSLR or astro camera. Dobsonians are amazing for cheap, large visual scopes and I loved mine, but the day I got into photography was the day I listed it for sale. You’ll generally be ok if the reflector is labeled as an imaging reflector and this usually doesn’t really command a larger cost so it isn’t that big of a deal. The category of reflector has a massive amount of depth to it from the world of Schmidt-Cassegrains to newtonians which each have massively different form factors, image quality, focal ratio and focal length. For beginners though, a 5 or 6 inch newtonian imaging reflector is a great start for good image quality, good light gathering and ease of use if you’re set on a reflector. SCT scopes are great for smaller targets like galaxies or something like an Edge-HD is basically like 3 scopes in one when paired with a hyper star.
Secondly is the refractor category. Here you’ll find two categories, the achromatic and apochromatic refractor and even deeper you’ll find terms like doublet, triplet, or even quadruplet. If you’re interested in photography, only look at apochromatic scopes. I used an achromatic scope early on because I thought I could process around the issues and I was getting a big refractor for a fairly low price, that was a mistake. In astrophotography, the size of the scope doesn’t matter too much so getting a smaller, apochromatic scope is the better than a large achromatic scope by quite a lot. That said, there isn’t zero benefit to having a bigger scope so get the biggest high quality apochromatic scope you can afford. I have been using an Astro-tech AT72ED and have been loving it due its small size and ease of use.
Telescopes are one of those things in astrophotography that seems very complex at first, but is honestly pretty easy to figure out. The hard part is finding out how you want to image and the results you want to create. If you choose a high quality image reflector or an apochromatic refractor, chances are you’ll be just fine and you’ll be able to take the gear far. The worst is spending way too little and outperforming your gear with your own knowledge too quickly. I learned the hard way and it cost me months of good images because my gear couldn’t live up to my own standards.