In general astronomy, aperture is king in a telescope. A larger aperture gathers more photons, and reveals more detail. However, be very careful not to fall under the spell of aperture fever for astrophotography. You do not need a large telescope to get started in astrophotography. For the beginner in astrophotography, a small telescope is much better than a large one. Large telescopes have more focal length and magnify errors. They are also much more expensive, and require larger and more expensive mounts. It is much easier to be successful with a small telescope when first starting out, and you can take some great pictures with a very modest-sized scope. Small Refractors for Astrophotography Many novice astrophotographers have great success with small refractors in the 70mm to 90mm aperture size. Refractors are good because they don't require any maintenance or collimation, and small refractors do not require massive, expensive mounts.
A refractor with a focal length of around 400-500mm can be used without the additional expense and complexity of guiding. While the stars may be trailed in some frames when used with the relatively inexpensive mounts we will consider, we can simply shoot a lot of frames and throw out the bad ones. With longer focal lengths, we would have a much lower percentage of usable frames with this method. A good doublet ED apochromatic refractor with about 70mm - 80mm of aperture and 400mm - 500mm of focal length at about f/6 is an excellent scope with which to get started in astrophotography. They are not that expensive. There are a lot of small apochromatic refractors out there. Almost all of the inexpensive ones are made in China. Of course, you can also get a high-end scope that is much more expensive. In general, as with anything else, you get what you pay for. However, the quality of the small, inexpensive Chinese-made apochromats is not that bad, and the prices are good. Below is a list of dealers that sell small apochromatic refractors. The list is not comprehensive and does not include every dealer out there, but it does include a representative selection.
Field Flatteners and Telecompressors If you buy a refractor for astrophotography, you are going to need either a field flattener or a telecompressor to deal with the curved focal plane that almost all refractors have. These optical devices attach to your camera's T-mount adapter and go into the back of the scope's focuser and sit just before the camera. A 1x-power field flattener keeps the scope at its original focal length and focal ratio.
A telecompressor, also called a focal reducer, makes the focal length shorter, and the field of view wider, and makes the optical system faster. Most telecompressors also flatten the field, but not all do, so check with the manufacturer before buying one.
Not all telescope manufacturers make a field flattener or telecompressor as these are used specifically for astrophotography and not visual use. This is something to consider when buying a refractor. If a particular manufacturer makes a field flattener or telecompressor for their scopes, they may work better than trying to mix and match ones from a different company. If you buy a scope from a manufacturer who doesn't make a telecompressor, you may have to test several different ones before you find one that works well with your particular scope. Note that there are many telecompressors that are made for Schmidt-Cassegrain telescopes. There are also some telecompressors that are made for small CCD cameras. Neither of these types will work well with a refractor and a DSLR. Inexpensive Alternatives? Not Really You may look at the prices of some of these refractors and think you can buy something like a Newtonian reflector much cheaper. You probably can. The problem is that the really inexpensive ones won't work very well for astrophotography with DSLR cameras. Some are made for visual use only and won't even focus a camera. You may see a small refractor on the shopping channel on TV and think you can get the telescope and a computerized mount for $200 for $300. You may think you can take astrophotos with it. You can't. It will be so wobbly that you won't be able to even focus it. And it will have a lot of other problems that make it unusable for astrophotography. Orion makes a StarBlast 4.5 inch f/4 Newtonian Reflector that is very inexpensive. However, it is made to illuminate a sensor that is only 8mm square. It will work very poorly with the 22 x 15mm sensor in most DSLRs because it does not fully illuminate the sensor. Orion also makes a 6 inch f/5 Newtonian reflector. It will also require a coma corrector such as the Baader Multi-Purpose Coma Corrector (MPCC). It will have 762mm of focal length, which is to the point where it will probably require guiding, which greatly complicates things for a beginning astrophotographer. Orion also used to sell a very nice Maksutov-Newtonian (a variation on the basic Newtonian design) that had 7.5 inches of aperture and was f/5.3 with 1,000mm of focal length. Although this design does not require any additional optical correctors, it cost more than $1,000 when it was available. At 1,000mm of focal length, it definitely required guiding.
Astro-Tech makes an interesting new 6-inch Imaging Newtonian that has 6 inches of aperture and is f/4 with 610 millimeters of focal length. It has a larger secondary mirror, so the focal plane of a DSLR should be fully illuminated. It will require a coma corrector such as the Baader Multi-Purpose Coma Corrector (MPCC). This is a potentially attractive instrument because it is specifically made for astrophotography and its large aperture and fast focal ratio will gather a lot of photons in a shorter amount of time than a slower refractor. The price with coma corrector is similar to a decent small refractor with field flattener or telecompressor. However, because it is larger and heavier than a small refractor, it will require a larger and more expensive mount.
If you already have a telescope, no matter what it is, then you should definitely try it first for astrophotography. But if you don't have a scope, and you are serious about beginning in astrophotography, you must realize that there is a minimum amount of cost and committment required to be successful. You may think you can start out cheaper, but you will find this is a false economy. This cheaper equipment just will not work very well for astrophotography, and you will end up frustrated, and eventually need to replace it if you intend to progress and succeed. You don't have to spend a fortune to have fun in the hobby of astrophotography. Modestly-priced equipment can take images that will give you immense satisfaction and provide years of fulfillment and enjoyment. If you are just starting and don't have any equipment, you can save money by buying good used equipment on AstroMart or Cloudy Nights.
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