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From what I've read, catadioptric telescopes seem to be superior to reflectors. They tend to require less frequent collimations, are generally shorter in physical length, and the combination of lenses and mirrors provide greater aberration correction.

Apart from their higher costs due to the presence of lenses, what other drawbacks might a catadioptric telescope have that would cause one to opt for a reflector instead?

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  • $\begingroup$ I updated the title to better reflect the body of the question - some people will vote to close before they even read a few sentences. $\endgroup$
    – uhoh
    May 2, 2023 at 3:50

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The main drawback of catadioptric telescopes is their high resultant ƒ ratio. Most SCTs are at or around ƒ/10, and most Maksutov combinations are around ƒ/13. This makes them bad for photography, as a longer exposure is needed to obtain the same amount of light (because light is spread out more—imagine trying to cover a whole wall with just a tiny can of paint, versus having a huge bucket of paint). With today’s CCD and CMOS cameras, though, it’s less of a problem.

In visual use, they will need a shorter eyepiece to achieve the same magnification. Considering many amateur astronomers enjoy viewing nebulae and galaxies, a very-short-focal eyepiece will be needed to make them fit wholly in the field of view—or sometimes, not even wholly. Short-focal-length eyepieces often have shorter eye relief, which means you need to put your eye much closer to the eyepiece than with longer-focal-length eyepieces. This may be difficult or even impossible if you wear glasses for observing. Again, modern eyepieces with short focal lengths and longer eye relief do exist, so it’s less of a problem as it used to be.

I would be hard-pressed to make a choice, though, as each amateur astronomer has their own wants and don’t-wants, dos and do-nots, so it’s really a matter of choice.

I currently do not own a catadioptric telescope (though I have a catadioptric camera lens), but I have owned an 8″ ƒ/10 SCT on a fork mount. I have traded it for a 10″ ƒ/4.7 Newtonian on a Dobson mount, as it suited my needs better. Then again, on the flip side, a friend of mine, who has the exact same 10″ telescope as I have, just got himself an 8″ ƒ/10 SCT, as it’s more easily transportable “for [his] old back.”

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You can essentially turn a catadioptric into a reflector by removing the secondary mirror and replacing it with something like a Hyperstar or RASA and a camera (but not really an eyepiece). Obviously at an extra expense. This typically turns an f/10 scope into an f/2 scope, while most reflectors tend to be in the f/5 range. So, optically, in theory, there's not an enormous difference if money is no object.

The catadioptrics are usually focused by moving the primary mirror, while a reflector will move the eyepiece. This gives the catadioptric a wide range of focus, but the downside is mirror flop. This results in the image shifting during focusing, and possibly as the telescope tracks across the sky. So, for photography, an off axis guider is normally used, which will have a very small field of view, making it much harder to find good guide stars.

Catadioptrics are also usually computer controlled, while reflectors are usually manual, though there's a lot of exceptions. That makes the reflector easier and faster to set up.

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Apart from higher costs, catadioptric telescopes have some drawbacks compared to Newtonian telescopes, which are among the best telescopes for amateur astronomers.

  1. Complexity: Catadioptric telescopes are more complex in their optical design compared to Newtonians. This complexity can make them more challenging to collimate and maintain, especially for beginners.

  2. Weight: Catadioptric telescopes tend to be heavier than Newtonian telescopes of similar aperture, making them less portable and more cumbersome to set up and transport to different observing locations.

  3. Cooling time: Catadioptric telescopes often require more time to cool down to ambient temperature before achieving optimal image quality. This can be a disadvantage for observers who want to start observing quickly.

  4. Obstruction: Most catadioptric designs have a secondary mirror obstruction, which reduces the overall contrast and may result in some light loss, affecting the quality of the observed images.

  5. Price: As mentioned earlier, catadioptric telescopes are generally more expensive than Newtonian telescopes of the same aperture size. This higher cost might be a limiting factor for some aspiring astronomers on a budget.

Despite these drawbacks, it's important to note that catadioptric telescopes still have their advantages, such as compactness and versatility, which make them suitable for various astronomical observations. The choice between different telescope designs ultimately depends on an individual's specific needs and preferences.

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