Refractor or Reflector – Which is “Best” for Astrophotography?

Started Nov 30, 2012 | Discussions thread
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RustierOne
RustierOne Veteran Member • Posts: 4,313
Refractor or Reflector – Which is “Best” for Astrophotography?
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Who dares to ask that question? Why there have been many spirited debates on the subject, which have discussed every detail and nuance of the question. Many times these debates only serve to fan the flames of the opposing camps. It is not my intention that this thread would have any such effect. My hope is that we can bring out the relevant points of comparison to help in choosing a telescope for astrophotography. Either design has its place in astro-imaging – the two designs are complementary and not necessarily in competition. It’s also worth noting there may be differences in how we answer the question, depending upon whether we are viewing or photographing our subject. Also it matters if we are photographing a wide view or narrow view (like planets).

Eventually this question was going to come up. So I hope we can do this in a way that is helpful rather than disturbing. I suggest that participants make every effort to avoid inflammatory statements that may offend others. If this thread should ever come to have many dozens of replies, I would expect that it has outlived its usefulness and has only become a vehicle for fruitless and perhaps unending debate.

It might be useful to limit ourselves to discussing the merits of two types of telescopes:

  • Refractors whose main image-forming element is a lens and
  • Reflectors, both Newtonian and compound, like Schmidt-Cassegrain, Maksutov, classical Cassegrain, etc. Basically we are talking about reflectors (telescopes that employ mirrors), obstructed with secondary mirrors in the light path.

Anyway, here’s my background: I own only 1 refractor (an old Dolland achromat made between ~1790 and 1840). Most of my experience has been with reflectors (6, 8 and 10-inch Newtonian) and Schmidt-Cassegrains (8 and 11-inch).

Now the source of the question is from Duncan Dovovan here:

http://forums.dpreview.com/forums/post/50354143

Duncan Dovovan wrote:

My goals are galaxies & nebula. I believe I read somewhere, that [1] refractors are excellent for planets and [2] SC-tubes are better for deep space? Is that true? What is your reason to prefer a refractor over a SC-tube?

RustierOne replies:

In answer, what follows is my opinion, based on my background, experience and viewpoint:

The statement about refractors and SC tubes is a loaded one. Here's why:

The statement that [1] "refractors are excellent for planets" is true. But it’s also true that much larger and much less expensive reflectors (like Schmidt-Cassegrains) are also excellent for planets. A smaller refractor (say 4-inch or less aperture) is a good choice if you want to explore what that aperture can provide and are willing to pay the price. An excellent 4-inch refractor will give a good showing of itself compared to a (less expensive) 6 or 8-inch reflector. But above the 6-inch size, the cost of a refractor is very high (if you can even find one), not to mention the cost of a mount to carry it.

Now the statement [2] "SC-tubes are better for deep space" is, strictly speaking, not true. But if you modify that statement to say “For the money, SC-tubes are better for deep space”, then maybe it’s true depending on your definition of “better”. Deep space objects are often faint and loaded with tiny image details, which can place a high demand on both light-gathering power and resolution. Both of these attributes come principally from aperture – the more aperture you have, the more resolution and light gathering power.

While there will probably always be the debate about the superiority of one design over the other, one of the hard facts pointed out recently by forum member wfektar is that aperture wins out when it comes to resolution [regardless of its design]. This is especially true if we are talking about high-contrast subjects like double stars. You can read his post here:

http://forums.dpreview.com/forums/post/50366723

While it’s true that a refractor (being unobstructed*) can out-perform other telescope designs of equal aperture, a refractor is much more expensive than most other designs, particularly above the 4-inch size. The problem is that the same amount of money will buy a lot more aperture in non- refractors. For example at Oceanside Photo and Telescope, a search for refractors sized between 111mm and 150mm (4.4 to 5.9-inch) aperture finds 37 telescopes ranging in price from $330 to $11,400. Now at most, you get what you pay for. Let’s choose a mid–priced refractor.

* Unobstructed means not having a secondary mirror in the light path in contrast to a Newtonian or Schmidt-Cassegrain reflector.

The Takahashi TOA 130NS (5.1-inch) at $6,500, will give an outstanding planetary view. But for $1,300, you can get a Celestron 9-¼" SCT optical tube, which will also give a great view. Even the Celestron 14" SCT is less than $4,000. Now of course any optical tube will need a mount. But you get the point - even medium sized refractors of good quality can be very expensive.

Now it must be pointed out that for a given aperture, refractors can outperform reflectors for low contrast planetary detail. But that advantage can be quantified as discussed in the next paragraphs. For high contrast images like double stars, there is much less difference in performance, if any. Given comparable quality, usually the larger aperture of whatever design, will excel on double stars.

For planetary viewing (often with much low contrast detail) the performance of an obstructed telescope is affected by (a) the aperture of the main mirror and (b) the size of the secondary mirror.

See articles in Sky and Telescope magazine by William Zmek, “Rules of Thumb for Planetary Scopes” (August, 1993 pg. 91 and September 1993, pg. 83).

For example, take the Celestron 9-¼" SCT with 235mm aperture and 85mm diameter secondary mirror. To find the diameter of an unobstructed telescope (a smaller aperture refractor) giving the same performance on (low contrast) planetary detail, one must subtract the secondary mirror diameter from the aperture. Thus:

Unobstructed Aperture = full aperture - obstruction diameter = 235mm - 85mm = 150mm

That is the Celestron 9-¼" SCT will give similar performance to a 150mm (5.9-inch, $11,400) refractor on low contrast planetary detail. Or to put it more bluntly, the $1,300, 9-¼" SCT optical tube will out-perform the $6,500, 130mm (5.1-inch) refractor, all else being equal. Of course all else is not equal. But that a subject for further discussion.

Now before I ignite a spirited discussion of why my conclusion should be modified in favor of the refractor, we are talking about a telescope (9-¼" SCT) which costs 1/5 of the cost of a much smaller, 130mm refractor ($1,300 vs. $6,500). Perhaps we can just leave it at that.

Now ignoring the price difference, if someone offered to give me the Takahashi TOA 130NS or a Celestron 14" SCT, without hesitation, I would take the refractor. It’s just a much more manageable telescope. The big S-C telescopes are somewhat common, while an excellent quality, mid-sized refractor is a rare possession indeed – one to be prized.

Of course there’s a lot more to be said in answer to Duncan’s questions. But I’ve had my say, and have probably said way too much. If anything I have said is incorrect or offensive, that was not my intention, and I will stand corrected.

So in answering Duncan's questions, what do y’all have to say? (My Dad was a Texan, so I can get away with that.) From your perspective, which design is “better” for astrophotography - Refractor or Reflector?

Best Regards,
Russ

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