Questioning Culture: Small Refractor Onlyism

Why should refractors dominate small telescope culture?

Why should refractors dominate grab ‘n’ go astronomical culture?

 

 

 

 

 

 

 

 

 

 

A man is rich in proportion to the number of things which he can afford to let alone.

From Walden by H. D. Thoreau (1854).

There has never been a better time to choose a small telescope for travel and leisure; short tube refractors have proven especially popular in this regard owing to their compact size, clean optics, and in their ED glass incarnations especially, can be used at high and low powers. But even though they are capable instruments, their high cost for a relatively small aperture has fuelled optical designers to seek more economical designs that can compare well with them. I’m thinking of course of the venerable catadioptric telescope – usually in the form of the Schmidt- and Maksutov Cassegrains. The Celestron C5 and the excellent 90, 102 and 127mm Maks are deservedly popular in this regard but their longer focal lengths in comparison to a refractor make them slightly less versatile and their fairly large central obstructions cut down on micro contrast on tough targets like the Moon and planets. But is that the end of the matter? Certainly not!

The deeply dishy Meniscus lens on the the 90mm SkyWatcher Maksutov.

The deeply dishy Meniscus lens on the the 90mm SkyWatcher Maksutov.

Ask a typical amateur why they like small refractors and they’ll usually list a number of attributes. They have low mass, allowing them to be mounted on lightweight tripods and mounts. Because they cool off quickly, their owners can engage with the night sky in typically no more than ten or 15 minutes. Short cool down times is widely cited as one of the main reasons why so many aspire to owning one over the aforementioned catadioptrics, the closed tube optics of which take significantly longer times to effectively acclimate. With their retractable dew shields and the ability to have their diagonals removed, apertures in the 60-90mm range are airline portable, allowing users to enjoy a decent level of performance in foreign climes.

But in the last few years, a new breed of ultra-portable Newtonians have been brought to market that completely open up the choices now available to the savvy amateur astronomer, and dare I say may even prove more versatile and desirable than the small refractor that has dominated this niche for the last two decades. I am thinking especially of the new line of mini Dobsonians offered by companies like SkyWatcher and Orion USA. To get decent performance on the range of objects catered for by small refractors, you’ll probably need to consider a minimum aperture of 4 inches (102mm). In this capacity, both these companies now offer high quality Newtonian optics with f/4 parabolic primary mirrors. One can pick up such a model in the UK for £89 plus shipping. Installation is easy: attach the finder, pop in an eyepiece, and you can be viewing the heavens in moments.

The SkyWatcher Heritage 100P Mini Dobsonian.

The SkyWatcher Heritage 100P Mini Dobsonian.

Tipping the scales at just 2.8 kilos (6.2 pounds) including the wooden mount, it’s small mirror will cool off super quick and you can be enjoying the night sky in no time at all. Users report good optics both at low and moderate power (up to 120x or so), so that you enjoy wide field views of deep sky objects and decent views of the vast lunar regolith and the brighter planets. Such a telescope would probably compare well with a 60 or 70mm refractor. The SkyWatcher 100P mini Dob model is showcased in this link.

To match or exceed the performance of a larger 80mm or 90mm refractor you need to increase the aperture of the Newtonian by a significant amount. And while this usually entails comprising on portability, it ain’t necessarily so. Enter the SkyWatcher Heritage 130P; a wonderful, collapsible Newtonian telescope, complete with a well-constructed lazy Susan Dobsonian mount, and all for a very modest price tag (£129 in the UK). It meets or exceeds all the usual requirements for airline portability, its upper tube assembly sliding inwards to shorten the OTA to just 38cm. It can be used with a variety of mounts too, just like a small refractor. It cools off rapidly and is capable of both low and high power applications. Out of the box, it has a smaller (29 per cent)  central obstruction than a SCT or typical Maksutov – important for good lunar and planetary images. Furthermore, as highlighted in the link above, the  telescope can be modified in a number of ways that would further improve its performance.

Though I cannot vouch for the optics, readers from Canada and the United States will note that there seems to exist a very highly thought of incarnation of this telescope supplied by Astronomers Without Borders (make sure to view the 9 minute youtube video by scrolling down this page). You can also read through the 100,000 + hits this telescope has thus far received on Cloudy Nights. This truly is Something for Nothing!

Plotina, my superlative 5.1" (130mm) f/5 ultra-portable Newtonian.

Plotina, my superlative 5.1″ (130mm) f/5 ultra-portable Newtonian.

My own tests during daylight show that the images remain razor sharp up to 366x, all thanks to a very well figured, and fully collimatable f/5 parabolic primary mirror. In extensive testing against a 90mm f/5.5 apochromatic refractor costing nearer £900, the instrument outperformed it on just about every target. To elaborate, it showed a little more detail on Jupiter, better and deeper views of celebrated deep sky objects such as M35, the Great Orion Nebula, the Double Cluster and the Pleiades, among many others. The telescope showed me clean, well resolved views of iota Cassiopeiae, eta Orionis and theta Aurigae; systems that can sometimes prove challenging for a 3- or 4–inch refractor. Lunar vistas are stunning at all reasonable magnifications.

Interestingly, enterprising companies like Kasai Trading Company Ltd, Japan, have gone a stage further to offer premium quality 6-and 8-inch Newtonians (at f/5) in an ultra-low mass format for travel. These have the potential to completely re-define what it is to do grab ‘n’ go astronomy. And while more expensive than mass-produced models, they still offer considerably more bang for the buck than small refractors.

That a Newtonian could meet or exceed the usual requirements cited by amateurs as being important for both grab ‘n’ go and travel came as a great surprise to me, but it is true. Sure, small refractor enthusiasts will cite other, more subjective reasons for choosing a smaller glass over a larger speculum. For example, “The refractor gives purer details in the image.” Yet, having enjoyed many hours of viewing with an excellent 8–inch for a whole year and more recently with the aforementioned 5.1 inch (130mm) Newtonian, I am not at all sure what they mean and would only ask; how can one claim the image shows purer details when one can’t even see them in the smaller glass? “The colours are purer in a refractor,” they claim, evidently unaware that even the best corrected telescopes in this genre can only approximate the true achromaticity of a reflector image. Or, “I like the aesthetics of a fine refractor tube.” Well, one will soon forget what a telescope looks like when one peers through the eyepiece. Some folk don’t like diffraction spikes. I understand that, but having once been steeply immersed in small refractor culture for quite some time, I feel that this is something that is easily surmountable and can be quickly ignored or unlearned, as it were. What counts are the details presented in those images.

Don’t get me wrong: I’m as much a refractor fan as the next guy, but nowadays there is no compelling reason to adopt small refractor ‘onlyism’ if all one wishes to do is grab ‘n’ go astronomy. And having that choice is surely a good thing.

Dr. Neil English is author of Grab ‘n’ Go Astronomy.

De Fideli

5 thoughts on “Questioning Culture: Small Refractor Onlyism

  1. Heritage is amazing scope 🙂 I am very happy with it. Only problems with mirrors is that they need more cleaning because of the dust:)

  2. And you said that u use 366x power? How is that possible. I think 5* barlow and some eyepiece?

  3. Hello Rici,

    The Heritage sure is an amazing telescope. Rest assured it will be getting a lot of use in the coming months and years!

    You are right about cleaning the mirrors; they will have to be done more often owing to the instrument’s open tube design, but it’s no big deal in the scheme of things.

    The 366x I quoted was done during daylight tests to see how far I could push the optics. The images at these powers remained very sharp, which, in itself, is a good sign that the mirrors are of high quality.

    366x may be obtained using the 650mm focal length Heritage 130P by coupling a 4mm Plossl to a Baader 2.25x Barlow.

    Although this power is well beyond what you’d normally need in most astronomical applications, it might prove useful for examining small planetary nebulae and tight double stars when conditions allow.

    Best wishes,

    Neil.

    • Hmm but wasn´t the picture too dark at 366? Becauce at 162 is darker a little bit. I thought about buying 5x barlow lens and use it with 10mm eyepiece so I will have 325 times magnification but due to 10 mm eyepiece more light will come to the eye so the picture will be better I think.

      • Hi Rici,

        It was only a test and it was carried out in bright daylight. Of course over magnifying will give a dimmer image. As I said before, you don’t need these uber high powers for most applications and I wouldn’t recommend you buying lenses just to fulfil that purpose. I think a great combination for field work is a 32mm Plossl and a Baader 8-24mm zoom and Barlow combo. That would cover all magnification ranges in two eyepieces.

        Best wishes,

        Neil.

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