Comparing Commercial Dobsonians
I recently realized that I have close to enough money to get an 8" or 10" full-sized ground Dob, which would be a serious upgrade from my 6" dob.
The only other two full-size Dobs I have had any experience with so far have been the SkyWatcher FlexTube 8", which we use outside at the Observatory, and my own Apertura DT6 which I received in March. In both cases I had little choice in which they were--the Eight Inchers were bought well before I started volunteering at the Observatory, and my Six Incher was bought during and open box sale at a significant discount--the only way I'd have been able to afford it at the time. It normally costs $300, but it only cost me about $230, which is quite a steal!
Now that I'm looking into an upgrade with a heftier budget, I'm in a position to make a more informed decision.
The majority of commercial dobs of a given aperture are all nearly identical in focal length. 6", 8", and 10" dobs all have focal lengths of 1200mm, and 12" dobs usually have close to 1500mm focal length. Until recently I was under the impression that they were in fact all exactly identical, using the same GSO mirrors used in Apertura, Zhumell, and GSO dobs, but I'm now fairly sure there are subtle differences of a few millimeters of aperture and a few centimeters of focal length between the dobs of different manufacturers. Still, they're all extremely similar. They all have coatings of about the same reflectivity (94%), they all have mirrors figured well enough that they are diffraction-limited, etc.
The main differences between different dobs is not optical, but mechanical. There are different focusers, different altitude bearings, and different weights. And different sets of accessories--which is somewhat less important if you're starting out with some or if you can afford just the upgrades you want.
I compiled a spreadsheet of 8" and 10" dobs comparing the accessories between several telescopes in a line. And then I decided for completeness to add 6" and 12" dobs too. 6", 8", and 10" dobs are all the same length and can fit across the back seat of even a fairly small car. However, 12" dobs are slightly longer and may not fit in the same places 6/8/10 dobs do.
And here's the spreadsheet I came up with. Bolded text represents desirable features. Apologies for the formatting, you'll have to side-scroll to see the whole chart and it does run off the page. If you'd prefer to see it in Google Spreadsheets, you can see it here.
A few notes about the desirable features.
Focuser: First of all, you want a 2" focuser if you can get it, because that allows for 2", wide-field eyepieces which aren't limited in fov by the barrel of a 1.25" eyepiece. Second, you want a Crayford focuser because it can provide a smoother focusing motion than a Rack & Pinion focuser. And if you can get a 2-speed focuser, that will provide precise control over your focusing, which is very useful especially in the faster, larger telescopes.
Altitude Bearings: A Dobsonian mount is designed so that you push it to wherever you want it to go and it just stays there. Traditional big Dobs use gravity alone to adhere themselves to their rockerbox, but this doesn't cut it for lighter dobs, so either a tension adjustment knob is used or a tension spring is used. Both work fine most of the time, but with my Apertura DT6, which uses springs, pointing very high up (above 80 degrees) or very low (about 10 degrees) results in the Dob tending to drift backwards or down. Tension knobs (which are also found on the Roller Bearing alt bearings) allow you to lock the scope down so it won't drift when in those rare unbalancing positions. Though a counterweight can also solve the problem.
Finderscope: Red dot finders are acceptable, and more comfortable to use than straight-thru finders, but at the magnifications a full-size Dob operates at, you really do need a magnifying finderscope. The Right Angle Correct Image (RACI) finder is vastly superior in comfort compared to craning your neck around in odd angles to get behind the eyepiece of the straight-thru finderscope. Ideally I think you'd have both--a red dot finder for pointing the dob, and the RACI for star hopping. But this is why people get high-end reflex finders like the Telrad.
Fans: These can usually be found as a separate accessory if you need them. They allow the mirror to acclimate to ambient air temperature, which vastly improves the quality of the image as tube-seeing is prevented.
Eyepieces: Most dobs come with a standard 25mm Plossl. This is more than acceptable for finding Deep Sky Objects at various magnifications and taking cursory glances at the Planets, and is great for seeing the Moon. However, a wider field of view will provide a better experience because more objects will be visible and with a lower magnification, they'll be brighter. And a higher magnification is nice to investigate planets and double stars. So many dobs include a 10mm or 9mm Plossl which is a little uncomfortable in the eye relief department, and some dobs which have 2" focusers also include 2" wide-angle eyepieces which are excellent for deep sky viewing and scanning starry fields. A Barlow is sometimes included, but you need two eyepieces to take advantage of the versatility a Barlow provides anyway.
Weight: This, along with the focuser, are mechanically built in to the telescope and aren't simple to upgrade. You want the telescope tube and dobsonian base to be light enough to carry around. An extremely heavy dob might find itself rarely used because it's left inside the house forever because it can't easily be taken outside. You usually need to carry the tube and the base separately if you're walking more than a few feet, but with a 6" dob it is at least possible to carry them both at once. Luckily my 6" dob is already at the heavy end, so if I get one of the lighter eight inchers I won't notice much of a difference in weight, and I could probably handle the weight of a 10 incher.
I'm still not sure what Dob I'll end up going with. I'd really like to be able to get a 10" dob, because it'd be a much bigger upgrade than an 8" dob. Since I'm now going through the process of moving to another house, I don't really want to make a decision until I've settled in and had a chance to see how the sky is from my new home, and if I need portability more than power. (As it is now, I have to travel every single time I observe due to trees)
Clear skies.
Skywatcher Classics actually come with Modified Kellner "Super" eyepieces, not Plossls. They can also be found much cheaper than MSRP. The 200p Classic (8 inch), in particular, is consistently available on Amazon for $355, which is $90 off and just more than the 6-inch MSRP.
ReplyDeleteThanks for the info.
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