Eyepieces

I’m quickly realizing the single 25mm Plössl eyepiece that this scope came with is not enough. The scope is an f/5.9 1200mm, with an 8″ reflector. So that gives me 1200/25 = 48x magnification. The specs are theoretically capable of 29x – 400x, with a practical upper limit of 200x or 300x. What I’m most missing is more magnification for planets.

Reading my book I think I understand the basics of eyepieces. The primary thing that matters is the focal length; this determines magnification. Divide the telescope’s focal length by the eyepiece focal length to get magnification (ie: 1200mm / 25mm = 48x magnification). There’s upper and lower limits on what a scope can handle. Also important is apparent Field of View, which is how wide the image you can see is. Bigger is always better here unless the resulting optics are compromised and distorted in some way. (Or simply cost too much). The Plössl eyepiece I got has 52° FOV which is sort of run-of-the-mill, you can get up to 100° FOV if you don’t care what it costs.

One secondary factor is barrel width. 1.25″ is standard, but 2″ allows more light in to the eyepiece. I’d think that’s always a good thing but apparently it doesn’t much matter unless you have a really long focal length wide lens. Most eyepieces seem to stick with 1.25″ unless they’re specialty products. My scope can use either.

Another complicating factor is the use of Barlow lenses, which give you 2x – 5x magnification as an inline bit of optics. The book swears up and down this doesn’t compromise image quality and strongly recommends using Barlows both for flexibility and because it avoids the small eye relief that can be an issue for high magnification lenses like an 8mm Plössl. Also the “Shorty Barlows” apparently work as well as the regular long ones, at least at 2x.

Bottom line, the book recommends someone like me start with a 25mm, a 17mm, and a 2x Barlow. That’ll give me 48x and 70x clean, and 96x and 140x with the Barlow. Also sounds like I should get a Yellow filter for looking at the moon. Other planetary filters and nebula filters sound like fun things that can wait.

All that being said I ordered the Orion planetary explorer’s kit which has a 2x Barlow, a 7.5mm eyepiece, 4 filters, and some crap I don’t want. The eyepiece is shorter than I had in mind but the price was great; $99 for $150 worth of stuff I wanted. I threw in an eyepatch to qualify for free shipping  🙂 I continue to buy inexpensive stuff just to get a feel for the hobby. I definitely appreciate why someone would pay $600 for a Nagler 100° FOV view window, but I’d rather start simple.

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