How important are precision optics in a home-built Dobsonian telescope?
mvsopen asked:
I’m going to be building a 12.5″ Dobsonian telescope. For the primary mirror, precision optics, of F5 or better and certified, are roughly $1600. Yet many websites sell Chinese imported mirrors, at prices between $500 and $800. Will the quality of the primary make a huge amount of difference in the finished product, or would investing in a better eyepiece yield the same results?
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I’m going to be building a 12.5″ Dobsonian telescope. For the primary mirror, precision optics, of F5 or better and certified, are roughly $1600. Yet many websites sell Chinese imported mirrors, at prices between $500 and $800. Will the quality of the primary make a huge amount of difference in the finished product, or would investing in a better eyepiece yield the same results?
March 7th, 2009 at 1:47 am
You need to get to 1/8 wave or better for “diffraction limited” performance, which means that the optical surface is no longer the “weak link” in image quality. If your Chinese mirror is 1/8 wave or better, the cost difference will be decisive.
The only other thing to be concerned about is the reflective coating: aluminum or silver? Nowadays almost everyone uses aluminum, which is fairly permanent. Silver is more expensive and tarninshes, but it does reflect better when the quality is good.
March 9th, 2009 at 12:33 am
A telescope is only as good as its weakest link, so you can’t make up for a bad primary with better eyepieces. The quality of the mirror will show up in subtle, low contrast details, like features on Jupiter and Mars, or texture in nebulae. Because premium optics scatter less light, you will also be able to see just a tiny bit deeper.
However, you’re talking about paying twice as much or more for a mirror that is maybe 10% better. The difference is very subtle in most situations. If you had the two mirrors side-by-side and the sky conditions were very good, you’d be able to pick the winner. But those Chinese mirrors are mostly well made, and will perform just fine.
Get the premium mirror if money’s no object, or if you have to have the very best. Otherwise, I think you’ll be happy with the Chinese mirror. I have a Taiwanese (Guan Sheng) mirror and it works great.
But do be sure you’re buying a good quality, diffraction-limited paraboloid, and that you can exchange it if you get a lemon. There are some dubious goods being offered for sale on eBay and other places.
March 10th, 2009 at 5:19 pm
You may want to look on, I occasionally see some mirrors made by R.F. Royce on there, which are supposed to be very high quality.
You can also look on in the classifieds for used (but pristine) mirrors.
I think a mirror from any fairly reputable company would be okay. Most people can’t afford the super “high quality” mirrors and to be honest, the atmospheric conditions don’t allow most telescopes to perform to their full potential.