Posts Made By: Brian Tung

Roy Troxel said:

This is probably old stuff to some of you, but after being into this hobby for the past year, I have come to the following conclusions:

Old stuff, old message, new response.

1. Reflectors can have as high a resolution as refractors of the same aperture.

They can, but resolution is a fairly constrained characterization of a telescope's discerning ability, emphasizing the ability to preserve contrast at high spatial frequencies (very small detail). This is not an unimportant ability, especially in the area of tight doubles, but there is more to optical image quality than just resolution. Telescope A can have a better limiting resolution than telescope B, yet reveal less detail at some scales.

The usual technical description of optical quality is the modulation transfer function, or MTF. It can be thought of as a graph of contrast versus detail size. In a perfect world, contrast would always be retained 100 percent, but the world is not perfect, because diffraction causes light in the image to be spread out from where it "ought" to go. As details get small to the point where diffraction matters, contrast decreases, until you get to the resolving limit of the telescope, where it reaches zero. Broadly speaking, the MTF drops continuously from 100 percent for very large details, to 0 percent at the resolution limit. There are, of course, exceptions.

Again broadly speaking, telescopes of the same size reach 0 percent contrast at essentially the same point (the same detail size). However, the way they get to 0 percent varies. Refractors, the simple beasts that they are (in principle, at least), get to 0 in a smooth curve. Obstructed designs, on the other hand, drop faster than refractors initially, and then they rejoin the refractor curve, sometimes even exceeding it a little bit (this last part is one reason why some people try adding obstructions when trying to observe tight doubles). The main difference is in that intermediate region, where details are a few times larger than the resolution limit (e.g., a couple of arcseconds in an 8-inch scope). That's where a lot of planetary detail resides, so this is not an insignificant consideration.

All that being said, the main factor is aperture. A 5-inch refractor's MTF may be above a 5-inch reflector's MTF for a large part of the range, but they are both trumped by a 10-inch reflector (assuming comparable manufacturing tolerances, of course). That's the sense in which aperture wins.

2. Refractors have better contrast, making it easier to see details on planets or galaxies. However, contrast is not the same thing as resolution.

Well, as I said, they're not the same thing, but they're not entirely distinct. Resolution is an expression of how small you can go before contrast vanishes.

3. Reflectors can have as good resolution as refractors, but you might have to strain your eyes a little more with the reflectors.

On high spatial frequency objects, such as tight doubles, there is no reason why this should be true, and I've never noticed it.

5. In order to be properly collimated, first both mirrors must be collimated by laser, then they must be star-collimated.

That might make things easier, but no, it certainly doesn't have to be done that way--obviously, since many Newtonians have been accurately collimated since before laser collimators became available.

The whole point is to get the axis of the primary to align with the axis of the eyepiece. Everything else is mechanism. Whether you do this by star-collimation or laser-collimation or X-collimation (for whatever value of X)--the only criterion is that they align well enough to provide diffraction-limited performance. That criterion is harder to achieve for faster mirrors, and it is here that lasers can help a lot, but they aren't inherently necessary.

Michael Frias said:

WHEN VIEWING THE SUN IN H-ALPHA, IS THERE ANY DIFFERENCE IN THE VIEWS OF SAY A $200.00 80MM ACHROMAT VS A $1200.00 APO? AND WHAT IS SO SPECIAL ABOUT THE DEDICATED SOLAR SCOPES?

I don't know specifically about the dedicated solar telescopes, but I can venture a guess: they are designed to correct for spherochromatism at the rather long wavelength of 656.3 nm.

In case I just spoke gobbledygook at you, spherochromatism also goes by the long-winded name of "chromatic variation of spherical aberration." What that means is that the degree to which a refractor exhibits spherical aberration varies by the wavelength of light going into it. (Reflectors are, of course, immune to spherochromatism.) It ordinarily makes sense to correct for spherochromatism near the middle of the visual range, since the eye is most sensitive to those wavelengths (about 550 nm).

For H-alpha viewing, such considerations are moot, since 550 nm light doesn't get to the eyepiece. Only light in a narrow range near 656.3 nm gets to the eyepiece, and if you are going to correct for spherochromatism in a H-alpha telescope, you should correct it for light of that wavelength.

As I said, I don't know specifically that this is what they mean when they say specially optimized. But that's what makes sense to me. If that's the case, then an apo will be better than an achromat, simply because they generally control spherochromatism better than an achromat. But you can make a specially optimized achromat for less than you can make a generally optimized apo.

Lee Spain said:
Does the face of the moon appear to rotate slowly as moon traces an arc across the sky?

Relative to the zenith, yes. Relative to the north celestial pole, no. You are seeing the effect of field rotation.

Harry Vanderburg said:

I tried my GSO 30 in my XT10i and loved it. Only negative thing is that it doesn't have enough focus and I must lift the ep. Nebulas looked nice in it. What eyepiece should I get so I could see star cluster's and galaxy's better? I was guessing around a 15mm for a 1200mm scope? At higher magnification, will a wide angle work good or am I better of just buying plossls(I can't spend too much)? Will any inexpensive wide angle(like the 1.25" 15mm GSO wv) in a fast scope(4.7) be ok or will I still run into distortion problems similar to the cheaper 2" 30ish wide angles?

The eye relief on Plossls is pretty short. If that's a problem for you, you may prefer something like a Radian, where all of the various eyepieces have the same eye relief. Alternatively, you could use a Barlow. Barlows are generally considered to reduce contrast slightly, though most of today's Barlow units are quite reasonable, optically speaking.

Coma is rather noticeable in a f/4.7 reflector. Tele Vue's Paracorr will reduce that significantly and make it possible to use less exotic eyepieces and still get good edge performance. It also increases the effective focal length by some 15 percent, to about 1400 mm on your telescope.

DSO plus star fields sounds like "fast and wide" to me. The EQ doesn't appeal to me, but it (or some kind of tracking mount) would be useful if you're seriously planning on longish-exposure astrophotography.

Since you mention returning to the hobby after a few decades, I'm expecting you'll be able to navigate the night sky without too much difficulty. I'd be thinking big dob, maybe 12 inches, maybe a bit more if it's a truss and you can handle the mirror box all right. Even with a tube dob, laying the tube in the rocker box doesn't require a lot of precision. The only real downside of the dob is that it's harder to do exposure photography. Otherwise, though, you can get a heck of a lot of quality/aperture for your $4,000 minus eyepieces.

Joe Hoener said:

Hello! My C8's instruction manual tells me when polar aligning to first orient the OTA such that the dec circle setting is 90 degrees and the OTA is parallel to the forks.

Goodness, why? What do you do with the OTA after this initialization?

It seems to me that there's plenty of possibility for pointing errors during this procedure: the OTA may not be perfectly parallel to the forks, and the dec circle might be misaligned. Are these kinds of errors tuned out during drift alignment? Thanks!

Yes. To expand on Rod's answer , drift alignment aligns the axis of symmetry of your mount with the celestial axis. Since any miscalibration of the declination circle and/or misalignment of the OTA/fork joint does not affect the axis of symmetry of the mount, they do not adversely impact the accuracy of drift alignment.

That's not to say that those miscalibrations won't have any effect. Miscalibration of the declination circle will affect any attempt to use the analog setting circles after polar alignment. And if the OTA is not parallel to the forks, there will be sections of the polar regions (probably very small) where the OTA cannot be pointed exactly. But these will not affect tracking over the vast majority of the celestial sphere.

Can you articulate why this SCT would be an advance upon a similarly well-made Newtonian? It seems to me that a Newtonian can be made that more than matches the optical quality of your SCT, at lower monetary cost. The trade-off is that the Newtonian would have somewhat worse off-axis performance, but if you're talking about high-resolution/high-contrast observing, this isn't much of an issue, providing you can get some nominal tracking (or you don't mind tracking by hand).

In other words, this seems like an attempt to make a jack of all trades into a master of one. Probably I'm missing something, but I need a nudge in the right direction to see what that is.

Paul Webb said:

I recently purchased a new C-14 OTA XLT. In all of the advertising and literature the info states that this OTA package comes with a 2" 40MM eyepiece. When I received mine it had a 1.25" instead. Anyone else get the shaft? Thanks, Paul

I haven't specifically received this particular shaft, but all the "base" 40 mm eyepieces I've seen have always been 1-1/4 inch. I'm not sure what the appeal of this eyepiece is supposed to be, because it yields the same true FOV as a 32 mm eyepiece (typically), and has lower magnification to boot.

Have you asked Celestron about this (if the dealer proves unhelpful)?

Rod Mollise said:

Have y'all seen the new "Celestrons"?

Along those lines, Rod, do you remember when the small Omni XLT's came out? Specifically, the one based on the C5 OTA?