Posts Made By: Brian Tung

Evan Chan said:
This morning, I went searching to see if I was crazy to think I could see Jr. with such small aperture. Apparently, people have reported seeing it with aperture as small as 80mm.

I know it was already difficult to cleanly pick out the Oval BA from the south polar region with the TEC6 (it was ify through the TSA). Yet people have done it with 80mm!

I'm never quite sure what to make of such reports. They strain credulity, and certainly some people are only too ready to report positive results, but on the other hand, I guess it's not quite impossible, and with the large number of people looking for BA, it's conceivable that just a handful have made it out in 80 mm.

That being said, it's phenomenally difficult. BA is about half the size of the GRS, true, but unlike the GRS, it does not sit in its own hollow in the SEB, so that one doesn't have a clear guide as to where to look. I saw it early on with my 5-inch SCT when it was still quite a ways clear from the GRS; a few days ago, I couldn't make it out. (Granted, the seeing was much worse a few days ago.)

I would say that unless one can see some hints of structure within the GRS, I would have a hard time accepting a positive BA report. It's just so close to the GRS right now.

Tom Lacovik said:
Can you guys recommend a lightweight binocular (about 26oz). I've found a couple out there in the 42mm aperture range. Probably no such thing as 50mm binocs that weigh less than 30 oz, and I might actually prefer a slighly smaller package than the standard 10x50 astro binocs everyone seems to settle on.

So a) is there a good 42mm binoc you could recommend (price is a factor)
and b) is there a good 28oz-ish (or lighter) binocular (of any size!) you would recommend.

Personally, I like the Canon 10x30 IS. If your eyes can accommodate a certain amount of miscollimation, I think this is worth a look. The miscollimation may relate to the fluid-filled prisms used to cancel out vibrations. The edge of field performance isn't horrible, and you can get refurbished specimens for the mid-200s. I got mine for even less, but you might not be able to duplicate my deal; I had to use my considerable grace and charm.

In any case, the image stabilization is really key for impromptu looks. I keep mine in my car for observing on short notice.

jess tauber said:
Has anyone ever reflectivized a Fresnel lens to make a telescope mirror? Can it even be done? Can diffractive zones be masked or otherwise treated? How closely spaced must they be to get a good image at low, high powers?

It can be done, but the image quality is going to be pretty poor, I suspect. Fresnel lenses are not in general high quality; they are used in environments where you really need the flatness. Mirrors are much harder to make in order to maintain the same wavefront accuracy; the same surface accuracy that would yield a 1/4-wave lens would yield about a 1-wave mirror (both of those being wavefront).

Remember that you cannot grind a Fresnel mirror; they must be molded (unless I'm missing some ingenious manufacturing process).

jon yimin said:
Ok, I just got my Ipaq and am in need of some software to control my Dave Ek's DSC for my XT8. Does anyone know of any software for Pocket PC, other than ThySky, that does telescope tracking with an interface like TheSky or Cartes, for, how do I say this, FREEEEE, or at least cheaper???

I'm not aware of anything that does this for free, but it's not my area of expertise. There's a PalmAstro Yahoo group that also covers Pocket PCs, but I see that the FAQ for Pocket PCs hasn't been updated in some time. Still, you might consider asking your question over there.

john boyce said:
Does anyone know the formula for converting RPM into focal length in the spin casting of mirrors? Any help would be welcome as always thank you.

I suspect that these forums aren't very good at formatting math, but I'll do what I can to make this readable. In case it doesn't go through, the character ² below is the superscript 2, for squaring a quantity. It should go through, but it might not.

The short answer is that the focal length of the mirror, in meters, is 450g/(pi*n)², where g is the acceleration due to gravity (about 9.8 m/s², pi is the usual 3.14159..., and n is the number of revolutions per minute. Thus, for example, for n = 20 (RPM), we get 450*9.8/(3.14*20)² = 1.1 m.

This strange looking formula comes ultimately from the centrifugal force field set up by the mirror's rotation. As a result of this field, each point on the mirror's surface is subject to two accelerations: it's mostly pulled downward by g, but it also has an outward component w²r, due to the rotation, where w is the angular velocity of rotation, and r is the distance of that point from the mirror's center. The slope of this force is therefore g/(w²r). The mirror's surface must be perpendicular to that, so its slope is one over that, or w²r/g.

Notice that the slope of the mirror at any point is proportional to the point's distance from the mirror's center. Such a shape must be a paraboloid, with the height y of the mirror above its bottom given by y = w²r²/(2g). The focal length of any such paraboloid is equal to 4 over the coefficient of r², so it must be g/(2w².

Now we're almost there, but w, unfortunately, is given in radians per second, which most people aren't comfortable with. So instead, we give it as rotations per minute. There are 2pi radians in a rotation, and 60 seconds in a minute, so the conversion factor from n RPM to w radians per second is pi/30. That's what accounts for the final form, given above.

Timothy McDougall said:
Orion's 120ED has a full page ad on the last page of Sept. S&T issue. F/7.5 and priced at $1999. I wonder if the orions are of the sky-watcher ed quality? Orion uses FPL-53, do the sky-watchers? I think its priced abit high, does 20mm more glass really more than double the price?(100ED is $899) Never quite understood the the jump in price for such a small difference in area. Maybe the price will "correct" after the newness wears off...it would then be a great buy if it can match the 80ED's quality.

I don't know. As Jon mentioned, the cost goes up as the cube of the aperture--even more than that if you take into consideration the need for homogeneity over a greater volume of glass. But that only applies to the raw cost of the glass. The whole scope shouldn't go up as the cube of the aperture.

The problem is that there's just so much that could vary here. Even if you assume a given quality of color correction (whatever it might be), there's still plenty else that can affect the cost. You can spend a lot of time to make sure the optics are smooth, or you can just do a little touch-up off the machine. Either way, it still uses FPL-53 glass, but I bet you'd see quite a difference on low-contrast detail.

The bottom line is that the price will react to the market. If, after a few of these get looked through, the word is that the optics are only so-so, it'll come down. Personally, I wouldn't be in a rush to purchase one of these, and at that price, I suspect others won't, either.

William Green said:
Does anyone sell a really-high-optical-quality Moon filter? In the past, I shied away from using Moon filters in the belief they would degrade the image.

In my opinion, they don't, really, but I rarely use one, because for the high powers where optical quality would matter, the light of the Moon is sufficiently attenuated to make observing comfortable.

dan hilts said:
In Ed Moreno's review of the Celestron 80ED he states "the collimation error is WELL within the 3 arc-minute limit required for high-contrast viewing."

This limit is something I have never heard of before. Can someone fill me in? Where does the limit come from? Any references to literature I can obtain?

If your telescope isn't collimated, it's akin to it having a broad error in the optical surface--except, of course, that you can fix the error in this case simply by collimating it. The extent of the error increases with the amount of miscollimation.

The reason you haven't heard of this limit before, most likely, is that it depends on the focal ratio; the faster the telescope, the better the collimation must be. So it's not always 3 arcminutes; for a slower telescope, it can be much higher. You can read a bit about this in Daniel Rickey's article in the June 2006 issue of Sky and Telescope.

But in my opinion, it's not a very interesting rule of thumb, since as far as I know, there's no simple way to measure the angular extent of your miscollimation. I suspect it's there in the review mostly to impress upon us in a quasi-quantitative way the accuracy of the manufacturing's optical and mechanical alignment. That's the sort of thing I could do without in my telescope review. Anyway, I would read that sentence basically as, "The telescope was well collimated."

Louis Scott Busby said:
I recently received my new TSA102 Super APO. I want to build an eyepiece set for this scope. I currently have a set of Meade 5000 series 60 degree FOV eyepieces. I'm looking for consensus on eyepiece choices to bring out optimum viewing with the TSA. Should I stick with Tak eyepieces? Or, go with something like the Televue Naglers?

Now, the TSA hasn't seen first light yet and I have no experience with the Tak or Televue eyepieces, but your experiences can send me in the right direction.

There's no reason to think that Takahashi eyepieces are inherently better mated to the telescope than any other brand. If a telescope produces a well-corrected image, any good eyepiece can be used to magnify it. It's not like a H-alpha scope, where you can argue that an eyepiece can be optimized for the H-alpha wavelength.

Which eyepiece you get depends on what you want to use it for. For DSO observing, sure, Naglers are quite good. I'm not a big ultra-wide eyepiece sort of guy myself, but I've looked through Naglers, and they are definitely useful for giving that wide apparent field of view. (And true field of view, too--lots of DSOs show up at higher magnifications, and Naglers allow you to get that and a fairly wide swath of sky too.)

On the other hand, if you're observing the planets and you have a driven mount, you might be more concerned about the center of the field than how big the field is. I think the Radians are pretty good in this respect, as are any of the Abbe orthos; for those more exotic-minded among us, you might try a monocentric. Doesn't Thomas Back sell some of those?

Anyway, my advice is to find the kind of power and field you want to use, and then get the best eyepiece you can afford at that power and field. Don't feel like you have to worry about full coverage of the range of focal lengths. Personally, I'd rather observe than be switching eyepieces all the time.

ed manspeaker said:
I thought this would be easy to find on the WEB, but no luck. How do I calculate the total magnification when I stack 2 or more Powermates? Is it different if they are upstream or downstream from the diagonal? Thanks for any info...

It isn't easy to find on the Web, because it's not a simple formula. What you get depends on the spacing. The amplification you get depends on the spacing between the Barlows/Powermates, and also between the rear Barlow/Powermate and the eyepiece that goes into it. Since those aren't fixed, neither is the formula.

If the Barlows are parfocal with each other and with the eyepiece, and you don't put anything else in between (like a diagonal), then you should be able to multiply the amplification factors together and get something like the correct result. But otherwise, all bets are off.