Posts Made By: Greg Nowell

Some people here are complaining about the difficulty in getting various parts from Orion. When the bearings on my son's star blast failed (the bearing on the handle) I didn't even try to replace it from Orion. I figured they'd send me another cheap piece of junk. I blew ten or fifteen dollars on two or three types of bearings from McMaster Carr and half the cost was the shipping. All the bearings I selected worked fine, I just wanted to see what worked best once I had gotten into it far enough to be looking through bearing options on line.

My thought is that when some of these things fail it is better to try to upgrade to something better than OEM parts.

I love the Star Blast and I think Orion's scopes are in general a service to the astronomy community, but as I say, if something is coming off or apart under ordinary service, there's no reason to think that another one just like it will do any better. So it's worth thinking about getting a new bearing from McMaster instead of Orion.

If one is totally clueless a plumber, car mechanic, or other handyman would be able to recommend alternatives in most cases. We're not talking CNC Astro-physics baffled tubing here. This stuff is pretty basic. Focuser, mirror cell, spider, OK, these are not typical handyman things. But the rest of the little scope has few mysteries to anyone who has used a wrench and a screwdriver. That's part of the beauty of the thing.

regards
Greg N

The issue on Chinese mounts has always been the machining; I think most of us are familiar with the "grease glue" alternative to precise machining, essentially the difference between a Vixen (real grease, close tolerances) and a CG5 (glue to take up tolerances the size of the grand canyon) in days of yore.

Though progress has been made in products from mainland China, they ain't there yet. One of the newer big name entries is the CGE Pro from Celestron. Rated at 90 lbs, these things cost $5k and come with a tripod. The uncorrected PE is 18 arc seconds peak to peak.

Seeing a mount at NEAF is not the same as a test drive. Seeing no polar scope on the mount, I was assured that you didn't need one because of the polar alignment protocol. Computerized modeling calculates how far off true N you are. You then go to a star and tel the computer you want to do a polar alignment correction. The computer looks at where the star is, relative to the mount, and where it would be if polar alignment where perfect. It moves the tube an offsetting distance.

The user then employs the AZ and ALT adjustments on the mount to get alignment star re-centered in your field of view. The motions shift your polar alignment to true N. Tell the scope that you have done that and presto you are polar aligned, or at least, better polar aligned.

Gemini users will recognize this subroutine which has been in their paddles for a decade.

What they won't recognize is the wrist breaking adjustment knob. I used a C14 on my G11 for years and even so it was always easy to adjust in altitude, even going "up" against the full weight of the C14. Not so with the CGE Pro loaded with a C14. Trying to turn the coarse threads was next to impossible. The handle is basically pushing down against a plate that looks glued to the mount and I am not sure how long it will stay there before it falls off. The solution, upon inspection, is that the handle has holes in it so you can stick something in to get some leverage.

This stick-in-handle solution first entered astronomy, so far as I know, via the AP900 and 1200 designs which are easy to turn (unlike the CGE pro) but hard to reach. The difficult-to-reach-altitude adjustment has been copied by an Italian company which essentially sand casted an early AP900 mount, painted it gold, and called it its own...with different drive mechanisms. And the hard-to-reach altitude adjustment is now being propagated by iOptron into an AP900 clone that will be streaming to our shores from China sometime soon--one was on display at NEAF. I note all this because Roland apparently saw the ease of use issue as important when designing the Mach 1 where forward placement of the altitude adjustment makes it as easy to use as on the Losmandy series. The Italians and Chinese are apparently not aware of these user issues, or are twenty years behind the times. ANYHOW...

By contrast the CGE Pro altitude adjustment is easy to reach but impossible to turn. I'm sorry to say that I have little faith in the casting of the altitude adjustment and would expect it to break in use. Since the adjustment will have to be used every time the mount is set up (except in an observatory) I find myself ill-at-ease with $5k on this piece of equipment and worry that if the critical altitude adjustment is sloppily executed that other parts I can't see will be sloppily executed too. So, on the one hand, we are light years ahead of the ancient CG5 of yesteryear, the one that made "astronomyboy.com" famous (he put up how to get the crappy grease out and de-burr the interior, with photos). But of course we are paying $5k for handles that won't turn rather than $500 for a mount that won't stay where it is pointed.

This is not what I would buy to get 90 lb capacity. I would go with a better mount and re-scale my optics to 50 lbs or so. Well: if all I wanted to do in the world is take web cam images of Jupiter and Saturn, this mount would do the trick. But for other stuff I'm not so sure.

That said, there is a significant workaround for a mount with a difficult altitude adjustment: level carefully every time you set up. When you do so Polaris is in the same altitude as last time you were out and most of your adjustment error will be in azimuth, which is easy. With the Losmandy mounts the altitude adjustment is so easy there is no real incentive to do anything more than a cursory leveling; it is a completely optional procedure, GEMs do not require being level. That said, the workaround of leveling works fine for an AP 900 or 1200 because once you have your hand on the adjustment knob you can in fact turn it. But the situation with the CGE Pro is considerably more difficult and I think we may infer from the 18 arc second peak to peak rating and the movement of the adjustments that this is a "rough" product.

regards
Greg N

Jon I'm signed up to do a presentation on equatorial mounts to my club this month. You seem to have a lot of pictures of yourself with GEM mounted Newts and I'd like to have a few of those, particularly a big one with you on a ladder (if possible), to use in my presentation.

I'm going to contact you in the astromart email with my home email which I prefer not to post for fear scanning robots will snatch it up.

Thanks
Greg N

Roland a while back on the "whiteness of saturn" thread you mentioned that the very slight off-white color in the C14 and FS128 were due to the same reason. Recently you posted in "one for roland" a more detailed appraisal of an ED refractor vs. an SCT. What is confusing me is that you suggested that the removal of a touch of violet form the in-focus image will cause a slight yellowing of the hue in an SCT and in an FS128. In the passage below however the discussion seems to indicate that the FS128 will have classic refractor purple (not as much as an achro, but a tad) but that the SCT will have a white light fringe. (And in fact I have seen fatter stars in luminance images with SCTs; the FS128 will show some color under "stress," but not as much as the ED Vixen doublet; the c14 has never shown color).

What is not penetrating my noggin is how both a purple fringe and a white light fringe can be equivalent in changing the hue of Saturn?

thanks
Greg N

p.s. I wanted to thank you for running all the graphs on c14 vs Mak, I am going to upload those into our archives at Yahoo! C14 for discussion. GN




Post from previous thread:


>Should this lead to purple fringed stars in SCT photographs>

The chromatic aberration of a doublet ED refractor is different than that of an SCT. In the SCT, the 70.7% zone has no color error, below that zone and above that zone there is color error, but of opposite sign (this is known as pure sphero-chromatism). The result is that for the shorter and longer wavelengths the light simply extends beyond the smallest Airy disc size to form a ring of white light that extends outward some 80 to 90 microns (Airy diameter for a C14 is ~14 microns). While the main light is concentrated inside the Airy disc, the extraneous light forms a faint halo that extends outward and decreases in brightness. In so doing it makes the diffraction rings brighter and lowers the brightness of the central disc. The result is an overall lowering of the contrast and an enlarging of the star diameters in a CCD image (over that of a perfect 14" optic).

In a refractor that has chromatic aberration, it generally occurs primarily in the blue and is a result of blue defocus plus sphero-chromatism. The result is a fairly bright blue-violet out-of-focus disc surrounding the Airy Disc.

Typically, sphero-chromatism produces a white halo while longitudinal chromatism produces a colored halo.

While sphero-chromatism in an SCT does not produce a colored halo, it does reduce the polychromatic Strehl. Visually it might not be all that important because the eye filters out the ends of the spectrum, especially for those over age 55 due to the eye lens becoming yellowed. Imaging wise, it does affect the star sizes and the potential resolution, except in the green where the Strehl can be 100% (if figured accurately). Nevertheless, a well made Mak can produce a polychromatic Strehl over 95% for the entire photo-visual wavelength range.

Rolando

I have just submitted an article to astromart documenting that the well known Vixen Super Polaris mount can be used in alt-az configuration. I think most folks are aware that the Polaris mount can be used this way (the small one). But I also think most folks don't know that the Vixen Super Polaris can be used this way. (I don't think the Great Polaris can).

Herewith I am posting the instructions on how to do this. The Super Polaris is a well made little mount and the prices it currently fetches on the used market are well below the quality of the product. I think many people who are spending quite a bit on different alt-az mounts would be very happy with a Super Polaris in alt-az.

As I mentioned in teh article, I am indebted to fellow club member Mike Mayer for educating me on this use of the SP mount and also for providing a pdf of the original instruction page.

regards
greg n

I seem to have encountered two kinds of eyepiece ghosting. One is the "skittering ghost" which apparently is the light reflecting off my eye. The worst case I ever had was a Russell inexpensive eyepiece. I also saw it in my former Nagler 17mm T4.

In my Zeiss 16.7 on very bright objects I get a different kind of ghost. It doesn't move but actually sits to one side of the bright object.

Are there different terms for these types of ghosting? Do they represent different kinds of weaknesses in eyepiece design?

thanks
Greg N

74.2 +/- 3.6 would be a range of 7.2

7.2/74.2=9.7%.

So where do we get 5%? Maybe they mean "peak to peak" 3.6 for an error range of +/- 1.8?

That would fit the headline.

Greg N

I'm not set up for bird photography, but I thought I would show this series of photos. They are of a common flicker. The difficulty here is the camera (Coolpix 995), the back window (not as clean as it might be), and the fact that everything is hand-held.

This includes the last shots, which are all hand held, where I'm working with the camera lens thrust into the eye socket of a Pentax 10x43 binocular.

1. The bird through the window at 1x.
2. The bird through the window using camera zoom.
3. The bird through the window using camera zoom at around 2x plus the binocular, all hand held
4. The bird through the window using camera zoom at 4x, with the binocular, with the bird moving just a tad
5. The same as #4 except the bird is digging deep into the ground and hold fairly still.

...maybe if I had a tripod I could do something!

Greg N

http://www.nytimes.com/2009/06/14/opinion/14rich.html?em

GN

Actually in the days of the "seven sisters" oil cartel we never had price fluctuations of this magnitude. And there was no futures market in oil.

We futures market began in 1984 some time after the majors lost control of world production in the mid-1970s. It's a response to uncertainty, but also an opportunity for speculation.

Anyhow there's a lot of money going into speculation that should be going into productive investment.

regards
Greg N