Posts Made By: Michael Connelley

Hello:

I use both the 31mm Nagler and a 55mm Plossl in my 12.5" f/15 classical cassegrain. I kind of like the fact that the exit pupil is small since it darkens the sky for those faint fuzzies and minimizes the effect of the astigmatism in my eyes. But yes, large, low surface brightness objects are harder. Things like the Horsehead Nebula seem harder to get in my cassegrain than my f/6 dobsonian (both using the 31mm). Targets like M31 are just out of the question, and I can only get a part of Eta Carina in at a time.

I suppose the answer is that I don't do anything about it. I still try to see these targets, but they are harder to get and you can't see as much at once. The higher magnification makes hunting for planetaries and little galaxies really nice. Planetary observing is a lot of fun, since a cheap 9mm eyepiece works great with a f/15 scope and gives very good contrast. I also learned how to use a finderscope with my charts to find faint fuzzies. The old "use the Telrad to point in the general direction of X and pan around until you hit it" method doesn't work so well with a classical cassegrain. The real solution, of course, is to convince Mr. Nagler to make a 60mm Nagler with a 4" barrel so that we can get a lower power AND wider fields of view.

Cheers
Mike Connelley

Hello:

Autoguiding is a technique used for long exposure astrophotography to keep the telescope accurately pointed at a target during the exposure. The mechanical precision of the telescope's drive and its polar alignment are never good enough to keep the image of a star on the same spot on the focal plane for many minutes. Thus, guiding is needed. Typically, a guidescope is mounted with the main telescope and is pointed at a nearby star and had a crosshair eyepiece. In the OLD days, a real person would sit at the guidescope eyepiece with the hand controller and make small corrections to the telescope mount to keep the star accurately centered on the crosshair. As you can imagine, doing this all night is boring and painful. So, an autoguider replaces the person with a small CCD camera. The camera takes an exposure every second or so and measures the position of a star on the CCD chip, and sends corrections to the mount to keep the star at that spot. Now the astronomer is free to do other things, like warm up in the car and fall asleep.

Cheers
Mike Connelley

Hello:
You can probably relax your 1/8th wave optical quality criterion. If your SCT is f/10, and the mirror is 2"-3" from the focal plane, then the beam size on the diagonal mirror is 0.2" to 0.3". If a 2" diagonal mirror has a smooth 1 wave error in its figure, then over a 0.2" patch this would be 1/10th wave! Errors in optics this close to the focus of a slow telescope mostly results in a slight amount of distortion and field curvature. For a SCT diagonal, a smooth polish, good reflectivity, and a true 90 degree angle are more important than 1/10th wave mirror figure.

Cheers
Mike Connelley

Hello:

I find the article to be highly dubious. I have observed Uranus several times, and I haven't seen the rings but have seen three of its moons. The moons are REALLY faint in a 16", but are very bright compared to the rings and farther away. The idea that colder temperatures freezing out water vapor and thus making it easier to see the ring is total nonsense. If that's the case, someone should have seen the rings a long time ago since there is no shortage of cold observing sites. The air over Mauna Kea is very clean, dry, and dark, but we don't see the rings from here.

The only time I've taken an image of the rings is with adaptive optics on a 3m telescope in the infrared! The article mentions Keck observatios that the rings have a red color. They are probably refering to the Keck AO image of Uranus, which was taken in the infrared! In the infrared tri-color image, Uranus looks aqua blue in the infrared for the same reason it looks aqua blue in the visible: methane absorption. The rings glow in the infrared due to thier thermal emission, so they're brightest at K-band (2.2 microns), which is used as the red channel.

The darkening of the rings of Saturn observed by Cassini is very slight, not the huge change in albedo that would be required to make the rings of Uranus visible. It also doesn't make sense that the rings of Saturn should be visible for much more than 400 years (the effect of the rings on Saturn's total brightness has been known since antiquity), but the visibility of the rings of Uranus comes and goes. I don't think the changing brightness argument holds water.

Given that it's hard for HST and large ground based scopes to observe the rings of Uranus, and that no one since has visually observed the rings, makes me very suspicious of the observations. I can't offer an explaination as to what he did observe, but I don't think it was the rings.

Cheers
Mike Connelley

Hello:

Lye can attack the glass, ruining the optical surface of the mirror. You definately do NOT want to use lye.

I've heard of people using ferric chloride (or whatever the PCB etchant they sell at radioshack is). Most folks use hydrochloric acid. This is also sold as muriatic acid. You may be able to get it at a local chemical supply shop, or shops that sell agricultural chemicals or swimming pool chemicals.

Cheers
Mike Connelley

Hello:

I vote for a 3.5". The increase in central obstruction is negligable, and people seem to worry about it too much. With a 3.5" secondary, your central obstruction (in percent) will be less than HST. The larger secondary gets you a wider fully illuminated field, and you won't be using as much of the edge of the mirror which can be bad.

Cheers
Mike
Who has a 10" mirror with a 5" secondary

Hello:

You can make a cassegrain of any f-ratio with a spherical primary. This is the Pressman-Carmichael variant. The secondary mirror needs to be a oblate spheroid to cancel the spherical aberration of the primary mirror, and is quite difficult to figure. And the coma is horrendous, which is why no one makes these things. But you could use a f-ratio typical of cassegrain primaries, such as f/3 or f/4. There is also a solution for a Gregorian with a spherical primary mirror, and as I recall the secondary is a concave ellipse. If you want to make a standard cassegrain, but don't want to parabolize the primary, then you have the same constraint as with a Newtonian, i.e. for a 6" primary mirror the primary mirror f-ratio should be around 10. Using such a primary mirror in a cassegrain would result in a very long focal length (around f/30) and a very long tube.

When making a cassegrain, the hardest part isn't so much the parabolization of a fast primary mirror but the making of a small, convex hyperboloid, and figuring the mirrors to match. This is one reason to make a Dall-Kirkham; the secondary is spherical and easier to make with a smooth figure.

Cheers
Mike Connelley, who has made a Cassegrain but didn't do the optics himself.

Hello:

There was an article a while ago in S&T about how the moons of Jupiter looked egg shaped in a Clark refractor. The cause was warm air along the tube wall since the tube tended to be very close to the light path for Clark refractors. All else being equal, I'd say go with the 12" tube to avoid this sort of problem. Also, a 12" diameter tube is 2 times stiffer than a 10" tube while being 20% heavier. If it means getting a new mount, then I suspect you can get away with the 10" tube if you mount the lens cell carefully to avoid vignetting but you'd have a worse time with tube currents and it'll be harder to baffle fully.

Cheers
Mike Connelley

Hello:

For the SCT owner, collimation is simple. Point to a bright star at high power and a little out of focus, and collimate the secondary mirror until you see the secondary mirror shadow in the center of the out-of-focus image of the star. In other words, the hole in the donut should be exactly in the center of the donut.

For the Dall-Kirkham, if it's only a little out of collimation, then adjusting the secondary mirror collimation (see above) should work fine. In order to check the primary mirror collimation, I do the following:

1) remove the secondary mirror from the spider
2) Put a collimating eyepiece/Cheshire into the focuser
3) Stand a few feet back from the front of the scope. Look into the scope, and move your head until you can see the center of the Cheshire through the center of the spider. With your head in this position, the reflection of the spider vanes should lie behind the spider vanes. If they do, then the primary mirror collimation is OK. If not, tilt the primary mirror until the reflection of the spider aligns with the spider.
4) Put the secondary mirror back in.
5) Look through the Cheshire at the secondary mirror. The reflection of the Cheshire should be in the middle of the secondary. If not, tilt the secondary until it's centered. Try not to get confused with reflections from the primary mirror.
6) Point the scope at a bright star, and collimate the secondary mirror on an out-of-focus star as described above.

Cheers
Mike Connelley

Hello:

NGC 7027 is a very dusty planetary nebula. Images taken in the infrared (from about 2 to 20 microns) shows a ring structure, almost exactly the same as M57. It's bright enough in the infrared that we were able to observe it in the day time!

Cheers
Mike Connelley