Posts Made By: Alex McConahay

Paul,

Coatings last a long time. As far as I know, anti-reflection coatings last forever. I've not heard much discussion, at any rate, about them fading or needing replacement. On the other hand, for the last many years, manufacturers have been "improving" these anti-reflection multi-coatings found on corrector plates, lenses, and eyepieces. Therefore, while the coatings on an old scope may be as good as new, the quality of their "new" would not match the current state of the art. What will happen in twenty years? I don't know. It could be that anti-reflection coatings have just about reached perfection, and there is not that much improvement left.

On the other hand, coatings on a mirror do deteriorate with time. (In protected SCT's probably less than in open newts's, and in scopes used at the ocean more than in scopes used in the desert.) How much they deteriorate depends on so many things that, in general, you should not buy a mirror depending on what condition it "might" be in, but what it "is" in. I've seen twenty year old mirrors in fine shape. I've seen ten year olds needing recoating. I haven't seen anything needing work in less than ten--although I have heard stories.

Alex

Portable--as much as any operation that large can be considered portable.

Much easier than setting up a large fork mounted equatorial on a wedge. And much easier polar aligning when moved.

Mine breaks into one toolbox to hold the weights, one suitcase (size of a roll-on airline bag) to hold the head and related electronics, and a medium sized Rubbermaid trunk for the legs. Then the top of the Tripod comes along without a case. The head is the heaviest of these things, but still not too bad.

Alex

Tom.

No, you don't need it. Please ship it to me. I will send you $50 to cover the cost of shipping!

Alex

P.S. I don't think guiding is absolutely necessary for most of what people use DSLR's for. They track and stack, and therefore are not generally dependent on autoguiding. Since their exposures are generally short, the stars do not need the long term convenience of the ST-4.

However, my buddies who use DSLR's tend to hand guide. This is tolerable at those exposure times.

But--why not use your ST-4. It does make it more convenient to expose your photos.

Where is the difficulty with the learning curve--with the ST-4 or with the rest of astrophotography? What is going wrong? The St-4 is a bit intimidating at first.....But once you ignore most of the settings and use it a while, you figure everything out.

I've tried the LPI-Autostar Suite--Haven't gotten it to work in about three sessions with an LXD 55 mount.

Ok--this means you probably won't be shipping me your ST-4.

Alex

My first mirror came out to be about 1/12th wave. It was an 8 inch F5.8. WOnderful little scope. Took about 20 hours to get it to polish, and another 40 to get it figured right. However, the last figuring session--when I finally "got it"--went from 1/2 wave to 1/12th in less than three sessions of five minutes each, and I predicted it would do exactly that.

Anybody can do it.

I should tell you that one of the many reasons I took up mirror making is because of my personality. I was a teacher and high school principal. All my life I worked with people and therefore success was always an approximation--never a dead on measurable goal. This extended to whatever project I was working on. Whenever I built anything, as long as it worked, I was satisfied.

But I could not do that with telescopes. Everything had to work. Precision mattered. So I had to change my personality.

I had to learn that when something was not working, I had to stop and regroup--not just bull my way through it.

When measurements did not come out right, I had to rework a piece, not just change the measurements.

I had to learn patience.

Anyway--I did it, and was really proud of the mirror and the changes in my personality.

In short--anybody with a modicum of intelligence can make a mirror.

I strongly agree with other advice you have in this thread--

If you want a telescope--buy an already finished mirror.

Get somebody to help you with the project.

Be patient. The first part comes remarkable easy. the last part (figuring and testing) torments.

You will never reach perfection. Let your next mirror be perfect. When this one gets to 1/8 th wave or so, put it in a scope. Then start on your next.

Alex

What you call fL/Diam. ratio is called the F ratio or focal ratio, as in F 10 or F 4.5.

Light gathering ability is dependent on aperture alone. (Diameter of the primary).

However, as another A-marter has described, on extended objects, a faster (lower number) F ratio will expose a given piece of film or ccd sensor quicker.

Alex

You should enjoy some wide fields--not too much magnification.

It should work. But don't expect the sharpest views. One thing that you mentioned-- 80% parabolic--indicates to me that an amateur made the mirror, and got it about 80% parabolized before stopping work. It is not wholly unusual that a mirror maker would write that on the back of a mirror, although usually he would come back and finish figuring it. (Although, technically, a true optician is never "finished" with a mirror, you just finally have to steal it from his Foucault tester and get it aluminized or he'll go on forever trying to tweak it just a bit more.)

Why don't you find a star party, and take it, and borrow some eyepieces and see how it works.

Alex

Mark,

This brings up a problem that I have always thought rather intriguing. Suicide is an example of what Catholics call "Mortal" sin--that which would send you straight to hell if unrepented. In order to commit a mortal sin, one must have knowledge that a deed is a mortal sin, and one must have free will--a chance to either do or not do it.

The problem I have always had with that analysis is that I do not believe a person with a free will could do a deed that they knew was a mortal sin. There are people who could kill, for instance, out of sheer meanness--but if they are that mean, can it be said they did it with a free will?

So--to suicide--can one in his right mind (which is necessary if you are considered to have free will) kill himself? No, I do not think so. Something so overwhelming must be compelling the act--and therefore, the person has no free will.

In short-No, I don't think the person is barred from heaven. They would go to the same place as the mentally incompetent and the innocent.

Alex

By the way--first concern with anybody wanting to talk about suicide is to make sure they are talking philosophically, and not planning anything.

I would not have spent any time discussing this if you had not answered already that you are just discussing philosophically. I would have, as Jason did in his first response, suggested you call somebody close at hand.

>>Can a ronchigram also be analyzed with computer software?

Not that I know of. But have you seen

http://members.aol.com/ronwin20/

It will give you something to campare with.

However, since the Ronchi is not all that quantitative, I don't see how it would lend itself to computerization.

Alex

>>If I'm not a Buddhist when I die I go to somewhere else besides heaven.

Even if you are a Buddhist, and a good one, if you die you don't get to go to heaven.

Alex

The issue could be any one of a number of things--but what is interesting is that you can focus indoors pointing out. This would indicate to me that you can focus when you are at a stable temperature--but when you take it outside, you have some thermal currents spoiling your view. Is the image steady or does it seem to move? If it moves, suspect temperature differentials. TO cure it, take the scope outside, and leave it there for an hour, then try to focus it.

If it does not work well after it has been outside for a while, then pick a lmedium bright star, and bring it to as close a focus as you can. Then take it in and out of focus. The image should have a circular fuzzball around it. If it is not circular, you have a collimation problem. Collimate your scope. (Do this with and without your diagonal, and see what you can get.) The circles should be round in either case.

If it still does not work, check you optical path for things like dirt, dew, and such.

Oh, and I am assuming you are not simply overpowering your scope.

Of course, then we go back to--why is it sharp inside the house, and not outside.

Alex