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Posts Made By: Alex McConahay

November 10, 2004 06:42 AM Forum: Telescope Making

Where does infinity start?

Posted By Alex McConahay

Okay, that is not really the question--just an interesting title.

The real question is how far away does a terrestrial object have to be before it has substantially the same focuser position as something at infinity?

Specifically, we have rebuilt a 22 inch F4.2 or so Dob, and will be setting it up for the day for the first time this weekend. I know with the work we did on the mirror cell that we will probably have to shorten the truss tubes (since the mirror cell and collimation system is now an inch and a half "shorter" and the mirror is that much further back).

There is a cross on the hill a mile away that we can focus on. How much difference will there be between the focuser position for that cross and a star at infinity?

If it is essentially the same, then I can focus as well as possible to determine where the focuser would need to be, shorten the poles during the day, and observe that night. Other wise, I will have to check things out at night, and not really be able to observe until then next night after modifications.

By the way--anybody have any helpful hints about how to do this operation?


November 10, 2004 03:19 PM Forum: Politics

Re: Eerie parallels between wars in Vietnam and Ir

Posted By Alex McConahay

Okay--Just for comic relief--

On the comparisons between Iraq and Vietnam, I want to remind us all of a posting from a week or two ago, I believe in this forum--

"The difference between Iraq and Vietnam was that Bush knew how to get out of Vietnam."


November 14, 2004 03:16 PM Forum: Telescope Making

How much collimation flex is there

Posted By Alex McConahay

One purpose of a telescope tube is to eliminate all flex so that when a telescope is collimated, it is collimated no matter where it points. However, I know that this can be a difficult job.

We just set up a newly reconstructed Truss tube dob. It is a 22 inch F 5.

When collimated in a down position (a few degrees above horizon), the laser spot is dead on. When you crank it up to zenith, the spot is maybe 1/8 inch from center.

I've inspected for the obvious things, mainly loose parts I suppose.

Is this within spec? Is there any margin for error? Is it a problem with big truss tube dobs?

What is your advice. And where should I look to make it better?


November 23, 2004 11:57 AM Forum: Equipment Talk


Posted By Alex McConahay


The thread was about green laser pointers--bringing bright lights to dark sky places.

It was not about you.

I certainly did not mean to give offense.


November 23, 2004 04:35 PM Forum: Equipment Talk


Posted By Alex McConahay

This isn't a response to Hal in particular, just a comment on the whole thread.

A clear distinction should be drawn between using a laser pointer for the public, and for individuals.

I don't know of many astronomers who would object when used a green laser is used to point out objects to a whole group, or for educational purposes when the star party is for those purposes. (Or during the early evenings when the public need that kind of pointing.) They do a better job than the 1 million candle-power flashlights we used to use with the general public and beginners.

But using a laser pointer at a dark site as a pointer where others have come seeking the darkest possible skies so that they can do some serious observing of faint fuzzies is not right. I don't know where exactly it ranks as far as distractions. There are others competing with it for the title of most annoying distractions (in my opinion):

flashing lights on your tripod legs so you will know where they are. (If you don't know where your tripod legs are--maybe you should consider another hobby.) If you must light them, at least make it steady red light.

Computer screens used for CCD work unless they are at the edge of the field, turned away from other observers, and properly filtered.


running generators

improperly filtered red flashlights--just cause it's red does not make it Okay!

Loud music of the kind I don't like (I like Mozart).

Of course, I am sure other people have other pet annoyances.....

It is just that I see no reason to have a bright light on--even momentarily--when I have gone to so much trouble to be in a dark site.


November 27, 2004 03:18 PM Forum: CCD Imaging and Processing/Solar System

F ratios on DSLR Lenses

Posted By Alex McConahay

Here's a question I thought about when looking into buying a DSLR to match my existing collection of Canon EOS lenses.

Okay, I know my existing lenses will fit the new D20. However, I will have to multiply their focal length to get the "equivalent" 35 mm focal length. I.E., my 300 is now a 480 mm lens.

Okay, so what happens to the F ratio?

The aperture has remained the same, the actual focal length of the lens has remained the same, and the angle of the last rays of light coming to the film plane (or chip in this case) has remained the same. So, the F ratio is the same as it is when using a 35mm film camera, right? But what, a third of the light is wasted?

How bright are the finder screens on a 20D compared to an Elan when using the same lens?

The 18-55 zoom they have on the 20D kit--Is it's F3.5 as bright as my 75-300 mm's F 3.5?

Let's have your thoughts.


December 12, 2004 05:41 PM Forum: Equipment Talk

Error in Losmandy G-11

Posted By Alex McConahay

Hi all,

Anybody have any experience with Losmandy G-11 and a strange error I am having.

When I set up normally, things run pretty well normal. Then after two or three minutes of normal use, with the star staying right on the crosshairs of the reticle, the star starts to drift ever so slightly, and then there is an audible click someplace down in the right ascension shaft or worm-ring interface, and the star snaps back to the crosshairs. The total drift is about from one side of the crosshairs to the other (12 mm reticle eyepiece in a Celestron 9.25) The cycle repeats with a click-correction every 27 seconds (or so).

I can limit the amount of error by seriously unbalancing the scope--but there is still some drift and correction, and the click still happens at about the same rate.

Here is a picture of a star trail in M-42. You can see gradual drift-then correction, then shift-correction, shift-correction.

I should say that this picture was taken last weekend. So I brought the mount home, changed the right ascension motor and electronics. Same error after this fix. I then took the worm off, checked for dirt, grit, etc in both worm and ring, and then shifted the ring halfway around before resetting the worm gear. Still same problem. Did it all over again. Same error. Of course I adjusted for backlash, etc. each time.

This is one of Losmandy's upgraded worms. I have been using it for a while, although, I cannot remember taking photos with it yet--and that is always more critical.

Any ideas?


December 18, 2004 07:26 AM Forum: Equipment Talk

Re: Dovetail plate and dovetail saddle plate

Posted By Alex McConahay

It would help if you specified the name of your mount, or included a picture.

I have made my own dovetails several times. They are simple pieces of aluminum. The tricky part may be making (or just buying) the "radius blocks" that attach to the scope to make a tangent to the circumference of the scope.


December 25, 2004 09:34 AM Forum: Beginning Astronomy?


Posted By Alex McConahay


Is the problem that you are getting sharper and sharper and it stops before it gets sharp or is it that you get it sharper and sharper then it becomes softer and softer without ever getting sharp in between?


December 26, 2004 02:03 PM Forum: Equipment Talk

LX90 Motor-Drive Problem

Posted By Alex McConahay

Anybody know where to start looking on a problem a friend of mine is having on his LX90?

He turns it on, and everything seems happy.

He tells it to start aligning, and it just stops and makes what he describes as a "god-awful noise," and he shuts it down before it blows up.

Now, I know most Meade mounts make "god awful noise" whenever they are slewing, and maybe this is all it is. That, and the combination of a loose drive gear, clutch lock, or or something. The scope has just been doing this for a little while, and he has had it three or four years, though, so I think he does have some real problem with it.

He's bringing it over my house in the next few days, and I thought maybe somebody would have some helpful hints as to where to look.