Posts Made By: Alex McConahay

Maybe I don't quite get what your experimental situation is, here, but if you are holding two eyepieces at arm's lenth (about 910mm?) and they both come to focus, then they are both 910 mm focal length.

And, yes, the easiest way to tell is to measure it.

Turn a lamp on. Remove the lampshade if you like. Get about a yard or two away from a wall on the other side of the room, and hold your lens up. YOu should see a focused image of the lamp (or bulb) on the wall behind you. If it is not focused, move the lens forward and backward against the wall. When you get focus, measure the distance between the lens and the wall. THat is your focal length.

(Note that this is not an exact measurement, because the lamp is not a star at infinity, and things focus closer when they are at infinity. But, it gets you in the ball park.)

Alex

>>>>What's my next step

Step backwards.

While it seems that getting a big scope is a good idea. It is actually very difficult to start with a large scope like the C14.

Alex

The easiest formula is to divide the focal length of your telescope by 55 for a full frame 35mm camera, or DSLR. For most DSLR's which use a smaller chip, divide by 35.

Traditionally, it was thought that a 55 mm lens was "normal" since it covered the diagonal (55mm) of a 35 mm film frame. This gave the perspective one would get with the human eye--1 to 1, or 1 power.

However, the smaller DSLR chips only need about 35mm on the diagonal, and therefore a 35 mm focal length lens gives the same perspective (multiply 35 x 1.6, and you get 55). This is the source of the extra apparent "focal length" of a standard camera lens (designed for 35mm camera). You have seen that 1.6 figure before. In early DSLR days it impressed people that they were getting that much more focal length out of their lenses when they switched to DSLR's.

However, before this discussion goes too far--it should be said that there is really no single measure of "power" when it comes to discussing astroimages. (You have to settle on a definition of what the viewing distance is first.)

Alex

Not that it matters much to the discussion, but we should note that the Clear Sky Clock has changed its name to Clear Sky Chart.

Alex

Jared,

Depends a lot on the club and conditions.

As for me and mine--An unfiltered laptop screen is verboten anywhere I have been at a dark sky site. This is true even for those with the screen dimmed, and even when on "Night Vision Mode." And this is true even if on for "only a couple of minutes."

At some, you can get by with a heavy red filter over the dimmed (and probably in night vision mode) laptop screen. But even here, it is good to have a box with three cardboard sides, a top, and an opaque cloth to hang down when you are not actually viewing....And keep viewing to a minimum.

You will know a lot more when you get to the site and see what other people are doing. But I would suggest that unless everyone else has lit up their laptops you keep yours in the car.

Perhaps there will be an area set aside for imagers and laptop users. I have heard that happens in some places.

And perhaps it won't matter at all---But I doubt it.

At any rate, it is best to see what others are doing, and ask before doing it.

Alex

Tom,

WHile the idea of adapting a dob mount for this purpose has an appeal to the ATM junkie in me, --I cannot imagine anything you build being any more packable than a small photo tripod with the proper scope attachment. And it sure would be a lot more flexible and easier to use.

Alex

I was camping in the Mojave Preserve last weekend. I could easily see it at 2:30 in the afternoon, but could not pick it out the next day at the visitor center in front of some people.

Alex

Star charts do not invert images--they are simply meant to be used above your head, not on a table!

Hold them up in the air, and they are fine!

Alex

Michael,

That could be dust on the sensor--but it looks out of focus enough that it could also be further out--like on a focal reducer or field flattener.

Alex

I agree with Jon, that you want to get some rings. (I think they will cost more than he says, but they are the way to go.)

However, should you want to do it without, try the following:

Determine the center of balance. The easiest way to do this is to completely "populate" the tube. That is, put in an eyepiece, finder, Tel-rad, and all that goes on the tube when you are using it. Then place a dowel (or broomstick, pipe, whatever) on the ground. Place the loaded tube on top, and shove back and forth till you get it to balance on the dowel. The point it touches the dowel should be the center of the dovetail. This still give you some leeway should things change later in terms of what you load on the tube.

As for how to mount it, yes-- you need to drill some holes in the dovetail. Whether you tap them or not depends on you. But you will need to countersink if you do not tap them. A through bolt with the head countersunk below the level of the dovetail surface will work with the nuts on the inside. (Or, if you have tapped, you can put the bolt from the inside screwed into the dovetail. Remember to have bolts short enough so they do not stick past the dovetail. I would assume you would want a large washer on each bolt inside the tube also to spread the load. Then blacken the nuts/bolts/washers inside.

Again--rings are better in the long run.

Alex