52 replies to this topic

[Skipper_52]

Where is a good place to learn about setting up a Dob with and using setting circles?


Thanks in advance.

[Skipper_52]

Never ending.... Can anyone tell me what the focal length of this f8 6" is? I can find it anywhere. I'm taking a guess at 1200mm.

[MistrBadgr]

You might look at this site and see if there are bits that might apply to what you are doing.....maybe, maybe not.

http://www.homebuilt...tingCircles.htm

[Skipper_52]

Thanks

[Skipper_52]

Let’s see if I finally understand this coordinate system and using a scope with circles whether they are RA (right ascension,time) or Azm (360 degrees).

1. If you’re going to be using either of these circles to find objects you have to have either your equatorial tripod or in my case Dobsonian base level. And Polaris set at 90 deg on your Alt so you know where the celestial equator (0 degrees) is.

Apparently someone a long time decided to base time/degrees off of the Royal Observatory in England and set that time/degree (longitude) as 0. If we divide the earth into 24 ‘hours’ or 360 deg. moving East to West we come up with RA or Right Ascension or Azm. Although we know that everything in the universe moves constantly we can assume that during our short lives, the coordinates that someone gave to the stars based on Greenwich meridian hasn’t moved one helluva lot. Although I’ve seen star coordinates change during my lifetime I’m not sure if this is due to more accurate instruments, the moving of the stars and us or all of the above. So, stars/objects have been given coordinates and they remain ‘fixed’ in space. They appear to move only because of the earth’s rotation which at the equator is moving at a pretty good clip of about 1000 mph. You can see this effect through a telescope. The less powerful the eyepiece the slower the object appears to move and the more powerful an eyepiece is the faster it moves due to the decrease of the field of vision. Normal field od vision for the human eye is somewhere around 50 deg. A more powerful eyepiece has less degrees of view. Look at the moon with a magnification of 150x and before your friend has a chance to see what you just did he’ll be lucky to see the moon at all and have to readjust the position of the scope. Basically a more power eyepiece is like putting on a set of blinders.

2. Once you level your base/tripod, locate a known star and set your circles to the correct settings you can pretty much be assured that if you know the coordinates of another object, you can aim your scope to the new coordinates and you’re going to see what you wanted to. However, go inside to use the bathroom and come back out and your object is gone from view. Not because it moved but because the earth is rotating and the stars position/coordinates have changed based on where the earth is now. So, to find it again, you have to go back to your target/home star, reset the current coordinates and dial it back in again. A stars position never changes but the earth’s rotation does therefore it's coordinates will change based on your position in 'time'. Example, focus on any star and set your circles to it’s coordinates. Wait 5 mins and look in the scope. It’s not there. It moved. Wrong, the earth rotated. Find it again without using your circles. Get it centered in the scope and look at the circle settings. They sure aren’t what you set them at before. But the position of the star didn’t change. Your viewing position did, so relative to your position in ‘time’ these alt/azm coordinates will change. So unless you’re just pointing and viewing you better have a program that tells you the coordinates of the object you want to look at based on the current time. The original coordinates aren't going to work unless you view it again tomorrow at the exact time. I won't get into realtime.

So, with my Alt/Azm mount and using a program for coordinates, I shouldn't have to reset my circles (Alt/Azm settings) once I initially setup the scope. (unless someone moves the Azm circle.

If I’ve got anything wrong Bill, please let me know.

[MistrBadgr]

I think you pretty much have it. For RA and Dec, equatorial, imagine tilting your Dobsonian base until the azimuth axis is parallel with the axis of the earth by pointing it at the celestial North pole, near Polaris. Since the position of your scope is very near the axis of the Earth in celestial terms, they are then the same and the azimuth axis of your scope becomes the axis of the Earth and is now "pinned" as the Earth spins, you only have to change the position on that now RA axis and your now azimuth position stays the same. If you know the RA coordinates of a star near the object of your choice, then doing what you stated will definitely work.

For a true Alt/Az situation, the only thing your scope is near is the center of the Earth and all the other objects or axis are rotating in some fashion. You have to have positions in Alt/Az for the known near star and your object at a particular time. At any other time, the relationship between the Alt/Az positions of the know star and your object change as well as the actual coordinates themselves.

When I was in engineering school, all of that was pretty much a "piece of cake," but over forty years later it makes my head hurt just trying to think about it, much less calculate the numbers. You have to know your lattitude and longitude. If I were doing it, I would enter the information into Astro Planner, let it calculate the Alt/Az values for a known star and my object of interest at a particular time I wanted to observe it and then use those values.

I guess in theory, you could pin point one star, at the particular time and then go immediately to another object anywhere in the sky, but I think you would be better off using a bright star that is fairly close to the object.

Hope this makes sense.

Bill Steen

[Skipper_52]

Sure does. can't wait to try it out.

[Skipper_52]

The base is pretty much done. I will try and send pictures tonight. I have to disassemble it, sand and paint but everything works as well as I can expect. There are some minor tweeks I have to make when I disassemble it. I bought a digital angle finder and it might be too sensitive as it will 'jump' sometimes between readings. I put it up against the old one and they are always within .1 degrees of each other. I beleive this might be close enough to find something under low power. I laminated the Azm scale to keep it dew proof and I can dial it to within 1/4 degree.

I found a free piece of software that gives you 'live' coordinates. I just need to ressurect an old laptop to make it portable. http://www.ap-i.net/skychart/start


If you were to use a green laser as a pointer which one would you recommend?

[Skipper_52]

The next pictures won't be here until it's painted, I really wanted the eyepiece holders up higher but there was too much in the way (bad planning I guess) but I wanted them on the same side as the adusting wheels.

As you can see the feet have levelers on them. They will have a locking nut on tob when finished. The Azm scale will be glued to wheel after it's painted.

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[d33d0gg]

I am in awe.

[MistrBadgr]

That is just SO SLICK!!!

As far as a green laser pointer goes, I think they are all about the same on the inside. If you buy one that is just for pointing in the sky and does not come with a mount, you can save some money. I am quite confident you can make a mount for one. Otherwise, get one that comes with a mount to attach to a scope. I bought one for use with sky talks and then took the mount for a 5X24mm finder, made some bushings to slide on the barrel of the pointer out of PVC pipe. I could slide it in place of a regular finder and adjust the alignment screws on the mount to set up the pointer.

I think a regular 6X30mm or even a 8X50mm finder would work well on your scope also.

Bill Steen

[Skipper_52]

Well I sure do thank you for the compliments. I had fun designing and building it. Can't say it didn't tax my brain more than once.

[d33d0gg]

How much does it weigh now?

[MistrBadgr]

Another thing about lasers. Get a green one, I have been told that the red ones are worse for eyes and the green ones are easier to see.

Bill Steen

[Skipper_52]

It's about 35 lbs in weight. Most of being the hardware (plate bearings etc). I got it apart, sanded and primed today. Cut off the long bolts and cleaned up the stuff I wanted to. Waiting on the white laquer to get here next week.

Everyone has said you can see the green lasers. I'm going to wait and let my son pick that up for me at Christmas. Easy gift and it won't break the bank. I can get a 100mW for about $32.00

[Skipper_52]

If I want to sell some extra things I have can I sell them here or not?

[MistrBadgr]

So far, that has not been an activity on this board. I think you would do much better anyway of you did it on Cloudy Nights.....more viewers.

Bill Steen

[d33d0gg]

Its not against the terms of service, or the code of conduct, but as bill said, you will get more eyes on it on cloudy nights.

But, if you are trying to get rid of some astro gear send me a message, and let me know what you got. I might be interested.

[Skipper_52]

Well the damn paint never got here this week.

[MistrBadgr]

I feel your pain! Try not to jump up and down, or wave your arms around, too much!

Bill Steen