Azimuth Setting Circle in Vector (SVG) Format

syousef · #1 23-03-2010, 03:17 PM

First release of a quick and dirty Java program for creating an Azimuth Setting Circle in SVG format. The idea being no quality loss if it is enlarged.

Feedback welcome but no promises about changing the program at all. I mostly wrote this for myself. It's released under the GPL and source is included in the zip, so feel free to fiddle with it if you code Java.

However you don't need to run the Java code if you're happy with the SVG files themselves.

Numbers might not be perfectly aligned depending on your SVG viewer. However the lines should be correct.

If you use it for anything, would appreciate you dropping me a note.

Sammy

Bruceaust · #2 23-03-2010, 05:09 PM

I know that setting circles are the usual way of setting azimuth on a dob.
However I have recently used a hand held compass placed on the bottom of the scope azimuth bearing. It has worked really well. I now plan to purchase a Digital Compass to do the same. I believe these compasses can be set to compensate for declination - so it seems a ideal solution.
At present I use a digital level on the altitude and it works fine.
With just a few minutes to set up using a level, the combination of the compass and digital level gives pretty good results. Plenty good enough for visual observing.

Am I barking up the wrong tree? Hope this helps.

Bruce

syousef · #3 23-03-2010, 06:59 PM

I've recently ordered a digital compass with just that in mind. It would have the advantage of not requiring the base to be positioned firmly, which means it can go on wheels. What I've found so far with my cheap analog hiking compass is that moving the large metal tube of the telescope also seems to cause the needle to move by as much as 15-20 degrees.

I'm hoping the digital compass would at least have the advantage of being accurate within 2 degrees without being as difficult to read in the dark. A larger compass might also do. However I am not as hopeful about eliminating deviation caused by the telescope itself.

My dob is currently on castors and has been for some years. Before I did that I use to be able to get within about 2-3 degrees with painstakingly hand drawn setting circles. I am planning to get off my proverbial sometime soon and find a good way of locking the wheels so there's no movement. I'm thinking feet or wedges will do the trick. When I do. Failing that I'll take the castors off and buy a hand truck. I miss the accuracy that gave me before I put the wheels on.

I think that with the amount of time/money/effort that goes into buying or building a dob, it's insane that good setting circles don't come as standard!

syousef · #4 23-03-2010, 07:38 PM

One other thing I forgot to add. If you use a spirit level to level the telescope all you have to do is align your altitude and asimuth setting circles well on a single star (at least 45 degrees from zenith), and check against a second.

Bruceaust · #5 23-03-2010, 08:32 PM

I forgot to mention, My Dob has a cardboard tube!

syousef · #6 23-03-2010, 09:21 PM

I suspect the reduction in metal would make all the difference. Probably much lighter too.

alistairsam · #7 04-10-2010, 03:09 PM

Hi,
could someone help with the alignment process for analog setting circles on a Dob mount?
I have an 8" Dob mount, and if I print out a graduated scale on the base, where should 0 deg point to?
I have a digital angle gauge for alt, so that should be straightforward, but was not clear on the azimuth.

JethroB76 (Jeff) · #8 04-10-2010, 04:03 PM

Quote:

Originally Posted by alistairsam

Hi,
could someone help with the alignment process for analog setting circles on a Dob mount?
I have an 8" Dob mount, and if I print out a graduated scale on the base, where should 0 deg point to?
I have a digital angle gauge for alt, so that should be straightforward, but was not clear on the azimuth.

As in, where on the base should the 0 deg be? Can be anywhere you like really.
The way I used to do it was to have a moveable pointer for this scale.
So I would setup the scope and zero the alt (electronic angle finder) on a level surface. Determine the az coord of my alignment star and centre it in an EP. Then clip the pointer onto the base to point at this az. Next check the alt was what it should be, and then check the overall alignment by "finding" another object.

syousef · #9 04-10-2010, 07:55 PM

Quote:

Originally Posted by alistairsam

Hi,
could someone help with the alignment process for analog setting circles on a Dob mount?
I have an 8" Dob mount, and if I print out a graduated scale on the base, where should 0 deg point to?
I have a digital angle gauge for alt, so that should be straightforward, but was not clear on the azimuth.

1. Get the base of your scope level. You need to find flat ground. Spirit levels help. The base of your scope must be fixed. (If it's on wheels you need a good way to lock them).

2. Find at least 2 bright stars.

3. Use software to find their current altitude and azimuth. Stellarium will do the job and is free. You'll need to make sure your position is set correctly.

4. Align the scope to one of the bright stars, so that you can see it clearly in the center of a medium power eyepiece. Read off it's altitude and azimuth and compare to your circles. If you have movable azimuth scales set them. If you don't rotate the base for azimuth, or note the offset and use it when you read positions.

5. Your altitude should correspond correctly. If it doesn't, go back to leveling your scope, or if you have an inclinometer zero it aimed at the star, then move your scope to negative what the altitude of the star is and zero again so that when it moves back to the star it moves correctly.

6. Repeat 3,4,5 for the second star. Hopefully you shouldn't need too much adjustment, if you do go back and repeat again for the first star.

Hope this helps.

alistairsam · #10 05-10-2010, 10:09 AM

thanks. when choosing reference stars, if the alignment is done with just sigma octanis, will that be reasonably adequate? do its co-ordinates ever change?

else, with two stars, when aligned to the second star, do you adjust the scales again, and then go back to star 1 and adjust again?

syousef · #11 05-10-2010, 11:17 PM

Quote:

Originally Posted by alistairsam

thanks. when choosing reference stars, if the alignment is done with just sigma octanis, will that be reasonably adequate? do its co-ordinates ever change?

else, with two stars, when aligned to the second star, do you adjust the scales again, and then go back to star 1 and adjust again?

The altitude and azimuth co-ordinates of objects in the sky constantly change as the sky rotates so you need to look these up for the exact time you are aligning the scope. Against the imaginary "celestial sphere" nearby stars do change but only very slowly and this is called proper motion. Google it.

Choose 2 bright stars as far apart in the sky as practical. You don't have to worry too much about proper motion for the purpose of alignment. You're only looking for accuracy no better thanabout 1-3 degrees - enough to get you in the ballpark with a low power eyepiece - the moon is about half a degree and comfortably fits in such an eyepiece in an 8 or 10 inch Dob. Your accuracy should improve with a little bit of practice but it's not going to get much better than a degree or so - standard Dob mounts and home made analogue circles just aren't made to high enough precision and will typically still have some play in them. So unless you're picking stars with very high proper motion and your software can't calculate it, entering your GPS coordinates and time accurately into good sky chart or sky simulator software and looking up altitude and azimuth should be fine. (If you were pointing a large professional instrument for photographic or instrument work, you'd need to be much more precise)

ZeroID (Brent) · #12 06-10-2010, 12:33 PM

Quote:

Originally Posted by Bruceaust

.... However I have recently used a hand held compass placed on the bottom of the scope azimuth bearing. It has worked really well. I now plan to purchase a Digital Compass to do the same. I believe these compasses can be set to compensate for declination - so it seems a ideal solution.
At present I use a digital level on the altitude and it works fine....

Bruce

Hmmm, just realised I have my old handheld Garmin GPS from my serious boating days. It has full compass functionality including preset on the declination and a range of output methods. Gets about 8 hours on a set of AA's. Add a digital level and the bases are all covered maybe. have to dig it out and see how it works. Glad I didn't sell it off now ..

syousef · #13 06-10-2010, 06:58 PM

Quote:

Originally Posted by ZeroID

Hmmm, just realised I have my old handheld Garmin GPS from my serious boating days. It has full compass functionality including preset on the declination and a range of output methods. Gets about 8 hours on a set of AA's. Add a digital level and the bases are all covered maybe. have to dig it out and see how it works. Glad I didn't sell it off now ..

If your scope tube is metal like mine it'll throw off a digital compass. I bought a cheap one for $20 to play with but quickly realised it won't work out. (That's okay I can use it with my Garmin GPS, which doesn't have a compass, though it still can display direction if moving)

Much more accurate to set azimuth based on a star.

I also bought an inclinometer. I think that will work out, but haven't used it much. This is the one I have.
http://www.wixey.com/anglegauge/index.html

alistairsam · #14 14-10-2010, 10:14 AM

Quote:

Originally Posted by syousef

The altitude and azimuth co-ordinates of objects in the sky constantly change as the sky rotates so you need to look these up for the exact time you are aligning the scope. Against the imaginary "celestial sphere" nearby stars do change but only very slowly and this is called proper motion. Google it.

So unless you're picking stars with very high proper motion and your software can't calculate it, entering your GPS coordinates and time accurately into good sky chart or sky simulator software and looking up altitude and azimuth should be fine. (If you were pointing a large professional instrument for photographic or instrument work, you'd need to be much more precise)

Hi,
I had been to the IISAC astrocamp and got to use a Dob mounted scope and the Argo Navis, and learnt how to do the two star alignment.
But for that, you need to know which star you're aligning with.

On the other hand, is it possible to achieve reasonable accuracy if we calibrate or align on the horizontal and vertical plane, thats 0 deg and 90 deg on the altitude axis say with a digital angle gauge, and use a compass to align the azimuth, so zero on the azimuth scale points to the pole?

just trying to understand, by aligning to the two stars, we're basically setting the offset from zero degrees in each axis? so can't we just align to zero in each axis if we have the right instruments?

syousef · #15 14-10-2010, 10:07 PM

In practice using a compass is difficult and error prone. A sufficiently accurate compass is going to be expensive, and will still be prone to inaccuracy if there's any metal about (especially on your telescope).

In contrast the stars have very well known and calculable positions. On a good night, it shouldn't be very difficult to find 2 bright stars that you can match up with a sky simulator. For that you'll need to know rough positions of course. It's a good reason to learn your constellations and do a little planning before you observe.