Build a Spectral Calibration Source

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Build a Semi-Permanent Spectral Calibration Source:

After I used the artificial star one time to calibrate my spectral images I loved it so much I build a semi-permanent one as I found out I needed to use it every time I took spectral pictures and wanted to know the wavelengths of the spectral lines I found. By making a setup that was semi permanent it wouldn't take very long to set everything up so I could take a calibration picture every time I took spectral pictures even if I didn't expect to do any Red Shift measurements. This way I could always go back on some later date if I changed my mind and wanted to do Red Shift measurements as I would have the spectrum pictures as well as the calibration pictures. Without calibration pictures it would be all but impossible.

We will start by buying the hardware needed. This is real easy so don’t get worried that it will be over your budget or be too complex for you to use. What you are going to build is an artificial star that you use at night and you will illuminated with a spectral light source (such as a gas laser or fluorescent tube). I use two different light sources. One is a very inexpensive fluorescent light source and the other is a Helium Neon gas laser. The laser works the very best and it will cost a little more but not much more.

Go to Ace hardware and buy a 1/4" steel ball bearing ($1.50) and a can of flat black spray paint ($4.00). Then go to Wal Mart and buy a 20 to 30 watt screw-in fluorescent bulb ($5) and a reflector holder ($10.00) to screw the bulb into. Then go to a craft store and buy a 14" x 10" piece of black 2 mm thick foam material ($3).

I Recommend that you also add the next item to your list. It may be a bit much but when you have it all set up you will be glad you have it.

Find a take-out/used Helium Neon gas laser at one of the surplus dealers on the internet ($50). It won't come with a power supply or connector, so go to Radio Shack and buy a 12 VDC at 1 Amp power supply ($20) and a connector that fits the power supply connector. This last connector will need to be soldered to the laser when you receive it. The red wire coming from the laser will be connected to the positive wire from the power supply and the black wire will go to the negative wire from the power supply. Lastly you will need to get a small lens from a surplus supplier, like American Science and Surplus. You need something small that you can mount to the front of the laser that will widen the beam so it will fully illuminate the entire steel ball. A focal length of 12 mm to 25 mm should work fine. Mine has a focal length of 18 mm and is 7.5 mm x 7.5 mm square and cost $1.50 + S/H.

Then find a clear location that you can see clearly from your telescope site, that is at least 10 times the focal length of your telescope from your telescope site. Pick a location where you can build your back drop and leave it, like an old wooden fence or other flat surface about two or three feet above the ground. I found an old wooden fence about 30 yards from my telescope site where I mounted my back drop.

Take the flat black spray paint and spray an area about a two feet in diameter around where you are going to build your calibrator. After the paint dries glue (using Silicone glue) the black foam, you purchased from the craft store, over the black area. This will give the background image a black featureless background that won't interfere with your calibration images.

I nailed a 2" x 2" piece of wood to the fence and painted it flat black as well. Then I cut a small piece of the foam and glued it on the front side (the side facing the telescope) of the 2" x 2" board. I then put a small circle of Silicone glue in the middle of this assembly on top of the 2" x 2" board. When the glue dried that is where I would put my steel ball when I was taking calibration pictures. These little balls have a tendency to roll around and drop to the ground and get lost. This way the ball would stay in the circle as I piled the glue up 1/8" high.

I put am old plastic lawn table (24" x 24" by 22" high - Wal Mart $9) several feet from the fence and that is where I set my light sources. The light source sits a bit below the level of the ball so the glue and top of the 2" x 2" board are not illuminated and don't show up in the calibration image.

You need to take the ball inside when you are not using it as it will rust, especially if it rains on it and its left out in the weather. When I bought my steel ball at Ace I bought several of each of the different sizes they carried. This way I can change the size of the ball to a larger size if the spectral image is too dim (a larger ball will brighten the image). I wouldn't buy any steel balls larger than 5/8" or smaller than 1/4". In the picture to the right I placed a 3/8" steel ball in the glue circle so you could see how it stays in place. I used the 3/8" ball as it showed up better in the picture than did the 1/4" ball.

Now, the back drop is finished so we need to set up the light sources. First you will need a piece of cardboard about 12" x 12". Then cut a 2" hole in the center of it and paint it flat black on both sides. This will be placed in front of the light source to cut the beam size down and cut down on back reflections that might get to the telescope.

Sit the fluorescent light on your table/chair, no closer than 2' to the steel ball. Put the piece of cardboard, you just painted, in front of the lamp. The fluorescent light won't get very hot but if you use a regular light bulb it will get very HOT. Be careful not to get the black cardboard so close to the bulb that it gets hot and catches on fire.

Then wait for a dark night. Turn on the fluorescent light, put the black cardboard in front of it and illuminate the steel ball and focus the telescope on the steel ball. Hook up your camera and focus on the spectrum (not the ball) then take a few short pictures. Do not use any filters on your software other than Dark Frames. Meade's AutoStar software has a set of built in software filters that come on automatically every time the application starts up, so be sure to go into the filter section of the application and turn all filtering off. Make a fresh set of Dark Frames before you start taking any calibration pictures that you are going to use. In the picture to the right I have inserted the fluorescent bulb through the cardboard mask. DO NOT do this if you are using an ordinary light bulb as the bulb will get so hot the cardboard will catch fire. The fluorescent bulb never got so hot I couldn't hold it with my hand.

What you are doing is building an artificial star and illuminating it with a spectral line source instead of a conventional light. As long as the image size of the light source, from the steel ball, is below the resolution of your scope this will work fine (the dimensions of the parts and set up I gave you above are good for scopes from 1" to 20" - Reflector or Refractor). If you need more information on building/using artificial stars read the book by Harold Richard Suiter - Star Testing Astronomical Telescopes, published by Willmann Bell - Chapter 5.

This will work really well if you are using a Type B or D - DG Filter and have adjusted it so that it fits tight in its threads when the spectral image is horizontal. If you have done all of this you will have a good reference spectrum that you can use over and over again. If you are using a Type A - DG Filter you will have to take a calibration picture each time you move or adjust the unit as the Type A Filters are held in place by friction alone.

The five main lines you will see from a fluorescent source, are:

 Hg Line          Intensity
Center (A)          Units  

 4046.56             1800
 4358.33             4000
 4916.07	       80
 5460.74             1100
 6149.50             1000

You may not see all of these lines form any one source. Generally you will see some of these lines but not all of them. The green line at 5460.74 Angstroms (A) will be present in all fluorescent lamps but not all of the other lines will be present. I have used different types of fluorescent sources and the only thing they all had in common was the bright green line.

To use the laser I glued (using Krazy Glue) a 1/4" x 20 common nut (ACE $0.05) to the center of the bottom of the unit. This is the standard size for a camera tripod's mounting screw. This way I can mount the laser on a small tripod (Wal Mark $12) and easily point it right at the steel ball. You don't use, or need, the black piece of cardboard you used with the fluorescent light, with the laser. After the laser is set up and pointing toward the steel ball I put the small positive lens in front of the laser beam to spread it out. If you don't do this the reflected beam is still too focused when it hits the steel ball and then you have to aim it on to the telescope which is a bit of a problem. By using the diverging lens you don't need to worry about lining it up at all as the divergent beam takes care of it all. In the picture on the right you can see the 1/4" x 20 nut that I glued to the bottom of the laser housing. The little blue thing in the picture is a small tripod used to hold and aim the laser at the steel ball.

The laser produces a much sharper much better defined spectrum then does any of the fluorescent sources. The big difference is in the image of the source itself (not its spectrum). The laser source breaks into diffraction rings while the fluorescent sources do not. Finding the center of the source when it has a dark circular hole is much easier than trying to find the center of a bright blob that may have wriggled some during the exposure. In the picture on the left you can see the brown envelope that the positive lens came in, and on the front of the laser unit is a small white cardboard tab that I glued the lens onto. I insert the cardboard tab all the way up in the channel which centers the lens in the laser's beam. When I am finished I take the white cardboard piece (with the lens on it) off of the laser and put it in the brown envelope for safe keeping. In this picture is also shown a small plastic container holding my assortment of small steel balls. The picture at the right shows the laser assembly set up and ready to go.

By having a dark spot in the center of the source image you can very quickly and very accurately find its center. This is not as easy with other sources that do not produce the diffraction ring pattern. Additionally, the spectrum from the gas laser is a sharp dot in contrast to a fuzzy dot which is what the fluorescent sources produce. All in all the laser allows me to nail down the reference distance to six place pixel accuracy very fast. Its difficult to get better than four places with the fluorescent sources.

The wavelength of Helium Neon gas lasers is 6328.165 A. The only problem I have with the laser, its a low power 1.5 mW, is that the reflected image is so bright the exposures with my DSI camera is close to 0.0004 seconds and with the LPI camera is around 0.008 seconds. Even then, with the Offset down to 45%, the laser calibration images are over exposed. I will probably need to get a shorter focus lens that will spread the beam out three to five times wider.

A short note on lasers: Do Not use any type of solid state laser pointer for this project, as these devices all seem to have a different operational wavelengths. To make matters worse their wavelength changes with temperature. Only use a gas laser as they are rock stable and remember the object of the game is to produce a very accurate calibration picture.

A TIP on taking good reference pictures: Do not take/stack more than is needed to get a good clean black background. If you have a fresh set of Dark Frames this could be as little as one picture. Do not try to stack 50 or so thinking it will enhance the calibration image, it won't. Keep it Simple as far as the calibration pictures go. All you need is a sharp well defined source image and a sharp well defined spectral line. This TIP is only for your calibration pictures not for the star spectrum pictures you take.

Disclaimer:

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