What to use as an artificial Star.

rumples riot · #1 10-03-2005, 09:49 PM

Hi all, well as some of you may know I am currently trying to get my collimation of my SCT to near perfection. I want great results for my planetary stuff.

Well do not have the time for using the laser, although I will get to it very soon, to confirm my results.

Well necessity being the mother of invention, I finally decided that I would make an artifical star for day light collimation.

The photos will show, that I used a couple of lengths of timber with a nail in it and a very old christmas decoration. Glass variety from my childhood. In fact I think it is older that I am.

There is a distance shot where I placed the Star on the end of our carport. Don't mind my neighbours yards (they really need to do something with them). Then there is a close up of the star in place.

Finally the infocus and out of focus shots. Right down to as far as the scope could go without becoming a star. These shots are at 416x. There is still a defect in the airy disk, but I tried to go in several direction to improve the defect, but it would get worse each time. Maybe I am doing something wrong, but I think that this is very good collimation. I am going to test it tonight on Jupiter and see what it is like.

Anyway, you don't need lots of money to make this and it works really well. I do recommend though, when directly observing the throught an eyepiece, to use a filter (moon)and a pair of decent sunglasses. It is very bright. It will not resolve to show the sun and thereby causing you eye damage. This is perfectly safe.

Besides you will only get to a point where direct observing become impossible and the webcam and laptop need to come out. With the webcam and laptop you can get the focus down to where you see I got to. You will not ever be able to see this with your own eyes. It is too small and too bright.

Finally, I am really chuffed about this and glad to pass on this knowledge to you all. Incidently, I had never seen the airy disk when collimating with a star. Now I know what it looks like.

Comments welcome.

rumples riot · #2 10-03-2005, 09:50 PM

Now the close

rumples riot · #3 10-03-2005, 09:51 PM

Now the rings.

rumples riot · #4 10-03-2005, 09:51 PM

And finally the airy disk.

Starkler (Geoff) · #5 10-03-2005, 10:05 PM

I think to be effective for quality testing of optics an artificial star must be a fair distance away , like 100's of metres.

But for testing collimation closer is ok . It appears the rings are nicely centred so I'd say your collimation is ok. The strange shapes are possibly due to imperfections in the surface of the glass ball which may be less apparent at greater distance.

rumples riot · #6 10-03-2005, 10:14 PM

You could be right there Geoff. I could if I wanted, move the star another twenty metres further away. I was already about 27 metres away, which should be enough to make it work well. Anyway I take your point and will try that later some time.

Starkler (Geoff) · #7 10-03-2005, 10:27 PM

The proof of the pudding of course, is how it looks when viewing a real star

MiG · #8 10-03-2005, 10:45 PM

I've read about using a ball bearing on a sunny day as an artificial star. The reflection of the sun off the very convex surface gives a tiny point of light.

Edit: Ahh, I only just noticed from the photo that you have used a shiny ball decoration. Same thing then.

janoskiss (Steve H) · #9 10-03-2005, 11:02 PM

I've seen artificial stars advertised for sale. I think they're just an LED behind a screen with a ~10 micron pinhole in it. Light from single mode optical fibre would do nicely too.

Paul, that picture of what is supposed to be an Airy disk looks like an upside down xmas decoration!

rumples riot · #10 10-03-2005, 11:23 PM

Well, looked at Orion and the trapezium, at 416 times all the stars are crisp and clear, nice tiny little sparkles of light. Jupiter at 20 degrees off the horizon looks very good, could see the bands quite clearly through the shimmer of the atmosphere. Under a star test the image looked really good. So I think that this is a gooer.

Astroman (Andrew Wall) · #11 11-03-2005, 06:42 AM

A friend of mine uses a stainless ball head nut and bolt pushed trough a piece of black square fabric, he has the thing about 100-200m (further the better) away for a better star image you can focus on. looks something like the image below when up. Although you do have to do it on a clear sunny day, which could result in tube warming too.

http://www.theastroweb.com/astronomy/ballcloth.jpg

Although what you have is very effective. I think the real test is the stars though.

gbeal · #12 11-03-2005, 12:40 PM

Paul,
I have tried a similar method, with a ball bearing. I used a considerabl distance though, perhaps 100 metres, and across/over grass.
More recently I built up an LED inside a film container, and with a piece of foil over one end, and a very fine needle hole in that foil.
Again at about 75 metres, and at night. Great project on a cloudy night. The LED was white, but I also thought to try red, green, and/or blue.
Gary