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artificial star for collimation

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#1 gazerjim

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Posted 15 February 2004 - 01:18 AM

Has anyone had experience with using an "artificial star" to collimate a SCT? I'm looking for a way to achieve optimum collimation for my Celestron 9.25 that is not dependent upon seeing conditions.

Thanks,
Jim Fisher

#2 Suk Lee

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Posted 15 February 2004 - 02:45 AM

I've got a "Picostar" on order, I'll let you know when it arrives...

Suk

#3 Gary BEAL

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Posted 15 February 2004 - 04:45 AM

Jim,
I have tried two different tricks.
I have an old ball bearing, and stick that to the top of a stick, and place a bit of black card behind it. At about 200 meters over grass, the suns reflection is a good source.

Second trick is a pinhole in foil with a coloured (I use green, but also have red and yellow) LED behind. The LED is inside an old black film container, and a hole in the cap, with the foil and pinhole on the front. Again a couple of hundred meters. One by day, and one at night.

Doesn't fully replicate a star, but it gets you in the ballpark, and teaches you the way to twek which screw.
Good luck,
Gary

#4 John P

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Posted 23 March 2004 - 06:14 AM

The sun's reflection off a ball bearing or small Christmas ornament works well as long as the diameter of the reflecting surface is small, it's located distant from the telescope, and you recenter the image of it after each "tweek" of the screws. It really helps to have a dark background. Use a reflecting surface too large in diameter, and you end up getting a real image, not a point source. Distance between the reflecting surface and telescope needs to be large...an old rule of thumb was I think, 1000x focal length. (?)

#5 EdZ

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Posted 23 March 2004 - 06:44 AM

I know Sidwick has an appendix that helps calculate the size of the pinhole or the distance that must be used from the reflection off of a ball. I believe Suiter also has a chapter on this.

At any rate, the reflection must be smaller than the spurious disk (central bright spot in the Airy disk) for the size telescope being tested. That is incredibly small. Otherwise, the point of light you are looking at will be observed as an extended object, not a point source. Sidwick recommends honing a needle on glass, checking the progress of the honing under a microscope. For the use of a ball bearing Sidgwick states the reflection of the Sun will be 0.0025x the diameter of the ball.

edz

#6 Bird

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Posted 23 March 2004 - 07:54 AM

While looking into this I found the following advertisment:

http://www.telescope...lighting_3.html

This claims a <125 micron pinhole, with a distance from scope of 47m for a 10". Maybe this is an older model or something, but it's the same price as the model mentioned here, and looks identical (same wording in the ad, etc) except for the different specifications.

This got me wondering how one might build something like this at home... I had a bit of a think about to create a tiny hole - my best guess so far would be to take a single thread from small multi-core wire (the thread would be about 100 microns in size), surround it with some sort of black goop that solidifies, then use etching solution to dissolve the wire away, leaving a nice hole. Keep the wire under tension while the goop dries and it should make a nice straight tunnel.

Put a high intensity white LED behind it and it might just do the trick.

Now, if 100u is too big, maybe you could consider putting an optical instrument between it and the telescope to "reduce" its size. Something as simple as one barrel of a pair of 10x50 binoculars, used in "reverse", would shrink the apparent size of the hole to 10u.

Now I have to try this out and see if I can make something...

Of course, spending about $200 australian dollars to buy a commercial unit is also a possibility... I'll be very interested to hear other peoples experiences with the picostar.

$0.02

Anthony

#7 EdZ

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Posted 23 March 2004 - 09:19 AM

Once the size of the resultant hole is known, the distance to the target must be calculated to provide an image smaller than the Central bright spot of the Airy disk for the diameter telescope being tested. The size of the hole is not as critical as the combination of hole diameter x distance = angular size.

edz

#8 Bird

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Posted 23 March 2004 - 08:14 PM

It would be nice to get something that wrked at <10m distance so could fit it into my backyard :-)

So I'm going to try and make the smallest pinhole I can...

Bird

#9 Suk Lee

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Posted 23 March 2004 - 09:14 PM

You can buy calibrated pinholes from Edmund Optics www.edmundoptics.com all the way down to 1 micron +- 0.5 microns.

Suk

#10 Rusty

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Posted 23 March 2004 - 10:22 PM


This got me wondering how one might build something like this at home... I had a bit of a think about to create a tiny hole - my best guess so far would be to take a single thread from small multi-core wire (the thread would be about 100 microns in size), surround it with some sort of black goop that solidifies, then use etching solution to dissolve the wire away, leaving a nice hole. Keep the wire under tension while the goop dries and it should make a nice straight tunnel.

Put a high intensity white LED behind it and it might just do the trick.

Now, if 100u is too big, maybe you could consider putting an optical instrument between it and the telescope to "reduce" its size. Something as simple as one barrel of a pair of 10x50 binoculars, used in "reverse", would shrink the apparent size of the hole to 10u.

Now I have to try this out and see if I can make something...

Of course, spending about $200 australian dollars to buy a commercial unit is also a possibility... I'll be very interested to hear other peoples experiences with the picostar.

$0.02

Anthony


You might find the technique for making pinhole irises for photography (used to give exceptional depth-of-field by stopping down the lens).

"Take a piece of brass sheet and with a sharp punch, make a deep indentation. Gently file or sand the deformation until you sand through". This makes a very thin iris, which helps diffraction. However, I think that if you used a needle to punch through without filing away enough material to make a hole, you'd have a very small hole. 125 microns is very small, however - 1/8 of a millimeter...

#11 Bryan Greer

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Posted 23 March 2004 - 10:49 PM

Hi Robert,

I'm a big fan of "artificial stars". The oft-cited warning about ensuring the artificial source is far enough away is valid if you are trying to test for spherical aberration and a few other aberrations. However, it is not a strict requirement for collimation testing. For collimating, all you care about is getting the diffraction rings concentric, and this can be done just fine with the point source rather close. (It can also deviate from a true diffraction point source as well.)

Sincerely,
Bryan Greer

#12 IDONTSEEIT

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Posted 23 March 2004 - 11:06 PM

Once the size of the resultant hole is known, the distance to the target must be calculated to provide an image smaller than the Central bright spot of the Airy disk for the diameter telescope being tested. The size of the hole is not as critical as the combination of hole diameter x distance = angular size.
edz


Ed,

How does one calculate the spurious disk size for different scope apertures?

Thanks,

#13 Stacy

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Posted 24 March 2004 - 01:51 AM

From apogeeinc.com:

PRECISION PINHOLE Made for Military Collimators. The precision hole is made of brass, and is .004" in diameter & the housing is 1" in diameter. Ideal for making an artificial star.
Price: $6

Would this work?

Regards,
Stacy

#14 Guest_**DONOTDELETE**_*

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Posted 24 March 2004 - 11:11 AM

Bryan,

I am interested by your comments about artificial star distance. I have noted the same thing. I used a Christmas tree ornament about 150 feet from my 9.25 SCT. First and foremost, the scope has to be thermally neutral. Then, I noted significant spherical aberation, no matter how I tweaked the collimation. So, I did the best I could in terms of making sure everything was concentric. Is this related to the fact that focusing a SCT changes the f-ratio? And, hence, any alignment object not at infinity could show spherical abberation?

That night I lucked out with some pretty good seeing. I did a star test and was quite pleased how the spherical abberation "disappeared"! Big relief!

Carl

#15 Tom L

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Posted 24 March 2004 - 03:25 PM

From apogeeinc.com:

PRECISION PINHOLE Made for Military Collimators. The precision hole is made of brass, and is .004" in diameter & the housing is 1" in diameter. Ideal for making an artificial star.
Price: $6

Would this work?

Regards,
Stacy


Good find, Stacy! Combined with a white LED flashlight or even a christmas tree light behind it

#16 EdZ

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Posted 24 March 2004 - 03:38 PM

While this conversation was going on over here the whole discussion of hole size and distance to target was taking place over here

Daylight collimation tool

I've posted a table for the calculation of hole size and distance to target for sizes of instruments from 60mm to 300mm. It varies with the intensity of the light.

edz


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