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Anyone made thier own Picostar

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#1 ammisco

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Posted 15 July 2016 - 11:56 AM

I was wondering if anyone has plans for a DIY picostart for collimating a SCT?



#2 ammisco

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Posted 15 July 2016 - 11:57 AM

I forgot to say thanks for suggestions.


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#3 Cometeer

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Posted 15 July 2016 - 12:06 PM

I forgot to say thanks for suggestions.

 

You can edit your original post btw.



#4 epee

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Posted 19 July 2016 - 07:41 AM

There are several ideas available on the web. I've built a model which is basically a backwards refractor; a tube, blackened on the inside with baffles, a reversed short focal length eyepiece at the viewing end and a bright LED flashlight with a pinhole in aluminum foil covering it's face. I drilled and tapped a hole so I could mount it on a photo tripod. 
The formula for the focal length of the eyepiece and length of the tube can be found on the web.

 

it works well in indoor light but is too dim for full sunshine. 


Edited by epee, 19 July 2016 - 07:43 AM.


#5 highertheflyer

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Posted 20 July 2016 - 09:22 AM

Do you have a source for the backward refractor on the web Jim?

Thanks,, Jim



#6 epee

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Posted 20 July 2016 - 02:06 PM

https://sites.google...ar(pointsource)

http://observatory.m...st/ArtStar.html

I primarily used the Google site design, but I substituted a cheap LED flashlight (inserted through a drilled out PVC end cap) for the hard wired LED.

 

I use mine for collimating my C5 indoors. Star testing is in theory best, but indoors I have perfect seeing, a non-moving star, and a lit, comfortable, environment. 


Edited by epee, 20 July 2016 - 02:10 PM.


#7 BGRE

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Posted 22 July 2016 - 12:57 AM

Whilst such an artificial star may be useful for alignment purposes, it is far from suitable for testing for optical aberrations as any axial aberrations imparted by  the eyepiece remain in the wavefront viewed by the telescope. A multimode optical fibre illuminated by an incoherent source is a better source for testing purposes (provided sufficient light can be coupled into the fibre - no more difficult than achieving sufficient light with your  setup) as achieving the required diameter is easy since 50 micron (~0.002 inches) and smaller fibres are readily available.



#8 totvos

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Posted 13 March 2018 - 12:06 PM

Resurrecting this thread because I, too, am in the market for a PicoStar. I know about the Hubble Optics 5-Star, but the fibre optic approach used by PicoStar, as BGRE notes, is a superior one, especially if the fibre is 9µ. It seems easy enough to do (especially after viewing Ed Jones' video https://www.youtube....h?v=nQlKohPAYXQ), but wonder if I am missing something important.

 

Also, where would be a good source for a small length of 9µ fibre? I think (but don't know) that simple audio patch fibre is not appropriate here, right?


Edited by totvos, 13 March 2018 - 12:06 PM.


#9 BGRE

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Posted 13 March 2018 - 02:39 PM

Audio optical leads usually comprise bundles of plastic fibers with core diameters somewhat larger than 9 microns.

Single mode fiber intended for use at 1500nm or so has a 9 micron core.

Butt coupling done properly is as effective as using optics to couple an LED to the fiber.

Thorlabs will sell you a 1m length (or longer) with or without connectors (see custom patch leads).

 

A quick search gives the following sources of OS2 9/125 micron patch cable:

https://www.fiberopt...smdup-stst.html

Ebay search for 9/125 micron fiber produces a few hits as well.



#10 ChristianG

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Posted 14 March 2018 - 01:57 AM

Another option:

 

Grab a Mini Maglite LED (without reflector, approx. 2 mm wide LED), a length of pipe and a cheap 4 mm Plossl eyepiece--and some duct tape. Mine puts the LED about 600 mm from the eyepiece, so the image of the LED is about 13 microns across, and appears just in front of the eyepiece. With longer pipe or a microscope objective, 'stars' aroung 9 microns can be created. Works well in daylight, and costs almost nothing.

 

--Christian


Edited by ChristianG, 14 March 2018 - 01:57 AM.


#11 peleuba

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Posted 14 March 2018 - 12:30 PM

Another option:

 

Grab a Mini Maglite LED (without reflector, approx. 2 mm wide LED), a length of pipe and a cheap 4 mm Plossl eyepiece--and some duct tape. Mine puts the LED about 600 mm from the eyepiece, so the image of the LED is about 13 microns across, and appears just in front of the eyepiece. With longer pipe or a microscope objective, 'stars' aroung 9 microns can be created. Works well in daylight, and costs almost nothing.

 

--Christian

This is excellent.  I create a collimated beam of light by running a light source backwards through a stack of Barlows placed into the eypeice of a Newtonian reflector.  

 

Could you post a picture of your set up?  I'd like to create something like this.

 

Thanks!



#12 BGRE

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Posted 14 March 2018 - 05:00 PM

Another option:

 

Grab a Mini Maglite LED (without reflector, approx. 2 mm wide LED), a length of pipe and a cheap 4 mm Plossl eyepiece--and some duct tape. Mine puts the LED about 600 mm from the eyepiece, so the image of the LED is about 13 microns across, and appears just in front of the eyepiece. With longer pipe or a microscope objective, 'stars' aroung 9 microns can be created. Works well in daylight, and costs almost nothing.

 

--Christian

A bad idea if you intend to use it to test for aberrations in the telescope.

The aberrations produced by the eyepiece will affect the results.



#13 ChristianG

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Posted 15 March 2018 - 01:25 PM

A bad idea if you intend to use it to test for aberrations in the telescope.

The aberrations produced by the eyepiece will affect the results.

Hi.

 

The eyepiece creates an image that can have any desired size in the few microns range, depending on eyepiece/tube combination.

 

The image is very bright and the whole thing can be placed as far as one needs for the 'star' to be well beyond the resolution of the telescope.

 

For instance, a C8 resolution is about 1.22*0.0000005/0.203 = 3e-6 radians.

 

A 3 micron 'star' image placed 1 m away is resolvable as an 'object' in the C8.

A 3 micron 'star' image placed 10 m away will already be 1/10 of the Airy disk and not resolvable.

A 9 micron 'star' image placed 30 m as well. Etc.

 

I don't have my copy of Suiter's book (it's at home), but he has guidelines as to when a 'star' is sufficiently small. Artificial stars in the micron range are just fine.

 

What the eyepiece does to the image of the LED is therefore irrelevant when the artificial star is suficiently far from the telescope and is well beyond the resolution of the instrument. The Mini Maglite LED is very bright and lasts hours, and one can place it quite far away, across a river at night for instance.

 

When I was only interested in the collimation of a 100 mm refractor, say, which has 1/2 the resolution of the C8 (1.5e-6 radians), the 13 micron 'star' placed 33 m away (I had a long hallway) subtended an angle of 4e-7 radians. That was still about 1/4 of the smallest resolvable feature. If I had been interested in measuring aberrations, I could have used a longer tube to get a smaller 'star', or ventured outside...

 

--Christian 



#14 sg6

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Posted 15 March 2018 - 01:56 PM

Many of these "stars" seem to really be a point source, and a point source at 5-10-20 meters is not a collimated source which is what an artifical star should be.

 

Epee describes what it should be -  a refractor in reverse. If you can get a 1mm or 1/2mm diameter hole placed at the focal plane of a lens and on the axis then you have an artifical star. Used to be able to get foil disks with such holes at one time.

 

The problem then of using one is to position it on the optical axis of the scope and maintaining it there.

 

If you are using it for an SCT the obvious problem is that even a 50mm lens will if central be aimed at the secondary obstruction. Life is kind of never easy is it. lol.gif



#15 ChristianG

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Posted 15 March 2018 - 02:40 PM

Yes, of course I agree that ideally, one should use a collimated beam. This is ideal--but costly. I built setups like this when doing laser spectroscopy.

 

Ideally you need to make a spatial filter with a pinhole at the focal point of the sender telescope, illuminate it with a bright source, and make sure the emerging beam is collimated--preferably with an optical flat. You need a good sender telescope too.

 

Not sure if this is within the reach of most amateurs.

 

I was suggesting a much simpler and cheaper approach. If a point source is sufficiently far, it's essentially as good. There are guidelines, I don't remember since my copy of Suiter's book is at home, but apparently star testing can be meaningfully done this way. And if one is interested in collimation, that's certainly sufficient.

 

Picostar, Hubble Optics and the arrangment below all make point-like sources which one needs to place a sufficient distance away. Hubble Optics smallest pinhole is 50 microns, Picostar is about the size of the core of a single-mode fiber, around 8 microns minimum, larger if multi-mode fiber is used. The benefits of the eyepiece-pipe-LED arrangment is that is can make a wide range of 'star' size, and most amateur astronomers already have all the parts required.

 

Anyway, here's a photo of a simple one, for the benefit of member peleuba:

 

ArtStar2_s.jpg

 

Cheers!

 

--Christian


Edited by ChristianG, 15 March 2018 - 02:42 PM.


#16 BGRE

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Posted 15 March 2018 - 04:21 PM

Hi.

 

The eyepiece creates an image that can have any desired size in the few microns range, depending on eyepiece/tube combination.

 

The image is very bright and the whole thing can be placed as far as one needs for the 'star' to be well beyond the resolution of the telescope.

 

For instance, a C8 resolution is about 1.22*0.0000005/0.203 = 3e-6 radians.

 

A 3 micron 'star' image placed 1 m away is resolvable as an 'object' in the C8.

A 3 micron 'star' image placed 10 m away will already be 1/10 of the Airy disk and not resolvable.

A 9 micron 'star' image placed 30 m as well. Etc.

 

I don't have my copy of Suiter's book (it's at home), but he has guidelines as to when a 'star' is sufficiently small. Artificial stars in the micron range are just fine.

 

What the eyepiece does to the image of the LED is therefore irrelevant when the artificial star is suficiently far from the telescope and is well beyond the resolution of the instrument. The Mini Maglite LED is very bright and lasts hours, and one can place it quite far away, across a river at night for instance.

 

When I was only interested in the collimation of a 100 mm refractor, say, which has 1/2 the resolution of the C8 (1.5e-6 radians), the 13 micron 'star' placed 33 m away (I had a long hallway) subtended an angle of 4e-7 radians. That was still about 1/4 of the smallest resolvable feature. If I had been interested in measuring aberrations, I could have used a longer tube to get a smaller 'star', or ventured outside...

 

--Christian 

Your reasoning is somewhat imprecise and the conclusion that the eyepiece aberrations do not matter is misleading. The finite aperture of the telescope acts as a filter that attenuates but doesn't entirely eliminate SA, astigmatism and other aberrations, which if they are large enough will be noticeable. Astigmatism in particular is attenuated far less than SA by the finite aperture of the telescope.

 

You actually have to show that the aberrations produced by such a setup are sufficiently small when filtered by the telescope.

Setting limits on the maximum SA, astigmatism etc and then determining that the particular eyepiece in use has aberrations well below these limits for the actual source conjugate in use is necessary if any confidence in the results is to be achieved.

 

The entire setup is unnecessarily complex and somewhat bulky compared to using an inexpensive optical fiber.



#17 ChristianG

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Posted 15 March 2018 - 10:41 PM

Hi.

 

Forgive me if my explanations seem imprecise (but what parts are imprecise exactly) or misleading.

 

--I am not the inventor of this setup, it is a fairly well known way to make a point-like source. Some designs call for a microscope objective, a short focal length eyepiece can do as well. The result is not significantly different from fiber optic ones, or pinhole ones. None of these are perfect point sources, and all need to be placed some distance away for them to function as point-like sources.

 

--The eyepiece's only purpose is to make a very small image of some other light source. Same purpose as the core of a fiber optic, or actual pinhole. Since the angle subtended by this image, as viewed from the telescope under test, can be made much smaller than the angular resolution of said telescope, I do not quite understand why one would be worried about the quality of the optics of the lens creating that image, or about the quality of the cleaved end of the optical fiber, or the exact shape of the pinhole, whatever the case may be. Are you saying that the light leaving the eyepiece, in the form of an image that is only about 20 wavelengths wide, retains a significant deviation from a spherical wavefront after it has travelled more than 10 m, i.e. 20 million wavelengths? And that a telescope, which can not possibly resolve any detail about the source, can detect this deviation from sphericity? That's interesting, I would not have expected that. And Mr. Suiter and others didn't mention that either. 

 

--I also do not understand why an eyepiece, a piece of pipe and a flashlight is more complex than some diode laser attached to one end of a fiber, with the other end cleaved (never perfectly) and exposed to air. It is bulkier, of course, but this is not important. One can be assembled with available parts, the other needs more exotic items (optical fiber, laser, or small enough pinhole, whatever the case may be). It is certainly not more complex than using another telescope as a spatial filter/beam expander.

 

Anyway, this thread is about a DIY fiberoptic artificial star, and I was merely suggesting another design that achieves the same result in a simple way. Regards,

 

--Christian



#18 BGRE

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Posted 16 March 2018 - 08:58 PM

The size of the image won't be that calculated from Gaussian optics especially if the eyepeice has significant aberrations.

The real question is how do such aberrations affect the wavefront incident at the telescopes entrance pupil?

Surely its a simple problem in physical optics to estimate the form and deviation from spherical across the telescope aperture?

It shouldn't take more than a few minutes to come up with a reliable estimates and hence the maximum acceptable SA, coma and astigmatism produced by the eyepiece.

 

Numbers are much more useful than qualitative hand waving.

Hint: The amplitudes of the astigmatism and coma terms are more critical than the SA term.

 

If one uses a fiber that's single mode at the wavelengths in use then the effect of typical cleving/polishing defects is usually undetectable.

A simple test is to set up a Young's interferometer using 2 coherently illuminated single mode fibers and analyse the resultant fringe pattern.

 

Multimode fibers and light guides produce  a more complex output wavefront.

With macro sized cylindrical light guides there are a prominent set of ring features in near and far field regimes.



#19 ChristianG

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Posted 18 March 2018 - 11:48 AM

Hi..

 

It's all very interesting, but not my point. The original poster's question was

 

"I was wondering if anyone has plans for a DIY picostart for collimating a SCT?"

 

A light source that is sufficiently small and placed sufficiently far away will be just fine for that purpose. Regards,

 

--Christian




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