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Artificial Star

Stub Mandrel

By Stub Mandrel
in DIY Astronomer

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Stub Mandrel

Stub Mandrel

Posted
Posted

No, I don't have several billion tons of hydrogen...

I was wondering if a little box with a ring of LEDs set to emulate the brightnesses of different magnitude stars could be useful. For example when assessing if a certain camera scope combination will be sensitive enough to find stars suitable for guiding, rather than wasting a precious night's viewing on a test.

My thought is that a pinhole over an LED might be the way to go?

Any suggestions on how to set/judge the brightness - as it would be used at a fair distance putting it 30 feet up in the air to compare with a real star is a bit of a non-starter.

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ronin

ronin

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Really I think you need LED, Pinhole and then a lens (+ve) so that the pinhole is at the focal length of the lens.

That way you produce a collimated image and the scope should focus it all back to a point.

The artifical star should be on the optical axis of the scope and parallel to it, which makes me think the secondary may get in the way for the ideal case.

You can buy an artifical star from Hubble Optics

Previous posts seem to be good concerning them, notr sure if you would get stung for VAT (imported item) and the PO administration charges however, which may make it somewhat more costly.

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Psychobilly

Psychobilly

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The Hubble optics work fine I have had a few of them .. IIRC nothing fancy in them just a white LED and a stainless plate with a range of different diameter pinholes etched in them and a little patch of material to cover over the holes you don't need - I never bothered with that bit...

The very first batch did overheat but they sorted it and sent out free replacements...

To get a clear view of it from the obs  SCT I had to put mine about 20 foot up in a tree at the far end of the garden,  so fixed it to the end of a fishing rod that was cable tied to a telescopic decorators pole!! and used a pair of walkie talkies to get a "willing" helper to move it around ...

Peter...

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cjdawson

cjdawson

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I bought an artificial star a while back.  My it's bascially a white LED, with a piece of fiberoptic to create the pin of light.  The fiberoptic fixed through the side of the box.  The brightness of the star can be adjusted using a knob of top (potentiometer)

the whole thing can be mounted on top of a camera tripod.

A pin hole will probably be fine.  The fibre optic does help to bring the light through the pinhole though. :)

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Stu

Stu

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You can use a ball bearing in sunlight too. I have a 1-inch ball on the end of a stick. The effective size is about 1/350th of the ball diameter.

What's sunlight? ;)

I've tried this too and it works quite well. I also have a Hubble artificial star which I use with the SCT at the end of the garden

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ollypenrice

ollypenrice

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You can use a ball bearing in sunlight too. I have a 1-inch ball on the end of a stick. The effective size is about 1/350th of the ball diameter.

If you go into a professional optical workshop you'll find ballbearings used for just this purpose. Last year I visited Dany Cardoen who has mirrors at Paranal and he said it's the universal and best solution.

You can illuminate a BB with a torch beam quite successfully. I've done this in a large warehouse but outside in the dark would also work. The near-point source it creates is of high quality and gives excellent diffraction rings if the optics are good. Why mess about with electronics when this is so simple?

Olly

PS you can mount the BB on a stick or glue it to flocking or matt black card. I used a much smaller one but Dany's was maybe an inch. Go with him!

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Tiki

Tiki

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Definitely the ball-bearing and torch method. Once you have it calibrated for one BB-torch distance it will be very easy to vary the magnitude in a controlled way by simply varying the BB-torch distance and applying the inverse square law.

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cjdawson

cjdawson

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Could try this one from Rother Valley Optics as an alternative to making one if it doesn't work out.  :smiley:

ooo, that's the same brand as mine.  But not the same.  I did a quick google search and pulled up this page.  The difference is that mine as a dimmer knob.  Can't remember it being that much to buy, and frankly. I'd be tempted get the cheaper one and change the switch to a dimmer like mine.  Don't see why it's £20 more for a different component that will cost about the same price.

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Stub Mandrel

Stub Mandrel

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Info on the  Hubble optics artificial star

It has 5 bright white LEDs with 5 precision pinholes (50/100/150/200/250 microns).

Have fun...

50 microns might be a challenge but small ones can be done by drilling something like number 80 (370 microns) in brass and then swaging it down, other way is to make a conical impression into a countersink then grind away until you get a hole - that's how they do espresso filters which are about 60 microns. Actually, the simple solution would be to use an expresso filter - could create an entire globular cluster that way!

But to be honest, if a fibre optic will do the job easy enough to make a thin tapered filament...

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ollypenrice

ollypenrice

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50 microns might be a challenge but small ones can be done by drilling something like number 80 (370 microns) in brass and then swaging it down, other way is to make a conical impression into a countersink then grind away until you get a hole - that's how they do espresso filters which are about 60 microns. Actually, the simple solution would be to use an expresso filter - could create an entire globular cluster that way!

But to be honest, if a fibre optic will do the job easy enough to make a thin tapered filament...

If the professionals use a ballbearing why are amateurs using anything else? Honest question.

Olly

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skybadger

skybadger

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Err - aren't we missing the requirement here to represent accurately various magnitudes of stars ?

Van Sutters book has an appendix which describes the size and distance limits for usable bearing 'stars' using the sun or other sources.

If you can measure the torch brightness then you can calculate the effective stellar apparent magnitude. So a led might be more useful with a calibration between current and brightness. Now only need to convert between lumens and Vflux..

Mike

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Scorpius

Scorpius

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How bout a ball bearing on a stick, inside a box with different size holes, illuminated by LED's on a dimmer switch with magnitude gauged by a light meter attched to the OTA? Too complicated you say? Well then, if the professionals use ball bearings - when in Rome, do as the Romans do - or so they say...

Of course when all else fails - there's a number of "real stars" suitable for collimation if you point your scope towards the sky on a clear night with good seeing...  :grin:

But seriously, I'd love to get one of these for collimating my SCT but too rich for my blood I'm afraid  :sad:

http://www.hotechusa.com/category-s/23.htm

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buzz

buzz

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Ball bearings and sunlight work well in sunlight for visual adjustment but are too bright for normal CCD cameras, even with a fraction of a second exposure.  For CCD use, I have a Maglight LED torch, which I diffuse with a tissue and place at a distance so that it subtends 0.5 degrees (as the sun does).

Now here is the fun part - for RCT collimation, you need a central and peripheral stars for star testing the primary and secondary mirror alignments.  I bought 9 balls and glued them to a 500-mm square black planter base (from a garden center). (One center ball and the others in a circle.) I place this at 25x the focal length of the RCT and it fills my 8300-based CCD just right for a good view of the balance of aberrations.

If you can, take a look at Harold Suiter's book on star testing. It has a lot of useful information.

regards

Chris Woodhouse

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