Author Topic: Artficial star distance?  (Read 520 times)

seucamthepo

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Artficial star distance?
« on: December 29, 2017, 05:10:43 AM »
Looking to begin star testing my 80mm f5 and need some clarity for the distance of my artificial star. I'm guessing a 5mm eyepiece and approximately at least 60ft minimum according to my figures but isn't that too close? Thanks



Duane Berhane

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Re: Artficial star distance?
« Reply #1 on: December 31, 2017, 01:37:30 AM »
<sub>50 feet or so is about right, if you got a long hallway the length of house can do it inside. My last house couldsetup scopeby front door aimed down hall, leave garage door open and put the hubble flashlight in dark on far wall, still needed an extension tube to get to focus. Stellarvue has a 4.75" for a reasonable price.</sub>

taibedaha

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Re: Artficial star distance?
« Reply #2 on: January 04, 2018, 06:34:40 PM »
Thanks. It just sounds too close. Am I correct in the 5mm eyepiece?

Tim Massey

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Re: Artficial star distance?
« Reply #3 on: January 08, 2018, 01:56:59 AM »
Are you testing the telescope's colour correction, its objective's spherical aberration, or just trying to see how well it performs?

For an optic designed to operate at infinity so close an artificial star might throw up a completely false result under test.

I keep meaning to put a Christmas tree bauble or shiny ball bearing on a post at 100 yards in sunlight to see if it can work for optical testing of achromats.

Even this distance is considered much too close for some compound instruments.

A brilliant Cree LED could be used to illuminate the shiny ball at night if daylight swamps the results in sunlight.

Very distant electrical insulators on overhead wires used to be favorite telescope targets in sunlight.

Eric Guffey

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Re: Artficial star distance?
« Reply #4 on: January 10, 2018, 12:49:00 PM »
It does in fact depend on what you are trying to do with that artificial star.
 If you are just collimating, then such short lengths work. If you are evaluating the quality of the optics you'll get some spherical aberration at that distance.

agtofonist

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Re: Artficial star distance?
« Reply #5 on: January 11, 2018, 12:28:15 PM »
There are two problems with the methods described here. One is air currents and the second one is wavefront overcorrection due to the source being too close. The simplest way to resolve both issues is to use a collimating telescope as show below.
Mladen


Tony Patton

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Re: Artficial star distance?
« Reply #6 on: January 14, 2018, 01:21:15 AM »
I'm needing to check collimation on my scope due to objective replacement.

Rob Freeman

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Re: Artficial star distance?
« Reply #7 on: January 16, 2018, 07:18:29 AM »
Well, if it's just for collimation, you could even use a finder scope or a pair of binoculars to produce an adequate artificial star from a flashlight. You likely have everything you need already.

Mike Heck

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Re: Artficial star distance?
« Reply #8 on: January 18, 2018, 08:16:54 AM »
An artifical star should be a light source, pin hole and a lens at a distance of the focal length.
That way the output from the unit is delivering close to collimated light, to check the output you will have toi be able to move the lens in and out a little either side of the focal length.

Problem then is getting the pin hole and the lens centered.
After that the whole lot has to sit on the optical axis of the scope.

James Etrheim

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Re: Artficial star distance?
« Reply #9 on: January 18, 2018, 10:44:56 AM »
I already have an artificial star. It's a Tech 2000 Nano-Star. The one thing I don't quite understand is that they tell you that you can bounce the light off of a ball bearing or similar to achieve a longer distance. Wouldn't that be the same as setting it up at the same distance as the ball bearing? Why not just use a dimmer light source? (Now I'm getting into a little bit of theory)

handvestlazo

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Re: Artficial star distance?
« Reply #10 on: January 21, 2018, 04:31:28 AM »
Simple principle - look at the reflection of a scene in a mirrored ball and compare to the angular diameter of the original scene. The convex reflector functions as a negative lens and shrinks everything including the size of the light source.

Joe Maillet

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Re: Artficial star distance?
« Reply #11 on: January 23, 2018, 09:15:18 AM »
Watch this video.

Ed

Marquise Nation

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Re: Artficial star distance?
« Reply #12 on: January 31, 2018, 05:01:07 AM »
Quote
There are two problems with the methods described here. One is air currents and the second one is wavefront overcorrection due to the source being too close. The simplest way to resolve both issues is to use a collimating telescope as show below.

infinity_focus_2.jpg

Mladen

Hi.

Some comments:

It should be mentionned that the collimating telescope needs have equal or lager aperture than the telescope to collimate.

An 80 mm f/5 has a 400 mm only. A long hallway in a school or university would do if it is longer than 20 f, as recommended in Harold Suiter's book.

Star test should ideally be done at higher magnification than 25X per inch of aperture, so a 5 mm eyepiece is not enough in my experience to really look at the diffraction pattern. When I collimated my ST-80, I used a 3.2 mm eyepiece. I would not use a barlow unless I knew it was near perfect! Too easy to blame the scope if the bad sfar test is caused by the barlow, as is the case with two of my cheaper barlows.

Have fun!

--Christian

noneanoncrag

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Re: Artficial star distance?
« Reply #13 on: January 31, 2018, 05:55:27 AM »
Quote
It should be mentionned that the collimating telescope needs have equal or lager aperture than the telescope to collimate.

In addition if the collimating scope has an obstruction you would need to go off axis to avoid the shadow. This places a limit on the aperture being tested.

For collimation only that can be relaxed. For a star test the incoming wavefront needs to be as flat as possible. This requires the pinhole to be placed at the infinity focal point rather accurately for generating the beam.

For a refractor of this type a long hallway is fine. Air currents will be seen, but longer distances are better as you dont want the drawtube extended too far as it might sag somewhat. Best to test closer to the intended design point, i.e. a longer distance to the art star.

A scope this size should have no problems with a real star even in average seeing.

Glenn

barlaliblo

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Re: Artficial star distance?
« Reply #14 on: February 02, 2018, 04:54:29 PM »
ChristianG wrote in response to my post #7

<p class="citation">Quote

It should be mentionned that the collimating telescope needs have equal or lager aperture than the telescope to collimate.
[/quote]

My drawing clearly shows exactly that.

<p class="citation">Quote

An 80 mm f/5 has a 400 mm only. A long hallway in a school or university would do if it is longer than 20 f, as recommended in Harold Suiter's book.
[/quote]

A collimating source doesn't have to be far at all -- provided the incoming light is collimated ("flat" wave).

Mladen