Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Min. distance to artificial star ?

  • Please log in to reply
26 replies to this topic

#1 Bob4BVM

Bob4BVM

    Vanguard

  • *****
  • topic starter
  • Posts: 2,437
  • Joined: 23 Mar 2015
  • Loc: W. Oregon

Posted 31 March 2017 - 12:20 PM

Out of necessity for a couple of projects I am in the middle of, I finally got around to making a pinhole artificial star.

Can someone tell me what is the minimum distance to it need for it to appear at infinity.

Thanks,

Bob



#2 ftwskies

ftwskies

    Viking 1

  • -----
  • Posts: 886
  • Joined: 29 Jan 2015
  • Loc: Under Fort Worth skies...

Posted 31 March 2017 - 12:38 PM

Hopefully the experts will chime in after me.  Last night I was reading an old thread where someone said something about 100 focal lengths away...?



#3 Joe1950

Joe1950

    Voyager 1

  • *****
  • Posts: 10,455
  • Joined: 22 Aug 2015

Posted 31 March 2017 - 12:59 PM

The usual accepted distance, though some would dissent is 20 to 25 times the focal length of your telescope's objective.

 

So, if you were testing a 6" f/8, your focal length is 48". 48x25= 1200" or 100ft. That is a safe distance so the spherical aberration due to the proximity of the artificial star will not be a contributing factor. Any longer would be a plus, of course.

 

For just collimation, you can use much less distance, but you have to be careful of back focus and vignetting. The closer you are, the further back you have to go to focus and the more likely your scope will vignette.

 

It has also been said by Suitor, in his famous book, that refractors are much less influenced by the proximity of the artificial star than are reflectors and CATs when doing a star test. So, If necessary, you could use less than 20 to 25x the focal length.

 

Again, however, back focus and vignetting could be a problem if you get too close.

 

Short answer, 25 times the scope's focal length or longer.

 

Hope this helps.

 

 

Added: Even at 25x the focal length or more, the pinhole won't focus the same as a star at infinity. The concern is spherical aberration that will show up in the test if you did the test too close, say 5 times the focal length or 10 times. But with 20 to 25 times the focal length, the induced SA is very low and would not factor into your assessment of the test.


Edited by Joe1950, 31 March 2017 - 01:07 PM.

  • Figment and xiando like this

#4 PPO

PPO

    Vendor - PrecisionPro Optics

  • *****
  • Vendors
  • Posts: 782
  • Joined: 10 Feb 2012
  • Loc: Big Island, Hawaii and Owens Valley, CA.

Posted 31 March 2017 - 01:12 PM

Out of necessity for a couple of projects I am in the middle of, I finally got around to making a pinhole artificial star.

Can someone tell me what is the minimum distance to it need for it to appear at infinity.

 

The answer is - infinity! Any distance closer than infinity will introduce SOME correction error, it is a continuous variation, not a discrete cutoff. So, it depends on how much you can tolerate, and how picky you are about the test. Just like there is no minimum level of radiation that doesn't cause some damage to your DNA.

 

If you set a 1/4 wavefront limit, then Suiter's table 5.2 gives you some distance numbers.



#5 Joe1950

Joe1950

    Voyager 1

  • *****
  • Posts: 10,455
  • Joined: 22 Aug 2015

Posted 31 March 2017 - 02:21 PM

The table shows that the larger the aperture and the lower the focal ratio, the higher the multiplier must be the limit the induced overcorrection in a parabolic mirror to 1/4λ. 

 

So it does matter what the specs of the scope you are testing are.

 

Some of the multipliers are over 20 to 25 for the above mentioned cases and some are well below the 20 to 25.

 

For example, your 6" f5 refractor calls for a multiplier of 16 times the focal length. So 16x30" = 480" or 40 feet. That would result in an induced SA wavefront error of 1/4λ. If however you were to place the pinhole twice that distance, at 80 feet, the induced SA would be 1/8λ. 160 feet, 1/16λ.

 

BUT. Suiter also goes on to say that refractors are not nearly as prone to the effects of proximity errors of spherical aberration. So the distances noted above will give significantly less induced SA if you are testing a refractor.

 

 

On the other side of the coin, if you want to test the 17.5", f/4.5 DOB, you may have to drive to the pinhole location. Using the formula rather than the table the pinhole would have to be 423 feet away to limit the induced SA to 1/4λ, or 846 feet away to limit the induced SA to 1/8λ.

 

The actual formula derived by Roger Sinnott of S&T by careful ray tracings, is as follows:

 

N [ft]  =  28 ( D/F )2

 

D is the diameter in inches

F is the focal ratio

N is the result in feet.

 

Again, this for an induced error of 1/4λ

 

If you want to limit the error to 1/8λ, double the distance N.

 

Finally, not to overstress the point, these distances and errors apply to reflectors and CATs, not refractors. Refractors are significantly less affected by the proximity of the pinhole than are the other types of scopes. I would say, as my opinion, if you use 25 times the focal length of the refractor to place the pinhole, or more if possible, the induced CA will be insignificant.

 

Don't let the answer prevent you from using an artificial star. Harold Suiter, himself makes a point of telling the reader how much more convenient it is to use and how much less atmospherics will disturb the image!

 

There is much less turbulence and  a much more usable image with an artificial star observed from 100', 200', 500'... than starlight passing through the entire atmosphere. I tried a star test a few nights ago with Polaris and seeing was so bad it was totally impossible to properly read or image.

 

good luck,

 

joe


Edited by Joe1950, 31 March 2017 - 02:23 PM.

  • AndresEsteban likes this

#6 MKV

MKV

    Cosmos

  • *****
  • Posts: 9,065
  • Joined: 20 Jan 2011
  • Loc: Southeast, USA

Posted 31 March 2017 - 04:12 PM

If you use a lensless laser (less then 5 mW power) running in non-lasing mode, it will be a point source. At 100 feet it will introduce an RMS optical path difference (OPD) residual of 1/20 wave pvw for a 6" f/8 perfect paraboloid. The bfl is pushed to 50". For a 6" f/4, with the light source at 50 feet, the RMS OPD will be only only 1/3 wave pvw, and the bfl is pushed back by 1" to 25". The bfl changes linearly, the wavefront error as a cube.  

 

Mladen


  • Joe1950 likes this

#7 steveastrouk

steveastrouk

    Apollo

  • *****
  • Vendors
  • Posts: 1,166
  • Joined: 01 Aug 2013
  • Loc: State College, Pa.

Posted 01 April 2017 - 08:33 AM

If you use a lensless laser (less then 5 mW power) running in non-lasing mode, it will be a point source.

Is it effectively a point source LED, because of the construction of the diode ?



#8 MKV

MKV

    Cosmos

  • *****
  • Posts: 9,065
  • Joined: 20 Jan 2011
  • Loc: Southeast, USA

Posted 01 April 2017 - 09:59 AM

 

If you use a lensless laser (less then 5 mW power) running in non-lasing mode, it will be a point source.

Is it effectively a point source LED, because of the construction of the diode ?

 

Yes, the laser light cavity is essentially only a few microns in diameter, which is essentially smaller than most Airy discs. 


  • Alpollo likes this

#9 Bob4BVM

Bob4BVM

    Vanguard

  • *****
  • topic starter
  • Posts: 2,437
  • Joined: 23 Mar 2015
  • Loc: W. Oregon

Posted 01 April 2017 - 03:31 PM

 

 

If you use a lensless laser (less then 5 mW power) running in non-lasing mode, it will be a point source.

Is it effectively a point source LED, because of the construction of the diode ?

 

Yes, the laser light cavity is essentially only a few microns in diameter, which is essentially smaller than most Airy discs. 

 

Will it be safe to view thru a lensed system as my point source ?

Where would I find such a thing? My lapse from a long-ago electronics career is showing :)

CS

Bob



#10 Bob4BVM

Bob4BVM

    Vanguard

  • *****
  • topic starter
  • Posts: 2,437
  • Joined: 23 Mar 2015
  • Loc: W. Oregon

Posted 01 April 2017 - 03:38 PM

 

Out of necessity for a couple of projects I am in the middle of, I finally got around to making a pinhole artificial star.

Can someone tell me what is the minimum distance to it need for it to appear at infinity.

 

The answer is - infinity! Any distance closer than infinity will introduce SOME correction error, it is a continuous variation, not a discrete cutoff. So, it depends on how much you can tolerate, and how picky you are about the test. Just like there is no minimum level of radiation that doesn't cause some damage to your DNA.

 

If you set a 1/4 wavefront limit, then Suiter's table 5.2 gives you some distance numbers.

 

If we say "The answer is - infinity!", then you do realize that real stars are too close to use, right ?

lol.gif

 

Anyway, so much for my hopes to use my artif star inside the house at 40 FEET undecided.gif  My goal was to have something to use with an optical bench setup. Maybe I can point the bench out an open window and put the artif star on a distant tree.

 

I will follow the guidelines you have all suggested here and then compare them to what I get with actual stars on a good night, even though they are not at "infinity"  smile.gif

 

Thanks all,

Bob


Edited by Bob4BVM, 01 April 2017 - 03:40 PM.


#11 precaud

precaud

    Fly Me to the Moon

  • *****
  • Posts: 5,010
  • Joined: 05 Dec 2012
  • Loc: north central New Mexico

Posted 01 April 2017 - 09:55 PM

There are very useful things you can do without the artificial star appearing to be at infinity. Using the Hubble artie (smallest pinhole) at about 35 feet, I have, and one can:

 

1) Test for astigmatism (fracs and newts), and adjusted refractor lens rotation for minimum astigmatism.

 

2) For Newts, fine-tuning the primary mirror rotation relative to the secondary mirror, at 50X/inch using the shape of the 1st ring just as it breaks away from focus on either side. This is an "old-school" trick that I learned from a local elder and from an optician. The theory is, since both mirrors inevitably have errors, there exists a rotational relationship between them where the errors are the least additive, and in some cases, even cancel to some extent. Rotate the primary until you see the most circular breakaway coming just off of focus.

 

In both cases, the results translate perfectly to outdoors under a star.

 

Oh, and 3) Test eyepieces or diagonals for scatter and out-of-field reflection problems.


Edited by precaud, 01 April 2017 - 10:03 PM.

  • Pierre Lemay, Joe1950 and Alpollo like this

#12 steveastrouk

steveastrouk

    Apollo

  • *****
  • Vendors
  • Posts: 1,166
  • Joined: 01 Aug 2013
  • Loc: State College, Pa.

Posted 04 April 2017 - 12:14 PM

Will it be safe to view thru a lensed system as my point source ?

Where would I find such a thing? My lapse from a long-ago electronics career is showing smile.gif

CS

Bob

 

If you keep it below the lasing threshold - the point at which it really develops a beam, and its unlensed, you will be fine. Shout if you want one. I have 10 in my desk drawer in my office.



#13 MKV

MKV

    Cosmos

  • *****
  • Posts: 9,065
  • Joined: 20 Jan 2011
  • Loc: Southeast, USA

Posted 04 April 2017 - 12:31 PM

 

Will it be safe to view thru a lensed system as my point source ?

Where would I find such a thing? My lapse from a long-ago electronics career is showing smile.gif

CS

Bob

 

If you keep it below the lasing threshold - the point at which it really develops a beam, and its unlensed, you will be fine. Shout if you want one. I have 10 in my desk drawer in my office.

 

The laser should also be less than 5 mW, preferably 1 mW. It's safer to observe with a camera LCD screen. 

 

Mladen



#14 Bob4BVM

Bob4BVM

    Vanguard

  • *****
  • topic starter
  • Posts: 2,437
  • Joined: 23 Mar 2015
  • Loc: W. Oregon

Posted 04 April 2017 - 12:50 PM

 

Will it be safe to view thru a lensed system as my point source ?

Where would I find such a thing? My lapse from a long-ago electronics career is showing smile.gif

CS

Bob

 

If you keep it below the lasing threshold - the point at which it really develops a beam, and its unlensed, you will be fine. Shout if you want one. I have 10 in my desk drawer in my office.

 

Sent you a PM Steve

CS

Bob



#15 Joepie

Joepie

    Explorer 1

  • -----
  • Posts: 80
  • Joined: 15 Oct 2012
  • Loc: The Netherlands

Posted 04 April 2017 - 03:52 PM

 

 

Will it be safe to view thru a lensed system as my point source ?

Where would I find such a thing? My lapse from a long-ago electronics career is showing smile.gif

CS

Bob

 

If you keep it below the lasing threshold - the point at which it really develops a beam, and its unlensed, you will be fine. Shout if you want one. I have 10 in my desk drawer in my office.

 

The laser should also be less than 5 mW, preferably 1 mW. It's safer to observe with a camera LCD screen. 

 

Mladen

 

So this one would be ok?

 

http://www.ebay.com/...A-/142334980746

 

And all I have to do is remove the lens? How do I make sure it doesn't lase, as in produce a beam?



#16 steveastrouk

steveastrouk

    Apollo

  • *****
  • Vendors
  • Posts: 1,166
  • Joined: 01 Aug 2013
  • Loc: State College, Pa.

Posted 04 April 2017 - 04:06 PM

 

 

 

Will it be safe to view thru a lensed system as my point source ?

Where would I find such a thing? My lapse from a long-ago electronics career is showing smile.gif

CS

Bob

 

If you keep it below the lasing threshold - the point at which it really develops a beam, and its unlensed, you will be fine. Shout if you want one. I have 10 in my desk drawer in my office.

 

The laser should also be less than 5 mW, preferably 1 mW. It's safer to observe with a camera LCD screen. 

 

Mladen

 

So this one would be ok?

 

http://www.ebay.com/...A-/142334980746

 

And all I have to do is remove the lens? How do I make sure it doesn't lase, as in produce a beam?

 

NO ABSOLUTELY NOT, that's a class III, you want a MUCH lower power than that. More like these

http://www.ebay.com/...sd=120921387106

 

Its says its 1mW laser, which is class II, but says its class IIIa... so do your homework yourself.

 

And you need an adjustable supply of around 1.6..2.2 Volts, certainly for the ones I tested this afternoon.  I'd actually advise a constant current supply, and set it to the current that you need to get the thing to below laser threshold.

 

Keep the power LOW.



#17 MKV

MKV

    Cosmos

  • *****
  • Posts: 9,065
  • Joined: 20 Jan 2011
  • Loc: Southeast, USA

Posted 04 April 2017 - 04:25 PM

 

 

 

Will it be safe to view thru a lensed system as my point source ?

Where would I find such a thing? My lapse from a long-ago electronics career is showing smile.gif

CS

Bob

 

If you keep it below the lasing threshold - the point at which it really develops a beam, and its unlensed, you will be fine. Shout if you want one. I have 10 in my desk drawer in my office.

 

The laser should also be less than 5 mW, preferably 1 mW. It's safer to observe with a camera LCD screen. 

 

Mladen

 

So this one would be ok?

 

http://www.ebay.com/...A-/142334980746

 

And all I have to do is remove the lens? How do I make sure it doesn't lase, as in produce a beam?

 

Jopie, steveastrouk pretty much answered it. You need to read up on lasers first. I would still never recommend anyone to look directly into a laser directly. A camera LCD screen is how I look at lensless laser light.

 

Mladen



#18 NHRob

NHRob

    Cosmos

  • *****
  • Posts: 8,107
  • Joined: 27 Aug 2004
  • Loc: Southern Cloud-Hampshire

Posted 08 April 2017 - 03:26 AM

Can one use geometric analysis to figure this out?

 

I am thinking the following ...  

 

1. you want to illuminate the entrance pupil (full aperture) with perfect plane-wave illumination.

2. consider the artificial star is a point source, projection the light as a spherical wavefront.

3. as the distance from the pojnt source increases, the spherical wavefront approaches a plane wave.

4. knowing the aperture of your scope, you can calculate the error of a spherical wavefront from a true plane wave ... at the edge of the aperture ... in terms of wavefronts .. as a function of distance from the source.

5.  apply some criteria for this .... .... < 1/8 wave of green



#19 Nils Olof Carlin

Nils Olof Carlin

    Vanguard

  • *****
  • In Memoriam
  • Posts: 2,227
  • Joined: 26 Jul 2004

Posted 08 April 2017 - 04:50 AM

Can one use geometric analysis to figure this out?

I did my best, many years ago:
http://web.telia.com...sc/nulltest.htm

#20 Nils Olof Carlin

Nils Olof Carlin

    Vanguard

  • *****
  • In Memoriam
  • Posts: 2,227
  • Joined: 26 Jul 2004

Posted 08 April 2017 - 08:14 AM

What has been said about an unlensed laser as point source applies to RED lasers, not to common 532 nm GREEN ones working on a totally different principle!

The cheap red laser pointers in my junkbox (and mostly many years old by now) electrically behave like red LEDs, with 3 LR44 size 1.5v cells in series driving the chip via a typically 70 ohm (IIRC, but check) series resistor for current limiting, at something like 30 mA (this I measured). Halving this current (again IIRC) will stop lasing. The difference is not hard to tell using a series potentiometer: at low currents, a paper will be evenly illuminated by incoherent red light, but increasing current will show a "diffraction ridge", stronger with increased current until it totally dominates. This ridge is at right angles to the chip, suggesting the laser radiating face on the chip edge is a small fraction of a wavelength in height, but a couple of wavelengths wide (this diffraction pattern seen close to the lens is circularly vignetted and often mistakely seen as "elliptic"). Thus, a pot of 100-200 ohms in series (with the original) ought to let you go between lasing and non-lasing. The radiating surface when non-lasing could presumably be a bit larger but not dramatically so.

In a Foucault tester, I have bypassed the extra resistance to get an easily visible return spot when setting up, before actual testing.

I think the chip is held by low-temp solder - never heat the base with a normally warm soldering iron.

 



#21 mconsidine

mconsidine

    Explorer 1

  • -----
  • Posts: 53
  • Joined: 16 Nov 2006

Posted 08 April 2017 - 08:07 PM

So ... should a green laser not be used at all or should it be dealt with in a different way?

#22 Ed Jones

Ed Jones

    Skylab

  • *****
  • Posts: 4,096
  • Joined: 06 Apr 2004
  • Loc: Sin-sin-atti

Posted 08 April 2017 - 11:00 PM

You can bounce a green laser of a Christmas tree ornament placed as far away as possible.

 

Large fast mirrors will still need the star to be quite a distance but one way to deal with the residual spherical is to place a null lens in the focuser like this:


  • Joe1950 likes this

#23 Joe1950

Joe1950

    Voyager 1

  • *****
  • Posts: 10,455
  • Joined: 22 Aug 2015

Posted 09 April 2017 - 07:22 AM

Good video Ed. I enjoy all the ones you do. Thanks!



#24 steveastrouk

steveastrouk

    Apollo

  • *****
  • Vendors
  • Posts: 1,166
  • Joined: 01 Aug 2013
  • Loc: State College, Pa.

Posted 09 April 2017 - 07:29 AM

The cheap red laser pointers in my junkbox (and mostly many years old by now) electrically behave like red LEDs, with 3 LR44 size 1.5v cells in series driving the chip via a typically 70 ohm (IIRC, but check) series resistor for current limiting, at something like 30 mA (this I measured). Halving this current (again IIRC) will stop lasing. The difference is not hard to tell using a series potentiometer: at low currents, a paper will be evenly illuminated by incoherent red light, but increasing current will show a "diffraction ridge", stronger with increased current until it totally dominates. This ridge is at right angles to the chip, suggesting the laser radiating face on the chip edge is a small fraction of a wavelength in height, but a couple of wavelengths wide (this diffraction pattern seen close to the lens is circularly vignetted and often mistakely seen as "elliptic"). Thus, a pot of 100-200 ohms in series (with the original) ought to let you go between lasing and non-lasing. The radiating surface when non-lasing could presumably be a bit larger but not dramatically so.

That agrees with measurements I made on my collection too. I think 10..12mA is suffcient. I ran mine with a constant current source made from an LM317 regulator. I put a 47 Ohm pot in series with a 100 Ohm, and got a trimmable current from 12 to 8mA. This cannot lase at any pot setting.



#25 MKV

MKV

    Cosmos

  • *****
  • Posts: 9,065
  • Joined: 20 Jan 2011
  • Loc: Southeast, USA

Posted 09 April 2017 - 06:37 PM

The difference is not hard to tell using a series potentiometer: at low currents, a paper will be evenly illuminated by incoherent red light, but increasing current will show a "diffraction ridge", stronger with increased current until it totally dominates. This ridge is at right angles to the chip, suggesting the laser radiating face on the chip edge is a small fraction of a wavelength in height, but a couple of wavelengths wide (this diffraction pattern seen close to the lens is circularly vignetted and often mistakely seen as "elliptic"). Thus, a pot of 100-200 ohms in series (with the original) ought to let you go between lasing and non-lasing. The radiating surface when non-lasing could presumably be a bit larger but not dramatically so.

In a Foucault tester, I have bypassed the extra resistance to get an easily visible return spot when setting up, before actual testing..

lasing_nonlasing.jpg

 

Mladen

 

PS Red dot lasers use symmetrical light cavity mirrors and produce a circular beam in non-lasing or lasing mode.




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics