Jump to content


Photo

Hubble Optics 5-Star Artificial Star review

  • Please log in to reply
16 replies to this topic

#1 Olivier Biot

Olivier Biot

    Amused

  • *****
  • Posts: 24946
  • Joined: 25 Apr 2005
  • Loc: 51°N (Belgium)

Posted 16 February 2012 - 05:45 AM

Hubble Optics 5-Star Artificial Star for Testing and Collimating Telescopes

By Steve Bennetsen.

#2 skyjim

skyjim

    Surveyor 1

  • *****
  • Posts: 1623
  • Joined: 13 Jan 2007
  • Loc: Carmel, NY

Posted 16 February 2012 - 04:00 PM

Hi Steve, nice reveiw, I have used there fake star for over a year now, I set it up inside, have a run between the far part of the living room going threw the house to the other end past the laundry room into the storage closet and thats were I place the star on a shelf, the runs about 75 feet and on my mac I use an 100mm extention onto the focuser, gets me right on the money and when checked outside when we dont have that much turbulance in the air its on the money. I try to do that mac real fine at very hi powers at the end and when we have decent seeing it does really well. I have also used it with other scope I have as well in the past like the C9.25 and to test my fracs out, it works really well and I am glade someone finally placed a reveiw, good job, Jim.

#3 Mark9473

Mark9473

    Cosmos

  • *****
  • Posts: 8644
  • Joined: 21 Jul 2005
  • Loc: 51°N 4°E

Posted 17 February 2012 - 10:17 AM

So is there a minimum (or maximum?) recommended working distance, depending on scope aperture and focal length?

Note: adjusting your red-dot finder on something as close as a target on your livingroom sofa, will only mean it is off for use on the stars.

#4 BSJ

BSJ

    Apollo

  • *****
  • Posts: 1268
  • Joined: 22 Dec 2008
  • Loc: Grand Isle, VT

Posted 24 February 2012 - 04:32 PM

So is there a minimum (or maximum?) recommended working distance, depending on scope aperture and focal length?

Note: adjusting your red-dot finder on something as close as a target on your livingroom sofa, will only mean it is off for use on the stars.


Depends on the design of the scope. The star can't be any closer than the scope can reach focus.

#5 MKV

MKV

    Mercury-Atlas

  • *****
  • Posts: 2573
  • Joined: 20 Jan 2011

Posted 28 February 2012 - 08:40 PM

In my experience, the best way to make an artificial star is to use a polished steel ball bearing or a lens relay system using the lens maker's formula 1/s1 + 1/s2 = 1/f. You can create an optical artificial star that is much smaller than any physical pinhole you can make, and brighter than any externally illuminated pinhole.

Using a steel ball is the easiest method. Use a flashlight to shine the light on the steel ball at some distance (say 6 feet) and observe the image of a bright dot on the steel ball. (see image below)

A more compact setup can be obtained using short focal length lenses, or eyepieces. All you need is a small planoconvex (pcx) lens, or a 25 mm eyepiece, and a 6 mm, or (preferably) a 4 mm eyepiece.

I use a 20 mm fl pcx lens and a 4 mm eyepiece. The light path length is 316 mm (about 12.45 inches) The star diagonal makes the setup more compact and stable. The light source is a super bright 3 mm LED, and the final image formed in front of the 4 mm eyepiece is 12 microns (0.012 mm) or 0.00005 inches, with practically all the brightness of the original LED compacted into that tiny image. It shows a clean, classic Airy disc pattern at high power.

You can replace eypieces with microscope objectives. They are actually easier to install then are the eyepieces, but most people have a set of eyepieces handy.



Mladen

Attached Files



#6 MKV

MKV

    Mercury-Atlas

  • *****
  • Posts: 2573
  • Joined: 20 Jan 2011

Posted 28 February 2012 - 08:46 PM

and here is the image of the alternate method.

Mladen

Attached Files



#7 MKV

MKV

    Mercury-Atlas

  • *****
  • Posts: 2573
  • Joined: 20 Jan 2011

Posted 28 February 2012 - 09:17 PM

and here is the component and lens schematic

Mladen

Attached Files



#8 bartine

bartine

    Apollo

  • *****
  • Posts: 1363
  • Joined: 03 Oct 2007
  • Loc: Potomac, MD

Posted 01 March 2012 - 09:14 AM

Could you shoot a pic of the front / star view of your diagonal and lens based artificial star?

And - to be clear, you have a 25mm eyepiece in the middle of the tube, and a 4mm eyepiece at the end?

Where do you see the star - some point in front of the 4mm eyepiece?

#9 MKV

MKV

    Mercury-Atlas

  • *****
  • Posts: 2573
  • Joined: 20 Jan 2011

Posted 01 March 2012 - 06:30 PM

Could you shoot a pic of the front / star view of your diagonal and lens based artificial star? And - to be clear, you have a 25mm eyepiece in the middle of the tube, and a 4mm eyepiece at the end? Where do you see the star - some point in front of the 4mm eyepiece?

Sure. The artifciial star image is formed in front of the 4 mm eyepiece. The separation (s1) being 88 mm and the fl = 4 mm, the image is formed at 4.19 mm in front of the eyepiece. It is extremely small and bright.

The intermediate lens is used because a 4 mm eyepiece will give me only a 38 micron images over 316 mm optical path length. I don't have anything with a shorter fl. Even a 100X microscope objective has about a 4 mm fl.

The only other option is to use longer PVC tubes, which is an effort with diminishing returns. A 600 mm (24 inches) PVC tube would give me a 20 micron image. A one meter (40 in) PVC tube would be required to give me 12 micron images. Clearly another lens can accomplish this in a much more compact package.

And 12 microns is the Airy disc that's acceptable for f/9 and slower...so the intermediate lens is indispensable for creating needed reduction in a compact package.

Mladen

Attached Files



#10 MKV

MKV

    Mercury-Atlas

  • *****
  • Posts: 2573
  • Joined: 20 Jan 2011

Posted 01 March 2012 - 06:35 PM

Rather than a 25 mm eyepiece, I use a 20 mm fl pcx lens in a machined plastic cell at the end of a white PVC tube.

Mladen

Attached Files



#11 MKV

MKV

    Mercury-Atlas

  • *****
  • Posts: 2573
  • Joined: 20 Jan 2011

Posted 02 March 2012 - 01:34 AM

I originally made the artificial star jig with a slightly shorter PVC tube for a Ritchey Common test for flats which requires a very bright near-diffraction-limited light source. Otherwise the test is not very sensitive with larger pupil sizes. You can tailor the image size by varying intermediate lenses and/or use different tube length.

The setup, as is, can use superbright LEDs or lasers. It's just that I don't like the speckled nature of the laser light, so I don't use lasers for such tests.

As a bonus, the PVC tube part of the artificial star setup (minus the pcx lens) can double as a laser collimating eyepiece for telescopes. Not bad considering a PVC tube and a laser pointer cost less than $10.

Mladen

Attached Files



#12 bartine

bartine

    Apollo

  • *****
  • Posts: 1363
  • Joined: 03 Oct 2007
  • Loc: Potomac, MD

Posted 02 March 2012 - 12:43 PM

That is totally cool. I have spare eyepieces, so will have to give this a shot.

I'll post feedback on how it works out...

#13 MKV

MKV

    Mercury-Atlas

  • *****
  • Posts: 2573
  • Joined: 20 Jan 2011

Posted 02 March 2012 - 10:21 PM

That is totally cool. I have spare eyepieces, so will have to give this a shot.

I'll post feedback on how it works out...

Make sure everything is properly aligned. Otherwise you'll see astigmatism and/or coma. To me it's of secondary importance to have a clean Airy disc, as long as the dot size is correct and its brightness sufficient for such trying tests as Ritchey-Common's test for flats.

#14 BSJ

BSJ

    Apollo

  • *****
  • Posts: 1268
  • Joined: 22 Dec 2008
  • Loc: Grand Isle, VT

Posted 03 March 2012 - 02:06 PM

Does there need to be a diagonal in the chain?

#15 MKV

MKV

    Mercury-Atlas

  • *****
  • Posts: 2573
  • Joined: 20 Jan 2011

Posted 03 March 2012 - 05:22 PM

Does there need to be a diagonal in the chain?

No, not at all. The diagonal "buys" you about 4 inches in optical path which is related to the degree of reduction in size of the light source.

The lens maker's equation is 1/s1 + 1/s2 = 1/f and magnification/reduction = s2/s1, where s1 is the object to lens distance, and s2 is the lens to focus distance. Thus, when you have a very large s1 compared to s2, i.e. s1 >> s2,, you are reducing the image size; and conversely when s2 >> s1, then you have enlargement (magnification) of the image size.

Sometimes I use the device straight through, and the diagonal is a convenient observing device, as shown below, so as not to get in the way of the long tube.

Mladen

Attached Files



#16 MKV

MKV

    Mercury-Atlas

  • *****
  • Posts: 2573
  • Joined: 20 Jan 2011

Posted 03 March 2012 - 05:24 PM

Of course, the most compact star tester is a steel ball setup. It's just flat, but it's not the easiest to align.

Mladen

Attached Files



#17 MKV

MKV

    Mercury-Atlas

  • *****
  • Posts: 2573
  • Joined: 20 Jan 2011

Posted 03 March 2012 - 05:26 PM

The steel ball is sitting on a sockehead screw, and is held by a magnet (see the washer placed there for easier identification).

Mladen

Attached Files








Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics