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-   -   star test with an artificial star (https://www.birdforum.net/showthread.php?t=280285)

Vespobuteo Monday 7th April 2014 09:42

star test with an artificial star
 
is it possible to set up an artificial star when doing a
star test?

I tried yesterday with a inverted pin hole camera,
a small box with a strong LED light inside
and a very fine pinhole in piece of black tape,
on the side of the box,

I managed to see some airy discs towards
the edge of the pinpoint light source,
but not in the center,
(just some strange patterns there…a bit like psychedelia patterns without colors..)

the light source looked otherwise circular,
looking of center, it became more distorted,

distance was about 10 meters/30 feet

could this test work and show something relevant?

mooreorless Monday 7th April 2014 09:57

Do you have any Christmas tree balls? I have used those outside on a sunny day.

Vespobuteo Monday 7th April 2014 10:08

Quote:

Originally Posted by mooreorless (Post 2963771)
Do you have any Christmas tree balls? I have used those outside on a sunny day.

are you pulling my leg now?
:)

or could you explain further?

Bob D Monday 7th April 2014 12:18

I've used a ball bearing with the sun reflecting off it.

Binoseeker Monday 7th April 2014 17:25

Is there a recommended ball-radius to minimum viewing distance ratio?

Anders

Vespobuteo Monday 7th April 2014 22:32

found this old thread, with good info

http://www.birdforum.net/showthread.php?t=230720&page=2

My set up is now a ice-cream spoon made by stainless steel (half sphere approx. 3 cm/1 inch in diameter) and then pointing my LED lamp to it,

and thanks for the tip on the christmas tree ball,

:)

Binoseeker Tuesday 8th April 2014 21:18

Yup, I did try to learn to star test with my Meopta in that thread :-)

Anders

mooreorless Tuesday 8th April 2014 22:32

Quote:

Originally Posted by Vespobuteo (Post 2963777)
are you pulling my leg now?
:)

or could you explain further?

Bob moreorless answered for me, I have used ball bearings as well, the glint off the Christmas ball in Sunlight.

Vespobuteo Tuesday 8th April 2014 22:45

Quote:

Originally Posted by Binoseeker (Post 2964713)
Yup, I did try to learn to star test with my Meopta in that thread :-)

Anders

are you still happy with your meopta?

Is it as good as it seems?

Vespobuteo Tuesday 8th April 2014 22:47

Quote:

Originally Posted by mooreorless (Post 2964766)
Bob moreorless answered for me, I have used ball bearings as well, the glint off the Christmas ball in Sunlight.

I tried some star testing with my ED50 but I only have the 20x eye piece
so I guess I need a booster of some kind?

Binoseeker Tuesday 8th April 2014 23:18

Quote:

Originally Posted by Vespobuteo (Post 2964772)
are you still happy with your meopta?

Is it as good as it seems?

Yes, I am happy with it. You should try before buy...if not possible, buy and return it if you donīt like it.

You can get it here...http://www.handladigitalt.se/tubkika...0_589_719.html (18490 kr with eye piece) or check eu-sites like:

Benel has it with eyepiece for 1848 Euro

opti-pro.de has it with eye piece for 1855 euro

If you fill in the form you get with the scope, and return the form to meopta within a month, you get 30 years warranty...you can register on-line also.

Anders

Vespobuteo Wednesday 9th April 2014 09:31

Quote:

Originally Posted by Binoseeker (Post 2964792)
Yes, I am happy with it. You should try before buy...if not possible, buy and return it if you donīt like it.

You can get it here...http://www.handladigitalt.se/tubkika...0_589_719.html (18490 kr with eye piece) or check eu-sites like:

Benel has it with eyepiece for 1848 Euro

opti-pro.de has it with eye piece for 1855 euro

If you fill in the form you get with the scope, and return the form to meopta within a month, you get 30 years warranty...you can register on-line also.

Anders

Ok,
thanks for info
:t:

30 years warranty is very good,

OPTIC_NUT Wednesday 9th July 2014 13:30

Quote:

Originally Posted by Vespobuteo (Post 2963768)
is it possible to set up an artificial star when doing a
star test?

I tried yesterday with a inverted pin hole camera,
a small box with a strong LED light inside
and a very fine pinhole in piece of black tape,
on the side of the box,

I managed to see some airy discs towards
the edge of the pinpoint light source,
but not in the center,
(just some strange patterns there…a bit like psychedelia patterns without colors..)

the light source looked otherwise circular,
looking of center, it became more distorted,

distance was about 10 meters/30 feet

could this test work and show something relevant?


You don't need to get fancy if you have distance.
I've found that a keychain LED light 50-200meters away does a great job
(at night). You get much more brightness with the LED light pointing
at your telescope from a distance.

The interesting patterns happen when you are de-focused in one side of
ideal focus but not the other.

Here are some illustrations:
http://www.telescope-optics.net/diff...berrations.htm

You can see various patterns. A cross-rippled pattern
usually is seeing some on-axis astigmatism.

A note about white LEDs: they are good for looking at chromatic
aberration, but there is a very strong violet part that will make
the chromatic aberration worse than normal.


At lower powers you might have a hard time seeing much.
You can magnify the image with a small ow-power monocular.

Vespobuteo Wednesday 9th July 2014 15:42

Quote:

Originally Posted by OPTIC_NUT (Post 3026419)
You don't need to get fancy if you have distance.
I've found that a keychain LED light 50-200meters away does a great job
(at night). You get much more brightness with the LED light pointing
at your telescope from a distance.

The interesting patterns happen when you are de-focused in one side of
ideal focus but not the other.

Here are some illustrations:
http://www.telescope-optics.net/diff...berrations.htm

You can see various patterns. A cross-rippled pattern
usually is seeing some on-axis astigmatism.

A note about white LEDs: they are good for looking at chromatic
aberration, but there is a very strong violet part that will make
the chromatic aberration worse than normal.


At lower powers you might have a hard time seeing much.
You can magnify the image with a small ow-power monocular.

thanks for info!
its much appreciated,
I think I will have to get a booster,
any ring pattern is hardly seen at 20x

OPTIC_NUT Wednesday 9th July 2014 17:45

Henry Link just suggested (on another thread) that Polaris makes a good star that doesn't move much.
My next try at an artificial, I think I might place it over 200m away. Single small LED light. (a big reflector or
lens is not good). You may want to boost 20x to a power equal to or over your aperature in mm
(approximately), I am told. Most monoculars are too powerful. 4x is useful, or 6x.
Low-vision supplies places have nice low-powered monoculars,
...like the 4x12s here:
http://www.lssproducts.com/category/other-monoculars

OPTIC_NUT Thursday 10th July 2014 04:01

Quote:

Originally Posted by Vespobuteo (Post 3026505)
thanks for info!
its much appreciated,
I think I will have to get a booster,
any ring pattern is hardly seen at 20x

Bear in mind....you only see interesting ring patterns at much higher
powers. Your observation is normal for 20X.

If you have 60mm up front, you need to look at 60 to 120x.
The booster at 20x might boost a boring pattern unless your
source is very small.

A ball bearing in the sun is interesting, since the sun's image would
be very tiny and very bright (diverging rays cause a small virtual sun
to appear in the ball bearing). You would need a lot of black around it
but making a tiny artificial sun that has enough light is hard to do.

If you commonly use 20x it seems a more practical measurement would
be different fonts at 50 ft or the USAF chart. A booster is still very handy
for those tests. It goes beyond your eye's resolution, which is a hint that
you can only use a certain amount of resolution.

OPTIC_NUT Saturday 16th August 2014 03:28

I finally got a night clear and stable enough for pinpoint stars.
At 117x, things were pretty sharp and clear, and the brightest point did generate a little donut.
It was quite symmetric, but I've learned this is not unusual with long barrels...I'm at
at 70mm-f/10. The longer the barrel the less critical the alignment. People worry about this
mainly for short refractors and very-short reflectors.
Things have a little color at 175x, but that's a useless power due to dimming.
That and the tripod falling short of ideal.

I got a nice porcelain mug with various font sizes for eyechart-style resolution testing.
I can read the .06 inch high font crisp and distinctly at 92 feet. That's 11.2 arc-seconds high.
Optometrists say you need 5 units of pair-seperation to recognize letters, so that puts
the resolution somewhere well below 2.2 arc-seconds. It was early dusk...I can extend the test distance
with some strong light.

I souped-up a 50mm - f/12 for fun..
that just barely resolves makes the font in question, with issues. The next font up is solid.
So...that's probably about 2.5 arc-seconds. Makes a crisp image at 40x. On a heavy Bogen
tripod (has 1/4" threads) it's actually a lot of fun.

You need decent eyepieces, of course. Kellner to 40x, Plossl to 60-100x, super-Plossl
to get a wide apparent field.


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