TheSkySearchers

Long ago i came across a mention of using a round Christmas tree ornament to reflect sunlight (Earth's very own star), using it as an artifical star. Can't find the reference to it now to save my life. Is this a viable option? As it is the Christmas season ornaments are easy to come by. If so, I would assume silver colored ones would be the item of choice.

Seasons Greetings and Peace in the New Year to ALL!

I have used a chrome ball bearing stuck to a fence post with modelers clay as an artificial star. Seems to me that a chrome ball bearing is just a very tiny Christmas tree ornament. Just be careful of the brightness reflected by that ornament. If it is large, it may just be giving you a dangerous amount of light.

Cheers,
JT

The main point about using an artificial star (aside from safety with lasers) is setting it at the right distance. For collimation one needn't worry. If you are evaluating optical quality an insufficient distance will look like spherical aberration that is under corrected. All you need to know is here :

https://www.amazon.com/Star-Testing-Ast ... 0943396905

I don't remember where I found it, but:

Artificial star (maximum) size: Ø ~ L/3000D
λ=0.00055mm, W=λ/20, L=distance from artificial star [mm], D=aperture [mm]

Artificial star distance: L ~ D^2/(512*W*F^2)
L= (minium) distance [mm], D=aperture [mm], W= P-V wavefront-error, F= F-number
"For instance, by substituting W=λ/20=0.0000275mm for λ=550nm, gives the distance for this error level for D=400mm, ƒ/4, as L=710m. "
W=λ/20=0.0000275mm for λ=550nm (= 0,00055 mm)
L = 160000 / (512*0.0000275*16) = 160 000 / 0,22528 = 710227,2 mm ~ 710 m

That is, the ornament just needs to be put quite far away.

Oh my turbo, your response is technically way over this guys head. Would it be possible to dumb it down for a person awash (noobie) when it comes to mathematical thing? i.e.: globe size. distance to the artificial star, or any ancillary requirements? To reference how a star test should appear is no mystery, plenty of pictures on the net for that.
My favored scope is the ES 102 listed in profile. I'm 100% visual and have no desire to tinker, and perhaps mess up collimation myself but simply wish to verify that I have the best i can get from my optics. (if adjustment is required I live within 1:15 drive time to the ES home store in Springdale Ar. so no big deal if needed) Not having to manually chase a chosen test star across the sky would so simplify things for novice as I've become endeared to my Orion SVP non-goto mount...

Well, if Turbo's math is correct, put your Christmas ornament/artificial star roughly 8 football fields distant from your scope. Oh, BTW, make sure you do this on a sunny day. I know that sounds dumb, but you can never tell what follow-on questions you may get!

Cheers,
JT

konza wrote: ↑Fri Dec 18, 2020 5:57 am Oh my turbo, your response is technically way over this guys head. Would it be possible to dumb it down for a person awash (noobie) when it comes to mathematical thing? i.e.: globe size. distance to the artificial star, or any ancillary requirements? To reference how a star test should appear is no mystery, plenty of pictures on the net for that.
My favored scope is the ES 102 listed in profile. I'm 100% visual and have no desire to tinker, and perhaps mess up collimation myself but simply wish to verify that I have the best i can get from my optics. (if adjustment is required I live within 1:15 drive time to the ES home store in Springdale Ar. so no big deal if needed) Not having to manually chase a chosen test star across the sky would so simplify things for novice as I've become endeared to my Orion SVP non-goto mount...

These: λ=0.00055mm, W=λ/20 mention about the validity of the first equation. It's valid for 1/20 wavelength of light when the light used is 550 nm (yellowish green). The eye is most sensitive to this.
Then the diameter of the artificial star should be L/3000*D where L is the distance from the ornament to the mirror/lens and D is the diameter of the lens (all in millimeters).

The second equation gives you the distance for the first equation:
L ~ D^2/(512*W*F^2)
The distance is about aperture squared and that divided by (512 * wavefront error * F-number squared)
The wavefront error here is 1/20 vavelength of that same 550 nm light (= 1/20 * 0.00055 mm). and if you have an F/6 telescope, the F^2 is 6*6 = 36.
You could, probably, do with bigger error, like 1/10 or even 1/4.

That is, the distance depends on your scope (diameter of the mirror/lens and F-number) as well as the diameter of your ornament.

Turbo, your explanation has aided understanding. Now need to study enough to put it all together. I did a web search on your first reply.

Artificial star (maximum) size: Ø ~ L/3000D
λ=0.00055mm, W=λ/20, L=distance from artificial star [mm], D=aperture [mm]

That led to a "sister forum" that also discussed this issue. If anyone is interested it may be worthwhile they check it out. Too bad the old AF is no more as that may have been where i first seen it.
Thank you

Ø ~ L/3000D
L = Ø * 3000 * D
If the ornament diameter is 60 mm and the telescope aperture is 102 mm the formula gives 60 * 3000 * 102 = 18360000 mm = 18360 m = 18.4 km = 11.5 miles.
For aperture of 127 mm it's 60 * 3000 * 127 = 22860000 mm = 23 km = 14.5 miles.

If I understood it right, the idea is to see the artificial star in an angle that corresponds to 1/4 of the diameter of the airy disk for 550 nm light.

Artificial star distance: L ~ D^2/(512*W*F^2)
For error W = λ/4 = 0.00055 mm / 4 = 0.000138 mm, D = 102 mm, F/7
L = 10404 / (512 * 0.000138 * 49) = 10404 / 3.462144 = 3005 mm
Ø = 3005/(3000 * 102) = 0.010 mm.
=> you could check the ES with 10 um artificial star 3005 mm away from the lens.

My source was https://www.telescope-optics.net/star_t ... escope.htm

Turbo I think you need to put the calculator aside!!

Konza if you are just wanting to check the collimation of your telescope then a rough approximation is 2 metres distance for every 1 inch of aperture +1 metre will do fine.
A 4" refractor would require 30 feet as a minimum distance. (4x2=8+1=9 metres)

Again if you just want to check collimation then purchase a refractor collimating eyepiece and you can check it anytime indoors and distance is not required.
https://agenaastro.com/agena-1-25-colli ... ctors.html

If you are looking to star test then a artificial star is handy but there are other factors to take into account.

Lady Fraktor wrote: ↑Sun Dec 20, 2020 8:21 pm Turbo I think you need to put the calculator aside!!

Konza if you are just wanting to check the collimation of your telescope then a rough approximation is 2 metres distance for every 1 inch of aperture +1 metre will do fine.
A 4" refractor would require 30 feet as a minimum distance. (4x2=8+1=9 metres)

Again if you just want to check collimation then purchase a refractor collimating eyepiece and you can check it anytime indoors and distance is not required.
https://agenaastro.com/agena-1-25-colli ... ctors.html

If you are looking to star test then a artificial star is handy but there are other factors to take into account.

I don't think 30 feet is enough if you're using a Christmas ornament as an artificial star.

So, I don't think a Christmas tree ornament is very handy. Rather, try a bearing ball, like JayTee mentioned.

30 feet is just fine for a minimum distance for collimation for a 100mm f/7 refractor.
Your results in miles are incredibly off, and I suspect you are using the wrong formula for figuring out collimation distances.
My artificial stars range from 0.5mm to 0.20mm for different telescopes.

A ornament will work but you need to know what size point source is required which is a different calculation than distance required.

I used a torch up the end of the garden (20m) with aluminium foil over it and the smallest hole I could make in the centre. It worked ok to collimate my 9.25 SCT.

Regards

Graeme

Lady Fraktor wrote: ↑Mon Dec 21, 2020 8:01 am 30 feet is just fine for a minimum distance for collimation for a 100mm f/7 refractor.
Your results in miles are incredibly off, and I suspect you are using the wrong formula for figuring out collimation distances.
My artificial stars range from 0.5mm to 0.20mm for different telescopes.

A ornament will work but you need to know what size point source is required which is a different calculation than distance required.

Yes. It seems those formuli are for checking the optics. I haven't found anything about requirements for collimation. Those formuli give the size and distance for point source that gives 550 nm light 1/4 airy disk size. Using Dawes limit giver pretty much the same results. Then again, 0.5mm is less than 1/100 of 60 mm.

I bought an arti star. It's basically a flash light with a cover and tiny pin holes in it.

Found out I could not use it as there was no where in my yard far enough away. I would have to put it about halfway out in my neighbor's yard... that aint gonna work.

Hmm, looks like the formuli were valid, but my understanding of using bearing ball or Christmas tree ornament wasn't.
I found somewhere in the net:

The focal length of the 2" ball bearing is 1/2" or 12.7mm.
Thus the diameter of the image formed of the sun is about d = 127 microns

The "object" is not the ball itself, but the image of the Sun on it.

UlteriorModem wrote: ↑Mon Dec 21, 2020 3:15 pm I bought an arti star. It's basically a flash light with a cover and tiny pin holes in it.

Found out I could not use it as there was no where in my yard far enough away. I would have to put it about halfway out in my neighbor's yard... that aint gonna work.

If you have figured out the distance you need to star test, shorten that distance for collimating to 70% of the distance and it will work fine.

What are the criteria for artificial star when used for collimation? I'd guess that it should be far enough for focusing, and its apparent size should be smaller than... what?

The spot size is half the airy disk.
You need to figure out the resolution of the telescope, I will see if I can find my book with the formulas.