Star testing - artificial star?

[erick]

Now I've read that star testing (and consequent collimation tweaking) is only effective in good seeing conditions. Given such conditions seem to be fleeting :sadeyes: and the problem of keeping the star centred in a non-tracking dob or alt-az SCT, seems it's about time I learnt how to collimate with an "artificial star". I read hints of capturing a reflection of sunlight - off a glass/ceramic insulator on the electrical wires seems to be a favourite. But how long is that present, given the movement of the Sun?

So how does everyone do it? How do you get a fine pinprick of bright light that is fairly stable in brightness and not moving with the earth's rotation? Is there a way to get that pinprick source located at different scope altitudes so collimation can be checked against OTA altitude? Can it be sourced close to the scope or does it need to be hundreds of metres away :shrug:

Looking forward to hearing your wisdom and tips and tricks. :)

[JethroB76]

Have you done a search for "artificial star" on IIS? There appears to be several threads and even an article in the projects section..

whether you have light shining through a pinprick in some alfoil or a lit piece of fibre optic or whatever, the light needs to be around 20x your focal length away from your scope IIRC

[Wavytone]

1. Reflection off a distant insulator, suffers from air turbulence and thermals.

2. A ball bearing - outdoors it can be placed where it is illuminated by the sun, (don't leave it, they rust), suffers from air turbulence and thermals as above.

3. Indoors a ball bearing can be illuminated by the reflection of a bright lamp - anything from a torch will do strongly recommended if you have a long corridor or garage or similar with still air and no thermal gradients.

4. Tiny hole in aluminium foil, illuminated by a small bright source, used indoors as above.

I've tried all four before and the best by far is #3, I used a body corporate underground carpark late at night.


If you have no choice except doing it outdoors, try to make sure the optical path is over ground that is a garden or grass, in shade. Try to avoid a path over ground that is in the sun, hot/cold paved areas (concrete, brick, tar) or a fence or wall heated by the sun.

Outdoors the turbulence in daytime is usually so bad it will limit you to grossly out of focus images which won't tell you much. The really interesting aspects are what you can see in a highpower eyepiece chosen such that you should normally be able to resolve the Airy disk and rings, only slightly in/out of focus. Using low power eyepieces will tell you nothing useful unless the scope is horrendously out of whack.

[erick]

Ooops! Sorry. :ashamed:

[erick]

OK, I made my own version. Basically a fairly bright yellow LED (whatever I had in my box of bits), powered by a couple of rechargeable AAA batteries through an approx 50 ohm resister. It shines onto a piece of optical fibre which turns about 60 deg and pokes through just to the front of a panel which is painted black. The optical fibre is cheap plastic stuff cut from an illuminated Christmas tree.

Have to use at night - not bright enough during the day. Tonight I could pick it up in my C8 at about 100x at a distance of about 100m. I could defocus and see stable diffraction rings, I think. Yes, I should have measured the diametre of the fibre and done lots of calculations based on resolving power and distance to target etc., but thought I'd just "suck it and see".

Now to try and follow the instructions to achieve more accurate collimation of my SCT. I've reached having the shadow of the secondary fairly well centred in the greatly defocussed blob. Now to move to the finer collimaton steps.

[erick]

Update: I have fiddled around with my home made artificial star for a while but have never got it particularly bright at distance. Usable, but....

Then I saw this commercially available version. the Hubble Optics 5-Star artificial star:-

http://www.hubble-optics.com/artificial-stars.html
http://www.hubbleoptics.com/files/Hubble5starFAQ.pdf

Reasonable price - USD 24.95 shipped

Tried it last night for collimating my old C8.

Found a darkish location - in tree shadow from some security lights. I set up my home made on a tripod about 50m from the scope. The Hubble 5-star I balanced beside the home made and pointed it at the scope.

Quickly determined I could see the 5-star much better so went with that. I chose the middle hole (perhaps the "Goldilocks" hole - not too bright and not too dull!) and used the supplied magnetic blank to cover all holes but that one.

After that it was easy to observe the diffraction rings and tweak the SCT collimation to a much improved condition.

Looks like a worthwhile accessory.

[DavidU]

I have a 10mm ball bearing on a shaft for day time star testing and an LED one I made for night .
I had them in my car at Snake Valley, I could have lent them to you.

[multiweb]

I use the astrozap from here: http://www.astrozap.com/pages/artificial-star.htm I use it a lot for various things.

[asimov]

Yeah, I made my own many moons ago. It was just a red LED light that was in a remote control hand piece.....of all things....A battery operating fart machine! Hung it on a barbwire fence about 700M away & collimated the C9.25.

[Harpspitfire]

i just use an LED flashlight , i put a 2mm (or smaller) fitting in front from an air tool, its all covered with electrical packing-- works just fine, i can do a 9.25 with it at over 500X without any issues

[dcalleja]

Like Marc, I too sucumbed to another gadget and bought the Astrozap. It's actually really good and I've used it several times now. My wife however couldn't believe it.

[citivolus]

I have the Astrozap one too, and find it works as well as my home made ones, but is much more compact. On the issue of the purchase, my wife hasn't been informed ;)

[Omaroo]

I just bought the Hubble-Optics version too. With 5 apertures to choose from (50/100/150/200 & 250 microns) I'm keen to use it for both collimation and multi-scope/camera/finder alignment with my side-by-side-on-top setup. Thanks Eric :thumbsup:

[casstony]

I bought the 'hubble 5 star' as well; it's construction is cheap and nasty but it works well. I collimated a C8 inside the house (at 280x) over a 20m distance, which is closer than recommended I think, but the image looked fine for collimation purposes. I think any distance would be okay so long as you can achieve focus. I checked my C6 and it still looks spot-on, even though it was purchased from the US; I have a set of Bob's Knobs for it but won't install them until the collimation goes out.