Artificial star - home made

[Stu]

Probably had too much time on my hands, but the mak needs collimating so I thought I'd have a go at making an artificial star to see if it makes things easier.

1 broken head torch

1 piece of card with a hole punch hole in it to cover all but one LED, placed inside the torch lens

1 old ep cover with a pin hole in it. Fits perfectly :-)

Tried it out in daylight and I could just about make out nice diffraction rings with my little frac so will try the mak once it's darker. Web cam is working now so that should make it easier too.

Garden is about 120 feet long so it might just work.

Will report back once I've tried it out. Might have to make a smaller hole but shouldn't be a problem

Stu

[ollypenrice]

Nifty. I wonder if your hole will be small enough?

The other way is to glue a tiny ballbearing onto a black card and point a bright beam at it. Only the nearest part of the steel ball reflects light towards the scope. I used this once on a repair project and it gave a perfect set of diffraction rings at quite a short distance (inside a condom warehouse to be absolutely accurate!!)

Olly

[Stu]

Thanks Olly. Not sure if it is, but I'm sure I can make a smaller one if needed.

Are you going to elaborate on exactly what you were doing in said warehouse? :-) sounds intriguing.....

Stu

[opticalpath]

Interesting method, Olly. Does it have to be a condom warehouse, or would any unlikely large building do? Couldn't be too large I suppose or you might lose your bearings.

Adrian

[Stu]

Adrian, that was awful :-) :-)

Just tried it out in the dark and it seems to work very well with the refractor. Haven't tried the mak yet, may be more of an issue with that but have made a smaller hole already so should be ok

[opticalpath]

Stu, I just realised that I recognise that head-torch. Got one just like it in the pound shop (but the switch broke after 1 day!)

Adrian

[Stu]

Yes, it certainly wasn't anything expensive! I think I got mine from a garage. It broke and would only point down towards the ground so was time to use for something else.

[Stu]

Tried this last night and of course I could get it to focus with the Mak. I had a mess around with various different configs but couldn't get close.

Dumb question, do I need extension tubes to stand more chance of focussing or to go the other way?

Stu

[cs1cjc]

I thought about making one but in the end have bought the Hubble Optics artificial star which has precision holes and is not expensive. It cost less than £20 and was delivered promptly from Hong Kong.

Artificial Star

It works well. The colour on my ED scope is easily visible and I am at last convinced that my newtonian is properly collimated and that the coma corrector is doing its job.

[Stu]

Thanks, will have a look

My problem now is that I can't get focus in the mak so wouldn't be able to use it anyway. May well just try on a star after all next time I get it setup

[opticalpath]

Thanks, will have a look

My problem now is that I can't get focus in the mak so wouldn't be able to use it anyway. May well just try on a star after all next time I get it setup

Stu, you have to rack out the focuser to reach focus on objects closer than infinity. But I just noticed the f.l. of your scope is 4000mm. It just ain't gonna work in your garden!

You can work out how far you'd have to rack out the focuser for any given distance. Example: your scope is 4m focal length and garden length - say 40 metres.

Expressing everything in metres .......

1/D = 1/4m (focal length) - 1/40m(object distance),

where D is the distance of the focal plane from the objective.

So 1/D = 9/40m

and D = 40/9m = 4.444m= 4444mm

Focusing on a star, the focal distance is 4000mm, so at 40m you would have to rack out the focuser a further 444 mm! You need to get a LOT further away from your artificial star to reduce the extra back focal distance.

As an approximation, if your object distance was 100x the focal length of your scope - i.e. 400 metres away - then the focus point would move back by 1/100th of the focal length of your scope, i.e. 40mm .... which might be more manageble. But no way at 40 metres I'm afraid.

Adrian

[Stu]

Thanks very much Adrian, that's really helpful. Helps me understand the calculations, and has also stopped me wasting any more time trying to get it to focus!

Am sure my artificial star will come in handy sometime, I'd love to get a 10 or 12 inch newt at some point with a more normal focal length :-)

Guess I'm back to collimating on a star then

Thanks again