Minimum focus and ball bearing artificial star

[Stonius]

Has anyone tried this?

I had a go at the ball-bearing artificial star the other night without success because 1) the reflection in the ball bearing was not bright enough to stand out and 2) minimum focus on the telescope would not allow focusing at that distance. I know you don't want focus for a star test, but you do want close to focus - this is nowhere near.

So I could venture into the driveway which has about a 70m clear run, but only a short window of full sun, or I could go to the park.

The main problem for me is not being able to achieve focus on anything that close. It would be good to know the minimum distance at which my scope will focus on an object. Apparently the formula for minimum focus for a simple lens system is 1/u + 1/v = 1/f.

u= distance from the subject to the lens
v = distance from the lens to the image
f = focal length

So I have a newtonian. I figure I can substitute 'mirror' for 'lens'. But is the focal length referring to the focal length of the scope or the eyepiece? and again is the image distance the focal length of the scope, or the eye relief of the eyepiece?

It's an 8" f/6 (1200mm), BTW.

***since the reflection of the sun is going to be way brighter than any star and still in essence, still looking at the sun do I need to treat this with the same safety precautions as with solar observing?****

Thanks for any geniuses who know the answer :-)

[jenchris]

Minimum focus is more to do with the travel on the focuser.
My 8 inch meade is about 6 metres......

[doppler]

If you want the scope to focus closer, all you need is an extension tube for the focuser.
http://www.bintel.com.au/Accessories/Extension-Tubes/47/catmenu.aspx

[Astroman]

If your only looking at a defocused light from the ball bearing you wont need filters for it. With my 8" f/4 I have to use extension tubes to see my "star" which is about 30 metres from the scope. I was thinking of putting one on my neighbours house when they aren't looking :D But yeah you will need the extension tubes for anything less than say 50-100 metres. Try do it when it isn't hot either, those heat currents can really be annoying.

[Stonius]

Thanks for the replies, Any comments on getting a bright enough highlight on the ball bearing indoors with artificial light? Is a 60W globe from a bedside lamp okay? Should it be close or far away from the ball bearing?

And is a really small ball bearing better for making the highlight unresolvable? Currently have a choice of a 3mm or 8mm.

Also, anyone know how to mathematically figure out minimum focus? I'd need to know this even to know how long the extension tube should be, surely?

Markus

[jenchris]

I used the back of a teaspoon jammed in the fork of a tree.

[kens]

And signed with a thumbnail dipped in tar

[Merlin66]

"Star Testing Astronomical Telescopes" by HR Suiter has all the details.
The "star" should be ideally x20 the telescope focal length and the size of the reflective ball can be 20-50mm to give a suitable "glitter" spot.
The artificial lamp (usually using a quartz halogen bulb) placed a couple of feet to the side of the reflective ball.
Try what you have and see.....

[jenchris]

I'd have used the front except it was full of air