Stars colours

[Lee]

I found a cool way to very easily demonstrate the different colours of stars - no fancy equipment required. Just my D70 and tripod.
Take long exposures (20sec or so) with a defocused lens. Then the star image is spread out and the colour really clearly demonstrated.
For example here is alpha/beta-centauri and crux.

This was done on purpose after finding out how this would happen accidentally last year - took a bunch of constellation photos only to find the focus was miles (light years in fact) off.......

[joe_smith]

it dose bring the colors out looks pretty cool

[Rodstar (Rod)]

You can do the same thing, just by defocusing the image, to visually observe the colours better. I must say, though, that imaging the defocused image makes the colour differences even plainer, and avoids the tricks our eyes can play on us when visually observing two contrasting stars within close proximity of each other.

[sheeny (Al)]

G'Day Lee,

It looks quite effective doesn't it.

I have a copy of David Malin's "A Celebration of Colour in Astronomy". The first two images are "similar but different"... They are images of the Orion and then Crux and the Pointers as you have done, but what David did was start off in focus, and periodically tweak the focus out a little bit more each time as the stars trailed. It looks quite effective, and gives a good indication of both colour and brightness. No doubt it takes a little bit of practice to get the timing / focus tweak right so it looks like a uniform progression!

To illustrate I've done a quick scan of the crux image - please note this image is copyright David Malin. I have only posted it here to demonstrate the technique - and please note the scan does not do the image justice! The original print is superb!

Al.

[ving (David)]

very interesting! might have to give it a go

[RAJAH235]

FWIW....There was an article on how to build a 'Automated Stepped Focuser Unit' for just such a procedure, back in Sept. '98 in US Sky & Telescope, by Mr. Ernie Piini. He used a simple base board to hold the camera & the drive assy., with limit switches at either end to stop & reverse it, etc etc etc. (Pic. explains more). The electronics controlled the stepper motor, time interval & how much the focus changed in each step.... Was very good set-up. Bit rough around the edges but gave very nice results.
See attached for few details...
Another interesting little project for those so inclined..... L.