Sky&Tel Sept - pictures p77 & p90, please explain

[Stu]

Has any one else got the september issue of Aus Sky and Telescope yet?

There are two pictures that have me a little confused. There's one of the trifid nebula on page 77 and other of an area in scorpio on page 90. Both shots use the basically the same type of gear: small APO's and big ass CCD's. But one shows stars that have big spikes coming out of them (like how kids would draw stars) and the other shows stars that are perfectly round.

Is this a telescope issue? Filter effect?

[iceman (Mike)]

Us newt owners get diffraction spikes (as they are called) because of the spider veins holding the secondary mirror in place.

The guys with APO refractors don't get diffraction spikes, but some people like the effect they add to an image, so they tie a piece of fishing line or wire in a cross pattern across the front of their lens.

It then gives you the diffraction spikes in a long exposure image.

I love the spikes, I think they look great.

[elusiver]

whats' RC?

el

[cventer]

That Triffid image was taken by Mike Sidonia with an Orion ED80.

He puts string across the front to make the spikes. He likes to take the **** out of the RC owners showing them sometimes comparable images from a $699 telescope.

Regards
Chris Venter

[iceman (Mike)]

El, the RC's are Richety-Cassegrains (I think that's how they're spelt). Very expensive telescopes.. but built for astrophotography.

[RAJAH235]

If I may add a small correction here, It's 'Ritchey Chretian'. They are, as Mike said, very expensive & mostly used/designed for Astrophotography. L.
Examples here > http://hiddenloft.darkhorizons.org/
98.

[iceman (Mike)]

Thanks Loz, knew I had it wrong

[acropolite (Phil)]

I don't like the spikes, I think they look chocolate boxy, I don't see spikey stars when I look with my naked eye so I prefer photo's to be the same.....

[Stu]

Thanks guys. I'm not sure whether I like them. But I can see how Mike could find the idea amusing, on the other hand it's also very creative.

I thought that most people only used refractors for photography because the contrast was better and something to do with the center of the image being clearer. But it looks like there is a diffraction spikes verses non-diffraction spikes debate as well.

So iceman, you are saying that if you do short exposures with a Dob or RC that do won't get the spikes?

[iceman (Mike)]

You might still get some, but they won't be as bright. The longer the exposure, the brighter/larger the diffraction spikes get.

I've even used the technique on my digicam, when taking long-exposure (7s or more) on a street scene for example, I held two pieces of flat grass in a cross pattern across the front of my camera lens, and out came a nice "star cross filter" effect on the bright street lights.

[seeker372011 (Narayan)]

Originally Posted by cventer

sometimes comparable images from a $699 telescope.

down below $600 if you go for the clones-Saxon, synta, bauer