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Spiky stars


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I bought a Sigma 70-300mm Apo lens for my Canon 1100d. I have been getting some nice results for a relatively inexpensive lens, but i have noticed the brighter stars are a bit spiky. I've had a careful look and it's a star shape. I'm guessing this is from the aperture blades in the lens. I believe one solution is to make a circular mask and put it on the end of the lens. So, does anyone know:

will this work?

how do I calculate the size of opening?

how to cut perfectly circular holes in card or plastic sheet?

are there any alternatives?

an example of the sort of images I am getting - have a look at the bright star near the bottom

OrionsSword_130329_300_8x10_zpsdb5c67d2.jpg

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Did you have the lens wide open?

Those spikes do look like aperture blade diffraction, and AFAIK adding an aperture mask in front of the lens won't make any difference as the aperture blades are deep inside.

If you do want to have a go at making a mask I would try a hole saw, preferably on a pillar drill, though you might get away with a hand held electric with care.

I'm not sure about the calculations, though you could start with the basic equation of Focal Length / F No.

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The stars you have shown here do indeed look as though they have been affected by diffraction artefacts from the lens iris. This is quite common and you either like 'em or hate 'em!! A circular aperture mask can help although I have found that this can simply lead to vignetting. A hole cut out of black card using a compass cutter like this one. I have also used step down rings for this purpose but I am not yet convinced that it works for all circumstances!

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As the tools and materials needed are inexpensive, it's worth a little experiment. I will have a go on a cloudy night, using the lights on radio masts as targets, after all, you can't afford to waste time on a clear night. As the lens covers a large focal length range it looks like I will have to make a numer of masks of different sizes.

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It did work for me and was easy to do. This should explain most of it. I'm sure Steve's right though. Lots will depend on the lens and I think I heard someone say it didn't work at short FL. I've never tried it.

APERTURE%20MASK-L.jpg

Draft excluder strip, black matt spray paint and a compass cutter from a graphics/art shop or the web.

The focal length is known, you decide on the F ratio you want and by dividing the focal length by the F ratio you get the aperture whch you need to cut. I'm sure that for complex optical reasons this will only be an approximation.

I don't like diff artefacts, really. I feel they are most obtrusive on widefield images because there are SO many stars in the field. Forget the Pleiades with aperture blade artefacts!!

Olly

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I use stepping rings on my telephoto lenses with no problem (wide angles will vignette), the 200mm L lens pictured below gives six point diffraction patterns if you stop it down but clean dots with the rings. I use a 72mm - 62mm ring to give me F3.2 and a 72mm - 55mm ring for F3.6, if you search on ebay you can buy these for £1.99 inc postage.

Mel

post-4305-0-07661400-1366029297_thumb.jp

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  • 1 year later...

I use stepping rings on my telephoto lenses with no problem (wide angles will vignette), the 200mm L lens pictured below gives six point diffraction patterns if you stop it down but clean dots with the rings. I use a 72mm - 62mm ring to give me F3.2 and a 72mm - 55mm ring for F3.6, if you search on ebay you can buy these for £1.99 inc postage.

Mel

hi a bit of a post revival, and this may sound like a daft question, but did you also then leave the F stop fully open. I tried a step down ring last night on my 200mm lens but forgot I had left the lens on at f3.6. It's coupled with an Atik 428ex OSC.

The step down ring I used was a 72 to 55mm but I still got some spiking, so I am thinking using the Fstop down and the ring caused the problem.

One from last night, although mot death from aperture blades it's heading in the wrong direction:)

post-11156-0-41447900-1411292982_thumb.p

cheers

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