An Experimental Contraption

JForiegner and myself have been conflabbing about a problem with using camera lenses for DSO photography. Because of the iris blades, which cause stars to throw off a wierd sprayed out refraction pattern.

Example - Look at the star below the Nebula.



To get rid of this, you can run the lens wide open. Unfortunately depending on the lens used, this can lead to CA being quite bad. The longer the exposure, the worse it will get.

This lead me to go gawking at Astrophotography forums. This lead to an answer, which lead to me creating this contraption...










It connsists of - step down rings from 86mm to 62mm. On top of that is a Cokin medium filter mount, with a Cokin ClearSky LPF. Then its all mounted on my Sigma 150-500mm, which in turn is stuffed on my K5ii. I have also stuffed on 3 Cokin lens hoods.

This lot will them be mounted on a tracked, and guided EQ mount.

The main bit of kit here is te step down rings. The reduction from 86mm to 62mm will reduce the aperture down to abuot f8 while leaving the actual lens aperture wide open. According to all the info I consumed, it "should" get rid of the distortion caused by the iris blades.

For those interested, and if I read it all correctly... to get the desired aperture you use this formula:

Lens focal length divided by the require aperture.

You then use the step down the rings to get close to that figure. So I am goin to use 500mm and want f8. So this gives me 62.5mm. Hence the step down to 62mm.

Images will be shot tonight, and results posted when I have processed them!!

Thank yiu for you attention, and you can now all bugger of so I can start fannying !!

PS....

For those interested.... There is zero vignetting!!

Aren't you in good company? Doesn't the JWST produce spikey stars due to its hexagonal mirrors, spikes have to be removed in post-processing?

Steve

That's because JST and Hubble are more or less Reflector scopes. Refractors and if it wasn't for the Iris blades, camera lenses too, do/would not produce them.

Small steps forward to improve your final product.

I have this problem with my images as well. I am going to approach this problem in a slightly different way. A large fat bloke dressed in red left me a 3d printer under the tree. My way of producing an aperture mask will be to print a donut shaped item that will either fit snug or be able to be threaded into the front of the lens.

[quote:3496ace15f="Lubbyman"]Aren't you in good company? Doesn't the JWST produce spikey stars due to its hexagonal mirrors, spikes have to be removed in post-processing?

Steve

By all means if someone wishes to correct me, please do. But my understanding of the hubble/jwst spikes is that they are caused by the veins in front of the mirror. It is an artifact of the design of these sorts of telescopes.

Gary