Strange image artifacts with Canon EF 18-55mm

[GuLinux]

Hi,

I know that the Canon EF 18-55mm is not exactly an excellent lens to start with, but I was planning to combine it with my ASI178mm and a SkyWatcher Star Adventurer to do some wide field astroimaging.

My first tests are a bit disappointing however: images show some strange halo around stars, sometimes even "V" shaped.

Attached a test image.

I can't close the aperture right now, I don't have a Canon body, but I might borrow one: do you think this could help solving the issue?

Thanks

 

[Knight of Clear Skies]

It will probably need stepping down a bit, at 18mm I use either f5.0 or f5.6. If you can't control the lens directly you could try a front aperture mask, I use step-down rings for this which screw into the filter thread at the front of the lens, a set only costs a few quid. Thirds-focussing can help, focus on a star a third of the way in from the edge of frame rather than one in the centre - this gives better focus across the whole frame.

The kit lenses are capable of quite good results, this is 20 minutes on the Milky Way at f5.0 with the STM kit lens, which i think is a little better than the earlier ones. Taken at a dark site with a modded DSLR.

The stars aren't perfect but aren't too bad either, it's a bit of a compromise between star-shapes and signal.

[GuLinux]

@Knight of Clear Skies Thank you for the tips,

Can you explain me a bit better the step down rings?

I found a few on amazon, right, but I miss how can they be used as an aperture mask... do you mean attaching some cardboard to it and then screwing it to the lens?

[Knight of Clear Skies]

No problem. Screw together a set of step down rings like this until you get the desired aperture:

Then screw the whole thing to the front of the lens using the filter thread. Cardboard with a hole of the correct size in front would be the alternative.

Unfortunately, looking at it in more detail I don't think a front aperture mask will work with a wide angle lens as the front element is so much larger than the entrance pupil. I've only done this with telephoto lenses.

I'm not sure how you could set the aperture for a kit lens on your camera. Even a camera body may not help, as I believe the aperture is only set when a photo is taken! Only thing I can think of is to set the aperture in manual mode, take an exposure of several seconds and remove the camera battery. Not sure that's a good idea and it may not work as the body may detect the opening of the battery compartment and close the shutter.

Tricky one, hopefully someone else can help. When using camera lenses on astro-cams people usually use front-aperture masks if required.

[GuLinux]

Yes, that's what I though... you would need a 5-6 mm ring to close the lens to the desired aperture, it's not possible.

I already saw a trick for closing the aperture with a canon body, basically you just need to remove the lens, or better to shut down the camera, while pressing the "depth preview" button.

I'm gonna try tomorrow.. if the guy owning the camera remembers to bring it to me

I'm also thinking of buying a second lens, like a Samyang 85mm, fully manual and, as far as they say, very good quality.

[Knight of Clear Skies]

Yes, when I worked out the actual aperture required I realised something was wrong. Reading up on it wide-angle lenses use different portions of the front element to form different parts of the image, so a front-aperture mask just wouldn't work.

Hope the 'depth preview' trick works for you.

[Alien 13]

I would have suggested old M42 fit lenses but good examples at short focal lengths are hard to come by, the use of Samyang lenses is a good way to go for AP and I myself are eyeing up the mighty 135 f/2.

Alan