Lens issue? Purple stars on one side

[twomonger]

Hi astrophotography forum members,

I took my Pentax K-1 + 35mm f/1.4 Sigma Art to take some pictures of the stars during the most recent new moon. I used this lens a few times before, but had previously stuck with my 24-70mm. I also hoped to make some stitched panorama landscapes. Unfortunately, post-processing uncovered a nasty issue in almost all of the images. The stars on one side of the lens were all very purple, almost like a bad case of chromatic aberration but not uniformly distributed.

This lens had worked great during the day, but shooting the night sky seemed to bring out this serious error. The two example shots below were taken on different days, and the purple stars on one side . The effect is even more prevalent in Raw, and basically made creating a panorama impossible - at least one without purple stars all over the place. I can send the RAW files to anyone interested.

Anyone encounter this problem? Should I send the lens for repair? Unfortunately it is a second hand lens. All help is very much appreciated.


10 sec, f/1.4, ISO 3200, OOC JPEG, no Astrotracer


10 sec, f/1.4, ISO 3200, Raw without processing


80 sec, f/1.8, ISO 1600, Astrotracer ON, OOC JPEG


80 sec, f/1.8, ISO 1600, Astrotracer ON, Raw

 

[Tristimulus]

Most likely the lens is not parallell to the image sensor (tilted lens).

Coma is rather evenly distributed in all corners so the optics seems to be well centered.

If getting chromatic aberration (purple fringing) all over the field, the lens is slightly out of best focus.

 

[Astrophotographer 10]

Most likely the lens is not parallell to the image sensor (tilted lens).

Coma is rather evenly distributed in all corners so the optics seems to be well centered.

If getting chromatic aberration (purple fringing) all over the field, the lens is slightly out of best focus.

This is typical of many fast lenses. Coma I the corners and chromatic aberration around the brighter stars usually purple/magenta or blue.

Use the purple fringing tool in Lightroom to correct or pull back purples in Photoshop or stop down the lens to the point it stops (probably about F2 at a guess).

There is nothing wrong with your lens, this is normal for most fast lenses.

Greg.

 

[twomonger]

Most likely the lens is not parallell to the image sensor (tilted lens).

Coma is rather evenly distributed in all corners so the optics seems to be well centered.

If getting chromatic aberration (purple fringing) all over the field, the lens is slightly out of best focus.

I am thinking its possibly a tilted element too.

Some images also show stars with green fringing on the other side of the frame, which to me hints that one side is focused too near, and one side too far. I'll try tightening down the screws of the lens - this 35mm art was sent back for mount repair for the K-1 (the well known scratching problem) so I'm hoping the uneven focal plane is just due to an issue with the mount. Or I'll have to send it back

Agreed about the focus - it can be tough to be sure out in the dark, even with the help of magnified live view. Although I'm somewhat confident that it isn't missed focus, since whenever I use the 24-70mm f/2.8 or 15-30mm f/2.8 I tend not to see similar issues with CA/purple fringing.

 

[twomonger]

Most likely the lens is not parallell to the image sensor (tilted lens).

Coma is rather evenly distributed in all corners so the optics seems to be well centered.

If getting chromatic aberration (purple fringing) all over the field, the lens is slightly out of best focus.

This is typical of many fast lenses. Coma I the corners and chromatic aberration around the brighter stars usually purple/magenta or blue.

Use the purple fringing tool in Lightroom to correct or pull back purples in Photoshop or stop down the lens to the point it stops (probably about F2 at a guess).

There is nothing wrong with your lens, this is normal for most fast lenses.

Greg.

I did try bringing it down to F/2. This was the result. Still a lot of purple stars, especially on one (right) side of the frame.


35mm @ f/2.0, 80 sec, Astrotracer ON

The tools for removing purple fringing and CA in PaintShop Pro and AfterShot (the programs I use in my workflow) have had little success with this type of fringing. I might have to give Adobe a try despite my dislike of them... am also using RawTherapee's tool, which has given some promising results so far. I'll try reprocessing my Raws in that

On an unrelated topic, I've still been trying to assemble panorama, purple stars and all, for practice purposes. I've run into two problems. For this particular panorama, I took two series of shots. One set tracked for the sky, and one set untracked for the foreground. However, merging foreground panorama with the sky panorama is much harder than doing so for a single foreground and sky shot. The issues with panorama projection and distortion are really frustrating... I spent all night trying to figure out how to do it. should I premerge all of my images with foreground before sending them into the panorama editor? Or is there any other way of doing it? I've tried both Hugin and Microsoft ICE.

Another problem is that I've encountered is with inconsistent stitching of the earth/sky border in some programs. Hugin does a poor job with it, while Microsoft ICE does much better.

I've attached all the Raws I shot in the RAR file linked below for those interested in taking a look.

Dropbox link