Star Trailing, Poor focus, Something else?? - Pleiades

[Icesheet]

Hi,

 

Can see me posting here quite regularly now so apologies in advance if my name keeps popping up on threads! I'm looking for some advice on what exactly is happening in my image below.

 

Last night was my second night out with Star Adventurer and DSLR (also have a Pentax 75 EDHF and GPCAM224 but not comfortable enough with that set-up yet). I decided to go for an 'easy' target in Pleiades. Subs were taken with following equipment and settings:

Equipment- Nikon D3200 with Sigma 70-200mm f2.8 HSM II

10*120s @ 185mm f3.5, ISO 800

10*120s @ 185mm f3.5, ISO 400

 

I've attached a sample sub from the ISO800 acquisitions. At the time it all looked good on the camera screen. Tracking seemed good, focus seemed good and the histogram sat perfectly at 1/4. I could even see some nebluosity coming through so I was a happy chappy.

 

When I got home and viewed things in light and a bigger screen I started to notice smearing of the stars. This seems most apparent to the left edge of the image but is also slightly evident in stars towards the centre. My first instinct was star trailing but they don't appear to trail in a uniform motion. At points the smears seem like they are almost perpendicular to others. Then I thought I must have lost focus somehow, but would this not affect the bigger brighter stars more? To my untrained eye they still seem fairly round. One other observation, zooming in on the brighter stars reveals a thin halo that to me does not seem like nebluosity. Is this CA showing up? 

So is this star trailing, poor focus- both? A problem with the lens? Most of all will I be able to do anything with these subs?

[Victor Boesen]

Great photo, but the stars are very odd as you say yourself.

[vlaiv]

It actually looks like focal plane tilt - so it is up to the lens. Maybe lens is to heavy and not properly supported on camera?

Shapes of stars that you are seeing are combination of coma and astigmatism, added to that they change aberration depending on position in frame - so left side is affected by coma and right side by astigmatism - this would imply tilt and improper collimation if it were telescope. With lens, I'm not sure, it might be a bit to inherent lens design (try using higher F/ratio if you are using lowest F/2.8) and make sure lens is properly seated in camera. Maybe add some support to lens itself.

[vlaiv]

To add to above, it looks like this lens is not really well suited for astrophotography. It looks it is Macro lens - optimized for close shots, and I guess there will be a lot of aberrations when focused at infinity.

This review, also states that there is significant "image softening" when using 200mm setting.

https://www.dpreview.com/reviews/sigma-70-200-2p8-n15/6

[alacant]

Hi. +1 with @vlaiv; if stopping down the lens (try f8) doesn't fix it, there are affordable 200mm lenses which don't suffer such aberrations. HTH.

[wxsatuser]

It will likely be quite good at 70mm but not so good at 135mm and bad at 200mm, this would be normal.

 

[Icesheet]

Thanks for the quick replies! Well I can take some comfort in the fact that it's less user error than equipment shortcomings! 

I picked up this and another Sigma lens (Sigma 50mm F1.4 EX DG HSM) for £150 second hand so it seemed like a good deal. Maybe it still is? If they are useful at shorter focal lengths then I might keep them. At least they should perform better than the kit lens I have. I plan to use my pentax telescope and GPCAM in the long term for longer focal lengths anyway.

37 minutes ago, vlaiv said:

To add to above, it looks like this lens is not really well suited for astrophotography. It looks it is Macro lens - optimized for close shots, and I guess there will be a lot of aberrations when focused at infinity.

This review, also states that there is significant "image softening" when using 200mm setting.

https://www.dpreview.com/reviews/sigma-70-200-2p8-n15/6

I did actually see this review and that's why I went to 185mm rather than 200mm. I'll try 100mm and stopping it down further like you, alacant and wxsatuser suggest. One of the main reasons I plumped for it (other than cost) was that there were some good examples using it on AstroBin. I've just checked back and perhaps those good examples weren't so good! Closer inspection on a few of them seem to show similar aberrations. 

 

However do you think there is still hope for this image, if I perhaps crop and try and bring out some of the nebluosity that is there? Anything I can do in processing to limit the aberrations?

 

Cheers

Chris

[vlaiv]

3 minutes ago, Icesheet said:

Anything I can do in processing to limit the aberrations?

Not sure that anything will help with star shapes in your case. To much of distortion. Usually only way to somewhat suppress out of shape stars is to bin / resize image to smaller size - star profiles become smaller and it is harder to see aberrations. Other than that, there might be some tools that "fix" stars - make them round by altering the image itself, but how successful that will be I have no idea.

[SteveNickolls]

Yeah, not all lenses are created equal where astro-imaging is involved. Here's a decent read regarding lenses for astro-photography-http://www.astropix.com/html/i_astrop/lenses.html There is a good lens review site here-https://www.lenstip.com/lenses_reviews.html which has a number of reviews for lenses suited for astro-imaging.

The Star Adventurer is a good mount and capable to returning good images. My own approach was to start with a 18mm lens and work upwards in FL learning all the time.

Good luck with your future imaging.

Cheers,
Steve

[wxsatuser]

1 hour ago, Icesheet said:

Thanks for the quick replies! Well I can take some comfort in the fact that it's less user error than equipment shortcomings! 

I picked up this and another Sigma lens (Sigma 50mm F1.4 EX DG HSM) for £150 second hand so it seemed like a good deal. Maybe it still is? If they are useful at shorter focal lengths then I might keep them. At least they should perform better than the kit lens I have. I plan to use my pentax telescope and GPCAM in the long term for longer focal lengths anyway.

I did actually see this review and that's why I went to 185mm rather than 200mm. I'll try 100mm and stopping it down further like you, alacant and wxsatuser suggest. One of the main reasons I plumped for it (other than cost) was that there were some good examples using it on AstroBin. I've just checked back and perhaps those good examples weren't so good! Closer inspection on a few of them seem to show similar aberrations. 

 

However do you think there is still hope for this image, if I perhaps crop and try and bring out some of the nebluosity that is there? Anything I can do in processing to limit the aberrations?

 

Cheers

Chris

It all depends on how the image is displayed, on the web resized they can look ok.
I tried my Canon version 50mm f/1.4 Sigma and it was quite poor.

The ultimate lens for astrophotography is the Samyang/Rokinon 135mm f/2, for the price it's a bargain.
You can easily image at f/2, mine is almost perfect across the field at f/2

[Icesheet]

18 hours ago, SteveNickolls said:

Yeah, not all lenses are created equal where astro-imaging is involved. Here's a decent read regarding lenses for astro-photography-http://www.astropix.com/html/i_astrop/lenses.html There is a good lens review site here-https://www.lenstip.com/lenses_reviews.html which has a number of reviews for lenses suited for astro-imaging.

The Star Adventurer is a good mount and capable to returning good images. My own approach was to start with a 18mm lens and work upwards in FL learning all the time.

Good luck with your future imaging.

Cheers,
Steve

Thanks, this was helpful. Seems all is not lost with my lenses and I'm still hopeful I can get something decent if I limit focal length and stop down.

 

17 hours ago, wxsatuser said:

It all depends on how the image is displayed, on the web resized they can look ok.
I tried my Canon version 50mm f/1.4 Sigma and it was quite poor.

The ultimate lens for astrophotography is the Samyang/Rokinon 135mm f/2, for the price it's a bargain.
You can easily image at f/2, mine is almost perfect across the field at f/2

Yes, I hear the Rokinon recommended frequently. I'll keep my eye out for one second hand, thanks for the tip.

[Icesheet]

There were clear skies last night so I wanted to try and test stopping down further and shooting at a shorter focal length. My plan was to do this on Pleiades to keep things as objective as possible of course, but when I got out Orion was on the horizon and I've always wanted to shoot that! So I thought I'd do that first and come back to Pleiades...unfortunately the Bergen weather showed its true colours and rain came in from nowhere. However, here is one of my subs of Orion constellation - 90sec @105mm ISO800 f5. To my eye it seems a lot sharper with less aberrations. Of course the shorter focal length plays into this I guess. Zooming in on the crop below reveals imperfections but I'm not sure how much that has to do with zooming in on a cropped image. 

As an aside I was like a wee boy at Christmas when I seen M42 pop up after the first exposure. Really did not expect to see so much detail from one exposure! I think for the time being I'll keep the lenses and work well within their capabilities, with one eye on a deal for a Rokinon if it pops up.

 

PS the light at the bottom left is either from a nearby street light or the clouds that were approaching bringing the rain

[rotatux]

Very nice subs, stars are ok now. Next step you want to learn stacking

[Icesheet]

Thank you! Hopefully giving it a go tonight