- This topic has 8 replies, 9 voices, and was last updated 1 year, 11 months ago by
.
-
Posts
-
What do people think to composite planetary images. The recent Lunar Occultation of Mars got me thinking as most images being posted are separate process of Mars and the moon, then stuck together in Photoshop. Sometimes the planetary images are taken well before Mars is Occulted by the Moon and simply inserted with part the planet obscured. Many having a lunar surface image take after Mars had gone behind it.
As a minimum the image should be clearly labelled it is a composite image, but often the image starting to be in the realm of artwork, albeit the final image is from data obtained approaching time of occultation. I attached two images one is a Composite and the other is a stack of the event. In practice I found you could only stack 40 frames (3 seconds of video) before the lunar limb starts to streak across Mars. The composite image is clearly nicer? but also a fake. The third image is a single frame during the occultation.Composites should always be marked as such so that it is clear that parts of the image have been processed separately and then recombined. People can then make a judgement about how “real” the image is. I wouldn’t say that a composite is a “fake” since, if done properly, it shows what would have been seen if the limitations of the telescope and camera had not been present. A good example is stacking a moving object, such as a comet, separately from the fixed stars. I agree though that this is open to abuse from people who are less than honest about how the image has been generated.
Hi,
This is my third – and final – attempt to post a comment. During my two previous attempts I vieweed them, clicked on Edit to change some punctuation – and my messages vanished! Argh! (Gone to a parallet universe?)
Try again…again…
The human eye and brain combine to help us perceive scenes with wide ranges of brightness and contrast. My 8-bit video of the occultation is a low quality representation of the view recorded by my biological sensor. I pondered whether to expose for the limb (and have a faint Mars) or for Mars (and have a washed out limb. I chose the latter.
Most observers who submitted composite images (bigger well depth, etc) mentioned that in their Comments. Almost all publicity images from HST and JWST are nothing like the original – we expect that and accept it. Likewise, many submissions to astrophotography competitions have been heavily processed – a combination of good seeing, a well-collimated ‘scope and the observer’s skills with image processing software.
It’s also nice to see members’ images that are ‘straight out of the can’.
Clear skies,
Alex.
Tip – Save a copy of your message to the Forum before clicking Submit.
I have received a number of images of the occultation and in a few cases the observer has selectively brightened Mars (for the reason stated above) and said so. This, provided it is stated, is no worse than if one did the same thing with a drawing. It seems to me that all the observers with whom I correspond do state clearly if they have modified an image. Because emails are not always kept, any modification should be stated UPON the image. The problem of surface brightness is of course more acute with Saturn when it is occulted by the Moon.
By the way, I am hoping I will receive all those nice images from all those who have posted on their member pages but did not formally send the image to the Mars (or Lunar!) Sections. Then your work will be acknowledged online and in any later report in print in the Journal.
Richard McKim
Where does manipulation start though ? For example I would venture to suggest almost every deep sky, comet and planetary image in the gallery has had undocumented processing which affects the scientific value (non linear stretch, sharpening, noise reduction star elimination etc etc). As a science based organisation though contributors should welcome challenge and be able to supply the raw data if required. (With the new website there doesn’t appear to be any simple way to make contact with other members any more though)
Cheers
RobinI agree that there should be very clear comments on what has been done to manipulate ANY image. For planetary this will usually be along the lines of “best 50% of 10,000 frames stacked, sharpened with wavelets and brightness, contrast and RGB adjusted”. The difficulty arises when dealing with images like this lunar occultation of Mars. I have seen several images of this occultation online where the processing comments should read along these lines:
1. Best 50% of 10,000 frames of Mars stacked (taken 60 minutes prior to lunar occultation). Derotated in Winjupos. Wavelets and RGB adjustments.
2. Best 50% of 10,000 frames of the lunar limb stacked (taken 60 minutes prior to the occultation of Mars). Wavelets and RGB adjustments.
3. Capture of lunar occultation taken during the event to show apparent spatial interaction between the Moon and Mars and to collect timing points.
4. Photoshop editing of two separate images (Mars and lunar limb) to merge into spatially correct apparent lunar occultation of Mars.Using a bit of lunar limb and a full-face Mars from Richard’s original data (https://britastro.org/observations/observation.php?id=20221211_181703_df51689fca6965b0) as shown below, it is possible to construct the attached animated gif, which is totally made up, but just a rough representation of what was observed in time-lapse. But this face of Mars wasn’t the one that got occulted by the Moon, it is just the same full face of Mars slipping behind a layer in Photoshop. The issue then is neither the actual lunar limb, nor the actual face of Mars are correct; if the Mars image was taken 60 minutes before the Martian features will be in the wrong place, and the illumination of the lunar limb will be subtly wrong too.
But this all boils down to what is the purpose of the image? If it is for scientific scrutiny then this made-up composite has little value. If it is for maximal visual impact of a lay audience then this method is likely the one to follow.
James
Attachments:
Where does manipulation start though ? For example I would venture to suggest almost every deep sky, comet and planetary image in the gallery has had undocumented processing which affects the scientific value (non linear stretch, sharpening, noise reduction star elimination etc etc). As a science based organisation though contributors should welcome challenge and be able to supply the raw data if required. (With the new website there doesn’t appear to be any simple way to make contact with other members any more though)
Cheers
RobinHallelujah!
Pretty pictures are, well, pretty but the original data should always be easily available upon request.
For the avoidance of doubt: I keep all the subs which I subsequently process, whether or not the result is published.
The hint is in the name. The vast majority of deep sky images are exactly that, pretty pictures. Theres no scientific rationale for taking them and no need for the person doing so to worry about gamma stretching or sharpening – its about showing the structures and elegance in an object in a tasteful and appealing way. Look at how many people return to an object (such as M42 or M16) every few years because improved skills, location or instrument mean they will be able to pick out more detail or create an image more eye catching. The target itself has not changed. It is a quest to get the maximum performance from the kit you can afford: we all know that if we had a PlaneWave 24″ in Chile, an Andor camera and unlimited time on it we could take better pictures, but most of us cannot afford one, so we strive to do the best we can.
With planets its also about detecting features (dust storm, cloud feature, polar cap) but, they change which makes life much more interesting.
Its the change aspect that makes variable stars, the Sun, aurora, meteors and comets interesting too. Its why I spend most my nights chasing variable nebulae.
I have seen people post pics in the BAA Gallery that were clearly composites and they have not mentioned it. That I must admit to disliking.
It seems to me that the purpose of all image processing is to create an image that makes certain aspects of the image available and clear to the human eye. For example choice of colour palette, highlighting edges etc.
They are not intended for making measurements on except in certain special cases eg. distaces between edges. Measurements would be done on the correctly processed linear image.
This is as true for an image prepared for the BAA Gallery as one prepared for a scientific journal.
Regards Andrew
- You must be logged in to reply to this topic.