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Posted by M C Butcher at 11:07 on 2011 May 12
Help! I would like to create an Analemma image. I understand how the photograph was created in the era of film cameras, I assume it must be easier with a digital camera and whilst I understand what I am trying to achieve I would welcome some advice on how to achieve it. I have a stock Canon 40D, a tripod and a remote controller for the camera, a computer and the necessary image processing software (Photoshop CS5). My queries are:1. I assume I can photograph the Sun without the use of a filter (I have a ‘White light’ filter that I use with my LX-90). If so what are the recommended settings for exposure/focal ratio/ISO etc so that I can both record the Sun’s disc and the terestial background?2. If I have to use a filter, then how can I align the different images of the Sun as none of the terrestial background would be recorded on the image and it is this that I would expect to use for aligning the images? Of course I could follow the film camera practise of clamping down the camera before each image was taken but this would seem to be little easier than with a film camera.Can anyone help?Martin Butcher
Posted by Andrea Tasselli at 18:09 on 2011 May 12
Hi Martin,First off analemmas as best created with cameralens as the field of view required is normally far too large for most scopes. Depending on the lens choice this will severely restricted the size of the sun disc on the image. If you won’t use filters (such as baader’s astrosolar) than you should aim at taking the sun’s shots fairly low on the horizon and use the shortest exposures @ f/22. ISO could be around 200. Using filters you’d probably need to open up the lens quite a bit and adjust exposure as required. I’d keep ISO as low/high as required. Just experiment a bit and you’ll find the right combo.Now for the second question. You’d need either to clamp down the camera or the camera tripod (assuming you can move the camera in and out of the tripod without disturbing the aim) for a whole year or you need to have a reference in the field of view that won’t move for a year and a way to exactly match the position of the reference object before taking the shot (this is quite difficult to do unless you use an alt-azimuth mount with slow motion controls). This can be done regardless of whether you shooting without filter or with filters as you’d just need to remove the filter, align the camera and put back the filter before taking the shot in the later case. The way to do is the analemma is to know or estimate the exact position of the sun at the winter solstice and at the summer solstice at the same UT at your location. This will set the requirement of the maximum focal length you can use (and hence the size of the sun’s disc on the sensor) and how you can emplace the whole set-up with regards to the local features you’d need either to include for reference purposes or for the landscape shot to be overlapped against the analemma.This is just to get started. The nitty-gritty details can be discussed later.Hope it helpsAndrea T.
Posted by M C Butcher at 10:19 on 2011 May 16
Andrea,Many thanks for your response. I was planning to use a 20mm f/2.8 lens. Unfortunately I think your second paragraph indicates that there is something about this that I don’t fully understand.I appreciate that to get the Analemma all images must be precisely aligned and taken at exactly the same time of day. With a film camera the only way to do this is to clamp the camera to the same spot, with the same alignment for each exposure. However, with a digital camera, provided there is an identifiable object that does not move (eg a house) in the field of view can the alignment of the images not be conducted in the computer using the photo processing software? After all I can routinely take photos of terrestial scenes which I then merge together to give a panoramic view. It was this technique that I assumed made taking Analemma photographs with a digital camera much easier than with film.Or have I totally misunderstood what modern technology can do for me?Many thanks.Martin
Posted by Andrea Tasselli at 17:34 on 2011 May 16
Hello Martin,Well, the problem that I see is if that you use a widefield lens or worse still a semi-fisheye, you’ll introduce certainly some degree of distortions in the field geometrical linearity (those can be of all sorts, barrel, pincushion, cylindrical…). By coarse repositioning the camera each time you’ll run the risk of that the actual position of the sun (assuming it being a near point-like object at that focal length) will be misplaced because of geometrical distortions with respect to the "real" position in the sky (according to the analemma). Now, you might be able to correct it by various software ( I think PS does it) but not really sure whether the reference points you’ll need to have in each shot will be enough to sort this out. Maybe it is nit-picking but better be safe than sorry, as they say! This is worsened (by a lot!) if you intend to use white-light filters, since in that case the correction needed must be extrapolated from other shots as you won’t have any references in each sun shot and I don’t kwow of any program that would allow you to do it (short of doing a lot of heavy mathematics and write the program yourself).Naturally it may be that all this sort of issues are not really of much of significance to you, i.e. you won’t mind if the analemma will come out with a bit of geometrical distortion and maybe some slight offset from the actual position in the sky. I just thought it was worth mentioning…Hope it answers your questions.Andrea T.
Posted by Graham Relf at 18:24 on 2011 May 16
I have discussed some of the maths involved in aligning images when the camera was not pointing in a fixed direction – hereThe algorithm is built into my own image processor, GRIP, which you can download from my site (from the link above). This is non-commercial software I wrote (and continue to develop) for making my own astrophotos. I’d be interested to know whether it helps in making analemmas.(I think your main problem though is going to be getting regular cloud-free days.)
Posted by Andrea Tasselli at 18:37 on 2011 May 16
Graham,The problem is not that of the projection but rather of the optical distortions the lens may have.Reg’sAndrea T.
Posted by M C Butcher at 10:11 on 2011 May 17
Andrea,Once again many thanks. I shall have to give it a try and see how it goes. If I achieve a result of which I am proud then I’ll let you know, otherwise I will have failed. Very many thanks for your help.Martin
Posted by M C Butcher at 10:27 on 2011 May 17
Graham,Many thanks for the introduction to your website. I shall give it a try and let you know how it goes. Of necessity it will be a long term project. Unfortunately I suspect that your final comment will probably be only too true. Fingers crossed!Martin
Posted by Graham Relf at 14:13 on 2011 Jun 01
Yes, GRIP does correct lens distortions, by first calibrating against a photo of a regular grid taken through the same optical arrangement. I have made this more accessible to end users in my latest version: initial description here: http://www.grelf.net/new.html
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