[Content moved from http://photographingtransitofvenus.wordpress.com/]
In my previous post, I mentioned my reasons for ordering the Baader AstroSolar film in preparation for photographing the transit of Venus. In this post, I will describe the process that I adopted for making a solar filter. The filter can be fitted to any D-SLR lens using a standard Z-PRO Cokin ND filter holder. My design choice was primarily dictated by the fact that I will be using my D-SLR with a telephoto zoom lens (most likely my Canon EF 70-300mm f/4.5-5.6 IS USM) instead of a telescope, and I already have the Z-PRO filter holder. The Baader film came with instructions on how to construct a Filter Cell for using any telescope; you could also follow their instructions to construct your own filter. I don’t think, from an image quality standpoint, that there is any fundamental difference between the two ways of mounting the filter in front of your lens. Their recommended way is suitable if you are using a telescope, or you don’t have a filter holder like one mentioned here.
The picture below shows all the material that you may need (I actually ended up using a blade too and didn’t use the gloves at all) to construct the filter. I was extra careful not to touch any part of the film with my figures. Things to note in the picture are two 4×6 size photo mat that I bought from Michaels, the AstroSolar film (unrolled slightly), and the Z-PRO Cokin filter holder on the top right.
The basic idea is to sandwich a piece of the AstroSolar film between two (appropriately sized) photo mat. As pictures are worth thousand words, I will mostly let the pictures speak for each step.
Step 1: As the mats were just a little too big to fit into the holder slots, they were first cut into the appropriate size.
Step 2: The two flaps that will be used to sandwich the film between them are shown below, which were cut into the appropriate sizes.
Step 3: Applying synthetic adhesive to one of the flaps.
Step 4: Carefully place the flap, with adhesive on its bottom surface, on the AstroSolar film. The AstroSolar film does not need to be “stretched”. In fact, Baader recommends that it is more desirable to have slight wrinkles than to stretch the material as stretching will damage the optical quality and the coatings. “Wrinkles, creases and folds in the material are normal and will not affect the function and performance in any way” [quoted from “Making an Inexpensive Filter Cell for BAADER AstroSolar(TM) Material”] (I am not completely sure about the accuracy of their claims, as I believe that parallel light rays incident on the surfaces which are at slightly random angles due to wrinkles, will not emerge parallel. However, without having done any experiments of my own, I will assume that their claims are true since the film is a very thin foil of about 0.012mm.)
Step 5: Cut out the frame with the film stuck to it from the rest of the AstroSolar film.
Step 6: Before sticking the bottom flap, a quick and dirty test was done to check for any pinholes or extreme scratches, that could potentially cause light leakage and loss in contrast, by placing the filter against a bright lamp. You can see the filament ring towards the center-left within the AstroSolar film area.
Step 7: Once I was sure that there weren’t any pinholes or scratches, the lower flap was glued to the rest of the sandwich to complete the filter.
Step 8: The filter was then carefully placed under the weight of two books on either sides and kept for a while till the adhesive dried.
Once the filter was ready, it was time for a quick test. The two photographs below show the completed solar filter fitted to the camera’s lens using a Cokin Z-PRO filter holder.
With the lens zoomed to 300mm, I took a hand-held photo of the afternoon Sun. The picture below shows the area that the image of the Sun occupies within the 22.4 mm x 14.8 mm frame (Canon APS-C sensor). [More on image size of the Sun with respect to the focal length and sensor size will be a topic for another blog post.]
The picture below is a cropped and digitally (zoomed-in) version of the above image. The diameter of the Sun’s image is 417 pixels across, which is approximately 2.67 mm (assuming square pixels) since the pixel pitch for the sensor is 6.4 microns. Although the photograph is not tack sharp (possibly because it was hand-held telephoto shot), one can still see the solar spots on the surface. No, they are not sensor dust, I have double checked that. (Please bear in mind that I wasn’t trying to get the sharpest possible photograph of the Sun in this experiment.)
I think I am pretty happy with what I have here in terms of the solar filter. I will be doing more experiments with it and will write about it soon. Please stay tuned for more.
[This content was originally posted by me at http://photographingtransitofvenus.wordpress.com/, a blog dedicated to the 2012 transit of Venus]