White Light Imaging
with
Ha' - Oiii - V - Rc - Solar Continum
filters
Solar Photosphere is something that can give the amateur Astronomers, many wonderfull views of our close star. Many try to image the Sunspots and other Photospheric phenomena with filters like the old Mylar and the newest Astrosolar. After years of observing we can say that the best filter to use for Photospheric imaging is a Hershel Wedge, most times and most observers, use that "prism" in combination with a green filter (the Green from LRGB set of filters) and some others us it with a 540nm Narrowband filter (also green).
In this article we are going to show you what happened when we tried to use not only the 540nm (Baader Solar Continum in this casse) but also other narrowband filters, like the Ha' and the OIII filters.
Above you can see the filters used for this test. All the images you are going to see beneath are taken through a 6" f/1200mm Acromatic Refractor, the Baader Hershel Wedge that University of Athens use and a 2.5x TeleVue Powermate with a DMK 31AU03AS monochromatic camera, also with all the filters tested here, we used them in combination with a UV-IR Cut.
Solar Continum filter (540nm)
With the "Solar Continum" filter, Sunspots are more than a nice thing to image. This filter makes the contrast of the image to be in the best possition for viewing the Sunspot with lot of details in Umbra and Penumbra regions.
Also the Faculae are bright and with lots of details vissible on them.
Also the Faculae are bright and with lots of details vissible on them.
Ha' 7nm filter
With the Ha' filter (used for CCD imaging of nebulas) the Sunspot gets a litle darker than with the Solar Continum filter. This makes the Granulation more easy to image and also details inside the penumbra more sharp.
Faculae are well vissibe in this filter also, but they seem to loose brightness, so the imager needs to be cearfull in the prossesing not to underexpose them.
Faculae are well vissibe in this filter also, but they seem to loose brightness, so the imager needs to be cearfull in the prossesing not to underexpose them.
OIII 10nm filter
OIII filter looks much with the Solar Continum filter as they are both green filters and with the pics of their spectral curves to be verry close to each other.
The difference with this filter is that it is more difficult to focus your scope and camera and you must be cearfull with the details of the Penumbra cause the get more dark than with the Solar Continum.
The difference with this filter is that it is more difficult to focus your scope and camera and you must be cearfull with the details of the Penumbra cause the get more dark than with the Solar Continum.
V- Visual Photometric filter
"V" (Visual) Photometry filter is also a green filter. With that filter, Sunspots loose a lot of contrast and also Faculae are more difficult to be separated from the other photosphere.
Granulation is not always vissible with that filter and there are no vissible details in the Penumbra region of the Sunspots.
Granulation is not always vissible with that filter and there are no vissible details in the Penumbra region of the Sunspots.
Rc - Red Photometric filter
Rc (Red) Photometry filter looks a lot like the Red imaging filter (from LRGB set) but has many differences.
With this filter you get similar results with the Ha' (7nm) filter, but it does not give us the same amount of details in the final image.
Here the Sunspots are a bit brighter than in the Ha' filter and this makes the whole image to loose some contrast.
Granulation is well vissible, but Faculae are difficult to separate from the Photosphere.
Also details inside the Penumbra region are not so distinqed as in the Ha' line filter.
With this filter you get similar results with the Ha' (7nm) filter, but it does not give us the same amount of details in the final image.
Here the Sunspots are a bit brighter than in the Ha' filter and this makes the whole image to loose some contrast.
Granulation is well vissible, but Faculae are difficult to separate from the Photosphere.
Also details inside the Penumbra region are not so distinqed as in the Ha' line filter.
Conclusion ...
After all the above we can say that from those filters compared in this article, the best two to be used with a Hershel Wedge to image the Photosphere, are the 540nm (Solar Continum) and the Ha' narrowband filters.
If now those filters are combined with a Mylar or a Astrosolar filter we can not say what will happen, but we thing it is worth trying.
Left image: Center of the Sun showing the Granulation in good resolution. Filter used here was the Solar Continum filter and colour was aded after the image asquision with PhotoShop CS4
All images above are taken at 09 January 2011 by J.D.Strikis "Elizabeth Observatory of Athens"