Solar photography

Here is some advice on solar photography, given by Franky Dubois (), Amateur observatory "Adonis", Langemark, Belgium
Any beginner wishing to make up solar photography is going to ask some questions about:" How can I reduce the intense light of the sun , which camera and film I have to use and how to develop these films". What follows is a brief explanation of the accessories and thechnics I use in my observatory with my 125 mm refractor. The light reducer.
One of the numerous possibilities for reducing the intense light of the sun is the use of a solarprism.The prism can be used just as well for solar photography as for visible observations. The solarprism used by me is a 2 inch Helioscope with Herschel Optical Wedge ( Carl Zeis wedge prism ) from Baader Planetarium in Mammendorf Germany.
The main characteristics of the prism are:
Entrance: 2 inch
Exit : for use of 2 or 1 1/4 inch oculairs and various camera adapters.
Method of reducing light and heat : reflection
Transmission of light : ± 90%
Absorption : ± 5% 
Total reduction of light : ± 95%
Type of image : reverted
The Prism is delivered with four neutral density filters ( D= 0.6 / 0.9 / 1.8 and 3.0 ) of B&W ( Schneider-Kreuznach ).
The camera.    In principle any single lens reflex camera can be used for solar photography. The camera used by me is a Olympus OM1 body provided with a varimagni finder and winder. The normal viewscreen is removed and a reticulated transparent focusing screen ( 1-12 ) is added. 
The Photographic Emulsion
    In order to photograph the smallest details the film must be exceptionally fine grained, and therefore you have to use films of low and middle speed. I have good experiences with the Agfa Ortho 25 film. Agfa Ortho is an orthochromatic film which means it is red-insensitive. The film can be purchased in 35 mm ( 36 exposures or 25 meterrolls). 
Developing
    For solar photography as in all astrophotography, the general rule is that all photo processing should be do-it-yourself to obtain the most satisfactory results. The film should be developed in an energetic developer to make the best use of the sensitivity and in order to get high contrast. I use Rodinal in a dilution of 1 to 15, this is one part developer and 15 parts of water. The film is developed 6 minutes at 20°C, agitation the first minute, after this every 30 seconds.
Results obtained

This photograph was made on the 1st of september 1989 with the 102mm Polarex f/15 refractor with a Hershell wedge exposed for 1/500th second on Agfa-Ortho 25.	These two pictures are of the same group in March 1989, made the same way of the above. This group was the largest group of the 22th solar cycle !
This picture was taken on the 17th of june 1989. The group was about 2340 MVH big and so the 7th gratest of the 22th solar cycle.	This pictures shows the partial solar eclipse of 12 October 1996 made with the 125mm f/20 refractor with a Hershell wedge exposed for 1/1000th second on Afga-Orho 25. Note the fine lunar details on the side ! No sunspots where visible that day.
After a relative long period of very low activity, finaly a bit movement on the sun. NOAA 7999 was visible from 21 november till 3 december 1996. This picture was made on the 25the with the 125mm refractor. The group was visible with the naked eye.	This is a recent picture of the sun. It was made on the 10th of september 1997 and it shows the three visible groups of that day. Several amateurs reported naked eye visibility of the two gratest groups.