Post by brianb on Apr 21, 2008 9:48:31 GMT
On Friday night, with a bright near-full Moon, I decided to try deep sky imaging using my unmodified Canon 5D and a B+W 091 (deep red, = Wratten 29) filter - the theory being that, as scattered moonlight is brightest in the blue, it might be possible to eliminate most of the troublesome background light. The cut off of the 091, unlike the 090 (Wratten 25A), is also far enough into the red to suppress sodium light pollution.
The theory seems to work reasonably well. Here's an image of the "Castor's foot" area of Gemini taken with a 200mm lens, 091 filter, stack of 6 x 2 min exposures at f/4, using Astrotrac driven mount:
Shows up M35 nicely, and there's even a hint of the emission nebula I.443 just above the chevron of stars between Mu and Eta Gem.
The image as in monochrome (obviously enough as it contains almost nothing except red light). This points out a problem I discovered with DSS - to get a decent image I had to convert to monochrome and save as TIFF rather than letting DSS use the RAW images. I don't know why but DSS just wouldn't dig into the filtered frames properly, the finished stack actually showed rather less than the individual raw frames.
If you have a modified DSLR the B+W 092 filter, which is very deep red to the eye and transmits mostly in the infra red, would probably work even better.
One problem I discovered when doing this test. Though sufficient light gets through the 091 to focus on a first magnitude star, it is very hard to see much in the viewfinder, making pointing at a target a problem. I've since manufactured the device illustrated below, from a very old miniature flash gun and one of those cheap and nasty 8x21 monoculars, though a small finder scope would work just as well. The wood wedges are to strengthen the joint, and help keep the thing aligned.
Obviously it just fits into the camera hotshoe! BTW the flash is non functional, the batteries have been removed!!!
The theory seems to work reasonably well. Here's an image of the "Castor's foot" area of Gemini taken with a 200mm lens, 091 filter, stack of 6 x 2 min exposures at f/4, using Astrotrac driven mount:
Shows up M35 nicely, and there's even a hint of the emission nebula I.443 just above the chevron of stars between Mu and Eta Gem.
The image as in monochrome (obviously enough as it contains almost nothing except red light). This points out a problem I discovered with DSS - to get a decent image I had to convert to monochrome and save as TIFF rather than letting DSS use the RAW images. I don't know why but DSS just wouldn't dig into the filtered frames properly, the finished stack actually showed rather less than the individual raw frames.
If you have a modified DSLR the B+W 092 filter, which is very deep red to the eye and transmits mostly in the infra red, would probably work even better.
One problem I discovered when doing this test. Though sufficient light gets through the 091 to focus on a first magnitude star, it is very hard to see much in the viewfinder, making pointing at a target a problem. I've since manufactured the device illustrated below, from a very old miniature flash gun and one of those cheap and nasty 8x21 monoculars, though a small finder scope would work just as well. The wood wedges are to strengthen the joint, and help keep the thing aligned.
Obviously it just fits into the camera hotshoe! BTW the flash is non functional, the batteries have been removed!!!
