in testing a tracking mount for the eclipse, i encountered an unpleasant surprise: the sun is brighter in the summer! so bright that the sun is now completely overexposed, blowing out any sunspots and surface detail. a few more filters and voila granulation and a few small sun spots (click to see surface detail at full size):
click for full size
Here's an image of the monster sunspot of October 2014 showing a close up of surface detail (click this link for more info on sunspots and granulation):
|AR 12912 10/25/2014|
click for full size
full disk filter discussion:
potential solutions to overexposure on the full disk:
1. additional filter in front of camera--too hard with takahashi adapters
2. stop down aperture--worked provisionally, but hate to throw out light
3. neutral density gel film in front of telescope--not optical quality so blurs image
4. larger filter in front of objective--an opportunity to test more filters :)
tested two types of filters:
-0.6 neutral density IR blocking
the advantage of this is that IR is poorly focused by refractors so it blurs the image a bit, so blocking IR should give a sharper image.
why green? turns out the solar granulation has the highest contrast with a green filter. furthermore, refractors tend to handle green light best.
fortunately, the 77 mm filters fit perfectly on the dewshield of my takahashi FS-60, though it's not threaded.
results: both filters blocked enough light to prevent overexposure. hard to say whether the ND or green filter gave a better image, but stacking the two gave the best results in terms of granulation contrast (subtle difference).
DMK 51 web cam, Takahashi FS-60C, 60 mm aperture at f/4.2 with a reducer. Baader solar film, Tiffen 77 mm green and IR ND.6 filters. The field of view is approximately 96x72 arc minutes. 20 second video capture at 12 fps, aligned in autostakkert, wavelets in registax.
Eastbluff, CA 6/24/2017
camera ZWO ASI 120 MC, telescope Celestron Nexstar 8 GPS (8" SCT), Baader film and IR/UV block filter.
20 second video at 13 fps, 5 ms exposure.
green channel extracted with PIPP, aligned in autostakkert, wavelets in registax.
Eastbluff, CA 10/25/2014