Comparison of White Light Solar Image Exposure Times

[JamesF]

I almost didn't post this because I can't quite believe the results are so poor for the second image, but here it is. I'll try again tomorrow if the weather holds out. A stack of 60 images, processed from RAW in PIPP, stacked in Registax v6 and finished in Photoshop. Identical processing other than the exposure time.

The first one is 1/1000th @ ISO100 which is what I normally use. This gives me a histogram peak about one third of the way across the graph in APT. The second is 1/320th @ ISO100, which I chose because it gave me a peak about two thirds of the way across.

To me, this second one looks out of focus but I didn't touch anything but the APT controls between taking the two:

And a comparison of the sunspot area at the bottom left:

I don't know if the brighter image caused Registax to have trouble aligning the frames, or if there's some other processing issue, but clearly the 1/320th sec exposure image looks awful. I'll post another test when I can do one, but on the the basis of this result I shall be keeping my histogram output well to the left.

James

[GlassWalker]

Since the air is typically so turbulent, I wonder if the "longer" exposure is catching more of that movement and thus giving you more blur. Also from the close up crops it looks like there is a directional blur. What camera did you use? A DSLR? Just wondering if vibration might be also a factor.

[JamesF]

DSLR (450D) on my 127 Mak mounted on the EQ3-2. It was a bit breezy at times so vibration may be a problem due to the longer exposure time, but all the images were centred and cropped down in PIPP first and I didn't notice any hideously bad frames. I shall keep an eye out for it next time though.

James

[Montana]

Not being an expert with a DSLR (never used one) I would think that the exposure is too long in the second one. You need very fast capture to get a moment of good seeing and with 1/1000 you will have more of a chance.

Fabulous first pic

Alexandra

[JamesF]

I think you're right, Alexandra. Given the piece that sologuitarist linked to last week however I felt the need to experiment for myself and see how it turned out. I'll give it another go just to make sure the poor results aren't a fluke, but my gut feeling is that on an object as large as the sun you really need a fast exposure and if the histogram doesn't get that far to the right then you just have to deal with it in processing.

James