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Hi Greg,

Is it possile to create a solar filter in ST4i to use to simulate imaging of the upcoming transit of Mercury on 11 Nov 2019? If so how would I go about creating a 16 to 18 stop solar filter for use in the Exposure Calculator? I'd like to use the filter with a DSLR camera + lens configuration.

I successfully used a 10 stop ND + IR blocking filter to photograph the transit of Venus in June 2012. The exposure was 1/3200 at f/11. I've since obtained an 18 stop solar filter that I'd like to use for the transit of Mercury.

The attached images of the Venus transit were shot with a Canon EOS 50D & EF 100-400L IS lens with & without a 1.4x tele converter for a 560mm & 400mm focal length, respectively. I used an earlier version of SkyTools (2 or 3 can't remember which) to help set up the shots.


Phil S.

A 10 stop filter reduces the intensity of the light by a factor of 1000x. So what you would do is to create a filter that is 0.001 at all wavelengths. SkyTools expects the data at regular wavelength intervals and the first and last pairs to be zero when reading data from a file. So it could look something like this:

300 0.0
350 0.001
400 0.001
450 0.001
500 0.001
550 0.001
600 0.001
650 0.001
700 0.001
750 0.001
800 0.001
850 0.001
900 0.001
950 0.001
1000 0.001
1100 0.0

To add the IR blocking filter you would need to know the IR cutoff wavelength. Enter zeros for the transmission after the cutoff. So if the cutoff was 930 nm, then 900 nm would be 0.001, and 950 nm would be 0.0. You would stop there.

You might want to use a smaller interval, such as 10 nm. But the idea is the same.

Nice pictures!
Hi Greg,

Thanks for your reply. The filter factor for a 10 stop=0.001, 16 stop=0.000015 and 18 stop=0.0000038. Should I combine the effect of the ND/solar filter with the color filter in the DSLR or just eliminate the color filter altogether? I don't plan to use the IR blocker with the solar filter, the IR blocker was needed because the 10 stop ND that I used in 2012 didn't block IR. At 10 stops of brightness reduction, the sun was still pretty bright. I bought the 18 stop solar filter for the 2017 solar eclipse, but was clouded out  Sad .

Will I need to define a new camera/solar filter for another Imaging System?

Glad you liked the pictures,

Phil S.

The color filter is just a clear filter that I named a color filter because calling it a clear filter confused things. :-)

You should be able to use the same ND filter for more than one camera, given that the color filter doesn't really do anything.
Hi Greg,

If I understand correctly, I should define the new solar filter with the appropriate filter factor then add the new solar filter to the cameras of each Imaging System. Save the modified Imaging Systems. Finally, select the solar filter in place of the color filter for solar imaging. ST4i will then take the filter attenuation into account when calculating the exposures of the sun. Sounds sweet!


Phil S.
Hi Greg,

I created the attached filter file to represent the 18 stop solar filter. The Help file indicated that the transmission should be input as percent transmission, so that's what I did. When I tried to read the filter file, ST4i ver.e crashed with no error message given.

Any suggestions on a fix?


Phil S.
Hi Phil,

Bad news. Looking at the code as currently implemented, there is a lower limit to the transmission of around 0.4%. Anything below that will become zero. I apologize for this. I didn't anticipate the requirements of a solar filter. At the moment there is no workaround.

Although it was a simple matter to stop the crash from happening, it may require major surgery and thorough testing to fix this because the filter data formats may all have to be changed. I do have an idea that might get around that, however. I will look into it and if we are lucky I can have a fix in the next update, which I expect around the end of the year.
Hi Greg,

Bummer, I don't think that the transit will wait until the end of the year  Rolleyes . Is there a way to combine the effects of 2 filters simultaneously to add up to 18 stops? From what you found, even a 10 stop filter with 0.1% transmission is below the allowable 0.4% limit. I think that the next transit occurs in 2023 or 2024, but may not be visible from Ohio in any case.

I guess that the thing to do is work from the 10 stop exposure of 1/3200 sec at f/11 and try 1/50 at f/5.6 for starters.

So far our autumn weather has been pretty clear, but November is pushing things.

I take it that my filter file was correctly formatted.

Thanks for checking this out,

Phil S.