Saturday, January 31, 2015

Star / Galaxy imaging with Stromgren Filters

Rob Hawley has a blog post about using Stromgren narrowband filters to image galaxies. As galaxy season is approaching, I thought that I give it a try.

Getting the filters wasn’t too easy. The wider Ha (32nm) filter wasn’t too hard - Baader has these filters. But the Stromgren filters were more difficult. I asked at several sites and nobody had them. Astrodon had them on their website, but doesn’t sell them anymore (why are they then still on the web site??!!) But Don from Astrodon told me that I should check with Omega Optical. They have a HUGE selection of filters!!! I asked, and indeed they could get me some. They had to cut them from square filters - which made it not too expensive!

A few weeks later, they were in the mail. Boy are they thick!!!
<photo of filters>

But I could insert them into the filter wheel without any problems. 
<photo of filters in filter wheel>

Of course, this will mean that their focal point is VERY different from the other filters! So, I measured their focal point and entered the different values into SGPro, so that it would at least adjust the focal position on a filter change - which should make autofocussing much faster.

My first targets were:
  1. A G2V star <which one??> for calibration
  2. The double cluster in Perseus to check how well the filter work for stars
  3. M82 and M<???> to check how galaxies come out (and compare to my image of M82 from last year that I made with LRGB filters)
  4. Markarian’s chain

My first challenge were the focus position changes. SGPro still has the issue that the FLI Atlas Focuser can’t handle multiple focus change commands (when it receives a second one, the FLI driver throws an exception that gets swallowed by SGPro, i.e. the second command doesn’t return properly). But I could deal with that (with lots of manual intervention :-(

First, I wanted to use the G2V star image to color calibrate the filters. But I’m not sure how to do this. The filters I have are:
  • Ha = Red
  • Stromgren B = Blue
  • Stromgren Y = Yellow
I.e. I need to figure out how to calculate the green channel from the Blue and the Yellow channel. Asked on various mailing lists how to do that. The main feedback was to just use Yellow as the green channel and see how bad/good it is.

Until I hear back, I worked on the other images. First the double cluster. I calibrated and stacked the images as usual and then used the StromgrenY filter as the green channel and used the colors as is. This is the result:

Compare this to other images, the colors don’t look too bad. The red stars come through, but blue seems to be too weak.

Another observation is the strange halo around brighter stars with the filters:

I hope, this is not because of the thickness of these filters. I asked on the ccd-newastro mailing list if somebody has an idea. Only Ron Wodawski replied, but he said that he needs more data.

Finally, I processed the image of M82 and <other galaxy>