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>

Flats issues - imaging train sagging?

Recently, I took for the first time LRGB images with the super reducer. And I noticed severe vignetting in the corners:

I guess, I should have used square filters and not the round ones… But anyway, I should be able to remove this vignetting with flats:

Calibrating my images with these flats seemed to work. But when I stacked these subs, I images like these:

In some corners, calibration worked well, but in others the flats clearly failed making the image either too bright or too dark.

I checked  the calibration, took new flats, took panel flats - but the result was always the same. I wonder if my imaging train is sagging and that causes the incorrect calibration.

Asked on the ccd-newastro mailing list. Ron Wodawski was incredibly helpful:

• he figured that the reducer should be able to illuminate the entire chip easily (it is specified as delivering a flat field of 65mm diameter - more then enough for my 16070 chip)
• he said to focus on all elements (correct orientation, correct spacers, correct location)
• he also said it could be because of the dew shield - that's easy to test.

APCC

I finally tried out APCC. Installation was fairly straight forward. Connection is now a little bit more complicated (first start and connect APCC, create virtual port and then connect the ASCOM driver to that one). But in the end it wasn’t too bad. At startup when I connect the first program to the mount, APCC starts automatically (good!) but neither does it connect automatically to the mount nor does it create the virtual port. If I’m quick enough, I can make that on startup, otherwise the connecting program times out and I have to attempt again.

One of the first things I tried was to track comet Lovejoy with Horizon. Ray Galack created a very good video that explains how to load the ephemerides from the JPL website and transfer them to the Horizon’s app. I then used the closed slew from TheSkyX to slew exactly to the comet and then tracked it. That worked really, really well. On top, I used PHD2 to guide on the comet itself. That resulted in a guiding errof of less then 1 arcsec!!! Although, I’m not sure if these two programs work really well together:

• Horizon adjusts the mount every couple of seconds (i.e. the scope moves a little bit away from the comet and then it brings it back).
• PHD2 constantly tries to bring the comet back.

I.e. PHD2 makes corrections to keep the comet centered, then comes Horizon and thinks that the comet already moved out and moves it back - which results in more corrections from PHD2.

I didn’t find the time time compare the different modes (only PHD2, only Horizon, both).

One of the most exciting features is the pointing model for astro-physics mounts. Will try to set that up soon - but it might be tricky because of the limited visibility from our backyard.

Unreliability sucks!!!

Somehow my nice, stable setup became insanely unreliable. The most common symptom is that either the Lodestar guider, the focuser or the filter wheel stop working. The get triggered to do something (take image, change filter, move focuser) and just throw exceptions. Restarting the software often helps, sometimes I have to restart the entire computer.

Tried to figure out what the problem is:

• USB connections?
• Tried a different USB hub
• Tried to connect cables directly to the computre
• Tried different USB cables
• Power supply?
• Tried to connect devices directly with original connectors
• Bought a new power supply  - well this was a good idea anyway. Now I don't have any 12V cigarette lighter plugs left. And the new power supply has up to 30A !!!

Nothing made a difference. Then I tried the devices themselves. I exchanged the FLI Atlas focuser for the Robofocus focuser - no difference (only that I experienced focuser slip again).

After observing it more closely, I realized that the guider always seemed to be involved (when it captures an image it throws an exception but only if another device is active at the same time!). Tried an unguided session - but that didn't work either.

At last I thought about the computer itself! I plugged all USB cables into the new USB hub that I bought and connected that back out to my laptop. NO ISSUES!!!

I tried this now several times, every time I use the NUC I get these issues. Every time I use the laptop, everything works!!!

Hmmmmmm, can I exchange the NUC? Get it repaired? A friend of mine pointed me to the DS87 - sligthly bigger but this one would have the advantage that it as a serial connector! (that would get rid of the serial-USB adapter).