Urb'n Imagin', ver 1.5
- Last Updated: 17 May 2015
By Dave Nakamoto
April 27th was not a good night. First quarter moon in the sky. Strong winds at dusk, and while things calmed down soon after sunset, the dust it raised caused more than the usual amount of sky glow. But bad nights for observing objects may still be good to test new equipment.
In this case, it was the autoguider I got way back in November of last year. The test was to see if stars would jump all over the place, or would they stay nice and round. For that, all I needed was to be able to image stars, so away I went !
Finally ! Autoguiding
So what is autoguiding? It's when you use an electronic device to detect motion of a star, and drive the telescope's mount in order to correct it. Since no mount perfectly tracks the sky, you need to guide, depending on the magnification of your setup and the size of the spurious motions. Imperfections in the gears, shafts, and bearings are the source of most tracking errors.
For my own gear, experience has shown that below 30 seconds, most images show no tracking errors, but this confines my efforts to brighter objects. Removing this restriction means that the only restriction left is the level of sky glow.
Autoguiders have been used by amateurs for decades, but they were custom devices built by the guy who needed it. I remember one such device being described in an issue of Sky and Telescope back in the 60s or 70s. It was pure electronics with no computer. I remember it used two disposable shaving razor blades that were placed at right angles to one another so they could detect the motion of the guide star in two different directions. They were positioned at the focal plane of the telescope. Two photodiodes were used to detect the motion of a guide star by a shift in its brightness. Then the rest of the electronics would generate a compensating signal to the mount's motors. It worked so well, the inventor started an exposure, only to fall asleep. When he woke up, he found that the autoguider had tracked the object right up to the horizon, at which point it tried to drive the mount and telescope further into the ground.
Of course, the original guiders were people, looking through a long focal length refractor, using a reticle in the eyepiece to see the minute changes in position of the star. They used manual controls to correct for those errors, for as long as the exposure took. I read somewhere that Milton Humason took spectra of distant galaxies on Mt Wilson's 100-inch Hooker reflector. Some of these spectra took 100 hours to record, meaning that Humason had to go night after night, perhaps 10 hours per night, guiding manually all the time.
Fortunately technology arrived to same both time and effort. The arrival of small CCD sensors and the personal computer meant that the guiding could be done without human intervention and with far more reliablility. The CCD sensor acts like a 4mm or SMALLER eyepiece, giving the high magnification needed for detecting those small tracking errors. This also means the telescope used for guiding can be smaller.
But this would be the first time I had tried to autoguide.
The First Night
I got mine from Woodland Hills Camera during last year's (2014) Nightfall star party at Palm Desert Resort in Borrego Springs. It consisted of an AstroOptics 50mm aperture 220mm focal length (f/4.4) finderscope with a SBIG ST-i monochromatic autoguider; see the images below.
The finder came with dual rings with centering screws mounted on a dovetail plate. This is a versatile setup. The telescope/camera combination can be used as a 1.5 degrees finderscope. The ST-i can be used as a monochromatic planetary imager through a longer focal length telescope, although for that purpose SBIG makes a color version of the ST-i. It even can do some deep sky work; the exposure times can be set quite long. The ST-i even has a mechanical shutter to take dark frames with.
Autoguiders have come a long way since the days of the ST-4, also from SBIG, about 20 years ago. In those days, you have to figure out how to wire your ST-4 to your mount, which of course needed motorized slow motion motors and controls on both axes. With the ST-i, the correcting motions are fed to the mount through what looks like an old fashioned phone jack, which coincidently, most motorized mounts sold these days have the receptacle for, like my Orion EQ-G mount.
So this is plug-and-play, right ? Well, there are TWO cables to plug in. One wire goes to the autoguider port on the mount. Another is a USB cable leading to your computer, to control the autoguider. Plug the two cables into their respective ports, install and open the ST-i software, then use the focusing option to take a rapid series of images to get the telescope and camera in focus. Then under the menu item "Track" select "Calibrate" to train the ST-i so it knows which mount control to associate with which motion a star makes in its field of view, to make the right corrections. Then under "Autoguide", I started my first track . . . on Capella. Well, I was already there since I used it as my alignment and focusing check star, so why do any extra movement? Besides, I wanted to make sure the autoguider was working before tackling a fainter object or guide star.
I took a 120 second exposure. From previous experience I knew that the mount's periodic error, and other errors, would show up within 60 seconds for sure.
The first image told the story. The star images were round ! Now to test it by imaging faint objects. But there was another piece of equipment I was going to test on the night of April 27th.
To Filter or not to Filter
When I gave my talk about urban imaging to the South Bay Astronomy club, someone asked me whether a light pollution filter would help reduce, or even get rid of, glow from lights. I had not tried filters, but it was worth experimenting with.
On this night, I tested a Lumicon "deep sky" filter I had gotten a few decades ago at RTMC. It allows light in at three selected wavelengths: 91% transmission @ 486nm, which is hydrogen-beta, the green line in the hydrogen spectrum and common in emission nebula; 92% transmission @ 501nm, oxygen-III or doubly ionized oxygen (O-III), also in the green part of the spectrum and commonly found in planetary nebula; and 95% transmission at 656nm, hydrogen alpha, a red line, also very common in emission nebula.
But I should have realized that the moon in the sky would be worse than any city lights. For the most part, city lights can be filtered out because they don't tend to emit light at some selected wavelengths that nebula and galaxies shine at. But the moon is basically reflected sunlight, which shines at all visible light wavelengths. So using a filter is practically worthless, but I hadn't figured that out at the time.
My first target was NGC-2158, a densely packed open cluster next to M35. M35 is too larger to capture in Orion's G3 color camera through my 10-inch f/4.5 Newtonian reflector, and NGC-2158 is both smaller and fainter.
But while a 120 second exposure showed nearly round stars, the image also showed a lot of sky glow. No small wonder, since the moon wasn't that far away.
So to give the filter a chance, I turned my telescope north away from the moon. I centered on a portion of Ursa Major that was west of the meridian. M81 or M82 ? Well, M81 is the tougher object. While it might be larger, overall it is fainter. I tried a 300 second (6 minute) exposure. The goals were two-fold; to check the tracking on longer exposures, and to see what the filter would do to the images. Here is the "raw" image. No dark or flat frame correction was applied. It is colored, and some contrast expansion applied, but nothing else.
I should mention that I didn't collimate the scope, which probably means the alignment was off. Still, not too bad. Only a small amount of tracking errors. I've seen far worse on exposures of a minute or less ! The small amount is probably due to the one second exposure I was using on the ST-i; a shorter exposure might have corrected this by allowing the autoguider to respond faster.
There are lots of hot pixels in the form of brightly colored dots, but this might have been due to the fact that I had not activated the camera's cooler. Oh well ! Due to the excessive sky glow, not many details were recorded, even with the filter. But the exposure is still remarkable, considering there was a first quarter moon in the sky, lots of haze, and I didn't turn off my driveway lights !
Even with all the technical problems, a lot can be done with this image, if you're willing to fudge things a bit. After loading it into PaintShopPro, I got lazy, and used the built-in one-step-photo fix, a texture preserving filter, some aggressive contrast adjusting, plus scratch removal to get rid of each of the bright colored spots one at a time and the artifact "donut", and got this. Since this is a test image, I don't mind the glow, caused by the Moon shining on the haze, at the bottom (south) of the image.
I don't know if the dark ring around the very central core is an artifact or real; I have not seen it in any images I could remember, and didn't look on the web for comparisons. But another arc of darkness lies outside the core on its western side, and some knots are visible in the spiral arms.
The next target was M-108. I used the same exposure levels and processing as for M-81, and produced this raw image.
Again, after somewhat aggressive image processing through PaintShopPro, I got this.
Finally, just for fun, I imaged the Owl Planetary, M-97. Again, I used the same exposure and processing to produce the initial raw image.
And after being aggressive with PaintShopPro . . .
But these images showed the promise of autoguiding. They also clearly showed the need to image during nights without a moon, and to take all precautions against outdoor lighting, turning off outdoor lights where possible, shielding your equipment from any residual lighting, and pointing where the sky glow is fainter. Even filters cannot correct for everything.
Next time, I hope to reduce the tracking errors more. I also want to see if my Lumicon ultra-high contrast filter would perform better than the galaxy and nebula filter. It allows light from three wavelengths in the green portion of the visible spectrum through. Stay tuned !