Moonstruck is a Christmas movie, you know. All together now,
"W h h h e e e e n n n n   the Moon hits your eye like a big pizza pie..."
it's time get the narrowband filter out.

 

December 4-5. The air is in the low 30's F, headed down into the mid-20's tonight and into the teens by the end of the week, so this evening it's easy to make the CCD chip a little piece of Siberia. It's holding -35C with only 50-60% cooling.

This spell of dry, cold weather ("cold" for the foothills of North Carolina anyway) has arrived with the Full Moon. I knew the CCD and H-a filter could work very well to defeat city light. Most city light is emission-line light from sodium and mercury vapor. The filter's window sits in a very dark gap in the skyglow. It has no such spectral advantage under the Moon, so I wasn't sure what could be done on this brilliant, moonstruck evening.

The Veil: The first data I took from the Veil surprised me for two seperate reasons: the darkness of the sky and the birghtness of the target. The brighter arc of the Veil was obvious in just five second exposures. I could actually focus on its structure and not just on the myriad field stars (I used Maxim's image inspection window to finish focusing based on the PSF).

To a good approximation, moonlight is just attenuated sunlight. Guestimate that the window behind the clear, IR-blocking filter is 4000-7000A. A 10nm H-a filter knocks out all but 100A of that spectrum centered on 6563A, so if moonlight is evenly spread across that continuum, then the filter knocks out about 97% of it. So I guess I shouldn't be surprised by the darkness of the H-a filtered sky under these conditions: the 10nm H-a filter reduces the Full Moon to 3% of its unfiltered brightness. A Moon with only 3% of the area of the Full Moon is a very thin Moon indeed. It's an astronomocally harmless Moon. Of course, the filter dims stars and reflection nebulae to the same degree, so it should offer no real advantage in shooting them (and a large advantage when stars are really just so much glare hiding faint fuzzies). However, the filter dims red emission nebulae hardly at all, so you get this huge, double whammy of a contrast boost with wisely chosen targets. Supernova remnants, for example, are just full of H-a light. (Rough and ready calculations above... see FWHM and break out your calclus if you want something closer.)

A 10 minute exposure of the Pleiades (14 degrees from the Full Moon) was instructive but not worth showing off. It showed not a hint of their nebulosity. Measurements of the total light in two 120s images confirm that the H-a frame contains about 3% of the total energy found in a Clear (IR blocked) frame. Lends some empirical confidence to my maunderings above.

The Horsehead: Using the 9x50 finder, I looked through the woods to center Sigma Orionis about an hour before it reached the opening in the trees above the old logging road, a portion of which is our driveway and which continues south and east from my open-air observatory. Sigma Orionis's UV light excites the nebula behind the dark Horsehead. I made a two minute exposure using a conifer exclusion filter to confirm the location and then went inside to do some webwork and to wait. I began taking ten minute frames when I thought the field was near the gap. The first frame was streaked by moonlight on the limbs of pines. That one has its charms, but the next few frames look great!

As I added each ten minute block of data to the image, more details became clear, and the fan of soft light reaching toward and beyond Sigma Orionis became smoother and more subtle. Eventually, the guidestar faded, and I knew the field had passed behind one of the tall pines west of the logging road. I went outside to shut down for the night but saw that in a little while I could sneak in another brief session by shooting between two treetops. The two frames added late contributed most of the field rotation visible in the larger images linked at bottom right, but they also added substantially to the quality of the light fan. Out of nine, ten-minute exposures, six contribute to the images shown here. LSAN (Large Scale Arboreal Noise) compromised the others either by streaking the field with uncontrolled moonlight or (once) by confusing the guider.

Guiding tonight was on magnitude 3.7 Sigma Orionis. Small fractions of a second would have sufficed. To avoid chasing seeing, I used half second and one second exposures. The software seems perfectly capable of finding a centroid in the extended point-spread. I could as easily have guided on 8.5 - 9.0 magnitude stars. Comparing guide exposures to Guide 8.0's charts, I am pretty sure I could have used eleventh magnitude stars. What's the limit with longer guide exposures? Doesn't seem to matter! The DSI finds guidestars anywhere I need them (as Tom said it would).

Next: make some mechanical provisions to insure that I can return the ST2000 to the same orientation so I can build up exposures on multiple nights. Then we'll think about tri-color and color-mapped imaging.

 

The Veil Nebula – SN Remnant in Cygnus
200mm EDIF F2 Nikkor @ F2.8,
SBIG ST2000XM at -35C w/10nm H-a filter
3x600s, darks applied, autocorrelated and
summed in Maxim, range stretched in Maximm
stretched, histogram eq'd, etc in Photoshop CS2

 

Same data, inverted to make
faint details more apparent

 

Flame & Horsehead Nebulae in Orion
200mm EDIF F2 Nikkor @ F2.8,
SBIG ST2000XM at -35C w/10nm H-a filter
6x600s, darks applied, autocorrelated and
summed in Maxim, range-stretched in Photoshop CS2

 

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Cameras behind Telescopes:
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Text & Photos by David Cortner
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