The “Scarlet Letter”

I had so much fun taking this image.  The target is huge and faint, with a field of view of about 7 degrees across (~14 full moons!).   In addition to the H-alpha emission nebula, there are some faint reflection nebulae/molecular clouds in the area.   These were hard to bring out of the luminance data, but they add depth to the image.

As shown below, the image has been rotated at two different angles, with the nebula outline highlighted and the book artwork from the 1850 edition of “The Scarlett Letter: A Romance” by Nathaniel Hawthorne (George Routledge and Sons, publisher) superimposed.  Based on its shape and color, it is obvious how this nebula got its name.





I hated the puritan cruelty and judgement this 19th century romance novel is centered around, but the celestial equivalent is just beautiful.  The stars are colorful, as are the dust and blue oxygen clouds that intermingle with the nebula.

Processing Notes

Data was collected on two moonless (but humid) summer nights in mid-August, with above average transparency and seeing in Bortle 4 skies.  My Unihedron handheld Sky Quality Meter readings were 20.78 and 20.81 for August 15 and 16, respectively.  High clouds set in on the first night and I had to toss 41 images, but was left with 25 good ones.  Night 2 yielded 38 good subframes, so I could piece together over 5 hours of integration time.  The final image is a stack of 63 sub frames, each taken using a ZWO ASI294MC camera set at 1x1 binning, 300s exposure; 120 gain, and -10°C sensor temp and UV-IR cut filter.  The manual auto-indexing Nikkor macro lens has a focal length of 105mm and the camera sensor a pixel size of 2.31 µm, so the image resolution is 4.576 arc-seconds/pixel.  The star field has a median FWHM of 1.715 pixels and median eccentricity of 42.07%.  The camera was rotated 35° counter clockwise from horizontal.

Sub-frames were registered and calibrated using PixInsight’s Weighted-Batch Pre-Processor (WBPP) with 2x drizzle.  PixInsight’s amazing and incredible Spectro-Photometric Color Calibration (SPCC) script was used to correct the color of the background, nebula, and stars.  The new artificial intelligence version of GraXpert was used to remove background gradients.

BlurXTerminator was applied in “correct first” mode with a PSF setting of 1.72 and 0% star reduction and 0% halo adjustment.  StarXTerminator was used to create starless and star versions while still in linear state. NoiseXTerminator (50% denoise; 15% detail values) also applied while image remained in a linear state.  Generalized Hyperbolic Stretch (GHS) used to create nonlinear versions of both stars and starless images, then luminance mask used to protect the background while slightly adjust the saturation of the nebula and dust lanes.  The new “ScreenStars” utility script (by Bill Blanchan and Mike Payne) was used to combine the stars and starless images into one.  I absolutely love that script along with its companion StarReduction script, also developed by Bill Blanchan and Mike Payne.

Field Notes

Here are some details about the gear used to take this image:

• ASI294MC camera with ZWO external dew heater adhered to front of camera body
• ZWO filter drawer (2”) with UV-IR cut filter
• Nikkor Micro 105mm f/2.8 camera lens (aperture set to 5.6 for this photo)
• ZWO 52mm ring with Arca-Swiss size dovetail base plate (comes with the ring)
• ZWO Mini guide scope (30mm focal length) with ZWO 120 mini camera
• Fotoconic CL-110N Quick Release Clamp for Arca-Swiss style base plates
• William Optics DSD210 8.27” dual-sided dovetail plate with Vixen/Synta dovetail to match my mount heads.  The rig is shown here on the iOptron SkyGuider Pro, but for this image I used my old Celestron AVX mount.  I wanted the goto and meridian flip capabilities of a equatorial mount to run unattended all night long.
• The plate solving capabilities of the ASIAir software work well with this lens, which is only 105mm.

Not shown is (a) Nikon HS-14 snap on 52mm lens hood, (b) ASIAir Pro mini computer, and (c) two 12VDC AstroZap™ dew heaters for the lens and guide scope.  Where I live, heavy dew is a major problem throughout the year and heaters straps and lens hoods are required practically every imaging session.  My ASI294MC camera is 5 years old and at the time I bought it, ZWO cameras did not feature built-in dew heaters.  It was an add-on accessory.  It works beautifully and plugs into the ASIAir’s power port and can be controlled using the ASIAir software app.   But the dew heater power cable attached to the heater strip needs to secured to the body of camera or else the fragile power leads break off (which has happened to my ASI1600MM camera).  The cable zip-tied to the camera body is a royal pain in the neck when you need to rotate the camera and frame a target.  The newer ASI2600MC has a built in dew heater, UV-IR cut filter, and much, much better resolution sensor.  I suspect it could be a perfect fit for this old Nikon 105mm and I am saving my pennies to get one that I can dedicate to camera lenses.