On three separate nights data were collected on the Soap Bubble nebula PN G75.5+1.7 .
The scope is a William Optics 81mm APO refractor fitted with a ZWO EAF focuser, an 0.8 reducer/flattener, an Altair magnetic filter holder v2 containing either a 7nm H-alpha filter or a 6nm OIII filter, a manual camera rotator and an SV605MC cooled monochrome, 14 bit CMOS camera. The equipment was mounted on a Celestron AVX mount. An SVBONY SV165 guide scope with a QHY-5II-M guide camera was used for guiding.
The imaging rig
On the first night 2h 10 min worth of 5 min exposures were captured in H-alpha light. On the second night 2h 10 min worth of 5 min exposures were captured in OIII light. On the third night 1h 10 minutes worth of 10 min exposures were captured in H-alpha light. These exposures were used to provide extra luminance data and reduce noise in the final image. This gave a total of 5 hours accumulated exposure time on this object.
As usual, the mount was placed on marks on the ground which gives a good polar alignment when care is taken with the placement of the tripod feet. With practice, the polar alignment achieved by this method can be very good.
AstroDMx Capture running on the imaging computer indoors sent the scope/mount, via an INDI server running on the same computer to the star Altair and AstroDMx Capture controlled the ZWO EAF and focused the star using a Bahtinov mask.
Focusing with a Bahtinov mask
AstroDMx Capture was used to send the mount to the star HD 228550 with repeated plate solves until an accuracy of 5 arc seconds. This star is immediately adjacent to the Soap Bubble nebula, so when the star is centred, the nebula is virtually at the centre of the field of view.
PHD2 was used for multi-star pulse auto-guiding and was controlled by a separate Linux computer indoors.
Once guiding has been established and guiding assistance performed, guiding was switched off and the scope/mount was again sent to HD 228550 with repeated plate solves until an accuracy of 5 arc seconds was acheived. Then guiding was switched on again. The reason for doing this is that after PHD2 has performed guiding callibration, it rarely places the field of view in precisely the same position as when it started. Re-centering the reference star or object enusures that is a meridian flip is performed with plate solving to centre the reference star or object, the reference star is centred in the field of view as it was before the meridian flip. This helps with the subsequent registration of the sub frames, minimising edge inaccuracies.
AstroDMx Capture was used to capture the data as described above.
Screenshot of AstroDMx Capture acquiring 10 minute sub frames
With a negative preview, which can help with the visualising of faint objects
Matching dark frames were captured in all cases along with Flat fields, Dark Flats and Bias frames.
The data were registered, calibrated and stacked in Deep Sky Stacker, using the best frame from the H-alpha 5 min exposures as the reference frame, to ensure the alignment of the different H-alpha and OIII channels.
The data were processed in the Gimp 2.10, the starnet++ gimp plugin, Neat Image, Photoscape X Pro and Image Magick.
The Soap Bubble region was cropped out of the centre of the overall image to reveal the final image.
Soap Bubble nebula PN G75.5+1.7 .
Wide field view including the Crescent nebula with the Soap bubble nebula in the centre of the image
