NGC4216, NGC4206, NGC4222
NGC4216 together with NGC4206 and NGC4222 Galaxies - Click here for full resolution
NGC 4216 is a metal-rich intermediate spiral galaxy located not far from the center of the Virgo Cluster of galaxies, roughly 55 million light-years away. It is seen nearly edge-on. NGC 4216 is one of the largest and brightest spiral galaxies of the Virgo Cluster, with an absolute magnitude that has been estimated to be −22 (i.e.: brighter than the Andromeda Galaxy). NGC4206 and NGC4222 are neighbouring galaxies and are also part of the Virgo cluster. They are 70 and 60 light-years away respectively.
source: Wikipedia
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NGC4216
UGC7284, PGC39246, Virgo Cluster
Galaxy
Virgo
12h 15m 54s
+13º 08’ 58”
13 Apr
66º S
Conditions
NGC4216 is a typical spring galaxy with best visibility around April. The reason for imaging this galaxy and its neighbours was that the object had been chosen as Object of the Month at my local astronomy club. This was the second target imaged from the Remote Observatory at IC Astronomy in Spain. Until around 04:00h it was visible above 30° altitude, giving about 6h of maximum observation time per night. The crescent moon was below the horizon during all imaging sessions.
Equipment
With this being the second image taken from the remote observatory, the rig was kept in its original configuration, with one exception. Between the first two and last two sessions, the shroud was taken off. As the sessions continued, more and more automation was applied in Voyager. A dragscript was developed that was designed to support a full night of imaging, with automatic startup and shutdown, automatic object selection based on visibility criteria, etc. By the time this object was finished still some tweaks had to be made, but overall the scripting worked very well in Voyager.
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Planewave CDK14, Optec Gemini Rotating focuser
10Micron GM2000HPS, custom pier
Moravian C3-61000 Pro, cooled to -10 ºC
Chroma 2” RGB unmounted, Moravian filterwheel “L”, 7-position
Unguided
Compulab Tensor I-22, Windows 11, Dragonfly, Pegasus Ultimate Powerbox v2
Voyager Advanced, Viking, Mountwizzard4, Astroplanner, PixInsight 1.8.9-2
Imaging
For exposures, the same settings were applied as with the previous M95 target. This involved 300s exposures at gain 0, using only R, G and B filters. No luminance was included. The overall field of view was perfect to include some neighbouring galaxies from the Virgo cluster. Over a total of four sessions during 1.5 week, more than 20h of exposure time was collected. This is one of the benefits of a remote observatory with favourable climate conditions, it is relatively easy to collect a lot of data on a target in a relatively short amount of time.
Resolution
Focal length
Pixel size
Resolution
Field of View
Image center
9530 × 6339 px (60.4 MP)
2563 mm @ f/7.2
3.8 µm
0.30 arcsec/px
47' 53.8" x 31' 51.6"
RA: 12h 15m 54.165s
Dec: +13° 09’ 00.12”
Within the main galaxy NGC4216, the bright supernova SN 2024gy is visible as a bright star. This supernova was discovered by Koichi Itagaki (Japan) on 04 January 2024 as a magnitude 16 star.
Supernova SN 2024gy is visible in NGC4216. Click the image for a full-screen view.
Processing
All images were calibrated using Darks (50), Flats (25), registered and integrated using the WeightedBatchPreProcessing (WBPP) script in PixInsight to R, G, B images. A very mild, mostly linear gradient was removed using the new PixInsight process GradientCorrection. Reports on this tool do vary, but in this case the tool worked well in the default settings. The individual colour channels were combined to an RGB image.
From here processing followed the standard pattern. SpectroPhotometricColorCalibration (SPCC) was used to get the correct colour balance. Before SPCC could be used, the image was plate-solved using ImageSolver. This time ImageSolver immediately gave a solution, in contrast to the previous M95 image, which was impossible to solve in PixInsight. When BlurXTerminator (BXT) was applied, a lot of detail in the galaxy’s resolved.
To stretch the image, first a very mild run of GeneralisedHyperbolicStretch (GHS) was applied. Mild here means local intensity set to around 2. When set to a higher value, star centers very quickly saturate. For the stretch of these images, HistogramTransformation on its own would do a great job. But GHS has the benefit of the ‘colour’ mode, which, blended at about 0.7, pulls out a lot of colour from the image, resulting in nice red and blue stars and a warm hue on the galaxy. A second stretch using HT finished stretching the image.
NGC4222 (click to enlarge)
NGC4216 (click to enlarge)
NGC4206 (click to enlarge)
A few CurvesTransformation runs were applied to add a little bit of contrast and saturation. Separately the ColourSaturation process was used to selectively boost the blues a bit, which was especially beneficial for NGC4206 and IC771. The next step in the processing was noise reduction using NoiseXTerminator (NXT) at about 0.7. Finally BackgroundNeutralization was used to set the background level to 0.07, so that between each other images all have a somewhat similar background level.
Little gems are all over the place. Here IC771, a small barred spiral galaxy.
There is a lot of detail available in this image, and it is a joy to scroll around. See above the individual galaxies cropped out of the main image. But also little gems, such as IC771 are very nice objects to discover. The best way to stroll through the image is by zooming into the full-resolution version, that you can find here.
Processing workflow (click to enlarge)
Star quality
With the shroud on (left two images), an extra diffraction spike is visible. When stacking images from both sides of the meridian, this gives the pattern of an extra set of spikes. The quality of focus/seeing does not seem to influence this much. With the shroud off (right two images), a ‘blob’ on the bottom side of the star is visible (third image) under poor focus/seeing conditions. As the focus/seeing improves (fourth image), the star is more symmetrical, but overall star surface is larger than with the shroud. Images past meridian are rotated 180°, to compare the same scope/camera positions.
During processing of the first image from the new remote observatory, M95, a double set of spikes were very apparent. One suggestion was that such a double set of spikes could be caused by the shroud bending inwards, blocking part of the light-path. This is a phenomenon that has been often described for the CDK-series of telescopes. To fix this, shroud spreaders have become available from both Planewave and Rouz Astro. I had opted for the latter, so in theory these double spikes should not come from the shroud. But to eliminate this option, the shroud and the rods were taken off completely. Interestingly, this revealed another curiosity in the star shape. As can be seen in the third image from the left, the star surface shows a bit of an asymmetrical ‘blob’, reaching out towards the bottom, in opposite direction where the spike occurs in the image with the shroud on. At much better focus/seeing conditions, this asymmetry largely disappears, but the overall tightness of the star is less than with the shroud on.
Interestingly, as I was writing down this data, a discussion on Astrobin was started by another astrophotographer finding the exact same thing using a CDK14. One conclusion from that discussion up to this point, is that the 1/4” holes in the front ring of the CDK14 truss-frame could act as little point-sources of light. This would have the potential of causing aberrations that might be able to make their way past the secondary mirror baffle, causing artefacts probably mostly off-axis. Taping off these holes showed indeed markedly improved star-shapes off-axis. So one improvement I will make is to close those holes as well. Hopefully in the thread more possible explanations and improvements will surface. If so, this page will be updated accordingly.
Overall, the differences between shroud on or off are not very big, so either way will probably work. The preference is to take the shroud off, as the scope becomes less sensitive to wind, and tube-seeing can be better controlled. To be continued…
This image has been published on Astrobin and received Top Pick Nomination.
