M3
M3 - Click here for full resolution
M3 (R.A.: 13h 42m 11.62s, Dec: +28º 22’ 38.2”), also known as NGC5272, is a globular cluster in the constellation Canes Venatici at a distance of 32,600 lightyear from Earth. It is the first Messier object that was actually discovered by Charles Messier himself. While he considered it originally as a nebula, William Herschel was able to identify it as a star cluster. After M13, it is the largest globular cluster visible in the Northern Hemisphere and contains half a billion stars. There is an uncommonly large amount of 274 variable stars.
Sky-plots with a FoV of 50º (left) and 5º (right). Click to enlarge
Conditions
Images were taken on in late February, early March 2022 from the backyard in Groningen, The Netherlands (53.18, 6.54). Moon was mostly absent. Visibility from this location is best during springtime. Observing conditions were good over the southern horizon, with altitudes up to 65 degrees.
Visibility charts showing annual 22:00h altitude (left) and during the session on 04 March, 2022 (right).
Weather was generally good, with temperatures around freezing. Humidity fluctuated between days with lows of 63% and highs of 96%. Visibility was reasonably good with SQM values just touching 20 mag/arcsec2.
Capturing
Equipment used to capture the images:
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Takahashi Mewlon-180c + 0.8x reducer, Esatto 2”
Rainbow Astro RST-135E, Berlebach Planet
QHY268c, cooled to -15 ºC
Astronomic L3 luminance filter, 2” mounted
Askar FMA180 guidescope, ZWO ASI290MM
Fitlet2 (Linux 20.04), Pegasus Powerbox Advance
KStars/Ekos 3.5.6, INDI Library 1.9.3, SkySafari 6.7.2, openweathermap.org, PixInsight 1.8.9
Frames that made it to the final image:
This image was pretty much first light for this setup. Both the telescope and the mount were new and required a lot of learning before being able to be used properly. This resulted in a total of 5 sessions with many discarded frames and a lot of trial and error. Some general issues and solutions that occurred on both the mount and the telescope:
RST-135E Mount
Guiding was difficult. A DEC oscillations kept playing up and sometimes also disappeared, as can be seen in the below graph. More detailed information and guiding settings can be found here.
Randomly appearing (left side) and disappearing (right side) DEC oscillations gave a lot of troubles in guiding the RST-135E.
The RST-135E mount is a harmonic drive mount, that supposedly can do without balancing and counterweights. From a pure motor-strength perspective, this is absolutely true, and the mount has no problems swinging this fairly substantial load around. But a very unbalanced load does lead to unpredictable flexures in the whole setup. It was decided to roughly balance the mount within the saddle and add the Rainbow Astro counterweight. It made polar alignment much smoother, resulted in better guiding, and overall gave more peace of mind that the whole setup would not topple over.
Originally the guid-scope was mounted on top of the telescope, but the Mewlon-180c does not have good connection opportunities. Therefore the telescope was mounted on a much longer baseplate, which allowed the guide scope to be mounted under the main tube of the Mewlon. This was a much more stable setup.
Mewlon-180c
Straight out of the box it turned out that the Mewlon was not 100% perfectly collimated. Collimating resulted in stars that were quite a bit sharper.
The autofocus routine in Ekos gave unpredictable outcomes. The Esatto has a ridiculously large number of 375,000 steps for a 15mm travel. Some backlash and a slow scope (f/9.8) made it very difficult to find a reliable autofocus setup. Using a semi-manual method with Bahtinov mask worked quite well. On this reflector, focus hardly changed overnight, so focusing once per session appeared sufficient..
The connection of the Mewlon reducer to the Esatto is with a compression ring which was not too solid and required careful positioning to not loosen during the session.
Some of the above-mentioned issues were solved during these sessions. Others have to be worked on separately. As a consequence, many of the frames had to be thrown away and only 178 frames eventually made it to the processing phase.
Image
When processing the images, a further selection was made. Using the SubframeSelector tool in PixInsight, a selection was made of frames with FWHM < 3 and eccentricity of < 0.55. That reduced the total set of frames from 178 to 119 frames that finally ended up in an image with an overall exposure time of 5.9h.
Since this was the first light of the Mewlon-180c, it may be good to spend a few words on overall experience with this telescope. The telescope is fairly light-weight and has a long focal length with moderate aperture. Ideal for small and/or bright objects. This includes moon and planets, but also smaller DSO’s that still have a decent brightness. The light-weight makes it a good companion for a ‘travel-mount’ like the RST-135E. This portability allows for easy travel to a location where there is access to planets or DSO’s that may be low on the horizon.
The telescope does not have a particular flat field suitable for imaging, but with the 0.8x focal reducer there is a decent 30mm image circle. When looking at the final image, it seems like the center of the image is quite sharp and contrasty. The corners however suffer from quite a bit of coma and chromatic aberration (see image below). It is difficult to draw a definite conclusion about image quality yet, given the issues that were experienced around focusing, collimating, and guiding. The image of M13, which is taken later, is already quite a bit better. Although also there, some coma is present in the corners. Overall the refractors are performing a lot better, but the long focal length of the Mewlon gives it its own place. It would be interesting to see what this telescope can do in planetary for example.
An aberration plot from the M3 image. In the center stars are fairly sharp and contrasty. In the corners the image suffers from some coma and chromatic aberration.
The 1760mm focal length of the Mewlon-180c with reducer leads to a quite narrow Field of View. In combination with the APS-C sized sensor of the QHY268c the field of view is only 0.53 x 0.40 degrees. For smaller galaxies and clusters, such as M3, this is actually perfect, and one of the reasons this telescope was purchased. Due to some image rotation issues between sessions, a little crop had to be applied, which resulted in a 16MP image.
Annotated image showing other deep sky objects, stars brighter than mag. 11 and the orientation.
Processing
All frames were calibrated with Bias (100), Dark (50) and Flat (25) frames, debayered and registered using the WeightedBatchPreprocessing script. With the SubframeSelection tool, only images with FWHM <3 and eccentricity <0.55 were selected. See the below results of that process. It essentially meant that whole sessions were discarded in the end. The remaining 119 frames were normalized and scaled using the NormalizeScaleGradient script and integrated using NSG parameters.
Frames with FWHM of >3 or eccentricity >0.55 were deleted from the final image.
Processing the integrated image followed a relatively straightforward path. The OSC camera resulted in an RGB image from the start. This RGB image was processed on its own, and in parallel, a luminance was created from the RGB, to do separate noise reduction and deconvolution on. All with the intention of obtaining the highest amount of detail in the in the image.
In the RGB image, a slight gradient was removed using DynamicBackgroundExtraction. Then using RGBWorkingSpace, the luminance coefficients of all colours were set to 100% to be able to extract the most realistic Luminance from it. Using BackgroundNeutralization and Color Calibration, the proper colours were dialed in. Then the image was stretched using the Generalised Hyperbolic Stretching script. Contrast was increased a touch by applying an S-curve using Curves Transformation.
In the Luminance channel noise was reduced using the MMT noise reduction method, and then deconvolution was applied to sharpen the stars a bit and enhance detail. The star Mask used for this was essentially a binarised and convoluted version of the same luminance image.
Luminance was brought back into the RGB image using LRGBCombination. Three very bright stars gave quite some artefacts when putting the two together, due to the 6 diffraction spikes (Mewlon-180c has three vines). Therefore the GAME script was used to create a simple circular mask around those three stars. The mask was applied and inverted before the Luminance was added. As a finishing touch, the saturation was bumped up a bit using the CurveTransformation tool, and also here the three bright stars with diffraction patterns were protected.
The final image was exported without further editing.
Processing workflow (click to enlarge)
The image as per the original processing, using the workflow described on the left.
Update
This image was originally shot and processed in 2022. However, in two years so much progress has been made in the processing software that the original image did not do justice to the Mewlon telescope. In 2024 this image was reprocessed, following a processing workflow similar to images like M56 and M10, resulting in a much better outcome.
This image has been published on Astrobin.
