M13 - Hercules Cluster
M13 or the Hercules Cluster - Click here for full resolution
M13 or the Hercules Cluster (R.A.: 16h 42m 28.34s, Dec: +36º 25’ 3.7”) is a globular cluster in the constellation Hercules at a distance of 23,000 lightyear from Earth. It is also known as NGC 6205. It is a massive cluster, with several hunderd thousand stars. Not only are there many, they are also very densely packed. When compared to the area of the Sun, the distance between stars in M13 is about 100 times less. This leads even to occasional collisions between stars, creating newly formed so-called “blue stragglers”. The dense nature of the cluster makes it difficult to separate individual stars when photographing the object, especially in the center.
Sky Plot (click to enlarge)
5º FoV + scope display (click to enlarge)
Conditions
Images were taken on March 23 and 24, 2022, from the backyard in Groningen, The Netherlands (53.18, 6.54). Moon was at around 60% illumination, but had set already by the time the images were taken. Visibility of M13 is best in spring and summer. On March 23, M13 rose in the East and was visible from around 21:30h onwards and climbed to an altitude of around 70 degrees towards the end of the night.
Visibility charts showing 22:00h altitude throughout the year (left) and the session on March 23, 2022 (right).
Weather was generally good, with low temperatures around 5 degrees Celsius. Humidity was quite well at 81% on the first night, but got pretty high on the second night of imaging. Visibility was not great with SQM values around 19 mag/arcsec2.
Capturing
The image was captured using the Takahashi Mewlon-180c in combination with the QHY268c camera. This combination creates a field of view of 0.78º x 0.52º. The long focal length of the Mewlon (1900mm) fits very well with a small object like M13, with an apparent size of 0.3º.
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Takahashi Mewlon-180c + 0.8x reducer/flattener, Esatto focuser
Rainbow Astro RST-135E, Berlebach Planet
QHY268c, cooled to -15 ºC
Astronomik-L3 2” IR/UV-cut filter
Askar 180/40 guidescope, ZWO ASI290MM
Fitlet2 (Linux 20.04), Pegasus Powerbox Advance, Gerd Neumann Aurora flatpanel
KStars/Ekos 3.5.7, INDI Library 1.9.4, PHD2 2.6.11 SkySafari 6.7.2, openweathermap.org
The camera was a one-shot-colour camera. An IR/UV cut filter was used to eliminate unwanted signal from the far IR and UV end of the spectrum.
With other cameras and telescopes, star-only images are usually captured at gain 0. It leverages maximum full well depth and helps to prevent star-centers to clip. Usually 1 minute exposures give a fairly decent image. But the telescope here is fairly slow at f/10. The one minute exposures during the first session were too short. Therefore in the second session, exposure was increased to 120s, which gave more detail in the cluster. In future cases, the gain could be further increased, e.g. to 26 to see even more details with lower noise levels.
Guiding the RST-135E has been quite challenging from time to time. In particular some sort of oscillation in DEC guiding has caused concern. Many people are reporting this behaviour, as can be seen in this thread on the Rainbow Astro forum, there is no single solution found yet. During the two sessions, different algorithms were applied in a quest for the optimal scenario.
During the first sessions, the Z Filter algorithm was applied for DEC guiding. Settings were Exposure Factor of 20 for 0.5s exposures and a minimum move of 0.1 arcsec. On average during the whole session an RMS of 1.06” was too high and resulted in deformed stars and a high number of rejected frames.
During the second session, the ResistSwitch algorithm was applied for DEC guiding, with minimal move value of 0.2, aggressiveness of 70 and with the FastSwitch option enabled. Throughout the evening much better guiding performance was achieved on the second night. The average RMS of 0.66” resulted in perfectly round stars.
The future will tell whether this was just luck, or whether this is indeed the right guiding algorithm with the proper settings for this mount in this setup.
Image
During the first session, two issues led to sub-optimal images. First there were guiding issues, resulting in sometimes weird star shapes. Secondly, the backfocus distance was based on a setup without a filter. So in session 2, an extra 0.5mm of backspace was added.
The consequence was quite a few frames of the first session did not look too great. Using the SubFrameSelector in PixInsight, stars with an eccentricity higher than 0.65 were eliminated. In total 43 out of 406 frames were deleted, resulting in a total of 363 frames that made it to the final image. This included exposure times of 60 and 120s, making up for a total exposure time of 7.9h.
In the corner of the frame was a very bright star, that created an asymmetrical diffraction pattern because of its off-axis positioning. The shape and brightness together made it a very distracting element. Therefore it was decided to apply a decent crop. This resulted in a final image with a resolution of 4544 x 3032 pixels, or 13.8 Megapixels. It covers a field of view of 0.5 degrees horizontally.
Annotated image showing M13 and the image’s orientation.
Processing
For each session, separate Flat frames (25) were created, and calibrated with Bias only. Using the SubframeSelector tool, all Light frames with eccentricity >6.5 were eliminated. The remaining frames were calibrated using darks (50) and the earlier created Flats. Then they were debayered and registered. All 363 images, regardless their exposure time, were then stacked into one RGB image, using noise estimates as weight factor.
Even though Flats had been applied, there was still a fairly strong remaining radial gradient present in the combined image. This could be reasonably eliminated using DynamicBackgroundExtraction. After cropping away the rough edges from the stacking process, the colour channels were calibrated using PhotometricColorCalibration.
In order to maintain star colours during the stretching process, a combination of ArcsinhStretch and Histogram Transformation was applied. This resulted already in a fairly good image. A very modest amount of noise reduction was applied using TGVDenoise on the background, under protection of an inverted star mask. With the same mask in place, a small green cast in the background was eliminated with SCNR. For final finishing touch, a star mask based on a range selection was created, which was a bit more inclusive. The blue and the red of the respective stars was enhanced slightly by specific color saturation enhancement. And a slight touch of sharpening was applied using UnsharpMask.
As mentioned before, until this stage, the very bright star in the background had been part of the image. Its weird diffraction spikes distracted just too much, so a tighter crop was applied for the final image.
Upon close inspection, the stars in the image, especially towards the edges, are not completely symmetrical, neither in shape, nor in colour. The shape is probably attributable to some guiding issues, especially in the first session. But colour is sometimes a bit unevenly spread across the diameter of the star. Especially in bright stars towards the edge of the frame, one side can have a bit of a blue-ish colour, with the opposite side a bit more towards the red. This may be related to the optics. The Dall-Kirkham design of the Mewlon in its original form does not have a flat field. The flattener/reducer is doing a great job, but may not be up to apochromatic standards. Overall it’s just lacking the pinpoint crispness of the TOA-130 and FSQ-106. Not sure if there’s gains to be made in processing, but worth exploring. And building further experience in focusing, guiding, optimal capture settings, etc might further enhance the output of this telescope. But even with the current output, it is a nice portable long focal length telescope, which is well suited for small bright objects, such as clusters and galaxies as well as moon and planets.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
