Mis à jour : 13 janv. 2020
Messier 71 is a rather strange star cluster located about 12,000 light-years away in the small constellation Sagitta. It is best photographed in the Summer season.
The image on the left was taken by the Hubble Space Telescope.
When looking at it, what do you see?
If you're like us, you will say that at first glance, you would think this is an open cluster. There is plenty of space between all the stars, there is no apparent core and it is overall not dense!
Well, you would be wrong, and so was the world from the day of M71's discovery to the 1970's!
M71 is strange because it looks exactly like an open star cluster when observed or photographed. In reality, Messier 71 has been officially classified as being a very loose open cluster. A similar object in the Messier catalog would be Messier 68 (although it does look like a globular cluster when photographed), which we yet have to capture.
We imaged M71 during a full moon night, in a Bortle 5 zone (worse than our usual imaging location which is Bortle 4), with a One Shot Color CMOS camera. We decided to try out the ZWO ASI 071MC camera to compare the results with our monochrome ASI 1600MM. This photo was taken in just 30 minutes and was more of a test than anything. We did not even bother plugin in the guiding camera!
We literally picked out this target at random and chose to slew our telescope at it because of it was the 71th number in Messier's catalog and we were trying the 071MC camera for the first time. We also had to crop a lot of it out because we did not achieve the right back focus with our coma corrector. With the moon shining at full power overhead and our setup being closer to light pollution than usual, we expected to trash all the files when we got home, but realized that it was actually worth saving!
Below you can see our final image. Let us know if this is good enough to stay for a while or if we should revisit it and switch it out as soon as possible!
Messier 71 with the ASI 071MC
Camera: ZWO ASI 071MC
Telescope: Orion 8" Astrograph f/3.9
Mount: Atlas EQ-G motorized Mount
Acquisition: ZWO ASIAIR
Total Exposure Time: 30 minutes
Exposure Time per frame: 30 seconds
Filters: N/A (OSC)
Locating Messier 71
Messier 71 can be found in the Northern hemisphere, in the third smallest constellation in our sky: Sagitta. M71 itself is also one of the smallest globular clusters discovered, as it "only" spans 27 light-years in diameter.
Being so small, spotting M71 may be difficult unless using a large instrument. Binoculars will only reveal a tiny blurry patch in the sky, and that's if you are observing from a very dark location. You should be able to see the object with a medium size telescope, but it will not be very clear through the eyepiece.
Single Shot & Processing of Messier 71
This was our first time using a OSC (One Shot Color, in RGB) cooled camera. We wanted an easy target that could be done in a small amount of time as we did not want to spend a long time out that night. A star cluster was a perfect choice, especially one that is so loose!
Being used to our cooled monochrome camera and its filter wheel, we assumed that we had to replace the space normally used by the filter wheel with an extra adapter. This was a mistake as the coma corrector was rendered completely useless, as you can see on our single shot of 30 seconds below. We had to crop a lot of it out but thankfully, M71 is so small that we did not have to sacrifice any good data.
We processed this image in a rush and did not expect it to turn out OK, but it did!
M71 is a very easy object to photograph. It is not a popular object for beginner astrophotographers, but can be imaged in very little time! The framing, the acquisition and the processing are all a walk in the park and we definitely recommend imaging this cluster if you're not sure what to pick as a target tonight!
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