Messier 13

The Great Hercules Cluster.

I’ve heard it said that when you’ve seen one globular cluster, then you’ve seen them all. I guess there’s some element of truth in that but I wanted to image M13, reputedly the best globular cluster in the Northern Hemisphere. It’s 6,000 light years further away than our glorious Southern  Omega Centauri and about half it’s physical diameter, so I didn’t expect too much.

It’s still a magnificent object, though, composed of hundreds of thousands of stars.

It’s a large  globular cluster, visible here in Sydney at a maximum height of 20° above the northern horizon. When I began imaging, it was already heading downwards through Sydney’s light glow towards a neighbour’s tree and I decided to use my long neglected Baader UHC-S dual band light pollution filter. (Goodness knows why I haven’t been using it recently, as my suburban sky continues to deteriorate).

Perhaps now that I’ve seen M13, Omega Centauri and 47-Tucanae I’ve seen them all……………

Time for a moan.

The current lunar cycle began last week with a number of consecutive clear nights and guess what?

 ✔ Night one: Clear. I managed to get the above image and a few frames of M16 before the Moon rose.

❌ Night two:  Clear – but a disaster as my laptop (or the SharpCap software, I’m not sure which) dropped every single image frame and I packed up with nothing imaged.   😨 

❌ Night 3: a  bit cloudy so I stayed indoors.  🤓 

❌❌❌❌ Nights 4-7: all clear nights – but I found myself house-bound with a nasty head cold and missed out on a field night too. 😵 

❓ Now recovered and of course the forecast is less promising for the rest of the cycle. Not happy!  😖

Object Details:

Designation: M13, The Hercules Cluster, NGC 6205
Constellation: Hercules
Visual magnitude:  +5.8
Apparent size:  20′
Diameter:  135 light years.
Distance:  23,000 light years.
Altitude during exposure:  16° above NNW horizon.

Also Visible:

Red giant star HD150998 (near top).

Technical stuff below.
Please feel free to jump to the end and like or comment.


Exposure:  26 min (52 frames 29.8 sec)
Gain: 128
Date:  2019-07-22
Location:  Outer suburban.
Conditions:  clear.
Moon: rise 10.00 pm.
Sky:  1.77 e/pixel/s  


Image acquisition software:  SharpCap, live stacked.
Image post-processing:  GIMP.
Cropping: two edges, to centre the object.


Telescope: SkyWatcher Esprit  Type: 120ED triplet refractor
Focal: 840 mm F/7 Mount: SkyWatcher EQ6-R Pro
Camera: ZWO ASI 071 MC Pro
Type: CMOS 28.4mm 16 Mpx
Optical aids: Flattener: Y; filter: LP Guiding: No
Polar aligning: QHYCCD PoleMaster Polar Error: 02’ 26” 

Geek Log:

[ZWO ASI071MC Pro]
Debayer Preview=On
Output Format=FITS files (*.fits)
Capture Area=4944×3284
Colour Space=RAW16
Hardware Binning=Off
Turbo USB=40
Frame Rate Limit=Maximum
Timestamp Frames=Off
White Bal (B)=60
White Bal (R)=60
Cooler Power=61
Target Temperature=-15
Auto Exp Max Gain=300
Auto Exp Max Exp M S=30000
Auto Exp Target Brightness=100
Mono Bin=Off
Anti Dew Heater=Off
Banding Threshold=35
Banding Suppression=0
Apply Flat=None
Subtract Dark=None
#Black Point
Display Black Point=0
#MidTone Point
Display MidTone Point=0.327798941363283
#White Point
Display White Point=0.910970869974874

Image © Roger Powell



  1. I didn’t know a head cold interferes with the functioning of astronomical observation equipment.

    Just kidding. Sorry to hear about the missed opportunities, but the capture looks good.

    Liked by 1 person

  2. I always enjoy a view of M13. Thank you for it.

    My wife and I volunteer weekly for a group of new members of our community from other countries. They are learning the ways of the English language. It is a rewarding time each week. Last Wednesday, I gave a lesson on the race to the Moon. Most are too young to have experienced it. I kept the story as simple as possible due to the language issues. They were very interested and asked questions.

    The skies were very clear that day. I invited them to a star party at a nearby school yard that evening. About 10 showed up. We viewed Jupiter and 3 moons, Saturn, Mizar and Alcor, Alberio, among other things. I enjoy hearing people exclaim when they see things in the eyepiece the first time. You’ve probably heard those words, too.

    Liked by 1 person

    1. As an astro-photographer, I gave up using the eyepiece several years ago.

      Except for the occasional public nights run my Society, when I revert to my 200mm alt-az SCT. It’s always fun to listen to people who have never viewed before. Their expectations are different to those of astronomers and they are less impressed with faint galaxies, more with bright objects.

      If the seeing is good I treat them to lunar craters and Jupiter’s GRS. Yes, Alberio too, the contrasting star colours are always a conversation starter.

      Liked by 1 person

  3. Great picture. That’s my favorite cluster, perhaps because it looks wonderful at the telescope even in my light polluted skies. It travels high overhead where I am, which helps. That you can take that picture and it rises a max of 20° at your location is amazing.


    1. Yes, an amazing sight and some of those northerly objects can be quite a challenge from SW Sydney – looking through the murky metro glow from between the trees…

      The brief window of opportunity is one of the reasons I’ve yet to image M31 through the telescope. The max altitude of only 15° is achievable but it’s also about the timing.


      Liked by 1 person

  4. Hi Roger, you really need to give Astrophotography Tool a go, I’ve had no issues with it since I’ve been using it with either my ZWO asi1600mm Pro or any of my Canon cameras. It’s not free software admittedly, but at 18.70 Euros it won’t break the bank, you also get emails informing you of any updates. I only use Sharpcap for planetary imaging and prefer to use Firecapture over Sharpcap. heres the link to APT.


  5. Just to be clear, Will, the issue I sometimes get is frame dropping. SharpCap expects an image after the allotted exposure period and doesn’t receive it. It’s not a live stack rejection by the SharpCap filters. It occurs even in preview with livestack off. .

    The day after the above event, I tested the camera in daylight and it went for over six hours without dropping a single frame. 🤔

    I then had it back on the telescope, imaging M16 and it functioned non-stop. All with the same cable.

    It’s a sporadic fault and mostly it works just fine and so I think it’s most likely an issue with the superspeed USB3 port or even MS Windows itself. It could be SharpCap though and my next step is to reinstall the software. Following that I might well consider alternate image acquisition software.


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