Globular Cluster Messier 71 in Sagitta
Fairly faint and hardly visually spectacular, Messier 71 is one of the nearest globular clusters to us at 13000 LY and is relatively young at about 10 billion years old.
Admiral Smyth described it in 1844 as… “of a very feeble light”.
It lies fairly close to the edge of the Milky Way so the stellar background is quite crowded which reduces contrast.
For many years, astronomers thought it looked more like an small open cluster than a globular until high resolution spectroscopy confirmed it was a globular.
This was my first image since March with my Esprit 120 scope. I had a lot of trouble with it out of the box back then with a faulty mechanical focuser and pinched optics from an overtightened lens unit - it took me most of last season to sort that out.
Given a 68% waning Moon, scudding cloud, a gusty wind and Saharan sand in the atmosphere (!), I thought I should go for something straightforward and its always nice to bag another Messier object!
I'd planned to get 30 x 2 minute subs over an hour but strong gusts of wind and scudding cloud left me with 25 x subs over 2 hours.
I'm just happy that my stars are round instead of hexagonal and the focuser was flawless.
Technical Card
840/120mm f/7 SkyWatcher Esprit 120ED triplet refractor.
SkyWatcher 1.0 x FF with 2 inch IDAS LPS P3 filter
ZWO ASI2600MC; 25 x 120 second subs, Gain 100, Offset 25, Temp = -10c.
EQ6 pro mount with Rowan belt drives. EQMOD control. Pegasus Astro Focus Cube electronic focuser.
Session control; SharpCap 4.0 on laptop with WiFi link to IPad.
Automated plate solving GOTO via ASTAP (4 secs exp at Gain 350)
FWHM multistar focusing - the best FWHM reading I could get was 4.3 - I didn't know it but next morning, the media had lots of pictures of dusty sunsets all over the UK - Saharan sand suspended in the atmosphere.
40 dark frames
60 flat frames (electroluminescent panel, 3000ms exposure at Gain 0).
Post processed in PixInsight 1.8.9.2.
Light Pollution and Weather:
Gusts of wind and scudding cloud. About half of all subs were discarded.
SQM (L) not measured due to waning Moon.
Polar Alignment:
PoleMaster alignment
Error measured by PHD2= 3.1 arc minute.
RA drift + 3.85 arcsec/min
Dec drift + 0.78 arcsec/min
Guiding:
PHD2 guiding with ZWO ASI290mm/Primalucelab
240/60mm guider.
RA RMS error 0.77 arcsec
Dec RMS error 0.74 arcsec
This deteriorated later in the evening as it got windier.
I'm thinking of changing to a 320mm f4 guide scope - I understand guide FL should be about 1/3 of scope FL.
Astrometry:
Focal distance: 862.40 mm
Pixel size: 3.76 um
Resolution: 0.9 arcsec/pxl.
Field of view:55' 34.6" x 34' 45.5"
Image centre: RA: 19 53 44.124 Dec: +18 46 01.89
Globular Cluster Messier 71 in Sagitta
Fairly faint and hardly visually spectacular, Messier 71 is one of the nearest globular clusters to us at 13000 LY and is relatively young at about 10 billion years old.
Admiral Smyth described it in 1844 as… “of a very feeble light”.
It lies fairly close to the edge of the Milky Way so the stellar background is quite crowded which reduces contrast.
For many years, astronomers thought it looked more like an small open cluster than a globular until high resolution spectroscopy confirmed it was a globular.
This was my first image since March with my Esprit 120 scope. I had a lot of trouble with it out of the box back then with a faulty mechanical focuser and pinched optics from an overtightened lens unit - it took me most of last season to sort that out.
Given a 68% waning Moon, scudding cloud, a gusty wind and Saharan sand in the atmosphere (!), I thought I should go for something straightforward and its always nice to bag another Messier object!
I'd planned to get 30 x 2 minute subs over an hour but strong gusts of wind and scudding cloud left me with 25 x subs over 2 hours.
I'm just happy that my stars are round instead of hexagonal and the focuser was flawless.
Technical Card
840/120mm f/7 SkyWatcher Esprit 120ED triplet refractor.
SkyWatcher 1.0 x FF with 2 inch IDAS LPS P3 filter
ZWO ASI2600MC; 25 x 120 second subs, Gain 100, Offset 25, Temp = -10c.
EQ6 pro mount with Rowan belt drives. EQMOD control. Pegasus Astro Focus Cube electronic focuser.
Session control; SharpCap 4.0 on laptop with WiFi link to IPad.
Automated plate solving GOTO via ASTAP (4 secs exp at Gain 350)
FWHM multistar focusing - the best FWHM reading I could get was 4.3 - I didn't know it but next morning, the media had lots of pictures of dusty sunsets all over the UK - Saharan sand suspended in the atmosphere.
40 dark frames
60 flat frames (electroluminescent panel, 3000ms exposure at Gain 0).
Post processed in PixInsight 1.8.9.2.
Light Pollution and Weather:
Gusts of wind and scudding cloud. About half of all subs were discarded.
SQM (L) not measured due to waning Moon.
Polar Alignment:
PoleMaster alignment
Error measured by PHD2= 3.1 arc minute.
RA drift + 3.85 arcsec/min
Dec drift + 0.78 arcsec/min
Guiding:
PHD2 guiding with ZWO ASI290mm/Primalucelab
240/60mm guider.
RA RMS error 0.77 arcsec
Dec RMS error 0.74 arcsec
This deteriorated later in the evening as it got windier.
I'm thinking of changing to a 320mm f4 guide scope - I understand guide FL should be about 1/3 of scope FL.
Astrometry:
Focal distance: 862.40 mm
Pixel size: 3.76 um
Resolution: 0.9 arcsec/pxl.
Field of view:55' 34.6" x 34' 45.5"
Image centre: RA: 19 53 44.124 Dec: +18 46 01.89