[QUOTE=sweety439;520138]Is this number "lucky prime" [/QUOTE]

By 13 lucky primes I meant the last 13 Mersenne Primes above 10M, because only ~6 are expected on average between p and 10*p, for example 10M-100M, but so far we found 13 between 10M and 100M.

[QUOTE=xilman;520187][quote=Dr Sardonicus;520175]
It is well known for huge system of globular clusters that can be easily seen on long exposure photographs.[/quote]Patience, caterpillar ...[/QUOTE]
Yeah, well, I was just quoting from the web page. But the admonition is apt notwithstanding, because (if I read things right) the jets show up well on [i]short[/i] exposures, while the globular clusters reqire [i]long[/i] exposures.

Years ago, I tried a few photos of night sky objects with a film camera on a tripod. One of them was a multiple exposure of the moon coming into and out of eclipse. I had to use daylight exposure so the moon wouldn't "burn out" on the film. It came out pretty well.

Another one was of the comet Hale-Bopp. I let in as much light as possible to limit the exposure time, because I didn't have a tracking drive and wanted to limit the motion blurring. A 30-second exposure worked reasonably well -- the motion blurring wasn't too bad, and both tails of the comet were clearly visible on the film.

I noted that, with the lens I was using, the moon's disk was almost exactly the same size as the little circle in the middle of the viewfinder. This provided an easy way to check that the "moon illusion" was just that -- an illusion. Even though the moon looks [i]way[/i] bigger on the horizon than when it's high in the sky, when I pointed my camera at it and got it centered in the viewfinder, it always filled that little circle just the same, no matter where it was in the sky.

[QUOTE=Spherical Cow;520133]Very nice one; you must be getting some good skies there. What sort of exposure time are we looking at?[/QUOTE]Apologies for the very late answer. I've been diverted. That image was taken only a couple of hours or so before the camera caught a severe attack of death. First light on its replacement was ~36 hours ago.

Selected chunks of the FITS headers answer your question and a few more besides.
[C]
SIMPLE = T / This is a FITS file
BITPIX = -32 /
NAXIS = 2 /
NAXIS1 = 900 / NUMBER OF ELEMENTS ALONG THIS AXIS
NAXIS2 = 666 / NUMBER OF ELEMENTS ALONG THIS AXIS
EXTEND = T / This file may contain FITS extensions
EQUINOX = 2000.00000000 / Mean equinox
MJD-OBS = 5.865900000000E+04 / Modified Julian date at start
RADESYS = 'ICRS ' / Astrometric system
CTYPE1 = 'RA---TAN' / WCS projection type for this axis
CUNIT1 = 'deg ' / Axis unit
CRVAL1 = 1.877035354096E+02 / World coordinate on this axis
CRPIX1 = 4.505000000000E+02 / Reference pixel on this axis
CD1_1 = -3.960297908634E-04 / Linear projection matrix
CD1_2 = 0.000000000000E+00 / Linear projection matrix
CTYPE2 = 'DEC--TAN' / WCS projection type for this axis
CUNIT2 = 'deg ' / Axis unit
CRVAL2 = 1.239356794190E+01 / World coordinate on this axis
CRPIX2 = 3.335000000000E+02 / Reference pixel on this axis
CD2_1 = 0.000000000000E+00 / Linear projection matrix
CD2_2 = 3.960297908634E-04 / Linear projection matrix
EXPTIME = 2.018000000000E+03 / Maximum equivalent exposure time (s)
FOCALLEN= 2614.0000000000000 /Focal length of telescope in mm
APTDIA = 400.00000000000000 /Aperture diameter of telescope in mm
APTAREA = 116502.82520771027 /Aperture area of telescope in mm^2
EGAIN = 2.6800000667572021 /Electronic gain in e-/ADU
OBJECT = 'M87 '
OBJCTRA = '12 30 52' / Nominal Right Ascension of center of image
OBJCTDEC= '+12 24 02' / Nominal Declination of center of image
OBJCTALT= ' 31.8082' / Nominal altitude of center of image
OBJCTAZ = '266.9463' / Nominal azimuth of center of image
OBJCTHA = ' 4.0192' / Nominal hour angle of center of image
SITELAT = '28 38 30' / Latitude of the imaging location
SITELONG= '-17 52 04' / Longitude of the imaging location
JD = 2458660.4782638890 /Julian Date at start of exposure
JD-HELIO= 2458660.4782602866 /Heliocentric Julian Date at exposure midpoint
AIRMASS = 1.8843917125197109 /Relative optical path length through atmosphere
TELESCOP= 'Dilworth Relay 0.4m f/6.5' /telescope used to acquire this image
INSTRUME= 'SBIG ST-8 Dual CCD Camera w/ AO'
OBSERVER= 'Paul Leyland, Tacande Observatory'[/C]

The EXPTIME is the sum of 69 subs, some of which were only 5 seconds but most were 30 seconds.

[QUOTE=Dr Sardonicus;520253]Yeah, well, I was just quoting from the web page. But the admonition is apt notwithstanding, because (if I read things right) the jets show up well on [i]short[/i] exposures, while the globular clusters reqire [i]long[/i] exposures.[/QUOTE]Once more I apologize for the lengthy delay.

Almost everything circular in this image is a globular cluster; very few are stars in our galaxy. AFAIK, everything else are Virgo cluster galaxies; five are obvious and you should be able to pick up more with careful examination. [url]http://aladin.u-strasbg.fr/AladinLite/[/url] may be helpful here.

The image is the result of a quick and dirty effort. Doubtless a much better one could be produced if more care were taken with the image processing

HVGC-1
 

[QUOTE=Dr Sardonicus;520253]Yeah, well, I was just quoting from the web page. But the admonition is apt notwithstanding, because (if I read things right) the jets show up well on [i]short[/i] exposures, while the globular clusters reqire [i]long[/i] exposures.[/QUOTE]This image was the last taken with the old camera. It consists 137 x 30 second subs, median co-added which comes to 4110 seconds total. No filter was used and the spectral response corresponds quite well to Gaia's g-band.

M87 is 16.4Mpc away, which corresponds to a distance modulus of 31.0. At that distance M13, which has an absolute magnitude of -8.5, would appear as a star of magnitude 22.5. The imaged globular cluster "Hyper-Velocity Globular Cluster 1" shines with a Gaia-g magnitude of 20.61. It would be an easy naked eye object at the distance of M13, comparable in brightness to ⍵ Centauri.

HVGC-1 is near to M87 but is technically not one of its globular clusters. It was ejected from the galaxy a long time ago, presumably by a gravitational sling shot from a black hole binary. It is now travelling at 1026 km/s towards us (i.e. it is blue shifted) and moving at a speed of 2200 km/s away from M87, faster than the escape velocity of the entire Virgo cluster of galaxies!

[QUOTE=xilman;524255]Apologies for the very late answer. I've been diverted. That image was taken only a couple of hours or so before the camera caught a severe attack of death. <snip>[/QUOTE]
My condolences on your loss.

The replacement's performance confirms the adage, "Patience is a virtue."

[QUOTE=Dr Sardonicus;524266]The replacement's performance confirms the adage, "Patience is a virtue."[/QUOTE]
Or as we Romanians say, any kick in the butt is a step ahead...

Waiting for more nicer photos...

[QUOTE=LaurV;524270]Or as we Romanians say, any kick in the butt is a step ahead... [/QUOTE]
Must add this to my repeartoire

[QUOTE=xilman;524255]
Selected chunks of the FITS headers answer your question and a few more besides.
[/QUOTE]

Excellent- thanks. I don't know why, but seeing the technology available to the non-professional astronomer is genuinely heart-warming. And your note about HVGC-1 is fascinating; have to read up on that.

Norm