Imaging a Black Hole (effects) with a Small Telescope

Hmmm….thinking about how to image a black hole with my telescope after last week’s release of the first black hole picture. I have a relatively small Celestron EdgeHD 8 inch (200 cm) telescope and a ZWO ASI 178 cmos camera for imaging. Imagers know that matching the angular size of a target to angular resolution of a telescope/camera is an important consideration, so I did that for the b\lack hole and my equipment.

The Hamburg Observatory released a nice image of the region around the black hole and its containing galaxy, M87, that shows some angular sizes.

The region around galaxy M87 and its black hole with angular sizes
Angular sizes of the region around the galaxy M87 and its black hole vs the Moon
Image credit: Hamburg Observatory, EHT collaboration, VLBA image – C. Walker,┬áVLA image – F. Owen, LOFAR image – F. de Gasperin

I added the full moon for some additional context but a quick summary is:

  • The full moon has an angular size of about 1,900 arc-seconds
  • The galaxy M87 has an angular size of about 400 arc-seconds in visible light, and a somewhat larger size in radio frequency observations.
  • The event horizon of the black hole is about 25 micro-arc-seconds!

My telescope/camera combination has a theoretical image scale of roughly 0.25 arc-seconds per pixel (Astronomy Tools has an online image scale calculator). So that means that the event horizon would cover

25 micro-arc-seconds / 0.25 arc-seconds/pixel = 0.0001 pixels

or one ten-thousandth of a pixel! Not likely to be visible, especially since the Earth’s atmosphere typically limits the practical resolution to about 1 to 2 arc-seconds.

Never the less, M87 is still a cool imaging target because it is possible to catch a jet of plasma shot out from the galaxy at relativistic speeds.

Image og galaxy M87 with relativistic jet
The Galaxy M87 with its relativistic jet pointing almost straight up.
Image credit: Ken Jackson, April 20th, 2019, Coquitlam, BC.

The above image was taken using a Celestron Edge HD 8 and ASI 178 camera in poor conditions – heavy light pollution in Bortle class 7 skies from my front yard in Coquitlam and a full moon rising in the east. It is just a single 30 second exposure. Still the jet can be seen pointing almost straight up from the galaxy!

The jet is thought to be powered by the black hole at the galaxy’s core and its surrounding accretion disk of material. In 1999, observations from the Hubble Space Telescope measured the speed of M87’s jet at four to six times the speed of light. This faster-than-light speed is actually an illusion caused by the relativistic velocity of the jet: the time interval between light pulses emitted by the jet is less than the actual interval due to the relativistic speed of the jet moving in the direction of the observer.

So we can’t see the black hole directly with smaller telescopes but we can see one of its cool effects.