GUIDE TO OBSERVING and IMAGING COMETS (updated August 2023)
Comets are SOLAR SYSTEM OBJECTS, not atmospheric
phenomena.
They require dark, moonless, rural skies to observe at their best.
Since most people live with less than perfect skies, the trend is towards
astrophotography rather than visual observing.
Todays advances in astronomical equipment, digital imaging and image
processing software have made it a lot easier.
The sensitivity can now capture comets in great detail, that the eye does not
see, especially colour detail.
Latest mobile phone cameras are capable of taking night sky photos
and would be able to capture a rare bright comet.
1. AVOID LIGHT POLLUTION
Ensure that the direction in which you observe the comet does
not overlook populated areas.
For many, this means travelling to a dark
observing site. Before you attempt this, refer to tips 2 and 3.
In Australia, the majority of people live in urban areas.
The urban sprawl and its demand for light
has resulted
in the night sky being lost to current and future generations.
This is the
main reason why comet Halley in 1986 was labelled
by the general public as a fizzer when in fact,
it was an impressive object when seen from a dark sky location.
2. AVOID MOONLIGHT
The Moon is the brightest object in the night sky and acts as a natural light
pollutant.
Therefore it is important to observe comets in periods free from
moonlight interference.
There are many free mobile apps or websites that provide you with local times
for moonrise & moonset.
The exception to this rule is of course the appearance of a bright comet, where
every opportunity to observe it should never be missed.
3. AVOID CLOUD
It's of no use to travel many kilometers to your dark observing location only
to find that cloud interrupts the view.
One of the best weather prediction
websites for Australian observers is the Bureau of Meteorology
http://www.bom.gov.au/
It has high resolution satellite animation at
http://satview.bom.gov.au
In Layers, select coastal, boundaries, roads, cities. Infrared greyscale for
night-time animation.
You can see in real time where the clouds are at with this animation.
Also highly recommended is the Skippy sky website at
https://www.skippysky.com.au/Australia/
If your location is dark blue for the forecast period, you have a great chance
of clear skies.
4. DARK ADAPTATION
An essential requirement for visually observing faint astronomical objects.
Dark Adaptation requires the observer to avoid direct light for at least 15
minutes prior to observing.
The eye in total darkness will progressively develop
a light sensitive pigment called Rhodopsin (a derivative of Vitamin A)
which
enables greater peripheral vision at night. Persons lacking in Vitamin A will
suffer from "night blindness".
To maintain dark adaptation during the night, avoid light eg. wear an
eyepatch and use a low light RED torch.
5. USE AVERTED VISION
A trick of the trade is to use the side of the eye rather than look directly
at the astronomical object
since your peripheral vision is much more sensitive
to faint light.
The most effective method for observing faint astronomical
objects is when you combine Dark adaptation and Averted vision with Movement.
i.e. wiggle a pair of binoculars or telescope.
6. OBSERVE AT THE END OF ASTRONOMICAL TWILIGHT
Your evening sky is not completely dark until it enters "end of astronomical
twilight". This is when the Sun reaches 18 degrees below your horizon.
The reverse is true for sunrise. Depending on your location, this is usually at
least 1 hour (more likely 1:20) after Sunset or before Sunrise.
Unfortunately comets are typically brightest when closest to Sun so you may not
have a choice.
7. USE OPTICAL AID
A bright comet will appear far more spectacular with optical aid. The best
instrument to use is a pair of 7x50mm binoculars.
This usually gives a field of
view of around 8 degrees, perfect for close inspection of the coma and tail.
10x50mm binoculars are preferable for observers over 50's due to aging eyes.
Through a telescope at
high power, you may witness dust jets or nuclear fragmentation.
8. KNOW EXACTLY WHERE AND WHEN TO LOOK
Predicting the brightness of a particular comet is educated guesswork at
best,
but we do know WHERE a comet will be located in the sky at a particular
time.
You should learn to recognize the stars and constellations in which a
comet will travel through, well before the event.
A torch wrapped in thick red cellophane is an essential observers tool for
reading maps at night.
9. WEAR APPROPRIATE CLOTHING
Spending an evening under the stars requires common sense. Wear appropriate
clothing to protect you from cold and wind.
10. ALTITUDE,ALTITUDE
This advice goes with any astronomical type of observing. You will see a
comet at its best when it reaches highest elevation in the sky.
This is not always possible as comets are brightest when near the Sun, and are
often seen low on the horizon,
but occasionally they are near the Earth. Try to observe the comet when it is
located highest in the sky.
Your local elevation also helps. Mountain views of the night sky will always
beat a view from sea level.
GUIDE TO IMAGING COMETS
(updated August 2023)
I am not an expert astrophotographer by any means but
Below is my method for image capture and post processing using Comet
C/2021 A1 Leonard as reference.
C/2021 A1 reached naked eye visibility during the second half of December 2021.
TELESCOPE SETUP:
Celestron C11 Rowe Ackermann Schmidt Astrograph,
Super Fast, wide-field 11 f/2.2 optical design. Not for visual use however.
Large image circle maintains pinpoint stars to the far corners of a full frame
camera
Fan to cool mirror running throughout the imaging session
Mount: Skywatcher AZEQ6 mount used equatorially / no autoguider
Dew Shield very important addition
Camera: Canon 6D Mark I unmodified
Image capture software: Canon EOS utility remote capture
Telescope control software GUIDE 9
Focus: Bahtinov mask, star at high altitude. Apply mirror lock after focussing
CAMERA LENS SETUP
Canon 60Da (or 6D) + Sigma 200mm lens
Skywatcher Star Adventurer tracking mount.
Processing: Deepsky stacker (free) to create calibrated tiff image
Then run through Maxim DL6 for final image processing.
WORKFLOW
First obtain Calibration frames. You must use RAW setting on Camera
1. Flat frames
are integral to good imaging because the camera body or lens will distort
the image.
take TWILIGHT FLATS (TIMING IS MOST IMPORTANT)
take images halfway between civil and nautical twilight (10 minute period)
directly overhead
Tracking must be off else you will introduce dark spots to the master frame
Take flat frames at the same speed as your light frames
Trial and Error: my setup works best at 10x2 second exposures at ISO3200.
Please note: Twilight flats create a gradient, which needs to be removed in post
processing.
A set of flats should last the season as long as you dont adjust the
positioning of the camera
2. Dark frames
can be taken any time (not wasted at the telescope)
ensure camera is in dark cupboard/room.
exposure and speed to match the light frames
3. Dark flat frames
10x2 second dark frames (or whatever time matches your flat frame exposure)
which will subsequently be subtracted from flat frames
4. Bias
10x very short dark exposures
5. Light Frames
Finally you image your target.
I take 10-20 30 second exposures for a total of 5-10 minutes
short exposures eliminate the need for autoguiding.
You can also throw out bad frames bad tracking or satellites passing through
the field.
6. Post Processing
Deep Sky Stacker free
http://deepskystacker.free.fr/english/index.html
is a fantastic free program that allows you to combine all 5 sets of frames as
discussed above.
Create and save the image as a tiff file 16 bit
You then import the tiff file into a post image processing software>
I use Maxim DL6
OR FREE software Fitswork
https://www.fitswork.de/software/softw_en.php
Ensure you flip image if using a RASA
Remove Gradient
White balance
Rotate to north up standard for comet imaging
Digital development Gaussian blur
Save as jpeg