Planning
to Observe a Comet with SkyTools 3
SkyTools offers
a variety of unique tools for planning comet observations.
This guide gives an overview of each. This guide is
for users of the Standard and Pro Editions; for
the Starter Edition see the Observing with SkyTools Guide
that came with your software.
Setting up
SkyTools is
driven by a scientific model, and that old saying about
computers, "garbage in, garbage out" still applies. So
before we start planning we need to make sure we have everything
set up properly.
Your observing
location includes some critical information. Click on your
location to open the Observing Sites dialog. It is a good
idea to set your air temperature and relative humidity to values
that are typical.
The most
important setting is your sky brightness. Click to the right of
where it says Sky Brightness at zenith (Pro Edition) or
Best Naked-eye magnitude (Standard). This value tells
SkyTools about how much light pollution you have. It is measured
on a dark night near the zenith. There are several ways to
estimate your sky brightness, such as the Bortle Scale, the
faintest star that can be seen overhead, or even the sky
brightness as measured using a sky meter device. The meter is
the most accurate. A close second is measuring the faintest star
you can see overhead. To do that, wait for a good, dark night. Allow your
eyes to become dark adapted. Then find the faintest star you can
see overhead. Locate this same star on a SkyTools chart and note
its V magnitude. Enter this value on the Basic tab of the
Light
Pollution dialog.
Once your
location is set up open the Nightly Planner and have a look at
the Nightbar at the top. If your longitude and time zone are set
correctly the dark part of the night should be near the middle
(see below).
If the dark period
is not in the middle, check the longitude of your location to make sure
it has
been entered correctly. West longitudes, such as in the
Americas, are entered as positive numbers. East longitudes, such
as commonly found in Europe, are entered as negative
numbers. Also check that your time zone is correct and that you
have properly set up daylight saving (summer) time.
It is important to set up your telescope, binoculars, and observer.
Check that your telescope has the correct aperture (and units)
and focal length, that you have added all your useful eyepieces,
and that your finding device displays the
correct field of view & orientation. The two important
settings for the observer are your pupil diameter and level of
experience. Enter your approximate age and it will estimate your
pupil diameter for you.
Download the
Current Comets List
Comet data changes
over time, so it is important to keep it fresh. The best way to
get the latest comet data is to update your current lists
from our server; our data is kept up to date with the latest
observations. With
the Nightly Planner open, click on the Observing List
menu and select Update "current" lists from web.
We update these lists with recent data as necessary.
We
recommend updating the current
lists prior to each planning session.
Get
an Overview via the Observing Synopsis
Select
the Nightly Planner tab.
Select
the Current observing list group and then the Current
Comets observing list (as shown below).
Set
the date at the top of the planner to today's and choose your
observing location and telescope/binoculars. Select a comet in the
list and double-click on it. When you
double-click on the comet, the Object Information window
will open. Select the Observing Synopsis tab. The
synopsis will tell you important dates and give you a summary of
the visibility of the comet from your location and in the
instrument you selected.
Planning
for a Night of Visual Observations
Once
you have selected a night to observe, use the Nightly Planner.
Always use Visual Mode on the planner. Enter the date of
the evening prior to the morning you wish to make your
observation, your location, telescope/binoculars, and observer.
We
have the difficulty filter set to Visible (at any difficulty)
above to filter out comets that aren't visible in the 7x50 binoculars,
leaving only comet ISON (from 2013) in the list. This is
typically the only filter required.
We
want to go out at around the Begin time of 04:47 AM so we
have time to get ready and find the field. We have a fairly long
window to observe (over an hour). The comet will be low on the
horizon and rising. As it rises we will get a better view
because we will be looking through less atmosphere. But the sun
is also going to rise (twilight begins at 5:17 AM) and the sky
will begin to brighten afterward. The scientific model that
SkyTools uses is predicting that optimum visibility -- the sweet
spot in the tradeoff between altitude and twilight -- will be at 5:33 AM. After
then the brightening sky will degrade our view.
When
you right-click on the comet above you have the choice of opening a naked
eye chart, or a chart for the 7x50's or any other telescope that
is entered into SkyTools. The chart will open (or print) at
the optimum time of 5:33 AM with the comet centered. Print
this chart and take it into the field with you to help you locate
the comet.
Tip:
there are two comet databases in SkyTools; the current (or
supplemental) database, and the historical (or primary)
database. Open the View Controls for each chart view and
ensure that the historical (primary) comet database is disabled
(no check in the left box) and that the current (supplemental)
database is enabled (check in right box). The historical comet
database is accurate in the past only; it is not for use in the
present. Be sure to click the Save button before you exit
the dialog.
Long
Range Planning Using the Nightly Optimal Viewing Ephemeris
The
Nightly Optimum Viewing Ephemeris is a powerful tool for
selecting the best nights to observe. It is designed primarily
for visual observing, but can also prove useful for imaging.
Select
the Ephemeris tab of the Planner. Set it up similarly to
how it is set up below:
Click
Compute. This tool will list the time the comet is
best viewed on each night, it's magnitude, and how difficult it
is to see in the selected telescope, as well as the altitude,
azimuth, and how dark the sky is at that optimum time. If you
don't see all of the columns above, click on the Ephemeris
menu to customize which columns are displayed.
You
can sort the results by clicking on any column header. Sometimes
it may be useful to sort on visual difficulty or "Vis"
(a combination of altitude and darkness) to put the very best
nights at the top. By default the listing is sorted by the
date.
Important
note: the date listed is the Date of Evening. For a
morning comet this with be the date of the evening before you go
out to observe. For example, September 2nd refers to the morning
of the 3rd. We use this convention in SkyTools so that
observations can be organized by a single date, even if they were
made before and after midnight.
If
there are many nights that the comet isn't detectable in the
selected instrument, it may be useful to filter them out by
selecting Visible (any difficulty) as the minimum
difficulty. Make
sure to experiment with your different telescopes, binoculars,
and the naked eye as your visual instrument.
To see a
chart for any night, right-click on the night in the list and
select the chart to view/print.
Nightly
Finder Chart
A
very powerful way to make a finder chart for more than one night
is to make a chart using the ephemeris. To do this click
the Ephemeris menu and select the chart you wish to view.
It is best to use the chart for the same instrument that you
created the ephemeris for. The position of the comet will be
marked on the chart at the optimum time to view the comet on
each night. This kind of chart works best over several days or a
week, and are particularly useful for Binoculars, which include
both a naked eye and binocular view. You can print this chart
and use it each night to find the comet.
The
naked-eye view is on the left and the binocular field is on the
right. The comet is plotted for each night and is labeled by the
date and time of each morning (not the previous evening). Note the
proximity to Mars, which should make the comet easy to spot.
Planning
for a Night of Imaging
Once
you have selected a night to observe, again use the Nightly Planner.
Select Imaging Mode. Enter the date of
the evening prior to the morning you wish to make your
observation, your location, telescope, camera, and filter. We
don't need to apply any filters to the list. Select a comet.
The
blue line on the NightBar displays the quality of the image you
can obtain vs. time. The comet rises before 3 AM, but it is too
low. In the circumstances above, the quality increases steadily as
the comet rises higher, until twilight begins, after which it
falls dramatically. Keep in mind that a different telescope,
camera, or filter, may behave differently, perhaps allowing for
exposure well into twilight. The size of the comet is a healthy
245 pixels, but this is for the coma only. This comet has a
growing tail, so overall the comet will appear larger. If we begin
imaging at 4:45 AM we can achieve a very good signal to noise
ratio in the 60 minutes that follow.
The
planner helps us pick targets for imaging and gives us an overview
for each. Once a target is selected, right-click on the target (in
this case ISON) and open the Exposure Calculator to explore
in more detail.
Make
sure to set the weather conditions appropriately. The program has
pre-selected a time period (the optimum exposure opportunity) for
the comet. The total SNR available during this 60-minute period is
a respectable 110, and the mean estimated effective resolution is
3.2" per pixel. Note that this is considerably worse than the
maximum effective resolution listed under Camera Data. This
is due to the altitude of the comet and the weather conditions
selected, particularly the seeing. There are many other exposure opportunities
listed in the table. These have been broken up by effective resolution,
which is going to improve as the comet rises. Note, for example,
that the effective resolution of the top opportunity, starting at
3:20 AM, is a much worse 7.4" per pixel.
If
you are going to use multiple filters, be sure to investigate each
one separately. You will find that the optimum time to expose will
change with the choice of filter; some filters are less affected
by twilight so you can expose later. The effective resolution
will also depend differently on altitude. Use the information you
gather for each filter to determine the optimum order in which to
use them. For the best results, try to match both the SNR and
effective resolution of the final image in each filter, which will allow
them to be better combined to make a color image.
 Finally,
note that the program has selected an optimum exposure time of 1
minute (the minimum we have allowed). Many modern imaging systems
are insensitive to the sub exposure time used; it doesn't really
matter if it's 30 seconds or 10 minutes. The program will still
suggest an optimum, but it is mathematical rather than practical,
and a very short sub exposure is often indicated in these
circumstances.
To
further investigate sub exposures, click the Auto button to
the off position (not depressed) on the Calculate SNR for Exposure
tool. Enter any sub exposure time to the right of the
button. Compare the final SNR for different sub-exposure times.
Note that in our case (right) we have entered a 16-minute sub
exposure and the SNR has remained at 110. Thus for this telescope
and camera, without a filter, under these conditions and for this
target, we can go ahead and use any sub exposure time that is
practical.
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