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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.