The Moon is probably the second most important thing in the sky when you’re imaging a target. The first most important thing is, of course, your target.

The proximity of the moon to your target is one of the factors that influence your image’s Signal to Noise Ratio (SNR). If the Moon is up in the sky during your imaging session, regardless of the Moon’s phase, chances are that it will negatively affect your image’s SNR.

Your goal is to have the Moon as far as possible from your target. Of course, and ideally, the Moon should be below the horizon during your imaging session, but that’s not always possible.

A general rule for maximizing your image’s SNR is to have the Moon at least 100 degrees away from your target during your imaging session; however, the further away, the better.

In this article I explain how to image targets that have the Moon at least 100 degrees away. I also show you a way to determine the proximity of the Moon for any target and at any time – this capability can work to dramatically improve your images and can help you more effectively plan your observing sessions.

Using Telescopius To Select Suitable Targets

Telescopius is a service that helps you select targets using complex queries that rival the capabilities of expensive planetarium software.

I discuss configuring and using Telescopius in my book. I recommend you read the sections that discuss Telesopius. I also discuss Telescopius in this tutorial about SNR.

Alternatively, point your browser to this location: https://telescopius.com , sign up for the service and configure it with the Slooh Canary Islands location for the purposes of this tutorial.

Select the Targets option and modify the Search Parameters so that targets are at least 30 degrees for at least 30 minutes and having a Moon proximity of at least 100 degrees. Select any Object Type.

The display will show you targets that meet the search criteria. I sort the list by Transit time so that I can plan my observing sessions. The display includes lots of information including the transit altitude of the object, rise time, and set time.

If you have a set of celestial coordinates in mind, you can enter them in the settings panel on the left to further limit the list of possible targets.

Remember to set the date for your observation session too.

While this approach works well, it is limited to the available targets. There are times when I want to know whether a specific object will be close to the Moon during my imaging session. Other times, all I have is a set of celestial coordinates and I want to figure out whether I can image that location without worrying about the Moon’s proximity.

In the next section, I show you how to use AstroImageJ to calculate the Moon’s proximity to any object, or any coordinates, at any time.

Using AstroImageJ to Investigate A Target

AstroImageJ is great for viewing, aligning, and stacking images. But AstroImageJ is also useful for figuring out the proximity of the Moon to any target at any time. This is useful in cases where you’re trying to figure out the best time to image an object or set of coordinates.

I discuss downloading and installing AstroImageJ in my book; however, if you are savvy you can figure it out on your own. Install AstroImageJ from this location: https://www.astro.louisville.edu/software/astroimagej/

Start AstroImageJ and select from the menu Plugins – Astronomy – Coordinate Converter:

This brings up a new window as shown:

Fill in the Geographic Location Of Observatory fields for the Slooh Canary Islands telescopes for this tutorial. The coordinates for AstroImageJ are +28:17:58.92, and -16:30:29.74 (East is positive).

Let’s investigate NGC 936 for this example – enter that in the SIMBAD Object ID box and hit enter.

The Moon’s phase and proximity to the target is displayed at the bottom of the window. The proximity is the bottom number and it is in degrees.

Change the time in the Epoch Of Interest area by changing the UTC time fields and press enter. The Moon’s phase and separation from your target change as you change the date.

You can investigate any coordinates by entering them in the Standard Coordinates fields – enter the RA and Dec in the respective boxes and press enter on your keyboard to update the display.

This screen includes a lot of useful information. For example, you can determine the airmass between the telescope and the target by viewing the value in the Airmass box near the bottom right of the window. You can also see the target’s altitude and azimuth above the Moon’s Proximity box.

Conclusion

In this tutorial you gained an understanding of how the Moon affects SNR, you learned the minimum separation between the Moon and your target for high SNR, and you learned how to use Telescopius and AstroImageJ to select targets and investigate specific targets.