Sorry for the length of this post, but there is a lot to know to get the best astro images.
The Noise Filter setting reduces noise in images by blurring them on a pixel scale. The Noise Reduction setting reduces noise by taking and subtracting a dark frame (shutter closed during exposure) from the light frame (shutter open). Because Noise Reduction exposes a dark frame in addition to the light frame, it takes twice as long. Only the subtracted image is saved to the memory card whether shooting JPG or RAW - the original light and dark frames are discarded.
In astrophotography, dark frames are taken independent of light frames, and the dark frames are subtracted in post-processing. The big advantage is that multiple dark frames can be statistically averaged, for better overall noise reduction than subtracting single dark frames.
For best results, leave Noise Reduction and Noise Filter turned off, and handle noise in post-processing. Take dark frames for the same shutter times as light frames, but with the lens cap on. The main requirement is that dark frames be taken at the same temperature as light frames, because noise approximately doubles with every 6 C increase of temperature. This is harder to do with a dSLR or MILC than a cooled astrocam because we don't have an easy way to see or control sensor temperature.
At least one app for astrophotography will correct for temperature differences between light and dark frames: PixInsight. This requires taking a third kind of shot, called a bias frame. Bias frames are easy to take: set the camera at its fastest shutter speed and take a dozen shots or so, which will be averaged and subtracted from the light and dark frames in post-processing. This leaves only the temperature-dependent part of signal noise, which can be adjusted according to the sensor temperature.
We don't usually see sensor temperature in EXIF data displayed by, e.g., Lightroom and Photoshop, but it is there and can be read by apps like EXIFTool. PixInsight reads this temperature value and adjusts to match the light and dark frames.
I don't have an E-P5 but I do own the E-M5 and E-M1. Before the info that the E-M1 uses a Panasonic sensor instead of the Sony sensor of the E-M5 became public, I saw the difference in long astro exposures of 1 to 5 minutes duration. The Panny sensor is noisier in this type of photography.
For that reason, I bought an E-PL5, which uses the same sensor as the E-M5 that I like for astrophotography, and sent it off to Life Pixel for the full-spectrum mod, for greater sensitivity to H-alpha from emission nebulae. You don't have to go this far, but my point is that the Sony sensor is better for long exposure astrophotography than the Panny sensor. I believe the E-P5 uses the same Sony sensor as the E-M5. Therefore, I would recommend the E-P5 for AP instead of the E-M1.
With good technique using light and dark frames, the E-M1 is still fine for AP (and superb for normal high-ISO photography), but the E-M5, E-PL5 and (assuming it uses the Sony sensor) the E-P5 will be a bit easier to correct for noise.
Optimum exposure settings depend on your target. Test shots are important. If you include a scenic foreground, expose to get that right. If you are shooting only the sky, expose to get the sky right. Expose for long enough to get some space between the histogram peak and the left (dark side) axis, for best noise reduction in pp. Watch what the few pixels at the bright end are doing. To preserve star colors, avoid overexposing them.
Most often, a sky image will have too great dynamic range and either show more noise in the dark sky or blow out star colors. Then the photographer must decide which end of the brightness range to preserve. HDR techniques can be used to extend the dynamic range.
