[mental4astro (Alexander)]

One thing needs to added to using a filter to suss out planetaries. Most planetary nebulae are tiny. Even the Blue Planetary, though it is actually quite bright, its angular size is actually very small. This is part of the reason that it is bright is that it is so small. If it were larger in diameter, it would appear dimmer as its light would be spread out over a larger area.

Anyway, PN's are not always immediately obvious, be it for their size, brightness, location, etc. And while an OIII filter is invaluable in identifying them, unless you know what to look for, or the PN is very obvious, in most instances an OIII filter won't help. The Blue Planetary is an example where it really requires a lot of magnification to bring out its disc size. So unless you were anticipating this, you'll run right over, even with your OIII filter.

One brilliantly easy tool you can make that is fantastic to chase out PN's is a blinking paddle. This paddle holds a 1.25" OIII filter that you then flick between your eye and the eyepiece. If there is a PN in the field of view (FOV), the filter will make the PN literally "POP" out from the background. Once you've identified where the PN is, you can either put the filter in an EP or filter slide, or whatever, and further your examination of it. The Blue Planetary is one PN that really benefits from 'blinking' it out first if you are not to confident in identifying it.

In other instances, blinking out a PN is the only way to identify it from its surroundings. The giant open cluster M7 holds no less than 6 planetary nebulae within its boundaries (not all associated with the cluster, if any). But because M7 lies in within a rich area of the Milky Way, and the PN's are so tiny, blinking them out initially is the only way to isolate them. And it staggers me just how much these PN's leap out by blinking, and impossible to identify without doing so. Even large PN's benefit from blinking as they can have a low surface brightness, and blinking will just help identify their location.

For what it's worth, also within the celestial boundary of M7 are four other open clusters and at least one globular cluster, none of which are associated with M7, and all are just a line of sight coincidence.

Pluto was similarly first discovered by blinking, though for Pluto the blinking was done between two photographs, and the 'moving star' that was how Pluto was noticed.