None.
OK, maybe one. Or two. But that’s it.
Here’s a rundown of Seestar accessories and whether or not they’re useful.
External Battery
I don’t consider this an optional accessory. If you’re going to use the Seestar continuously for more than 3-4 hours on a full charge, you’re going to need some kind of external power source. I use a Jackery 300 that will run it for 2-3 nights. Others use 10-30 watt-hour power banks. Whatever you’ve got will work.
The Seestar always draws from external sources first. So it will be charging its internal battery and running from your external battery or power supply until that power is exhausted, then it will switch to internal. Remember the internal battery will not charge when the temperature is below 32F/0C, though you may find that yours stops charging when the temperature gets below about 50F/10C.
Eq Wedge
You aren’t required to use your Seestar in equatorial mode, but if you do, you’ll need a wedge. I don’t consider this an “accessory” per se, as it is an essential part of imaging in eq mode. So it isn’t quite on point for our discussion here.
By the way, the Seestar wedge is very good; the Williams Optics wedge is perhaps the best; and the Sky-Watcher wedge is “good enough”.
While we’re talking about wedges, I’d be remiss if I didn’t bring up the Peterson Engineering balance arm and and eq mounts. The balance arm offsets the eq wedge so that the center of gravity of your Seestar remains centered above the tripod, making it more stable. The Peterson Engineering eq mounts are customized to your location and eliminate the need to adjust the angle of the wedge while also adjusting azimuth.
If you’re using an eq wedge, strongly consider the balance arm. And if you haven’t purchased a wedge yet, strongly consider their eq mount.
Tripod
The S50 comes with an excellent tripod. It’s sturdy, portable, and low to the ground. The simple fact of its short legs make it more stable than some more expensive, taller tripods. And you need something stable, especially in eq mode. The S50 tripod is absolutely perfect.
The S30 has a less stable set of plastic legs. They work perfectly well in alt-az mode on a solid surface, but if you’re operating in eq mode you’ll need a real tripod. Consider Apertura’s $100 carbon fiber tripod made especially for the S30.
Leveler
A lot of people choose to add a leveler between the Seestar and their tripod. It makes it easier to make fine adjustments to get the Seestar precisely level. I’ve found that the Seestar’s inclinometer isn’t as stable and sensitive as any leveler I’ve used, and I can get good-enough results by making minor adjustments to the lengths of the legs.
If your tripod is right on the verge of being stable due to the height and weight of the Seestar, especially when it’s windy, then a leveler adds to the problem by raising the Seestar another inch or two from the mount. I found after a few nights of using a leveler that I just stopped using it. Instead, I set my tripod legs so that when I set the tripod down in the same spot it was always setting on my sidewalk, it would be level. Then I just skipped calibration altogether.
Bahtinov Mask
A Bahtinov mask is a device that fits over the objective end of your telescope and adds spikes to stars to make it easier to focus. These are from the old days of manual focus. Your Seestar has an auto-focus function that uses the power of math and science to find the perfect focus. You don’t need to ever manually focus.
Diffraction Spikes Mask
One of the benefits of a refractor (like your Seestar) is that it does not generate “spikes” on your stars. Those are an artifact of the way reflector telescopes work. They have a mirror mounted out on the business end of the telescope, and it is held in place by four or more arms that form an X on the end of the scope. These diffract the light and cause those spikes you see extending from stars in children’s books.
Diffraction spikes are not natural, and in fact are a down-side of using a reflector-style optical design. You paid good money for a refractor, don’t ruin it by distorting all the images you capture with it.
Stars in the sky don’t have spikes. The stars in your images shouldn’t, either.
Lens Cap
Your Seestar lens is protected when the arm is in its closed position. You do not need a lens cap.
External Filter Mount and Filters
The Seestars come with all the filters they need. They have a UV/IR cut filter for broadband imaging and a dual narrowband filter for emission nebulae. If you add an external filter you would want to turn off all the filters in the Seestar itself, and there’s not a way to do that. Stacking filters isn’t a good idea in general, and it’s especially not a good idea in a system where you don’t have full control over the light path.
Dew Shield
A dew shield is like a toilet paper tube that is placed on the objective lens of the Seestar. It hides the lens from the heat-sucking effects of the open sky and helps prevent dew formation. It has a secondary benefit of shading the lens from any nearby bright light sources.
The Seestar has a good dew heater, making the dew-prevention aspect of a dew shield unnecessary. But if you have a nearby street light or porch light that you can’t protect the Seestar from, you might benefit from a dew shield for that purpose.
When using an external dew shield, make sure it either is loose enough to detach when the arm closes or you’re standing right there to remove it before closing the arm. Otherwise you will hear an awful noise as the internal gears shred themselves trying to close the arm.
I have the luxury of my observing spot being in the shadow of a tree that blocks the street light near my house (and also blocks a good piece of the northern sky, thank you very much), so I’ve never bothered with a dew shield.
TL;DR
If you use an eq wedge, consider the balance arm (or eq mounts) from Peterson Engineering.
If you have unwanted bright light sources nearby, consider a dew shield to shade your lens.