The 'Focus on Stars' Telephoto Filter is a focusing aid, similar to a bahtinov mask. This 100mm glass filter can be held in place with either a 100mm square filter holder, or by hand. Once you've got the filter ready, aim your lens up to a bright star and turn on Live View. You should now see a clear diffraction spike pattern in real-time! Adjust your focus ring slightly until the middle spike is centered up, this indicates the lens is perfectly focused.

https://focusonstars.com/termek/focus...

Keep in mind, the diffraction spike pattern may not be clearly visible unless you have a moderately bright star somewhere in the field of view. Let's say you aren't seeing a diffraction spike on your live view, and you've got your nebula or galaxy ready to photograph. You could always take a 5 - 10 second test photo with a high ISO. This should let you see a diffraction spike on a few stars in the image. You can then do the slow method of adjusting the focus ring slightly, taking a photo, checking the diffraction spike, and repeating until it's sharp. This is the process I use with my dedicated astro camera, telescope, and bahtinov mask. It's slow, but it works.

Don't forget about dew heaters! These will keep your telescope at the same temperature all night long, and prevent the focus from shifting. Without a dew heater, you'll likely need to refocus every ~20 minutes. I just bought a cheap USB powered dew heater, and that works well.

https://www.amazon.com/Defogging-Cond...

Review:

After testing out the Focus on Stars Filter, I was impressed how bright the diffraction spikes were. This made it very fast and easy to achieve perfect focus using Live View. Since I already had a 100mm filter system from Lee Filters, I was able to quickly slot the filter and begin shooting. However, if you don't have a 100mm filter holder, this could be a problem. They aren't cheap, and the alternative is to hand-hold the filter. You can certainly do this, but it might be hard to keep the filter steady in front of the lens if you're taking those 5 second long test photos.

Another potential problem with this filter is that you may not have a visible diffraction spike in the field of view when photographing a nebula or galaxy. If you can't see any spikes, you may have to aim your lens up towards a nearby bright star instead and focus on that. This would be fast and easy with a go-to mount, but very frustrating with a star tracker.

I do have one final bit of advice. I've been shooting with a RedCat 51mm telescope for the past 4 years. It has a bahtinov mask built into the lens cap. This makes it very easy to focus at night, and I know for sure when the stars are sharp. But this week I was testing out a new telescope that did not have a bahtinov mask. I spent over 20 minutes moving the focuser back and forth, taking test photos, and struggling to achieve focus. I almost gave up. I didn't realize just how much I took the bahtinov mask for granted until that night. Therefore, I'd highly recommend the Focus on Stars filter to anyone with a telephoto lens and DSLR. It will make focusing much easier! For those using telescopes, get a bahtinov mask if you don't have one already. After one focusing session you'll wonder how you ever got by without one.