The moon is really a great object to observe. You can see lots of details and you can see it even in areas with lots of light pollution. The brightness of the moon may effect your vision when viewed through a telescope.
When viewing the moon, you may temporarily lose vision, the same way you do when you look at bright headlights on a dark night of driving. Your vision will return as your eyes adjust to the sudden change of brightness. Most agree that the brightness of the moon through a telescope is not enough light to permanently damage your eyes.
If you notice any changes in your vision after a night of observing, please visit an optometrist immediately. Since you are paying close attention to your vision you may notice issues you had been ignoring.
Telescopes work by collecting and focusing light to make dim objects visible to the human eye. Telescopes do not produce any light on their own, they just collect the available light. This focusing of the light makes objects appear brighter.
The moon is one of the brightest objects in the night sky, it is uncomfortable to view the moon without a filter.
Moon filters, which work like sunglasses for your telescope, will dim the brightness of the moon and allow you to observe more detail.
Moon filters can be purchased for about $10. They make viewing the moon much more comfortable and increase the contrast. This makes it easier to see the amazing detail of lunar geography.
What filters should I use to observe the moon?
Using a filter to dim the brightness of the moon makes it more comfortable to observe. Moon filters are inexpensive, a 1.25” moon filter can be purchased for about $10. The gold standard filter for viewing the moon or other bright objects is a Variable Polarizing FIlter, Orion has one for about $60.
Filters sit before your eyepiece and block certain types of light. I have a few filters that I use to increase contrast and make dim objects more visible, but for the moon you want a light blocking filter. There are filters that help when you are dealing with lots of light pollution by increasing contrast. Look into filters if you are not satisfied with what you are seeing in your scope.
Moon filters are the obvious choice for observing the moon. These filters block light and help you to see greater detail when observing the moon. You can also use other dimming filters like a Neutral Density filter or Variable Polarizing filter.
A Variable Polarizing Filter allows the observer to adjust the amount of light allowed into the telescope. It allows you to adjust your brightness depending on what your are observing.
There are other options for filters when viewing the moon and other planets.
The Baader Moon and SkyGlow filter is about $150. I have a friend who won’t even view planets without this filter. He finds that it brings out a lot of detail and brings the brightness down. It also helps with light pollution.
Neutral Density filters come in various strengths.
I only have one.
I asked for recommendations for filters in my favorite astronomy facebook group, and they recommended a Neutral density filter, its a 13% filter, and with my 10 inch that was an ok filter.
I wish they had recommended a Variable Polarizing Filter. This is like a dimmer switch for your telescope. You twist the two layers of the filters and change the amount of light the filter allows to enter the scope. They come in 1.25 and 2 inch sizes and can be used on many different types of telescopes.
Tricks to observing the moon without a filter:
If you do not have any filters yet and you would like to observe the moon, there are a few tricks that can make it more comfortable
Keep the Lights On
Keeping your eyes exposed to white light will allow your eyes to comfortably view the bright moon without a filter.
Normally when you observe you want to avoid light and allow your eyes to become dark adapted. Dark adapted vision allows you to see dimmer lights than you would normally be able to see.
Dark adaptation is very interesting and there are lots of tips and tricks to help you get the best views through your telescope. You can read a bunch more about dark adaptation in this post.
Increase Magnification
By increasing the magnification you will find that the image is dimmer.
There is only so much brightness coming from an object and if you focus on a smaller area of the object, you can only see the brightness coming from that smaller area. This makes the object dimmer and more comfortable to view.
Some people prefer this method to the use of filters.
I prefer to just use filters, but for the sake of minimalism, this will work.
Decrease Aperture
The larger your aperture, the brighter the objects you are observing, so if you have a smaller scope you will collect less light and the objects will be dimmer.
Obviously, this may not be a very practical tip. A moon filter is much less expensive than a new telescope.
Can you go blind looking at the moon?
Telescopes collect light and direct it to the human eye. They do not create brightness, they collect it.
While there are conflicting opinions, in most situations you cannot go blind looking through a telescope. The brightest object that we observe from earth is the Sun. This should only be observed with especially designed solar telescopes. The moon is the next brightest object that we observe from earth and it should be observed with a moon filter for more comfortable viewing.
If you notice any changes in your vision after using a telescope, visit your optometrist as soon as possible. You may find that there is an issue that you had not previously noticed. When using a telescope you pay much more attention to your vision than you do in your daily life.
Your optometrist can help you understand the best way for you to observe safely.
So, did you know that people observe the sun with special telescopes? My wife was really surprised to learn that. They use specific protections that keep their eyes safe.
Solar observing is done with devices such as the Herschel Wedge and layers of neutral density filters.
