Best Telescope For Beginners: The Maximum Usable Magnification (MUM Factor)

best telescope for beginners

What is the best telescope for beginners?

There is a long debate amongst the astronomers that debate back and forth over the topic of picking and choosing the telescope for beginners. As with anything technical, there are a number of factors, variables, requirements, etc that should be considered. In particular, this article is one of a series to help provide definition and focus to a very specific topic that is key whenever a discussion of choosing a telescope comes into the spotlight. For this article, we’ll be focusing on something called the MUM Factor.

Maybe you’re new to astronomy and you’ve never heard of the MUM Factor. Or maybe you’re an established hobby astronomer and you’ve heard of the MUM Factor, but you’re not really sure what it stands for and what it is all about.  In either case, this article is going to define it, break it down, and then build it back up by using a few examples so help illustrate the concepts.


MUM is an acronym that stands for, Maximum Useable Magnification. It represents the amount of magnification that a telescope is capable of enlarging a viewed object. But more specifically, it is the maximum magnification amount that can be applied with negatively distorting the viewed object image.


Now that we’ve defined MUM, let’s go one step further and define MUM Factor. This is a numeric value that that is the result from a calculation based upon the certain characteristics of a telescope. This value usually represented the numeric value followed by the letter ‘x’.  For example, perhaps you’ve seen magnification power values listed as 100x or 140x or 600x. In this instance, “magnification power” represented by the MUM Factor is the maximum magnification that you can expect to use with a telescope with satisfactory results.


The importance of the MUM Factor falls into two areas of astronomical viewing. These are listed in terms of technical and viewing session satisfaction.

  1. The MUM Factor specifies the maximum amount of magnification that can be configured  in a telescope so that the viewed image is clear and sharp, without any noticeable distortion. This is the technical limitation of a telescope’s capability.
  2. By keeping the applied magnification of a telescope at or below the MUM Factor, you can expect good results for the viewing session and avoid any unnecessary disappointment (e.g., image is too blurry to see well).

By this point, you should begin to see how the MUM Factor can be very important when discussing the best telescope for beginners.


Using some very simple math, it is very easy to calculate the MUM Factor for any telescope. First, let’s take a look at the basic formula:

MUM Factor = Telescope’s Aperture Size (in inches) x 50

As you can see, a simple multiplication is the only mathematics required for computing the MUM Factor. There is one variable (the telescope’s aperture size) and it is multiplied by a constant, the value of 50. If you want to know more about the aperture size of a telescope, be sure to check out our telescope guide, “Telescope 101“, as well as one of our feature articles, “Telescope Aperture Explained – Is Bigger Aperture Better?

Now let’s walk through a few examples to help illustrate how the calculation is executed and reinforce the concept.

EXAMPLE 1: Refracting Telescope with a 70mm Objective Lens

best telescope for beginners

First, we need to convert the 70mm (millimeter) objective lens into the equivalent measurement in inches. This is accomplished by dividing the fixed conversion value of 25.4 into the size of the lens. So, in this case it is 70mm / 25.4 = 2.8 inches.   The aperture size in this example is computed as 2.8 inches. The MUM Factor is may now be computed:

MUM Factor = 2.8 inches x 50 = 140

So the computed MUM Factor for the refracting telescope used in this example is 140. Which means that the maximum useable magnification that should be used with this particular telescope is 140x.

EXAMPLE 2: 5.1-inch Reflecting Telescope

best telescope for beginners

Since the aperture size the mirror is already listed in inches, we can jump immediately into the MUM Factor calculation:

MUM Factor = 5.1 inches x 50 = 255

So the computed MUM Factor for the reflecting telescope used in this example is 255. Which means that the maximum useable magnification that should be used with this particular telescope is 255x.

Example 3: 3.5-inch Compound (Catadioptric) Telescope

best telescope for beginners


Since the aperture size is already listed in inches, we can jump immediately into the MUM Factor calculation:

MUM Factor = 3.5 inches x 50 = 175

So the computed MUM Factor for the reflecting telescope used in this example is 175. Which means that the maximum useable magnification that should be used with this particular telescope is 175x.


So we’ve walked through the definition of the MUM Factor, we’ve shown the formula and how to calculate it, and then we followed it up with a few examples. Now that you’ve a better understanding you may be asking yourself one of these questions,

“Isn’t the MUM Factor and the maximum power listed in the description same thing?”

“The magnification power displayed on the box is different than the MUM Factor calculation.  What is going on?”

“How come the telescope’s power rating is so much bigger than the MUM Factor?”

Often times individuals configure a telescope with a magnification setting that exceeds the MUM Factor, only to find that they are disappointed with the results. The plain truth is that the MUM Factor and the maximum magnification power listed for a telescope are two very different things.  You may see magnification powers for a telescope shown in an ad, or displayed on a box, or listed in a description that are hyped up to catch the attention of buyers that just don’t know or understand the optical limitations of a telescope. People that don’t know about the MUM Factor, get drawn into the big magnification numbers and automatically think that the larger value means a better telescope.

In actuality, the MUM Factor and the maximum power are different. We’ve addressed the definition of the MUM Factor, so let’s look at the definition of a telescope’s maximum power. The maximum power is the raw technical capability of the telescope’s physical characteristics (focal length) coupled with the focal length of the eyepieces used to view the objects. This value usually exceeds the MUM Factor.


Viewing an object through a telescope whose applied magnification power exceeds the MUM Factor can yield disappointing results. There are number of factors that can influence the viewing results.

But if you really want to try here are a couple bits of information we’ve collected over the years:

MUM Factor +10%: There is a general rule of thumb that states that the configured magnification setting should not exceed 10%. In other words, if the applied magnification power exceeds the MUM Factor by 10% or less, the result can be satisfactory.

MUM Factor +15%: This range of extra magnification has been Hit or Miss for us.  It some cases we have used telescopes that possess a little bit extra capacity beyond the specifications and so it ended up working for sessions.

MUM Factor + More than 15%: In has been our experience that anything over 15% has been a waste of time.  There a been a few case where we were able to push the the limit to almost 19% and the viewing was usable.  But for the majority of the instances, the objects being viewed were very poor quality with almost zero clarity.


You should make it a point to calculate and know the MUM Factor for every telescope that you own or use. Having that number will help to ensure that your experience using the telescope for that viewing session will be productive and enjoyable. Otherwise, you’ll run the risk of possible configuring the telescope with too much magnification and ending up with a blurry image.  So how do you avoid this situation?  It’s just a simple calculation after you ascertain the telescope’s aperture.


Maybe you’ve already made a purchase and don’t need to discuss the best telescope for beginners.  That’s okay, the information in this article is still germane.  If you haven’t done so already, take a minute and compute the MUM Factor for your scopes so that you you’ll know just how far you can push your equipment.  Knowing the limits of your telescopes will help to ensure that your viewing sessions are both productive and enjoyable.

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