Why can't I see a planet as a big image?

5 August 2020

People have sometimes asked me why they can't see a planet as a big round image that nearly fills the field of view. Sometimes, they've got quite cranky with me, convinced that either the telescope is faulty or that they've been sold something that doesn't do the job.

The first part - that the telescope is faulty - is not the case (well, I guess there's always the possibility that the telescope really is faulty, but that's a warranty issue).

The second part - that they've got the impression that the telescope they're buying would give them gigantic image - may be fair.

Customer expectations

It's very common that people come to the store looking for a telescope that can see the planets. Most of the time, it's for their kids.

These people have normally been on the website and found a telescope like a 60mm refractor with a 700mm focal length. Alternatively, they've found a reflector, something like a 76mm aperture and the same 700mm focal length. These scopes are normally in the sub $200 price bracket (that's Australian dollars).

In addition to this, for every person who actually comes into the shop, there are about a dozen people who buy off the website without ever having spoken with anyone about the scope.

Now, it's been my experience that these customers are thinking they're going to see planets that fill the field of view. This is where we have a problem.

In some cases, the pictures on the box the telescope comes in does not help - at all.

The reality

I don't really consider myself a telescope salesman, but rather a technical expert, and as such, I always try to give the customers an objective idea of what to expect from their equipment.

To put it bluntly, I have to manage their expectations down.

Let's look at a field of view simulator. (https://www.12dstring.me.uk/fovcalc.php)

First, if you get a basic scope with a 700mm focal length and a 10mm eyepiece (this is a pretty standard lens to start with), this is what you're going to see of Saturn.

As you can see, it's a dot. If you've got sharp eyes, you might - might - be able to make out a ring. It's not the sort of image that's awe-inspiring, and that will propel a kid into a deep love of astronomy and STEM subjects, is it?

Alternatively, let's see what we can drag out of the telescope. Different scopes come with different equipment (eyepieces, Barlows, etc.). This is what you'd get if you used a 4mm eyepiece and a 3x Barlow.


It's better, but remember, this just simulates the magnification in terms of the field of view. The quality of the image is going to be terrible. It'll be dim, hard to focus, and wobbly with atmospheric activity. That cheap Barlow isn't going to help, as the chromatic aberration it's going to produce will be so bad it's really not worth using.

What would you need?

Turning away from the kids' telescopes, let's have a look to see what equipment you'd need to get a decent image of Saturn. We'll worry about cost in a second.


First, we know that we're going to need something with a massive focal length (which determines the basic magnification of the scope). The Celestron 11'" Cassegrain range fits the bill, with a focal length of 2800mm.


Because we've got an 11" aperture, we'll be able to get enough light in to support a very short focal length eyepiece, but it's going to have to be good quality. My guess is that the aperture we have will support a saxon Cielo 4.5mm, which is a good ED glass equipped lens.


Finally, we can double the magnification using a high-quality Barlow. We could go a Powermate, but the Celestron X-Cel LX will probably do. Let's not go 3x, that'd be pushing it.

Here is the simulated image. Remember, this is all about field of view, not quality of image. My bet is that this will be a way, way better image, one that you'd actually describe as awe-inspiring.

What's that going to cost?

What does this set-up cost? The basic Celestron 11" Schmidt-Cassegrain (the CPC 11) is in the order of $7000. The eyepiece and the Barlow are relatively cheap in comparison, at $140 and $200 respectively.

That's a bit more than the $160-odd for the kids' telescope.

Is there a solution?

I understand that parents who are buying their kids a telescope don't have a detailed understanding of fields of view or magnification. It's up to the industry (including me) to manage their expectations.

We have a laptop at the counter, and whenever possible, I go through the field of view simulation with customers. People are reasonable, and naturally understand that a $160 telescope isn't going to give them $7000 images.

However, this is only the minority of customers. I'm not sure how to provide people who buy off the website with a good understanding of what they're going to see. Specifications on web pages are largely just numbers. And I don't think we have the legal right to use images from the simulator on our web pages.

I've also got to say that companies that put Hubble images on the side of kids' telescope boxes should have something very painful applied to their more tender areas.


  1. Replies
    1. Remember that they're just a representation of the magnification you'd expect using the equipment entered into the simulation. The actual image may be (probably will be) way inferior to the simulation.


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