How to configure your telescope and camera for an object in the sky

Simply grabbing a telescope and a camera to shoot any object in the night sky is quite a bit of a gamble. Most likely the object will be to small or it won't fit in your view and that is exactly what this is about: the field of view or 'FOV'. You need to get your optical train in order. In other words you have to fit the scope, the lenses (barlows and reducers) and camera (size of the pixels) together with the size of the celestial object expressed in 'Arc seconds'.

This sure is a challenge as some objects are quite large like the Andromeda Galaxy. To the naked eye this 'M31' is a faint star. But the actual size is 6 times that of the full moon! Bernards Loop which is a sort of a borderline just east of Orion is as large as Orion itself and a telescope is therefor not your optic of choice which rather is a normal camera with something like an 85mm lens.

But first thing to know is your maximum magnification for visual observation and which eyepiece you can use. This is a quite simple thing: just the width of your telescope in millimeters x 2 so my Celestron EDGE HD 8" is 203mm wide and the max is a magnification of objects by 406 times. So my Lunt LS60MT which is 60mm wide can enlarge up to 120x. You can of course go beyond this point but the thing is that you will not get more details because of the overlapping Airy discs.

Example: my Lunt LS60MT has a focal length of 420mm, it can max magnify 120x. The shortest possible focal length of the eyepiece therefor is 420/120=3.5mm. As I do not have a 3.5mm eyepiece I have to achieve that in combination with a 2x of 5x Barlow lens that I both have.
So the 2x Barlow multiplies that 3.5 by 2 = 7mm eyepiece and the 5x Barlow asks for a 3.5x5=17.5mm eyepiece
The latter is possible because I have a zoom eyepiece from 24-8mm and the first will stick at 8mm giving a max magnification of 420/8x2=105 times
My second eyepiece is a 5mm which gives me 420/5=84 which is maximum as multiplying it by 2 will already take me beyond 120.
Which combination to use will depend on the combination of the lenses by each of their qualities and no less and foremost by the seeing of the moment.
For my Celestron the numbers would be:
203x2=406 times max magnification. 2032/406=5,00493 so my 5mm eyepiece is the smallest I can use and gives me a magnification of 2032/5=406.4 times.

So... where to start?

Speed of telescopes by The Space Koala.

How to read, interpret and understand camera specifications: this explanation by Cuiv.

Camera pixels per focal length by Sky Story

Then start with this explanation by Cuiv.

And the calculator that he refers to.


Three different fields of view, my scope only,my scope with the 0.7 reducer an my scope with the Hyperstar V4.


Play with it yourself