Magnification complications...

[Greymouser]

After reading @Mark Daniels thread from just over a year ago, about using the Evoguide 50 with a diagonal, my interest was increased. I have been intrigued by this little scope and recently got one, not primarily to use visually, but as a guide scope, in the near future, as I dip my toes into AP. However now I have it I thought, why not, lets have a play! Terrestrially only for now.

Out came the diagonal, cheap end of a 2x barlow inserted: Lo and behold a good image! Yeh!  Tried with an erecting prism and success too!  ( In fact a clearer image in the prism, which I will have to think on... )

The thing that now occurs is how do I work out magnification? I have no idea what increase the barlow end will give in the end of the diagonal. Clearly the magnification is different in the erecting prism too, because I presume of the different length?

Can someone please  explain to me how to work it all out?

I was using a cheap Celestron diagonal; a cheap Skywatcher prism and a cheap 2x barlow, with a 16 mm eyepiece. The focal length of the EVO is 242 mm.

Any guidance is greatly appreciated, as I am clearly a little dim when it comes to working out magnification, as I always assumed just the basics, without looking deeper. After all if the view is pleasing, why bother?

I am however starting to consider this little scope will have multiple uses...

[vlaiv]

There are a lot of options to figure out magnification:

1. On "paper" - you need to know focal length of barlow element and distance to eyepiece (not easy thing to know or measure) - there is barlow formula for that: magnification = 1 - D/F (where D is distance and F is focal length of barlow element - negative value! because barlow is negative lens element). This is more useful for imaging.

2. Using "ruler" of sorts - usable during the day. Find a brick wall or any other texture that provides some sort of linear division - stairs, ladder, anything with "steps". Take eyepiece of known focal length and insert without a barlow element - for example 25mm eyepiece which otherwise gives ~x10 magnification. Count number of "steps" that you see with that eyepiece on a distant object. Insert same eyepiece with barlow element and again count number of "steps". Divide the two and you will get how much barlow amplifies and effective magnification will be ~x10 times that ratio. For example you count 20 bricks from top to bottom of FOV with eyepiece alone and 8 bricks with barlow. Barlow gives 20/8 = x2.5 magnification and you are observing at ~ x10 * 2.5 = ~x25 magnification when using barlow like that.

3. Using stopwatch - same as above but usable at night. Find a bright star near meridian and equator and time how long does it take to traverse FOV from one side to other if scope is not tracking the sky (just sitting there on a mount). Insert barlow, repeat measurement and this time divide time intervals. Make sure star traverses at same elevation - near the center of FOV. This can be achieved if you have alt az mount and choose star near/at meridian and move scope in az only once you change configuration.

I guess there are other options to do it, but above should be enough to get you going?