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Eyepiece /Telescope Calculator Tool

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Hi all

When looking at buying a new EP, or new scope, I tend to first do a few calculations based on the excellent Warthog Tutorial which many of you are sure to know, and a few other basic astronomy formulas - in order to get a good feel of whether a prospective EP will work with my current scopes and vice versa.

This happens a lot (looking is free, right?!), consequently I have a load of text files knocking about with my notes in them. So, I decided to build it into a spreadsheet based tool, and I figured other folks might get some use out of it, and here it is! It's a basic XLSX file with no VB script or anything weird. Should be able to open it in Excel 2007 onwards with no issue (shout up if you need a converted copy for 2003, I should be able to provde that).


First up let me say - credit for the original work should go to Warthog - I'm just standing on the shoulders of giants :) If you haven't read the tutorial, You should go and do that now, as this tool will make a lot more sense with it!

Second - an overview:

- You can add a list of scopes (up to 20) and the main attributes (Aperture, Focal Length), plus a barlow 

- You can then select a scope from this list and the Warthog tutorial formulas will be applied, giving you a list of the 4 ideal basic set of EPs, along with their 'extended' attributes

- You can also add a list of your own EPs (up to 30) and the main attributes (Focal Length, AFOV) and it will generate the extended attributes for those as well (in respect to the chosen scope)

- The full list of extended attributes generated are: Mags, Power/Inch Aperture, Barlowed Mags, True FOV (Degrees), True FOV (Arcminutes), Exit Pupil

Third - here is how it works:

- Input a list of your scopes (or prospective purchases!) on the 'Owned Scopes' sheet

- Input a list of owned (or prospective) EPs on the 'Owned EPs' sheet

- Adjust the "Barlow Power" setting on the 'Calculator' sheet if needed. I personally own a 2.5x, many are 2x. Maybe useful if you're thinking of buying a different power Barlow and want to know how it will affect your EPs

- Adjust the "Fast F Ratio Limit" if you wish. This affects the calculations only set up for the auto-generated 'ideal' EPs. Any scope you input that is faster than the ratio given will have a bigger focal ratio multiplier for Med-Low, Med-High and High as per the Warthog Tutorial. I think 6.5 is probably the slowest Fast scope(!) but you may want to adjust.

- Select your scope from the drop down list on the 'Calculator' Page, and that's it!

Finally, I have set up a few generic scopes and EPs in there so you can play with the data and see how it comes out. There is one small bug - if you only populate the scopes list with a few scopes you'll see blanks, which is annoying but the coding required to fix that didn't seem worth the hassle. Just don'e select the blanks :) 

If you'd like to share or change the sheet feel free - but if you repost anywhere please just give me a nod :) If you have an suggestions of things to add I'm happy to do so.

Thanks - hope it's useful!!

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Some time ago, I also created a spreadsheet for computing a few parameters I am interested in. 

It is shared and people can simply download a copy of it and customise with their own equipment of course. 

Whether this sort of thing is useful or not, it is subjective. To me, I don't know what I would do without it, although many other people just prefer to try eyepieces more practically. Everyone has his/ her own preferences. 

Anyway, here is the link: 


Hope this is useful, 


p.s. I don't own the second telescope listed in the spreadsheet, but I bought my eyepieces thinking about what my future dso telescope will be. That is another example why I find this spreadsheet useful ... as exit pupil and fov are 'portable' concepts. 

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Thanks Piero. Looks a lot more advanced than mine - I'm just really going for the basics.

Hmm, I wonder if this might be better in the beginners/getting started section?

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Thanks Andy, Charic. Yeah I have used both of those - the Scopemath one in particular. This spreadsheet is really just a local or offline version of those and the Warthog tutorial stuff but with the ability to save multiple setups in one place.

I've found it really handy being able to see everything in one place without having to keep referring back to the sites and re-inputting everything again, so I thought others might too.

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I had tried a number of sites or spreadsheets and came to a conclusion that Stellarium is the best simulator. What is most important it has a huge object database and you can try or change your setup immediately. And the views are more or less close to reality. I use Stellarium when planning on buying some new eyepieces or preparing my observing plan for every specific night. It works for me best.

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I just run the numbers through my head.

Magnification is easy, Fs/Fep

FoV is then simply EP View/Mag.

Exit Pupil is Dia/Mag.

Nothing needs to be accurate to 2dp, being honest with magnification +/- 5 either way is adaquate. Sort of a 9mm EP in the 558mm scope will give about 60x is close enough. The numbers would be worked as 560/10 then add on 1/10 of that result. So 56+5.6, say 61x or 62x, but 60x is close enough.

That will give about 1 degree view in most EP's and that will fit M42 in just.

In reverse: M31 is 3 degrees, so that means a Mag = EP Field/3 = about 20x (for the ones I use)

A Mag of 20x is an EP = 560 (close to 558)/20 so about 28mm.

So I might try a 32mm TV plossl or take a bit of a chance on a 25mm BST.

Does no-one do it themselves?

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Hi Ronin

Yeah, I mean I didn't want to go too overboard so most stuff, including mags, is rounded :) I have always previously tended to work most stuff out in my head and note it down for future reference.

A couple of years ago I started writing out text files with my scopes and EPs just to refer back to, and then thought it would be useful to be able to see at a glance all of your scopes in relation to your EP collection so I knocked this up in am hour a night or two ago. Just thought it might be useful for others but everyone is different I guess :)



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