My requirements: Being able to do astronomical calculations while observing through my telescope during cold nights while not hurting my night vision.

With a smart phone, I’d have to use touch gloves or take off my gloves to press “buttons” on the screen. And then I’d have the hassle of filter programs to dim the screen and make it red to keep it from impacting my night vision.

With my HP-41, I’d have to use a led light to see the screen.

But with an old red led calculator, I could get all my requirements at once. Except there are few of these old pre-1980 calculators that would be capable of doing all the calculations I need; Date -> Julian Date -> Date, Sun rise/set, Moon rise/set, Moon Phase, Field of View calculations fro various eyepieces, etc. Fiddling with magnetic cards for the HP-67 would not be ideal.

HP-25E to the rescue! Bernhard Emese (Panamatik) has created a true piece of art with his “brain transplant” for the old HP-25 calculators. His HP-25E boast a 100x increase in programming memory, on-board GPS with time, Latitude, Longitude, heading, speed and more as well as a stop watch, chess clock, hexadecimal conversion and much, much more. It’s the ideal calculator for my requirements. Except it still had only 50 program steps memory per program. Although it had the possibility of storing 100 HP-25 programs in a constant memory (not lost when you turn the calc off – unlike the original HP-25), you couldn’t write programs with more than 50 steps. And this was way too small for the programs I need.

Talking to Berhard about the possibility of “stringing together” different “pages” of 50 program steps, we came up with a neat way of solving this issue. By using some available rare codes, the HP-25E can now jump between various programs – potentially creating programs with thousands of programming steps.

And so I embarked on the journey of creating the AstroCalc – the ideal tool for the amateur astronomer. So far, the GitHub repository only includes the calculation of Julian Date from a date and time and the backward conversion, from Julian Date to date and time. With the HP-25E’s possibility of constantly updating the time, this makes writing down the exact Julian Date on my observations a breeze.

I will add the Sun/Moon rise/set, Moon Phase and eyepiece calculations soon. Just check my HP-25E_astro Github page for updates.

I fucking love this calc.