Excel spreadsheet to calculate eyepiece sets...can you help?

[CumbrianGadgey]

Hi to all you good people at SGL,

After lots of tinkering around and heartache trying to decide what eyepieces to get for my first scope, I decided to write a spreadsheet to calculate a recommended set for any scope.

This I have partly completed. The aim is to take into account the age of the telescope owner and their scope specification and recommend a complete set of focal lengths that will be most useful to them.

It will all work with sliders to input age, aperture, focal length or f-number (the user selects which data they want to use for this one) and even has a slider to vary the minimum exit pupil somewhat depending how daring people want to be with maximum magnification.

I have done the easy part and the spreadsheet tells the user what maximum and minimum focal lengths they need for their scope and age. Aperture range for the program is 50mm to 450mm. Age range is 0 to 100 years old. f number is 2 to 12, or by focal length 200 to 1200mm. Exit pupil can be ranged from 0.3mm to 0.7mm with 0.5mm being recommended. The largest focal length eyepiece is based on the maximum pupil dilation for the user's age.

Where I am struggling is with the options in between the two extreme eyepieces. First, I simply suggested four EPs in total, but then noticed that with a 450mm aperture, the gaps in magnification were collossal from one EP to the next. I then tried altering the number of EPs based on a maximum jump of x90 from one to the next but this offered ten EPs for a big aperture scope and only 2 for a 50mm scope, rising to 4 from 110mm to 150mm.

The other difficulty I am having is just how to split up the intermediate EPs. As an example, for my 130mm scope, 4 EPs are suggested. With the spreadsheet calculating equal jumps in magnification, the selection suggested is 3.5mm, 5mm, 9mm and 40mm (Age set to 47, exit pupil to 0.5mm), giving x22, x100, x180 and x260. (Eyepiece sizes are rounded to the nearest 0.1mm)

My questions are this:

1. Would such a spreadsheet be of any use, for beginners, advanced users or both?
2. Is it reasonable to suggest so many EPs for the larger apertures, or should the number be cut down?
3. Is it sensible to divide up the EPs based on equal jumps in magnification?

If you feel like you want to contribute, my ultimate aim is to factor in the use of Barlow lenses from x1.5 to x5. This should go some way to alleviate the problem of so many EPs for bigger scopes.

Cheers.

[Tinker1947]

You could install Stellarium and input your type of scope and EP's you have or may want to buy, Stellarium will show you a FOV and any part of the sky so you get to see what they would look like....

[ronin]

You will often find that equal jumps in magnification are not what is used.

The most common is a progression of 1.6 or similar.

If your "main" magnification is say 100x then the next eyepiece would give 100/1.6 or 62x, then next being therefore 40x.

The problem is different people want/like different progressions.

I would prefer something down at 1.3 or even 1.2.

I tend to get the set not a mix of eyepieces, BST do 5mm, 8mm, 12mm  and the rest, I cannot get anything else so coming up with 5mm, 7mm, 10mm as the required eyepieces is irrelevant

Also where do you start? I always start at an eyepiece equal to the f number so an exit pupil of 1mm.

100x on my Mak is easy, on a ST80 it may be its limit.

Like wise you talk of aperture mainly, an ST80 and an 90mm Mak are close but the magnification in practical terms is vastly different. Do not get into the rule of 2x diameter, I have seen 1,5x dia, 2x dia, 2,5x dia and 3x dia. All from different and fairly well known reliable manufacturers.

I would not expect an ST80 to be able to supply the magnification that my GT-81 can, same diameter, both refractors, utterly different expectations.

[Moonshane]

I created an excel sheet for this sort of thing but cannot access it here at work. I'll try and remember to upload it later on.

[CumbrianGadgey]

Thanks for the replies so far.

It is not an easy task, I know.

Ronin, your suggestion of using a progression of x1.6 etc, is a good one. It gives more of the sort of range of EPs that people want, rather than going in equal steps of magnification. I could also incorporate a control to let people alter the rate of progression, say from 1.2 up to 1.8. They could then view the number of EPs that are recommended and tailor the outcome to their budget, or buy some of the focal lengths with a view to getting the rest later.

This sort of thing will never be perfect...it is intended as a guide, if it works well. I could even include a box to choose what type of observations the user is primarily interested in, ie planetary or DSO. This could bias the EP selection to have more shorter focal lengths for planetary, etc.

I think I just like playing with spreadsheets!

[CumbrianGadgey]

You will often find that equal jumps in magnification are not what is used.

The most common is a progression of 1.6 or similar.

If your "main" magnification is say 100x then the next eyepiece would give 100/1.6 or 62x, then next being therefore 40x.

The problem is different people want/like different progressions.

I would prefer something down at 1.3 or even 1.2.

I tend to get the set not a mix of eyepieces, BST do 5mm, 8mm, 12mm  and the rest, I cannot get anything else so coming up with 5mm, 7mm, 10mm as the required eyepieces is irrelevant

Also where do you start? I always start at an eyepiece equal to the f number so an exit pupil of 1mm.

100x on my Mak is easy, on a ST80 it may be its limit.

Like wise you talk of aperture mainly, an ST80 and an 90mm Mak are close but the magnification in practical terms is vastly different. Do not get into the rule of 2x diameter, I have seen 1,5x dia, 2x dia, 2,5x dia and 3x dia. All from different and fairly well known reliable manufacturers.

I would not expect an ST80 to be able to supply the magnification that my GT-81 can, same diameter, both refractors, utterly different expectations.

I just noticed that I forgot to comment on the differences between scopes. I've already found that my scope may get better than the x2 aperture rule by going to a smaller focal length than recommended with reasonable results. It's a good point. also, it depends what you want to look at and where you are in the country! Clearer skies, more or less light pollution etc. Let's not forget the quality of the scope, too. I tried a cheap scope and found it to be totally incapable of getting near the x2 rule of thumb, whereas a decent bit of kit the same size would have done.

Some of this will just have to be covered with a bit of explanation. I could do some research into what people practically achieve both on low and high maglimits of magnification with each type, Mak, refractor, Newtonian etc.

I guess I've got my work cut out to produce something really useful, but it's fun, so I don't mind.

[John]

This is the one I use. Its not that sophisticated and Its got my data in it so feel free to muck about with it and improve it as much as you like:

ScopeCalculator.xls

[Joves]

Great spreadsheet, John. Very useful!

[rwilkey]

Hi there, I can't see much use in the spreadsheet personally, however, I notice your exit pupil parameters are not realistic as 'from 0.3mm to 0.7mm with 0.5mm being recommended'.  More realistically exit pupil should not ideally go below 0.5 and up to 7mm is quite acceptable for those of a young disposition.  For older people like myself I would prefer not to go beyond 5mm exit pupil, though I can cope with 7mm.  For checking the maths of any purchase I use the following website which I find very good: http://www.stargazing.net/naa/scopemath.htm

[Moonshane]

here's the one I use with the eyepieces I have and my most used scope. you could obviously replace the paracorr box with a barlow value for the same effect on focal ratio, exit pupil and magnification etc

eyepieces and scopes.xls

[bodtwo]

More stuff to help with research and delay purchase. Maybe i should just go out and get something and use it!

Very helpful, thank you.

[John]

More stuff to help with research and delay purchase. Maybe i should just go out and get something and use it!

Very helpful, thank you.

It's worth using something like this just to check that a prospective purchase is actually going to be useful and fill a real gap rather than a perceived one

[CumbrianGadgey]

Hi there, I can't see much use in the spreadsheet personally, however, I notice your exit pupil parameters are not realistic as 'from 0.3mm to 0.7mm with 0.5mm being recommended'.  More realistically exit pupil should not ideally go below 0.5 and up to 7mm is quite acceptable for those of a young disposition.  For older people like myself I would prefer not to go beyond 5mm exit pupil, though I can cope with 7mm.  For checking the maths of any purchase I use the following website which I find very good: http://www.stargazing.net/naa/scopemath.htm

I take the point about exit pupil on the low side but some people say they can get away with 0.3mm. On the top side that is what the age slider does because it has an equation built into it for how maximum pupil dilation changes with age. It is that bit that I use with the scope parameters to decide the maximum focal length of the eyepieces. I'll see if I gat any other comments about 0.3 being too low and might simply have this fixed at 0.5mm.

Thanks for the comments.

[CumbrianGadgey]

here's the one I use with the eyepieces I have and my most used scope. you could obviously replace the paracorr box with a barlow value for the same effect on focal ratio, exit pupil and magnification etc

eyepieces and scopes.xls

Thanks.

I was intending to incorporate Barlows and am working on that bit now. i'll let you know how I get on.

[John]

My only caution would be to concentrate on getting the spreadsheet to show data on scope / eyepiece combinations but not to build in too many "rules" such as efficient exit pupils etc as these things could end up constraining it's usefulness rather than adding to it.

Computers don't deal with "grey" areas too well and many of these factors have lots of "grey" around them which gives us humans room to use discretion and experiment, which contributes a lot to the fun. They are guidelines and, as such, the saying "Your Mileage May Vary" is very applicable 

[Moonshane]

one advantage of buying used of course is the ability to buy, try and sell at almost no loss in the majority or cases.

[CumbrianGadgey]

Hi there, I can't see much use in the spreadsheet personally, however, I notice your exit pupil parameters are not realistic as 'from 0.3mm to 0.7mm with 0.5mm being recommended'.  More realistically exit pupil should not ideally go below 0.5 and up to 7mm is quite acceptable for those of a young disposition.  For older people like myself I would prefer not to go beyond 5mm exit pupil, though I can cope with 7mm.  For checking the maths of any purchase I use the following website which I find very good: http://www.stargazing.net/naa/scopemath.htm

Just another word on this,

It was a simple bit of maths to get the spreadsheet to recommend EPs on a ratio of magnifications as you originally pointed out.

When I did it and ran it for my own scope and age...hey presto! It suggested the exact range of EPs that I found that I liked best, after all the humming and hawing, scratching of head and experimenting, etc.

I also found another useful point that with the 1.6 ratio especially, a x2 Barlow lens assembly screwed directly to the EPs enabled me to cut out alternate EPs and get the selection i wanted plus an extra one that strayed a bit below the 0.5mm exit pupil for the odd occasion when seeing etc made it worth a try.

Thanks a lot for the tip. I've made great progress because of it, I think. It will be very simple to add a slider control for those who can afford or ultimately want a closer set of eyepieces with the magnification ratio down to 1.3, or conversely want to go up and have fewer EPs.

I'll have something to let people try a little sooner now.

[Naemeth]

here's the one I use with the eyepieces I have and my most used scope. you could obviously replace the paracorr box with a barlow value for the same effect on focal ratio, exit pupil and magnification etc

eyepieces and scopes.xls

Made my own starting with Shane's and added things I found useful...

Feel free to play around with it. Yes, I do have that many eyepieces

[rwilkey]

Just another word on this,

It was a simple bit of maths to get the spreadsheet to recommend EPs on a ratio of magnifications as you originally pointed out.

When I did it and ran it for my own scope and age...hey presto! It suggested the exact range of EPs that I found that I liked best, after all the humming and hawing, scratching of head and experimenting, etc.

I also found another useful point that with the 1.6 ratio especially, a x2 Barlow lens assembly screwed directly to the EPs enabled me to cut out alternate EPs and get the selection i wanted plus an extra one that strayed a bit below the 0.5mm exit pupil for the odd occasion when seeing etc made it worth a try.

Thanks a lot for the tip. I've made great progress because of it, I think. It will be very simple to add a slider control for those who can afford or ultimately want a closer set of eyepieces with the magnification ratio down to 1.3, or conversely want to go up and have fewer EPs.

I'll have something to let people try a little sooner now.

Sounds good to me!

[AlexB67]

I take the point about exit pupil on the low side but some people say they can get away with 0.3mm. On the top side that is what the age slider does because it has an equation built into it for how maximum pupil dilation changes with age. It is that bit that I use with the scope parameters to decide the maximum focal length of the eyepieces. I'll see if I gat any other comments about 0.3 being too low and might simply have this fixed at 0.5mm.

Thanks for the comments.

It is worth bearing in mind the age eyepupil is a very approximate of studies of averages.  Useful nevertheless, allowing one to input the eye pupil where known is a useful option. There are several relationships you can use

8.1-(0.04*age)

I prefer the one by Schaefer

7 e^{-age*age / 2000}

The latter is more conservative, and if in doubt better be on the conservative side IMHO, the first one as used here http://www.stargazing.net/naa/scopemath.htm is a bit more optimistic if not on the side of too optimistic IMHO. 

Now with regard to the 0.5 exit pupil being a limit, if memory serves me right, there is another reason behind this why it can be argued anything smaller is of little use when tied in with the Rayleigh limit, assuming no atmospheric interference no more extra detail can be observed in any case, but just on a bigger scale.   It has been a while since I worked through the derivation but when you do so the factor comes out something like 1.97 * aperture IIRC, effectively 2 * aperture in round figures.  That is not to say that a larger scale may not be easier to look at and in some cases be useful, it is a rather idealised case and guideline.

For a bigger scope the max magnification  would come out at 500 or more, in practice that it is of no use in pretty much most cases. 

The way i'd look at a starter set at it is by exit pupil largely as a first rule, something in the 2 - 4 exit pupil range considered optimum for the eye, something near 200 - 250 as max if the scope can take it, and something slightly above the minimum where the exit pupil is no bigger than the dilated eyepupil, that drops out pretty well in most cases.

Taking it to the next level, for more specialised circumstances a rule set base on the power per inch criterion and considering the scope aperture size may be useful given that the demands for DSOs and double stars will be vastly different two extremes on the opposite scale, so you could have groups, a double star set, a DSO set and so on. 

[Moonshane]

I think with double stars you are looking at a special case as you are not looking for more detail, just a clean split. upping the power in good seeing does often help with this.

[CumbrianGadgey]

Well,
Thanks for all the advice and comments so far. I now have a prototype without the addition of a bit I'd like to play around with for Barlow lenses, to let people see if they can get the recommended lenses cheaper by buying one and if so, what magnification should it be.
 
If I can attach it, give it a whirl for your own scope(s), please and see if it's reasonable or not!
 
I have not password protected anything yet, since it's a pain to do, so you will be able to enter things where I don't want you to! There are instructions with it though. Basically just use the sliders and pull-down menus.
 
I'm particularly concerned about how I've treated Maks, etc. I'm happy with Newtonians and refractors.
 
As you can see, I've steered clear of users specifying exit pupil, but it is worked out and stated. Because of the way I've treated the optical efficiencies, it will almost never specify a minimum exit pupil as low as 0.5 now and is often at 1 or more.
 
You'll notice that optional higher magnifications are suggested that go above x200 but these have warnings. I would be interested to hear from those of you who feel you have had mileage out of such EPs. I've even written to the observatory at Kielder Forest, to see what magnification they have usefully used. I hope they answer.
 
Anyway, give it a go and see what you think.
 
Remember, it's a work in progress. I can still alter it!

Doh! All I need now is to find out how to insert the flipping Excel Spreadsheet! Help?

[John]

Doh! All I need now is to find out how to insert the flipping Excel Spreadsheet! Help?

Use the "More Reply Options" button at the lower right of the reply screen. That brings up a more "feature rich" editing window with an "Attach Files" option at the bottom left. Excel files are supported by this feature, as I found out when I attached the spreadsheet I've been using earlier in this thread.

For what it's worth I regularly use 225x and 257x with my 120mm ED refractor and 250x - 300x is fairly standard with my 300mm F/5.3 dobsonian.

It's only my 102mm ED refractor that seems to "run out of puff" at around 200x.

[Joves]

I've just this afternoon entered my scope and eyepiece details into this spreadsheet also and it has worked a charm. Nice to see the different attributes and combinations laid out in an easy to view fashion.

Thanks again for sharing, John.

[CumbrianGadgey]

Use the "More Reply Options" button at the lower right of the reply screen. That brings up a more "feature rich" editing window with an "Attach Files" option at the bottom left. Excel files are supported by this feature, as I found out when I attached the spreadsheet I've been using earlier in this thread.

For what it's worth I regularly use 225x and 257x with my 120mm ED refractor and 250x - 300x is fairly standard with my 300mm F/5.3 dobsonian.

It's only my 102mm ED refractor that seems to "run out of puff" at around 200x.

It's worth a lot. Whilst it doesn't exactly explode the myth that you cannot exceed x200 in the Uk, it lays down the dynamite and lights the fuse.

Thanks for the bit about how to upload a file. I can design process plant and machinery, work out how long it would take to fall through the centre of the Earth and reach the other side, if it were possible to make the tunnel (around 40 minutes)...but I can't figure out how to attach a file!

To save me repeating everything I put in the last thread, take a peek at it, if you can spare the time and give the spreadsheet a whirl. All constructive criticism is welcome!

calculate eyepieces.xlsx