@acer Thank you for a prompt and informative reply

best wishs

@Kitonum 

Problem solved. Thank you.

@acer 

All is now clear. May thanks

@John May Many thnaks for your input. I have implemented the first method as in image, a bit disappointing that I need the 100 to perform the conversion to centimetres. Here is the file TelescopeMethodA.mw

@mmcdara 

Many thanks.It even works now with lamda:=600nm, and with sIn(theta) not just theta in the  definition of Diam.

Re your PS: are you sayinf units normally get used in document mode?

best wishes from Nova Scotia

@Kitonum 

I must get better specs or work with zoom in Maple!

@Kitonum 

Many thnaks to you both

@Kitonum 

many thnaks. must have had a senior moment not to notice that

@Rouben Rostamian  

That just drews a line at y=0; the axis sltays at the base of the plot.

but thanks

@Kitonum 

Many thanks. I need to study this to see hwo it works. |I had no luck with the location option.

But I want the boxed effect!

Many thanks to both

@acer 

many thanks, I think I understand it now

@acer 

Thank you for this but it does not explain why !!! (exceucte entrie workbook)  changes the anwser from correct vlaue of 26.4 to incorrect value of -13

@Carl Love 

Will add instantiate to my Maple vocablary!

Like the idea of t-final :=1

Thanks agina