I've been thinking a little more about this. There is another option, and one that has several advantages going for it, and may prove cheaper too.
If you have a goto mount, such as and HEQ5 or and NEQ6, it will provide a very accurate set of position coordinates that change as you slew the scope. This is the first part.
The second part is a decent, but inexpensive eyepiece, such as a TMB Planetary Type II from ebay -
follow this link for a selection of them. You can disassemble the two lens assemblies (top and bottom groups) and place an optic fiber pointer into the field stop, and re-assemble the eyepiece (easy to do).
I did this to a 9mm TMB eyepiece myself. I mainly use it for outreach to point out features on the Moon. The first pic below shows the 9mm TMB next to a 9mm plossl - a 9mm plossl is one difficult eyepiece to use. The next pic shows the pointer as seen through the EP.
,
Now, pop the eyepiece into the scope, and position the "home" star on the tip of the pointer and note down the coordinate position of this star. Next slew the scope to the next star, and note its coords. Simple trig calculations will give you the angular separation - which you will need to do anyhow. This method will prove extremely accurate, especially with such short focal length eyepieces.
Why use one of these particular eyepieces? First they are inexpensive, they have a great AFOV (58° compared to the pissy 40° of most reticle eyepieces, and 52° with plossls), a huge eye lens to look into, and a very generous 20mm eye relief in ALL focal lengths. Also, these are actually very good eyepieces, especially for their price! Far superior to
any reticle eyepiece. Far superior too to any plossl eyepiece of the same focal length.
If you prefer a longer focal length eyepiece, you can do the same pointer system for say a 25mm plossl. But note that if you want to accurately measure angular separations, you need some eyepiece grunt.
Optic fiber strands are not hard to come by, and there are a few "ogre" tricks to strip the insulating sheaths around them.
You could go to the expense of a dedicated reticle eyepiece, but look at what you are producing: you are producing a sketch, so this alone has an inherent amount of inaccuracy. You cannot rely on a sketch to measure angles. Not accurately. You can position the stars to a good approximation on paper, but the measurements you would be better off reading coords from the computerised mount and doing your calculations. Much more accurate for record keeping.
Alex.
Oh, and
to IIS, Chris!
