Re: [ATM] What is that Image size in Calculations

[Mike Lockwood <melockwo@uiuc.edu>]

George,

Geogre Nikolidakis wrote:
> This is my first mail to the list so let me introduce myself
> My name is George Nikolidakis and I am an amateur astronomer from Greece 
> Reentry I manage to finish a permanent roll-off observatory to house a 12"
> LX200 Mainly for astrophotography purpose.
> You can take a look for the observatory here
> http://www.geocities.com/ginik_gr/NewObservetory
> And also in some deep sky photos here
> http://www.geocities.com/ginik_gr/EosDeepSky.htm
> My problem is now that I don't have any telescope for optical observation
> (star parties and campaigns)
> So I decided to order a 20" f/4.5 primary from Pegasus optics in order to
> build a truss tube telescope.
> In my calculation about the size of secondary mirror one of the parameters
> is the image size...... I am not sure what it means, for example I like to
 > full illuminate a 2" eyepiece.
> Is that means the image size it will be 2"?
> Also L = the distance from the Secondary to focal plane .
> The point for the focal plane it will be the base, I mean the low end of the
> eyepiece ?

That's a beautiful observatory you have, and some excellent images you
have taken.

It is not necessary to fully illuminate an eyepiece if you are just 
doing visual observations for fun.  The field stop of a 2" eyepiece, 
its largest clear diameter, is usually around 1.4" or so.  Still, you 
do not need to fully illuminate that like you do with an imaging 
sensor.  A fully illuminated field 0.5" in diameter is my recommendation.

Maybe this will help with your secondary sizing:
   http://bi-staff.beckman.uiuc.edu/~melockwo/diagonals.html

Read the introduction, then just scroll down to a 20" F/4.5 mirror and 
then choose the proper column according to L (which I call d2) and 
then the row according to the fully illuminated field your require. 
If you illuminated slightly less than a 0.5" diameter field, you would 
likely never notice the difference.

For example, if the focal plane is 15" from the center of the 
secondary mirror, then for a 0.5"-diameter fully illuminated field 
(FIF) you would need a 3.75" minor axis diagonal.

As Vladimir mentioned, assuming that the focal plane is near the top 
of the focuser is a good place to start.  The value I typically use is 
0.25" above the top of the focuser (read my tip #3 near the top of the 
web page above), but that is for the eyepieces I use.

	Mike Lockwood


_______________________________________________
ATM mailing list http://www.atmlist.net/