Quote:
Originally Posted by Saturnine
Hi Jethro
Use this Newt for the Web calculator to work out your scopes dimensions.
https://stellafane.org/tm/newt-web/newt-web.html
The focal length of your scope doesn't change by moving the mirrors, the focal length is governed by the radius of curvature of the primary mirror which doesn't change unless you regrind the mirror. By moving the primary up the tube , all you are doing is moving the point of focus further outside the tube, as reflected by the secondary.
What also happens then is that not all the light from the primary is being concentrated on to the secondary. Some light will pass it and go straight out the front of the tube so you're effectively losing aperture. Your focuser will need to rack out an extra 28mm for focus to be reached. Doesn't matter if they are the original TAL eyepieces with a 32mm barrel or the standard 31.7mm ( 1.25" ) of most common eyepieces , the focal plane of the scope stays in the same place, it's the eyepieces that have slightly varying focal planes, that's why different eyepieces need to be re-focused slightly. Unless of course you have a par-focal set. |
Thanks, that is helpful. So, the f/ratio stays the same.
>By moving the primary up the tube , all you are doing is moving the point of focus further outside the tube, as reflected by the secondary.
So, this is interesting - after moving up the primary mirror - I basically created a "problem" - that now some light reflected by the primary was missing the secondary - to compensate, I need a larger mirror. Is that right?
>Your focuser will need to rack out an extra 28mm for focus to be reached.
Previously, the 1.25" eyepieces could not be focused - only the original 32mm versions. After the mod, most eyepieces can be focused using the standard focuser as is.
What does this mean for my secondary mirror size calculations?
Well, I put the optics data in newt-web (Primary 4.3, Focal Ratio 7.3) and it calculated it as "Diagonal Range = 0.95 - 1.43"
A 1.5" mirror would give me a 35% obstruction, the original mirror (1.1") 26%, and it recommends trying to get 20%.
I was thinking of getting a 1.3" - Would that be advisable in your opinion given my situation?