Re: Focal length of scope vs focal length of eyepiece vs brightness of object
Posted: Fri Nov 05, 2021 12:47 am
if 2 scopes "same diameter" then same magnification will give you same brightness or "exit pupil diameter"
the other factors that can change the "real world effect"
1) mirror may be about 90% in reflectivity fresh from factory and slowly age 1% per year to 80-81% region , secondary mirror is another loss , central obstruction is another 10% loss from surface area
total there are triple loss of brightness compared to a fully glass lens refractor design
2) the smaller MM eyepiece at beginner price point tend to have small at the eye lens (uncomfortable to align your eyeball) and less eye relief distance (unfriendly to spectacle wearer)
3) the longer focal ratio telescope will be more friendly to lower price eyepieces (less distortion away from center) at the tradeoff of losing wide open & bright view
4) diameter specification may not be true , the edges clipped off by bad design in lens/mirror holder and diagonal
i remember the testing method is shine laser pointer into eyepiece , the light goes in reverse out the front of scope , measure the spread-out disk of laser light on a white paper arm's length infront of scope on a table
I used this tool often in the past
http://www.stargazing.net/naa/scopemath.htm
the other factors that can change the "real world effect"
1) mirror may be about 90% in reflectivity fresh from factory and slowly age 1% per year to 80-81% region , secondary mirror is another loss , central obstruction is another 10% loss from surface area
total there are triple loss of brightness compared to a fully glass lens refractor design
2) the smaller MM eyepiece at beginner price point tend to have small at the eye lens (uncomfortable to align your eyeball) and less eye relief distance (unfriendly to spectacle wearer)
3) the longer focal ratio telescope will be more friendly to lower price eyepieces (less distortion away from center) at the tradeoff of losing wide open & bright view
4) diameter specification may not be true , the edges clipped off by bad design in lens/mirror holder and diagonal
i remember the testing method is shine laser pointer into eyepiece , the light goes in reverse out the front of scope , measure the spread-out disk of laser light on a white paper arm's length infront of scope on a table
I used this tool often in the past
http://www.stargazing.net/naa/scopemath.htm