Calculating Full illumination circle

[bmitchell82]

Im trying to work out the full illumination circle of my Skywatcher 10" dobsonian to find the culprit of the vingetting (bigger secondary, bigger focuser, more efficient apperature to camera/focuser interface).

Im using Newt which is a raytrace program and tonight im going to use vernier callipers to get all the correct distances to give the exact ray trace.

Does anybody know a formula/method for calculating the illumination circle?

Your help will be apprciated.

Brendan:thumbsup:

[Garyh]

Don`t know the formula etc but have you looked at Mel Bartels site for calculating diagnal size?
Gives you the illumination off axis ..
Hope that helps a little.

http://www.bbastrodesigns.com/diagonal.htm

[bmitchell82]

thanks gary for that.

i actually printed out the newt raytrace prog and in that print out it has the illuminated field at the focal plane. grrr...

interesting to note the Skywatcher 10" dobsonian (254mm primary) will illuminate:

8.5mm diameter @ 100%
35.6mm diameter @ 75%

of course this is a gradient from 100 -->75

another interesting thing that the tube is actually too small in its current format and blocks light at the 75% range... very interesting indeed!

To illuminate a 30mm diameter at 100% i would require a 2.5" focuser and a 70mm secondary! im thinking of hitting the prototyping lab and finding some bits and pieces...

[bmitchell82]

Okay, got the spot sorted, next question does anybody know if the vingetting of a 75% illumination spot on the focal plane is important if the 100% full illumination spot on the focal plane covers the CCD sensor?

[Garyh]

If you have 100% illumination over your ccd then you should be sweet.
On my 8" newt it works out around 92% illumination in the corners of a APS-C sized sensor. I have attached a flat which was taken with the 40D a few months back..
Even when stretched it`s hard to see..
cheers Gary

[bmitchell82]

Gary your a bloody legend! thats exactly what i wanted to hear. Everything i was reading was telling me that everything was good but i just needed that human repsonse that a book cant give! well... im getting the 10" SW where it needs to be for a imaging rig!

for the calculation of % illumination of a 40d Cmos sensor will be approximately 89% total coverage with a 27% central obs on a 254mm mirror. mmmm tasty! :) thankyou once agian

[Manav]

Let us know how you go with this...