Image of the day

From the
ATWB Customer Gallery

pileated woodpecker

My Account

New to Astromart?

Register an account...

Need Help?

Central obstruction / light grasp question.

Started by Doombot!, 01/22/2005 06:07PM
Posted 01/22/2005 06:07PM Opening Post
I'm not really a beginner, but figured this would be the best place for this.

I currently own a N11 GPS, and I'm trading it in for a new 15.5" dob. I know that the straight-up light grasp calculation would be 11*11=121 and 15.5*15.5=~241 and 241/121=~1.99.

The new mirror will have and ultimate light grasp increase of nearly double, but shouldn’t I get further increases from a smaller central obstruction and fewer air/glass surfaces?

Am I on target? Would any of you be able to tell me about how much the perceived increase would be?

Posted 01/22/2005 06:13PM #1
Not so much light grasp increase from this, but increase in contrast will make it seem like more light grasp. The contrast is what makes DSO observing so rewarding. Details will be better.
You will also have an increase in resolution with the larger aperture. So that you will resolve globular clusters much better.

[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue grin
Amateur Imager
Posted 01/22/2005 06:22PM #2

Simply take the diameter of the C.O. in inches and square it just as you did for the diameters of your primary mirrors. Subtract each C.O. squared diameter from the corresponding squared primary diameter before doing your ratio.

You will probably find it doesn't make much difference in area ratio. Where it will show up is in image quality. See H.R. Suiter's Star Testing Astronomical Telescopes for a good explanation of this.

With regard to elimination of the corrector, it theoretically would help but reflectivity differences in the mirrors could make as much difference.

All other things being equal, a 2x area ratio will result in 2.5log(2)=0.75 magnitude difference in light grasp.