Dust Cap

[vindictive666 (John)]

hi all

i forgot to mention the other day
that the dob has an dust cap on it

in which has two holes like a face i can remove the two caps and i wind up with two holes i not really sure on what the purpose is can some one possibly point me in the right direction ?


exscuse the rough pic the centre one hasnt been cut out ?

[westsky]

Hi. with the holes you have a built in Hartman mask, or depending on the hole size they could be used as an aperture mask

David.

[rumples riot]

A hartman mask is used for focusing images correctly. If you leave it in and focus on a star with the holes open, the stars will form separately and as you get closer to focus the stars will converge until they over lap each other. At this point the image is in focus. This system can be utilised for visuage and more importantly for astrophotography.

It s a bonus, keep it.

[blackpearl]

Vindictiv666
My 12' Dob dust cap is the same, except only one hole has a removeable cap.
So it has a useful purpose?
Thanks Paul

[RAJAH235]

Margo, The smaller cap is used as an aperture stop. When viewing the moon, or other bright objects, use the small hole. This cuts down the intensity of light, but increases the focal length and gives much better contrast to enable better planetary viewing. HTH. L.

[vindictive666 (John)]

thanks for the reply people

both holes are the same size each are near the edge of the cap if you know what i mean like a pair of eyes