Baffles

The amount of vignetting depends on ttwo things - the size of the front aperture of the telescope and the size of the secondary mirror.

ALL telescopes experience vignetting (a drop in illumination) the further towards the edge of the field you go and the larger the field of view.

ALL telescope designs are a series of trade-offs geared to giving you an 'acceptable' image. Generally most Newt tubes are sized to offer a 1/2 degree field of view at the front aperture - that is to say that the primary mirror can see about 1/2 a degree either side of the optical axis... so about one degree in total. This is a convenient number as it also gives a reasonable space between the inside of the tube and the primary mirror so that heat currents coming off the primary don't interfere with the light path too much.

The size and position of the secondary is critical to your application. Again, most commercial Newts are a trade-off - central obstruction v fully-illuminated field of view. If you are going for planetary work you generally plum for a small secondary and live with the resulting radical vignetting towards the edge of the field. If you are going for wide/rich field then you use a larger seondary to give you a larger fully illuminated FOV but with a loss of contrast.

Paul - if you can give me the full output from Newt for your scope I'll try to interpret it for you.

Phil.

Thanks Philip.

Primary 300
focal length 1479
f 4.93
tube id 320
tube wall 10
focuser height 80
focuser id 50
focus extra travel 50
minor axis 65
diagonal offset 3.2961
100% illumination diametre7.6829
75% illumination diameter 39.256
front aperture 333.22 ( in red).
mirror to focus hole 1189.0
focuser to front 80
mirror face to back 80
tube length 1349.0

Reducing the secondary does not remove the Vignetting of 75% warning. increasing the tube diameter cures the problem ,but requires baffles.
I do not have a bigger tube to hand . thinking on truss rod type design made of spool piece of alloy tube and end plates.


Thanks for the Help
Paul.

Thanks Philip.

Reducing the secondary does not remove the Vignetting of 75% warning. increasing the tube diameter cures the problem ,but requires baffles.
I do not have a bigger tube to hand . thinking on truss rod type design made of spool piece of alloy tube and end plates.

Ok, that's pretty much what I expected. If you can get hold of a larger diameter tube then you can eliminate vignetting. You can overcome the baffeling problem by simply making the tube longer.

To be honest you will probably not notice the vignetting when you look through the eyepiece. It *might* become noticable if you use the scope for photography, but you can mitigate this effect by taking flat fields and subtracting the gradient. So, unless you want to delve into building a truss type tube (with all the associated problems!) then I'd make do with the solid tube you have (or can get) and live with the numbers you are getting.

Phil.

Thanks again Philip.

Have tracked down some A.B.S tube with 275mm i.d .
It works o k on the newt program.I have to make some baffles, but thats no problem.

Paul.