Dis Adv Small secondary

[dannat (Daniel)]

If i were to put a smaller secondary into a 12" newt what are the disadvantages?

[pgc hunter]

As long as you can see the entire primary in the secondary at the focal plane, you'll be right. But if you can't, the scope is effectively working at a smaller aperture as the secondary doesn't capture the entire light cone of the primary.

[Robh (Rob)]

What are you hoping to gain?
By my calculations, the width of the light cone from a 12" f/5 scope when it reaches the secondary is about 63mm. What size is your secondary mirror? A 70mm secondary allows 3.5mm of play either side due to imperfect allignment. By reducing the secondary obstruction from 70mm to 63mm, the light gain is only 1% due to areas being relatively smaller at the centre of the primary mirror.
A 63mm secondary is most likely going to spill the light off edge and therefore lose more than was gained by the smaller diameter of its obstruction.

Regards, Rob.

[Starkler (Geoff)]

Yep. the secondary needs to be large enough to catch the entire light cone from the primary, and then a little bit more to maintain illumination off axis.

This is far more critical in an astrograph than a visual scope.

[dannat (Daniel)]

yes its just for visual - i figure losing a little bit of light for better contrast is a good trade off

[Starkler (Geoff)]

Have a look at this and see if you reckon its worth it. Scroll down the page for some sample images.

Personally unless you are building the scope to be a planetary special, i wouldnt bother changing out for a reduced size unless its a lot over 25%.

You will get far better gains paying close attention to collimation and thermal controls.