Dewshield Length

[Nick P]

Can anyone tell me if there is a formula for calculating the optimal length of a dew shield for any given OTA/mirror diameter/focal length?

With Autumn approaching I am expecting my newt to start suffering soon so intend making one.

[GazOC]

Rule of thumb is at least half as long again as it is wide.

[brianb]

Rule of thumb is at least half as long again as it is wide.

1.5D really is minimal ... 3D or 4D works better in still, damp conditions but quickly becomes unmanageable in a breeze. If you make your own (camping mat) you can have a choice of lengths. The commercial ones are mostly far too short, though a short one does reduce the power needed by an objective/corrector plate heating tape by a useful amount.

[George]

Wouldn't a long one cause vignetting if your imaging?

[brianb]

Wouldn't a long one cause vignetting if your imaging?

The base diameter of a dewhield is usually at least a bit more than the objective diameter. If you are imaging a field of diameter 1 degree, your dew shield can overhang by very nearly 5 feet for every inch of excess of dew shield diameter without obstructing any light ... assuming the dew shield is accurately coaxial with the optical components.

In practise, it's not a significant issue, except for wide field imaging ... in which case an electrical heater system is going to be required as a conical dew shield will not give the front element of the lens much protection.

[Nick P]

Thanks for the advice all, and just so that I am clear, with an 8" scope the dew shield should extend past the OTA by at least 12"?

Thanks

Nick

[dweller25]

My 8" Newt has never suffered from dew problems, even when the tube has been frozen over the mirrors were fine.

[brianb]

My 8" Newt has never suffered from dew problems

A solid tube Newt should provide adequate protection for the primary mirror but dewing of the secondary can be an issue. Lattice tube Newts tend to suffer very badly from dewing - and from heat plumes from the observer if you stand on the upwind side of the tube - a shroud can help a lot.