How long to make a dew shield

[Noddy81]

How long is the right length I have a cpc 11 inch

[glend]

I seem to recall that there is a formula, something like 1.5x the diametre, but i could be wrong.

[multiweb]

My C11 dewshield is the same length as the tube, so the corrector would be approx. halfway between aperture and primary. Seems to keep it dry.

[billdan]

As Glen said, the length should be 1.5 times the mirror diameter, any longer and you could get vignetting. However you can flare the dew shield (wider at the front) to overcome the vignetting.
So for a C11 you would need about 16 inches (400mm) in length.

EDIT: They don't help when pointing near vertical though.

[glend]

If your sticking on a C11, don't forget the heater.

[multiweb]

Sounds about right. 450mm which is close to my body length. I flocked mine. Helps with stray light also easier to keep the glass heated. I have a big wire running 12V on the corrector cell on the outside of the scope rim. Here's a pic (https://wbsqpw-dm2305.files.1drv.com/y4mspn9-X7a-2gbnncJ4DMNZzyO58jRjZU4H0qybzRxi-p3G5G7TVIJo6d1XH_3Kan9xl_6JEQc-u9mWH03FrhE5i_yKbNncJ9k-gIvLuekHR1gA7-4kFv86nWJJwSoYl1dyuH12hFTtIanhUa7VI RJ5YdY1tphLJMS0ZiP1CsTMdYTy6nwRut_9 XzsnBvg1jgJqStb_jzzTjyWey383gew1A/C11_HS3.jpg?psid=1) of the rig in hyperstar config.

[Merlin66]

Yeah,
400mm in front of the corrector would be good....plus a dew heater!

[ChrisV]

Where can you get the stuff to make dew shields yourself ?

I've got a bought one from my old SCT, but for my current scopes/finders I'm using cardboard which isn't great when it gets soggy.

[Noddy81]

My mate is a printer and had a hole heap of this flexible plastic but still strong so going to paint or stick black mat fabric on the in side and paint the out side and put Velcro on it

[gary]

Hi Chris,

Thin, foam rollup camping mats work well.
This type of thing :-
https://www.tentworld.com.au/buy-sale/oztrail-camper-earth-mat-8mm

Cut to size and can be secured with a Velcro strap.

They are light weight but their insulating performance is good to help trap
heat produced by the heater.

If you pay more than $10 for one, you have paid too much.

[Wavytone]

Another source are FoamBooth shops.

[glend]

Yep, flat black foam yoga matts work well and are cheap as chips on ebay.