Dew shield for dobs

[slice of heaven]

That's one serious equation stringscope.

Can I just send you the measurements for my scopes?

Slice

[bytor666]

Thanks for the info on this issue. My dewshield extends exactly 15" beyond the end of my tube. The only thing I found was when using my 2 " 32mm widefield eyepiece is a slight "darkening" near the fieldstop. I don't know if this is the extension causing this and I will check the view without the shield on the end of the tube the next time I go observing.
I highly doubt the slight darkening near the field stop is the dewshield, but like I said I am going to check it.
Thanks for the math but I need to see it in layman terms with out all the crazy stuff..it makes no sense to me the way you layed it out.
a link would have been much easier to understand, or pictures.
Thanks for the tip though , you're obviously way more intelligent than I am at this stuff. (lol).
---------------------------------
Mark
12" gso scope

[stringscope (Ian)]

Sorry about the maths guys. If I knew how to do it I would have used a diagram but I don't know how to add a diagram into a forum reply.

Mark, Interesting re the darkening near your fieldstop. You don't mention what FoV this EP gives you. I did a quick check making some assumptions:

Assuming a BW Optik 30mm/80 degree AFoV EP with a 300mm F5 scope this will give you 50X and 1.6 degree TFoV. Assuming the end of the dew shield is 2000mm from the mirror, the min ID for the dew shield to avoid vignetting with this EP is 352mm or 13.9".

Got to fly..................cheers,

[bytor666]

Originally posted by stringscope
Sorry about the maths guys. If I knew how to do it I would have used a diagram but I don't know how to add a diagram into a forum reply.

Mark, Interesting re the darkening near your fieldstop. You don't mention what FoV this EP gives you. I did a quick check making some assumptions:

Assuming a BW Optik 30mm/80 degree AFoV EP with a 300mm F5 scope this will give you 50X and 1.6 degree TFoV. Assuming the end of the dew shield is 2000mm from the mirror, the min ID for the dew shield to avoid vignetting with this EP is 352mm or 13.9".

Got to fly..................cheers,

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Excellent. this is a lot better . I just measured how far out the extension tube is, (right out to the very end of it) ,from my mirror.

from the end of the mirror (roughly) it extends out 71".
the inside diameter of the dewshield is 14.25"
The eyepiece I was using was a 2" 32mm erfle with a 70 degree apparent field.

does this tell you anything??? , it looks to me like I'm not in the "red" so to speak ......thanks.
----------------------------
mark
12" gso scope

[Starkler (Geoff)]

Wouldnt it be as simple as this ?

L = Distance from mirror to end of tube
D = diameter of primary mirror.
M = max fov.

Tube diameter required = D + L tan(M)

[Thiink]

Starkler: that doesn't appear simple to me, it involves using a button on my calculator I haven't touched since highschool!

Another option for cheap dew shields is to make one from sonotube/formatube if it can be got cheaply locally. There is a guide for making them here.

Ultimately, though, a metal one like mick pinner makes would be better if you have the $$.

[bytor666]

Metal is far too heavy for a dob. it would throw the balance way out of whack.
Mine probably doesnt even way 2 ounces, I just tried putting it on a scale & I can't even see the scale budge.
My shield is also rigid enough that it keeps it's shape when used too.
I think I paid about $15.00 for mine with tons of material left over.
--------------------------------
Mark
12" Gso scope