Dew shield for dobs

[Thiink]

I'm interested in seeing if anyone (I know bytor has one) is using a dew shield for a dob. Mainly interested in knowing what effect it has, and how it was made/what it is made of.

Also would a dew sheild help block out light? I have a light constantly on at night next door (back door light) which I would love to try and block out as much as I can (turning it off isn't an option).

Any other comments welcome.

[ballaratdragons]

Mick Pinner (forum member) makes and sells them! See his thread in this section under 'Dew Shield'.

Yes, they are good for blocking light as well as dew.

[Striker]

Its really easy...I just went to my Local foam company and bought a matt black 5ml sheet of closed foam...it was $17 and will be heaps to make 3 shields if wanted......

A dew shield will temporarily slow down the dew process but as you have a Dob...you dont have to worry about the corrector plate like us SCT owners.......The dew shield will help with light polution from next door......it stops some of that unwanted light entering your tube.

Here is a picture of mine I made 2 weeks ago....

[ving]

dont know much bout these ken, I assume they only cut stray light out and not from the object the scope is pointed at right?
in which case your could probably get more contrast?

[ballaratdragons]

Yes David, stray side-light.

They can be made of lots of things (like Tony's). Mick pinners are very proffessional looking and made to fit.

[Starkler]

Yep in general dew isnt a problem for a dob style scope, although some large truss dobs users have a heater on their secondary mirror.

A tube extension however is a good idea for blocking stray light.
Stray light can be seen from the focuser from out the end of the tube in many dobs (including GSO dobs) and it can also illuminate the spider vanes which can also be seen from the focuser.

A nicely blackened tube extension will improve contrast.

[mick pinner]

Thiink, if you like you can contact me and l could make you something up if you like.

[ving]

I might whip one up then :)
thanks for the tread thiink :)

[Thiink]

No worries, let me (us?) know how you go with yours. If it even blocks a little bit of stray light I am going to be happy.

[Striker]

I should of listened to Ian from NT from the start.....Just dropped into Sirus Optics and picked up a Orion flexible dew shield for my 10" for $45.....I am very impressed with the Quality as well....I will post a pic....

All others were marked $59 but this one was marked $45 so I grapped it......Looks very durable and is fitted with Velcro and a cushioning pad.....I know I just said it but it looks and feel great......

Look similar to the foam one I made but its nothing like it.

[gaa_ian]

Glad you found that useful Tony :)

[frogman]

dob boys....... trying this one tonight....

[ving]

let us know how it goes :)

[iceman]

What's it made out of, anthony?

[GrampianStars]

G'day STRIKER

That home made Sheild you've there looks to be tooooooooo long :confuse3:

Your FOV would be reduced considerably :whistle:

[ving]

doesnt matter robert... he is scopeless now! :(

[frogman]

Ummm its a camping bed roll cut down... $5 from cheapo shop at tuggerah

Bit o sticky velcro to keep it on

need any photos ?

[ving]

photos that show, step-by-step how you cut the bed roll would be nice :lol:

[Striker]

As ving said....I dont have it anymore Robert and it got replaced with the Orion dew shield which is shorter...as seen in the above picture.

[mick pinner]

l know that the flexible shields are cheap but if anyone is interested l can custom make a rigid shield painted any colour you like any length you like for $120.00 + post eg: Perth $15.00, Q/Land $10.00.

[GrampianStars]

G'day Mick
Great looking sheild
Nice setup as well

What's that rail your using for the Orion 80 ed ?

Say what have you made them out of ?

I used blue poly but after a coupla years it cracked

Just interested for a 14" LX 200 on peir mount

[frogman]

I think im going to get my next prototype made out of aluminium or something thin... but i think wieght might be a problem.

just a thought

Anthony

Page of how to (sorts) will be up soon

[frogman]

Bed Roll Dew Shield (http://my.integritynet.com.au/anthonylord/bedroll_dew_shield.htm)

Rough copy of the page any suggestions welcome.
\
Anthony

[Starkler]

Ive bought a bed roll which looks the same as yours and black on the other side.
Is the velcro holding ok ? I was thinking maybe it wouldnt due to the porous and stretchy nature of the foam. I was considering holding it to the tube with a luggage strap.

[frogman]

Yeah just discovered that......... i just added another strip to the middle so well see if it will hold this time.

theres got to be a better way than a hockey strap.. cance of scratching would be high wouldnt they ?

maybe sewing it with twine of some variety
But wow it blocks out a lot of light.... im looking over the house with the lights on and it looks the same as if the lights were off !

Anthony

[mick pinner]

hi Robert, they're made from 24 gauge galvanised steel, with 1/4" locating thumb screws. the weight for a 12' shield is just over 1200 grams which is enough to offset such add ons as cameras etc up the viewing end. There is 2 problems with aluminium 1. the only decent threaded rivets will chemically react with the aluminium over time and it will deteriorate 2. joining the tube once it is rolled, welding is almost impossible because of the thickness and pop riveting just looks terrible and if you can find the size you want in a solid tube it will cost a fortune, l researched a few ways to do these and found the gal the only real option especially if you want a professional finish. The scope rail is also home made.

[Starkler]

Thats my main reason for doing this. The GSO dob lets a lot of light in through the front due the the tube not being long enough.

[frogman]

well it held up well other than needing another strip o velcro in the middle. great amount of light reduction (not that its needed out here) and no dew...

Dosent stop your EP's fogging tho ! :( next project .......

still toying with the idea of stitching it together.

Not bad for around $10

Anthony

[ving]

interesting, thanks anthony. I'll put one of those on my to-do-list too :)

[ving]

btw, are the magnets absolutely needed?

[frogman]

it is when you put the grenade in to view

umm grenade = 1RPD 30mm 2"

[frogman]

can i get a closer look at your dob please the shield actually??????
Please

[bytor666]

ok froggy LOL....here's a few pics of the shield. I didn't want to take it apart where the vecro meets because it fits perfectly right now and loosening it will cause me to find the right circumference again and the first time it was difficult finding that. This first pic shows the shields inside. The white goes over the tube & the black part is where I joined the seam from the inside.
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Mark
12" gso scope

[bytor666]

here's pic #2. It shows where I've made a seam . Under that seam is the velcro with adhesive backing on the outside & the inside. That space you see on the top is to make room for my Telrad when I slide the light shield onto the tube.
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Mark

[bytor666]

Pic #3...shows the end you would see if the shield were on the telescope tube..

[bytor666]

finally...pic #4 shows you a close up of the end that slides onto the telescope tube again.

I hope this helps you....If you need more information..just send me a PM or reply on this post !!!!!
--------------------------------
Mark
12" GSO SCOPE

[Starkler]

Great stuff Mark, does it sit nice and square and not wobble out of position etc?

[bytor666]

yes it sits perfectly square. If it didnt i wouldn't be using it.
-----------------------------
Mark
12" gso scope

[bytor666]

Mine is made specifically by myself, but it is made of that bubble-wrap pipe insulation.
Just get it from your local hardware type store and measure 1.5 inches longer than your primary mirror size and wrap around scope tube , then cut to length.
I then flocked the inside with a felt-like substance with adhesive on it, and I put it together with velcro also with adhesive on the back of it.
It doesn't affect the weight of your scope because it is super light stuff.
all I do to attach it is leave it in it's "circular" form and slide it onto the tube. This stuff is slightly "flexible" also , so when you put it onto the tube, you can easily slide it over all 4 of the spider vane screw attachments on the tube itself.

It helps greatly for blocking stray light and it will also slow the process of having your secondary mirror from fogging up.

I also use 4 poles and tarps that surround me on my platform from those pesky "security" lights that go on in my neighbors' yard every time a cat goes by them.
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Mark
12" Gso scope

[stringscope]

The one item that hasn't yet been discussed in detail is vignetting caused by extended dew shields. Due to the larger achievable FoV, this is probably more of an issue for smaller dobs rather then SCT's.

In particular it may be an issue for minimum diameter metal tubed scopes. The example I have used below is for my "sonotube" dob which has a 10" tube for an 8" mirror. For those with GSO scopes I believe the tube ID for the 200mm dobs is closer to 240mm. This may cause vignetting if you combine very wide FoV ep's with extended dew shields of the same diameter as the tube.

I have used the simple 1 in 60 rule as a double check on scale drawings to assist me determine the dimensions of the "obstacle free area" in my dob. The way I have applied it is as follows:

8" (203mm) F6 dob/2" focusser = 2.2 degrees max FoV = 1.1 degrees either side of optical axis originating at the edge of the mirror that should be free of any protrusions (with the exception of the secondary spider).

The focusser is close to 1050mm from the primary mirror. 1.1 degrees = 1.1mm in 60mm = 19.3mm + 101.5mm (1/2 mirror D) clear @ focusser = 121mm. 250mm ID tube = 4mm max focusser protrusion into tube when eyepiece is focussed.

Scope as supplied my manufacturer: Tube ID 250mm. 200mm extension forward of secondary. Front of tube = 1250mm from primary mirror. 1.1 degrees = 1.1mm in 60mm = 23mm @ 1250mm. The minimum ID radius for the tube @ 1250mm = 102 (1/2 mirror D) +23mm =125mm = min tube ID @ 1250mm is 250 mm to avoid vignetting at max possibe FoV. As this is the tube ID, any further extension must have a greater ID if I want to avoid vignetting with certain eyepieces.

My current dew/light shield extends 430mm forward of the secondary mirror (or 1480mm forward of the primary) and has an ID of 280mm. My widest FoV ep is a 2" Meade 40mm SWA which gives a TFoV of 2.2 degrees. This eyepiece requires a modest level of "out focus" and the focusser tube does not impinge on the light path. At 1480mm 1.1 degrees = 27.14mm (say 28mm). The min dew shield ID at this distance is therefore; 56mm (28X2) + 203mm = 259mm which is less than my existing dew shield = no vignetting. Note -- This is larger than the ID of the telescope tube ---


Cheers,

[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]

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]

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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.
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mark
12" gso scope

[Starkler]

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 (http://www.cloudynights.com/howto/dew2.htm).

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