Light/Dew Shield Question...

[wavelandscott]

I am home from work today fighting the flu and as such I feel terrible and my brain is not working particularly well...(however, my wife says she can see no difference in my level of brain function/intelligence)

I am interested in putting together and/or buying a light/dew shield for my 8 inch Bintel/GSO Dob...

I tried to read through some of the other posts and got myself a bit lost in the process (I am blaming the flu for my lack of understanding)...hence this post

My basic question is how far should it extend beyond the end of the OTA...

I've read about the foam/bed roll with velcro and that is something that my meagre skills are capable of...

Does anyone sell shields commercially for dobs?

I've found many sites for CATs on the web but not for NEWTs...will a shield for a Cat work?

For those of you who have built/bought one, are there any helpful hints/tips that you can pass on?

Thanks!

[iceman]

I got my wife to buy me a bedroll today, from target for $10. It's blue. I'll line the inside of it with black felt.

I plan to use velcro strips to keep it in a tube shape so it fits snuggly over the end of my OTA.

As for how far past the end, I plan on going about 300mm.

[wavelandscott]

Thanks Ice,

I am curious how you go with it...especially what you affix the felt and the velcro with...

I saw some blue bed rolls at the reject shop for $5...but, the surface texture on them felt like it might be difficult to get anything to stick to them...

Cheers!

[iceman]

Those reject shop ones is the same as frogman bought.. i'm sure he'll chip in with how the sticky on the velcro worked.

If that fails, i'll just sew them onto it.

They're nice and light and shouldn't disrupt the balance too much, but i do need some counterweights anyway because of the Meade 14mmUWA.

[Thiink]

I'm currently in the process of trying to find somewhere in AU (anywhere, I've tried everywhere locally) that sells sonotube/formatube, hopefully for a reasonable price. I plan to make a light/dew shield from it as per this (http://www.cloudynights.com/howto/dew2.htm) how-to page (also mentioned in this thread (http://www.iceinspace.com.au/forum/showthread.php?s=&threadid=1231&perpage=20&pagenumber=3)). I'm planning to make mine approximately 300mm longer than the end of my OTA.

[[1ponders]]

I may be wrong and I can't find the reference again, but I'm sure I read/heard somewhere that a dew shield should protrude past the end of the scope at least one and a half times the aperture. If I'm remembering correctly that was for a CAT and refractor, can't imagine it would be much different for a Newt. I've just measured the one on my refractor and it is exactly that.

[wavelandscott]

After much surfing today I did find mention of the 1.5X aperture as the correct length past the end of OTA...as you mentioned Paul...

Thanks all for comments and thoughts...

[Starkler]

Or is that 1.5x Diameter past the secondary mirror ?

[slice of heaven]

Past the secondary is right.

[RAJAH235]

300mm or so, of sonotube will be very heavy on the front of your 8" DOB. Might be better off with the lighter bedroll foam. Just a thought. :D L.

[rmcpb]

I wouldn't use sonotube for an extension as it would be very heavy and need to be counter balanced with weights. Try cardboard or the bedroll foam onstead, I can say the foam works very well and hardly weighs anything.

Cheers

[wavelandscott]

I went back and re-read again...

As corrected, it is 1.5x aperture from secondary mirror...

Sorry for the error and thanks for the correction...

[iceman]

I made mine last night, with a velcro strip along the edge to join it up. Just need to adhere some black felt to the inside and it's done. Will post pics when I've done that.

It works fine in theory, haven't tested it under real dew conditions but it's light and easy to put on/take off.

[ausastronomer]

Sorry should have posted this earlier.

A good material to build your dew shield from is .5mm black HIPS (High Impact Polystyrene Sheet). This is a light strong plastic material that bends nicely into the shape of your tube. It can be purchased from any plastics supplier, I got mine from All Plastics Engineering at Chatswood. I attached it with velcro and covered the inside with self adhesive flocking paper, notwithstanding the inside is a matt black anyway. I will post a picture tonight.

CS-John B

[slice of heaven]

What do you guys use for flocking paper?

Slice

[iceman]

I've got some black velvet from Lincraft.

John, how much did it cost?

[ausastronomer]

Slice,

I posted this response about 2 mins ago to Mike's question on "spray adhesive" so I am duplicating it below:-

I bought some "spray adhesive" labelled as such from Spotlight Tuggerah (Gosford would also have it) for about $10 for a large spray can. This will also do a good job of sticking your flocking material down.

Another option for flocking, is to buy the self adhesive black flocking paper from Spotlight. It is already cut into sheets about 300mm x 250mm which is a comfortable size for handling and laying. The glue on this stuff is the best "self sticking" glue I have ever seen. Once its stuck to your tube and given an hour or two to dry it's stuck for good. I once needed to remove a small piece and had to soak it with turps. You also need to be carefull with this stuff not to let it fold back and stick to itself glue side to glue side, if you do its in the garbage bin, thats why the small sheets are comfortable to work with. Another tip is to cut any smaller pieces required to size before you remove the backing paper. It costs about $2-50 a sheet, total cost is about $35 to completely baffle a 10" scope with this stuff but it does a good job.

CS-John B

[ausastronomer]

Mike,

It cost $15 for a sheet large enough to do 2 x 10" scopes and 1 x 8" scope. I actually have a shield already made that you can have, which will fit perfectly. All you need to do is attach some velcro to your scope and its done (the dewshield already has the velcro on it). You may also want to cut it down an inch or two (can cut with good scissors), because I made it a fraction too long for my liking, you may like it that length. If you want to come up to my place late this afternoon, tonite or before 12 tomorrow and pick it up you can have it, you could also take the other piece which someone may wish to claim tomorrow night at the star party to make a dewshield for an 8" scope.

CS-John B

[iceman]

Thanks John.

I'll try and get up there tomorrow morning, it's unlikely i'll get up there this arvo or tonight. I'll give you a call to arrange.

[slice of heaven]

Thanks Aus
I'll hunt it down

Slice

[Thiink]

I gave up on the sonotube idea and went and brought a blue bed roll from BigW for $9.

Just quickly; is there any reason you guys are using velcro to keep the sides together instead of just gluing it together? I imagine if you glued it together it would be easy enough to slip it over the end of your OTA, but would look a heck of a lot neater.

[rmcpb]

Velcro lets you open it and store it flat. Really great when packing into the car.

Cheers