Dew shield for dobs

[GrampianStars (Rob)]

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 (Anthony Lord)]

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 (Anthony Lord)]

Bed Roll Dew Shield

Rough copy of the page any suggestions welcome.
\
Anthony

[Starkler (Geoff)]

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 (Anthony Lord)]

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 (Geoff)]

Originally posted by frogman

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 !

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 (Anthony Lord)]

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 (David)]

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

[ving (David)]

btw, are the magnets absolutely needed?

[frogman (Anthony Lord)]

it is when you put the grenade in to view

umm grenade = 1RPD 30mm 2"

[frogman (Anthony Lord)]

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.
----------------------------
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.
------------------------------
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 (Geoff)]

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]

Originally posted by 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.

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.
--------------------------------
Mark
12" Gso scope

[stringscope (Ian)]

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,