DIY 12" FlexTube Truss Shroud Mk II

[russ.will]

Having created what I though was a really neat Skywatcher FlexTube light-shroud in this thread, a smart Alec (okay, jimmyjamjoejoe) burst my balloon by pointing out that, a Heritage 130p owner had created a similar device that retracted with the top tube. Thus, I cannot claim this is my idea (darn!) but I can't see that anyone else has tried it on this scale. Thus, my ego is massaged!

The FlexTube 300p has a 305mm diametre mirror, with the narrowest point of the castings that cap each OTA, being 342mm. Divided by 2, this means there is 18mm of clear space around the outside of the primary mirror. The camping mat I had to hand is 10mm thick, leaving 8mm for velcro to fix it. two layers of Velcro is more like 3-4mm, so as far as I was concerned, it was job done on the material front.

Happily, the camping mat is 600mm wide whilst the drop to the primary mirror, allowing for the width of the velco at the top, is 650mm - Retracted, the shroud stops 50mm short of the primary mirror. Equally, fully extended, the 600mm wide camping mat sits about 50mm inside the top of the bottom tube.

If you're going to make a similar shroud, but for a different sized scope, you need to measure the distance from the the top of the velcro on the top OTA, to just below the casting at the top of the bottom OTA. If this is LESS than the measuremnt the FlexTube contracts by, you're okay. I suspect, that 350 & 400p owners will need to source something wider than a standard 600mm camping mat as they extend by more than that amount.

As for diametre, well Pi x 342mm = 1074mm long, so I cut it to 1070mm assuming that the reduction in diametre provided by the Velcro would provide a nice tight interference fit. It does, but this stuff is so easy to cut with a steel edge and Stanley knife, go a bit longer if you're unsure. Slightly tight is better than loose as the OTA keeps the shroud in shape under compression, but 10mm camping mat holds it's own shape pretty well.

Anyhow, pictures:

Pardon the slightly slack Velcro alignment - I'd stuffed it all together for the photos:

In case anybody is worried about camping matt being a bit shiny - 1. Remember that I'm using a flash and 2. Compare the brightness of the matt to the flocking opposite the focuser. It's a non-issue:

and from without

The eagle eyed will spot two pieces of tape in the photo above. I used parcel tape to make a neat joint and then removed a couple of pieces at a time as I superglued it in segements from within and from without. The foam sucks up a lot of glue, so it does not go off in 20 seconds - more like 20 minutes, so you have to do it in stages.

Perfect. Not quite. The bolts that hold the OTA mountings protrude, with sharp edges, into the OTA. They snag the shroud and if you're not careful, scratch it, producing lumps of foam that will fall on to the primary mirror. I've covered them with tape, but I've got some dome head nuts on order to cap them plus, I will run two strips of tape up the outside of the shroud to provide a smooth surface for them to run on.

Hope this provides inpiration as it really is about £25 of material all in. It's WAY lighter than one of the commercial shrouds, which is a big balancing plus and looks a whole lot neater into the bargain. Cool.

Russell

[JamesF]

That looks very nice indeed. Well done.

James

[russ.will]

I was about to say give it a go, but this was overtaken in short order by the fact that you have EIGHT scopes!!! Blimey, there's only been 4 1/2 decent nights this year - Which ones did you get time to use?

Russell

[JamesF]

I know, a few more and I'll almost have enough

Actually, I quite often have two out if I'm imaging, and the children like to use them too so we often have three out. They get used during the day for solar viewing when it's possible, too.

James

[VigdisVZ]

Well done! Camping mats are great for many astro-related things.

[russ.will]

Indeed. I'm working on a design for a pop up observatory, using camping mats, Hornby train sets and double sided tape.

Russell

[knobby]

Haven't I seen your work before... Blue peter I think it was!

Seriously though, nice work

[DaveJW]

Very nice indeed, I will be copying, thanks!

[russ.will]

I should note that I've had to do a couple of small mods to this shroud.

The heads of the screws that fix the lower OTA to the alt bearings are sharp and scrape the shroud. I've simply glued some rubber nubbins on the end of the screws, but a cap head nut would work just as well. To make it truely belt and braces, I've also run some black duck tape up the sides of the shrounds where these screws run, just in case repeated abrasion waers some foam off - It's likely to end up on your primary if it does. Otherwise, it all works just fine.

Russell

[kirkster501]

Russell, how are you getting the velcro strip to stick to the camping mat? I always find it unsticks in pretty short order when i have made dew shields for my SCT.... May I ask where you got your camping mat?

I must say your results look most professional - well done

[russ.will]

You are quite correct. I find it unsticks at low temperatures - presumably the adhesive goes brittle? - so I assemble it all when warm, then don't touch it. I haven't had anything actually come unstuck when cold as a result. I must also confess that once I'm happy with the dimensions of were everything is stuck, I tend to run a bead of Cyanoacrylate glue around it. This is basically Super Glue, but I tend to use the slow/medium stuff like this. A slower bond, means a harder, tougher bond and the glue isn't prone to drying out like a tube of Super Glue. The tube I have, is about two years old and gets used for all sorts of stuff.

Russell

[SnakeyJ]

Russel,

Nice sensible and elegant solution and a whole lot cheaper than most mods. How reflective is the camping mat - any mileage in flocking the inside as well? The flocking might also help hold it together and stop it crumbling on to the mirror with age.

Jake

[russ.will]

I think the sixth picture down illustrates how reflective it is, as you can see both the mat, standard black paint and flocking opposite the focuser. As it's outside of the areas visible from the focuser tube, or reflected in the secondary it's theoretically a non-issue. I can't see that flocking it could possibly hurt though.

If it gets old, I'll just buy another mat!

Russell

[SnakeyJ]

If it gets old, I'll just buy another mat!

Fair point - the flocking material would cost more than the mat!

[russ.will]

At 300mm, it certainly does.

Russell