Lining a Newtonian OTA

[Moonshane]

Hi all

My current dob is part flocked but I am wondering if I should remove this, add a 2mm liner of 2mm expanded foam sheet and then re-do the flocking on top of this. The logic is that this will prevent the thin aluminium wall of the OTA getting colder than the air outside, creating currents in the tube and affecting the view. Once the mirror and tube reach the ambient, then they stay there and (in the case of the tube) don't get even colder. This logic follows a few articles online that I have read recently.

Am I talking rubbish or is this a worthwhile exercise? I'm confident that I can do it, it's just a question of is it worth it.

I think I once saw someone had flocked with velour. Is there a reason I should but the expensive flocking sheets (I am saying expensive but I don't actually know how much they are!).

Thanks for any comments

[sbooder]

I don't know about the insulating bit, but here is a link for a possible source of self adhesive flocking material. Self Adhesive Flock Material

I asked them if the flock had a mat finish to it, and they sent me some swatches. The black swatch was just enough to do the inside of my 1.25 focuser tube. I have not yet tried it out...waiting for a clear night!

I will let you know how it performs

[brianb]

Insulating the (thick aluminimum) tube of the 6" f/8 Newtonian I had long ago was the best upgrade I ever did. 3mm expanded polystyrene sheet painted matt black using emulsion paint - ordinary paints dissolve EP.

[Moonshane]

thanks all

I reckon I'll have a go at this. seems reasonable to me!

[acey]

The logic is that this will prevent the thin aluminium wall of the OTA getting colder than the air outside, creating currents in the tube and affecting the view.

Interesting idea, but if metal or glass gets colder than the surrounding air temperature then dew forms on it, and you never get dewing inside a telescope tube (except maybe at the skywards end if there's no dewshield) so I wouldn't worry about tube currents caused by a cold interior. You do, though, get tube currents caused by the mirror being warmer than ambient temperature. One solution is to have holes in the tube (or a truss design), another is to have a fan, either behind the mirror or else blowing air across the face.

Regarding baffling/flocking, some people have advocated putting baffle rings inside a Newtonian tube, analogous to a refractor, but there's an argument that this worsens any tube currents.

In my experience, the most important part to flock is the interior of the focusser and the part of the tube wall directly visible through the focusser. Your aim is to block stray light paths that can go directly into the focusser. The dewshield also plays a very important role here.

I've personally never found tube currents much of a bother, either with my 8" or 12" scopes, neither of which has a fan, though I should add that I'm a DSO rather than planetary viewer. All depends how critical your requirements are, and how much work you're willing to do in order to get what might be a small improvement in performance.

[Moonshane]

cheers Acey

my tube is already flocked from the top end to about a foot from the bottom (whoever did it presumably ran out!). Not done the inside of the focuser but might try this too. therefore I have covered most of the 'angles' I suppose.

my theory was based on an excellent article on cloudy nights which was specifically referring to planetary/high power performance. they have oversized tubes with foam/cork lining and painted matt black. the comments were that the image was rock steady and noticably more so than without the lining. fans helped too. my scope currently has a side fan blowing across the primary but I have just added a fan at the back fitted to the tube cap. I intend to run the side one/both to cool and only the rear one while observing but at a lower speed. All experimentation of course and in the UK the skies will never or rarely, be as steady as on a mountain top in central USA.

I don't mind spending time on things as you can often do so for nothing or at low cost but if I could have, say, another eyepiece of a newly lined and flocked tube, I reckon I'd go for the former at the minute. will see if the rear fan makes a difference than decide on which way to go.

cheers again.

Shane

[davew]

Hi Shane,

" Oversize " Important word that I think. You need some room round the mirror to get all the light in or you MAY have a steady view that's slightly less than it should be.

Have a go at - Dale Keller's Newt Software page and do the calculations first.

Dave.

[acey]

My guess is that any improvement from insulating the interior would be by reducing heat convection inside the tube. If a telescope has been set up in daytime in a hot place then there's heat to be shed when night falls and the temperature drops. Most of it will come from the primary but there's going to be a contribution from the tube too. This might be more of an issue for people in places like Arizona rather than Britain, but if you're aiming for ultimate planetary performance then it sounds like it's worth a try.

[Moonshane]

cheers Dave

good point and I've downloaded this program but not used it yet.

I was only going to use 2mm foam and then flocking (so maybe 6mm all round reduction max) but it could make a difference.

[Moonshane]

cheers Acey - I agree that massive drops of temp in the desert will matter more. trouble is I want my cake and I want to eat it! if I can improve the performance for not much cash then I'll certainly be willing to have a go.

[davew]

Hi Shane,

From reading your other posts you sound like you want things right. Please report back when it's done. Interesting stuff

Dave.

[Moonshane]

cheers Dave

I'm just a fussy sod! The trouble is I only want to spend if it's going to make a difference. This might and the additional outlay for the foam is negligible so if I can get the flocking at a decent price I'll go for it and report back.

[brianb]

if metal or glass gets colder than the surrounding air temperature then dew forms on it, and you never get dewing inside a telescope tube (except maybe at the skywards end if there's no dewshield) so I wouldn't worry about tube currents caused by a cold interior.

The issue here is that the sky facing top of the tube cools below ambient by radiation, if the tube is conductive then it causes the air in the top of the tube to overcool, you get a "river" of dense cold air running down the tube.

One solution is to have holes in the tube (or a truss design)

Bad idea. Heat currents from the observer blow through the holes.

[Moonshane]

cheers Brian

this is a great learning curve for me. I have just contacted someone re the 'best' flocking and they advise that there's a new version of it with flocking on one side glued to a plastic board. this can be held off the metal with thin foam (maybe double sided foam) and thus creates an air gap which might provide enough insulation to prevent this problem. I gauge the cost and see what happens - either way I'll update this thread as things advance

[sbooder]

I don't know about the insulating bit, but here is a link for a possible source of self adhesive flocking material. Self Adhesive Flock Material

I asked them if the flock had a mat finish to it, and they sent me some swatches. The black swatch was just enough to do the inside of my 1.25 focuser tube. I have not yet tried it out...waiting for a clear night!

I will let you know how it performs

The flocking from the link above that I lined my focuser tube with works very well. I had my 10" newt with the flocked focuser and my 8" newt without the flocking side by side to compare views.

I viewed M57, M31, and Jupiter (in the early morning) and the difference in contrast with only the focuser tube flocked was quite pronounced.

On the strength of this, I have ordered their 5meter(L) 650mm(W) roll.

I will take a photographic record of the flocking process for the forum when I do it.

[sbooder]

The flocking arrived from ABM today. I will post photos after I have flocked the OTA.

[sbooder]

Ok, I have now flocked the OTA.

With the help of my wife, one of us at either end, we took the 6” wide strips I had pre cut and with the scope protected by a towel and on the table placed the first strip upside down the length of the tube at the bottom of the curve.

We then striped off the backing paper and discarded it. Taking one corner each with opposite hands to the corner we chose, we lifted the material up so it was vertical and not touching any part of the OTA (this stuff gives you no second chances, once stuck it is stuck!), we then took the opposing corners with out free hands, and lifted it so the flocking now resembled a long open bag with the sticky surface facing downwards.

We then let the bottom length of the bag attach to the bottom of the curve and then let the sides lower and follow a natural opening outwards motion, while maintaining a certain amount tension on the length of the material. This worked a treat and attached to the scope with out one wrinkle or crease which was the objective.

The first strip was purposefully placed so it covered the seam of the OTA, I did not want two pieces to meet there, it would be lumpy and more likely to come unattached.

Initially I wanted to butt the next piece to the first, but this proved impossible when trying it on a dry run with the backing paper still attached. So my wife asked “Why can’t it overlap?” “Because…because…because…actually there is no reason why…Dough!” I replied.

So we overlapped the following pieces using the same method as the first, until the entire OTA was covered, trimmed the over hangs and pressed and smoothed over the flock a final time, and then went over it with a hair remover roller.

I reassembled the OTA with focuser, secondary and primary; all that is left to do is re-balance the scope.

[TopHouse]

Be sure to also do the inside of your focuser tube. Looking good.

[sbooder]

The focuser tube was flocked with the sample swatch they sent me about 3 weeks ago. Mind you I am not short of flocking material now. I have about 2.5 meters left.

[TopHouse]

Perhaps some blackboard paint now for that outside focuser tube and any other shiney bits. When i flocked mine I did it in two pieces, didn't have enough length to do it in one (fnarr fnarr) but it's definitely a scary process, that glue on some of the flocking is like that mousetrap paper they used to use years ago!

[sbooder]

My only worry about painting the outside of the focuser tube is it rubbing off on the outer tube and depositing paint on the primary.

[TopHouse]

On mine, my inner doesn't come into contact with the outer except for tracks where it runs on bearings, so I left the tracks unpainted, you can see one in bottom pic here, but I don't know how your focuser is constructed :-

http://stargazerslounge.com/astro-lounge/99648-project-stealth.html

[sbooder]

Sounds like you have a crayford. I still have a R&P on mine, it is contact with the outer all over the place.

[TopHouse]

Yes, mines the crayford, on second thought, perhaps leave it alone then lol

[Moonshane]

cracking job Simon

I have bought some approx 1.5mmx450x300 sheets of black 'fab foam' and will be lining my 12" dob with this before getting some flocking material to go over it. I am hoping this will give a more settled environment in the tube.

I'll send across photos when I get around to doing the job. I am thinking about getting a 9 point cell for the dob so will do it then.

cheers for the pics

do you find the 10" is better (or as good) on planets/moon/doubles than/as your 8"? I am asking as I am thinking about whether or not it's worth getting an 8" f8 dob as well as my 12".

cheers

Shane