Hi all,
Have just been reading up on tube currents in Mak scopes and where they emanate from, and ways people go about to reduce them for faster usage.
It seems the easiest way to reduce currents is to insulate the tube. Has anyone tried insulating the rear of the primary as well as the dew shield? Have you tried different types of insulation?
A link to one of the threads. https://groups.io/g/tec-scopes/topic/2844940
Article on dew https://www.skyandtelescope.com/astr...ling-with-dew/
Last edited by Wilso; 14-11-2019 at 06:46 PM.
Reason: Add
1. insulate around the corrector cell and most of the OTA barrel, this kills the thermal transfer via the walls of the tube which is the main problem as - without insulation - the ota barrel cools quickly while the central baffle does not.
2. do not insulate the back of the scope - this is where you do want to lose heat as the baffle cools by transferring heat to the back plate. This in turn cools the primary which is mounted on the baffle.
This is also borne out by my 10” APM mak which has a rear backplate with more holes than Swiss cheese - clearly intended to cool the back and the baffle effectively. This does work - on the nights I’ve had it out no tube current was visible at all by the time I’m set up and ready to observe (30 mins).
I think the posts on CN where people have totally encased SCTs probably has more to do with them being in a far colder environment (below zero) where keeping the scope warm is both necessary and a challenge.
3. I do use an insulated dewcap, made of 6mm coreflute.
4. If dewing is an issue either use a 12V hair dryer, or you could put a heater strap around the OTA inside the dew cap, just behind the corrector. To be honest I’ve never really felt the need on several maks, unlike SCTs.
I made a wrap around insulator out of air cell insulation used in a car windscreen heat shield that you sit on your dash from supercheap auto. Enough in one to make two or three depending on the size of your scope. Mine is only a 5 inch so I got 3. Works very well and cheap as well. I cover my ota and dewshield with it in one not the rear. I used self adhesive stick on felt draw lining to reinforce around attachment holes on the inside, stick on velcro tabs to hold it on once wrapped around. Easy to instal and when not in use it folds up flat.
An insulating-dewshield made out of Coreflute is very effective. I did also try the car windshield cover too, but I found the Coreflute worked better after trying out both, but I guess it depends on your expectations and scope as to which you use and prefer. The Coreflute is also more rigid and less delicate than the windscreen cover - this is better for me is because I need to set up and take down the scope every session.
Below are some pics of the shield I made for my 7" Intes Mak. It does not totally stop dew formation, but it does stay dew free for more than four hours in my dew-central location at home, and most nights for even longer, and then the dew is very slow to form. The last pic shows it rolled up nice and tight for storage.
With this insulating-shield, I can take the Mak straight up to high power without needing to wait for it to reach thermal equillibrium. For a big Mak (say 7" and larger, it would take such an instrument many hours to cool down, especially if there scope is experiencing a big temperature differential between where it's been kept and the ambient temperature. Insulating the scope eliminates this need to wait for it to cool.
Oh, and DO NOT USE A BLACK MATERIAL TO MAKE THE INSULATING DEWSHIELD FROM!
Black at night becomes a heat sink, sucking up all the heat from its surrounds. Black surfaces are the very first to cool and have dew form on them, and will actually be cooler than the ambient temperature. I've often seen frost form on black surfaces when every other non-black surface was not even wet with from dew.
In the case of an insulative perspective, a black wrap will actually suck the heat out of the OTA rather than keep it in, and in turn creating the very opposite situation what you are seeking.
The dewshield end of my Coreflute wrap has black felt inside it. On dewy nights the outside of the wrap that is around the black felt has dew start to form on it, while the rest of the wrap stay bone dry. Yes, there is not the heat of the OTA keeping it warm, but it is more the case here of the black felt actually sucking the heat out of the Coreflute around it and plunging its temperature than the lack of heat.
I also made an insulating dewshield out of an old yoga mat for my 8" SCT, and this was also very effective!
Last edited by mental4astro; 14-11-2019 at 09:27 AM.
Thanks guys for the input, much appreciated.
Nice pictures by the way!
I was actually thinking of making it out of wet suit material. Should have good thermal qualities.
Neoprene open pore centre with the 70d nylon outer, relatively light weight. Now you have me 2nd guessing it Alex as all the thicker material seems to come only in black.
I’ll have to see what I can find.
Last edited by Wilso; 14-11-2019 at 05:03 PM.
Reason: Add
Hmmm, I didn't realise it had an adhesive backing, didn't really want that.
I coated my C9.25 SCT with the black neoprene foam above, but now it appears that this was the wrong decision. I thought that the black rubber foam would have insulated the OTA from the cold surrounds, keeping the exterior of the scope at the same temp as the interior and reducing internal currents?
I'll look into Corflute instead, I assume the white corflute is preferred over the black?
There is no "wrong" decision. There are only solutions, and of these some are better than others. Yeah, that sounds dicky, but we are all doing our best and we should encourage experimentation, creativity and having a go, and we all start from zero. Good on ya for starting as you did, mate!
Yes, white Coreflute is far better than black. A better wrap design is to have it it extend from end to end, dewshield over the front aperture and the back end to reach to the rear cell holding the primary mirror. The lens shade/dewshield extension is an intergral part of the devise to help control dew formation. The Coreflute can also be cut so it will accommodate things like finderscope blocks, dovetail mounting blocks, etc. That's what I did with the wrap for the 7" Mak, and used Velcro to hold it in place around the scope (see pic).
Coreflute is not the only material that can be used. A couple of others have been mentioned. Use your imagination! Sometimes even a found item could prove perfect! Phillip's solution is excellent (see his post in the link in his post).
If you want to use a dew heating strap, it is best to install it under the wrap. The wrap will also help make the dew strap more energy efficient, helping keep its heat more contained.
OK, so now I'm a bit confused. I read this on one of the links above The tube thickness of SCT's is very thin, as is the corrector plate. An SCT needs better insulation because is has such a small heat mass in its glass and tube. A Mak has a thick corrector, so it holds its heat better.
Does an SCT (like mine) need different insulation to a Mak? Is black foam better for SCTs than white Corflute?
[EDIT] I just noticed you replied just before me. I should have also added that I use the same black foam as a dew-shield which works really well, but it's separate to the OTA "cosy" that is normally always fitted. A photo of the full catastophe is attached.
I have some white Corflute at home (used for advertising purposes), I'll look into using that instead.
If you see anyone getting a new roof on their house, it's well worth a chat to the builder for some leftovers, or at least partake in some shameless dumpster diving.
In making a dewshield for my C9.25, I adhered some matt black curtain blockout fabric, and my Wife kindly hemmed the edges and sewed on some velcro. Looks like a bought one. We even made a mini one for the guidescope.
Since reading this thread, I'd be more partial to a white external fabric, with matt black internal to block stray light.
Looks like you’ve done a great job there Andrew.
If you haven’t found any issues with thermal currents or dewing up I wouldn’t bother changing it. It’ll be cheap to compare the corflute one if you decide to go ahead and make one anyhow.
Black is black and it will create thermal and dew problems - but if you are having good results with it without dew issues, good on ya! By the look of things dew may not be a problem for you - you lucky bugger!
I had not seen your photos before placing this post, so I'm altering it.
Look to use materials and colours that will give you the best result you can achieve. I use Coreflute because I find it practical for me. Yes it costs a bit more than other stuff, but cost should not be the prime motivator. Not to say that more expensive materials are better - they certainly will not be in this niche application. All manner of materials will be possible to use. It's about being creative and resourceful, but also understanding a little about what it is that one is trying to achieve, a little about materials science, and a little about thermodynamics.
Look to avoid Black though. This goes for black OTAs regardless of make or design. A black OTA will always be a dew & frost magnet before any other colour. You may be able to count on one hand all the professional scopes that are painted black, and have fingers to spare. You only see amatuer scopes painted black.
Andrew, if you do find your scope and dewsheild dewing up, you will now know one thing you will be able to tackle to improve things.
That most foams come in black means WE need to look for materials that are white or silver - if the material has black foam filling is of no consequence. Just at least not the outside face of the wrap. Like I said earlier, if dew has not been a problem for you, you are most fortunate.
Yes, SCTs have thinner walls and corrector. What that quote says is just fine in saying an SCT needs even more consideration than a Mak. What is the prime motivation regardless of Mak or SCT, mass production or small batch, it is to slow down the heat loss as much as practical. No confusion to be had. In reality it makes no difference to have "thicker" stuff for an SCT - the same stuff will also work with a Mak.
It is all just about slowing the rate of cooling. If the tube is not allowed to get cold, you eliminate a heat differential being created inside the OTA between the cold tube and warm primary and baffle, which means tube currents are also stopped from being generated from the warm primary and baffle.
Alex.
Last edited by mental4astro; 15-11-2019 at 05:47 PM.
This is one for all you telescope insulated corflute people.
See if sealing the end of the corflute with foil tape helps improve the insulation factor. (Similar to double glazing ) The air gap is the key, the glass isn’t.
Just roll the tape along the edge first sealing and then folding over. In theory should work sealing the warm air in
Readily available from your local hardware.
It won't make any difference. I've already done the calculations - the viscosity of air is enough to effectively stop any convection within the length of the cells. Apologies - I am an e.n.g.i.n.e.e.r and I know fluid flow maths inside out.
My dewcap is 6mm Coreflute effectively what this means is a captive 6mm layer of air between the OTA and external. 2.5mm Coreflute would be fine too, BTW. Best part is this stuff is tough, yet slitting every second cell lengthwise makes it so flexible as to roll up as a sausage, or wrap around an OTA. The lengthwise slits imply a slight leak too, but its not enough to matter - in practice this stuff works.
Even better - in my case the corrector cell is slightly larger than the OTA and this means there’s another 3mm air gap between the coreflute and the OTA, so more insulation, effectively.
Reflectix or that silver stuff from Clarke Rubber might be better but in my case the Coreflute was left-over offset (free) and does the job fine. No need to obsess over this.
Black is black and it will create thermal and dew problems - but if you are having good results with it without dew issues, good on ya! By the look of things dew may not be a problem for you - you lucky bugger!
Alex.
Thanks for your very detailed response Alex. I only upgraded my OTA a few months ago from a 6" Evolution SCT (which is actually painted silver) to my current C9.25 SCT which is painted black. I had previously used a black foam rubber dew shield on the 6" to good effect, and the one time I've needed to use a similar shield on the 9.25" it worked fine, no dew to be seen. To be sure, the black foam was damp with dew, but the corrector plate was clear.
Regarding the OTA cover, I don't have enough experience with my 9.25 to say one way or the other whether the black foam cover is helping or hurting the tube currents. I didn't use anything on the (silver) 6", just set it up early and waited a couple of hours to equalise. With the 9.25, the planets have been so low (and dropping fast) that the images I've taken of them were not made under ideal conditions (which would include tube currents), but I've had some success with Saturn recently.
As luck would have it, I have some large sheets of white Corflute here at home in both 5mm and 3mm thicknesses, a free weekend, and a good weather forecast for Monday night with Neptune and Uranus nice and high in the sky. It also looks like dew wont be problem as the dew point is supposed to be 6* lower than ambient. I'm assuming the 5mm Corflute will be better than the 3mm, a little extra insulation will help there. I also assume I will still need to blacken the inside of the dew shield section to remove stray light reflections, or maybe at the moment I'll just use the foam rubber shield I already have. I think I'll make the dew shield separate from the OTA cover.
By the look of things dew may not be a problem for you - you lucky bugger!
