180mm Mak - use of thermal blanket avoids cool down time?

[GazOC]

I've got to admit I've never seen the e and f trap stars in it (or in my ED120), either my eyes or skies aren't up to it. ?

As a planetary scope I find it gives noticeably sharper views than the ED120 but it's obviously not as good an all rounder as the ED. I only really observe  a few of the more popular doubles so I  wouldn't like to pass comment on it as DS scope.

You see them knocking about for sale around £400 second-hand which I think is a bit of bargain if you are happy with having a scope pretty much just dedicated to lunar/ planets/ DS and have other scope(s) for more general observing.

If the planets aren't visible then I can quite easily go months without using the Mak as I find my ED120 or 80mm Equinox are good enough and quicker to setup if I just fancy getting out for a quick look at the moon.

 

 

 

[chiltonstar]

As Stu indicates, I've been using an insulating jacket on my 180 Mak for six months, with generally a large reduction in cool-down problems. The o-o-f thermal plume is absent or much reduced, and a close test double almost perfectly resolved within a minute or so of setting the scope up. I do keep mine outside though in a hermetically sealed box with silica gel sachets to keep the air inside the scope dry, so it is always close to ambient temperature.

Clearly, on evenings when the seeing is poor, the insulating jacket cannot improve the view, but it helps a lot particularly on nights when the the temperature is dropping fast. An additional benefit is that dewing is much reduced, in fact I have it more often with my RACI and RDF than the scope itself - I presume the heat held inside the scope keeps the body above ambient.

A view of the setup:-

Chris

[JamesF]

What material did you use for your insulating jacket, Chris?  I've seen some made from what looks to be aluminised bubble wrap, but the surface of yours looks too smooth for that?

James

[Stu]

@chiltonstar Where did you get your insulation fro.? Looks much nicer than my bubbly stuff.....

[Stu]

2 minutes ago, JamesF said:

What material did you use for your insulating jacket, Chris?  I've seen some made from what looks to be aluminised bubble wrap, but the surface of yours looks too smooth for that?

James

Snap

 

[John]

Interesting topic this. A lot of what I have read about scope cooling and temperature control seems to indicate, to me at least, that the challenge to image quality comes when you have bodies of air of differing temperatures within the optical path and also where there is differential cooling of bodies of air within the optical path going on.

Chris's approach seems to be (if I've understood it right) about keeping as much of the air in the optical path at a consistent temperature and reducing or eliminating differential cooling within the tube while also ensuring that the scope is stored close to ambient temperature which itself should reduce the temperature vatiations within the tube when the scope is setup for use.

What I'm less sure about is the extent to which the themal wrapped approach would help in the case of an optical tube stored at room temperature and taken outside with a temperature drop of, say, 5-10 degrees C. Is it possible that in those circumstances the insulation might hinder rather than help ?

 

[JamesF]

I could have written that myself, John :)

In the case where the OTA is already at or very near to ambient temperature then I can see a plausible explanation for why it would work.

In the case where the OTA is taken out in the cold from a warm house I'd have guessed that you could get big air currents inside the OTA as the warm air at the top of the tube cooled due to contact with the corrector (which will probably cool quite rapidly when facing a clear sky at a significantly lower temperature) and then sinks through the remainder of the air in the OTA which is warmer because it can't lose heat through the sides.

I'm not saying that it can't work in the latter case, nor doubting the experience of those who have found it works for them.  I just don't understand it :)

James

[Stu]

I have read that even when taking the scope from inside a house the tube currents are much reduced by insulating. I suspect though it is a case of ensuring you do everything possible to equalise the mirror and corrector temperature to ambient and that the insulation stops or reduces tube currents caused by radiation of heat from the top of the tube giving unequal cooling.

Hopefully I will find out soon! 

[John]

Maybe a scope that keeps all of the air within it at a warm and even temperature would work well ?

You could warm yourself up on it as well !

 

[markclaire50]

Thanks everyone for their responses so far. Assuming I obtain a 180mm mak (moderately likely, given second hand prices of £400 or so) I would store in garage (I doubt I would get permission to store it indoors anyway....). So perhaps the insulation method would work. Having said that, with my 127mm mak, I tend to leave it outside for 1.5-2 hrs before mounting and it's pretty ice cold to the touch by then with pin point stars and perfect concentric rings on out of focus star. I wonder if it would be less hassle to follow the same modus operandi with a 180mm mak. 

@ John I've just ordered a 10mm bco for my mak on basis of your review I read, to see if I can have yet another crack at the Pup at 150x and 225x as soon as it arrives, weather willing. I've read other very good reviews after buying it, regarding the tighter star image it can give. I live in hope. I think it will get personal with the pup! 

[chiltonstar]

8 hours ago, JamesF said:

What material did you use for your insulating jacket, Chris?  I've seen some made from what looks to be aluminised bubble wrap, but the surface of yours looks too smooth for that?

James

I tried the thicker bubble material, but it made mounting the scope onto the dovetail difficult, so I used the thinner material that B&Q sell for insulating walls behind radiators, and fixed it together with aluminium plumbers' tape.

Chris

[chiltonstar]

7 hours ago, JamesF said:

I could have written that myself, John

In the case where the OTA is already at or very near to ambient temperature then I can see a plausible explanation for why it would work.

In the case where the OTA is taken out in the cold from a warm house I'd have guessed that you could get big air currents inside the OTA as the warm air at the top of the tube cooled due to contact with the corrector (which will probably cool quite rapidly when facing a clear sky at a significantly lower temperature) and then sinks through the remainder of the air in the OTA which is warmer because it can't lose heat through the sides.

I'm not saying that it can't work in the latter case, nor doubting the experience of those who have found it works for them.  I just don't understand it

James

I have noticed several times that on occasions when the heat plume is all but absent, tipping the scope up towards the zenith has caused the plume to re-appear slightly. Maybe trapped heat rising from the front corrector?

Another issue worth mentioning is heat bleed through the 180 Mak back plate. This is a seriously solid chunk of metal, attached effectively to the mirror by three sets of bolts set at 120 degrees. Without a jacket, on nights when the temperature is dropping fast, heat appears to bleed from the mirror to the colder backplate, creating three colder zones on the mirror, giving rise to the slight three bladed "propeller" effect seen in the diff rings of a captured image at high magnification (less obvious visually). Judging by images with other, different scopes, this is quite a common effect in dobs as well. Covering the backplate of the Mak does seem to reduce it.

Chris

[John]

I think the more solid the build, the more mass to cool down generally. Thicker glass, thicker metal all hold the heat and release it more slowly I suspect. Meade used to put great clonking counter weights of metal right next to the mirrors of their newts and mak-casses to counter balance the tube on the mount. This slowed cooldown times a lot with those and they were frequently removed by keen owners as an early mod.

 

[JamesF]

2 hours ago, chiltonstar said:

I tried the thicker bubble material, but it made mounting the scope onto the dovetail difficult, so I used the thinner material that B&Q sell for insulating walls behind radiators, and fixed it together with aluminium plumbers' tape.

Chris

Hmmm.  I have some camping mat with an aluminised surface that I used for insulating the water heater in the beer shack.  I reckon that might do the job.

James

[chiltonstar]

11 minutes ago, JamesF said:

Hmmm.  I have some camping mat with an aluminised surface that I used for insulating the water heater in the beer shack.  I reckon that might do the job.

James

I tried a camping mat at one stage, but again it was too thick unless I made the hole for the dovetail fairly large. How thick is yours?

Chris

[JamesF]

I think it's only about 5mm.  I'd have to go out and check.  And as the Sainsbury's/Tesco/whatever deliveryman just arrived looking like a drowned rat I think I shall leave that until later

James