Alternative to reflectix

[Trentend]

Am going to try insulating my C11 but am struggling to find any Retlectix in the U.K. Is there is an equivalent product recommended or would any cheap foil do the job (similar to that below)?

 

https://www.screwfix.com/p/radiator-reflector-foil-470mm-x-4m-1-88m-/88629?tc=CT8&ds_kid=92700055281954514&ds_rl=1249404&gclid=EAIaIQobChMIsr-Ww76f8wIVEdGyCh3eqgqIEAQYAyABEgIWhvD_BwE&gclsrc=aw.ds

[Trentend]

Also, presume just covering the metal tube should suffice as a starter excluding the plastic ends?

[CraigT82]

That screwfix stuff is exactly the stuff I have used on my Fullerscope and my SW300p (two layers of it).

If it were me I'd cover everything, but no harm to try doing just the metal tube at first and see if you get some benefit.

[MarkRadice]

This is what I bought for my C11.  https://www.diy.com/departments/diall-reflective-bubble-insulation-roll-l-10m-w-0-6m-t-3mm/1934806_BQ.prd 

And I didn't cover the focuser end (nor the upper end 🙂) primarily as it would affect my deep sky observing and secondly, with the scope pointing upwards, I am assuming that radiative cooling to the upper atmosphere is lessened. 

[Louis D]

I would think you would want a combination of foil underneath and closed cell foam above to combat both radiational and convective cooling, respectively.

[Trentend]

Finally got around to buying some reflective foil. Now to work out the best way to secure this to the scope. I see there’s plenty of space underneath the orange dovetail bar so I could go around the whole scope and effectively secure underneath the dovetail itself rather than reverting to duck tape (the foil is pretty thin so wouldn’t impact anything that I can see). There are 4 screws securing the dovetail to either end of the ota; presume no issue if I remove these to temporarily lift out the dovetail (seem to recall seeing a blog about the mak127 warning against this as items might drop off inside)?

[newbie alert]

Can I ask why?

[dweller25]

1 hour ago, Trentend said:

Finally got around to buying some reflective foil. Now to work out the best way to secure this to the scope. I see there’s plenty of space underneath the orange dovetail bar so I could go around the whole scope and effectively secure underneath the dovetail itself rather than reverting to duck tape (the foil is pretty thin so wouldn’t impact anything that I can see). There are 4 screws securing the dovetail to either end of the ota; presume no issue if I remove these to temporarily lift out the dovetail (seem to recall seeing a blog about the mak127 warning against this as items might drop off inside)?

You should be able to just fit two layers without removing the dovetail.

I found it essential to also cover as much of the rear as possible.

Edited October 24, 2021 by dweller25

[Deadlake]

If you use reflective foil then no need for a fan in the UK?

[CraigT82]

3 hours ago, newbie alert said:

Can I ask why?

The idea is to stop the tube walls of the OTA from cooling below ambient (via radiative emission)

The tube cooling below ambient is bad news because the air inside the tube which is in contact with the tube wall will also cool and become a source of temp deltas inside the OTA which as you probably know is a situation to be avoided. 

Other sources of temp deltas within the tube are the mirrors, and the warmer air rising off them and this is where fans can help.

Tue holy grail of scope cooling is to have ALL the air inside the tube at the same ambient temperature as the outside air.

 

[newbie alert]

How can it be cooler than ambient,  it's not being cooled,  the ambient air is cooling it..

I retract that comment Craig as I've just looked up radioactive emission,  thanks for bringing it to our attention...

Yes, this works. It's called radiative cooling. This phenomena has been known for a long time, considering the ancient Egyptians used to make ice this way.

Ideally, something open only to a clear sky would "see" the temperature of space, which is the microwave background radiation. In practise there is enough stuff in our atmosphere that radiates so that it won't get anywhere near that cold.

Edited October 24, 2021 by newbie alert
Added info

[Stu]

1 hour ago, newbie alert said:

radioactive emission

Just for the record it is radiative, not radioactive! I guess you have spell checker to thank for that one 🤪👍

[CraigT82]

Just to add… if you get dew on the scope tube during a session, that’s an indicator that the tube has cooled below ambient temp and has hit the dew point temp which may be a good couple of degrees cooler than ambient, depending on the humidity. I’ve read somewhere that even temp deltas in the tube of only 0.5 degrees can start to noticeable degrade the wavefront.

My old Fullerscope was terrible for radiating heat, the plastic tubing must have had a v. high emissivity as I used to get dew soaking the inside the tube as well as on the outside. Wrapping it up with the screwfix radiator stuff linked to earlier in the thread put a stop to that and helped the scope perform at it’s best - in my opinion that is, as I didn’t do any proper testing or data gathering to quantify the benefit of the insulation. 

Edited October 24, 2021 by CraigT82

[Merlin]

Perfectionists might wince at this, but I’ve made my  20cm Newtonian a closed tube ‘scope. I went to Asda’s to see if I could find a clock with a glass front big enough to play the part of an optical window. 

Having found a suitable  clock for just £6, I removed the very thin glass front and its bright aluminium collar and found that they fitted snuggly over the front of the ‘scope. The rear end of the tube was closed off with a piece of foam.

Now to try it out.. . The Moon was the first target. The new arrangement yielded sharp images at quite high magnifications. Then, I tried the stars in the Pleiades. Again, the images were sharp and, surprisingly, there was no sign of astigmatism. Finally, I made a dew shield from  a piece of rolled camping mat.

I haven’t tried the ‘scope yet with the highest magnification of 400x  as the primary needs a recoat, but it looks promising.

It’s been suggested that front aperture glass solar filters with their coatings removed would make good optical windows. A 20cm solar filter can be bought for about £100, but the problem is knowing how to remove the protective coating.

[Louis D]

On 24/10/2021 at 15:04, Merlin said:

Now to try it out.. . The Moon was the first target. The new arrangement yielded sharp images at quite high magnifications. Then, I tried the stars in the Pleiades. Again, the images were sharp and, surprisingly, there was no sign of astigmatism. Finally, I made a dew shield from  a piece of rolled camping mat.

I haven’t tried the ‘scope yet with the highest magnification of 400x  as the primary needs a recoat, but it looks promising.

Try A/B testing by viewing both with and without the optical window on low contrast targets like Jupiter.  Look to see if faint features become more difficult to make out either way.