57 replies to this topic

[Stargazer3236]

I read on another posting that wrapping Reflectix on your scope prevents dewing and dewing on the corrector, is that true?

[donstim]

Works well for me in the Pacific Northwest.  Celestron 8-inch SCT.

[alphatripleplus]

Moving to Cats & Casses for a better fit.

[Mike Mc]

Works well for me, too. I've avoided dew on my corrector even on nights when I must keep my eyepiece case closed to protect against dew, and when my car is so dewed over I need to run wipers and defoggers to see to leave. I'm visual, only, and don't stay out for more than about three hours at a time. 

Edited by Mike Mc, 17 August 2020 - 02:50 PM.

[SeattleScott]

I haven’t had any dew problems in Seattle, although if I am going out for more than an hour I still use dew shield to be safe.

To me the real benefit is eliminating cool down. Dew control is the icing on the cake.

Scott

[stuart keenor]

Best thing I’ve done to my 12” zen maksutov 👍

[KTAZ]

I haven’t had any dew problems in Seattle, although if I am going out for more than an hour I still use dew shield to be safe.

To me the real benefit is eliminating cool down. Dew control is the icing on the cake.

Scott

Can you explain how it "eliminates cool down"? Are you saying that your scope retains it's temp longer instead of giving off thermals to match the outdoor temp? Or do you just get reduced thermals for a longer period?

[Peter Besenbruch]

Reflectix eliminated cool down issues on a 7" Mak, here. Dew protection is a side benefit.

[CeeKay]

Have it on my Mak 127, C8 and C9.25...  it's a good investment.

[WadeH237]

I use it on my C14.  It allows me to do high magnification viewing much earlier in the evening than if I let it cool down naturally.  Also, it goes a long way to prevent dew on the corrector, but I still have a dew heater.  There have only been a couple of cases in the last few years where I picked up a small amount of dew (even with the Reflectix) for which I needed the heater.

 

Note that use a flexible Astrozap dew shield as well as Reflectix.  The dew shield extends the length of the scope by 18 to 20 inches or so.  When the scope is wrapped, the dew shield extends just about .5 inches beyond the Reflectix.  You don't want shiny material on the inside.  So if you don't use a dew shield like I do, you would need to do something to dull the inside of the Reflectix where it extends past the front of the scope.  You would probably get some nasty reflections and stray light otherwise.

[Bean614]

Can you explain how it "eliminates cool down"? Are you saying that your scope retains it's temp longer instead of giving off thermals to match the outdoor temp? Or do you just get reduced thermals for a longer period?

I think this might have been explained to you previously,  but, in a simplified version, the answer is Yes, that's what it does.

The internal thermals in a sealed SCT or Mak, which cause the blurry, mirage like views with the wavy verticals, are caused by the warm (or cold) mirror trying to get to the ambient (in this case, outside) temperature.   That natural process is slowed CONSIDERABLY by having the tube insulated (Reflectix). Thus, no thermals.  This equates to sharp, steady, distortion free views as soon as you take your scope outside and start viewing.   Even from a 70F home into 10F air, and even from an air conditioned 70F home into 90F air.  Going from warm home to cool outside also has the benefit of dew prevention. 

[KTAZ]

I think this might have been explained to you previously,  but, in a simplified version, the answer is Yes, that's what it does.

The internal thermals in a sealed SCT or Mak, which cause the blurry, mirage like views with the wavy verticals, are caused by the warm (or cold) mirror trying to get to the ambient (in this case, outside) temperature.   That natural process is slowed CONSIDERABLY by having the tube insulated (Reflectix). Thus, no thermals.  This equates to sharp, steady, distortion free views as soon as you take your scope outside and start viewing.   Even from a 70F home into 10F air, and even from an air conditioned 70F home into 90F air.  Going from warm home to cool outside also has the benefit of dew prevention. 

Well, it looks like I need a Reflectix blanket for my 9.25 SCT then. My routine is to keep it inside around 80F, but outside this time of year is at least 95F what I step outside after dark.

 

And frankly, I was always curious if thermals are generated as strongly by a mirror coming up to temp versus coming down to temp.

Edited by KTAZ, 17 August 2020 - 09:08 PM.

[Bean614]

If it's 15F WARMER outside than in, this is advisable:  First, of course, make sure you have the dew shield on, and both the OTA and dew shield wrapped in Reflectix BEFORE you bring it out!!!!  Then, just slip a little plastic 'baggie' over the front (you know, the kind that grocery stores used to provide) before you bring it out.   No need to use your front 'cover'.  Leave the baggie on for a few minutes to impede the dew, then take it off.

[Stargazer3236]

What about Leaving the scope outside all the time to acclimate to the outside air. Then you put on your dew shield then the Reflectix on over that. Will that also help keep the dew off the corrector?

[luxo II]

Can you explain how it "eliminates cool down"? Are you saying that your scope retains it's temp longer instead of giving off thermals to match the outdoor temp? Or do you just get reduced thermals for a longer period?

Ok there's an older thread that contains the detailed explanation but I'll spare you 17 pages as it caused quite a kerfuffle when this was first described.

 

What started it all was the old mantra of "must cool the scope", while true to reduce thermal currents in open-tube scopes (newtonians, RC and classical cassegrains), for scopes with closed tubes (SCT and maks) it creates another problem - the corrector dews quickly. So people then apply heat (dewstraps) in the hope of warming the corrector, which begs the question - was cooling it really the smartest thing to do ?

 

There is, however, an alternative solution, and the clue lies in identifying the source of the thermal plume seen in SCT's and maksutovs - the central baffle tube. When the scope is taken outside a metal OTA exposed to the night air cools rapidly by both convection and radiative cooling (to the dark sky) creating a temperature gradient with the internal air circulating between the cold OTA and the warmer baffle inside. It is this thermal gradient that drives the thermal plume. What's more, the primary mirror holds a lot of heat that is also being dissipated, mostly into the central baffle.

 

If insulation is applied to the OTA to reduce the rate it loses heat, the result is the thermal plume inside is likewise reduced and the result is visible in minutes.

 

There are two side benefits - retaining heat means the corrector doesn't cool so fast and will stay dew-free for longer and the insulation can be extended out the front to form a dew-cap. You could even put a heater strap inside the insulation and its effectiveness will be increased.

 

Win-win-win.

Edited by luxo II, 18 August 2020 - 03:36 AM.

[Bean614]

What about Leaving the scope outside all the time to acclimate to the outside air. Then you put on your dew shield then the Reflectix on over that. Will that also help keep the dew off the corrector?

NO!!!  Follow please---- If it's colder outside than inside, the Insulation is EXACTLY like a winter coat YOU would wear.  You put the Coat on INSIDE, before you go out!  You don't go outside and then put your coat on!

The Insulation Preserves the warmth inside the tube.

[Jethro7]

Hello everyone,

The thought of insulting a SCT is counter intuitive at best but after reading postings and articles by Yellowbeard, I said what the heck it's cheap and not permanent. So I bought Reflextix and covered my C8 Edge HD and what do you know it is ready for use immediately after set up  and as the night cools down I cannot detect any loss of  performance simply because it slows the telescopes temperature  equilibrium down to a point that it acts like it is stable all the time.  Doing a star test the airy disc does not have much of a pronounced  tube current signature. Lately I have used my SCT with just a dew shield without dew heaters and have zero dew issues. Unbelievable, it actually works and as a bonus unlike most Astro gear accessories it is cheap.

 

HAPPY SKIES AND KEEP LOOKING UP Jethro

 

 

Edited by Jethro7, 18 August 2020 - 10:42 PM.

[luxo II]

What about Leaving the scope outside all the time to acclimate to the outside air. Then you put on your dew shield then the Reflectix on over that. Will that also help keep the dew off the corrector?

In that situation the scope is already cold so there wont be an internal tube current - but it will be covered in dew as the scope will also be below the dew point. If you aren't storing it in a warm place, you'll need to heat the OTA gently. The insulation will also be beneficial with a heater strap, by reducing the heat losses and distributing the heat evenly.

 

Think Different™

Edited by luxo II, 19 August 2020 - 12:29 AM.

[Rock22]

Reflectix works for me, too.  I have a 180mm Orion mak and I insulated the OTA (including the rear) and dew shield.

 

I found that my dew shield needed to be aired out (brought to ambient temp) before using it, if it was noticeably warmer in the house than outside.  It's a homemade dew shield made from stiff felt and poster board, spray painted inside with flat black primer.  It was causing heat plumes, so air it out (still wrapped in Reflectix) as I set up my mount, then put dew shield back on the scope.  I just do this every time I use my mak, and I can view right away.  I love using the mak - for me, most fun to use out of all of my scopes.  Probably would not have been the case without insulation.

 

 

Just be open to experimenting to find out what works for your scope in your environment and observing conditions.  My OTA is double insulated with Reflectix, as is my dew shield.  The rear has only one layer.  Good when I observe spring, summer, fall, and winter.

[gnowellsct]

Can you explain how it "eliminates cool down"? Are you saying that your scope retains it's temp longer instead of giving off thermals to match the outdoor temp? Or do you just get reduced thermals for a longer period?

It keeps the scope from cooling down and the heat which is thus retained rises to the corrector plate and prevents dew formation.

The other issue is that on hard Rock surfaces like an asphalt or cement driveway or even a gravely rocky surface like you find in desert parks in the southwest, you get tremendous radiation of heat. The radiated heat is transferred to the tube which then re-radiates the heat to the sky. That adds up to a lot of thermal energy transfer and so reflectix can help with that.

observing on grass in the deciduous northeast I have never had a problem with cooldown. In fact the biggest problem I have is keeping my scope warm enough to prevent dew formation. I used insulation for a year and a half or two years and then decided to install a dew heater inside the scope. It sounds like a terrible thermal intrusion but actually it's 20 watts distributed over 150 square inches of corrector plate and is a very minor visual disturbance.

Insulation and observing over a grass surface are the two major things that you can do to control thermals in my opinion. My asphalt driveway runs 10 to 15° hotter than the adjacent grass for long hours into the night.

Edited by gnowellsct, 21 August 2020 - 04:38 PM.

[KTAZ]

What is this "dew" that you speak of?

 

Sorry, when you live in a place whose relative humidity averages in the single digits most of the year. it is hard to resist!

[gnowellsct]

What is this "dew" that you speak of?

 

Sorry, when you live in a place whose relative humidity averages in the single digits most of the year. it is hard to resist!

Well it would be funny except that much of the terrain east of the Mississippi is deciduous and humid and lush, and much of the Midwest as well.  Yet we regularly get lectured that making the scope COOLER is the main problem with using an SCT.  I don't think that's true.  Cool down is a hard rock The main problem operational problem is warming it up just over the dew point.

 

I would be very interested to drag my C14 out to Colorado and see how it handles.

 

Greg N

[billhinge]

I'm going to try this but instead of Reflectix, out of curiosity would an aerogel blanket be even better or overkill? (waterproof and much better R value)

 

How long does the Reflectix work for per night?

[Jeff B]

I read on another posting that wrapping Reflectix on your scope prevents dewing and dewing on the corrector, is that true?

Yes.

[elwaine]

Another consideration, in addition to preventing dew formation and thermals during initial cool down, is that insulation keeps a closed tube reflector free of thermals and dew formation during periods of rapid changes in ambient temperature during cold winter nights. That may not be much of an issue for visual only folks, but dealing with rapid temp. deltas can be a royal pain for astrophotographers. 

 

After first trying out insulation on my former TEC 6" Mak, I knew I would never go back to "naked" OTAs again. I currently use an insulation jacket on my 7" TEC Mak. I wraped a thin sheet of a white rubber foam material over a Reflectix-like insulating material. I did that for aesthetic reasons only. Reflectix alone will do the job.

 

Depending on the diameter of your OTA, and your environment, you will find that either one or two wraps of reflectix are needed. With an 8" SCT, in Waltham, MA, a single layer of Reflectix should work... unless the winters are colder now than they were when I lived in Southern New Hampshire.